CaltechAUTHORS: Article
https://feeds.library.caltech.edu/people/Feng-Yejun/article.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenFri, 17 May 2024 13:26:27 -0700Energy dispersive x-ray diffraction of charge density waves via chemical filtering
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163028495
Year: 2005
DOI: 10.1063/1.1938954
Pressure tuning of phase transitions is a powerful tool in condensed matter physics, permitting high-resolution studies while preserving fundamental symmetries. At the highest pressures, energy dispersive x-ray diffraction (EDXD) has been a critical method for geometrically confined diamond anvil cell experiments. We develop a chemical filter technique complementary to EDXD that permits the study of satellite peaks as weak as 10^(-4) of the crystal Bragg diffraction. In particular, we map out the temperature dependence of the incommensurate charge density wave diffraction from single-crystal, elemental chromium. This technique provides the potential for future GPa pressure studies of many-body effects in a broad range of solid state systems.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163028495Quantum and classical relaxation in the proton glass
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163028104
Year: 2006
DOI: 10.1103/PhysRevLett.97.145501
The hydrogen-bond network formed from a crystalline solution of ferroelectric RbH_2PO_4 and antiferroelectric NH_4H_2PO_4 demonstrates glassy behavior, with proton tunneling the dominant mechanism for relaxation at low temperature. We characterize the dielectric response over seven decades of frequency and quantitatively fit the long-time relaxation by directly measuring the local potential energy landscape via neutron Compton scattering. The collective motion of protons rearranges the hydrogen bonds in the network. By analogy with vortex tunneling in superconductors, we relate the logarithmic decay of the polarization to the quantum-mechanical action.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163028104Direct measurement of antiferromagnetic domain fluctuations
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163027265
Year: 2007
DOI: 10.1038/nature05776
Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago, and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present a direct measurement of the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves associated with antiferromagnetism in elemental chromium. The technique used is X-ray photon correlation spectroscopy, where coherent X-ray diffraction produces a speckle pattern that serves as a 'fingerprint' of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micrometre distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it is not so at lower temperatures, and indeed has a rate that saturates at a finite value - consistent with quantum fluctuations - on cooling below 40 K.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163027265Pressure-tuned spin and charge ordering in an itinerant antiferromagnet
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163026620
Year: 2007
DOI: 10.1103/PhysRevLett.99.137201
Elemental chromium orders antiferromagnetically near room temperature, but the ordering temperature can be driven to zero by applying large pressures. We combine diamond anvil cell and synchrotron x-ray diffraction techniques to measure directly the spin and charge order in the pure metal at the approach to its quantum critical point. Both spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. By comparing the development of the order parameter with that of the magnetic wave vector, it is possible to ascribe the destruction of antiferromagnetism to the growth in electron kinetic energy relative to the underlying magnetic exchange interaction.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163026620Chromium at high pressures: Weak coupling and strong fluctuations in an itinerant antiferromagnet
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163026508
Year: 2008
DOI: 10.1103/PhysRevB.77.184418
The spin- and charge-density-wave order parameters of the itinerant antiferromagnet chromium are directly measured with nonresonant x-ray diffraction as the system is driven toward its quantum critical point with high pressure using a diamond anvil cell. The exponential decrease of the spin and charge diffraction intensities with pressure confirms the harmonic scaling of spin and charge, while the evolution of the incommensurate ordering vector provides important insight into the difference between pressure and chemical doping as means of driving quantum phase transitions. Measurement of the charge density wave over more than two orders of magnitude of diffraction intensity provides the clearest demonstration to date of a weakly coupled BCS-type ground state. Evidence for the coexistence of this weakly coupled ground state with high-energy excitations and pseudogap formation above the ordering temperature in chromium, the charge-ordered perovskite manganites, and the blue bronzes, among other such systems, raises fundamental questions about the distinctions between weak and strong coupling.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163026508Invited Article: High-pressure techniques for condensed matter physics at low temperature
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025580
Year: 2010
DOI: 10.1063/1.3400212
