Article records
https://feeds.library.caltech.edu/people/Nadj-Perge-S/article.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenFri, 08 Dec 2023 12:29:08 +0000Bistability in superconducting rings containing an inhomogeneous Josephson junction
https://resolver.caltech.edu/CaltechAUTHORS:20160112-142643730
Authors: Gaass, M.; Nadj-Perge, S.; Radović, Z.; Bauer, A.; Aprili, M.; Wegscheider, W.; Strunk, C.
Year: 2008
DOI: 10.1103/PhysRevB.77.024506
We investigate the magnetic response of a superconducting Nb ring containing a ferromagnetic PdNi Josephson junction and a tunnel junction in parallel. Doubling of the switching frequency is observed within certain intervals of the external magnetic field. For sinusoidal current-phase relations in both junctions, our model of a double superconducting quantum interference device (a small two-junction loop that interrupts the larger ring) explains this feature by a sequence of current reversals in the ferromagnetic section of the junction in these field intervals. The switching anomalies are induced by the coupling between the magnetic fluxes in the two superconducting loops.https://authors.library.caltech.edu/records/3z1wg-r3981Tunable double quantum dots in InAs nanowires
https://resolver.caltech.edu/CaltechAUTHORS:20160112-112717199
Authors: Scheffler, Marc; Nadj-Perge, Stevan; Kouwenhoven, Leo P.; Borgström, Magnus T.; Bakkers, Erik P. A. M.
Year: 2008
DOI: 10.1016/j.physe.2007.08.033
Semiconductor nanowires offer a very versatile approach to create tunable quantum dots. Of the different semiconductor materials that can be grown as nanowires, InAs is particularly interesting due to the large spin–orbit coupling and furthermore promising for devices due to the comparably easy processing for Ohmic contacts. Here we study the electronic transport through gateable InAs nanowire devices at low temperatures. The nanowires are grown by MOVPE, and horizontal devices are individually fabricated using electron-beam lithography. We use local top gates to create barriers that can be used to define tunable quantum dots. Towards our final goal of spin manipulation of single electrons, we focus on tunable double dots. We measure the electronic transport through double quantum dots for the different accessible regimes: we present stability diagrams that demonstrate the tunability from two independent dots to one combined dot, including the particularly interesting region of two interacting quantum dots.https://authors.library.caltech.edu/records/n3get-s9n63Diameter-dependent conductance of InAs nanowires
https://resolver.caltech.edu/CaltechAUTHORS:20160112-104948904
Authors: Scheffler, Marc; Nadj-Perge, Stevan; Kouwenhoven, Leo P.; Borgström, Magnus T.; Bakkers, Erik P. A. M.
Year: 2009
DOI: 10.1063/1.3270259
Electrical conductance through InAsnanowires is relevant for electronic applications as well as for fundamental quantum experiments. Here, we employ nominally undoped, slightly tapered InAsnanowires to study the diameter dependence of their conductance. By contacting multiple sections of each wire, we can study the diameter dependence within individual wires without the need to compare different nanowire batches. At room temperature, we find a diameter-independent conductivity for diameters larger than 40 nm, indicative of three-dimensional diffusive transport. For smaller diameters, the resistance increases considerably, in coincidence with a strong suppression of the mobility. From an analysis of the effective charge carrier density, we find indications for a surface accumulation layer.https://authors.library.caltech.edu/records/5jrft-p2q08Disentangling the effects of spin-orbit and hyperfine interactions on spin blockade
https://resolver.caltech.edu/CaltechAUTHORS:20160113-085622730
Authors: Nadj-Perge, S.; Frolov, S. M.; van Tilburg, J. W. W.; Danon, J.; Nazarov, Yu. V.; Algra, R.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.
Year: 2010
DOI: 10.1103/PhysRevB.81.201305
We have achieved the few-electron regime in InAs nanowire double quantum dots. Spin blockade is observed for the first two half-filled orbitals, where the transport cycle is interrupted by forbidden transitions between triplet and singlet states. Partial lifting of spin blockade is explained by spin-orbit and hyperfine mechanisms that enable triplet to singlet transitions. The measurements over a wide range of interdot coupling and tunneling rates to the leads are well reproduced by a simple transport model. This allows us to separate and quantify the contributions of the spin-orbit and hyperfine interactions.https://authors.library.caltech.edu/records/yxbg1-8gj13Spin–orbit qubit in a semiconductor nanowire
https://resolver.caltech.edu/CaltechAUTHORS:20160113-085621870
Authors: Nadj-Perge, S.; Frolov, S. M.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.
Year: 2010
DOI: 10.1038/nature09682
Motion of electrons can influence their spins through a fundamental effect called spin–orbit interaction. This interaction provides a way to control spins electrically and thus lies at the foundation of spintronics. Even at the level of single electrons, the spin–orbit interaction has proven promising for coherent spin rotations. Here we implement a spin–orbit quantum bit (qubit) in an indium arsenide nanowire, where the spin–orbit interaction is so strong that spin and motion can no longer be separated. In this regime, we realize fast qubit rotations and universal single-qubit control using only electric fields; the qubits are hosted in single-electron quantum dots that are individually addressable. We enhance coherence by dynamically decoupling the qubits from the environment. Nanowires offer various advantages for quantum computing: they can serve as one-dimensional templates for scalable qubit registers, and it is possible to vary the material even during wire growth. Such flexibility can be used to design wires with suppressed decoherence and to push semiconductor qubit fidelities towards error correction levels. Furthermore, electrical dots can be integrated with optical dots in p–n junction nanowires. The coherence times achieved here are sufficient for the conversion of an electronic qubit into a photon, which can serve as a flying qubit for long-distance quantum communication.https://authors.library.caltech.edu/records/dty4n-k2c60From InSb Nanowires to Nanocubes: Looking for the Sweet Spot
https://resolver.caltech.edu/CaltechAUTHORS:20160113-085622213
Authors: Plissard, Sébastien R.; Slapak, Dorris R.; Verheijen, Marcel A.; Hocevar, Moïra; Immink, George W. G.; van Weperen, Ilse; Nadj-Perge, Stevan; Frolov, Sergey M.; Kouwenhoven, Leo P.; Bakkers, Erik P. A. M.
