CaltechAUTHORS: Article
https://feeds.library.caltech.edu/people/Eisenstein-J-P/article.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 12 Sep 2024 18:58:27 -0700Observations of a Critical Current in 3He-B
https://resolver.caltech.edu/CaltechAUTHORS:EISprl79
Year: 1979
DOI: 10.1103/PhysRevLett.43.1676
The flow of 3He-B is studied in a U-tube geometry. The current is found to saturate at a critical value, Js,c, which is independent of driving force. The current Js,c scales with temperature as (1-T/Tc)^3/2 in agreement with Ginzburg-Landau theory.https://resolver.caltech.edu/CaltechAUTHORS:EISprl79Effect of the Quasiparticle Mean Free Path on Poiseuille Flow in Normal Liquid 3He
https://resolver.caltech.edu/CaltechAUTHORS:EISprl80
Year: 1980
DOI: 10.1103/PhysRevLett.45.1199
Direct observations of the effect of quasiparticle mean free path on the hydrodynamics of normal liquid 3He are presented. Both the viscosity and the mean free path are found to vary as T^-2 down to 1.5 mK. The relevance of these observations for other 3He experiments is mentioned.https://resolver.caltech.edu/CaltechAUTHORS:EISprl80Measurement of Anisotropy in the Dielectric Constant of 3He-A
https://resolver.caltech.edu/CaltechAUTHORS:SWIprl80
Year: 1980
DOI: 10.1103/PhysRevLett.45.1955
The first observation of anisotropy in the dielectric constant of superfluid 3He-A is reported. The measurement is made using parallel-plate capacitors immersed in 3He. A magnetic field is used to orient the anisotropy axis l-^. At TAB and 29 bars, the anisotropy is only εl-^⊥E→-εl-^∥E→=5×10^-11.https://resolver.caltech.edu/CaltechAUTHORS:SWIprl80Observation of Flow Dissipation in 3He-B
https://resolver.caltech.edu/CaltechAUTHORS:EISprl82
Year: 1982
DOI: 10.1103/PhysRevLett.49.564
Anomalous dissipation is observed in 3He-B flowing in a U-tube device. The dissipation is of unknown origin and persists to the lowest measured velocity. The position of this result in the framework of other 3He-B flow experiments is discussed.https://resolver.caltech.edu/CaltechAUTHORS:EISprl82Effect of Inversion Symmetry on the Band Structure of Semiconductor Heterostructures
https://resolver.caltech.edu/CaltechAUTHORS:EISprl84
Year: 1984
DOI: 10.1103/PhysRevLett.53.2579
Two classes of artificial semiconductor heterostructures, differing only in the inversion symmetry of their internal quantum wells, are studied via magnetotransport. The samples consist of GaAs/(AlGa) As layered structures containing two-dimensional hole systems. The results reveal a lifting of the spin degeneracy of the lowest hole subband in the samples with inversion asymmetric quantum wells. In those structures with symmetric wells the subband remains doubly degenerate.https://resolver.caltech.edu/CaltechAUTHORS:EISprl84High-precision torsional magnetometer: Application to two-dimensional electron systems
https://resolver.caltech.edu/CaltechAUTHORS:EISapl85
Year: 1985
DOI: 10.1063/1.95478
A dc torsional magnetometer for use in high magnetic fields is described. With a resolution of 10^–12 J/T at 5 T and excellent rejection of background moments, this device has been used to study the de Haas–van Alphen effect in two-dimensional electron systems. This resolution is about 100 times that obtained with a commercially available superconducting quantum interference device magnetometer. The device is useful over a wide temperature range including that below 1 K.https://resolver.caltech.edu/CaltechAUTHORS:EISapl85Density of States and de Haas—van Alphen Effect in Two-Dimensional Electron Systems
https://resolver.caltech.edu/CaltechAUTHORS:EISprl85
Year: 1985
DOI: 10.1103/PhysRevLett.55.875
The density of states of two-dimensional electron systems in GaAs/AlGaAs single-layer and multilayer heterostructures has been determined through measurements of the high-field magnetization. Our results reveal a substantial density of states between Landau levels, even in high-mobility single quantum wells. There is no existing theoretical explanation for this anomaly.https://resolver.caltech.edu/CaltechAUTHORS:EISprl85Quantization of the Hall effect in an anisotropic three-dimensional electronic system
https://resolver.caltech.edu/CaltechAUTHORS:STOprl86
Year: 1986
DOI: 10.1103/PhysRevLett.56.85
Quantization of the Hall effect and concomitantly vanishing magnetoresistance are observed in a GaAs/(AlGa)As superlattice structure whose electronic spectrum exhibits dispersion in all three spatial dimensions.https://resolver.caltech.edu/CaltechAUTHORS:STOprl86Observation of One-Dimensional Second Sound in Superfluid Helium
https://resolver.caltech.edu/CaltechAUTHORS:EISprl86
Year: 1986
DOI: 10.1103/PhysRevLett.57.839
A new collective mode of heat transport has been observed in superfluid helium at low temperatures and pressures. Unlike ordinary second sound, this new mode travels at essentially the acoustic-sound velocity and is characterized by the nearly collinear phonon relaxation arising from anomalous dispersion. The mode has been found to persist into the collisionless regime, ωτ>1.https://resolver.caltech.edu/CaltechAUTHORS:EISprl86Quantum oscillations in the thermal conductance of GaAs/AlGaAs heterostructures
https://resolver.caltech.edu/CaltechAUTHORS:EISprl87
Year: 1987
DOI: 10.1103/PhysRevLett.59.1341
Magneto-oscillations have, for the first time, been observed in the thermal conductance of GaAs/AlGaAs heterostructures containing two-dimensional electron systems. The oscillations result from a modulation of the thermal-phonon lifetime via coupling to the Landau-quantized electrons confined in quantum wells near the sample surface. These thermal-conductance measurements provide a new avenue for study of both the high-field density of states and the electron-phonon interaction in these semiconductor systems.https://resolver.caltech.edu/CaltechAUTHORS:EISprl87Observation of an Even-Denominator Quantum Number in the Fractional Quantum Hall Effect
https://resolver.caltech.edu/CaltechAUTHORS:20120626-114637114
Year: 1987
DOI: 10.1103/PhysRevLett.59.1776
An even-denominator rational quantum number has been observed in the Hall resistance of a two-dimensional electron system. At partial filling of the second Landau level ν= 2+(1/2) = 5/2 and at temperatures below 100 mK, a fractional Hall plateau develops at ρ_(xy)=(h/e^2)/(5/2) defined to better than 0.5%. Equivalent even-denominator quantization is absent in the lowest Landau level under comparable conditions.https://resolver.caltech.edu/CaltechAUTHORS:20120626-114637114Collapse of the Even-Denominator Fractional Quantum Hall Effect in Tilted Fields
https://resolver.caltech.edu/CaltechAUTHORS:EISprl88
Year: 1988
DOI: 10.1103/PhysRevLett.61.997
The newly discovered even-denominator fractional quantum Hall effect at filling factor ν=5/2 is found to collapse rapidly as the magnetic field is tilted away from the normal to the two-dimensional-electron-gas plane. No similar effect has been reported in tilted-field studies of the spin-polarized, odd-denominator effect at ν=2/3 and 1/3. Since a primary result of such tilting is increased spin splittings, the collapse of the ν=5/2 effect strongly suggests that the underlying state is not spin polarized. Condensation into such a quantum liquid must involve substantial spin reversal.https://resolver.caltech.edu/CaltechAUTHORS:EISprl88Ultralow-temperature behavior of the ν=5/2 fractional quantum hall effect
https://resolver.caltech.edu/CaltechAUTHORS:GAMprb88
Year: 1988
DOI: 10.1103/PhysRevB.38.10128
The newly discovered even-denominator fractional quantum Hall effect at filling factor ν=5/2 has been studied at ultralow temperatures. While ρxx is not found to vanish in the temperature range studied, the minimum in ρxx is seen to drop at the lowest temperatures. While this drop is insufficient to determine the energy gap, Δ, it may be combined with the temperature dependence of the background resistivity to give a value of Δ∼26 mK. Because of the high electron-phonon relaxation rate, τε^-1=(2.9×10^3)T^3 sec^-1K^-3, a minimum electron temperature of 9 mK could be obtained with a residual heat leak of 8×10^-14 W. It appears likely that ρxx approaches zero as T→0.https://resolver.caltech.edu/CaltechAUTHORS:GAMprb88Evidence for a phase transition in the fractional quantum Hall effect
https://resolver.caltech.edu/CaltechAUTHORS:EISprl89
Year: 1989
DOI: 10.1103/PhysRevLett.62.1540
We observe a novel transition between distinct fractional quantum Hall states sharing the same filling fraction ν=8/5. The transition is driven by tilting the two-dimensional electron-gas sample relative to the external magnetic field and is manifested by a sharp change in the dependence of the measured activation energy on tilt angle. After an initial decline, the activation energy abruptly begins to increase as the tilt angle exceeds about 30°. A plausible model for these results implies a transition from a spin-unpolarized quantum fluid at small angles to a polarized one at higher angles.https://resolver.caltech.edu/CaltechAUTHORS:EISprl89Evidence for a spin transition in the ν=2/3 fractional quantum Hall effect
https://resolver.caltech.edu/CaltechAUTHORS:EISprb90
Year: 1990
DOI: 10.1103/PhysRevB.41.7910
Magnetotransport measurements on a low-density two-dimensional electron system have revealed a re-entrant dependence of the activation energy on magnetic field for the fractional quantum Hall state at 2/3 filling of the lowest Landau level. The data are consistent with a change in the spin structure of the ground state at 2/3 filling, but do not provide a simple picture of the quasiparticle excitation process. As yet we find no similar effect for the ν=2/5 state.https://resolver.caltech.edu/CaltechAUTHORS:EISprb90Formation of a high quality two-dimensional electron gas on cleaved GaAs
https://resolver.caltech.edu/CaltechAUTHORS:PFEapl90
Year: 1990
DOI: 10.1063/1.103121