Condensed matter experiments at high pressure accentuate the need for accurate pressure scales over a broad range of temperatures, as well as placing a premium on a homogeneous pressure environment. However, challenges remain in diamond anvil cell technology, including both the quality of various pressure transmitting media and the accuracy of secondary pressure scales at low temperature. We directly calibrate the ruby fluorescence R1 line shift with pressure at T=4.5 K using high-resolution x-ray powder diffraction measurements of the silver lattice constant and its known equation of state up to P=16 GPa. Our results reveal a ruby pressure scale at low temperatures that differs by 6% from the best available ruby scale at room T. We also use ruby fluorescence to characterize the pressure inhomogeneity and anisotropy in two representative and commonly used pressure media, helium and methanol:ethanol 4:1, under the same preparation conditions for pressures up to 20 GPa at T=5 K. Contrary to the accepted wisdom, both media show equal levels of pressure inhomogeneity measured over the same area, with a consistent Delta P/P per unit area of +/- 1.8 %/(10^(4) µm^(2)) from 0 to 20 GPa. The helium medium shows an essentially constant deviatoric stress of 0.021 +/- 0.011 GPa up to 16 GPa, while the methanol:ethanol mixture shows a similar level of anisotropy up to 10 GPa, above which the anisotropy increases. The quality of both pressure media is further examined under the more stringent requirements of single crystal x-ray diffraction at cryogenic temperature. For such experiments we conclude that the ratio of sample-to-pressure chamber volume is a critical parameter in maintaining sample quality at high pressure, and may affect the choice of pressure medium.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025580Diffraction line-shapes, Fermi surface nesting, and quantum criticality in antiferromagnetic chromium at high pressure (invited)
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025440
Year: 2010
DOI: 10.1063/1.3364062
We explore the behavior of the nested bandstructure of chromium as a function of temperature and pressure to the point where magnetism disappears. X-ray diffraction measurements of the charge order parameter suggest that the nesting condition is maintained at high pressure, where the spin density wave ground state is destabilized by a continuous quantum phase transition. By comparing diffraction line-shapes measured throughout the temperature-pressure phase diagram we are able to identify and describe three regimes: thermal near-critical, weak coupling ground state, and quantum critical.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025440Signatures of quantum criticality in pure Cr at high pressure
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025309
Year: 2010
DOI: 10.1073/pnas.1005036107
PMCID: PMC2922236
The elemental antiferromagnet Cr at high pressure presents a new type of naked quantum critical point that is free of disorder and symmetry-breaking fields. Here we measure magnetotransport in fine detail around the critical pressure, P_c ~ 10 GPa, in a diamond anvil cell and reveal the role of quantum critical fluctuations at the phase transition. As the magnetism disappears and T → 0, the magntotransport scaling converges to a non-mean-field form that illustrates the reconstruction of the magnetic Fermi surface, and is distinct from the critical scaling measured in chemically disordered Cr:V under pressure. The breakdown of itinerant antiferromagnetism only comes clearly into view in the clean limit, establishing disorder as a relevant variable at a quantum phase transition.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025309Magnetism, structure, and charge correlation at a pressure-induced Mott-Hubbard insulator-metal transition
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025146
Year: 2011
DOI: 10.1103/PhysRevB.83.035106
We use synchrotron x-ray diffraction and electrical transport under pressure to probe both the magnetism and the structure of single-crystal NiS_2 across its Mott-Hubbard transition. In the insulator, the low-temperature antiferromagnetic order results from superexchange among correlated electrons and couples to a (1/2, 1/2, 1/2) superlattice distortion. Applying pressure suppresses the insulating state, but enhances the magnetism as the superexchange increases with decreasing lattice constant. By comparing our results under pressure to previous studies of doped crystals, we show that this dependence of the magnetism on the lattice constant is consistent for both band broadening and band filling. In the high-pressure metallic phase the lattice symmetry is reduced from cubic to monoclinic, pointing to the primary influence of charge correlations at the transition. There exists a wide regime of phase separation that may be a general characteristic of correlated quantum matter.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025146Order parameter fluctuations at a buried quantum critical point
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163024695
Year: 2012
DOI: 10.1073/pnas.1202434109
PMCID: PMC3358829
