Person: Halperin, Bertrand
Loading...
Email Address
AA Acceptance Date
Birth Date
Research Projects
Organizational Units
Job Title
Last Name
Halperin
First Name
Bertrand
Name
Halperin, Bertrand
38 results
Search Results
Now showing 1 - 10 of 38
Publication Topological Superconductivity in a Planar Josephson Junction(American Physical Society (APS), 2017-05-30) Pientka, Falko; Keselman, Anna; Berg, Erez; Yacoby, Amir; Stern, Ady; Halperin, BertrandWe consider a two-dimensional electron gas with strong spin-orbit coupling contacted by two superconducting leads, forming a Josephson junction. We show that in the presence of an in-plane Zeeman field, the quasi-one-dimensional region between the two superconductors can support a topological superconducting phase hosting Majorana bound states at its ends. We study the phase diagram of the system as a function of the Zeeman field and the phase difference between the two superconductors (treated as an externally controlled parameter). Remarkably, at a phase difference of π, the topological phase is obtained for almost any value of the Zeeman field and chemical potential. In a setup where the phase is not controlled externally, we find that the system undergoes a first-order topological phase transition when the Zeeman field is varied. At the transition, the phase difference in the ground state changes abruptly from a value close to zero, at which the system is trivial, to a value close to π, at which the system is topological. The critical current through the junction exhibits a sharp minimum at the critical Zeeman field and is therefore a natural diagnostic of the transition. We point out that in the presence of a symmetry under a mirror reflection followed by time reversal, the system belongs to a higher symmetry class, and the phase diagram as a function of the phase difference and the Zeeman field becomes richer.Publication Mach-Zehnder interferometry using spin- and valley-polarized quantum Hall edge states in graphene(American Association for the Advancement of Science, 2017) Wei, Di; van der Sar, Toeno; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi; Jarillo-Herrero, Pablo; Halperin, Bertrand; Yacoby, AmirConfined to a two-dimensional plane, electrons in a strong magnetic field travel along the edge in one-dimensional quantum Hall channels that are protected against backscattering. These channels can be used as solid-state analogs of monochromatic beams of light, providing a unique platform for studying electron interference. Electron interferometry is regarded as one of the most promising routes for studying fractional and non-Abelian statistics and quantum entanglement via two-particle interference. However, creating an edge-channel interferometer in which electron-electron interactions play an important role requires a clean system and long phase coherence lengths. We realize electronic Mach-Zehnder interferometers with record visibilities of up to 98% using spin- and valley-polarized edge channels that copropagate along a pn junction in graphene. We find that interchannel scattering between same-spin edge channels along the physical graphene edge can be used to form beamsplitters, whereas the absence of interchannel scattering along gate-defined interfaces can be used to form isolated interferometer arms. Surprisingly, our interferometer is robust to dephasing effects at energies an order of magnitude larger than those observed in pioneering experiments on GaAs/AlGaAs quantum wells. Our results shed light on the nature of edge-channel equilibration and open up new possibilities for studying exotic electron statistics and quantum phenomena.Publication Electron-hole asymmetric integer and fractional quantum Hall effect in bilayer graphene(American Association for the Advancement of Science (AAAS), 2014) Kou, Angela; Feldman, Benjamin Ezekiel; Levin, Andrei; Halperin, Bertrand; Watanabe, Kenji; Taniguchi, Takashi; Yacoby, AmirThe nature of fractional quantum Hall (FQH) states is determined by the interplay between the Coulomb interaction and the symmetries of the system. The unique combination of spin, valley, and orbital degeneracies in bilayer graphene is predicted to produce novel and tunable FQH ground states. Here we present local electronic compressibility measurements of the FQH effect in the lowest Landau level of bilayer graphene. We observe incompressible FQH states at filling factors \(\nu = 2p + 2/3\) with hints of additional states appearing at \(\nu = 2p + 3/5\), where p = -2,-1, 0, and 1. This sequence of states breaks particle-hole symmetry and instead obeys a \(\nu \rightarrow \nu + 2\) symmetry, which highlights the importance of the orbital degeneracy for many-body states in bilayer graphene.Publication Fractional Quantum Hall Phase Transitions and Four-Flux States in Graphene(American Physical Society (APS), 2013) Feldman, Benjamin Ezekiel; Levin, Andrei; Krauss, Benjamin; Abanin, Dmitry; Halperin, Bertrand; Smet, Jurgen H.; Yacoby, AmirGraphene and its multilayers have attracted considerable interest because their fourfold spin and valley degeneracy enables a rich variety of broken-symmetry states arising from electron-electron interactions, and raises the prospect of controlled phase transitions among them. Here we report local electronic compressibility measurements of ultraclean suspended graphene that reveal a multitude of fractional quantum Hall states surrounding filling factors \(\nu =−1/2\) and \(−1/4\). Several of these states exhibit phase transitions that indicate abrupt changes in the underlying order, and we observe many additional oscillations in compressibility as \(\nu \) approaches \(−1/2\), suggesting further changes in spin and/or valley polarization. We use a simple model based on crossing Landau levels of composite fermions with different internal degrees of freedom to explain many qualitative features of the experimental data. Our results add to the diverse array of many-body states observed in graphene and demonstrate substantial control over their order parameters.Publication Detecting Non-Abelian Anyons by Charging Spectroscopy(American Physical Society (APS), 2013) Ben-Shach, Gilad; Laumann, Chris; Neder, I.; Yacoby, Amir; Halperin, BertrandObservation of non-Abelian statistics for the \(e/4\) quasiparticles in the \(\nu =\frac{5}{2}\) fractional quantum Hall state remains an