Year: 2012
DOI: 10.1021/nl203846g
High aspect ratios are highly desired to fully exploit the one-dimensional properties of indium antimonide nanowires. Here we systematically investigate the growth mechanisms and find parameters leading to long and thin nanowires. Variation of the V/III ratio and the nanowire density are found to have the same influence on the "local" growth conditions and can control the InSb shape from thin nanowires to nanocubes. We propose that the V/III ratio controls the droplet composition and the radial growth rate and these parameters determine the nanowire shape. A sweet spot is found for nanowire interdistances around 500 nm leading to aspect ratios up to 35. High electron mobilities up to 3.5 × 10^4 cm^2 V^(–1) s^(–1) enable the realization of complex spintronic and topological devices.https://authors.library.caltech.edu/records/qbzfa-cha13Spectroscopy of Spin-Orbit Quantum Bits in Indium Antimonide Nanowires
https://resolver.caltech.edu/CaltechAUTHORS:20160113-070931708
Authors: Nadj-Perge, S.; Pribiag, V. S.; van den Berg, J. W. G.; Zuo, K.; Plissard, S. R.; Bakkers, E. P. A. M.; Frolov, S. M.; Kouwenhoven, L. P.
Year: 2012
DOI: 10.1103/PhysRevLett.108.166801
A double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlet-triplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlet-triplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.https://authors.library.caltech.edu/records/37qm7-2sz44Suppression of Zeeman Gradients by Nuclear Polarization in Double Quantum Dots
https://resolver.caltech.edu/CaltechAUTHORS:20160113-070931989
Authors: Frolov, S. M.; Danon, J.; Nadj-Perge, S.; Zuo, K.; van Tilburg, J. W. W.; Pribiag, V. S.; van den Berg, J. W. G.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.
Year: 2012
DOI: 10.1103/PhysRevLett.109.236805
We use electric dipole spin resonance to measure dynamic nuclear polarization in InAs nanowire quantum dots. The resonance shifts in frequency when the system transitions between metastable high and low current states, indicating the presence of nuclear polarization. We propose that the low and the high current states correspond to different total Zeeman energy gradients between the two quantum dots. In the low current state, dynamic nuclear polarization efficiently compensates the Zeeman gradient due to the g-factor mismatch, resulting in a suppressed total Zeeman gradient. We present a theoretical model of electron-nuclear feedback that demonstrates a fixed point in nuclear polarization for nearly equal Zeeman splittings in the two dots and predicts a narrowed hyperfine gradient distribution.https://authors.library.caltech.edu/records/1s8s0-08j21Fast Spin-Orbit Qubit in an Indium Antimonide Nanowire
https://resolver.caltech.edu/CaltechAUTHORS:20160112-144541381
Authors: van den Berg, J. W. G.; Nadj-Perge, S.; Pribiag, V. S.; Plissard, S. R.; Bakkers, E. P. A. M.; Frolov, S. M.; Kouwenhoven, L. P.
Year: 2013
DOI: 10.1103/PhysRevLett.110.066806
Because of the strong spin-orbit interaction in indium antimonide, orbital motion and spin are no longer separated. This enables fast manipulation of qubit states by means of microwave electric fields. We report Rabi oscillation frequencies exceeding 100 MHz for spin-orbit qubits in InSb nanowires. Individual qubits can be selectively addressed due to intrinsic differences in their g factors. Based on Ramsey fringe measurements, we extract a coherence time T_2^*=8±1 ns at a driving frequency of 18.65 GHz. Applying a Hahn echo sequence extends this coherence time to 34 ns.https://authors.library.caltech.edu/records/xbtwe-yp677Electrical control of single hole spins in nanowire quantum dots
https://resolver.caltech.edu/CaltechAUTHORS:20160112-115130944
Authors: Pribiag, V. S.; Nadj-Perge, S.; Frolov, S. M.; van den Berg, J. W. G.; van Weperen, I.; Plissard, S. R.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.
