Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T13: Focus Session: Topological Materials - Quasi 1-dimensional |
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Sponsoring Units: DMP Chair: Joel Moore, University of California, Berkeley Room: 315 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T13.00001: Transition from fractional to Majorana fermions in Rashba nanowires Jelena Klinovaja, Peter Stano, Daniel Loss We study hybrid superconducting-semiconducting nanowires in the presence of Rashba spin-orbit interaction as well as helical magnetic fields.[1] We show that the interplay between them leads to a competition of phases with two topological gaps closing and reopening, resulting in unexpected reentrance behavior. Besides the topological phase with localized Majorana fermions (MFs) we find new phases characterized by fractionally charged fermion (FF) bound states of Jackiw-Rebbi type. The system can be fully gapped by the magnetic fields alone, giving rise to FFs that transmute into MFs upon turning on superconductivity. We find explicit analytical solutions for MF and FF bound states and determine the phase diagram numerically by determining the corresponding Wronskian null space. We show by renormalization group arguments that electron-electron interactions enhance the Zeeman gaps opened by the fields. \\[4pt] [1] J. Klinovaja, P. Stano, and D. Loss, arXiv:1207.7322 (2012). [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T13.00002: Majorana fermions in topological insulator nanoribons with multiband occupancy Piyapong Sitthison, Tudor Stanescu We present the phase diagram of a topological insulator nanoribbon with proximity-induced superconductivity as function of the chemical potential and the Zeeman field applied parallel to the ribbon. We find that, in doped topological insulator systems, both surface-like and bulk-like states contribute to the low-energy physics and that proximity-induced quantities, such as the induced superconducting pair potential, have different energy scales in these channels. We study the effect of this band-specific proximity coupling on the properties and the stability of Majorana zero-energy bound states in multiband topological insulator nanoribbons. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T13.00003: Time Reversal Invariant Topological Superconductors and Majorana Pairs Fan Zhang, Eugene Mele, Charles Kane We propose a feasible route to engineer two dimensional (2D) and one dimensional (1D) time reversal invariant topological superconductors via proximity effects. At a boundary of the 2D (1D) topological superconductor, a time reversal pair of Majorana edge (bound) states emerge as the localized midgap states. We analyze how the Majorana pair evolves in the presence of a Zeeman field, as the superconductor undergoes the symmetry class change as well as the topological phase transitions. A fractional Josephson effect with time reversal symmetry occurs in the presence of a mirror symmetry, realizing a topological crystalline superconducting state. We also briefly discuss the possible realization in materials and the unique signature in experiments. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 9:12AM |
T13.00004: Ripple modulated electronic structure of a 3D topological insulator Invited Speaker: Vidya Madhavan Many of the unusual properties of topological insulators can only be realized through a delicate tuning of the Dirac surface state rendering their detection thus far elusive. We have discovered that the surface state dispersion of a prototypical topological insulator can be continuously tuned via a novel topographical route. STM images of Bi$_{2}$Te$_{3}$ show one-dimensional (striped) ripples with 100nm periodicity. By combining information from Landau level spectra [1] and Fourier transform of interference patterns [2] we show that the ripples induce spatial modulations in the surface state dispersion. We describe how the ripples create topological channels for chiral spin modes at the boundaries such that placing the Fermi energy between the Landau levels of these periodic stripes would result in the first experimental realization of the ideal 1D dissipationless quantum wire. This ability to tune the surface state dispersion locally opens the door to a host of new phenomena in topological insulators.\\[4pt] [1] Yoshinori Okada, Wenwen Zhou, C. Dhital, D. Walkup, Ying Ran, Z. Wang, Stephen D. Wilson {\&} V. Madhavan, Visualizing Landau levels of Dirac electrons in a one dimensional potential, Phys. Rev. Lett. 109, 166407 (2012).