Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A45: 2D Topological SuperconductorsFocus
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Sponsoring Units: DMP GMAG Room: 392 |
Monday, March 13, 2017 8:00AM - 8:36AM |
A45.00001: Unconventional Electron Pairing and Topological Superconductivity in Proximitized HgTe Quantum Wells Invited Speaker: Hechen Ren Coupling s-wave superconductors to systems with exotic Fermi surface spin textures has been recently proposed as a way to manipulate the nature of the paired state, in some cases even leading to a topological phase transition. Recently, we studied the behavior of Fraunhofer interference in HgTe quantum well-based Josephson junctions, in the presence of a magnetic field applied in the plane of the quantum well. Here we theoretically analyze our system and compare the predicted behavior to our experimental results. We find that the in-plane magnetic field tunes the momentum of Cooper pairs in the quantum well, directly reflecting the response of the spin-dependent Fermi surfaces. This momentum tuning depends crucially on the type of spin-orbit coupling in the system. In the high electron density regime, the induced superconductivity evolves with electron density in agreement with our model based on the Hamiltonian of Bernevig, Hughes and Zhang. This agreement provides a quantitative value for gÞ/v$_{\mathrm{F}}$, where g Þ is the effective g-factor and v$_{\mathrm{F}}$ is the Fermi velocity. Our new understanding of the interplay between spin physics and superconductivity introduces a way to spatially engineer the order parameter from singlet to triplet pairing, and in general allows investigation of electronic spin texture at the Fermi surface of materials. [Preview Abstract] |
Monday, March 13, 2017 8:36AM - 8:48AM |
A45.00002: Controlled finite momentum pairing and spatially varying order parameter in proximitized HgTe quantum wells Sean Hart, Hechen Ren, Michael Kosowsky, Gilad Ben-Shach, Philipp Leubner, Christoph Bruene, Hartmut Buhmann, Laurens Molenkamp, Bertrand Halperin, Amir Yacoby Conventional $s$-wave superconductivity arises from singlet pairing of electrons with opposite Fermi momenta, forming Cooper pairs with zero net momentum. Recent studies have focused on coupling $s$-wave superconductors to systems with an unusual configuration of electronic spin and momentum at the Fermi surface, where the nature of the paired state can be modified and the system may even undergo a topological phase transition. Here we present measurements on Josephson junctions based on HgTe quantum wells coupled to aluminum or niobium superconductors, and subject to a magnetic field in the plane of the quantum well. We observe that the in-plane magnetic field modulates the Fraunhofer interference pattern, and that this modulation depends both on electron density and on the direction of the in-plane field with respect to the junction. However, the orientation of the junction with respect to the underlying crystal lattice does not impact the measurements. These findings suggest that spin-orbit coupling plays a role in the observed behavior, and that measurements of Josephson junctions in the presence of an in-plane field can elucidate the Fermi surface properties of the weak link material. [Preview Abstract] |
Monday, March 13, 2017 8:48AM - 9:00AM |
A45.00003: Demonstration of InSb quantum wells on InSb substrates Mihir Pendharkar, Joon Sue Lee, Borzoyeh Shojaei, Daniel J. Pennachio, Anthony P. McFadden, Chris J. Palmstrom Highly spin orbit coupled InSb material system has been central to the realization of novel phenomenon, fundamental for topological quantum computation. Quantum confined electrons in InSb/AlInSb heterostructures have until now been plagued with a very high density of defects and dislocations, due to their growth on lattice mismatched GaAs and GaSb substrates. In this work, Molecular Beam Epitaxy growth of InSb quantum wells on InSb substrates has been demonstrated. Low temperature magneto-transport measurements of the quantum wells showed an onset of Shubnikov-deHaas oscillations at 0.2 Tesla, corresponding to the quantum mobility of 50,000 cm2/Vs which is believed to be the highest reported to date. HAADF-STEM of epilayers grown, showed abrupt interfaces while AFM was used to confirm a dramatic reduction in screw dislocation density on the surface. This work paves the way for investigation of gate control and lithographically defined nanostructures necessary for scalable topological quantum computation on an InSb platform. [Preview Abstract] |
Monday, March 13, 2017 9:00AM - 9:12AM |
A45.00004: Strain Energy and Epitaxy Relation between Superconducting Thin Films and Semiconductors Klea Dhimitri, Jospeh Yuan, Javad Shabani Existence of an \textit{epitaxy} relation and domain matching between semiconductors and superconductors offer the ultimate flat and uniform interfaces. Pristine interfaces are much needed for the realization of topological superconductivity and quantum computation. Motivated by recent studies on Al-InAs, we have investigated the interface strain energies in two-dimension for a number of superconductors (e.g. Al and Pb) and semiconductors (e.g. InAs and Si). The strain energy between each pair has been calculated and analyzed up to 100 configurations .Guided by these studies, thin films of Al~(111), Al~(110), Al~(100) on InAs~(100), InAs~(110), InAs~(111) and Si have been grown by molecular beam epitaxy. Our numerical results are compared with details of crystal in-plane directions derived from x-ray diffraction patterns and transmission electron microscope images. \\ [Preview Abstract] |