Quantum criticality is a central concept in condensed matter physics, but the direct observation of quantum critical fluctuations has remained elusive. Here we present an X-ray diffraction study of the charge density wave (CDW) in 2H-NbSe_2 at high pressure and low temperature, where we observe a broad regime of order parameter fluctuations that are controlled by proximity to a quantum critical point. X-rays can track the CDW despite the fact that the quantum critical regime is shrouded inside a superconducting phase; and in contrast to transport probes, allow direct measurement of the critical fluctuations of the charge order. Concurrent measurements of the crystal lattice point to a critical transition that is continuous in nature. Our results confirm the long-standing expectations of enhanced quantum fluctuations in low-dimensional systems, and may help to constrain theories of the quantum critical Fermi surface.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163024695Pressure tuning of competing magnetic interactions in intermetallic CeFe_2
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163024336
Year: 2012
DOI: 10.1103/PhysRevB.86.014422
We use high-pressure magnetic x-ray diffraction and numerical simulation to determine the low-temperature magnetic phase diagram of stoichiometric CeFe_2. Near 1.5 GPa we find a transition from ferromagnetism to antiferromagnetism, accompanied by a rhombohedral distortion of the cubic Laves crystal lattice. By comparing pressure and chemical substitution we find that the phase transition is controlled by a shift of magnetic frustration from the Ce-Ce to the Fe-Fe sublattice. Notably the dominant Ce-Fe magnetic interaction, which sets the temperature scale for the onset of long-range order, remains satisfied throughout the phase diagram but does not determine the magnetic ground state. Our results illustrate the complexity of a system with multiple competing magnetic energy scales and lead to a general model for magnetism in cubic Laves phase intermetallic compounds.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163024336Four-probe electrical measurements with a liquid pressure medium in a diamond anvil cell
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163024196
Year: 2012
DOI: 10.1063/1.4757178
We describe a technique for making electrical transport measurements in a diamond anvil cell using an alcohol pressure medium, permitting acute sensitivity while preserving sample fidelity. The sample is suspended in the liquid medium by four gold leads that are electrically isolated by a composite gasket made of stainless steel and an alumina-loaded epoxy. We demonstrate the technique with four-probe resistivity measurements of chromium single crystals at temperatures down to 4 K and pressures above 10 GPa. Our assembly is optimized for making high precision measurements of the magnetic phase diagram and quantum critical regime of chromium, which require repeated temperature sweeps and fine pressure steps while maintaining high sample quality. The high sample quality enabled by the quasi-hydrostatic pressure medium is evidenced by the residual resistivity below 0.1 μΩ cm and the relative resistivity ratio ρ(120 K)/ρ(5 K) = 15.9 at 11.4 GPa. By studying the quality of Cr's antiferromagnetic transition over a range of pressures, we show that the pressure inhomogeneity experienced by the sample is always below 5%. Finally, we solve for the Debye temperature of Cr up to 11.4 GPa using the Bloch-Gruneisen formula and find it to be independent of pressure.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163024196Incommensurate antiferromagnetism in a pure spin system via cooperative organization of local and itinerant moments
https://resolver.caltech.edu/CaltechAUTHORS:20140707-131949022
Year: 2013
DOI: 10.1073/pnas.1217292110
PMCID: PMC3587267
Materials with strong correlations are prone to spin and charge instabilities, driven by Coulomb, magnetic, and lattice interactions. In materials that have significant localized and itinerant spins, it is not obvious which will induce order. We combine electrical transport, X-ray magnetic diffraction, and photoemission studies with band structure calculations to characterize successive anti-ferromagnetic transitions in GdSi. GdSi has both sizable local moments and a partially nested Fermi surface, without confounding contributions from orbital effects. We identify a route to incommensurate order where neither type of moment dominates, but is rooted in cooperative feedback between them. The nested Fermi surface of the itinerant electrons induces strong interactions between local moments at the nesting vector, whereas the ordered local moments in turn provide the necessary coupling for a spin-density wave to form among the itinerant electrons. This mechanism echoes the cooperative interactions between electrons and ions in charge-density-wave materials, and should be germane across a spectrum of transition-metal and rare-earth intermetallic compounds.https://resolver.caltech.edu/CaltechAUTHORS:20140707-131949022Charge transfer and multiple density waves in the rare earth tellurides
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163023829
Year: 2013
DOI: 10.1103/PhysRevB.87.155131
We use high-resolution synchrotron x-ray diffraction to uncover a second, low-temperature, charge density wave (CDW) in TbTe_3. Its T_(c2) = 41.0 +/- 0.4 K is the lowest discovered so far in the rare earth telluride series. The CDW wave vectors of the high temperature and low temperature states differ significantly and evolve in opposite directions with temperature, indicating that the two nested Fermi surfaces are separated and the CDWs coexist independently. Both the in-plane and out-of-plane correlation lengths are robust, implying that the density waves on different Te layers are well coupled through the TbTe layers. Finally, we rule out any low-temperature CDW in GdTe_3 for temperatures above 8 K, an energy scale sufficiently low to make pressure tuning of incipient CDW order a realistic possibility.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163023829Evolution of incommensurate spin order with magnetic field and temperature in the itinerant antiferromagnet GdSi