\\[0pt] [2] Yoshinori Okada, Wenwen Zhou, C. Dhital, D. Walkup, Stephen D. Wilson {\&} V. Madhavan, Ripple modulated electronic structure of a 3D topological insulator, Nature Communications 3 1158, (2012). [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T13.00005: Novel giant Rashba spin splitting of holes in semiconductor nanowires for Majorana Fermions Jun-Wei Luo, Lijun Zhang, Alex Zunger Majorana Fermions (MFs) are particles identical to their own antiparticles that have been first theoretical predicted and then experimentally observed in hybrid superconductor-semiconductor nanowire devices. The appearance of MFs requires (spin-orbit-induced) giant nanowire spin splitting (SS) to exceed the topological superconductor gap, a condition realized by tuning the magnetic field. Because the SS due to the conventional Dresselhaus or Rashba mechanisms is inversely proportional to the wire diameter, these mechanisms contribute but vanishing SS ($\ll1$ meV {\AA}) for wide ($\sim100$ nm) wires that are appropriate to device uses--a significant disadvantage of nanowire for this application. Our atomistic pseudopotential calculation predicted a novel large Rashba SS in GaAs/AlAs wires under electric field [1], which increases as the wire diameter to the potential benefit of nanowire MF device. This emerged automatically when the ordinary Schr\"odinger equation is solved in the presence of spin-orbit interaction. We will report such giant Rashba SS coefficient of the order of $\sim200$ meV{\AA} in a number of semiconductor wire materials $\sim100$ nm wide.\\[4pt] [1] J.W. Luo, L. Zhang, and A. Zunger, Phys. Rev. B 84, 121303(R) (2011) see Ref.25. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T13.00006: Classification of the 2D topological insulator/ superconductors through their 1D Dirac edge Hamiltonians Yi-Ting Hsu, Abolhassan Vaezi, Eun-Ah Kim Ref [1] analyzes the consequences of discrete symmetries for 1D Dirac Hamiltonians as candidate description of 2D topological insulators/superconductors(TI/TS), formally revealed that there are multiple inequivalent representations of time reversal symmetry as required by $\mathbf{T}^\dagger H T=H^*$. This is special to 1D Dirac edge Hamiltonians and leads to additional possibilities in the classification of 2D TI/TS. In this talk, we present physical implications of the multiple representations through additional hidden symmetries $X_i$ implicit in the 1D Dirac Hamiltonians. When $X_i$ do not commute with any of the existing discrete symmetries, it is necessary to consider $X_i$ alone as individual symmetries for the purpose of classifying the edge theory which usually extends its classification. Graphene-based topological insulators are physical examples of a resulting new Z-type topological phase obtained through imposing an additional $U(1)$ symmetry due to the absence of inter-valley scattering. [1] D. Bernard, E.-A. Kim, and A. LeClair, ArXiv:1202.5040 (2012) [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T13.00007: Topological pi Josephson effect and Majorana states in Rashba wires Teemu Ojanen Rashba-based topological superconductor nanowires, where the spin-orbit coupling may change its sign, support three topological phases protected by chiral symmetry. When a superconducting phase gradient is applied over the interface of the two nontrivial phases, the Andreev spectrum is qualitatively phase shifted by $\pi$ compared to usual Majorana weak links. The topological $\pi$-junction has the striking property of exhibiting maximum supercurrent in the vicinity of vanishing phase difference.The studied system could be realized by local gating of the wire or by an appropriate stacking of permanent magnets in synthetic Rashba systems. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T13.00008: Majorana fermions in hybrid superconductor-semiconductor nanowire devices Vincent Mourik, Kun Zuo, David van Woerkom, Sergey Frolov, Sebastien Plissard, Erik Bakkers, Leo Kouwenhoven Our recent experiment carried out in hybrid superconductor-semiconductor nanowire devices gave the first experimental evidence for the existence of Majorana fermions [1]. However, some open questions need to be answered. Majorana fermions have to come in pairs, but before we were only capable of probing one Majorana fermion. Besides, Majorana fermions should be fully gate controllable, which could not be demonstrated very convincingly. Furthermore the observed conductance peak was only at 5{\%} of the theoretically expected height of 2e\textasciicircum 2/h. Currently we are performing new experiments in similar but improved devices. We study three terminal normal-superconductor-normal InSb nanowire devices. This enables the possibility to simultaneously probe both Majorana fermions occurring at the ends of the superconducting contact by using tunneling spectroscopy from normal to superconducting contact. Furthermore, the devices have an improved gate design enabling more efficient gating under the superconducting contact. The first measurements already give a larger peak amplitude and the peak is visible in a larger magnetic field range. [1] V. Mourik, K. Zuo et al., Science, Vol. 336 no. 6084 pp. 1003-1007 [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T13.00009: Detecting Majorana fermions in quasi-1D topological phases using non-local order parameters Yasaman Bahri, Ashvin Vishwanath There has been much recent interest in realizing Majorana fermions in solid-state or cold atom systems. A primary goal has been to identify the topological phases which host them and propose routes towards their experimental detection. Such topological phases cannot be distinguished via local order parameters. Instead, we propose non-local string order parameters to distinguish 1D topological phases hosting Majorana zero modes. We also discuss potential cold atom measurements of string order, based on recent experimental developments, as a new and alternative route towards their detection. We further consider N identical chains of interacting fermions and use the group cohomology approach to construct non-local order parameters to distinguish topological phases of this quasi-1D system. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T13.00010: Gate-defined wires in HgTe quantum wells as a robust Majorana platform Johannes Reuther, Jason Alicea, Amir Yacoby We propose a new quasi-1D platform for Majorana zero-modes based on gate-defined wires in HgTe. Due to the Dirac-like band structure for HgTe such wires exhibit several remarkable properties. Most strikingly, modest gate-tuning allows one to modulate the Rashba spin-orbit energy from zero up to $\sim30K$, and the effective g-factor from zero up to giant values of $\sim600$. The large achievable spin-orbit coupling and g-factor together allow one to access Majorana modes in this setting at exceptionally low magnetic fields while maintaining robustness against disorder. Moreover, gate-defined wires may facilitate the fabrication of networks required for realizing non-Abelian statistics and quantum information devices. The exquisite tunablity of parameters further suggests applications in spintronics. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T13.00011: Hints of hybridizing Majorana fermions in a nanowire coupled to superconducting leads A.D.K. Finck, D.J. Van Harlingen, P.K. Mohseni, K. Jung, X. Li It has been proposed that a nanowire with strong spin-orbit coupling that is contacted with a conventional superconductor and subjected to a large magnetic field can be driven through a topological phase transition. In this regime, the two ends of the nanowire together host a pair of quasi-particles known as Majorana fermions (MFs). A key feature of MFs is that they are pinned to zero energy when the topological nanowire is long enough such that the wave functions of the two MFs do not overlap significantly, resulting in a zero bias anomaly (ZBA). It has been recently predicted that changes in external parameters can vary the wave function overlap and cause the MFs to hybridize in an oscillatory fashion. This would lead to a non-monotonic splitting or broadening of the ZBA and help distinguish MF transport signatures from a Kondo effect. Here, we present transport studies of an InAs nanowire contacted with niobium nitride leads in high magnetic fields. We observe a number of robust ZBAs that can persist for a wide range of back gate bias and magnetic field strength. Under certain conditions, we find that the height and width of the ZBA can oscillate with back gate bias or magnetic field. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T13.00012: Using InAs quantum wells to navigate the Majorana parameter space Peter O'Malley, Pedram Roushan, Yu Chen, Brooks Campbell, Borzoyeh Shojaei, Javad Shabani, Brian Shultz, Chris Palmstrom, John Martinis Although superconducting contacts laid down on self-assembled nanowires have produced impressive experimental results, the desire to build complex and scalable devices using Majorana modes leads us to want to develop lithographically defined nanowires. Our strategy is to deposit a superconducting layer in situ on an MBE-grown InAs 2DEG, and etch nanowires in subsequent microfabrication. This allows control over nanowire properties as well as the ability to vary the superconductor-semiconductor coupling strength in a precise manner. We plan to present measurements of both Nb coupled to an InAs 2DEG and nanowires fabricated out of two-dimensional InAs systems. We then discuss where these measurements put our system in the parameter space needed to observe the Majorana fermion, and propose a path forward. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T13.00013: High-Performance Topological Insulator Bi$_2$Se$_3$ Nanowire Field Effect Transistors Hao Zhu, Curt Richter, Erhai Zhao, Hui Yuan, Haitao Li, Dimitris Ioannou, Qiliang Li Single crystal topological insulator Bi$_{2}$Se$_{3}$ nanowires were synthesized by Vapor-Liquid-Solid (VLS) mechanism. Bi$_{2}$Se$_{3}$ NW field-effect transistors were fabricated by using self-alignment method with HfO$_{2}$ as the gate dielectric. Bi$_{2}$Se$_{3}$ NWFETs were measured in vacuum at different temperatures. Excellent MOSFET characteristics were achieved: smooth and well-saturated output characteristics, large On/Off ratio (10$^{7})$, zero Off-state current and good subthreshold slope in transfer characteristics. We have observed linear behavior of the saturation current extracted from the I$_{\mathrm{ds}}$-V$_{\mathrm{ds}}$ curves as a function of the overthreshold voltage (V$_{\mathrm{g}}$-V$_{\mathrm{th}})$, which indicated the main role of the metallic surface conduction at Bi$_{2}$Se$_{3}$ nanowire channel. Both effective mobility and field-effect mobility have been extracted. Very good effective mobility (\textgreater\ 5000 cm$^{2}$V$^{-1}$s$^{-1}$ at 77 K) was obtained under a low gate voltage. From off-state current we calculated the band gap of bulk about 0.33 eV, which is in a good agreement with reported value of 0.35 eV. [Preview Abstract] |
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