arXiv subjects

Rekha Kumari

Publications and source records attributed to Rekha Kumari.

6 recordsLinked to original sources

arXiv2026

Anomalous topological phases in a Chern insulator connected to leads in a cylindrical geometry

The observed robustly quantized Hall conductance in quantum Hall systems and Chern insulators (CI) is normally understood in terms of the bulk topology of isolated systems, not coupled to leads. It is assumed that the leads act as inert reservoirs. Within a model of a CI coupled to leads with a cylindrical geometry, we show that this is not always true. We identify the Hall conductance with a boundary invariant, the winding number of the phase of the reflection coefficient. We find anomalous topological phases where the boundary invariant is not the same as the bulk Chern number, even in the limit of weak lead coupling.

cond-mat.mes-hall↗

arXiv2026

Quantized Transport in Floquet Topological Insulators

We study quantum transport in a periodically driven (Floquet) topological system coupled to static fermionic reservoirs. Using the Floquet nonequilibrium Green's-function (NEGF) formalism we show, from exact numerics for a strip geometry, that the two-terminal (longitudinal) conductance is quantized as $|W_{\varepsilon}|\,e^2/h$, while the Hall (transverse) conductance is quantized as $W_{\varepsilon}\,e^2/h$, where $W_{\varepsilon}$ is the Floquet winding invariant associated with the quasienergy gap at $\varepsilon = 0$ or $\varepsilon = Ω/2$. Quantization is achieved only after summing over the contribution of all Floquet sidebands. We provide an analytic understanding of this Floquet conductance sum rule, by considering the Hall conductance in the weak coupling limit. In that limit, we show that the Floquet Hall conductance gets contributions from the Floquet sidebands, which includes the signs of the velocities of the edge modes. Their sum yields exact quantization, as predicted by the Floquet sum rule. We find that in a wide range of parameter regime, the convergence is fast, making observation of the sum rule and Floquet winding numbers accessible to experiments.

cond-mat.mes-hall↗

arXiv2025

Transport signatures of single and multiple Floquet Majorana modes in one-dimensional Rashba nanowire and Shiba chain

We theoretically investigate the transport signature of single and multiple Floquet Majorana end modes~(FMEMs), appearing in an experimentally feasible setup with Rashba nanowire~(NW) placed in closed proximity to a conventional $s$-wave superconductor, in the presence of an external Zeeman field. Periodic drive causes the anomalous $π$-modes to emerge in addition to the regular $0$-modes in the driven system where the former does not exhibit any static analog. For single $0$- and/or $π$-FMEM, differential conductance exhibits a quantized value of $2e^{2}/h$ while we consider the sum over all the photon sectors, supporting Floquet sum rule. We examine the stability of this summed conductance against random onsite disorder. We further investigate the summed conductance in several cases hosting multiple~(more than one) $0$- or $π$-modes at the end of the NW. In these cases, we obtain quantized values of $n_{M}\times 2e^{2}/h$ in summed differential conductance with $n_{M}$ being the number of modes~($0$ or $π$) localized at one end of the NW. We repeat our analysis for another experimentally realizable model system known as helical Shiba chain. Moreover, we corroborate our results by computing the differential conductance for FMEMs using non-equilibrium Green's function method. Our work opens up the possibility of studying the transport signatures of FMEMs in these realistic models.

cond-mat.mes-hall↗

arXiv2024

Fermi-arcs mediated transport in surface Josephson junctions of Weyl semimetal

This study presents Fermi-arcs mediated transport in a Weyl semimetal thin slab, interfacing two $s$-wave superconductors. We present detailed study with both time-reversal and inversion symmetry broken Weyl semimetals under grounding, orbital magnetic fields, and Zeeman fields. An orbital magnetic field induces energy level oscillations, while a Zeeman field give rise to the periodic anomalous oscillations in the Josephson current. These anomalous oscillations correlate with the separation of Weyl nodes in momentum space, junction length, and system symmetries. Additionally, we present an explanation by scattering theory modeling the Fermi-arcs as a network model.

cond-mat.mes-hall↗

arXiv2024

Valley filtering and valley valves in irradiated pristine graphene

We theoretically study valley-filtering in pristine graphene irradiated by bicircular counter-rotating laser drive. The dynamical symmetry of the graphene and laser drive disrupts graphene's inversion symmetry, which results distinct quasi-energy states and Floquet band occupations in the two valleys. Controlling the relative phase between the bicircular laser drive ultimately allows to blocks the contribution from one valley while allowing the opposite valley currents in the system. For practical realization of valley-based device, we propose configurational setup for valley filters and valley valve consisting of two graphene nanoribbons irradiated by two bicircular counter-rotating laser drives with a relative phase shift. It is observed that the relative phase between the two bicircular laser drives offer a control knob to generate valley-selective currents and transport responses with very high efficiency by an all-optical way. In addition, our findings about valley filter and valley valve are robust against moderate disorder and modest changes in driving laser parameters. Present work opens an avenue to realise light-based valleytronics devices in reality.

cond-mat.mes-hall↗

arXiv2023

Josephson-Current Signatures of Unpaired Floquet Majorana Bound States

We theoretically study the transport signatures of unpaired Floquet Majorana bound states in the Josephson current of weakly linked, periodically driven topological superconductors. We obtain the occupation of the Floquet Majorana modes in the presence of weak coupling to thermal leads analytically, and show that, similar to static superconductors, the Josephson current involving Floquet Majorana bound states is also $4π$-periodic in the phase difference across the junction, and also depends linearly on the coupling between superconductors. Moreover, unlike the static case, the amplitude of the Josephson current can be tuned by setting the unbiased chemical potential of the driven superconductors at multiple harmonics of the drive frequency. As a result, we uncover a Josephson Floquet sum rule for driven superconductors. We confirm our analytical expressions for Josephson current, the occupation of Floquet bands, and a perturbative analysis of the quasienergies with numerically exact results.

cond-mat.mes-hall↗