We have succeeded in fabricating a two-dimensional electron gas (2DEG) on the cleaved (110) edge of a GaAs wafer by molecular beam epitaxy (MBE). A (100) wafer previously prepared by MBE growth is reinstalled in the MBE chamber so that an in situ cleave exposes a fresh (110) GaAs edge for further MBE overgrowth. A sequence of Si-doped AlGaAs layers completes the modulation-doped structure at the cleaved edge. Mobilities as high as 6.1×10^5 cm^2/V s are measured in the 2DEG at the cleaved interface.https://resolver.caltech.edu/CaltechAUTHORS:PFEapl90Independently contacted two-dimensional electron systems in double quantum wells
https://resolver.caltech.edu/CaltechAUTHORS:EISapl90
Year: 1990
DOI: 10.1063/1.103882
A new technique for creating independent ohmic contacts to closely spaced two-dimensional electron systems in double quantum well (DQW) structures is described. Without use of shallow diffusion or precisely controlled etching methods, the present technique results in low-resistance contacts which can be electrostatically switched between the two-conducting layers. The method is demonstrated with a DQW consisting of two 200 Å GaAs quantum wells separated by a 175 Å AlGaAs barrier. A wide variety of experiments on Coulomb and tunnel-coupled 2D electron systems is now accessible.https://resolver.caltech.edu/CaltechAUTHORS:EISapl90Mutual friction between parallel two-dimensional electron systems
https://resolver.caltech.edu/CaltechAUTHORS:GRAprl91
Year: 1991
DOI: 10.1103/PhysRevLett.66.1216
Frictional drag between isolated two-dimensional electron gases separated by a thin barrier has been observed at low temperatures in GaAs/AlGaAs double-quantum-well structures. Separate electrical connection to the two electron systems allows the injection of current into one and the detection of a small drag voltage across the other. The drag voltage is a direct measure of the interwell momentum relaxation rate. Measurements of this rate are in qualitative agreement with calculations of an interwell Coulomb scattering model.https://resolver.caltech.edu/CaltechAUTHORS:GRAprl91Field-induced resonant tunneling between parallel two-dimensional electron systems
https://resolver.caltech.edu/CaltechAUTHORS:EISapl91
Year: 1991
DOI: 10.1063/1.105157
Resonant tunneling between two high-mobility two-dimensional (2D) electron systems in a double quantum well structure has been induced by the action of an external Schottky gate field. Using one 2D electron gas as source and the other as drain, the tunnel conductance between them shows a strong resonance when the gate field aligns the ground subband edges of the two quantum wells.https://resolver.caltech.edu/CaltechAUTHORS:EISapl91Probing a two-dimensional Fermi surface by tunneling
https://resolver.caltech.edu/CaltechAUTHORS:EISprb91
Year: 1991
DOI: 10.1103/PhysRevB.44.6511
Equilibrium tunneling between parallel two-dimensional electron systems is measured as a function of in-plane magnetic field and sheet density. For equal two-dimensional densities the tunnel conductance is sharply peaked around zero field, but varies slowly at intermediate fields. Around 6 T the conductance exhibits a weaker peak followed by an abrupt drop to zero. A simple model of two displaced, but intersecting, Fermi circles explains these results.https://resolver.caltech.edu/CaltechAUTHORS:EISprb91Negative compressibility of interacting two-dimensional electron and quasiparticle gases
https://resolver.caltech.edu/CaltechAUTHORS:EISprl92a
Year: 1992
DOI: 10.1103/PhysRevLett.68.674
Direct measurements of the compressibility K of the 2D electron gas at both zero magnetic field B and in the high-field extreme quantum limit, ν<1, are reported. A new method is used to determine both the magnitude and sign of K. At B=0 we find K^-1 changes sign and becomes negative as the 2D density is reduced. At high B, K^-1 exhibits negative anomalies near both ν=0 and 1. Compressibility features near the fractional fillings ν=1/3 and 2/3 give thermodynamic evidence for dilute interacting quasiparticle gases.https://resolver.caltech.edu/CaltechAUTHORS:EISprl92aNew fractional quantum Hall state in double-layer two-dimensional electron systems
https://resolver.caltech.edu/CaltechAUTHORS:EISprl92b
Year: 1992
DOI: 10.1103/PhysRevLett.68.1383
Transport studies on bilayer two-dimensional electron systems in GaAs double quantum wells have revealed a new fractional quantum Hall state that has no known counterpart in a single-layer 2D system. At total filling fraction ν=1/2 we observe a deep minimum in the diagonal resistivity and a flat Hall plateau within 1.5% of 2h/e^2. Studies of this new state in several samples with varying densities and layer separations strongly suggest this new state arises from interlayer Coulomb correlations. The data also suggest that the ν=1 quantum Hall effect seen in these samples has a similar origin.https://resolver.caltech.edu/CaltechAUTHORS:EISprl92bMultiquantum well structure with an average electron mobility of 4.0×10^6 cm^2/V s
https://resolver.caltech.edu/CaltechAUTHORS:PFEapl92
Year: 1992
DOI: 10.1063/1.107597
We report a modulation-doped multiquantum well structure which suppresses the usual ambient light effect associated with modulation doping. Ten GaAs quantum wells 300-Å wide are symmetrically modulation doped using Si δ doping at the center of 3600-Å-wide Al0.1Ga0.9As barriers. The low field mobility of each well is 4.0×10^6 cm/V s at a density of 6.4×10^10 cm^−2 measured at 0.3 K either in the dark, or during, or after, exposure to light. This mobility is an order of magnitude improvement over previous work on multiwells.https://resolver.caltech.edu/CaltechAUTHORS:PFEapl92Coulomb barrier to tunneling between parallel two-dimensional electron systems
https://resolver.caltech.edu/CaltechAUTHORS:EISprl92c
Year: 1992
DOI: 10.1103/PhysRevLett.69.3804
Tunneling between parallel two-dimensional electron gases in double quantum wells is examined at both zero and high perpendicular magnetic field. The measured I-V characteristics show the magnetic field qualitatively alters the tunneling density of states, creating both a wide gap at the Fermi level and a much broader density of final states than is present at zero field. We suggest the origin of these effects lies in the strong Coulomb correlations characteristic of Landau quantized two-dimensional systems.https://resolver.caltech.edu/CaltechAUTHORS:EISprl92cEvidence for virtual-phonon exchange in semiconductor heterostructures
https://resolver.caltech.edu/CaltechAUTHORS:GRAprb93
Year: 1993
DOI: 10.1103/PhysRevB.47.12957
Measurements of frictional drag between adjacent electron gases in double quantum wells provide strong evidence for phonon-mediated electron-electron interactions. These interactions, which dominate the contribution from simple Coulomb scattering at layer spacings larger than a few hundred Å, are between 20 and 100 times stronger than expected for sequential emission and absorption of real phonons. The observed range of the interactions, substantially smaller than the acoustic phonon mean free path, points to a novel electron scattering mechanism involving virtual-phonon exchange.https://resolver.caltech.edu/CaltechAUTHORS:GRAprb93Many-body integer quantum Hall effect: Evidence for new phase transitions
https://resolver.caltech.edu/CaltechAUTHORS:MURprl94
Year: 1994
DOI: 10.1103/PhysRevLett.72.728
The ν=1 quantum Hall effect in bilayer 2D electron systems is shown to continuously evolve from a regime dominated by single-particle tunneling into one where interlayer Coulomb interactions stabilize the state. This many-body integer quantum Hall state exhibits a phase transition to a compressible state at large layer separation. We also find evidence for an intriguing and unexpected second transition to a new incompressible state, driven by an in-plane magnetic field. While the origin of this last result is unclear, we discuss a recent model of a pseudospin textural phase transition.https://resolver.caltech.edu/CaltechAUTHORS:MURprl94Compressibility of the two-dimensional electron gas: Measurements of the zero-field exchange energy and fractional quantum Hall gap
https://resolver.caltech.edu/CaltechAUTHORS:EISprb94
Year: 1994
DOI: 10.1103/PhysRevB.50.1760
A quantitative study of the compressibility of the two-dimensional electron gas in GaAs heterostrutures is reported. Using a recently developed capacitive technique that avoids the large offset signals characteristic of conventional methods, high-precision compressibility data at both zero and high magnetic field has been obtained. The curious negative sign of the compressibility in certain regimes is shown to be a consequence of electron-electron interactions. Detailed numerical calculations show that the zero-field data are fully consistent with the known exchange energy, provided the finite thickness of the electron gas is properly included. At high magnetic fields, in the extreme quantum limit, the integrated compressibility signal is used to obtain a quantitative measure of the chemical potential discontinuity associated with the nu = 1/3 fractional quantum Hall effect. Comparison with a theoretical model which includes quasiparticle interactions has allowed a determination of the inhomogeneous broadening due to density fluctuations and has provided evidence of a second, apparently distinct, source of disorder. While the origin of this disorder is not fully understood, the data are consistent with simple lifetime broadening of the quasiparticle states.https://resolver.caltech.edu/CaltechAUTHORS:EISprb94Separately contacted electron-hole double layer in a GaAs/AlxGa1−xAs heterostructure
https://resolver.caltech.edu/CaltechAUTHORS:KANapl94
Year: 1994
DOI: 10.1063/1.112432
