arXiv subjects

Nancy P. Sandler

Publications and source records attributed to Nancy P. Sandler.

6 recordsLinked to original sources

arXiv2026

Floquet Superlattices and Edge States in Graphene Nanoribbons

Structured light provides a route to imprint spatially patterned Floquet potentials onto quantum materials. As a particular example, we study a zigzag graphene nanoribbon driven by two coherent tilted beams, whose interference creates a periodic polarization pattern that gives rise to a photo-induced superlattice. The matching between the periodicity of the optical field and the nanoribbon width leads to two regimes in the quasienergy spectrum: matched profiles preserve degenerate edge branches, while mismatched profiles yield a boundary-induced gap that survives in wide ribbons. We propose a two-edge model that captures this splitting through residual hybridization and the boundary-sampled optical field. The quasienergy gap reverses between valleys, leading to a valley-selective boundary response. Our results establish light-induced superlattices as a flexible method for valley selectivity in finite-size Dirac-like materials through tunable edge-state quasienergy splitting.

cond-mat.mes-hall↗

arXiv2026

Vortex-Beam-Driven Dirac Materials: Impurity and Polarization Effects on Light-Induced Vortex and Edge States

We study impurity scattering and polarization detuning in finite-size vortex-light-beam-driven massive Dirac systems. In finite geometries, circularly polarized vortex light opens a dynamical gap where topological edge states coexist with photoinduced multiply quantized vortex states. We analyze how finite-size effects, vorticity, and effective particle-hole symmetry manifest in the quasienergy spectrum, real-space states, and local density of states. We show that angular-momentum mixing due to localized impurities and impurity clusters reshape vortex states, while when produced by circular polarization, it leads to a gradual filling of the dynamical gap with bulk-derived states. Our results indicate that both vortex and edge signatures remain observable in the presence of impurities and realistic polarization deviations, providing guidance for experimental realizations.

cond-mat.mes-hall↗

arXiv2024

Photo-induced Multiply Quantized Vortex States in Dirac-like Materials

Subjecting a massive two-dimensional Dirac material to a vortex light beam provides a mechanism for the photo-induction of multiply quantized vortices. Using Floquet theory, we show that electronic vortices, characterized by their total angular momentum, are exclusive to circularly polarized vortex beams. The equations for the driven system at the one photon-resonance are mapped to the Bogoliubov-de Gennes equations of $s$-wave superconductors with multiply quantized vortices. This mapping provides valuable analytical tools for the analysis of the system's spectral properties.

cond-mat.mes-hall↗

arXiv2013

Spin-polarized conductance in double quantum dots: Interplay of Kondo, Zeeman and interference effects

We study the effect of an external magnetic field in the Kondo regime of a double-quantum-dot system in which a strongly correlated dot (the "hanging dot") is coupled to a second, noninteracting dot that also bridges the gap between two external leads. In zero field, the spectral function of the hanging dot has previously been shown to exhibit a split-peak structure near the Fermi level due to "Kondo resonance filtering" by the bridging dot. We show using the numerical renormalization group that application of a magnetic field leads to a subtle interplay between electronic interference, Kondo physics, and Zeeman splitting with nontrivial consequences for the spectral and transport properties. The value of the hanging-dot spectral function at the Fermi level exhibits a nonuniversal field dependence that can be explained using a generalized Friedel sum rule for a Kondo system with energy-dependent hybridization. The magnetic field also accentuates the exchange-mediated interdot coupling, which dominates the ground state at intermediate fields leading to the formation of antiparallel magnetic moments on the dots. By tuning gate voltages and the magnetic field, one can achieve complete spin polarization of the linear conductance between the leads, raising the prospect of applications of the device as a highly tunable spin filter. The system's low-energy properties are qualitatively unchanged by the presence of weak on-site Coulomb repulsion within the bridging dot.

cond-mat.mes-hall↗

arXiv1998

What do noise measurements reveal about fractional charge in FQH liquids?

We present a calculation of noise in the tunneling current through junctions between two two-dimensional electron gases (2DEG) in inequivalent Laughlin fractional quantum Hall (FQH) states, as a function of voltage and temperature. We discuss the interpretation of measurements of suppressed shot noise levels of tunneling currents through a quantum point contact (QPC) in terms of tunneling of fractionally charged states. We show that although this interpretation is always possible, for junctions between different FQH states the fractionally charged states involved in the tunneling process are not the Laughlin quasiparticles of the isolated FQH states that make up the junction, and should be regarded instead as solitons of the coupled system. The charge of the soliton is, in units of the electron charge, the harmonic average of the filling fractions of the individual Laughlin states, which also coincides with the saturation value of the differential conductance of the QPC. For the especially interesting case of a QPC between states at filling fractions $ν=1$ and $ν={1/3}$, we calculate the noise in the tunneling current exactly for all voltages and temperatures and investigate the crossovers. These results can be tested by noise experiments on $(1,{1/3})$ QPCs. We present a generalization of these results for QPC's of arbitrary Laughlin fractions in their weak and strong coupling regimes. We also introduce generalized Wilson ratios for the noise in the shot and thermal limits. These ratios are universal scaling functions of $V/T$ that can be measured experimentally in a general QPC geometry.

cond-mat.mes-hall↗

arXiv1998

Andreev reflection in the fractional quantum Hall effect

We study the reflection of electrons and quasiparticles on point-contact interfaces between fractional quantum Hall (FQH) states and normal metals (leads), as well as interfaces between two FQH states with mismatched filling fractions. We classify the processes taking place at the interface in the strong coupling limit. In this regime a set of quasiparticles can decay into quasiholes on the FQH side and charge excitations on the other side of the junction. This process is analogous to an Andreev reflection in normal-metal/superconductor (N-S) interfaces.

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