outstanding experimental problem. The non-Abelian statistics are linked to the presence of additional low energy states in a system with localized quasiparticles, and, hence, an additional low temperature entropy. Recent experiments, which detect changes in the number of quasiparticles trapped in a local potential well as a function of an applied gate voltage, VG, provide a possibility for measuring this entropy, if carried out over a suitable range of temperatures, T. We present a microscopic model for quasiparticles in a potential well and study the effects of non-Abelian statistics on the charge stability diagram in the VG−T plane, including broadening at finite temperature. We predict a measurable slope for the first quasiparticle charging line and an even-odd effect in the diagram, which is a signature of non-Abelian statistics.Publication Quenching of dynamic nuclear polarization by spin–orbit coupling in GaAs quantum dots(Nature Pub. Group, 2015) Nichol, John M.; Harvey, Shannon; Shulman, Michael D.; Pal, Arijeet; Umansky, Vladimir; Rashba, Emmanuel; Halperin, Bertrand; Yacoby, AmirThe central-spin problem is a widely studied model of quantum decoherence. Dynamic nuclear polarization occurs in central-spin systems when electronic angular momentum is transferred to nuclear spins and is exploited in quantum information processing for coherent spin manipulation. However, the mechanisms limiting this process remain only partially understood. Here we show that spin–orbit coupling can quench dynamic nuclear polarization in a GaAs quantum dot, because spin conservation is violated in the electron–nuclear system, despite weak spin–orbit coupling in GaAs. Using Landau–Zener sweeps to measure static and dynamic properties of the electron spin–flip probability, we observe that the size of the spin–orbit and hyperfine interactions depends on the magnitude and direction of applied magnetic field. We find that dynamic nuclear polarization is quenched when the spin–orbit contribution exceeds the hyperfine, in agreement with a theoretical model. Our results shed light on the surprisingly strong effect of spin–orbit coupling in central-spin systems.Publication Superfluid spin transport through antiferromagnetic insulators(American Physical Society (APS), 2014) Takei, So; Halperin, Bertrand; Yacoby, Amir; Tserkovnyak, YaroslavA theoretical proposal for realizing and detecting spin supercurrent in an isotropic antiferromagnetic insulator is reported. Superfluid spin transport is achieved by inserting the antiferromagnet between two metallic reservoirs and establishing a spin accumulation in one reservoir such that a spin bias is applied across the magnet. We consider a class of bipartite antiferromagnets with Néel ground states, and temperatures well below the ordering temperature, where spin transport is mediated essentially by the condensate. Landau-Lifshitz and magnetocircuit theories are used to directly relate spin current in different parts of the heterostructure to the spin-mixing conductances characterizing the antiferromagnet∣∣metal interfaces and the antiferromagnet bulk damping parameters, quantities all obtainable from experiments. We study the efficiency of spin angular-momentum transfer at an antiferromagnet∣∣metal interface by developing a microscopic scattering theory for the interface and extracting the spin-mixing conductance for a simple model. Within the model, a quantitative comparison between the spin-mixing conductances obtained for the antiferromagnet∣∣metal and ferromagnet∣∣metal interfaces is made.Publication Theory of Spin Hall Conductivity in n-Doped GaAs(American Physical Society (APS), 2005) Engel, Hans-Andreas; Halperin, Bertrand; Rashba, EmmanuelWe develop a theory of extrinsic spin currents in semiconductors, resulting from spin-orbit coupling at charged scatterers, which leads to skew-scattering and side-jump contributions to the spin-Hall conductivity. Applying the theory to bulk n-GaAs, without any free parameters, we find spin currents that are in reasonable agreement with experiments by Kato et al.Publication Superfluid-Insulator Transition in a Moving System of Interacting Bosons(American Physical Society (APS), 2005) Altman, E.; Polkovnikov, A.; Demler, Eugene; Halperin, Bertrand; Lukin, MikhailWe analyze the stability of superfluid currents in a system of strongly interacting ultracold atoms in an optical lattice. We show that such a system undergoes a dynamic, irreversible phase transition at a critical phase gradient that depends on the interaction strength between atoms. At commensurate filling, the phase boundary continuously interpolates between the classical modulation instability of a weakly interacting condensate and the equilibrium quantum phase transition into a Mott insulator state at which the critical current vanishes. We argue that quantum fluctuations smear the transition boundary in low dimensional systems. Finally we discuss the implications to realistic experiments.Publication Driven Nonlinear Dynamics of Two Coupled Exchange-Only Qubits(American Physical Society (APS), 2014) Pal, Arijeet; Rashba, Emmanuel; Halperin, BertrandInspired by the creation of a fast exchange-only qubit [Medford et al., Phys. Rev. Lett. 111, 050501 (2013)], we develop a theory describing the nonlinear dynamics of two such qubits that are capacitively coupled, when one of them is driven resonantly at a frequency equal to its level splitting. We include conditions of strong driving, where the Rabi frequency is a significant fraction of the level splitting, and we consider situations where the splitting for the second qubit may be the same as or different than the first. We demonstrate that coupling between qubits can be detected by reading the response of the second qubit, even when the coupling between them is only of about 1% of their level splittings, and we calculate entanglement between qubits. Patterns of nonlinear dynamics of coupled qubits and their entanglement are strongly dependent on the geometry of the system, and the specific mechanism of interqubit coupling deeply influences dynamics of both qubits. In particular, we describe the development of irregular dynamics in a two-qubit system, explore approaches for inhibiting it, and demonstrate the existence of an optimal range of coupling strength maintaining stability during the operational time.