Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y13: Majorana Bound States IV |
Hide Abstracts |
Sponsoring Units: DMP Chair: Ichiro Takeuchi, University of Maryland Room: LACC 304A |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y13.00001: Stabilizing a Majorana Zero Mode through Frustration Gu Zhang, Harold Baranger We show that a Majorana zero mode can be stabilized by hybridizing its partner with a two channel Kondo (2CK) state. Frustration inherent in the 2CK state leads to a decoupled Majorana degree of freedom which can naturally hybridize with one of a pair of Majorana modes in a topological superconducting wire, thereby leaving an unpaired Majorana zero mode at the other end of the wire. Concretely, the system that we study consists of two (left and right) on-resonant quantum dots that couple to two interacting Majorana fermions on the same grounded topological superconducting wire. The right dot is connected to two dissipative leads with $R = R_Q$, thus creating an effective 2CK state with an isolated Majorana zero mode on the dot. The left dot is connected to two regular leads and the transmission through it is monitored. We find that the coupling between the isolated dot Majorana and the Majorana in the superconducting wire stabilizes the non-trivial Majorana transmission signature of the left system. This also provides a clear signature of the presence of the 2CK Majorana. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y13.00002: Effective Multi-orbital 1D Models of Majorana Nanowires including Electronic Interactions Benjamin Woods, Tudor Stanescu Semiconductor-nanowires proximity coupled to superconductors are a promising platform for hosting and manipulating Majorana zero energy modes. The occupation of multiple confinement-induced bands and the associated electrostatic effects play a key role in these systems. We propose an efficient method to construct effective multi-orbital 1D models starting from microscopic 3D Hamiltonians, which incorporates the multiband physics as well as electronic interactions at the mean field level. The “orbitals” of the effective model are associated with transverse low-energy modes that are self-consistent solutions of the corresponding infinite wire problem with parameters matching the local conditions. This leads to band- and position-dependent effective parameters, such as effective potentials, spin-orbit coefficients, diagonal and off-diagonal hopping, etc. The electron-electron interactions are efficiently incorporated by finding a Green’s function, either numerically or analytically, and storing the results. Using this approach, we investigate various properties of the nanowire system, including the response to an applied magnetic field and the features induced by wire inhomogeneities. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y13.00003: Tailoring the Superconductor-Semiconductor Hybridization in Majorana Devices Panagiotis Kotetes, August Mikkelsen, Peter Krogstrup, Karsten Flensberg Recent experiments have provided solid evidence of Majorana quasiparticles in hybrid superconductor-semiconductor (Super-Semi) devices. However, the desired exploitation of Majoranas for quantum manipulations requires a high-degree of device-controllability and -tunability. More importantly, vital aspects of the Super-Semi hybridization which controls the above features, still remain unexplored. In this work, we employ a self-consistent Schrödinger-Poisson (SP) approach for addressing these key issues for Al-InAs interfaces. We obtain the electrostatic potential and the bandstructure via a numerical finite-difference self-consistent SP-method, using experimentally inferred parameters as an input. We also infer the band-resolved: (i) hybridization degree, (ii) Landé g-factor and (iii) Rashba spin-orbit coupling strength. We reveal that the interface coupling not only modifies the preexisting InAs levels, but most importantly, it gives rise to a new band near the Fermi level, solely due to the Al-InAs hybridization. We discuss how to tune the bandstructure characteristics via varying the width of the Al-layer, as also the backgate-voltage. Our numerical analysis is backed by an analytical approach, elucidating further aspects of the hybridization. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y13.00004: Detection of Majorana non-locality in hybrid dot-nanowire systems I: theory Ramon Aguado, Mingtang Deng, Saulius Vaitiekenas, Elsa Prada, Pablo San-Jose, Jesper Nygard, Peter Krogstrup, Charles Marcus The spatial non-locality nature of Majorana bound states (MBS) is the key ingredient for their topological protection. While several tunneling experiments have reported Majorana signatures in the form of a zero-bias conductance peak, a demonstration of non-local Majoranas as opposed to local Andreev levels coalescing near zero energy is still lacking. Here we demonstrate that a local transport measurement can be used to quantify the degree of Majorana non-locality associated to a zero-bias peak. This seemingly contradicting result can be achieved by measuring the sensitivity of the zero-energy state to a local perturbation created by a gate-tunable quantum dot located at the end of a nanowire, which simultaneously acts as a tunnel spectroscopic probe. The degree of sensitivity is then connected to the Majorana wavefunction overlap using theoretical tools [1]. Using this approach, we observed zero-bias peaks from low to high degree of wavefunction non-locality [2]. The first part of this talk (I) will focus on theory aspects while the second part (II) will present the experimental data. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y13.00005: Majorana bound states in non-homogeneous semiconductor nanowires Christopher Moore, Tudor Stanescu, Sumanta Tewari We demonstrate that partially overlapping Majorana bound states (MBSs) represent a generic low-energy feature that emerges in non-homogeneous semiconductor nanowires coupled to super conductors in the presence of a Zeeman field. The emergence of these low-energy modes is not correlated with any topological quantum phase transition that the system may undergo as the Zeeman field and other control parameters are varied. Increasing the characteristic length scale of the variations in the potential leads to a continuous evolution from strongly overlapping MBSs, which can be viewed as “regular” Andreev bound states (ABSs) that cross zero energy, to weakly coupled MBSs, which have nearly zero energy in a significant range of parameters and generate signatures similar to the Majorana zero modes (MZMs) that emerge in the topological superconducting phase. We show that using charge (or spin) transport measurements it is virtually impossible to distinguish MZMs from weakly overlapping MBSs emerging in the topologically-trivial regime and suggest a two-lead experiment which will distinguish between these two types of low energy modes in semiconductor-superconductor heterostructures. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y13.00006: Detection of Majorana non-locality in hybrid dot-nanowire systems II: Experiments Mingtang Deng, Ramon Aguado, Saulius Vaitiekenas, Elsa Prada, Pablo San-Jose, Jesper Nygard, Charles Marcus The spatial non-locality nature of Majorana bound states (MBS) is the key ingredient for their topological protection. While several tunneling experiments have reported Majorana signatures in the form of a zero- bias conductance peak, a demonstration of non-local Majoranas as opposed to local Andreev levels coalescing near zero energy is still lacking. Here we demonstrate that a local transport measurement can be used to quantify the degree of Majorana non-locality associated to a zero-bias peak. This seemingly contradicting result can be achieved by measuring the sensitivity of the zero-energy state to a local perturbation created by a gate-tunable quantum dot located at the end of a nanowire, which simultaneously acts as a tunnel spectroscopic probe. The degree of sensitivity is then connected to the Majorana wavefunction overlap using theoretical tools [1]. Using this approach, we observed zero-bias peaks from low to high degree of wavefunction non-locality [2]. The first part of this talk (I) will focus on theory aspects while the second part (II) will present the experimental data. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y13.00007: Two electron periodic Coulomb blockade and spin-orbit coupling in hybrid InAs-Al quantum dots Eoin O'Farrell, Asbjørn Drachmann, Fabrizio Nichele, Antonio Fornieri, Alexander Whiticar, Tiantian Wang, Geoffrey Gardner, Candice Thomas, Anthony Hatke, Michael Manfra, Charles Marcus Proximity induced superconductivity in semiconductors with a strong spin-orbit interaction has recently been the subject of intense theoretical and experimental research, this is driven by the possibility of inducing topological superconductivity. We present measurements of one-dimensional hybrid superconductor-semiconductor quantum dots fabricated on an InAs two dimensional electron gas with an epitaxial Al layer. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y13.00008: Andreev bound states versus Majorana bound states in quantum dot-nanowire-superconductor hybrid structures: Trivial versus topological zero-bias conductance peaks Chun-Xiao Liu, Jay Sau, Tudor Stanescu, Sankar Das Sarma Motivated by an important recent experiment, we theoretically consider the interplay between Andreev bound states(ABSs) and Majorana bound states(MBSs) in quantum dot-nanowire semiconductor systems with proximity-induced superconductivity(SC), spin-orbit coupling and Zeeman splitting. The dot induces ABSs in the SC nanowire which show complex behavior, and the specific question is whether two such ABSs can come together forming a topological MBS. We find that the ABSs indeed tend to coalesce together producing near-zero-energy states, but this mostly happens in the trivial regime although there are situations where the ABSs could come together forming a MBS. The two scenarios are difficult to distinguish by tunnel conductance. The "sticking together" propensity of ABSs to produce a zero-energy state is generic in class D systems, and by itself says nothing about the topological nature of the SC nanowire. Thus conductance measurements with the midgap sticking-together behavior of ABSs cannot be construed as definitive evidence for topological SC. We also suggest experimental techniques for distinguishing between trivial and topological ZBCPs. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y13.00009: Signature of Majorana Zero Mode in Atomic Chains on Proximitized Bi(110) Surface Yonglong Xie, Berthold Jaeck, Sangjun Jeon, Chao Lei, Allan MacDonald, Ali Yazdani Ferromagnetic chains of Fe atoms on top of a superconducting Pb substrate have proven to be a valuable platform to host Majorana zero modes(MZM) in different experiments[1,2,3]. Realizing new platforms, which combine ferromagnetism, strong Rashba spin-orbit coupling and superconductivity, is highly desirable to further study the emergence of MZM and to find systems with the largest topological gap. We have successfully grown expitaxial Bi(110) thin film on a superconducting substrate that shows robust proximity induced superconductivity with a hard gap. Examining the properties of such films in magnetic fields, we have visualized vortices and shown the persistence of surface superconductivity to fields above the bulk critical field. Introducing single magnetic atoms gives rise to Shiba states inside the superconducting gap on the Bi surface. Furthermore, we have successfully fabricated magnetic chains on the proximized Bi surface. Using high resolution spectroscopic measurements, we find distinct localized zero bias peaks at both ends, consistent with the formation of MZM in our atomic chains. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y13.00010: Appearance and disappearance of Majorana Zero Modes with magnetic field tilt in superconducting FeTe_{1-x}Se_{x} Areg Ghazaryan, Pavan Hosur, Ashvin Vishwanath, Pouyan Ghaemi Recently it was proposed that iron-chalcogenide material FeTe_{1-x}Se_{x} can host Majorana zero modes at the ends of vortex lines for a range of compositions. Due to the low density of states in FeTe_{1-x}Se_{x}, Majorana zero modes in superconducting vortices are expected to be separated by a relatively large gap from other vortex modes. On the experimental side, observation of Majorana zero modes at the ends of vortex lines has been debated. In this talk, we show that the presence of the vortex Majorana zero modes strongly depends on the direction of the vortex with respect to the crystal. By rotating the magnetic field, the vortex will go through a topological phase transition which is apparent from the appearance and disappearance of the Majorana zero modes at the ends of the vortex. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y13.00011: Topological Response for Majorana Fermions on the Boundary of Topological Superconductors Chao Xu, Wang Yang, Congjun Wu The Goldstone-Wilczek formula describes the topological response in (1+1) D fermionic systems, and was generalized to higher dimensions. We study the topological response of Majorana fermions on the boundary of topological superconductors. In particular analogues of soliton charge and charge pumping are discussed for Majorana fermions in the absence of U(1) charge conservation. The implication on thermal transport is also discussed in this work. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y13.00012: Braiding Majorana zero modes through Majorana spin Xin Liu, Xun-Jiang Luo, Xiong-Jun Liu The Majorana physics in a spin-full system is generally beyond the simplified spin-less model in the sense that the manipulation, detection and braiding of MZMs are in principle allowed through the addtional spin degree of freedom. In this work, we propose a new spin-based scheme to braid Majorana zero modes (MZMs) in a generic spin-full Majorana system. Due to the spin-triplet nature of MZMs, the worldlines, tracking the braiding history of MZMs, should be extended to worldribbons in spin space. The braiding of two MZMs is shown to be equivalent to the self-rotation of their spin by $\pi$ which can be depicted as twisting the two associated worldribbons. Although this scheme is general for any spin-full Majorana system, we explicitly show its realization based on a concrete model, in which the Fermion parity switch due to braiding MZMs is rooted from the topological charge pumping. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y13.00013: From Interacting Majorana to Universal Topological Defects Sharmistha Sahoo, Jeffrey Teo Ising anyons, Majorana fermions (MF) and zero energy Majorana bound states (MBS) have promising prospects in topological quantum computing (TQC) because of their ability to store quantum states non-locally in space and insensitivity to local decoherence. They appear in topological phases, such as a Pfaffian quantum Hall state and a topological superconductor. There are generalizations of MBS as twist defects, also known as parafermion or fractional MBS, in Abelian topological phases with global symmetries. Unfortunately, these objects are not powerful enough to assemble a TQC that can perform universal operations using topological braiding operations alone. On the other hand, there are anyonic quasiparticles, like the Fibonacci anyon in a Read-Rezayi quantum Hall state, that are universal in the TQC sense. However, these are quantum dynamical excitations, which can be challenging to spatially manipulate and susceptible to temperature fluctuations in a thermodynamic system. We propose new universal topological defects, which are semi-classical and static, in superconducting systems of many-body interacting MFs. |
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