arXiv subjects

Xiaosen Yang

Publications and source records attributed to Xiaosen Yang.

At least 37 records · Page 2Linked to original sources

arXiv2024

Enhancement in temperature sensing of a reservoir by Kerr-nonlinear resonator

The challenge of developing high-precision temperature sensors is an important issue that has recently received a lot of attention. In this work, we introduce an estimation technique to precisely measure the temperature of a quantum reservoir using a Kerr-nonlinear resonator with drive. Thermalization in our suggested protocol is assessed using Uhlmann-Jozsa fidelity, and then we utilize quantum Fisher information to evaluate the metrological potential of our thermometry scheme. We observe that increasing the Kerr nonlinearity coefficient and driving amplitude significantly enhances precision in the temperature estimation. Furthermore, we also explore the underlying physical mechanisms by analyzing probe purity in the steady state and evaluating the performance of homodyne versus heterodyne detection methods. Our results demonstrate that neither of these Gaussian measurements is optimal; instead, optimal homodyne detection always surpasses heterodyne detection.

quant-ph↗

arXiv2024

Tunable optical amplification and group delay in cavity magnomechanics

In this work, we theoretically investigate the controllable output probe transmission and group delay in a hybrid cavity magnomechanics (CMM) system. The setup comprises a gain (active) cavity and a passive (loss) cavity, which incorporates an optical parametric amplifier (OPA) and two yttrium iron garnet spheres to facilitate magnon-photon coupling. Unlike the single transparency window typically resulting from magnon-photon interactions, we also observe magnomechanically induced transparency due to nonlinear magnon-phonon interactions. Additionally, two absorption dips on either side of the central absorption dip can be asymmetrically modulated into amplification and absorption by varying different system parameters. A PT-symmetric to broken-PT-symmetric phase transition is observed in both balanced and unbalanced gain-to-loss scenarios. Notably, replacing the second passive cavity with an active one mitigates high absorption and introduces effective gain into the system. Our findings reveal that the group delay of the probe light can be adjusted between positive and negative values by modifying various system parameters. This study provides a robust platform for controlling light propagation in CMM systems, highlighting potential applications in optical communication and signal processing.

physics.optics↗

arXiv2024

Enhanced second-order sideband generation and slow-fast light via coupled opto- and magnomechanical microspheres

In this research, we investigate second-order sideband generation (SSG) and slow-fast light using a hybrid system comprised of two coupled opto- and magnomechanical microspheres, namely a YIG sphere and a silica sphere. The YIG sphere hosts a magnon mode and a vibration mode induced by magnetostriction, whereas the silica sphere has an optical whispering gallery mode and a mechanical mode coupled via optomechanical interaction. The mechanical modes of both spheres are close in frequency and are coherently coupled by the straightway physical contact between the two microspheres. We use a perturbation approach to solve the Heisenberg-Langevin equations, offering an analytical framework for transmission rate and SSG. Using experimentally feasible settings, we demonstrate that the transmission rate and SSG are strongly dependent on the magnomechanical, optomechanical, and mechanics mechanics coupling strengths (MMCS) between the two microspheres. The numerical results show that increasing the MMCS can enhance both the transmission rate and SSG efficiency, resulting in gain within our system. Our findings, in particular, reveal that the efficiency of the SSG can be effectively controlled by cavity detuning, decay rate, and pump power. Notably, our findings suggest that modifying the system parameters can alter the group delay, thereby regulating the transition between fast and slow light propagation, and vice versa. Our protocol provides guidelines for manipulating nonlinear optical properties and controlling light propagation, with applications including optical switching, information storage, and precise measurement of weak signals.

physics.optics↗

arXiv2024

Compasslike states in a thermal reservoir and fragility of their nonclassical features

Superposed photon-added and photon-subtracted squeezed-vacuum states exhibit sub-Planck phase-space structures and metrological potential similar to the original compass states (superposition of four coherent states), but are more closely tied to modern experiments. Here, we observe that these compasslike states are highly susceptible to loss of quantum coherence when placed in contact with a thermal reservoir; that is, the interaction with the thermal reservoir causes decoherence, which progressively suppresses the capacity of these states to exhibit interference traits. We focus on the sub-Planck structures of these states and find that decoherence effects on these features are stronger with increasing the average thermal photon number of the reservoir, the squeezing parameter, or the quantity of added (or subtracted) photons to the squeezed-vacuum states. Furthermore, we observe that the sub-Planck structures of the photon-subtracted case survive comparatively longer in the thermal reservoir than their counterparts in the photon-added case, and prolonged contact with the thermal reservoir converts these compasslike states into a classical state.

quant-ph↗

arXiv2024

Generalized bulk-boundary correspondence in periodically driven non-Hermitian systems

We present a pedagogical review of the periodically driven non-Hermitian systems, particularly on the rich interplay between the non-Hermitian skin effect and the topology. We start by reviewing the non-Bloch band theory of the static non-Hermitian systems and discuss the establishment of its generalized bulk-boundary correspondence. Ultimately, we focus on the non-Bloch band theory of two typical periodically driven non-Hermitian systems: harmonically driven non-Hermitian system and periodically quenched non-Hermitian system. The non-Bloch topological invariants were defined on the generalized Brillouin zone and the real space wave functions to characterize the Floquet non-Hermtian topological phases. Then, the generalized bulk-boundary correspondence was established for the two typical periodically driven non-Hermitian systems. Additionally, we review novel phenomena in the higher-dimensional periodically driven non-Hermitian systems, including Floquet non-Hermitian higher-order topological phases and Floquet hybrid skin-topological modes. The experimental realizations and recent advances have also been surveyed. Finally, we end with a summarization and hope this pedagogical review can motivate further research on Floquet non-Hermtian topological physics.

