Sumanta Tewari

Clemson University, Clemson, South Carolina, United States

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Publications (88)284.02 Total impact

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    ABSTRACT: Motivated by a recent experiment in which zero-bias peaks have been observed in scanning tunneling microscopy (STM) experiments performed on chains of magnetic atoms on a superconductor, we show, by generalizing earlier work, that a multichannel ferromagnetic wire deposited on a spin-orbit coupled superconducting substrate can realize a non-trivial chiral topological superconducting state with Majorana bound states localized at the wire ends. The non-trivial topological state occurs for generic parameters requiring no fine tuning, at least for very large exchange spin splitting in the wire. We theoretically obtain the signatures which appear in the presence of an arbitrary number of Majorana modes in multi-wire systems incorporating the role of finite temperature, finite potential barrier at the STM tip, and finite wire length. These signatures are presented in terms of spatial profiles of STM differential conductance which clearly reveal zero energy Majorana end modes and the prediction of a multiple Majorana based fractional Josephson effect. A critical comparison of our results with the experimental data shows a basic inconsistency in the interpretation of the Fe nanowire STM experiment in terms of Majorana zero modes-- in particular, the observation of the precise localization of the Majorana zero modes at the wire ends cannot be reconciled with the extremely small topological superconducting gap (and the associated extremely weak Majorana tunneling peak) observed simultaneously. Other than this rather disturbing basic incompatibility, for which we can offer no resolution at this stage, most other aspects of the experimental phenomenology are reasonably well explained by our theory.
    10/2014;
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    ABSTRACT: We establish theoretically that a ferromagnetic wire deposited on the surface of a spin-orbit coupled $s$-wave superconductor generically supports zero-energy topological Majorana fermion (MF) excitations near the two ends of the wire. Depending on the strength of the ferromagnetic moment in the wire, the number of MFs at each end, $n$, can be either one or two, and should be revealed by a robust zero-bias peak (ZBP) of height $2ne^2/h$ in scanning tunneling microscopy (STM) measurements which would show strong (weak) signals at the ends (middle) of the wire. The underlying physics of this system, which is distinct from that of the well-studied spin-orbit-coupled semiconductor-superconductor heterostructures in the presence of an applied magnetic field, is discussed in terms of a topological chiral symmetry in BDI class with an integer topological invariant. Our work introduces a new platform, namely a magnetic metal-superconductor heterostructure, in the race for finding a Majorana-carrying topological superconductor.
    07/2014;
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    ABSTRACT: Multiple zero-energy Majorana fermions (MFs) with spatially overlapping wave functions can survive only if their splitting is prevented by an underlying symmetry. Here we show that, in quasi-one-dimensional (Q1D) time reversal invariant topological superconductors (class DIII), a realistic model for superconducting lithium molybdenum purple bronze and certain families of organic superconductors, multiple Majorana-Kramers pairs with strongly overlapping wave functions persist at zero energy even in the absence of an easily identifiable symmetry. We find that similar results hold in the case of Q1D semiconductor-superconductor heterostructures (class D) with transverse hopping t_{perp} much smaller than longitudinal hopping t_x. Our results, explained in terms of special properties of the Hamiltonian and wave functions, underscore the importance of hidden accidental symmetries in topological superconductors.
    03/2014;
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    Kangjun Seo, Sumanta Tewari
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    ABSTRACT: The recent discovery of an incipient charge density wave (CDW) instability competing with superconductivity in a class of high temperature cuprate superconductors has brought the role of charge order in the phase diagram of the cuprates under renewed focus. Here we take a mean field (Q = 2pi/3,2pi/3) CDW state and calculate the Fermi surface topology and the resulting Hall and Seebeck coefficients as a function of temperature and hole doping. We show that, in the appropriate doping ranges where the low temperature state (in the absence of superconductivity) is a CDW, the Fermi surface consists of electron pockets, resulting in the Hall and Seebeck coefficients becoming negative at low temperatures, as seen in experiments.
