Publications (92)293.82 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: Motivated by a recent experiment in which zerobias 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 spinorbit coupled superconducting substrate can realize a nontrivial chiral topological superconducting state with Majorana bound states localized at the wire ends. The nontrivial 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 multiwire 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;  [Show abstract] [Hide abstract]
ABSTRACT: We establish theoretically that a ferromagnetic wire deposited on the surface of a spinorbit coupled $s$wave superconductor generically supports zeroenergy 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 zerobias 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 wellstudied spinorbitcoupled semiconductorsuperconductor 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 metalsuperconductor heterostructure, in the race for finding a Majoranacarrying topological superconductor.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We identify the dynamic chiral magnetic effect and the optical gyrotropy as the manifestations of the same physical phenomenon, namely the dynamic magnetoelectric effect. Therefore, the measurement of optical gyrotropy through Kerr effect is provides a confirmation of the existence of the dynamic chiral magnetic effect. We derive a general formula for the gyrotropic conductivity of a noncentrosymmetric metal in the high frequency limit, and apply our results to the special cases of inversion symmetry breaking Weyl semimetals and noncentrosymmetric cubic metals.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Multiple zeroenergy 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 quasionedimensional (Q1D) time reversal invariant topological superconductors (class DIII), a realistic model for superconducting lithium molybdenum purple bronze and certain families of organic superconductors, multiple MajoranaKramers 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 semiconductorsuperconductor 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;  [Show abstract] [Hide abstract]
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;  [Show abstract] [Hide abstract]
ABSTRACT: From a direct calculation of the anomalous Hall conductivity and an effective electromagnetic action obtained via Fujikawa's chiral rotation technique, we conclude that an axionic field theory with a nonquantized coefficient describes the electromagnetic response of the (3+1)dimensional Weyl semimetal. The coefficient is proportional to the momentum space separation of the Weyl nodes. Akin to the ChernSimons field theory of quantum Hall effect, the axion field theory violates gauge invariance in the presence of the boundary, which is cured by the chiral anomaly of the surface states via the CallanHarvey mechanism. This provides a unique solution for the radiatively induced CPTodd term in the electromagnetic polarization tensor of the Lorentz violating spinor electrodynamics, where the source of the Lorentz violation is a constant axial 4vector term for the Dirac fermion. A direct linear response calculation also establishes anomalous thermal Hall effect and a WiedemannFranz law, but thermal Hall conductivity does not directly follow from the well known formula for the gravitational chiral anomaly.Physical Review B 12/2013; · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider the quasionedimensional (quasi1D) system realized by an array of weakly coupled parallel onedimensional “tubes” in a twodimensional lattice which permits free motion of atoms in an axial direction in the presence of a Zeeman field, Rashba type spinorbit coupling (SOC), and an swave attractive interaction, while the radial motion is tightly confined. We solve the zerotemperature (T=0) Bogoliubovde Gennes (BdG) equations for the quasi1D Fermi gas with the dispersion modified by tunneling between the tubes and show that the T=0 phase diagram hosts the FuldeFerrellLarkinOvchinnikov (FFLO) phase with nonzero centerofmass 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 centerofmass 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 twodimensional optical lattice strength in this model allows one to explore the crossover behaviors of the phases during the transition between the threedimensional 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 Zeemantuned 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; · 2.99 Impact Factor 
Article: Chiral magnetic effect of Weyl fermions and its applications to cubic noncentrosymmetric metals
[Show abstract] [Hide abstract]
ABSTRACT: When the right and the left handed Weyl points are separated in energy, they give rise to a nondissipative charge current along the direction of a uniform applied magnetic field, even in the absence of an external electric field. This effect is known as the chiral magnetic effect and is a hallmark of the underlying chiral anomaly of the Weyl fermions. According to the linearized continuum theory of Weyl fermions, the induced current is proportional to the magnetic field strength and the energy separation with a universal coefficient $e^2/h^2$. By considering a generic tight binding model for the cubic noncentrosymmetric metals, we show that such a system naturally supports a set of Weyl points, which are separated in energies. We also show the existence of the chiral magnetic effect for generic band parameters, and recover the universal result of the continuum Weyl fermions for a restricted parameter regime. Therefore, cubic noncentrosymmetric metals can serve as suitable platforms for realizing Weyl fermions and the exotic chiral elctrodynamic phenomena, which have promising technological applications.11/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We show that onedimensional 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 DIIIinvariant 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" TRinvariant TS, whose Majorana Kramer's pair at each end is split by an applied Zeeman field in arbitrary directions in spin space.10/2013;  [Show abstract] [Hide abstract]
