Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X45: Two-Dimensional Topological Superconductors: IIFocus
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Sponsoring Units: DMP Room: 392 |
Friday, March 17, 2017 8:00AM - 8:36AM |
X45.00001: Manipulating Majorana Bound States with Tunable Magnetic Textures Invited Speaker: Alex Matos Abiague In condensed-matter systems Majorana bound states (MBSs) are emergent quasiparticles obeying non-Abelian statistics. While realizing the non-Abelian braiding statistics would provide both an ultimate proof for MBSs existence and a key element for fault-tolerant topological quantum computing, even theoretical schemes imply a significant complexity to implement such braiding. The first experimental evidences of MBSs formation were measured in one-dimensional (1D) systems [1]. However, since braiding statistics are ill-defined in 1D, complex wire networks must be used for directly probing the non-Abelian character of the MBSs [2]. In this talk I will discuss the possibility of creating and manipulating MBSs in two-dimensional (2D) systems by using tunable magnetic textures generated by an array of magnetic tunnel junctions (MTJs) located on a 2D superconductor-semiconductor heterostructure [3]. Magnetic textures can provide not only effective spin-orbit and Zeeman fields [4] -two important ingredients for the creation of MBSs- but also spatial confinement [5]. The underlying magnetic texture produced by the MTJs array leads to the formation of effective topological wires supporting MBSs formation. The effective wires can be re-shaped and re-oriented by properly changing the magnetic texture, allowing for the transportation of the MBSs in 2D [5]. I will then show how the proposed platform can be used to measure the non-Abelian statistics of MBSs through braiding and discuss the main challenges regarding materials, scalability, and detection. The effects of the coexistence of native and magnetically-induced spin-orbit fields on the MBSs as well as the possibility of using other magnetic textures will also be addressed.\newline [1] V. Mourik et al., Science 336, 1003 (2012); S. Nadj-Perge et al., Science 346, 602 (2014). [2] J. Alicea et al., Nat. Phys. 7, 412 (2011); D. Aasen et al., Phys. Rev. X 6, 031016 (2016). [3] J. Shabani et al., Phys. Rev. B 93, 155402 (2016). [4] Kjaergaard et al., Phys. Rev. B 85, 020503 (2012); J. Klinovaja et al., Phys. Rev. Lett. 111, 186805 (2013); G. Yang et al., Phys. Rev. B 93, 224505 (2016). [5] G. L. Fatin, A. Matos-Abiague, B. Scharf, and I. Zutic, Phys. Rev. Lett. 117, 077002 (2016). [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X45.00002: Transport of Majorana zero modes in ferromagnet-superconductor junction Jian-Xiao Zhang, Chao-Xing Liu We study the charge conductance and reflectance at the interface of a controlled ferromagnetic lead and a topological superconductor (TSC). In the N=1 phase of TSC, the magnetization-dependent anisotropic reflectance showed agreement with the polarization of the chiral Majorana mode occupying the interface layer. Further studies proposed on a realistic model of materials suggest spin-polarized normal and cross Andreev reflections could serve as a probe or a gate to the Majorana mode. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X45.00003: Odd-frequency Pairing of Majorana Fermions Severin Sjomark, Alexander Balatsky Advances in theoretical condensed matter physics in the last two decades has opened up the stage for Majorana fermions in topological systems. The prime example is that of Majorana zero-modes at the ends of 1D wires. Now that there are likely experimental realizations of Majorana modes one can ask questions about the stability of an ensemble of interacting Majorana fermions. We present a study of the effects of both a static and dynamic four-point interaction. In the static case it is found that the interaction opens up a gap in the dispersion, but that their Green’s function remain odd-frequency. The dynamic case is investigated in the mean-field regime with a retarded interaction. It is found that an odd-frequency pairing parameter has a non-dispersive effect, and consequently that the Majorana fermions remain as zero-modes. The Green’s function of Majorana fermions are odd-frequency as a result of being zero-modes. Since Majorana particles are their own antiparticles the Green’s function can be viewed as the Gor'kov anomalous function. We thus conclude that Majorana fermions have a natural propensity to form odd frequency pairing, and that their zero-mode nature is robust against odd-frequency pairing. We also investigate even-frequency pairing, which is found to open a gap. [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X45.00004: Yu-Shiba-Rusinov states and topological superconductivity in Ising paired superconductors Girish Sharma, Sumanta Tewari An unusual form of superconductivity, called Ising superconductivity, has recently been uncovered in mono- and few-layered transition metal dichalcogenides. This 2D superconducting state is characterized by the so- called Ising spin-orbit coupling (SOC), which produces strong oppositely oriented effective Zeeman fields perpendicular to the 2D layer in opposite momentum space valleys. We examine the Yu-Shiba-Rusinov (YSR) bound states localized at magnetic impurities in Ising superconductors and show that the unusual SOC manifests itself in unusually strong anisotropy in magnetic field response observable in STM experiments. For a chain of magnetic impurities with moments parallel to the plane of Ising superconductors we show that the low energy YSR band hosts topological superconductivity and Majorana excitations as a direct manifestation of topological effects induced by Ising spin-orbit coupling. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X45.00005: Josephson junctions of candidate topological crystalline insulator Pb1-xSnxTe Rodney Snyder, Christie Trimble, Patrick Taylor, James Williams Incorporating superconducting ordering through proximity effects in topological states of matter offers potential routes to novel excitations with properties beyond that of simple electrons. Topological crystalline insulators TCI offer alternative routes to topological states of matter with surface states of distinct character to those in more common 3d topological insulators. We report on the fabrication Josephson junctions using MBE-grown candidate TCI material — Pb-doped SnTe — as weak links and characterize the departures from conventional junctions using combined DC and RF techniques. Opportunities to create junction weak links from materials possessing electronic interactions will be discussed. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X45.00006: Nodal Topological Superconductivity in Monolayer NbSe{\$}\textunderscore 2{\$} Wen-Yu He, Benjamin T. Zhou, James J. He, Noah F. Q. Yuan, Ting Zhang, K. T. Law Recently, it was shown that the in-plane upper critical field {\$}H\textunderscore \textbraceleft c2\textbraceright {\$} of superconducting monolayer NbSe{\$}\textunderscore 2{\$} can be six times higher than the Pauli limit field. This is due to the Ising spin-orbit coupling (SOC) which pins electron spins to the out-of-plane directions and protects the electron spins from being aligned to the in-plane directions. In this work, we show that in a wide range of experimentally accessible regimes where the in-plane magnetic field is higher than the Pauli limit field but lower than {\$}H\textunderscore \textbraceleft c2\textbraceright {\$}, a monolayer NbSe{\$}\textunderscore 2{\$} becomes a nodal topological superconductor. The bulk nodal points appear on the {\$}$\backslash $Gamma- M{\$} lines of the Brillouin zone where the Ising SOC vanishes. The nodal points are connected by Majorana flat bands, similar to the Weyl points being connected by surface Fermi arcs in Weyl semimetals. The Majorana flat bands are associated with a large number of zero energy Majorana fermion edge modes which induce spin-triplet Cooper pairs. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X45.00007: Interplay of superconductivity and electrically controlled band structure in silicene – 0-$\pi$ transitions, $\varphi_0$-junctions, Majorana bound states, and odd-frequency superconductivity Dushko Kuzmanovski, Annica Black-Schaffer, Jacob Linder Silicene, the Si-atom analog of graphene, is a viable candidate for experimental realization of non-trivial topological phases due to the larger spin-orbit coupling. Also, owing to the buckled structure, it allows for tuning of its various band gaps by an applied electric field. An intriguing prospect is to consider effects due to the interplay between the non-trivial band structure and superconducting correlations in silicene, and to study the external control of such unusual phenomena via an electric field. We demonstrate theoretically that proximity-induced superconductivity in silicene offers the possibility to exert strong quantum ground state control. We show that electrically controlled $0$-$\pi$ transitions occur in Josephson junctions in the presence of an exchange field. We also discover that zigzag-oriented interfaces, featuring intervalley scattering, cause a $\varphi_0$ state with an applied electric field. Additionally, we demonstrate that Majorana bound states along the silicene edge are tunable via the edge orientation, electric, and in-plane spin exchange fields. Finally, we investigate odd-frequency superconducting pair amplitudes in both bulk silicene, and nanoribbons with two kinds of edges. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X45.00008: Josephson Effect in Graphene: a Tunneling Spectroscopy Study Landry Bretheau, Joel I-Jan Wang, Riccardo Pisoni, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero A normal conductor placed in good contact with a superconductor can inherit its electronic properties. This proximity effect in the conductor originates from the formation of entangled electron-hole states, called Andreev states. Spectroscopic studies of Andreev states have been performed in just a handful of systems. Graphene provides a novel platform for studying Andreev physics in two dimensions because of its large mobility, ease of access and electrostatically tunable carrier density. Using a full van der Waals heterostructure, we have performed direct tunnelling spectroscopy of proximitized graphene. The measured energy spectra, which depend on the phase difference between the superconductors and on graphene carrier density, reveal a continuum of Andreev bound states. We further infer the supercurrent they carry from the phase dependence of the spectra, thus relating Andreev physics and the Josephson effect. As graphene's extended two-dimensional nature enables one to combine superconductivity and the quantum Hall effect, this platform is promising for the detection of Majorana modes, key ingredients for topologically protected quantum computation. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X45.00009: Antiferromagnetic nuclear spin helix and topological superconductivity in $^{13}$C nanotubes Chen-Hsuan Hsu, Peter Stano, Jelena Klinovaja, Daniel Loss We investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction arising from the hyperfine coupling between localized nuclear spins and conduction electrons in interacting carbon nanotubes made of $^{13}$C$~$[1]. Using the Tomonaga-Luttinger liquid formalism, we show that the RKKY interaction is sublattice dependent, consistent with the spin susceptibility calculation in noninteracting nanotubes. The RKKY interaction forms $q=\pm 2 k_{F}$ peaks with the Fermi wave number $k_{F}$, and induces a novel antiferromagnetic nuclear spin helix with a spatial period $\pi/k_{F}$. Due to the feedback effect through the Overhauser field from the ordered nuclear spins, the transition temperature reaches up to several tens of mK. The nuclear spin helix, combining spin and charge degrees of freedom, results in a synthetic spin-orbit interaction, leading to nontrivial topology. In the presence of the proximity-induced superconductivity, this system has a potential to realize Majorana fermions without fine-tuning the external parameters, such as chemical potential and external magnetic field. Ref: [1] C.-H. Hsu, P. Stano, J. Klinovaja, and D. Loss, Phys. Rev. B 92, 235435 (2015). [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X45.00010: Engineering Of Majorana Modes In Carbon Nanotubes Contamin Lauriane Engineering of majorana modes in condensed matter systems could allow one to study excitations with particle/antiparticle duality and non-abelian statistics. Most of the experimental setups using nanoscale circuit use semiconducting nanowire connected to superconductors under a finte magnetic field. Several theoretical proposals have suggested to use single wall carbon nanotubes coupled to a magnetic texture and to superconductors. One interest of single wall carbon nanotubes is that they naturally exhibit few conducting channels contrarily to nanowires. This work, we demonstrate such a platform. Single wall carbon nanotubes are stamped over a magnetically textured gate and coupled to two superconducting electrode. We observe in the conductance of such devices subgap states and perform a detailed study of their magnetic field evolution. The interpretation of these modes in the majorana fermion context will be discussed. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X45.00011: A Density Matrix Embedding Theory Study of Superconductivity in Spin-Valley Locked Systems Jordan Venderley, Jason Iaconis, Leon Balents, Eun-Ah Kim A recent perturbative renormalization group study (1) raised the possibility that interplay between spin-valley locking and repulsive interactions in p-doped type VI transition metal dichalcogenides can lead to topological superconductivity. However, a more recent density matrix renormalization group (DMRG) study indicated that there is a threshold interaction strength for the pair field at the edge to reveal robust p-wave pairing tendencies in the bulk. Unfortunately, the DMRG study was limited to a quasi-one dimensional system of limited size as is standard for DMRG. In order to mitigate the finite size effects and the constraints of the quasi-one-dimensional system and to elucidate the possibility of topological superconductivity in spin-valley locked system with finite interaction strength, we turn to density matrix embedding theory (DMET). Herein, we spontaneously break particle-number symmetry according to the different irreducible representations of the lattice and observe how the associated order parameters evolve under the DMET self-consistency procedure. This enables us to investigate the phase diagram in a truly two-dimensional fashion and more faithfully determine the dominant superconducting instabilities. (1) Hsu et al. ArXiv e-prints (2016), 1606.00857. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X45.00012: Topological phase transitions in a doped Sr$_2$IrO$_4$ / metal heterostructure Mats Horsdal, Timo Hyart Doped Sr$_2$IrO$_4$ is predicted to be a high $T_C$ $d$-wave superconductor. As opposed to the cuprates, the Cooper pairs are not spin singlets, but rather pseudospin singlets. The pseudospin describes highly entangled spin and orbital degrees of freedom. This difference is not apparent when considering Sr$_2$IrO$_4$ as an isolated system. However, when tunnel coupled to a metallic $t_{2g}$ electron system this gives rise to a rich topological phase diagram, which has been mapped out and will be presented. [Preview Abstract] |
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