We describe a method for creating closely spaced parallel two-dimensional electron and hole gases confined in 200 Å GaAs wells separated by a 200 Å wide AlxGa1−xAs barrier. Low-temperature ohmic contacts are made to both the electrons and holes, whose densities are individually adjustable between 10^(10)/cm^2 to greater than 10^(11)/cm^2.https://resolver.caltech.edu/CaltechAUTHORS:KANapl94Lifetime of two-dimensional electrons measured by tunneling spectroscopy
https://resolver.caltech.edu/CaltechAUTHORS:20190521-085840789
Year: 1995
DOI: 10.1103/physrevb.52.14825
For electrons tunneling between parallel two-dimensional electron systems, conservation of in-plane momentum produces sharply resonant current-voltage characteristics and provides a uniquely sensitive probe of the underlying electronic spectral functions. We report here the application of this technique to accurate measurements of the temperature dependence of the electron-electron scattering rate in clean two-dimensional systems. Our results are in qualitative agreement with existing calculations.https://resolver.caltech.edu/CaltechAUTHORS:20190521-085840789Evidence for Skyrmions and single spin flips in the integer quantized Hall effect
https://resolver.caltech.edu/CaltechAUTHORS:SCHMprl95
Year: 1995
DOI: 10.1103/PhysRevLett.75.4290
We have employed tilted-field magnetotransport measurements of the energy gap for the odd-integer quantized Hall states at Landau level filling factors ν=1, 3, and 5 to determine the spin of thermally excited quasielectron-quasihole pairs. At ν=1 our data show that as many as 7 electron spin flips accompany such excitations, while at ν=3 and 5 apparently only a single spin flips. These results lend support to the recent suggestions that "Skyrmionic" quasiparticles are the lowest-lying charged excitations of the fully polarized ν=1 quantum Hall fluid but are not at the higher odd-integer fillings.https://resolver.caltech.edu/CaltechAUTHORS:SCHMprl95Coulomb drag in the extreme quantum limit
https://resolver.caltech.edu/CaltechAUTHORS:LILprl98
Year: 1998
DOI: 10.1103/PhysRevLett.80.1714
Coulomb drag resulting from interlayer electron-electron scattering in double layer 2D electron systems in a high magnetic field has been measured. Within the lowest Landau level the observed drag resistance exceeds its zero magnetic value by factors of typically 1000. At half-filling of the lowest Landau level in each layer (ν = 1/2) the data suggest that our bilayer systems are much more strongly correlated than recent theoretical models based on perturbatively coupled composite fermion metals.https://resolver.caltech.edu/CaltechAUTHORS:LILprl98Evidence for an anisotropic state of two-dimensional electrons in high Landau levels
https://resolver.caltech.edu/CaltechAUTHORS:LILprl99a
Year: 1999
DOI: 10.1103/PhysRevLett.82.394
Magnetotransport experiments on high mobility two-dimensional electron gases in GaAs/AlGaAs heterostructures have revealed striking anomalies near half filling of several spin-resolved, yet highly excited, Landau levels. These anomalies include strong anisotropies and nonlinearities of the longitudinal resistivity ρxx which commence only below about 150 mK. These phenomena are not seen in the ground state or first excited Landau level but begin abruptly in the third level. Although their origin remains unclear, we speculate that they reflect the spontaneous development of a generic anisotropic many-electron state.https://resolver.caltech.edu/CaltechAUTHORS:LILprl99aAnisotropic States of Two-Dimensional Electron Systems in High Landau Levels: Effect of an In-Plane Magnetic Field
https://resolver.caltech.edu/CaltechAUTHORS:LILprl99b
Year: 1999
DOI: 10.1103/PhysRevLett.83.824
We report the observation of an acute sensitivity of the anisotropic longitudinal resistivity of two-dimensional electron systems in half-filled high Landau levels to the magnitude and orientation of an in-plane magnetic field. In the third and higher Landau levels, at filling fractions ν = 9/2, 11/2, etc., the in-plane field can lead to a striking interchange of the "hard" and "easy" transport directions. In the second Landau level the normally isotropic resistivity and the weak ν = 5/2 quantized Hall state are destroyed by a large in-plane field and the transport becomes highly anisotropic.https://resolver.caltech.edu/CaltechAUTHORS:LILprl99bInsulating phases of two-dimensional electrons in high Landau levels: Observation of sharp thresholds to conduction
https://resolver.caltech.edu/CaltechAUTHORS:COOprb99
Year: 1999
DOI: 10.1103/PhysRevB.60.R11285
The intriguing re-entrant integer quantized Hall states recently discovered in high Landau levels of high-mobility 2D electron systems are found to exhibit extremely nonlinear transport. At small currents these states reflect insulating behavior of the electrons in the uppermost Landau level. At larger currents, however, a discontinuous and hysteretic transition to a conducting state is observed. These phenomena, found only in very narrow magnetic field ranges, are suggestive of the depinning of a charge density wave state, but other explanations can also be constructed.https://resolver.caltech.edu/CaltechAUTHORS:COOprb99New physics in high Landau levels
https://resolver.caltech.edu/CaltechAUTHORS:20190521-083359877
Year: 2000
DOI: 10.1016/s1386-9477(99)00043-0
Recent magneto-transport experiments on ultra-high mobility 2D electron systems in GaAs/AlGaAs heterostructures have revealed the existence of whole new classes of correlated many-electron states in highly excited Landau levels. These new states, which appear only at extremely low temperatures, are distinctly different from the familiar fractional quantum Hall liquids of the lowest Landau level. Prominent among the recent findings are the discoveries of giant anisotropies in the resistivity near half-filling of the third and higher Landau levels and the observation of re-entrant integer quantum Hall states in the flanks of these same levels. This contribution will survey the present status of this emerging field.https://resolver.caltech.edu/CaltechAUTHORS:20190521-083359877High frequency conductivity of the high-mobility two-dimensional electron gas
https://resolver.caltech.edu/CaltechAUTHORS:BURapl00b
Year: 2000
DOI: 10.1063/1.125881
We measure the real and imaginary conductivity sigma(k = 0,omega) of a high-mobility two-dimensional electron gas (2DEG) system at frequencies below and above the momentum scattering rate. The imaginary part of the 2DEG impedance is observed to be inductive, consistent with the Drude model. Using this kinetic inductance, we construct a transmission line by capacitively coupling the 2DEG to an Al Schottky barrier gate separated by 5000 Å from the 2DEG. The measured wave velocity and temperature-dependent damping of this transmission line are in good agreement with a simple Drude model. Exciting these modes is equivalent to exciting a 2D plasma mode strongly modified by the interaction between the 2DEG and the gate.https://resolver.caltech.edu/CaltechAUTHORS:BURapl00bResonantly Enhanced Tunneling in a Double Layer Quantum Hall Ferromagnet
https://resolver.caltech.edu/CaltechAUTHORS:SPIprl00
Year: 2000
DOI: 10.1103/PhysRevLett.84.5808
The tunneling conductance between two parallel 2D electron systems has been measured in a regime of strong interlayer Coulomb correlations. At total Landau level filling νT = 1 the tunnel spectrum changes qualitatively when the boundary separating the compressible phase from the ferromagnetic quantized Hall state is crossed. A huge resonant enhancement replaces the strongly suppressed equilibrium tunneling characteristic of weakly coupled layers. The possible relationship of this enhancement to the Goldstone mode of the broken symmetry ground state is discussed.https://resolver.caltech.edu/CaltechAUTHORS:SPIprl00New collective states of 2D electrons in high Landau levels
https://resolver.caltech.edu/CaltechAUTHORS:20190521-084707623
Year: 2001
DOI: 10.1016/s1386-9477(00)00170-3
A brief summary of the emerging evidence for a new class of collective states of two-dimensional electrons in partially occupied excited Landau levels is presented. Among the most dramatic phenomena described are the large anisotropies of the resistivity observed at very low temperatures near half-filling of the third and higher Landau levels and the non-linear character of the re-entrant integer quantized Hall states in the flanks of the same levels. The degree to which these findings support recent theoretical predictions of charge density wave ground states is discussed and a preliminary comparison to recent transport theories is made.https://resolver.caltech.edu/CaltechAUTHORS:20190521-084707623An investigation of orientational symmetry-breaking mechanisms in high Landau levels
https://resolver.caltech.edu/CaltechAUTHORS:20190521-091750235
Year: 2001
DOI: 10.1016/s0038-1098(01)00212-5
The principal axes of the recently discovered anisotropic phases of 2D electron systems at high Landau level occupancy are consistently oriented relative to the crystal axes of the host semiconductor. The nature of the native rotational symmetry breaking field responsible for this preferential orientation remains unknown. Here we report on experiments designed to investigate the origin and magnitude of this symmetry breaking field. Our results suggest that neither micron-scale surface roughness features nor the precise symmetry of the quantum well potential confining the 2D system are important factors. By combining tilted field transport measurements with detailed self-consistent calculations we estimate that the native anisotropy energy, whatever its origin, is typically ∼1 mK per electron.https://resolver.caltech.edu/CaltechAUTHORS:20190521-091750235Observation of a linearly dispersing collective mode in a quantum Hall ferromagnet
https://resolver.caltech.edu/CaltechAUTHORS:SPIprl01
Year: 2001
DOI: 10.1103/PhysRevLett.87.036803
