arXiv subjects

Longwen Zhou

Publications and source records attributed to Longwen Zhou.

65 records · Page 4Linked to original sources

arXiv2017

Photoinduced $\frac{1}{2}\frac{e^2}{h}$ and $\frac{3}{2}\frac{e^2}{h}$ conductance plateaus in topological insulator-superconductor heterostructures

The past few years have witnessed increased attention to the quest for Majorana-like excitations in the condensed matter community. As a promising candidate in this race, the one-dimensional chiral Majorana edge mode (CMEM) in topological insulator-superconductor heterostructures has gathered renewed interests during recent months after an experimental breakthrough. In this paper, we study the quantum transport of topological insulator-superconductor hybrid devices subject to light-matter interaction or general time-periodic modulation. We report half-integer quantized conductance plateaus at $\frac{1}{2}\frac{e^2}{h}$ and $\frac{3}{2}\frac{e^2}{h}$ upon applying the so-called sum rule in the theory of quantum transport in Floquet topological matter. In particular, in a photoinduced topological superconductor sandwiched between two Floquet Chern insulators, it is found that for each Floquet sideband, the CMEM admits equal probability for normal transmission and local Andreev reflection over a wide range of parameter regimes, yielding half-integer quantized plateaus that resist static and time-periodic disorder. The $\frac{3}{2}\frac{e^2}{h}$ plateau has not yet been computationally or experimentally observed in any other superconducting system, and indicates the possibility to simultaneously create and manipulate multiple pairs of CMEMs by light. The robust half-quantized conductance plateaus, due to CMEMs at quasienergies zero or half the driving frequency, are both fascinating and subtle because they only emerge after a summation over contributions from all Floquet sidebands. Such a distinctive transport signature can thus serve as a hallmark of photoinduced CMEMs in topological insulator-superconductor junctions.

cond-mat.mes-hall↗

arXiv2017

A computational study of two-terminal transport of Floquet quantum Hall insulators

Periodic driving fields can induce topological phase transitions, resulting in Floquet topological phases with intriguing properties such as very large Chern numbers and unusual edge states. Whether such Floquet topological phases could generate robust edge state conductance much larger than their static counterparts is an interesting question. In this paper, working under the Keldysh formalism, we study two-lead transport via the edge states of irradiated quantum Hall insulators using the method of recursive Floquet-Green's functions. Focusing on a harmonically-driven Hofstadter model, we show that quantized Hall conductance as large as $8e^2/h$ can be realized, but only after applying the so-called Floquet sum rule. To assess the robustness of edge state transport, we analyze the DC conductance, time-averaged current profile and local density of states. It is found that co-propagating chiral edge modes are more robust against disorder and defects as compared with the remarkable counter-propagating edge modes, as well as certain symmetry-restricted Floquet edge modes. Furthermore, we go beyond the wide-band limit, which is often assumed for the leads, to study how the conductance quantization (after applying the Floquet sum rule) of Floquet edge states can be affected if the leads have finite bandwidths. These results may be useful for the design of transport devices based on Floquet topological matter.

cond-mat.mes-hall↗

arXiv2017

Nonlinear Dirac Cones

Physics arising from two-dimensional~(2D) Dirac cones has been a topic of great theoretical and experimental interest to studies of gapless topological phases and to simulations of relativistic systems. Such $2$D Dirac cones are often characterized by a $π$ Berry phase and are destroyed by a perturbative mass term. By considering mean-field nonlinearity in a minimal two-band Chern insulator model, we obtain a novel type of Dirac cones that are robust to local perturbations without symmetry restrictions. Due to a different pseudo-spin texture, the Berry phase of the Dirac cone is no longer quantized in $π$, and can be continuously tuned as an order parameter. Furthermore, in an Aharonov-Bohm~(AB) interference setup to detect such Dirac cones, the adiabatic AB phase is found to be $π$ both theoretically and computationally, offering an observable topological invariant and a fascinating example where the Berry phase and AB phase are fundamentally different. We hence discover a nonlinearity-induced quantum phase transition from a known topological insulating phase to an unusual gapless topological phase.