    12/2013;
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    Kangjun Seo, Chuanwei Zhang, Sumanta Tewari
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    ABSTRACT: We consider the quasi-one-dimensional (quasi-1D) system realized by an array of weakly coupled parallel one-dimensional “tubes” in a two-dimensional lattice which permits free motion of atoms in an axial direction in the presence of a Zeeman field, Rashba type spin-orbit coupling (SOC), and an s-wave attractive interaction, while the radial motion is tightly confined. We solve the zero-temperature (T=0) Bogoliubov-de Gennes (BdG) equations for the quasi-1D Fermi gas with the dispersion modified by tunneling between the tubes and show that the T=0 phase diagram hosts the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase with nonzero center-of-mass momentum Cooper pairs for small values of the SOC while for larger values of the SOC and high Zeeman fields the uniform superfluid phase with zero center-of-mass momentum Cooper pairs has an instability towards the topological uniform superfluid phase with Majorana fermions at the tube ends. Also, we show that tuning the two-dimensional optical lattice strength in this model allows one to explore the crossover behaviors of the phases during the transition between the three-dimensional and 1D systems and in general the FFLO (for small SOC) and the topological uniform superfluid phase (for large SOC) are favored as the system becomes more one dimensional. We also find evidence of the existence of a Zeeman-tuned topological quantum phase transition (TQPT) within the FFLO phase itself and, for large values of the Zeeman field and small SOC, the TQPT gives rise to a topologically distinct FFLO phase.
    Physical Review A 12/2013; · 3.04 Impact Factor
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    Eugene Dumitrescu, Jay D. Sau, Sumanta Tewari
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    ABSTRACT: We show that one-dimensional time reversal (TR) symmetric (class DIII) topological superconductors (TS) frequently possess a coexisting chiral symmetry allowing one to define an underlying integer chirality invariant. For unbroken TR and chirality invariance the parameter regimes for nontrivial values of the Z_2 DIII-invariant and the Z chiral invariant coincide. However, broken TR may or may not be accompanied by broken chirality, and if chiral symmetry is unbroken, the pair of Majorana fermions (MFs) at a given end survives the loss of TR symmetry. Conversely, we show that broken chirality may or may not be accompanied by broken TR, and if TR is unbroken, the pair of MFs survives the loss of broken chirality. In addition to solving the outstanding problem of the anomalous magnetic field response of all the DIII class topological superconductors proposed in the literature so far, we also provide a realistic route to engineer a "true" TR-invariant TS, whose Majorana Kramer's pair at each end is split by an applied Zeeman field in arbitrary directions in spin space.
    10/2013;
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    ABSTRACT: The realization of spin-orbit coupling (SOC) in ultracold atomic gases has opened the door for observing Majorana fermions (MFs) in cold atom systems. We show that MFs exist in three dimensional (3D) fermionic optical lattices with strictly one dimensional (1D) SOC which has already been realized in experiments. The presence of an in-plane Zeeman field drives the system from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Fulde-Ferrell (FF) superfluid phase. We find that both phases support multiple MFs at each end of quasi-one dimensional (quasi-1D) optical lattices with a weak transverse tunneling. In the generalization to 3D, the multiple MFs form a zero energy flat band. Our results are useful to guide the experimentalists on searching for MFs in the context of ultracold fermionic atoms.
    10/2013;
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    Tudor D. Stanescu, Sumanta Tewari
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    ABSTRACT: We show that the topologically trivial zero bias peak (ZBP) emerging in semiconductor Majorana wires due to soft confinement exhibits correlated splitting oscillations as function of the applied Zeeman field, similar to the correlated splitting of the Majorana ZBP. Also, we find that the presence of a strong impurity can effectively cut the wire in two and destroy the correlated splitting in both the trivial and the Majorana regimes. We identify a strong non-local effect that operates only in the topologically trivial regime and demonstrate that the dependence of the ZBP on the confining gate potential at the opposite end in Majorana wires with two normal metal end-contacts represents a powerful tool for discriminating between topologically trivial and non-trivial ZBPs.