Year: 2013
DOI: 10.1038/nnano.2013.5
The development of viable quantum computation devices will require the ability to preserve the coherence of quantum bits (qubits). Single electron spins in semiconductor quantum dots are a versatile platform for quantum information processing, but controlling decoherence remains a considerable challenge. Hole spins in III–V semiconductors have unique properties, such as a strong spin–orbit interaction and weak coupling to nuclear spins, and therefore, have the potential for enhanced spin control and longer coherence times. A weaker hyperfine interaction has previously been reported in self-assembled quantum dots using quantum optics techniques, but the development of hole–spin-based electronic devices in conventional III-V heterostructures has been limited by fabrication challenges. Here, we show that gate-tunable hole quantum dots can be formed in InSb nanowires and used to demonstrate Pauli spin blockade and electrical control of single hole spins. The devices are fully tunable between hole and electron quantum dots, which allows the hyperfine interaction strengths, g-factors and spin blockade anisotropies to be compared directly in the two regimes.https://authors.library.caltech.edu/records/6b3n7-c0c58Proposal for realizing Majorana fermions in chains of magnetic atoms on a superconductor
https://resolver.caltech.edu/CaltechAUTHORS:20160112-132320239
Authors: Nadj-Perge, S.; Drozdov, I. K.; Bernevig, B. A.; Yazdani, Ali
Year: 2013
DOI: 10.1103/PhysRevB.88.020407
We propose an easy-to-build easy-to-detect scheme for realizing Majorana fermions at the ends of a chain of magnetic atoms on the surface of a superconductor. Model calculations show that such chains can be easily tuned between trivial and topological ground states. In the latter, spatially resolved spectroscopy can be used to probe the Majorana fermion end states. Decoupled Majorana bound states can form even in short magnetic chains consisting of only tens of atoms. We propose scanning tunneling microscopy as the ideal technique to fabricate such systems and to probe their topological properties.https://authors.library.caltech.edu/records/h6rwy-kxv38Termination-dependent topological surface states of the natural superlattice phase Bi_4Se_3
https://resolver.caltech.edu/CaltechAUTHORS:20160113-085622977
Authors: Gibson, Q. D.; Schoop, L. M.; Weber, A. P.; Ji, Huiwen; Nadj-Perge, S.; Drozdov, I. K.; Beidenkopf, H.; Sadowski, J. T.; Fedorov, A.; Yazdani, A.; Valla, T.; Cava, R. J.
Year: 2013
DOI: 10.1103/PhysRevB.88.081108
We describe the topological surface states of Bi_4Se_3, a compound in the infinitely adaptive Bi_2-Bi_2Se_3 natural superlattice phase series, determined by a combination of experimental and theoretical methods. Two observable cleavage surfaces, terminating at Bi or Se, are characterized by angle-resolved photoelectron spectroscopy and scanning tunneling microscopy, and modeled by ab initio density functional theory calculations. Topological surface states are observed on both surfaces, but with markedly different dispersions and Kramers point energies. Bi_4Se_3 therefore represents the only known compound with different topological states on differently terminated, easily distinguished and stable surfaces.https://authors.library.caltech.edu/records/9z3mt-7nb55Quasiparticle interference on the surface of the topological crystalline insulator Pb_(1−x)Sn_xSe
https://resolver.caltech.edu/CaltechAUTHORS:20160113-070931212
Authors: Gyenis, A.; Drozdov, I. K.; Nadj-Perge, S.; Jeong, O. B.; Seo, J.; Pletikosić, I.; Valla, T.; Gu, G. D.; Yazdani, A.
Year: 2013
DOI: 10.1103/PhysRevB.88.125414
Topological crystalline insulators represent a novel topological phase of matter in which the surface states are protected by discrete point group symmetries of the underlying lattice. Rock-salt lead-tin-selenide alloy is one possible realization of this phase, which undergoes a topological phase transition upon changing the lead content. We used scanning tunneling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES) to probe the surface states on (001) Pb_(1−x)Sn_xSe in the topologically nontrivial (x=0.23) and topologically trivial (x=0) phases. We observed quasiparticle interference with STM on the surface of the topological crystalline insulator and demonstrated that the measured interference can be understood from ARPES studies and a simple band structure model. Furthermore, our findings support the fact that Pb_(0.77)Sn_(0.23)Se and PbSe have different topological nature.https://authors.library.caltech.edu/records/mn1sc-4wx19Quantum computing based on semiconductor nanowires
https://resolver.caltech.edu/CaltechAUTHORS:20160113-070932327
Authors: Frolov, Sergey M.; Plissard, Sébastien R.; Nadj-Perge, Stevan; Kouwenhoven, Leo P.; Bakkers, Erik P.A.M.
Year: 2013
DOI: 10.1557/mrs.2013.205
A quantum computer will have computational power beyond that of conventional computers, which can be exploited for solving important and complex problems, such as predicting the conformations of large biological molecules. Materials play a major role in this emerging technology, as they can enable sophisticated operations, such as control over single degrees of freedom and their quantum states, as well as preservation and coherent transfer of these states between distant nodes. Here we assess the potential of semiconductor nanowires grown from the bottom-up as a materials platform for a quantum computer. We review recent experiments in which small bandgap nanowires are used to manipulate single spins in quantum dots and experiments on Majorana fermions, which are quasiparticles relevant for topological quantum computing.https://authors.library.caltech.edu/records/8s2h3-qjj07One-dimensional topological edge states of bismuth bilayers
https://resolver.caltech.edu/CaltechAUTHORS:20160113-085622475
Authors: Drozdov, Ilya K.; Alexandradinata, A.; Jeon, Sangjun; Nadj-Perge, Stevan; Ji, Huiwen; Cava, R. J.; Bernevig, B. Andrei; Yazdani, Ali
Year: 2014
DOI: 10.1038/nphys3048
The hallmark of a topologically insulating state of matter in two dimensions protected by time-reversal symmetry is the existence of chiral edge modes propagating along the perimeter of the sample. Among the first systems predicted to be a two-dimensional topological insulator are bilayers of bismuth. Here we report scanning tunnelling microscopy experiments on bulk Bi crystals that show that a subset of the predicted Bi-bilayers' edge states are decoupled from the states of the substrate and provide direct spectroscopic evidence of their one-dimensional nature. Moreover, by visualizing the quantum interference of edge-mode quasi-particles in confined geometries, we demonstrate their remarkable coherent propagation along the edge with scattering properties consistent with strong suppression of backscattering as predicted for the propagating topological edge states.https://authors.library.caltech.edu/records/atpfy-zjz59Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor
https://resolver.caltech.edu/CaltechAUTHORS:20160113-070932588
Authors: Nadj-Perge, Stevan; Drozdov, Ilya K.; Li, Jian; Chen, Hua; Jeon, Sangjun; Seo, Jungpil; MacDonald, Allan H.; Bernevig, B. Andrei; Yazdani, Ali
Year: 2014
DOI: 10.1126/science.1259327
Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.https://authors.library.caltech.edu/records/vxm1d-t1n80Electric and Magnetic Tuning Between the Trivial and Topological Phases in InAs/GaSb Double Quantum Wells
https://resolver.caltech.edu/CaltechAUTHORS:20160112-140347480
Authors: Qu, Fanming; Beukman, Arjan J. A.; Nadj-Perge, Stevan; Wimmer, Michael; Nguyen, Binh-Minh; Yi, Wei; Thorp, Jacob; Sokolich, Marko; Kiselev, Andrey A.; Manfra, Michael J.; Marcus, Charles M.; Kouwenhoven, Leo P.