cond-mat.mes-hall↗

arXiv2017

Notes on steady state current through a noninteracting quantum dot

A pedagogical introduction to matrix Green's function, focusing on its application to steady state transport through discrete-level quantum systems. Topics covered in the notes: 1. Retarded Green's function, spectral function and density of states 2. LCR system 3. Green's function and spectral function of LCR systems 4. Landauer formula and transmission coefficient 5. Semi-infinite tight-binding leads and wide-band limit 6. Linear conductance at zero temperature 7. Steady state current in a quantum dot: resonance and coherence 8. Steady state current across a tight-binding chain: metal-insulator transitions 9. Steady state current along the edge of a two-dimensional lattice: topologically quantized transport

cond-mat.mes-hall↗

arXiv2016

Floquet Semimetal with Floquet-band Holonomy

Exotic topological states of matter such as Floquet topological insulator or Floquet Weyl semimetal can be induced by periodic driving. This work proposes a Floquet semimetal with Floquet-band holonomy. That is, the system is gapless, but as a periodic parameter viewed as quasimomentum of a synthetic dimension completes an adiabatic cycle, each Floquet band as a whole exchanges with other Floquet bands. The dynamical manifestations of such Floquet-band holonomy are studied. Under open boundary conditions, we discover anomalous chiral edge modes localized only at one edge, winding around the entire quasienergy Brillouin zone, well separated from bulk states, and possessing holonomy different from bulk states. These remarkable properties are further exploited to realize quantized or half-quantized edge state pumping.

cond-mat.mes-hall↗

arXiv2016

Floquet Weyl Phases in a Three Dimensional Network Model

We study the topological properties of 3D Floquet bandstructures, which are defined using unitary evolution matrices rather than Hamiltonians. Previously, 2D bandstructures of this sort have been shown to exhibit anomalous topological behaviors, such as topologically-nontrivial zero-Chern-number phases. We show that the bandstructure of a 3D network model can exhibit Weyl phases, which feature "Fermi arc" surface states like those found in Weyl semimetals. Tuning the network's coupling parameters can induce transitions between Weyl phases and various topologically distinct gapped phases. We identify a connection between the topology of the gapped phases and the topology of Weyl point trajectories in k-space. The model is feasible to realize in custom electromagnetic networks, where the Weyl point trajectories can be probed by scattering parameter measurements.

cond-mat.mes-hall↗

arXiv2015

Floquet topological semimetal phases of an extended kicked Harper model

Recent discoveries on topological characterization of gapless systems have attracted interest in both theoretical studies and experimental realizations. Examples of such gapless topological phases are Weyl semimetals, which exhibit 3D Dirac cones (Weyl points), and nodal line semimetals, which are characterized by line nodes (two bands touching along a line). Inspired by our previous discoveries that the kicked Harper model exhibits many fascinating features of Floquet topological phases, in this manuscript we consider a generalization of the model, where two additional periodic system parameters are introduced into the Hamiltonian to serve as artificial dimensions, so as to simulate a 3D system. We observe that by increasing the hopping strength and the kicking strength of the system, many new band touching points at Floquet quasienergies $0$ and $π$ will start to appear. Some of them are Weyl points, while the others form line nodes in the parameter space. By taking open boundary conditions along the physical dimension, edge states analogues to Fermi arcs in static Weyl semimetal systems are observed. Finally, by designing an adiabatic pumping scheme, the chirality of the Weyl points and the $π$ Berry phase around line nodes can be manifested.

nlin.CD↗

arXiv2015

Finite-time Landau-Zener processes and counter-diabatic driving in open systems: beyond Born, Markov and Rotating-wave approximations