    10/2013;
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    E. Dumitrescu, Sumanta Tewari
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    ABSTRACT: We show that the pair of Majorana modes at each end of a 1D spin triplet superconductor with total Cooper pair spin S_x=0 (i.e., Delta_{up,up} = -Delta_{down,down} = p*Delta_0; two uncoupled time reversed copies of the Kitaev p-wave chain) are topologically robust to perturbations such as mixing by the S_z=0 component of the order parameter (Delta_{up,down}=Delta_{down,up}), transverse hopping (in quasi-1D systems), non-magnetic disorder, and also, most importantly, to time reversal breaking perturbations such as applied Zeeman fields/magnetic impurities and the mixing by the S_y=0 component of the triplet order parameter (Delta_{up,up}=Delta_{down,down}). We show that the robustness to time reversal breaking results from a hidden chiral symmetry which places the system in the BDI topological class with an integer Z invariant. Our work has important implications for the quasi-1D organic superconductors (TMTSF)_2X (X=PF_6, CIO_4) (Bechgaard salts) which have been proposed as triplet superconductors with equal spin pairing (Delta_{up,up},Delta_{down,down} \neq 0, Delta_{up,down}=0) in applied magnetic fields.
    Physical Review B 09/2013; 88(22). · 3.66 Impact Factor
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    Jay D. Sau, Sumanta Tewari
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    ABSTRACT: We show that carbon nanotubes (CNTs) are good candidates for one-dimensional topological superconductivity with Majorana fermions (MFs) localized at the tube ends. Such states can potentially be useful for topological quantum computation using a nanotube network. The physics behind topological superconductivity in CNTs is novel and mediated by a recently reported curvature-induced spin-orbit coupling which itself has a topological origin. In addition to the spin-orbit coupling, an important new requirement for a robust topological state is broken chirality symmetry. We use topological arguments, calculations of the topological gap, and explicit numerical solutions of the Bogoliubov–de Gennes equations to show that, for recently reported strengths of spin-orbit coupling and broken chirality symmetry, MFs and a robust topological gap ∼500 mK are achievable in chiral carbon nanotubes.
    Physical Review B. 08/2013; 88(5).
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    ABSTRACT: We calculate the spectrum of the amplitude mode, the analog of the Higgs mode in high-energy physics, for the d-density wave state proposed to describe the pseudogap phase of the high-Tc cuprates. Even though the state breaks translational symmetry by a lattice spacing and is described by a particle-hole singlet order parameter at the wave vector q=Q=(π,π), remarkably, we find that the amplitude mode spectrum can have peaks at both q=(0,0) and q=Q=(π,π); we shall set lattice spacing to unity. In general, the spectrum is nonuniversal and, depending on the microscopic parameters, can have one or two peaks in the Brillouin zone, signifying the existence of two kinds of magnetic excitations. Our theory sheds important light on how multiple inelastic neutron peaks at different wave vectors can, in principle, arise even with an order parameter that condenses at Q=(π,π).
    Physical review. B, Condensed matter 06/2013; 87(22). · 3.66 Impact Factor
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    T D Stanescu, S Tewari
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    ABSTRACT: After a recent series of rapid and exciting developments, the long search for the Majorana fermion-the elusive quantum entity at the border between particles and antiparticles-has produced the first positive experimental results, but is not over yet. Originally proposed by E Majorana in the context of particle physics, Majorana fermions have a condensed matter analogue in the zero-energy bound states emerging in topological superconductors. A promising route to engineering topological superconductors capable of hosting Majorana zero modes consists of proximity coupling semiconductor thin films or nanowires with strong spin-orbit interaction to conventional s-wave superconductors in the presence of an external Zeeman field. The Majorana zero mode is predicted to emerge above a certain critical Zeeman field as a zero-energy state localized near the order parameter defects, namely, vortices for thin films and wire ends for the nanowire. These Majorana bound states are expected to manifest non-Abelian quantum statistics, which makes them ideal building blocks for fault-tolerant topological quantum computation. This review provides an update on the current status of the search for Majorana fermions in semiconductor nanowires by focusing on the recent developments, in particular the period following the first reports of experimental signatures consistent with the realization of Majorana bound states in semiconductor nanowire-superconductor hybrid structures. We start with a discussion of the fundamental aspects of the subject, followed by considerations on the realistic modeling, which is a critical bridge between theoretical predictions based on idealized conditions and the real world, as probed experimentally. The last part is dedicated to a few intriguing issues that were brought to the fore by the recent encouraging experimental advances.