Year: 2015
DOI: 10.1103/PhysRevLett.115.036803
Among the theoretically predicted two-dimensional topological insulators, InAs/GaSb double quantum wells (DQWs) have a unique double-layered structure with electron and hole gases separated in two layers, which enables tuning of the band alignment via electric and magnetic fields. However, the rich trivial-topological phase diagram has yet to be experimentally explored. We present an in situ and continuous tuning between the trivial and topological insulating phases in InAs/GaSb DQWs through electrical dual gating. Furthermore, we show that an in-plane magnetic field shifts the electron and hole bands relatively to each other in momentum space, functioning as a powerful tool to discriminate between the topologically distinct states.https://authors.library.caltech.edu/records/q66eq-wkp04Josephson φ_0-junction in nanowire quantum dots
https://resolver.caltech.edu/CaltechAUTHORS:20160113-101657551
Authors: Szombati, D. B.; Nadj-Perge, S.; Car, D.; Plissard, S. R.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.
Year: 2016
DOI: 10.1038/NPHYS3742
The Josephson effect describes supercurrent flowing through a junction connecting two superconducting leads by a thin barrier. This current is driven by a superconducting phase difference ϕ between the leads. In the presence of chiral and time-reversal symmetry of the Cooper pair tunnelling process2, the current is strictly zero when ϕ vanishes. Only if these underlying symmetries are broken can the supercurrent for ϕ = 0 be finite. This corresponds to a ground state of the junction being offset by a phase ϕ_0, different from 0 or π. Here, we report such a Josephson ϕ0-junction based on a nanowire quantum dot. We use a quantum interferometer device to investigate phase offsets and demonstrate that ϕ_0 can be controlled by electrostatic gating. Our results may have far-reaching implications for superconducting flux- and phase-defined quantum bits as well as for exploring topological superconductivity in quantum dot systems.https://authors.library.caltech.edu/records/xn7ay-jnb52Edge Transport in the Trivial Phase of InAs/GaSb
https://resolver.caltech.edu/CaltechAUTHORS:20160112-150307051
Authors: Nichele, Fabrizio; Suominen, Henri J.; Kjaergaard, Morten; Marcus, Charles M.; Sajadi, Ebrahim; Folk, Joshua A.; Qu, Fanming; Beukman, Arjan J. A.; de Vries, Folkert K.; van Veen, Jasper; Nadj-Perge, Stevan; Kouwenhoven, Leo P.; Nguyen, Binh-Minh; Kiselev, Andrey A.; Yi, Wei; Sokolich, Marko; Manfra, Michael J.; Spanton, Eric M.; Moler, Kathryn A.