Double-layer two-dimensional electron systems can exhibit a fascinating collective phase believed to display both quantum ferromagnetism and excitonic superfluidity. This unusual phase has recently been found to exhibit tunneling phenomena reminiscent of the Josephson effect. A key element of the theoretical understanding of this bizarre quantum fluid is the existence of linearly dispersing Goldstone collective modes. Using the method of tunneling spectroscopy, we have demonstrated the existence of these modes. We find the measured velocity to be in reasonable agreement with theoretical estimates.https://resolver.caltech.edu/CaltechAUTHORS:SPIprl01An all-cryogenic THz transmission spectrometer
https://resolver.caltech.edu/CaltechAUTHORS:BURrsi02
Year: 2002
DOI: 10.1063/1.1426231
This article describes a THz transmission spectrometer for the spectral range of 2–65 cm–1 (100 GHz to 2 THz) with a spectral resolution of at least 1.8 cm–1 (50 GHz) where the source, sample, and detector are all fully contained in a cryogenic environment. Cyclotron emission from a two-dimensional electron gas (2DEG) heated with an electrical current serves as a magnetic-field tunable source. The spectrometer is demonstrated at 4.2 K by measuring the resonant cyclotron absorption of a second 2DEG. Unique aspects of the spectrometer are that (1) an ultrabroadband detector is used, and (2) the emitter is run quasicontinuously with a chopping frequency of only 1 Hz. Since optical coupling to room temperature components is not necessary, this technique is compatible with ultralow temperature (sub-100 mK) operation.https://resolver.caltech.edu/CaltechAUTHORS:BURrsi02Insulating and Fractional Quantum Hall States in the First Excited Landau Level
https://resolver.caltech.edu/CaltechAUTHORS:EISprl02
Year: 2002
DOI: 10.1103/PhysRevLett.88.076801
The observation of new insulating phases of two-dimensional electrons in the first excited Landau level is reported. These states, which are manifested as reentrant integer quantized Hall effects, exist alongside well-developed even-denominator fractional quantized Hall states at ν = 7/2 and 5/2 and new odd-denominator states at ν = 3+1/5 and 3+4/5.https://resolver.caltech.edu/CaltechAUTHORS:EISprl02Observation of Quantized Hall Drag in a Strongly Correlated Bilayer Electron System
https://resolver.caltech.edu/CaltechAUTHORS:KELprl02
Year: 2002
DOI: 10.1103/PhysRevLett.88.126804
The frictional drag between parallel two-dimensional electron systems has been measured in a regime of strong interlayer correlations. When the bilayer system enters the excitonic quantized Hall state at total Landau level filling factor νT = 1, the longitudinal component of the drag vanishes but a strong Hall component develops. The Hall drag resistance is observed to be accurately quantized at h/e^2.https://resolver.caltech.edu/CaltechAUTHORS:KELprl02Onset of anisotropic transport of two-dimensional electrons in high Landau levels: Possible isotropic-to-nematic liquid-crystal phase transition
https://resolver.caltech.edu/CaltechAUTHORS:COOprb02
Year: 2002
DOI: 10.1103/PhysRevB.65.241313
The recently discovered anisotropy of the longitudinal resistance of two-dimensional electrons near half filling of high Landau levels is found to persist to much higher temperatures T when a large in-plane magnetic field B‖ is applied. Under these conditions we find that the longitudinal resistivity scales quasilinearly with B‖/T. These observations support the notion that the onset of anisotropy at B‖=0 does not reflect the spontaneous development of charge density modulations but may instead signal an isotropic-to-nematic liquid-crystal phase transition.https://resolver.caltech.edu/CaltechAUTHORS:COOprb02Terahertz photoconductivity and plasmon modes in double-quantum-well field-effect transistors
https://resolver.caltech.edu/CaltechAUTHORS:PERapl02
Year: 2002
DOI: 10.1063/1.1497433
Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity. The voltage tuned resonance is determined by the plasma oscillations of the composite structure. The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood. The phenomenon is potentially important for fast, tunable terahertz detectors.https://resolver.caltech.edu/CaltechAUTHORS:PERapl02Evidence for 2k_F electron–electron scattering processes in Coulomb drag
https://resolver.caltech.edu/CaltechAUTHORS:20190521-081802621
Year: 2002
DOI: 10.1016/s0038-1098(02)00426-x
Measurements and calculations of Coulomb drag between two low density, closely spaced, two-dimensional electron systems are reported. The experimentally measured drag exceeds that calculated in the random phase approximation by a significant, and density dependent, factor. Studies of the dependence of the measured drag on the difference in density between the two layers clearly demonstrate that previously ignored q=2k_F scattering processes can be very important to the drag at low densities and small layer separations.https://resolver.caltech.edu/CaltechAUTHORS:20190521-081802621Resistivity of dilute 2D electrons in an undoped GaAs heterostructure
https://resolver.caltech.edu/CaltechAUTHORS:LILprl03
Year: 2003
DOI: 10.1103/PhysRevLett.90.056806
We report resistivity measurements from 0.03 to 10 K in a dilute high mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a gated field-effect transistor geometry, a wide range of densities, 0.16×10^(10) to 7.5×10^(10) cm^–2, are explored. For high densities, the results are quantitatively shown to be due to scattering by acoustic phonons and impurities. In an intermediate range of densities, a peak in the resistivity is observed for temperatures below 1 K. This nonmonotonic resistivity can be understood by considering the known scattering mechanisms of phonons, bulk, and interface ionized impurities. Still lower densities appear insulating to the lowest temperature measured.https://resolver.caltech.edu/CaltechAUTHORS:LILprl03Observation of narrow-band noise accompanying the breakdown of insulating states in high Landau levels
https://resolver.caltech.edu/CaltechAUTHORS:COOprl03
Year: 2003
DOI: 10.1103/PhysRevLett.90.226803
Recent magnetotransport experiments on high mobility two-dimensional electron systems have revealed many-body electron states unique to high Landau levels. Among these are reentrant integer quantum Hall states which undergo sharp transitions to conduction above some threshold field. Here we report that these transitions are often accompanied by narrow- and broad-band noise with frequencies which are strongly dependent on the magnitude of the applied dc current.https://resolver.caltech.edu/CaltechAUTHORS:COOprl03Bilayer Quantum Hall Systems at nuT = 1: Coulomb Drag and the Transition from Weak to Strong Interlayer Coupling
https://resolver.caltech.edu/CaltechAUTHORS:KELprl03
Year: 2003
DOI: 10.1103/PhysRevLett.90.246801
Measurements revealing anomalously large frictional drag at the transition between the weakly and strongly coupled regimes of a bilayer two-dimensional electron system at total Landau level filling factor nuT = 1 are reported. This result suggests the existence of fluctuations, either static or dynamic, near the phase boundary separating the quantized Hall state at small layer separations from the compressible state at larger separations. Interestingly, the anomalies in drag seem to persist to larger layer separations than does interlayer phase coherence as detected in tunneling.https://resolver.caltech.edu/CaltechAUTHORS:KELprl03Metastable resistance-anisotropy orientation of two-dimensional electrons in high Landau levels
https://resolver.caltech.edu/CaltechAUTHORS:COOprl04
Year: 2004
DOI: 10.1103/PhysRevLett.92.026806
In half-filled high Landau levels, two-dimensional electron systems possess collective phases which exhibit a strongly anisotropic resistivity tensor. A weak, but as yet unknown, rotational symmetry-breaking potential native to the host semiconductor structure is necessary to orient these phases in macroscopic samples. Making use of the known external symmetry-breaking effect of an in-plane magnetic field, we find that the native potential can have two orthogonal local minima. It is possible to initialize the system in the higher minimum and then observe its relaxation toward equilibrium.https://resolver.caltech.edu/CaltechAUTHORS:COOprl04Vanishing Hall Resistance at High Magnetic Field in a Double-Layer Two-Dimensional Electron System
https://resolver.caltech.edu/CaltechAUTHORS:KELprl04
Year: 2004
DOI: 10.1103/PhysRevLett.93.036801
At total Landau level filling factor nutot = 1 a double-layer two-dimensional electron system with small interlayer separation supports a collective state possessing spontaneous interlayer phase coherence. This state exhibits the quantized Hall effect when equal electrical currents flow in parallel through the two layers. In contrast, if the currents in the two layers are equal, but oppositely directed, both the longitudinal and Hall resistances of each layer vanish in the low-temperature limit. This finding supports the prediction that the ground state at nutot = 1 is an excitonic superfluid.https://resolver.caltech.edu/CaltechAUTHORS:KELprl04Onset of interlayer phase coherence in a bilayer two-dimensional electron system: Effect of layer density imbalance
https://resolver.caltech.edu/CaltechAUTHORS:SPIprb04
Year: 2004
DOI: 10.1103/PhysRevB.70.081303
Tunneling and Coulomb drag are sensitive probes of spontaneous interlayer phase coherence in bilayer two-dimensional electron systems at total Landau level filling factor nu(T)=1. We find that the phase boundary between the interlayer phase coherent state and the weakly coupled compressible phase moves to larger layer separations as the electron density distribution in the bilayer is imbalanced. The critical layer separation increases quadratically with layer density difference.https://resolver.caltech.edu/CaltechAUTHORS:SPIprb04Half Full or Half Empty?