Monday, March 13, 2017 9:12AM - 9:24AM |
A45.00005: Microwave properties of epitaxial superconductor-semiconductor interfaces Joseph Yuan, Klea Dhimitri, Aaron Somoroff, Jesse Kanter, Javad Shabani A key challenge in fabrication of hybrid semiconductor-superconductor devices is forming highly transparent contacts between the active electrons in the semiconductor and the superconducting metal. It has been shown that a near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on InAs [1]. We have grown in-situ aluminum thin films on InAs (100), InAs (110), InAs (111), and Si (111) after oxide removal and regrowth. Guided by our numerical studies, we have isolated the optimal growth orientations to minimize the strain energy at the interface. The interfaces are studied using x-ray diffraction patterns and transmission electron microscope imaging. Field-effect Josephson junctions have been fabricated and studied in microwave regime. [1] Shabani et al. PRB 2016. [Preview Abstract] |
Monday, March 13, 2017 9:24AM - 9:36AM |
A45.00006: Experimental Observation of Topological Superconductivity and Majorana Zero Modes on $\beta $-Bi$_{2}$Pd Thin Films. Yanfeng Lyu By using a cryogenic scanning tunneling microscope, we reveal a nodeless superconducting gap on epitaxial crystalline $\beta $-Bi$_{2}$Pd films, which is much larger than that of bulk superconducting $\beta $-Bi$_{2}$Pd. The newly emerging superconducting gap is found to originate from Dirac-fermion enhanced parity mixing of the surface pairing potential, thereby indicates topological superconductivity with spinless odd-parity pairing near the film surface. Majorana zero modes, supported by such a superconducting state, are unequivocally identified by directly probing quasiparticle density of states within the vortex cores under magnetic field. The superconductivity and Majorana zero modes are immune to intrinsic point and linear defects, characteristic of a time-reversal-invariant topological superconductor. [Preview Abstract] |
Monday, March 13, 2017 9:36AM - 9:48AM |
A45.00007: Kerr effect from diffractive skew scattering in chiral $p_x \pm i p_y$ superconductors Elio K\"onig, Alex Levchenko We calculate the temperature dependent anomalous ac Hall conductance $\sigma_H(\Omega, T)$ for a two-dimensional chiral $p$-wave superconductor. This quantity determines the polar Kerr effect, as it was observed in Sr$_2$RuO$_4$ [J. Xia \textit{et al.}, Phys.~Rev.~Lett.~\textbf{97}, 167002 (2006)]. We concentrate on a single band model with arbitrary isotropic dispersion relation subjected to rare, weak impurities treated in the Born approximation. As we explicitly show by detailed computation, previously omitted contributions to extrinsic part of an anomalous Hall response, physically originating from diffractive skew scattering on quantum impurity complexes, appear to the leading order in impurity concentration. By direct comparison with published results from the literature we demonstrate the relevance of our findings for the interpretation of the Kerr effect measurements in superconductors. [Preview Abstract] |
Monday, March 13, 2017 9:48AM - 10:00AM |
A45.00008: Topological superconductivity in an ultrathin, magnetically-doped topological insulator proximity coupled to a conventional superconductor Youngseok Kim, Timothy M. Philip, Moon Jip Park, Matthew J. Gilbert As a promising candidate system to realize topological superconductivity (SC), 3D time-reversal invariant topological insulators (TI) proximity-coupled to $s$-wave superconductors have been intensively studied. Recent experiments on proximity-coupled TI have shown that superconductivity may be induced in ultrathin TI. One proposal to observe the topological SC in proximity-coupled ultrathin TI system is to add magnetic dopants to the TI\footnote{PRB 92, 064520}. However, detailed study on the impact of the experimental parameters on possible topological phase is sparse. In this work, we investigate ultrathin, magnetically-doped, proximity-coupled TI in order to determine the experimentally relevant parameters needed to observe topological SC. We find that, due to the spin-momentum locked nature of the surface states in TI, the induced $s$-wave order parameter within the surface states persists even at large magnitudes of the Zeeman energy, allowing us to explore the system in parameter space. We elucidate the phase diagram as a function of: the hybridization gap, Zeeman energy, and chemical potential of the TI system. Our findings provide a useful guide in choosing relevant parameters to facilitate the observation of topological SC in thin film TI-superconductor hybrid systems. [Preview Abstract] |
Monday, March 13, 2017 10:00AM - 10:12AM |