Year: 2016
DOI: 10.1088/1367-2630/18/8/083005
We present transport and scanning SQUID measurements on InAs/GaSb double quantum wells, a system predicted to be a two-dimensional topological insulator. Top and back gates allow independent control of density and band offset, allowing tuning from the trivial to the topological regime. In the trivial regime, bulk conductivity is quenched but transport persists along the edges, superficially resembling the predicted helical edge-channels in the topological regime. We characterize edge conduction in the trivial regime in a wide variety of sample geometries and measurement configurations, as a function of temperature, magnetic field, and edge length. Despite similarities to studies claiming measurements of helical edge channels, our characterization points to a non-topological origin for these observations.https://authors.library.caltech.edu/records/68s7z-jjk90Decoupling Edge Versus Bulk Conductance in the Trivial Regime of an InAs/GaSb Double Quantum Well Using Corbino Ring Geometry
https://resolver.caltech.edu/CaltechAUTHORS:20170505-112142473
Authors: Nguyen, Binh-Minh; Kiselev, Andrey A.; Noah, Ramsey; Yi, Wei; Qu, Fanming; Beukman, Arjan J. A.; de Vries, Folkert K.; van Veen, Jasper; Nadj-Perge, Stevan; Kouwenhoven, Leo P.; Kjaergaard, Morten; Suominen, Henri J.; Nichele, Fabrizio; Marcus, Charles M.; Manfra, Michael J.; Sokolich, Marko
Year: 2016
DOI: 10.1103/PhysRevLett.117.077701
A Corbino ring geometry is utilized to analyze edge and bulk conductance of InAs/GaSb quantum well structures. We show that edge conductance exists in the trivial regime of this theoretically predicted topological system with a temperature-insensitive linear resistivity per unit length in the range of 2 kΩ/μm. A resistor network model of the device is developed to decouple the edge conductance from the bulk conductance, providing a quantitative technique to further investigate the nature of this trivial edge conductance, conclusively identified here as being of n type.https://authors.library.caltech.edu/records/91m3q-qeb90Electronic correlations in twisted bilayer graphene near the magic angle
https://resolver.caltech.edu/CaltechAUTHORS:20190426-084652512
Authors: Choi, Youngjoon; Kemmer, Jeannette; Peng, Yang; Thomson, Alex; Arora, Harpreet; Polski, Robert; Zhang, Yiran; Ren, Hechen; Alicea, Jason; Refael, Gil; von Oppen, Felix; Watanabe, Kenji; Taniguchi, Takashi; Nadj-Perge, Stevan
Year: 2019
DOI: 10.1038/s41567-019-0606-5
Twisted bilayer graphene with a twist angle of around 1.1° features a pair of isolated flat electronic bands and forms a platform for investigating strongly correlated electrons. Here, we use scanning tunnelling microscopy to probe the local properties of highly tunable twisted bilayer graphene devices and show that the flat bands deform when aligned with the Fermi level. When the bands are half-filled, we observe the development of gaps originating from correlated insulating states. Near charge neutrality, we find a previously unidentified correlated regime featuring an enhanced splitting of the flat bands. We describe this within a microscopic model that predicts a strong tendency towards nematic ordering. Our results provide insights into symmetry-breaking correlation effects and highlight the importance of electronic interactions for all filling fractions in twisted bilayer graphene.https://authors.library.caltech.edu/records/a69vz-3xp68Superconductivity in metallic twisted bilayer graphene stabilized by WSe₂
https://resolver.caltech.edu/CaltechAUTHORS:20200225-102617489
Authors: Arora, Harpreet Singh; Polski, Robert; Zhang, Yiran; Thomson, Alex; Choi, Youngjoon; Kim, Hyunjin; Lin, Zhong; Wilson, Ilham Zaky; Xu, Xiaodong; Chu, Jiun-Haw; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; Nadj-Perge, Stevan
Year: 2020
DOI: 10.1038/s41586-020-2473-8
Magic-angle twisted bilayer graphene (TBG), with rotational misalignment close to 1.1 degrees, features isolated flat electronic bands that host a rich phase diagram of correlated insulating, superconducting, ferromagnetic and topological phases. Correlated insulators and superconductivity have been previously observed only for angles within 0.1 degree of the magic angle and occur in adjacent or overlapping electron-density ranges; nevertheless, the origins of these states and the relation between them remain unclear, owing to their sensitivity to microscopic details. Beyond twist angle and strain, the dependence of the TBG phase diagram on the alignment and thickness of the insulating hexagonal boron nitride (hBN) used to encapsulate the graphene sheets indicates the importance of the microscopic dielectric environment. Here we show that adding an insulating tungsten diselenide (WSe₂) monolayer between the hBN and the TBG stabilizes superconductivity at twist angles much smaller than the magic angle. For the smallest twist angle of 0.79 degrees, superconductivity is still observed despite the TBG exhibiting metallic behaviour across the whole range of electron densities. Finite-magnetic-field measurements further reveal weak antilocalization signatures as well as breaking of fourfold spin–valley symmetry, consistent with spin–orbit coupling induced in the TBG via its proximity to WSe₂. Our results constrain theoretical explanations for the emergence of superconductivity in TBG and open up avenues towards engineering quantum phases in moiré systems.https://authors.library.caltech.edu/records/7yxgs-9qf25Spin-orbit-enhanced magnetic surface second-harmonic generation in Sr₂IrO₄
https://resolver.caltech.edu/CaltechAUTHORS:20201113-100152383
Authors: Seyler, K. L.; de la Torre, A.; Porter, Z.; Zoghlin, E.; Polski, R.; Nguyen, M.; Nadj-Perge, S.; Wilson, S. D.; Hsieh, D.
Year: 2020
DOI: 10.1103/physrevb.102.201113
An anomalous optical second-harmonic generation (SHG) signal was previously reported in Sr₂IrO₄ and attributed to a hidden odd-parity bulk magnetic state. Here we investigate the origin of this SHG signal using a combination of bulk magnetic susceptibility, magnetic-field-dependent SHG rotational anisotropy, and overlapping wide-field SHG imaging and atomic force microscopy measurements. We find that the anomalous SHG signal exhibits a twofold rotational symmetry as a function of in-plane magnetic field orientation that is associated with a crystallographic distortion. We also show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern. While we do not rule out the existence of hidden order in Sr₂IrO₄, our results altogether show that the anomalous SHG signal in parent Sr₂IrO₄ originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.https://authors.library.caltech.edu/records/sj520-zwr83Correlation-driven topological phases in magic-angle twisted bilayer graphene
https://resolver.caltech.edu/CaltechAUTHORS:20200922-103631989
Authors: Choi, Youngjoon; Kim, Hyunjin; Peng, Yang; Thomson, Alex; Lewandowski, Cyprian; Polski, Robert; Zhang, Yiran; Arora, Harpreet Singh; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; Nadj-Perge, Stevan
Year: 2021
DOI: 10.1038/s41586-020-03159-7
Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron–electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when ±1, ±2 and ±3 electrons occupy each moiré unit cell, and lead to the formation of various correlated phases. Although some phases have been shown to have a non-zero Chern number, the local microscopic properties and topological character of many other phases have not yet been determined. Here we introduce a set of techniques that use scanning tunnelling microscopy to map the topological phases that emerge in MATBG in a finite magnetic field. By following the evolution of the local density of states at the Fermi level with electrostatic doping and magnetic field, we create a local Landau fan diagram that enables us to assign Chern numbers directly to all observed phases. We uncover the existence of six topological phases that arise from integer fillings in finite fields and that originate from a cascade of symmetry-breaking transitions driven by correlations. These topological phases can form only for a small range of twist angles around the magic angle, which further differentiates them from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions, exhibits prominent electron–hole asymmetry, and features an unexpectedly large splitting between zero Landau levels (about 3 to 5 millielectronvolts). Our results show how strong electronic interactions affect the MATBG band structure and lead to correlation-enabled topological phases.https://authors.library.caltech.edu/records/zb5x8-zyx34Does filling-dependent band renormalization aid pairing in twisted bilayer graphene?