    Journal of Physics Condensed Matter 05/2013; 25(23):233201. · 2.22 Impact Factor
  • T. D. Stanescu, Sumanta Tewari
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    ABSTRACT: Majorana fermions (MFs) are predicted to occur as zero-energy bound states in semiconductor nanowire-superconductor structures. However, in the presence of disorder or smooth confining potentials, these structures can also host nontopological nearly zero-energy states. Here, we demonstrate that the MFs and the nearly zero topologically trivial states have different characteristic signatures in a tunneling conductance measurement, which allows to clearly discriminate between them. We also show that low-energy nontopological states can strongly hybridize with metallic states from the leads, which generates the smooth background that characterizes the soft superconducting gap measured in tunneling experiments and produces an additional decoherence mechanism for the Majorana mode. Our results pave the way for the conclusive identification of MFs in a solid state system and provide directions for minimizing quantum decoherence in Majorana wires.
    Physical review. B, Condensed matter 04/2013; 87(14). · 3.66 Impact Factor
  • Li Mao, Sumanta Tewari, Chuanwei Zhang
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    ABSTRACT: It has been theoretically predicted that a nanostructure composing of a semiconductor nanowire with strong spin-orbit coupling and an s-wave superconductor can support two Majorana fermions at the ends of the nanowire in the presence of a Zeeman field. Recently, following the theoretical proposals, some preliminary experimental signatures (e.g., zero-bias conductance peak) which may be related to the existence of Majorana fermions have been observed in the charge transport experiments. Here we investigate the Josephson currents with the zero-bias voltage in the topologically trivial region of a superconductor-insulator-superconductor junction in the presence of strong spin-orbit coupling and Zeeman field. This structure may be relevant to the Delft experiment by considering the possible proximity effect of the superconductor lead to the normal part of the nanowire. Our results indicate that the experimentally observed zero-bias conductance peak may not originate from Majorana fermions.
    03/2013;
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    Kangjun Seo, Chuanwei Zhang, Sumanta Tewari
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    ABSTRACT: We discuss the thermodynamic signatures for the topological phase transitions into Majorana and Weyl superfluid phases in ultracold Fermi gases in two and three dimensions in the presence of Rashba spin-orbit coupling and a Zeeman field. We analyze the thermodynamic properties exhibiting the distinct nature of the topological phase transitions linked with the Majorana fermions (2D Fermi gas) and Weyl fermions (3D Fermi gas) which can be observed experimentally, including pressure, chemical potential, isothermal compressibility, entropy, and specific heat, as a function of the interaction and the Zeeman field at both zero and finite temperatures. We conclude that among the various thermodynamic quantities, the isothermal compressibility and the chemical potential as a function of the artificial Zeeman field have the strongest signatures of the topological transitions in both two and three dimensions.
    Physical Review A 02/2013; 87(6). · 3.04 Impact Factor
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    ABSTRACT: We investigate theoretically the low-energy physics of semiconductor Majorana wires in the vicinity of a magnetic field-driven topological quantum phase transition (TQPT). The local density of states at the end of the wire, which is directly related to the differential conductance in the limit of point-contact tunneling, is calculated numerically. We find that the dependence of the end-of-wire local density of states on the magnetic field is nonuniversal and that the signatures associated with the closing of the superconducting gap at the Majorana TQPT are essentially invisible within a significant range of experimentally relevant parameters. Our results provide a possible explanation for the recent observation of the apparent nonclosure of the gap at the Majorana TQPT in semiconductor nanowires.
    Physical Review Letters 12/2012; 109(26):266402. · 7.73 Impact Factor
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    Jay D. Sau, Brian Swingle, Sumanta Tewari
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    ABSTRACT: Topological Majorana fermion (MF) quasiparticles have been recently shown to exist in semiconductor quantum wires with proximity induced superconductivity and a Zeeman field. Although the experimentally observed zero bias tunneling peak and a fractional ac-Josephson effect can be taken as necessary signatures of MFs, neither of them constitutes a sufficient "smoking gun" experiment. Since one pair of Majorana fermions share a single conventional fermionic degree of freedom, MFs are in a sense fractionalized excitations. Based on this fractionalization we propose a tunneling experiment that furnishes a nearly unique signature of end state MFs in semiconductor quantum wires. In particular, we show that a "teleportation"-like experiment is not enough to distinguish MFs from accidental conventional zero energy states, but our proposed tunneling experiment, in principle, can make this distinction.