https://resolver.caltech.edu/CaltechAUTHORS:20141117-110642548
Year: 2004
DOI: 10.1126/science.1099386
Excitons--that is, pairings of electrons with holes in a semiconductor--are examples of quantum particles called bosons. Like other bosons, such as those studied in ultracold atomic vapors, excitons should form under Bose condensation and fall into a single quantum state. In his Perspective, Eisenstein discusses recent efforts to observe excitonic Bose condensation in layered materials. The major obstacle has been the tendency of the electron and the hole to simply recombine and disappear. By applying a magnetic field, however, the excitons can be stabilized, leading to increased hopes that a clear case of condensation will eventually be observed.https://resolver.caltech.edu/CaltechAUTHORS:20141117-110642548Bose-Einstein condensation of excitons in bilayer electron systems
https://resolver.caltech.edu/CaltechAUTHORS:20150401-102532343
Year: 2004
DOI: 10.1038/nature03081
An exciton is the particle-like entity that forms when an electron is bound to a positively charged 'hole'. An ordered electronic state in which excitons condense into a single quantum state was proposed as a theoretical possibility many years ago. We review recent studies of semiconductor bilayer systems that provide clear evidence for this phenomenon and explain why exciton condensation in the quantum Hall regime, where these experiments were performed, is as likely to occur in electron–electron bilayers as in electron–hole bilayers. In current quantum Hall excitonic condensates, disorder induces mobile vortices that flow in response to a supercurrent and limit the extremely large bilayer counterflow conductivity.https://resolver.caltech.edu/CaltechAUTHORS:20150401-102532343Spin transition in strongly correlated bilayer two-dimensional electron systems
https://resolver.caltech.edu/CaltechAUTHORS:SPIprl05
Year: 2005
DOI: 10.1103/PhysRevLett.94.076803
Using a combination of heat pulse and nuclear magnetic resonance techniques, we demonstrate that the phase boundary separating the interlayer phase coherent quantum Hall effect at nu(T)=1 in bilayer electron gases from the weakly coupled compressible phase depends upon the spin polarization of the nuclei in the host semiconductor crystal. Our results strongly suggest that, contrary to the usual assumption, the transition is attended by a change in the electronic spin polarization.https://resolver.caltech.edu/CaltechAUTHORS:SPIprl05Surface acoustic wave propagation and inhomogeneities in low-density two-dimensional electron systems near the metal–insulator transition
https://resolver.caltech.edu/CaltechAUTHORS:20190521-090435854
Year: 2006
DOI: 10.1016/j.ssc.2005.10.028
We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs heterostructure containing a two-dimensional electron system (2DES) in a low-density regime (<10^(10) cm^(−2)) at zero magnetic field. The interaction of the surface acoustic wave with the 2DES is not well described by a simple model using low-frequency conductivity measurements. We speculate that this conflict is a result of inhomogeneities in the 2DES, which become very important at low density. This has implications for the putative metal–insulator transition in two dimensions.https://resolver.caltech.edu/CaltechAUTHORS:20190521-090435854Resistively detected NMR in a two-dimensional electron system near ν = 1: Clues to the origin of the dispersive lineshape
https://resolver.caltech.edu/CaltechAUTHORS:TRAprb06
Year: 2006
DOI: 10.1103/PhysRevB.73.121306
Resistively detected nuclear magnetic resonance (NMR) measurements on 2D electron systems near the nu=1 quantum Hall state are reported. In contrast to recent results of Gervais et al. [Phys. Rev. Lett. 94, 196803 (2005)], a dispersive line shape is found at all radio-frequency powers studied and Korringa-type nuclear spin-lattice relaxation is observed. The shape of the unexplained dispersive line shape is found to invert when the temperature derivative of the longitudinal resistance changes sign. This suggests that both Zeeman and thermal effects are important to resistively detected NMR in this regime.https://resolver.caltech.edu/CaltechAUTHORS:TRAprb06Korringa-like nuclear spin–lattice relaxation in a 2DES at ν=1/2
https://resolver.caltech.edu/CaltechAUTHORS:20110602-091813982
Year: 2006
DOI: 10.1016/j.physe.2006.03.038
Via a resistively detected NMR technique, the nuclear spin–lattice relaxation time T_1 of ^(71)Ga has been measured in a GaAs/AlGaAs
heterostructure containing two weakly coupled 2D electron systems (2DES) at low temperatures, each at Landau level filling v=1/2.
Incomplete electronic spin polarization, which has been reported previously for low density 2DESs at v=1/2, should facilitate hyperfine-coupled
nuclear spin relaxation owing to the presence of both electron spin states at the Fermi level. Composite fermion theory suggests a
Korringa-law temperature dependence: T_1T = constant is expected for temperatures T<1K. Our measurements show that for
temperatures in the range 35mKhttps://resolver.caltech.edu/CaltechAUTHORS:20110602-091813982Spin Transition in the Half-Filled Landau Level
https://resolver.caltech.edu/CaltechAUTHORS:TRAprl07
Year: 2007
DOI: 10.1103/PhysRevLett.98.086801
The transition from partial to complete spin polarization of two-dimensional electrons at half filling of the lowest Landau level has been studied using resistively detected nuclear magnetic resonance (RDNMR). The nuclear spin-lattice relaxation time is observed to be density independent in the partially polarized phase but to increase sharply at the transition to full polarization. At low temperatures the RDNMR signal exhibits a strong maximum near the critical density.https://resolver.caltech.edu/CaltechAUTHORS:TRAprl07Quantum lifetime of two-dimensional holes
https://resolver.caltech.edu/CaltechAUTHORS:20190521-083908547
Year: 2007
DOI: 10.1016/j.ssc.2007.06.010
The quantum lifetime of two-dimensional holes in a GaAs/AlGaAs double quantum well is determined via tunneling spectroscopy. At low temperatures the lifetime is limited by impurity scattering but at higher temperatures hole–hole Coulomb scattering dominates. Our results are consistent with the Fermi liquid theory, at least up to r_s = 11. At the highest temperatures the measured width of the hole spectral function becomes comparable to the Fermi energy. A new, tunneling–spectroscopic method for determining the in-plane effective mass of the holes is also demonstrated.https://resolver.caltech.edu/CaltechAUTHORS:20190521-083908547Evidence for a Finite-Temperature Phase Transition in a Bilayer Quantum Hall System
https://resolver.caltech.edu/CaltechAUTHORS:CHAprl08
Year: 2008
DOI: 10.1103/PhysRevLett.100.096801
We study the Josephson-like interlayer tunneling signature of the strongly correlated nuT=1 quantum Hall phase in bilayer two-dimensional electron systems as a function of the layer separation, temperature, and interlayer charge imbalance. Our results offer strong evidence that a finite temperature phase transition separates the interlayer coherent phase from incoherent phases which lack strong interlayer correlations. The transition temperature is dependent on both the layer spacing and charge imbalance between the layers.https://resolver.caltech.edu/CaltechAUTHORS:CHAprl08Area dependence of interlayer tunneling in strongly correlated bilayer two-dimensional electron systems at ν_T = 1
https://resolver.caltech.edu/CaltechAUTHORS:FINprb08
Year: 2008
DOI: 10.1103/PhysRevB.78.075302
The area and perimeter dependence of the Josephson-like interlayer tunneling signature of the coherent ν_T = 1 quantum Hall phase in bilayer two-dimensional electron systems is examined. Electrostatic top gates of various sizes and shapes are used to locally define distinct ν_T = 1 regions in the same sample. Near the phase boundary with the incoherent ν_T = 1 state at large layer separation, our results demonstrate that the tunneling conductance in the coherent phase is closely proportional to the total area of the tunneling region. This implies that tunneling at ν_T = 1 is a bulk phenomenon in this regime.https://resolver.caltech.edu/CaltechAUTHORS:FINprb08Charge imbalance and bilayer two-dimensional electron systems at ν_T=1
https://resolver.caltech.edu/CaltechAUTHORS:CHAprb08
Year: 2008
DOI: 10.1103/PhysRevB.78.205310
We use interlayer tunneling to study bilayer two-dimensional electron systems at ν_T=1 over a wide range of charge-density imbalance Δν=ν_1−ν_2 between the two layers. We find that the strongly enhanced tunneling associated with the coherent excitonic ν_T=1 phase at small layer separation can survive at least up to an imbalance of Δν=0.5, i.e., (ν_1,ν_2)=(3/4,1/4). Phase transitions between the excitonic ν_T=1 state and bilayer states which lack significant interlayer correlations can be induced in three different ways: by increasing the effective interlayer spacing d/ℓ, the temperature T, or the charge imbalance Δν. We observe that close to the phase boundary the coherent ν_T=1 phase can be absent at Δν=0, present at intermediate Δν, and then absent again at large Δν, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. At zero imbalance, the critical d/ℓ shifts linearly with temperature, while at Δν=1/3 the critical d/ℓ is only weakly dependent on T. At Δν=1/3 we report on an observation of a direct phase transition between the coherent excitonic ν_T=1bilayer integer quantum Hall phase and the pair of single-layer fractional quantized Hall states at ν_1=2/3 and ν_2=1/3.https://resolver.caltech.edu/CaltechAUTHORS:CHAprb08Observation of chiral heat transport in the quantum Hall regime