https://resolver.caltech.edu/CaltechAUTHORS:20210316-084002156
Authors: Lewandowski, Cyprian; Nadj-Perge, Stevan; Chowdhury, Debanjan
Year: 2021
DOI: 10.1038/s41535-021-00379-6
Magic-angle twisted bilayer graphene (MATBG) exhibits a panoply of many-body phenomena that are intimately tied to the appearance of narrow and well-isolated electronic bands. The microscopic ingredients that are responsible for the complex experimental phenomenology include electron–electron (phonon) interactions and nontrivial Bloch wavefunctions associated with the narrow bands. Inspired by recent experiments, we focus on two independent quantities that are considerably modified by Coulomb interaction-driven band renormalization, namely the density of states and the minimal spatial extent associated with the Wannier functions. First, we show that a filling-dependent enhancement of the density of states, caused by band flattening, in combination with phonon-mediated attraction due to electron-phonon umklapp processes, increases the tendency towards superconducting pairing in a range of angles around magic-angle. Second, we demonstrate that the minimal spatial extent associated with the Wannier functions, which contributes towards increasing the superconducting phase stiffness, also develops a nontrivial enhancement due to the interaction-induced renormalization of the Bloch wavefunctions. While our modeling of superconductivity (SC) assumes a weak electron-phonon coupling and does not consider many of the likely relevant correlation effects, it explains simply the experimentally observed robustness of SC in the wide range of angles that occurs in the relevant range of fillings.https://authors.library.caltech.edu/records/gfmjy-0nw09Interaction-driven band flattening and correlated phases in twisted bilayer graphene
https://resolver.caltech.edu/CaltechAUTHORS:20210302-094400352
Authors: Choi, Youngjoon; Kim, Hyunjin; Lewandowski, Cyprian; Peng, Yang; Thomson, Alex; Polski, Robert; Zhang, Yiran; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; Nadj-Perge, Stevan
Year: 2021
DOI: 10.1038/s41567-021-01359-0
Flat electronic bands, characteristic of 'magic-angle' twisted bilayer graphene, host many correlated phenomena. Nevertheless, many properties of these bands and emerging symmetry-broken phases are still poorly understood. Here we use scanning tunnelling spectroscopy to examine the evolution of the twisted bilayer graphene bands and related gapped phases as the twist angle between the two graphene layers changes. We detect filling-dependent flattening of the bands that is appreciable even when the angle is well above the magic angle value and so the material is nominally in a weakly correlated regime. Upon approaching the magic angle, we further show that the most prominent correlated gaps begin to emerge when band flattening is maximized around certain integer fillings of electrons per moiré unit cell. Our observations are consistent with a model that suggests that a significant enhancement of the density of states caused by the band flattening triggers a cascade of symmetry-breaking transitions. Finally, we explore the temperature dependence of the cascade and identify gapped features that develop in a broad range of band fillings where superconductivity is expected. Our results highlight the role of interaction-driven band flattening in defining the electronic properties of twisted bilayer graphene.https://authors.library.caltech.edu/records/xvc1e-83a03Gate-defined wires in twisted bilayer graphene: From electrical detection of intervalley coherence to internally engineered Majorana modes
https://resolver.caltech.edu/CaltechAUTHORS:20220104-233136449
Authors: Thomson, Alex; Sorensen, Ina M.; Nadj-Perge, Stevan; Alicea, Jason
Year: 2022
DOI: 10.1103/PhysRevB.105.L081405
Twisted bilayer graphene (TBG) realizes a highly tunable, strongly interacting system featuring superconductivity and various correlated insulating states. We establish gate-defined wires in TBG with proximity-induced spin-orbit coupling as (i) a tool for revealing the nature of correlated insulators and (ii) a platform for Majorana-based topological qubits. We show that the band structure of a gate-defined wire immersed in an intervalley coherent correlated insulator inherits electrically detectable fingerprints of symmetry breaking native to the latter. Surrounding the wire by a superconducting TBG region on one side and an intervalley coherent correlated insulator on the other further enables the formation of Majorana zero modes—possibly even at zero magnetic field depending on the precise symmetry-breaking order present. Our proposal not only introduces a highly gate-tunable topological qubit medium relying on internally generated proximity effects but can also shed light on the Cooper-pairing mechanism in TBG.https://authors.library.caltech.edu/records/j3avd-tf451Evidence for unconventional superconductivity in twisted trilayer graphene
https://resolver.caltech.edu/CaltechAUTHORS:20220621-285198100
Authors: Kim, Hyunjin; Choi, Youngjoon; Lewandowski, Cyprian; Thomson, Alex; Zhang, Yiran; Polski, Robert; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; Nadj-Perge, Stevan
Year: 2022
DOI: 10.1038/s41586-022-04715-z