We investigate Landau-Zener processes modeled by a two-level quantum system, with its finite bias energy varied in time and in the presence of a single broadened cavity mode at zero temperature. By applying the hierarchy equation method to the Landau-Zener problem, we computationally study the survival fidelity of adiabatic states without Born, Markov, rotating-wave or other perturbative approximations. With this treatment it also becomes possible to investigate cases with very strong system-bath coupling. Different from a previous study of infinite-time Landau-Zener processes, the fidelity of the time-evolving state as compared with instantaneous adiabatic states shows non-monotonic dependence on the system-bath coupling and on the sweep rate of the bias. We then consider the effect of applying a counter-diabatic driving field, which is found to be useful in improving the fidelity only for sufficiently short Landau-Zener processes. Numerically exact results show that different counter-diabatic driving fields can have much different robustness against environment effects. Lastly, using a case study we discuss the possibility of introducing a dynamical decoupling field in order to eliminate the decoherence effect of the environment and at the same time to retain the positive role of a counter-diabatic field. Our work indicates that finite-time Landau-Zener processes with counter-diabatic driving offer a fruitful test bed to understand controlled adiabatic processes in open systems.

quant-ph↗

arXiv2015

Effects of Dephasing on Quantum Adiabatic Pumping with Nonequilibrium Initial States

Thouless's quantum adiabatic pumping is of fundamental interest to condensed-matter physics. It originally considered a zero-temperature equilibrium state uniformly occupying all the bands below a Fermi surface. In the light of recent direct simulations of Thouless's concept in cold-atom systems, this work investigates the dynamics of quantum adiabatic pumping subject to dephasing, for rather general initial states with nonuniform populations and possibly interband coherence. Using a theory based on pure-dephasing Lindblad evolution, we find that the pumping is contributed by two parts of different nature, a dephasing-modified geometric part weighted by initial Bloch state populations, and an interband-coherence-induced part compromised by dephasing, both of them being independent of the pumping time scale. The overall pumping reflects an interplay of the band topology, initial state populations, initial state coherence, and dephasing. Theoretical results are carefully checked in a Chern insulator model coupled to a pure-dephasing environment, providing a useful starting point to understand and coherently control quantum adiabatic pumping in general situations.

quant-ph↗

arXiv2015

Interband Coherence Induced Correction to Adiabatic Pumping in Periodically Driven Systems

Periodic driving can create topological phases of matter absent in static systems. In terms of the displacement of the position expectation value of a time-evolving wavepacket in a closed system, a type of adiabatic dynamics in periodically driven systems is studied for general initial states possessing coherence between different Floquet bands. Under one symmetry assumption, the displacement of the wavepacket center over one adiabatic cycle is found to be comprised by two components independent of the time scale of the adiabatic cycle: a weighted integral of the Berry curvature summed over all Floquet bands, plus an interband coherence induced correction. The found correction is beyond a naive application of the quantum adiabatic theorem but survives in the adiabatic limit due to interband coherence. Our theoretical results are hence of general interest towards an improved understanding of the quantum adiabatic theorem. Our theory is checked using a periodically driven superlattice model with nontrivial topological phases. In addition to probing topological phase transitions, the adiabatic dynamics studied in this work is now also anticipated to be useful in manifesting coherence and decoherence effects in the representation of Floquet bands.

cond-mat.mes-hall↗

arXiv2014

Aspects of Floquet Bands and Topological Phase Transitions in a Continuously Driven Superlattice

Recently the creation of novel topological states of matter by a periodic driving field has attracted great attention. To motivate further experimental and theoretical studies, we investigate interesting aspects of Floquet bands and topological phase transitions in a continuously driven Harper model. In such a continuously driven system with an odd number of Floquet bands, the bands are found to have nonzero Chern numbers in general and topological phase transitions take place as we tune various system parameters, such as the amplitude or the period of the driving field. The nontrivial Floquet band topology results in a quantized transport of Wannier states in the lattice space. For certain parameter choices, very flat yet topologically nontrivial Floquet bands may also emerge, a feature that is potentially useful for the simulation of physics of strongly correlated systems. Some cases with an even number of Floquet bands may also have intriguing Dirac cones in the spectrum. Under open boundary conditions, anomalous counter-propagating chiral edge modes and degenerate zero modes are also found as the system parameters are tuned. These results should be of experimental interest because a continuously driven system is easier to realize than a periodically kicked system.

cond-mat.quant-gas↗