    10/2012;
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    Sumanta Tewari, Jay D Sau
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    ABSTRACT: We show that the Hamiltonian of a multiband spin-orbit coupled semiconductor nanowire with Zeeman splitting and s-wave superconductivity is approximately chiral symmetric. The chiral symmetry becomes exact when only one pair of confinement bands is occupied and the Zeeman splitting is parallel to the nanowire. In this idealized case the Hamiltonian is in the BDI symmetry class of the topological classification of band Hamiltonians, allowing an arbitrary integer number of zero-energy Majorana fermion modes at each end. In the realistic case of multiband wires (Zeeman splitting still parallel to the length) the chiral symmetry is approximate and results in multiple near-zero-energy end states with increasing Zeeman splitting. The existence of such low energy end states implies the vanishing of the minigap with increased Zeeman splitting which can only be restored by breaking the approximate chiral symmetry by a second Zeeman field.
    Physical Review Letters 10/2012; 109(15):150408. · 7.73 Impact Factor
  • Li Mao, Ming Gong, Sumanta Tewari, Chuanwei Zhang
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    ABSTRACT: Spin-orbit coupled semiconductor nanowires with Zeeman splitting in proximity contact with bulk s-wave superconductivity have recently been proposed as a promising platform for realizing Majorana fermions. However, in this setup the chemical potential of the nanowire is generally pinned by the Fermi surface of the superconductor. This makes the tuning of the chemical potential by external electrical gates, a crucial requirement for unambiguous detection of Majorana fermions, very challenging in experiments. Here we show that tunable topological superconducting regime supporting Majorana fermions can be realized in semiconductor nanowires using uniaxial stress. For n-type nanowires the uniaxial stress tunes the effective chemical potential, while for p-type systems the effective pairing may also be modified by stress, thus significantly enhancing the topological minigap. We show that the required stress, of the order of 0.1%, is within current experimental reach using conventional piezo crystals.
    10/2012;
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    T. D. Stanescu, Sumanta Tewari
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    ABSTRACT: A proposed signature for the Majorana zero-energy quasiparticle predicted to occur in semiconductor nanowires proximity-coupled to an s-wave superconductor is the zero-bias conductance peak (ZBCP) for tunneling into the end of the wire. Recently, it has been shown that, in the presence of a smooth confining potential, nearly ZBCPs can occur even in the topologically trivial phase. Here we show that, for a smooth confinement, the emergence of the nearly ZBCP at Zeeman fields corresponding to the topologically trivial phase is necessarily accompanied by a gap closing signature in the end-of-wire local density of state (LDOS). A similar behavior is found for nearly ZBCPs that appear in the presence of strong disorder. Our results strengthen the identification of the ZBCP observed in the recent Delft measurements, which show no gap-closing signatures, with topological Majorana fermions localized at the ends of the wire.
    08/2012;

Publication Stats

2k Citations
284.02 Total Impact Points

Institutions

  • 2008–2014
    • Clemson University
      • Department of Physics and Astronomy
      Clemson, South Carolina, United States
  • 2012–2013
    • West Virginia University
      • Department of Physics
      Morgantown, WV, United States
  • 2011–2012
    • Washington State University
      • Department of Physics and Astronomy
      Pullman, WA, United States
  • 2005–2011
    • University of Maryland, College Park
      • • Department of Physics
      • • Institute for Physical Science and Technology
      College Park, MD, United States
  • 2006
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, CA, United States
  • 2004
    • University of Oregon
      • Department of Physics
      Eugene, OR, United States
  • 2001–2002
    • University of California, Los Angeles
      • Department of Physics and Astronomy
      Los Angeles, CA, United States