Magic-angle twisted trilayer graphene (MATTG) has emerged as a moiré material that exhibits strong electronic correlations and unconventional superconductivity. However, local spectroscopic studies of this system are still lacking. Here we perform high-resolution scanning tunnelling microscopy and spectroscopy of MATTG that reveal extensive regions of atomic reconstruction favouring mirror-symmetric stacking. In these regions, we observe symmetry-breaking electronic transitions and doping-dependent band-structure deformations similar to those in magic-angle bilayers, as expected theoretically given the commonality of flat bands. Most notably in a density window spanning two to three holes per moiré unit cell, the spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunnelling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped superconductor to a nodal superconductor, which is theoretically compatible with a sharp transition from a Bardeen–Cooper–Schrieffer superconductor to a Bose–Einstein-condensation superconductor with a nodal order parameter. Within this doping window, we also detect peak–dip–hump structures that suggest that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results will enable further understanding of superconductivity and correlated states in graphene-based moiré structures beyond twisted bilayers.https://authors.library.caltech.edu/records/4z37b-7n608Promotion of superconductivity in magic-angle graphene multilayers
https://resolver.caltech.edu/CaltechAUTHORS:20221215-427737300.1
Authors: Zhang, Yiran; Polski, Robert; Lewandowski, Cyprian; Thomson, Alex; Peng, Yang; Choi, Youngjoon; Kim, Hyunjin; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; von Oppen, Felix; Refael, Gil; Nadj-Perge, Stevan
Year: 2022
DOI: 10.1126/science.abn8585
Graphene moiré superlattices show an abundance of correlated insulating, topological, and superconducting phases. Whereas the origins of strong correlations and nontrivial topology can be directly linked to flat bands, the nature of superconductivity remains enigmatic. We demonstrate that magic-angle devices made of twisted tri-, quadri-, and pentalayer graphene placed on monolayer tungsten diselenide exhibit flavor polarization and superconductivity. We also observe insulating states in the tril- and quadrilayer arising at finite electric displacement fields. As the number of layers increases, superconductivity emerges over an enhanced filling-factor range, and in the pentalayer it extends well beyond the filling of four electrons per moiré unit cell. Our results highlight the role of the interplay between flat and more dispersive bands in extending superconducting regions in graphene moiré superlattices.https://authors.library.caltech.edu/records/202s5-wxx58Enhanced superconductivity in spin–orbit proximitized bilayer graphene
https://resolver.caltech.edu/CaltechAUTHORS:20230223-726426000.4
Authors: Zhang, Yiran; Polski, Robert; Thomson, Alex; Lantagne-Hurtubise, Étienne; Lewandowski, Cyprian; Zhou, Haoxin; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; Nadj-Perge, Stevan
Year: 2023
DOI: 10.1038/s41586-022-05446-x
In the presence of a large perpendicular electric field, Bernal-stacked bilayer graphene (BLG) features several broken-symmetry metallic phases as well as magnetic-field-induced superconductivity1. The superconducting state is quite fragile, however, appearing only in a narrow window of density and with a maximum critical temperature T꜀ ≈ 30 mK. Here we show that placing monolayer tungsten diselenide (WSe₂) on BLG promotes Cooper pairing to an extraordinary degree: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in T꜀ and occurs over a density range that is wider by a factor of eight. By mapping quantum oscillations in BLG–WSe₂ as a function of electric field and doping, we establish that superconductivity emerges throughout a region for which the normal state is polarized, with two out of four spin-valley flavours predominantly populated. In-plane magnetic field measurements further reveal that superconductivity in BLG–WSe₂ can exhibit striking dependence of the critical field on doping, with the Chandrasekhar–Clogston (Pauli) limit roughly obeyed on one end of the superconducting dome, yet sharply violated on the other. Moreover, the superconductivity arises only for perpendicular electric fields that push BLG hole wavefunctions towards WSe₂, indicating that proximity-induced (Ising) spin–orbit coupling plays a key role in stabilizing the pairing. Our results pave the way for engineering robust, highly tunable and ultra-clean graphene-based superconductors.https://authors.library.caltech.edu/records/yz6kc-3m611Hot Carrier Thermalization and Josephson Inductance Thermometry in a Graphene-Based Microwave Circuit
https://resolver.caltech.edu/CaltechAUTHORS:20230530-441768000.65
Authors: Katti, Raj; Arora, Harpreet Singh; Saira, Olli-Pentti; Watanabe, Kenji; Taniguchi, Takashi; Schwab, Keith C.; Roukes, Michael Lee; Nadj-Perge, Stevan
Year: 2023
DOI: 10.1021/acs.nanolett.2c04791
Due to its exceptional electronic and thermal properties, graphene is a key material for bolometry, calorimetry, and photon detection. However, despite graphene's relatively simple electronic structure, the physical processes responsible for the heat transport from the electrons to the lattice are experimentally still elusive. Here, we measure the thermal response of low-disorder graphene encapsulated in hexagonal boron nitride by integrating it within a multiterminal superconducting microwave resonator. The device geometry allows us to simultaneously apply Joule heat power to the graphene flake while performing calibrated readout of the electron temperature. We probe the thermalization rates of both electrons and holes with high precision and observe a thermalization scaling exponent not consistent with cooling through the graphene bulk and argue that instead it can be attributed to processes at the graphene – aluminum interface. Our technique provides new insights into the thermalization pathways essential for the next-generation graphene thermal detectors.https://authors.library.caltech.edu/records/5384z-b6e89Gate-Defined Topological Josephson Junctions in Bernal Bilayer Graphene
