Sumanta Tewari

Clemson University, CEU, South Carolina, United States

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Publications (101)322.57 Total impact

  • Source
    Eugene Dumitrescu · Girish Sharma · Jay D. Sau · Sumanta Tewari
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    ABSTRACT: Recently it has been proposed that a unitary topological mirror symmetry can stabilize multiple zero energy Majorana fermion modes in one dimensional (1D) time reversal (TR) invariant topological superconductors. Here we establish an exact equivalence between 1D "topological mirror superconductivity" and chiral topological superconductivity in BDI class which can also stabilize multiple Majorana-Kramers pairs in 1D TR-invariant topological superconductors. The equivalence proves that topological mirror superconductivity can be understood as chiral superconductivity in the BDI symmetry class co-existing with time-reversal symmetry. Furthermore, we show that the mirror Berry phase coincides with the chiral winding invariant of the BDI symmetry class, which is independent of the presence of the time-reversal symmetry. Thus, the time-reversal invariant topological mirror superconducting state may be viewed as a special case of the BDI symmetry class in the well-known Altland-Zirnbauer periodic table of free fermionic phases. We illustrate the results with the examples of 1D spin-orbit coupled quantum wires in the presence of nodeless s_{\pm} superconductivity and the recently discussed experimental system of ferromagnetic atom (Fe) chains embedded on a lead (Pb) superconductor.
    Physical Review B 07/2015; 92(4):045421. DOI:10.1103/PhysRevB.92.045421 · 3.74 Impact Factor
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    Girish Sharma · Pallab Goswami · Sumanta Tewari
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    ABSTRACT: Weyl semimetals (WSM) are topologically protected three dimensional materials whose low energy excitations are linearly dispersing massless Dirac fermions, possessing a non-trivial Berry curvature. Using semi-classical Boltzmann dynamics in the relaxation time approximation for a lattice model of time reversal (TR) symmetry broken WSM, we compute both magnetic field dependent and anomalous contributions to the Nernst coefficient. In addition to the magnetic field dependent Nernst response, which is present in both Dirac and Weyl semimetals, we show that, contrary to previous reports, the TR-broken WSM also has an anomalous Nernst response due to a non-vanishing Berry curvature. We also compute the thermal conductivities of a WSM in the Nernst (${\nabla T} \perp \mathbf{B}$) and the longitudinal (${\nabla T} \parallel \mathbf{B}$) set-up and confirm from our lattice model that in the parallel set-up, the Wiedemann-Franz law is violated between the longitudinal thermal and electrical conductivities due to chiral anomaly.
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    ABSTRACT: A mechanism is proposed for the tantalizing evidence of polar Kerr effect in a class of high temperature superconductors--the signs of the Kerr angle from two opposite faces of the same sample are identical and magnetic field training is non-existent. The mechanism does not break global time reversal symmetry, as in an antiferromagnet, and results in zero Faraday effect. It is best understood in a phenomenological model of bilayer cuprates, such as YBCO, in which intra-bilayer tunneling nucleates a chiral d-density wave such that the individual layers have opposite chirality. Although specific to the chiral d-density wave, the mechanism may be more general to any quasi-two-dimensional orbital antiferromagnet in which time reversal symmetry is broken in each plane, but not when averaged macroscopically.
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    ABSTRACT: Majorana fermion (MF) excitations in solid state system have non-Abelian statistics which is essential for topological quantum computation. Previous proposals to realize MF, however, generally requires fine-tuning of parameters. Here we explore a platform which avoids the fine-tuning problem, namely a ferromagnetic chain deposited on the surface of a spin-orbit coupled s-wave superconductor. We show that it generically supports zero-energy topological MF excitations near the two ends of the chain with minimal fine-tuning. Depending on the strength of the ferromagnetic moment in the chain, 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 2 ne(2)/h in scanning tunneling microscopy (STM) measurements which would show strong (weak) signals at the ends (middle) of the chain. The role of an approximate chiral symmetry which gives an integer topological invariant to the system is discussed.
    Scientific Reports 03/2015; 5:8880. DOI:10.1038/srep08880 · 5.58 Impact Factor
  • Eugene Dumitrescu · Tudor D. Stanescu · Sumanta Tewari
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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 ${(\mathrm{Li}}_{0.9}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17})$ 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 ${t}_{$\perp${}}$\ll${}t$, where ${t}_{$\perp${}}$ and $t$ are the transverse and longitudinal hoppings, respectively. Our results, explained in terms of special properties of the Hamiltonian and wave functions, underscore the importance of hidden accidental symmetries in topological superconductors.
    Physical Review B 03/2015; 91(12). DOI:10.1103/PhysRevB.91.121413 · 3.74 Impact Factor
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    Girish Sharma · Chunxiao Liu · Kangjun Seo · J. D. Sau · Sumanta Tewari
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    ABSTRACT: The role of charge order in the phase diagram of high temperature cuprate superconductors has been recently re-emphasized by the experimental discovery of an incipient bi-directional charge density wave (CDW) phase in a class of underdoped cuprates. In a subset of the experiments, the CDW has been found to be accompanied by a d-wave intra-unit-cell form factor, indicating modulation of charge density on the oxygen orbitals sandwiched between neighboring Cu atoms on the CuO planes (the so-called bond-density wave (BDW) phase). Here we take a mean field Q_1=(2\pi/3,0) and Q_2=(0,2\pi/3) bi-directional BDW phase with a d-wave form factor, which closely resembles the experimentally observed charge ordered states in underdoped cuprates, and calculate the Fermi surface topology and the resulting quasiparticle Nernst coefficient as a function of temperature and doping. We establish that, in the appropriate doping ranges where the low temperature phase (in the absence of superconductivity) is a BDW, the Fermi surface consists of an electron and a hole pocket, resulting in a low temperature negative Nernst coefficient as observed in experiments.