https://resolver.caltech.edu/CaltechAUTHORS:20140707-131948529
Year: 2013
DOI: 10.1103/PhysRevB.88.134404
GdSi exhibits spin-density-wave (SDW) order arising from the cooperative interplay of sizeable local moments and a partially nested Fermi sea of itinerant electrons. Using magnetotransport, magnetization, and nonresonant magnetic x-ray diffraction techniques, we determine the H-T phase diagrams of GdSi for magnetic fields up to 21 T, where antiferromagnetic order is no longer stable, and field directions along each of the three major crystal axes. While the incommensurate magnetic ordering vector that characterizes the SDW is robust under magnetic field, the multiple spin structures of this compound are highly flexible and rotate relative to the applied field via either canting or spin-flop processes. The antiferromagnetic spin densities always arrange themselves transverse to the applied magnetic field direction. The phase diagrams are delineated by two types of phase boundaries: one separates a collinear from a planar spin structure associated with a lattice structural transition, and the other defines a spin flop transition that is only weakly temperature dependent. The major features of the phase diagrams along each of the crystal axes can be explained by the combination of local moment and global Fermi surface physics at play.https://resolver.caltech.edu/CaltechAUTHORS:20140707-131948529A compact bellows-driven diamond anvil cell for high-pressure, low-temperature magnetic measurements
https://resolver.caltech.edu/CaltechAUTHORS:20140707-131948390
Year: 2014
DOI: 10.1063/1.4867078
We present the design of an efficient bellows-controlled diamond anvil cell that is optimized for use inside the bores of high-field superconducting magnets in helium-3 cryostats, dilution refrigerators, and commercial physical property measurement systems. Design of this non-magnetic pressure cell focuses on in situ pressure tuning and measurement by means of a helium-filled bellows actuator and fiber-coupled ruby fluorescence spectroscopy, respectively. We demonstrate the utility of this pressure cell with ac susceptibility measurements of superconducting, ferromagnetic, and antiferromagnetic phase transitions to pressures exceeding 8 GPa. This cell provides an opportunity to probe charge and magnetic order continuously and with high resolution in the three-dimensional Magnetic Field-Pressure-Temperature parameter space.https://resolver.caltech.edu/CaltechAUTHORS:20140707-131948390Breakdown of the Bardeen-Cooper-Schrieffer ground state at a quantum phase transition
https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025841
Year: 2014
DOI: 10.1038/nature08008
Advances in solid-state and atomic physics are exposing the hidden relationships between conventional and exotic states of quantum matter. Prominent examples include the discovery of exotic superconductivity proximate to conventional spin and charge order, and the crossover from long-range phase order to preformed pairs achieved in gases of cold fermions and inferred for copper oxide superconductors. The unifying theme is that incompatible ground states can be connected by quantum phase transitions. Quantum fluctuations about the transition are manifestations of the competition between qualitatively distinct organizing principles, such as a long-wavelength density wave and a short-coherence-length condensate. They may even give rise to 'protected' phases, like fluctuation-mediated superconductivity that survives only in the vicinity of an antiferromagnetic quantum critical point. However, few model systems that demonstrate continuous quantum phase transitions have been identified, and the complex nature of many systems of interest hinders efforts to more fully understand correlations and fluctuations near a zero-temperature instability. Here we report the suppression of magnetism by hydrostatic pressure in elemental chromium, a simple cubic metal that demonstrates a subtle form of itinerant antiferromagnetism formally equivalent to the Bardeen-Cooper-Schrieffer (BCS) state in conventional superconductors. By directly measuring the associated charge order in a diamond anvil cell at low temperatures, we find a phase transition at pressures of similar to 10 GPa driven by fluctuations that destroy the BCS-like state but preserve the strong magnetic interaction between itinerant electrons and holes. Chromium is unique among stoichiometric magnetic metals studied so far in that the quantum phase transition is continuous, allowing experimental access to the quantum singularity and a direct probe of the competition between conventional and exotic order in a theoretically tractable material.https://resolver.caltech.edu/CaltechAUTHORS:20140707-163025841Direct probe of Fermi surface evolution across a pressure-induced quantum phase transition
https://resolver.caltech.edu/CaltechAUTHORS:20150519-092300781
Year: 2015
DOI: 10.1103/PhysRevB.91.155142
