arXiv subjects

J. Tworzydło

Publications and source records attributed to J. Tworzydło.

23 records · Page 2Linked to original sources

arXiv2011

Majorana bound states without vortices in topological superconductors with electrostatic defects

Vortices in two-dimensional superconductors with broken time-reversal and spin-rotation symmetry can bind states at zero excitation energy. These socalled Majorana bound states transform a thermal insulator into a thermal metal and may be used to encode topologically protected qubits. We identify an alternative mechanism for the formation of Majorana bound states, akin to the way in which Shockley states are formed on metal surfaces: An atomic-scale electrostatic line defect can have a pair of Majorana bound states at the end points. The Shockley mechanism explains the appearance of a thermal metal in vortex-free lattice models of chiral p-wave superconductors and (unlike the vortex mechanism) is also operative in the topologically trivial phase.

cond-mat.mes-hall↗

arXiv2010

Effective mass and tricritical point for lattice fermions localized by a random mass

This is a numerical study of quasiparticle localization in symmetry class \textit{BD} (realized, for example, in chiral \textit{p}-wave superconductors), by means of a staggered-fermion lattice model for two-dimensional Dirac fermions with a random mass. For sufficiently weak disorder, the system size dependence of the average (thermal) conductivity $σ$ is well described by an effective mass $M_{\rm eff}$, dependent on the first two moments of the random mass $M(\bm{r})$. The effective mass vanishes linearly when the average mass $\bar{M}\to 0$, reproducing the known insulator-insulator phase boundary with a scale invariant dimensionless conductivity $σ_{c}=1/π$ and critical exponent $ν=1$. For strong disorder a transition to a metallic phase appears, with larger $σ_{c}$ but the same $ν$. The intersection of the metal-insulator and insulator-insulator phase boundaries is identified as a \textit{repulsive} tricritical point.

cond-mat.mes-hall↗

arXiv2009

Theory of the topological Anderson insulator

We present an effective medium theory that explains the disorder-induced transition into a phase of quantized conductance, discovered in computer simulations of HgTe quantum wells. It is the combination of a random potential and quadratic corrections proportional to p^2 sigma_z to the Dirac Hamiltonian that can drive an ordinary band insulator into a topological insulator (having an inverted band gap). We calculate the location of the phase boundary at weak disorder and show that it corresponds to the crossing of a band edge rather than a mobility edge. Our mechanism for the formation of a topological Anderson insulator is generic, and would apply as well to three-dimensional semiconductors with strong spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2009

Maximal positive cross shot noise from Andreev reflection

The current flowing from a superconductor to a two-terminal setup describing a nanostructure connected to normal-metal leads is studied. We provide an example of scattering matrix giving ideal splitting off electrons from a Cooper pair by means of Cauchy-Bunyakovsky-Schwarz inequality. The proposal of the junction and its possible variants are discussed in a context of possible experiments.

cond-mat.supr-con↗

arXiv2007

Demonstration of one-parameter scaling at the Dirac point in graphene

We numerically calculate the conductivity $σ$ of an undoped graphene sheet (size $L$) in the limit of vanishingly small lattice constant. We demonstrate one-parameter scaling for random impurity scattering and determine the scaling function $β(σ)=d\lnσ/d\ln L$. Contrary to a recent prediction, the scaling flow has no fixed point ($β>0$) for conductivities up to and beyond the symplectic metal-insulator transition. Instead, the data supports an alternative scaling flow for which the conductivity at the Dirac point increases logarithmically with sample size in the absence of intervalley scattering -- without reaching a scale-invariant limit.

cond-mat.mes-hall↗