Person: Ben-Shach, Gilad
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Ben-Shach
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Gilad
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Ben-Shach, Gilad
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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 Detecting Majorana modes in one-dimensional wires by charge sensing(American Physical Society (APS), 2015) Ben-Shach, Gilad; Haim, Arbel; Appelbaum, Ian; Oreg, Yuval; Yacoby, Amir; Halperin, BertrandThe electron number parity of the ground state of a semiconductor nanowire proximity coupled to a bulk superconductor can alternate between the quantized values ±1 if parameters such as the wire length L, the chemical potential μ, or the magnetic field B are varied inside the topological superconductor phase. The parity jumps, which may be interpreted as changes in the occupancy of the fermion state formed from the pair of Majorana modes at opposite ends of the wire, are accompanied by jumps δN in the charge of the nanowire, whose values decrease exponentially with the wire length. We study theoretically the dependence of δN on system parameters, and compare the locations in the μ−B plane of parity jumps when the nanowire is or is not proximity coupled to a bulk superconductor. We show that, despite the fact that the wave functions of the Majorana modes are localized near the two ends of the wire, the charge-density jumps have spatial distributions that are essentially uniform along the wire length, being proportional to the product of the two Majorana wave functions. We explain how charge measurements, say by an external single-electron transistor, could reveal these effects. Whereas existing experimental methods require direct contact to the wire for tunneling measurements, charge sensing avoids this issue and provides an orthogonal measurement to confirm recent experimental developments. Furthermore, by comparing density of states measurements which show Majorana features at the wire ends with the uniformly distributed charge measurements, one can rule out alternative explanations for earlier results. We shed light on a parameter regime for these wire-superconductor hybrid systems, and propose a related experiment to measure spin density.Publication Controlled finite momentum pairing and spatially varying order parameter in proximitized HgTe quantum wells(Springer Nature, 2016) Hart, Sean; Ren, Hechen; Kosowsky, Michael; Ben-Shach, Gilad; Leubner, Philipp; Brune, Christopher; Buhmann, Hartmut; Molenkamp, Laurens; Halperin, Bertrand; Yacoby, AmirConventional s-wave superconductivity is understood to arise from singlet pairing of electrons with opposite Fermi momenta, forming Cooper pairs whose net momentum is zero[1]. Several recent studies have focused on structures where such conventional s-wave superconductors are coupled to systems with an unusual configuration of electronic spin and momentum at the Fermi surface. Under these conditions, the nature of the paired state can be modified and the system may even undergo a topological phase transition [2, 3]. Here we present measurements and theoretical calculations of several HgTe quantum wells coupled to either aluminum or niobium superconductors and subject to a magnetic field in the plane of the quantum well. By studying the oscillatory response of Josephson interference to the magnitude of the in-plane magnetic field, we find that the induced pairing within the quantum well is spatially varying. Cooper pairs acquire a tunable momentum that grows with magnetic field strength, directly reflecting the response of the spin dependent Fermi surfaces to the in-plane magnetic field. In addition, in the regime of high electron density, nodes in the induced superconductivity evolve with the electron density in agreement with our model based on the Hamiltonian of Bernevig, Hughes, and Zhang [4]. This agreement allows us to quantitatively extract the value of g/v ˜ F , where g˜ is the effective g-factor and vF is the Fermi velocity. However, at low density our measurements do not agree with our model in detail. Our new understanding of the interplay between spin physics and superconductivity introduces a way to spatially engineer the order parameter, as well as a general framework within which to investigate electronic spin texture at the Fermi surface of materials.Publication Theoretical Considerations for Experiments to Create and Detect Localised Majorana Modes in Electronic Systems(2015-01-06) Ben-Shach, Gilad; Demler, EugeneThis thesis presents our work on building a bridge between the theoretical proposals for the condensed matter realisation of peculiar localised excitations, known as Majorana modes, and experiments to search for them. The main focus in the first two sections is on charge sensing of localised Majorana modes in two distinct systems. First, we address the properties of charged quasiparticles in the $\nu=5/2$ fractional quantum Hall regime. In particular, we focus on the case where these particles are trapped by disorder, often in close proximity to one another. Next, we consider one-dimensional semi-conducting wires with strong spin-orbit coupling and proximity-induced superconductivity. The Majorana modes in this system are predicted to be charge-neutral. We show, however, that when the wire is short enough, there is a uniform charge distribution along the wire, and we show how the presence of this charge depends on system parameters. A third portion is related to HgTe quantum wells, another system predicted to host Majorana modes when coupled to a superconductor. Here we consider a HgTe well in the metallic regime, coupled to two superconducting strips. We compute the Josephson coupling in the presence of spin-orbit interactions and in-plane external magnetic fields.