A45.00009: Thermoelectric transport through Majorana bound states and violation of Wiedemann-Franz law Juan Pablo Ramos-Andrade, Oscar \'Avalos-Ovando, Pedro Orellana, Sergio Ulloa Our work model a system composed by two current leads and a one-dimensional topological nanowire (NW) hosting Majorana bound states (MBS) at each edge and coupled between them with a strength $\varepsilon_{M}$. The NW is coupled symmetrically to the leads at temperature difference $\Delta T$. We study thermoelectric transport across the NW, for two different configurations: A) When only one MBS is connected to the leads and B) when both MBS are connected to the leads. We find a noticeable violation of the Wiedemann-Franz law, which leads to obtaining sizable values of thermoelectric efficiency, measured by the figure of merit, and also an $\varepsilon_{M}$-independent behavior of the Seebeck coefficient for configuration A. We believe our findings could lead to interesting thermoelectric-based Majorana detection devices.\\~ [1] M. Leijnse, New J. Phys. \textbf{16}, 015029 (2014).\\~ [2] R. L\'opez et al., Phys. Rev. B \textbf{89}, 205418 (2014).\\~ [3] J. P. Ramos-Andrade et al., Phys. Rev. B \textbf{94}, 155436 (2016). [Preview Abstract] |
Monday, March 13, 2017 10:12AM - 10:24AM |
A45.00010: Thermal transport in a superconducting/normal/superconducting topological insulator junction with mixed singlet and triplet pairing states Hai Li, C. S. Ting In the frame of the Bogoliubov-de Gennes equation, we theoretically investigate the thermal transport properties in a superconducting/normal/superconducting topological insulator junction with mixed singlet and triplet pairing states. Owing to the helical spin textures of the topological surface states, the thermal conductance does not decay with the interface potential barrier. Remarkably, it is revealed that the thermal conductance is strongly sensitive to the components of the pairing states. In the singlet state dominated regime, the thermal conductance profoundly depends on the phase deference and decays with increasing the junction length. While for the triplet state dominated situation, the thermal conductance just oscillates with the junction length, but does not exhibit a decaying envelop. Moreover, in the triplet state dominated regime the thermal conductance performs a negligible oscillating characteristic with respect to the phase deference. These intriguing results would provide a novel approach for distinguishing the pairing states of the topological surface states. [Preview Abstract] |
Monday, March 13, 2017 10:24AM - 10:36AM |
A45.00011: Disorder-enhanced topological protection and universal quantum criticality in a spin-3/2 time-reversal invariant topological superconductor Sayed Ali Akbar Ghorashi, Seth Davis, Matthew S. Foster We study spin-3/2 topological superconductors that have been proposed as generalizations of Helium 3B, with potential applications to ultracold atoms and the half-Heusler compounds. A model with p-wave pairing has winding number 4, and exhibits both linear and cubically dispersing surface bands. Via an RG analysis, we show that for a clean surface, interactions are in fact only marginally relevant for an attractive coupling strength despite the cubic band van Hove singularity. Attractive interactions lead to a BCS-type instability that spontaneously breaks time-reversal symmetry at the surface. By contrast, quenched disorder is a strongly relevant perturbation. Numerically, we show that disorder drives the system to a stable fixed point of the RG flow governed by the same type of conformal field theory (CFT) that appears for spin-1/2 TSCs. In particular, the critical behavior of the global density of states and the multifractality of the surface wavefunctions are entirely universal (independent of disorder or band structure parameters, in agreement with CFT), and the system is stabilized against interactions. In this case, we conclude that topological protection is enhanced by disorder. [Preview Abstract] |
Monday, March 13, 2017 10:36AM - 10:48AM |
A45.00012: Laser-induced topological superconductivity in cuprate thin films Kazuaki Takasan, Akito Daido, Norio Kawakami, Youichi Yanase We propose a possible way to realize topological superconductivity with application of laser light to superconducting cuprate thin films. Applying Floquet theory to a model of $d$-wave superconductors with Rashba spin-orbit coupling, we derive the effective model and discuss its topological nature. Interplay of the Rashba spin-orbit coupling and the laser light effect induces the synthetic magnetic fields, thus making the system gapped. Then the system acquires the topologically non-trivial nature which is characterized by Chern number. The synthetic magnetic fields do not create the vortices in superconductors, and thus the proposed scheme provides a promising way to dynamically realize a topological superconductor in cuprates. We also discuss an experimental way to detect the signature. [Preview Abstract] |
Monday, March 13, 2017 10:48AM - 11:00AM |
A45.00013: Momentum-space spin texture of a topological superconductor Arno Kampf, Florian Loder, Thilo Kopp, Daniel Braak A conventional superconductor with spin-orbit coupling turns into a topological superconductor beyond a critical strength of the Zeeman coupling. The spin-expectation values ${\mathbf S}({\mathbf k})$ in momentum space trace this transition via a characteristic change in the topological character of the spin texture within the Brillouin zone. At the transition the skyrmion counting number switches from 0 to 1/2 identifying the topological superconductor via its meron-like spin texture. The change in the skyrmion counting number is crucially controlled by singular points of the map ${\mathbf S}({\mathbf k})/|{\mathbf S}({\mathbf k})|$ from the Brillouin zone, i.e. a torus, to the unit sphere. The complexity of this spin-map is discussed at zero temperature as well as for the extension to finite temperatures. [Preview Abstract] |
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