ABSTRACT: The realization of spinorbit 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 inplane Zeeman field drives the system from a BardeenCooperSchrieffer (BCS) superfluid to a FuldeFerrell (FF) superfluid phase. We find that both phases support multiple MFs at each end of quasione dimensional (quasi1D) 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;  [Show abstract] [Hide abstract]
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 nonlocal 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 endcontacts represents a powerful tool for discriminating between topologically trivial and nontrivial ZBPs.Physical Review B 10/2013; 89(22). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
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 pwave 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 quasi1D systems), nonmagnetic 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 quasi1D 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  [Show abstract] [Hide abstract]
ABSTRACT: We show that carbon nanotubes (CNTs) are good candidates for onedimensional 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 curvatureinduced spinorbit coupling which itself has a topological origin. In addition to the spinorbit 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 spinorbit 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).  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the spectrum of the amplitude mode, the analog of the Higgs mode in highenergy physics, for the ddensity wave state proposed to describe the pseudogap phase of the highTc cuprates. Even though the state breaks translational symmetry by a lattice spacing and is described by a particlehole 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  [Show abstract] [Hide abstract]
ABSTRACT: After a recent series of rapid and exciting developments, the long search for the Majorana fermionthe elusive quantum entity at the border between particles and antiparticleshas 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 zeroenergy 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 spinorbit interaction to conventional swave 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 zeroenergy 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 nonAbelian quantum statistics, which makes them ideal building blocks for faulttolerant 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 nanowiresuperconductor 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  [Show abstract] [Hide abstract]
ABSTRACT: Majorana fermions (MFs) are predicted to occur as zeroenergy bound states in semiconductor nanowiresuperconductor structures. However, in the presence of disorder or smooth confining potentials, these structures can also host nontopological nearly zeroenergy 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 lowenergy 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  [Show abstract] [Hide abstract]
ABSTRACT: It has been theoretically predicted that a nanostructure composing of a semiconductor nanowire with strong spinorbit coupling and an swave 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., zerobias 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 zerobias voltage in the topologically trivial region of a superconductorinsulatorsuperconductor junction in the presence of strong spinorbit 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 zerobias conductance peak may not originate from Majorana fermions.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: From a direct calculation of the anomalous Hall conductivity and an effective electromagnetic action obtained via Fujikawa's chiral rotation technique, we conclude that an axionic field theory with a nonquantized coefficient describes the electromagnetic response of the (3+1)dimensional Weyl semimetal. The coefficient is proportional to the momentum space separation of the Weyl nodes. Akin to the ChernSimons field theory of quantum Hall effect, the axion field theory violates gauge invariance in the presence of the boundary, which is cured by the chiral anomaly of the surface states via the CallanHarvey mechanism. A direct linear response calculation also establishes an anomalous thermal Hall effect and a WiedemannFranz law. But, thermal Hall conductivity does not directly follow from the well known formula for the (3+1)dimensional gravitational chiral anomaly. By calculating the gravitational chiral anomaly at finite temperature we show the existence of a new term, which correctly accounts for the thermal Hall effect in (3+1)dimensional Weyl materials, topological insulators and topological superconductors.03/2013;  [Show abstract] [Hide abstract]
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 spinorbit 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). · 2.99 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate theoretically the lowenergy physics of semiconductor Majorana wires in the vicinity of a magnetic fielddriven 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 pointcontact tunneling, is calculated numerically. We find that the dependence of the endofwire 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
Publication Stats
2k  Citations  
293.82  Total Impact Points  
Top Journals
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