The nature of a material's Fermi surface is crucial to understanding its electronic, magnetic, optical, and thermal characteristics. Traditional measurements such as angle-resolved photoemission spectroscopy and de Haas–van Alphen quantum oscillations can be difficult to perform in the vicinity of a pressure-driven quantum phase transition, although the evolution of the Fermi surface may be tied to the emergence of exotic phenomena. We demonstrate here that magnetic x-ray diffraction in combination with Hall effect measurements in a diamond anvil cell can provide valuable insight into the Fermi surface evolution in spin- and charge-density-wave systems near quantum phase transitions. In particular, we track the gradual evolution of the Fermi surface in elemental chromium and delineate the critical pressure and absence of Fermi surface reconstruction at the spin-flip transition.https://resolver.caltech.edu/CaltechAUTHORS:20150519-092300781Sub-Kelvin magnetic and electrical measurements in a diamond anvil cell with in situ tunability
https://resolver.caltech.edu/CaltechAUTHORS:20151029-110749806
Year: 2015
DOI: 10.1063/1.4929861
We discuss techniques for performing continuous measurements across a wide range of pressure–field–temperature phase space, combining the milli-Kelvin temperatures of a helium dilution refrigerator with the giga-Pascal pressures of a diamond anvil cell and the Tesla magnetic fields of a superconducting magnet. With a view towards minimizing remnant magnetic fields and background magnetic susceptibility, we characterize high-strength superalloy materials for the pressure cell assembly, which allows high fidelity measurements of low-field phenomena such as superconductivity below 100 mK at pressures above 10 GPa. In situ tunability and measurement of the pressure permit experiments over a wide range of pressure, while at the same time making possible precise steps across abrupt phase transitions such as those from insulator to metal.https://resolver.caltech.edu/CaltechAUTHORS:20151029-110749806Itinerant density wave instabilities at classical and quantum critical points
https://resolver.caltech.edu/CaltechAUTHORS:20150625-121656270
Year: 2015
DOI: 10.1038/nphys3416
Charge ordering in metals is a fundamental instability of the electron sea, occurring in a host of materials and often linked to other collective ground states such as superconductivity. What is difficult to parse, however, is whether the charge order originates among the itinerant electrons or whether it arises from the ionic lattice. Here we employ high-resolution X-ray diffraction, combined with high-pressure and low-temperature techniques and theoretical modelling, to trace the evolution of the ordering wavevector Q in charge and spin density wave systems at the approach to both thermal and quantum phase transitions. The non-monotonic behaviour of Q with pressure and the limiting sinusoidal form of the density wave point to the dominant role of the itinerant instability in the vicinity of the critical points, with little influence from the lattice. Fluctuations rather than disorder seem to disrupt coherence.https://resolver.caltech.edu/CaltechAUTHORS:20150625-121656270Spiral magnetic order and pressure-induced superconductivity in transition metal compounds
https://resolver.caltech.edu/CaltechAUTHORS:20161011-134922256
Year: 2016
DOI: 10.1038/ncomms13037
PMCID: PMC5059728
Magnetic and superconducting ground states can compete, cooperate and coexist. MnP provides a compelling and potentially generalizable example of a material where superconductivity and magnetism may be intertwined. Using a synchrotron-based non-resonant X-ray magnetic diffraction technique, we reveal a spiral spin order in MnP and trace its pressure evolution towards superconducting order via measurements in a diamond anvil cell. Judging from the magnetostriction, ordered moments vanish at the quantum phase transition as pressure increases the electron kinetic energy. Spins remain local in the disordered phase, and the promotion of superconductivity is likely to emerge from an enhanced coupling to residual spiral spin fluctuations and their concomitant suppression of phonon-mediated superconductivity. As the pitch of the spiral order varies across the 3d transition metal compounds in the MnP family, the magnetic ground state switches between antiferromagnet and ferromagnet, providing an additional tuning parameter in probing spin-fluctuation-induced superconductivity.https://resolver.caltech.edu/CaltechAUTHORS:20161011-134922256Multiple superconducting states induced by pressure in Mo_3Sb_7
https://resolver.caltech.edu/CaltechAUTHORS:20170301-083224304
Year: 2017
DOI: 10.1103/PhysRevB.95.125102
Tuning competing ordering mechanisms with hydrostatic pressure in the 4d intermetallic compound Mo_3Sb_7 reveals an intricate interplay of structure, magnetism, and superconductivity. Synchrotron x-ray diffraction and magnetic susceptibility measurements, both employing diamond anvil cell technologies, link a first-order structural phase transition to a doubling of the superconducting transition temperature. In contrast to the spin-dimer picture for Mo_3Sb_7, we deduce from x-ray absorption near-edge structure and dc magnetization measurements at ambient pressure that Mo_3Sb_7 should possess only very small, itinerant magnetic moments. The pressure evolution of the superconducting transition temperature strongly suggests its enhancement is due to a difference in the phonon density-of-states with changed crystal symmetry.https://resolver.caltech.edu/CaltechAUTHORS:20170301-083224304Strongly-coupled quantum critical point in an all-in-all-out antiferromagnet