https://resolver.caltech.edu/CaltechAUTHORS:20090528-075110597
Year: 2009
DOI: 10.1103/PhysRevLett.102.086803
Heat transport in the quantum Hall regime is investigated using micron-scale heaters and thermometers positioned along the edge of a millimeter-scale two dimensional electron system (2DES). The heaters rely on localized current injection into the 2DES, while the thermometers are based on the thermoelectric effect. In the v=1 integer quantized Hall state, a thermoelectric signal appears at an edge thermometer only when it is "downstream," in the sense of electronic edge transport, from the heater. When the distance between the heater and the thermometer is increased, the thermoelectric signal is reduced, showing that the electrons cool as they propagate along the edge.https://resolver.caltech.edu/CaltechAUTHORS:20090528-075110597Edge heat transport in the quantum Hall regime
https://resolver.caltech.edu/CaltechAUTHORS:20090911-154848940
Year: 2009
DOI: 10.1142/S0217979209062074
We investigate the transport of heat in the integer quantized Hall regime. We make use of quantum point contacts (QPC's) positioned along the edge of a large quantum Hall droplet to both locally heat and locally detect temperature rises at the edge of the droplet. The detection scheme is thermoelectric, in essence identical to one introduced by Molenkamp, et al. in the early 1990's for heat transport experiments at zero magnetic field.https://resolver.caltech.edu/CaltechAUTHORS:20090911-154848940Hot-electron thermocouple and the diffusion thermopower of two-dimensional electrons in GaAs
https://resolver.caltech.edu/CaltechAUTHORS:20090808-142459891
Year: 2009
DOI: 10.1103/PhysRevLett.103.046807
A simple hot-electron thermocouple is realized in a two-dimensional electron system (2DES) and used to measure the diffusion thermopower of the 2DES at zero magnetic field. This hot-electron technique, which requires no micron-scale patterning of the 2DES, is much less sensitive than conventional methods to phonon-drag effects. Our thermopower results are in good agreement with the Mott formula for diffusion thermopower for temperatures up to T~2 K.https://resolver.caltech.edu/CaltechAUTHORS:20090808-142459891Breakdown of particle-hole symmetry in the lowest Landau level revealed by tunneling spectroscopy
https://resolver.caltech.edu/CaltechAUTHORS:20100115-084107891
Year: 2009
DOI: 10.1016/j.ssc.2009.08.004
Tunneling measurements on 2D electron gases at high magnetic field reveal a qualitative difference between the two spin sublevels of the lowest Landau level. While the tunneling current–voltage characteristic at filling factor ν=1/2 is a single peak shifted from zero bias by a Coulomb pseudogap, the spectrum at ν=3/2 shows a well-resolved double peak structure. This difference is present regardless of whether ν=1/2 and ν=3/2 occur at the same or different magnetic fields. No analogous effect is seen at ν=5/2 and 7/2 in the first excited Landau level.https://resolver.caltech.edu/CaltechAUTHORS:20100115-084107891Quantum Hall Exciton Condensation at Full Spin Polarization
https://resolver.caltech.edu/CaltechAUTHORS:20100217-093146277
Year: 2010
DOI: 10.1103/PhysRevLett.104.016801
Using Coulomb drag as a probe, we explore the excitonic phase transition in quantum Hall bilayers at ν_T = 1 as a function of Zeeman energy E_Z. The critical layer separation (d/ℓ)_c for exciton condensation initially increases rapidly with E_Z, but then reaches a maximum and begins a gentle decline. At high E_Z, where both the excitonic phase at small d/ℓ and the compressible phase at large d/ℓ are fully spin polarized, we find that the width of the transition, as a function of d/ℓ, is much larger than at small E_Z and persists in the limit of zero temperature. We discuss these results in the context of two models in which the system contains a mixture of the two fluids.https://resolver.caltech.edu/CaltechAUTHORS:20100217-093146277Clausius-Clapeyron relations for first-order phase transitions in bilayer quantum Hall systems
https://resolver.caltech.edu/CaltechAUTHORS:20100615-143356094
Year: 2010
DOI: 10.1103/PhysRevB.81.205313
A bilayer system of two-dimensional electron gases in a perpendicular magnetic field exhibits rich phenomena. At total filling factor ν_(tot)=1, as one increases the layer separation, the bilayer system goes from an interlayer-coherent exciton condensed state to an incoherent phase of, most likely, two decoupled composite-fermion Fermi liquids. Many questions still remain as to the nature of the transition between these two phases. Recent experiments have demonstrated that spin plays an important role in this transition. Assuming that there is a direct first-order transition between the spin-polarized interlayer-coherent quantum Hall state and spin partially polarized composite Fermi-liquid state, we calculate the phase boundary (d/l)_c as a function of parallel magnetic field, NMR/heat pulse, temperature, and density imbalance, and compare with experimental results. Remarkably good agreement is found between theory and various experiments.https://resolver.caltech.edu/CaltechAUTHORS:20100615-143356094Thermopower of two-dimensional electrons at filling factors ν = 3/2 and 5/2
https://resolver.caltech.edu/CaltechAUTHORS:20100712-144828553
Year: 2010
DOI: 10.1103/PhysRevB.81.245319
The longitudinal thermopower of ultrahigh mobility two-dimensional (2D) electrons has been measured at both zero magnetic field and at high fields in the compressible metallic state at filling factor ν=3/2 and the incompressible fractional quantized Hall state at ν=5/2. At zero field our results demonstrate that the thermopower is dominated by electron diffusion for temperatures below about T=150 mK. A diffusion-dominated thermopower is also observed at ν=3/2 and allows us to extract an estimate of the composite fermion effective mass. At ν=5/2 both the temperature and magnetic field dependence of the observed thermopower clearly signal the presence of the energy gap of this fractional quantized Hall state. We find that the thermopower in the vicinity of ν=5/2 exceeds that recently predicted under the assumption that the entropy of the 2D system is dominated by non-Abelian quasiparticle exchange statistics.https://resolver.caltech.edu/CaltechAUTHORS:20100712-144828553Measurement of the electronic compressibility of bilayer graphene
https://resolver.caltech.edu/CaltechAUTHORS:20100817-085603958
Year: 2010
DOI: 10.1103/PhysRevB.82.041412
We present measurements of the electronic compressibility, K, of bilayer graphene in both zero and finite magnetic fields up to 14 T, and as a function of both the carrier density and electric field perpendicular to the graphene sheet. The low-energy hyperbolic band structure of bilayer graphene is clearly revealed in the data, as well as a sizable asymmetry between the conduction and valence bands. A sharp increase in K^(−1) near zero density is observed for increasing electric field strength, signaling the controlled opening of a gap between these bands. At high magnetic fields, broad Landau level (LL) oscillations are observed, directly revealing the doubled degeneracy of the lowest LL and allowing for a determination of the disorder broadening of the levels.https://resolver.caltech.edu/CaltechAUTHORS:20100817-085603958Nematic Fermi Fluids in Condensed Matter Physics
https://resolver.caltech.edu/CaltechAUTHORS:20101025-095402593
Year: 2010
DOI: 10.1146/annurev-conmatphys-070909-103925
Correlated electron fluids can exhibit a startling array of complex phases, among which one of the more surprising is the electron nematic, a translationally invariant metallic phase with a spontaneously generated spatial anisotropy. Classical nematics generally occur in liquids of rod-like molecules; given that electrons are point like, the initial theoretical motivation for contemplating electron nematics came from thinking of the electron fluid as a quantum melted electron crystal, rather than a strongly interacting descendent of a Fermi gas. Dramatic transport experiments in ultra-clean quantum Hall systems in 1999 and in Sr_3Ru_2O_7 in a strong magnetic field in 2007 established that such phases exist in nature. In this article, we briefly review the theoretical considerations governing nematic order, summarize the quantum Hall and Sr_3Ru_2O_7 experiments that unambiguously establish the existence of this phase, and survey some of the current evidence for such a phase in the cuprate and Fe-based high temperature superconductors.https://resolver.caltech.edu/CaltechAUTHORS:20101025-095402593Observations of nascent superfluidity in a bilayer two-dimensional electron system at v_T = 1
https://resolver.caltech.edu/CaltechAUTHORS:20110120-085852426
Year: 2010
DOI: 10.1016/j.physe.2006.02.011
Single-layer longitudinal and Hall resistances have been measured in a bilayer two-dimensional electron system at v_T = 1 with equal
but oppositely directed currents flowing in the two layers. At small effective layer separation and low temperature, the bilayer system
enters an interlayer coherent state expected to exhibit superfluid properties. We detect this nascent superfluidity through the vanishing of
both resistances as the temperature is reduced. This corresponds to the counterflow conductivity rising rapidly as the temperature falls,