https://authors.library.caltech.edu/records/e8bce-b9w69
Authors: Xie, Ying-Ming; Lantagne-Hurtubise, Étienne; Young, Andrea F.; Nadj-Perge, Stevan; Alicea, Jason
Year: 2023
DOI: 10.1103/physrevlett.131.146601
<p>Recent experiments on Bernal bilayer graphene (BLG) deposited on monolayer WSe₂ revealed robust, ultraclean superconductivity coexisting with sizable induced spin-orbit coupling. Here, we propose BLG/WSe₂ as a platform to engineer <i>gate-defined</i> planar topological Josephson junctions, where the normal and superconducting regions descend from a common material. More precisely, we show that if superconductivity in BLG/WSe₂ is gapped and emerges from a parent state with intervalley coherence, then Majorana zero-energy modes can form in the barrier region upon applying weak in-plane magnetic fields. Our results spotlight a potential pathway for "internally engineered" topological superconductivity that minimizes detrimental disorder and orbital-magnetic-field effects.</p>https://authors.library.caltech.edu/records/e8bce-b9w69Imaging inter-valley coherent order in magic-angle twisted trilayer graphene
https://authors.library.caltech.edu/records/07zqt-kcp68
Authors: Kim, Hyunjin; Choi, Youngjoon; Lantagne-Hurtubise, Étienne; Lewandowski, Cyprian; Thomson, Alex; Kong, Lingyuan; Zhou, Haoxin; Baum, Eli; Zhang, Yiran; Holleis, Ludwig; Watanabe, Kenji; Taniguchi, Takashi; Young, Andrea F.; Alicea, Jason; Nadj-Perge, Stevan
Year: 2023
DOI: 10.1038/s41586-023-06663-8
<p>Magic-angle twisted trilayer graphene (MATTG) exhibits a range of strongly correlated electronic phases that spontaneously break its underlying symmetries<a href="https://www.nature.com/articles/s41586-023-06663-8#ref-CR1">1</a>,<a href="https://www.nature.com/articles/s41586-023-06663-8#ref-CR2">2</a>. Here we investigate the correlated phases of MATTG using scanning tunnelling microscopy and identify marked signatures of interaction-driven spatial symmetry breaking. In low-strain samples, over a filling range of about two to three electrons or holes per moiré unit cell, we observe atomic-scale reconstruction of the graphene lattice that accompanies a correlated gap in the tunnelling spectrum. This short-scale restructuring appears as a Kekulé supercell—implying spontaneous inter-valley coherence between electrons—and persists in a wide range of magnetic fields and temperatures that coincide with the development of the gap. Large-scale maps covering several moiré unit cells further reveal a slow evolution of the Kekulé pattern, indicating that atomic-scale reconstruction coexists with translation symmetry breaking at a much longer moiré scale. We use auto-correlation and Fourier analyses to extract the intrinsic periodicity of these phases and find that they are consistent with the theoretically proposed incommensurate Kekulé spiral order<a href="https://www.nature.com/articles/s41586-023-06663-8#ref-CR3">3</a>,<a href="https://www.nature.com/articles/s41586-023-06663-8#ref-CR4">4</a>. Moreover, we find that the wavelength characterizing moiré-scale modulations monotonically decreases with hole doping away from half-filling of the bands and depends weakly on the magnetic field. Our results provide essential insights into the nature of the correlated phases of MATTG in the presence of strain and indicate that superconductivity can emerge from an inter-valley coherent parent state.</p>https://authors.library.caltech.edu/records/07zqt-kcp68Electrostatic fate of N-layer moiré graphene
https://authors.library.caltech.edu/records/120n7-1ft96
Authors: Kolář, Kryštof; Zhang, Yiran; Nadj-Perge, Stevan; von Oppen, Felix; Lewandowski, Cyprian
Year: 2023
DOI: 10.1103/physrevb.108.195148
<p>Twisted <i>N</i>-layer graphene (TNG) moiré structures have recently been shown to exhibit robust superconductivity similar to twisted bilayer graphene (TBG). In particular for <i>N </i>= 4 and <i>N </i>= 5, the phase diagram features a superconducting pocket that extends beyond the nominal full filling of the flat band. These observations are seemingly at odds with the canonical understanding of the low-energy theory of TNG, wherein the TNG Hamiltonian consists of one flat-band sector and accompanying dispersive bands. Using a self-consistent Hartree-Fock treatment, we explain how the phenomenology of TNG can be understood through an interplay of in-plane Hartree and inhomogeneous layer potentials, which cause a reshuffling of electronic bands. We extend our understanding beyond the case of <i>N </i>= 5 realized in experiment so far. We describe how the Hartree and layer potentials control the phase diagram for devices with <i>N </i>> 5 and tend to preclude exchange-driven correlated phenomena in this limit. To circumvent these electrostatic constraints, we propose a flat-band paradigm that could be realized in large-<i>N</i> devices by taking advantage of two nearly flat sectors acting together to enhance the importance of exchange effects.</p>https://authors.library.caltech.edu/records/120n7-1ft96