https://resolver.caltech.edu/CaltechAUTHORS:20180521-095351019
Year: 2018
DOI: 10.1038/s41467-018-05435-7
PMCID: PMC6063849
Dimensionality and symmetry play deterministic roles in the laws of Nature. They are important tools to characterize and understand quantum phase transitions, especially in the limit of strong correlations between spin, orbit, charge, and structural degrees of freedom. Here, using newly-developed, high-pressure resonant X-ray magnetic and charge diffraction techniques, we have discovered a quantum critical point in Cd₂Os₂O₇ as the all-in-all-out antiferromagnetic order is continuously suppressed to zero temperature and, concomitantly, the cubic lattice structure continuously changes from space group Fd-3m to F-43m. Surrounded by three phases of different time reversal and spatial inversion symmetries, the quantum critical region anchors two phase lines of opposite curvature, with striking departures from a mean-field form at high pressure. As spin fluctuations, lattice breathing modes, and quasiparticle excitations interact in the quantum critical region, we argue that they present the necessary components for strongly-coupled quantum criticality in this three-dimensional compound.https://resolver.caltech.edu/CaltechAUTHORS:20180521-095351019Linear magnetoresistance in the low-field limit in density-wave materials
https://resolver.caltech.edu/CaltechAUTHORS:20190412-092526240
Year: 2019
DOI: 10.1073/pnas.1820092116
PMCID: PMC6561266
The magnetoresistance (MR) of a material is typically insensitive to reversing the applied field direction and varies quadratically with magnetic field in the low-field limit. Quantum effects, unusual topological band structures, and inhomogeneities that lead to wandering current paths can induce a cross-over from quadratic to linear MR with increasing magnetic field. Here we explore a series of metallic charge- and spin-density-wave systems that exhibit extremely large positive linear MR. By contrast to other linear MR mechanisms, this effect remains robust down to miniscule magnetic fields of tens of Oersted at low temperature. We frame an explanation of this phenomenon in a semiclassical narrative for a broad category of materials with partially gapped Fermi surfaces due to density waves.https://resolver.caltech.edu/CaltechAUTHORS:20190412-092526240X-ray magnetic diffraction under high pressure
https://resolver.caltech.edu/CaltechAUTHORS:20190718-135728678
Year: 2019
DOI: 10.1107/s2052252519007061
PMCID: PMC6608628
Advances in both non-resonant and resonant X-ray magnetic diffraction since the 1980s have provided researchers with a powerful tool for exploring the spin, orbital and ion degrees of freedom in magnetic solids, as well as parsing their interplay. Here, we discuss key issues for performing X-ray magnetic diffraction on single-crystal samples under high pressure (above 40 GPa) and at cryogenic temperatures (4 K). We present case studies of both non-resonant and resonant X-ray magnetic diffraction under pressure for a spin-flip transition in an incommensurate spin-density-wave material and a continuous quantum phase transition of a commensurate all-in–all-out antiferromagnet. Both cases use diamond-anvil-cell technologies at third-generation synchrotron radiation sources. In addition to the exploration of the athermal emergence and evolution of antiferromagnetism discussed here, these techniques can be applied to the study of the pressure evolution of weak charge order such as charge-density waves, antiferro-type orbital order, the charge anisotropic tensor susceptibility and charge superlattices associated with either primary spin order or softened phonons.https://resolver.caltech.edu/CaltechAUTHORS:20190718-135728678Antisymmetric linear magnetoresistance and the planar Hall effect
https://resolver.caltech.edu/CaltechAUTHORS:20190624-074905872
Year: 2020
DOI: 10.1038/s41467-019-14057-6
PMCID: PMC6954222
The phenomena of antisymmetric magnetoresistance and the planar Hall effect are deeply entwined with ferromagnetism. The intrinsic magnetization of the ordered state permits these unusual and rarely observed manifestations of Onsager's theorem when time reversal symmetry is broken at zero applied field. Here we study two classes of ferromagnetic materials, rare-earth magnets with high intrinsic coercivity and antiferromagnetic pyrochlores with strongly-pinned ferromagnetic domain walls, which both exhibit antisymmetric magnetoresistive behavior. By mapping out the peculiar angular variation of the antisymmetric galvanomagnetic response with respect to the relative alignments of the magnetization, magnetic field, and electrical current, we experimentally distinguish two distinct underlying microscopic mechanisms: namely, spin-dependent scattering of a Zeeman-shifted Fermi surface and anomalous electron velocities. Our work demonstrates that the anomalous electron velocity physics typically associated with the anomalous Hall effect is prevalent beyond the ρ_(xy)(H_z) channel, and should be understood as a part of the general galvanomagnetic behavior.https://resolver.caltech.edu/CaltechAUTHORS:20190624-074905872Approaching the quantum critical point in a highly-correlated all-in-all-out antiferromagnet
https://resolver.caltech.edu/CaltechAUTHORS:20200224-111840247
Year: 2020
DOI: 10.1103/PhysRevB.101.220404
