Sergey S. Pershoguba

Spin-polarized tunneling current through a thin film of a topological insulator in a parallel magnetic field

We calculate the tunneling conductance \sigma between the surface states on the opposite sides of the ultra-thin film of a topological insulator in a parallel magnetic field B_y. The parallel magnetic produces a relative shift of the in-plane momenta of the two surfaces states. An overlap between the shifted Fermi circles and their spin structure define an unusual dependence of the tunneling conductance \sigma(B_y) on the magnetic field. Because the spin of the electronic surface states in topological insulators is locked with momentum, the spin-polarization of the tunneling current can be controlled by magnetic field B_y.

Currents induced by magnetic impurities in superconductors with spin-orbit coupling

We show that superconducting currents are generated around magnetic impurities and ferromagnetic islands proximity coupled to superconductors with finite spin-orbit coupling. Using the Ginzburg-Landau theory, T-matrix calculation, as well as self-consistent numerical simulation on a lattice, we find a strong dependence of the current on the direction and magnitude of the magnetic moment. We establish that in the case of point magnetic impurities, the current is carried by the induced Yu-Shiba-Rusinov (YSR) subgap states. In the vicinity of the phase transition, where the YSR states cross at zero energy, the current increases dramatically. Furthermore, we show that the currents are orthogonal to the local spin polarization and, thus, can be probed by measuring the spin-polarized local density of states.

Dirac Magnons in Honeycomb Ferromagnets

The discovery of the Dirac electron dispersion in graphene [A. H. Castro Neto, The Electronic Properties of Graphene, Rev. Mod. Phys. 81, 109 (2009)RMPHAT0034-686110.1103/RevModPhys.81.109] led to ...

Spin-polarized edge currents and Majorana fermions in one- and two-dimensional topological superconductors

We investigate the persistent currents, spin-polarized local density of states, and spectral functions of topological superconductors constructed by placing ferromagnetic impurities on top of an s- ...

Proposed chiral texture of the magnetic moments of unit-cell loop currents in the pseudogap phase of cuprate superconductors.

We propose a novel chiral order parameter to explain the unusual polar Kerr effect in underdoped cuprates. It is based on the loop-current model by Varma, which is characterized by the in-plane anapole moment N and exhibits the magnetoelectric effect. We propose a helical structure where the vector N(n) in the layer n is twisted by the angle π/2 relative to N(n-1), thus breaking inversion symmetry. We show that coupling between magnetoelectric terms in the neighboring layers for this structure produces optical gyrotropy, which results in circular dichroism and the polar Kerr effect.

Energy spectrum of graphene multilayers in a parallel magnetic field

We study the orbital effect of a strong magnetic field parallel to the layers on the energy spectrum of the Bernal-stacked graphene bilayer and multilayers, including graphite. We consider the minimal model with the electron tunneling between the nearest sites in the plane and out of the plane. Using the semiclassical analytical approximation and exact numerical diagonalization, we find that the energy spectrum consists of two domains. In the low- and high-energy domains, the semiclassical electron orbits are closed and open, so the spectra are discrete and continuous, correspondingly. The discrete energy levels are the analogs of the Landau levels for the parallel magnetic field. They can be detected experimentally using electron tunneling and optical spectroscopy. In both domains, the electron wave functions are localized on a finite number of graphene layers, so the results can be applied to graphene multilayers of a finite thickness.

Effects of a tilted magnetic field in a Dirac double layer

We calculate the energy spectrum of a Dirac double layer, where each layer has the Dirac electronic dispersion, in the presence of a tilted magnetic field and small interlayer tunneling. We show that the energy splitting between the Landau levels has an oscillatory dependence on the in-plane magnetic field and vanishes at a series of special tilt angles of the magnetic field. Using a semiclassical analysis, we show that these special tilt angles are determined by the Berry phase of the Dirac Hamiltonian. The interlayer tunneling conductance also exhibits an oscillatory dependence on the magnetic field tilt angle, known as the angular magnetoresistance oscillations (AMRO). Our results are applicable to graphene double layers and thin films of topological insulators.

Erratum: Proposed Chiral Texture of the Magnetic Moments of Unit-Cell Loop Currents in the Pseudogap Phase of Cuprate Superconductors [Phys. Rev. Lett.111, 047005 (2013)]

Originally, observation of the polar Kerr effect in cuprates [1] was interpreted as the evidence for spontaneous timereversal symmetry breaking. Then, it was proposed in Refs. [2–6], as well as in the earlier paper [7], that the polar Kerr effect in cuprates can be explained by a chiral gyrotropic order that breaks inversion symmetry, but preserves time-reversal symmetry. However, it was shown in a general form using reciprocity arguments by Halperin [8] and confirmed by recent papers [9, 10] that the reflection matrix of light must be symmetric for a time-reversal-invariant system, so the polar Kerr effect must vanish. This prompted retractions [11, 12] of the proposals that a chiral order without time-reversal-symmetry breaking can explain the polar Kerr effect. In this Erratum, we show that, while the electromagnetic constituent relations are correctly derived in our paper [4] and do contain a bulk gyrotropic term, the reflection matrix of light is, nevertheless, symmetric (in agreement with Refs. [8–12]), so our proposed model [4] cannot explain the experimental observation [1] of the polar Kerr effect in cuprates [13]. The confusion stems from different treatments of a surface contribution to the constituent relations in different papers. Ref. [7] employed the bulk relation 4πP = γ∇ ×E, where E and P are the electric field and polarization, and γ is the coefficient of natural optical activity [14]. However, for a system occupying semi-infinite space z > 0 in contact with vacuum at z < 0, the coefficient γ(z) has a step-wise dependence on coordinate z: γ(z) = 0 for z < 0 and γ(z) 6= 0 for z > 0. Ref. [15] proposed the following relation 4πP = ∇ × (γE) = γ∇ ×E + (∇zγ) ×E containing the delta-function surface term γδ(z)ẑ × E. Substituting these relations into Maxwell’s equations, Refs. [7] and [15] obtained opposite signs for the polar Kerr effect. However, both relations are wrong, as pointed out in Ref. [10], and the correct relation is 4πP = γ∇ × E + (1/2)(∇zγ) × E, as employed in Ref. [11] after correcting an arithmetic error in Ref. [16]. This relation can be obtained by variation P = δS/δE of the effective action S = (1/8π) ∫ dω dr γ(z)E · (∇×E), and it gives zero polar Kerr effect [11]. Equation (4) in our paper [4] utilized the incorrect formula from Ref. [15] claiming a non-zero Kerr angle. However, our microscopic derivation of the effective action for a helical structure of loop currents is correct. Moreover, the advantage of our discrete lattice model over continuos models is that the correct surface term in the constituent relations can be derived unambiguously without confusion. The electromagnetic action in our model is given by Eq. (9) in Ref. [4]

Interlayer tunneling spectroscopy of graphite at high magnetic field oriented parallel to the layers

Interlayer tunneling in graphite mesa-type structures is studied at a strong in-plane magnetic field H up to 55 T and low temperature T = 1.4 K. The tunneling spectrum dI/dV vs. V has a pronounced peak at a finite voltage V0. The peak position V0 increases linearly with H. To explain the experiment, we develop a theoretical model of graphite in the crossed electric E and magnetic H fields. When the fields satisfy the resonant condition E = vH, where V is the velocity of the two-dimensional Dirac electrons in graphene, the wave functions delocalize and give rise to the peak in the tunneling spectrum observed in the experiment.