reaching σ^(CF)_(xx) = 580(e^2/h) by T = 35 mK. This supports the prediction that the ground state of this system is an excitonic superfluid.https://resolver.caltech.edu/CaltechAUTHORS:20110120-085852426Tilt-Induced Anisotropic to Isotropic Phase Transition at ν = 5/2
https://resolver.caltech.edu/CaltechAUTHORS:20101109-090441714
Year: 2010
DOI: 10.1103/PhysRevLett.105.176807
A modest in-plane magnetic field B_(∥) is sufficient to destroy the fractional quantized Hall states at ν=5/2 and 7/2 and replace them with anisotropic compressible phases. Remarkably, we find that at larger B_(∥) these anisotropic phases can themselves be replaced by isotropic compressible phases reminiscent of the composite fermion fluid at ν=1/2. We present strong evidence that this transition is a consequence of the mixing of Landau levels from different electric subbands. We also report surprising dependences of the energy gaps at ν=5/2 and 7/3 on the width of the confinement potential.https://resolver.caltech.edu/CaltechAUTHORS:20101109-090441714Exciton Transport and Andreev Reflection in a Bilayer Quantum Hall System
https://resolver.caltech.edu/CaltechAUTHORS:20110624-092746822
Year: 2011
DOI: 10.1103/PhysRevLett.106.236807
We demonstrate that counterflowing electrical currents can move through the bulk of the excitonic quantized Hall phase found in bilayer two-dimensional electron systems (2DES) even as charged excitations cannot. These counterflowing currents are transported by neutral excitons which are emitted and absorbed at the inner and outer boundaries of an annular 2DES via Andreev reflection.https://resolver.caltech.edu/CaltechAUTHORS:20110624-092746822Evidence for a fractionally quantized Hall state with anisotropic longitudinal transport
https://resolver.caltech.edu/CaltechAUTHORS:20111213-104452350
Year: 2011
DOI: 10.1038/NPHYS2118
At high magnetic fields, where the Fermi level lies in the N=0 lowest Landau level (LL), a clean two-dimensional electron system (2DES) shows numerous incompressible liquid phases which exhibit the fractional quantum Hall effect (FQHE; ref. 1). These liquid phases do not break rotational symmetry, exhibiting resistivities which are isotropic in the plane. In contrast, at lower fields, when the Fermi level lies in the N ≥ 2 third and several higher LLs, the 2DES exhibits a distinctly different class of collective states. In particular, near half-filling of these high LLs the 2DES exhibits a strongly anisotropic longitudinal resistance at low temperatures. These 'stripe' phases, which do not exhibit the quantized Hall effect, resemble nematic liquid crystals, possessing broken rotational symmetry and orientational order. Here we report a surprising new observation: an electronic configuration in the N=1 LL, the resistivity tensor of which simultaneously exhibits a robust fractionally quantized Hall plateau and a strongly anisotropic longitudinal resistance resembling that of the stripe phases.https://resolver.caltech.edu/CaltechAUTHORS:20111213-104452350Quantum Hall Effect and Semimetallic Behavior of Dual-Gated ABA-Stacked Trilayer Graphene
https://resolver.caltech.edu/CaltechAUTHORS:20120208-130025715
Year: 2012
DOI: 10.1103/PhysRevX.2.011004
The electronic structure of multilayer graphenes depends strongly on the number of layers as well as the stacking order. Here we explore the electronic transport of purely ABA-stacked trilayer graphenes in a dual-gated field-effect device configuration. We find both that the zero-magnetic-field transport and the quantum Hall effect at high magnetic fields are distinctly different from the monolayer and bilayer graphenes, and that they show electron-hole asymmetries that are strongly suggestive of a semimetallic band overlap. When the ABA trilayers are subjected to an electric field perpendicular to the sheet, Landau-level splittings due to a lifting of the valley degeneracy are clearly observed.https://resolver.caltech.edu/CaltechAUTHORS:20120208-130025715Exciton condensation and perfect Coulomb drag
https://resolver.caltech.edu/CaltechAUTHORS:20120907-084644566
Year: 2012
DOI: 10.1038/nature11302
Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, double quantum well systems can support exciton condensates, which consist of electrons in one well tightly bound to holes in the other. 'Perfect' drag is therefore expected; a steady transport current of electrons driven through one quantum well should be accompanied by an equal current of holes in the other7. Here we demonstrate this effect, taking care to ensure that the electron–hole pairs dominate the transport and that tunnelling of charge between the quantum wells, which can readily compromise drag measurements, is negligible. We note that, from an electrical engineering perspective, perfect Coulomb drag is analogous to an electrical transformer that functions at zero frequency.https://resolver.caltech.edu/CaltechAUTHORS:20120907-084644566Thermoelectric response of fractional quantized Hall and reentrant insulating states in the N=1 Landau level
https://resolver.caltech.edu/CaltechAUTHORS:20121203-084756804
Year: 2013
DOI: 10.1103/PhysRevB.87.075302
Detailed measurements of the longitudinal thermopower of two-dimensional electrons in the first excited Landau level are reported. Clear signatures of numerous fractional quantized Hall states, including those at ν=5/2 and 7/3, are observed in the magnetic field and temperature dependence of the thermopower. An abrupt collapse of the thermopower is observed below about T=40 mK at those filling factors where reentrant insulating electronic states have been observed in conventional resistive transport studies. The thermopower observed at ν=5/2 is discussed in the context of recent theories which incorporate non-Abelian quasiparticle exchange statistics.https://resolver.caltech.edu/CaltechAUTHORS:20121203-084756804Tunneling at v_T = 1 in Quantum Hall Bilayers
https://resolver.caltech.edu/CaltechAUTHORS:20130925-121324886
Year: 2013
DOI: 10.1103/PhysRevB.88.165308
Interlayer tunneling measurements in the strongly correlated bilayer quantized Hall phase at ν_T = 1 are reported. The maximum, or critical, current for tunneling at ν_T = 1 is shown to be a well-defined global property of the coherent phase, insensitive to extrinsic circuit effects and the precise configuration used to measure it, but also exhibiting a surprising scaling behavior with temperature. Comparisons between the experimentally observed tunneling characteristics and a recent theory are favorable at high temperatures, but not at low temperatures where the tunneling closely resembles the dc Josephson effect. The zero-bias tunneling resistance becomes extremely small at low temperatures, vastly less than that observed at zero magnetic field, but nonetheless remains finite. The temperature dependence of this tunneling resistance is similar to that of the ordinary in-plane resistivity of the quantum Hall phase.https://resolver.caltech.edu/CaltechAUTHORS:20130925-121324886Exciton Condensation in Bilayer Quantum Hall Systems
https://resolver.caltech.edu/CaltechAUTHORS:20130618-085925694
Year: 2014
DOI: 10.1146/annurev-conmatphys-031113-133832
The condensation of excitons, bound electron-hole pairs in a solid, into a coherent collective electronic state was predicted more than 50 years ago. Perhaps surprisingly, the phenomenon was first observed in a system consisting of two closely spaced parallel two-dimensional electron gases in a semiconductor double quantum well. At an appropriate high magnetic field and low temperature, the bilayer electron system condenses into a state resembling a superconductor, only with the Cooper pairs replaced by excitons consisting of electrons in one layer bound to holes in the other. In spite of being charge neutral, the transport of excitons within the condensate gives rise to several spectacular electrical effects. This article describes these phenomena and examines how they inform our understanding of this unique phase of quantum electronic matter.https://resolver.caltech.edu/CaltechAUTHORS:20130618-085925694Transport in indium-decorated graphene
https://resolver.caltech.edu/CaltechAUTHORS:20150427-105709975
Year: 2015
DOI: 10.1103/PhysRevB.91.245402
The electronic transport properties of single layer graphene having a dilute coating of indium adatoms has been investigated. Our studies establish that isolated indium atoms donate electrons to graphene and become a source of charged impurity scattering, affecting the conductivity as well as magnetotransport properties of the pristine graphene. Notably, a positive magnetoresistance is observed over a wide density range after In doping. The low field magnetoresistance carries signatures of quantum interference effects which are significantly altered by the adatoms.https://resolver.caltech.edu/CaltechAUTHORS:20150427-105709975Heterostructure symmetry and the orientation of the quantum Hall nematic phases
https://resolver.caltech.edu/CaltechAUTHORS:20151002-093843583
Year: 2015
DOI: 10.1103/PhysRevB.92.115410