Continuous quantum phase transition involving all-in–all-out (AIAO) antiferromagnetic order in strongly spin-orbit-coupled 5d compounds could give rise to various exotic electronic phases and strongly-coupled quantum critical phenomena. Here we experimentally trace the AIAO spin order in Sm₂Ir₂O₇ using direct resonant x-ray magnetic diffraction techniques under high pressure. The magnetic order is suppressed at a critical pressure P_c=6.30GPa, while the lattice symmetry remains in the cubic Fd−3m space group across the quantum critical point. Comparing pressure tuning and the chemical series R₂Ir₂O₇ reveals that the approach to the AIAO quantum phase transition is characterized by contrasting evolutions of the pyrochlore lattice constant a and the trigonal distortion surrounding individual Ir moments, which affects the 5d bandwidth and the Ising anisotropy, respectively. We posit that the opposite effects of pressure and chemical tuning lead to spin fluctuations with different Ising and Heisenberg character in the quantum critical region. Finally, the observed low pressure scale of the AIAO quantum phase transition in Sm₂Ir₂O₇ identifies a circumscribed region of P-T space for investigating the putative magnetic Weyl semimetal state.https://resolver.caltech.edu/CaltechAUTHORS:20200224-111840247Optical Raman measurements of low frequency magnons under high pressure
https://resolver.caltech.edu/CaltechAUTHORS:20201104-103219829
Year: 2020
DOI: 10.1063/5.0026311
The application of giga-Pascal scale pressures has been widely used as a tool to systematically tune the properties of materials in order to access such general questions as the driving mechanisms underlying phase transitions. While there is a large and growing set of experimental tools successfully applied to high-pressure environments, the compatibility between diamond anvil cells and optical probes offers further potential for examining lattice, magnetic, and electronic states, along with their excitations. Here, we describe the construction of a highly efficient optical Raman spectrometer that enables measurements of magnetic excitations in single crystals down to energies of 9 cm⁻¹ (1.1 meV or 13 K) at cryogenic temperatures and under pressures of tens of GPa.https://resolver.caltech.edu/CaltechAUTHORS:20201104-103219829A continuous metal-insulator transition driven by spin correlations
https://resolver.caltech.edu/CaltechAUTHORS:20201111-082624574
Year: 2021
DOI: 10.1038/s41467-021-23039-6
PMCID: PMC8119431
While Mott insulators induced by Coulomb interactions are a well-recognized class of metal-insulator transitions, insulators purely driven by spin correlations are much less common, as the reduced energy scale often invites competition from other degrees of freedom. Here, we demonstrate a clean example of a spin-correlation-driven metal-insulator transition in the all-in-all-out pyrochlore antiferromagnet Cd₂Os₂O₇, where the lattice symmetry is preserved by the antiferromagnetism. After the antisymmetric linear magnetoresistance from conductive, ferromagnetic domain walls is removed experimentally, the bulk Hall coefficient reveals four Fermi surfaces of both electron and hole types, sequentially departing the Fermi level with decreasing temperature below the Néel temperature, T_N = 227 K. In Cd₂Os₂O₇, the charge gap of a continuous metal-insulator transition opens only at T ~ 10 K << T_N. The insulating mechanism parallels the Slater picture, but without a folded Brillouin zone, and contrasts sharply with Mott insulators and spin density waves, where the electronic gap opens above and at T_N, respectively.https://resolver.caltech.edu/CaltechAUTHORS:20201111-082624574Magnetic order, disorder, and excitations under pressure in the Mott Insulator Sr₂IrO₄
https://resolver.caltech.edu/CaltechAUTHORS:20211130-215730151
Year: 2021
DOI: 10.1103/PhysRevB.104.L201111
Protected by the interplay of on-site Coulomb interactions and spin-orbit coupling, Sr₂IrO₄ at high pressure is a rare example of a Mott insulator with a paramagnetic ground state. Here, using optical Raman scattering, we measure both the phonon and magnon evolution in Sr₂IrO₄ under pressure, and identify three different magnetically-ordered phases, culminating in a spin-disordered state beyond 18 GPa. A strong first-order structural phase transition drives the magnetic evolution at ∼10 GPa with reduced structural anisotropy in the IrO₆ cages, leading to increasingly isotropic exchange interactions between the Heisenberg spins and a spin-flip transition to c-axis-aligned antiferromagnetic order. In the disordered phase of Heisenberg J_(eff) = 1/2 pseudospins, the spin excitations are quasi-elastic and continuous to 10 meV, potentially hosting a gapless quantum spin liquid in Sr₂IrO₄.https://resolver.caltech.edu/CaltechAUTHORS:20211130-215730151Three-dimensional checkerboard spin structure on a breathing pyrochlore lattice
https://authors.library.caltech.edu/records/dd5ce-2dk24
Year: 2024
DOI: 10.1103/physrevb.109.064421