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    Ipsita Mandal · Sumanta Tewari
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    ABSTRACT: We show that certain singularities of the Hamiltonian in the complex wave vector space can be used to identify topological quantum phase transitions for $1D$ chiral topological superconductors/superfluids in the BDI class. These singularities fall into the category of the so-called exceptional points ($EP$'s) studied in the context of non-Hermitian Hamiltonians describing open quantum systems. We also propose a generic formula in terms of the properties of the $EP$'s to quantify the exact number of Majorana zero modes in a particular chiral topological superconducting phase, given the values of the parameters appearing in the Hamiltonian. This formula serves as an alternative to the familiar integer ($\mathbb{Z}$) winding number invariant characterizing topological superconductor/superfluid phases in the chiral BDI class.
  • Eugene Dumitrescu · Jay D. Sau · Sumanta Tewari
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    ABSTRACT: We study the magnetic field response of Majorana-Kramers pairs of one-dimensional time-reversal (TR)-invariant topological superconductors (class DIII) with or without a coexisting chirality symmetry. In addition to explaining the anomalous magnetic field response of all the DIII class topological superconducting systems proposed in the literature, we provide a realistic route to engineer a ``true'' TR-invariant topological superconductor, i.e., one whose pair of Majorana bound states at each end is split by an applied Zeeman field in arbitrary direction. We also prove that quite generally the splitting of the Majorana bound states in a time-reversal-invariant topological superconductor by time-reversal breaking fields is highly anisotropic in spin space.
    Physical Review B 12/2014; 90(24). DOI:10.1103/PhysRevB.90.245438 · 3.74 Impact Factor
  • 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 ${Q}_{1}=(2$\pi${}/3,0)$ and ${Q}_{2}=(0,2$\pi${}/3)$ biaxial CDW state and calculate the Fermi-surface topology and the resulting Hall and Seebeck coefficients as a function of temperature and hole doping. We establish 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.
    Physical Review B 11/2014; 90(17). DOI:10.1103/PhysRevB.90.174503 · 3.74 Impact Factor
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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.
    Physical Review B 10/2014; 91(9). DOI:10.1103/PhysRevB.91.094505 · 3.74 Impact Factor
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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.
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    Pallab Goswami · Girish Sharma · Sumanta Tewari
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    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.
  • Source
    E. Dumitrescu · T. D. Stanescu · S. Tewari
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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.
  • Source
    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.
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    Pallab Goswami · Sumanta Tewari
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    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 Chern-Simons 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 Callan-Harvey mechanism. This provides a unique solution for the radiatively induced CPT-odd term in the electromagnetic polarization tensor of the Lorentz violating spinor electrodynamics, where the source of the Lorentz violation is a constant axial 4-vector term for the Dirac fermion. A direct linear response calculation also establishes anomalous thermal Hall effect and a Wiedemann-Franz law, but thermal Hall conductivity does not directly follow from the well known formula for the gravitational chiral anomaly.
    Physical Review B 12/2013; 88(24):245107-. DOI:10.1103/PhysRevB.88.245107 · 3.74 Impact Factor
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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; 88(6):63601-. DOI:10.1103/PhysRevA.88.063601 · 2.99 Impact Factor
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    Pallab Goswami · Sumanta Tewari
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    ABSTRACT: When the right and the left handed Weyl points are separated in energy, they give rise to a non-dissipative 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.
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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.
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    Chunlei Qu · Ming Gong · Yong Xu · Sumanta Tewari · Chuanwei Zhang
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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.
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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.
    Physical Review B 10/2013; 89(22). DOI:10.1103/PhysRevB.89.220507 · 3.74 Impact Factor

Publication Stats

2k Citations
322.57 Total Impact Points

Institutions

  • 2007–2015
    • Clemson University
      • Department of Physics and Astronomy
      CEU, South Carolina, United States
  • 2005–2015
    • University of Maryland, College Park
      • • Department of Physics
      • • Institute for Physical Science and Technology
      CGS, Maryland, United States
  • 2006
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States
  • 2004
    • University of Oregon
      • Department of Physics
      Eugene, OR, United States
  • 2001–2003
    • University of California, Los Angeles
      • Department of Physics and Astronomy
      Los Angeles, CA, United States