Clean two-dimensional electron systems in GaAs/AlGaAs heterostructures exhibit anisotropic collective phases, the quantum Hall nematics, at high Landau-level occupancy and low temperatures. An as yet unknown native symmetry-breaking potential consistently orients these phases relative to the crystalline axes of the host material. Here we report an extensive set of measurements examining the role of the structural symmetries of the heterostructure in determining the orientation of the nematics. In single quantum well samples we find that neither the local symmetry of the confinement potential nor the distance between the electron system and the sample surface dictates the orientation of the nematic. In remarkable contrast, for two-dimensional electrons confined at a single heterointerface between GaAs and AlGaAs, the nematic orientation depends on the depth of the two-dimensional electron system beneath the sample surface.https://resolver.caltech.edu/CaltechAUTHORS:20151002-093843583Evidence for defect-mediated tunneling in hexagonal boron nitride-based junctions
https://resolver.caltech.edu/CaltechAUTHORS:20151015-094529168
Year: 2015
DOI: 10.1021/acs.nanolett.5b02625
We investigate electron tunneling through atomically thin layers of hexagonal boron nitride (hBN). Metal (Cr/Au) and semimetal (graphite) counter-electrodes are employed. While the direct tunneling resistance increases nearly exponentially with barrier thickness as expected, the thicker junctions also exhibit clear signatures of Coulomb blockade, including strong suppression of the tunnel current around zero bias and step-like features in the current at larger biases. The voltage separation of these steps suggests that single-electron charging of nanometer-scale defects in the hBN barrier layer are responsible for these signatures. We find that annealing the metal–hBN–metal junctions removes these defects and the Coulomb blockade signatures in the tunneling current.https://resolver.caltech.edu/CaltechAUTHORS:20151015-094529168Spin and the Coulomb gap in the half-filled lowest Landau level
https://resolver.caltech.edu/CaltechAUTHORS:20160908-103416522
Year: 2016
DOI: 10.1103/PhysRevB.94.125409
The Coulomb gap observed in tunneling between parallel two-dimensional electron systems, each at half-filling of the lowest Landau level, is found to depend sensitively on the presence of an in-plane magnetic field. Especially at low electron density, the width of the Coulomb gap at first increases sharply with in-plane field, but then abruptly levels off. This behavior appears to coincide with the known transition from partial to complete spin polarization of the half-filled lowest Landau level. The tunneling gap therefore opens a window onto the spin configuration of two-dimensional electron systems at high magnetic field.https://resolver.caltech.edu/CaltechAUTHORS:20160908-103416522Signatures of phonon and defect-assisted tunneling in planar metal-hexagonal boron nitride-graphene junctions
https://resolver.caltech.edu/CaltechAUTHORS:20161129-090026289
Year: 2016
DOI: 10.1021/acs.nanolett.6b04369
Electron tunneling spectroscopy measurements on van der Waals heterostructures consisting of metal and graphene (or graphite) electrodes separated by atomically thin hexagonal boron nitride tunnel barriers are reported. The tunneling conductance, dI/dV, at low voltages is relatively weak, with a strong enhancement reproducibly observed to occur at around |V| ≈ 50 mV. While the weak tunneling at low energies is attributed to the absence of substantial overlap, in momentum space, of the metal and graphene Fermi surfaces, the enhancement at higher energies signals the onset of inelastic processes in which phonons in the heterostructure provide the momentum necessary to link the Fermi surfaces. Pronounced peaks in the second derivative of the tunnel current, d2I/dV2, are observed at voltages where known phonon modes in the tunnel junction have a high density of states. In addition, features in the tunneling conductance attributed to single electron charging of nanometer-scale defects in the boron nitride are also observed in these devices. The small electronic density of states of graphene allows the charging spectra of these defect states to be electrostatically tuned, leading to "Coulomb diamonds" in the tunneling conductance.https://resolver.caltech.edu/CaltechAUTHORS:20161129-090026289Charge metastability and hysteresis in the quantum Hall regime
https://resolver.caltech.edu/CaltechAUTHORS:20161206-075018469
Year: 2016
DOI: 10.1103/PhysRevB.94.245440
We report simultaneous quasi-dc magnetotransport and high-frequency surface acoustic wave measurements on bilayer two-dimensional electron systems in GaAs. Near strong integer quantized Hall states, a strong magnetic-field-sweep hysteresis in the velocity of the acoustic waves is observed at low temperatures. This hysteresis indicates the presence of a metastable state with anomalously high conductivity in the interior of the sample. This nonequilibrium state is not revealed by conventional low-frequency transport measurements which are dominated by dissipationless transport at the edge of the two-dimensional system. We find that a field-cooling technique allows the equilibrium charge configuration within the interior of the sample to be established. A simple model for this behavior is discussed.https://resolver.caltech.edu/CaltechAUTHORS:20161206-075018469Quantum Hall Spin Diode
https://resolver.caltech.edu/CaltechAUTHORS:20170505-110632677
Year: 2017
DOI: 10.1103/PhysRevLett.118.186801
Double layer two-dimensional electron systems at high perpendicular magnetic field are used to realize magnetic tunnel junctions in which the electrons at the Fermi level in the two layers have either parallel or antiparallel spin magnetizations. In the antiparallel case the tunnel junction, at low temperatures, behaves as a nearly ideal spin diode. At elevated temperatures the diode character degrades as long-wavelength spin waves are thermally excited. These tunnel junctions provide a demonstration that the spin polarization of the electrons in the N=1 Landau level at filling factors ν=5/2 and 7/2 is essentially complete, and, with the aid of an in-plane magnetic field component, that Landau level mixing at these filling factors is weak in the samples studied.https://resolver.caltech.edu/CaltechAUTHORS:20170505-110632677Tunnel transport and interlayer excitons in bilayer fractional quantum Hall systems
https://resolver.caltech.edu/CaltechAUTHORS:20170503-105416560
Year: 2017
DOI: 10.1103/PhysRevB.95.195105
In a bilayer system consisting of a composite-fermion Fermi sea in each layer, the tunnel current is exponentially suppressed at zero bias, followed by a strong peak at a finite bias voltage V_(max). This behavior, which is qualitatively different from that observed for the electron Fermi sea, provides fundamental insight into the strongly correlated non-Fermi liquid nature of the CF Fermi sea and, in particular, offers a window into the short-distance high-energy physics of this state. We identify the exciton responsible for the peak current and provide a quantitative account of the value of V_(max). The excitonic attraction is shown to be quantitatively significant, and its variation accounts for the increase of V_(max) with the application of an in-plane magnetic field. We also estimate the critical Zeeman energy where transition occurs from a fully spin polarized composite fermion Fermi sea to a partially spin polarized one, carefully incorporating corrections due to finite width and Landau level mixing, and find it to be in satisfactory agreement with the Zeeman energy where a qualitative change has been observed for the onset bias voltage [Eisenstein et al., Phys. Rev. B 94, 125409 (2016)]. For fractional quantum Hall states, we predict a substantial discontinuous jump in V_(max) when the system undergoes a transition from a fully spin polarized state to a spin singlet or a partially spin polarized state.https://resolver.caltech.edu/CaltechAUTHORS:20170503-105416560Interlayer Interactions and the Fermi Energy of Bilayer Composite Fermion Metals
https://resolver.caltech.edu/CaltechAUTHORS:20181009-080813831
Year: 2018
DOI: 10.1103/PhysRevB.98.201406
When two two-dimensional electron gas layers, each at Landau-level filling factor ν=1/2, are sufficiently close together, a condensate of interlayer excitons emerges at low temperature. Although the excitonic phase is qualitatively well understood, the incoherent phase just above the critical layer separation is not. Using a combination of tunneling spectroscopy and conventional transport, we explore the incoherent phase in samples both near the phase boundary and further from it. In the more closely spaced bilayers we find the electronic spectral functions narrower and the Fermi energy of the ν=1/2 composite fermion metal smaller than in the more widely separated bilayers. We attribute these effects to a softening of the intralayer Coulomb interaction due to interlayer screening.https://resolver.caltech.edu/CaltechAUTHORS:20181009-080813831Precursors to Exciton Condensation in Quantum Hall Bilayers
https://resolver.caltech.edu/CaltechAUTHORS:20190513-075922050
Year: 2019
DOI: 10.1103/PhysRevLett.123.066802
Tunneling spectroscopy reveals evidence for interlayer electron-hole correlations in quantum Hall bilayer two-dimensional electron systems at layer separations near, but above, the transition to the incompressible exciton condensate at total Landau level filling ν_T = 1. These correlations are manifested by a nonlinear suppression of the Coulomb pseudogap which inhibits low energy interlayer tunneling in weakly coupled bilayers. The pseudogap suppression is strongest at ν_T = 1 and grows rapidly as the critical layer separation for exciton condensation is approached from above.https://resolver.caltech.edu/CaltechAUTHORS:20190513-075922050