<p>The standard approach to realize a spin-liquid state is through magnetically frustrated states, relying on ingredients such as the lattice geometry, dimensionality, and magnetic interaction type of the spins. While Heisenberg spins on a pyrochlore lattice with only antiferromagnetic nearest-neighbor interactions are theoretically proven disordered, spins in real systems generally include longer-range interactions. The spatial correlations at longer distances typically stabilize a long-range order rather than enhancing a spin-liquid state. Both states can, however, be destroyed by short-range static correlations introduced by chemical disorder. Here, using disorder-free specimens with a clear long-range antiferromagnetic order, we refine the spin structure of the Heisenberg spinel <span class="mjx-chtml MathJax_CHTML"><span class="mjx-math"><span class="mjx-mrow"><span class="mjx-mrow"><span class="mjx-msub"><span class="mjx-base"><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R">ZnFe</span></span></span><span class="mjx-sub"><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">2</span></span></span></span><span class="mjx-msub MJXc-space1"><span class="mjx-base"><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R">O</span></span></span><span class="mjx-sub"><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">4</span></span></span></span></span></span></span></span> through neutron magnetic diffraction. The unique wave vector <span class="mjx-chtml MathJax_CHTML"><span class="mjx-math"><span class="mjx-mrow"><span class="mjx-mrow"><span class="mjx-mo"><span class="mjx-char MJXc-TeX-size2-R">(</span></span><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">1</span></span><span class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span class="mjx-mn MJXc-space1"><span class="mjx-char MJXc-TeX-main-R">0</span></span><span class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span class="mjx-mfrac MJXc-space1"><span class="mjx-box MJXc-stacked"><span class="mjx-numerator"><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">1/</span></span></span><span class="mjx-denominator"><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">2</span></span></span></span></span><span class="mjx-mo"><span class="mjx-char MJXc-TeX-size2-R">)</span></span></span></span></span></span> leads to a spin structure that can be viewed as alternatively stacked ferromagnetic and antiferromagnetic tetrahedra in a three-dimensional checkerboard form. Stable coexistence of these opposing types of clusters is enabled by the bipartite breathing pyrochlore crystal structure, leading to a second-order phase transition at 10 K. The diffraction intensity of <span class="mjx-chtml MathJax_CHTML"><span class="mjx-math"><span class="mjx-mrow"><span class="mjx-mrow"><span class="mjx-msub"><span class="mjx-base"><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R">ZnFe₂</span></span></span></span><span class="mjx-msub MJXc-space1"><span class="mjx-base"><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R">O</span></span></span><span class="mjx-sub"><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">₄</span></span></span></span></span></span></span></span> is an exact complement to the inelastic scattering intensity of several chromate spinel systems which are regarded as model classical spin liquids. Our results challenge this attribution, and suggest instead of the six-spin ring mode, spin excitations in chromate spinels are closely related to the <span class="mjx-chtml MathJax_CHTML"><span class="mjx-math"><span class="mjx-mrow"><span class="mjx-mrow"><span class="mjx-mo"><span class="mjx-char MJXc-TeX-size2-R">(</span></span><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">1</span></span><span class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span class="mjx-mn MJXc-space1"><span class="mjx-char MJXc-TeX-main-R">0</span></span><span class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span class="mjx-mfrac MJXc-space1"><span class="mjx-box MJXc-stacked"><span class="mjx-numerator"><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">1/</span></span></span><span class="mjx-denominator"><span class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">2</span></span></span></span></span><span class="mjx-mo"><span class="mjx-char MJXc-TeX-size2-R">)</span></span></span></span></span></span> type of spin order and the four-spin ferromagnetic cluster locally at one tetrahedron.</p>https://authors.library.caltech.edu/records/dd5ce-2dk24Quantum interference in superposed lattices
https://authors.library.caltech.edu/records/ahvch-yk006
Year: 2024
DOI: 10.1073/pnas.2315787121
PMCID: PMC10873631
<div>Charge transport in solids at low temperature reveals a material’s mesoscopic properties and structure. Under a magnetic field, Shubnikov–de Haas (SdH) oscillations inform complex quantum transport phenomena that are not limited by the ground state characteristics and have facilitated extensive explorations of quantum and topological interest in two- and three-dimensional materials. Here, in elemental metal Cr with two incommensurately superposed lattices of ions and a spin-density-wave ground state, we reveal that the phases of several low-frequency SdH oscillations in σ<em>ₓₓ</em><span> (ρ<em>ₓₓ</em>)</span> and σ<em>ᵧᵧ</em><span> (ρ<em>ᵧᵧ</em>)</span> are no longer identical but opposite. These relationships contrast with the SdH oscillations from normal cyclotron orbits that maintain identical phases between σ<em>ₓₓ</em><span> (ρ<em>ₓₓ</em>)</span> and σ<em>ᵧᵧ</em><span> (ρ<em>ᵧᵧ</em>)</span> . We trace the origin of the low-frequency SdH oscillations to quantum interference effects arising from the incommensurate orbits of Cr’s superposed reciprocal lattices and explain the observed <span>π</span>-phase shift by the reconnection of anisotropic joint open and closed orbits.</div>
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</div>https://authors.library.caltech.edu/records/ahvch-yk006