cond-mat.mes-hall↗

arXiv2024

Non-Hermitian second-order topological superconductors

The topology of non-Hermitian systems is fundamentally changed by the non-Hermitian skin effect, which leads to the generalized bulk-boundary correspondence. Based on the non-Bloch band theory, we get insight into the interplay between the non-Hermiticity and the second-order topological superconductors in two spatial dimensions. We investigate that the non-Hermiticity drives both the bulk states and topological edge modes to accumulate toward opposite corners of the system depending on the particle and hole degrees of freedom protected by the particle-hole symmetry. Furthermore, the degeneracy of the Majorana corner modes can be broken in terms of both the eigenenergies and the eigenstates. Through an edge theory analysis, we elucidate the impact of non-Hermiticity and enable the extension of higher-order topological superconductors to the realm of non-Hermitian systems. We show that $Z_2$ skin effect and $Z_2$ skin-topological modes reveal the universal characteristics of non-Hermitian second-order topological superconductors and the generalized bulk-boundary correspondence is further enriched by the particle-hole symmetry.

cond-mat.supr-con↗

arXiv2023

Ab Initio Studies on Interactions in K$_3$C$_{60}$ under High Pressure

Fullerene solids doped with alkali metals (A$_3$C$_{60}$, A = K, Rb, Cs) exhibit a superconducting transition temperature ($T_c$) as high as 40 K, and their unconventional superconducting properties have been a subject of debate. With application of high pressure on K$_3$C$_{60}$ and Rb$_3$C$_{60}$, the experiments demonstrate the decrease of $T_c$. In this paper, we focus on K$_3$C$_{60}$ and derive the structure of K$_3$C$_{60}$ under different pressures based on first-principles calculations, exploring the trends of Coulomb interactions at various pressures. By utilizing the Maximally Localized Wannier function approach, Constrained Density Functional Perturbation Theory (cDFPT), and Constrained Random Phase Approximation (cRPA), we construct a microscopic low-energy model near the Fermi level. Our results strongly indicate that, in the K$_3$C$_{60}$ system, as pressure increases, the effect of phonons is the key to intraorbital electron pairing. There is a dominance of the phonon-driven superconducting mechanism at high pressure.

cond-mat.supr-con↗

arXiv2022

Topological energy braiding of the non-Bloch bands

The non-Hermitian skin effect, as a unique feature of non-Hermitian systems, will break the topological energy braiding of the Bloch bands in open boundary systems. Going beyond the Bloch band theory, we unveil the energy braiding of the non-Bloch bands by introducing a one-dimensional non-Hermitian tight-binding model. We find an entirely new generic class of topological non-Bloch bands such as Hopf link, which is generally generated by the non-Hermitian skin effect. The energy braiding is topologically robust against any perturbations without gap closing. Furthermore, non-Bloch topological invariants are proposed based on the generalized Brillouin zone to characterize the topology of these non-Bloch bands. The topological phase transition between the distinct phases occurs with the non-Bloch bands touching at exceptional points. We hope that our work can shed light on the topological energy braiding of the non-Bloch bands for non-Hermitian systems.

cond-mat.mes-hall↗

arXiv2021

Chiral Majorana Edge Modes and Vortex Majorana Zero Modes in Superconducting Antiferromagnetic Topological Insulator

The antiferromagnetic topological insulator (AFTI) is topologically protected by the combined time-reversal and translational symmetry $\mathcal{T}_c$. In this paper we investigate the effects of the $s$-wave superconducting pairings on the multilayers of AFTI, which breaks $\mathcal{T}_c$ symmetry and can realize quantum anomalous Hall insulator with unit Chern number. For the weakly coupled pairings, the system corresponds to the topological superconductor (TSC) with the Chern number $C=\pm 2$. We answer the following questions whether the local Chern numbers and chiral Majorana edge modes of such a TSC distribute around the surface layers. By the numerical calculations based on a theoretic model of AFTI, we find that when the local Chern numbers are always dominated by the surface layers, the wavefunctions of chiral Majorana edge modes must not localize on the surface layers and show a smooth crossover from spatially occupying all layers to only distributing near the surface layers, similar to the hinge states in a three dimensional second-order topological phases. The latter phase can be distinguished from the former phase by the measurements of the local density of state. In addition we also study the superconducting vortex phase transition in this system and find that the exchange field in the AFTI not only enlarges the phase space of topological vortex phase but also enhances its topological stability. These conclusions will stimulate the investigations on superconducting effects of AFTI and drive the studies on chiral Majorana edge modes and vortex Majorana zero modes into a new era.

cond-mat.supr-con↗

arXiv2021

Topological superfluids in two-dimensional Fermi gas with Rashba spin-orbit coupling

The realization of spin-orbit coupling (SOC) in ultracold atoms has triggered an intensive exploring of topological superfluids in the degenerate Fermi gases based on mean-field theory, which has not yet been reported in experiments. Here, we demonstrate the topological phase transitions in the system via the numerically exact quantum Monte Carlo method. Without prior assumptions, our unbiased real-space calculation shows that spin-orbit coupling can stabilize an unconventional pairing in the weak SOC regime, in which the Fulde-Ferrell-Larkin-Ovchinnikov pairing coexists with the Bardeen-Cooper-Schrieffer pairing. Furthermore, we use the jumps in the spin polarization at the time-reversal invariant momenta to qualify the topological phase transition, where we find the critical exponent deviated from the mean-field theory. Our results pave the way for the searching of unconventional pairing and topological superfluids with degenerate Fermi gases.

cond-mat.quant-gas↗

arXiv2021

Universal characteristics of one-dimensional non-Hermitian superconductors

We establish a non-Bloch band theory for one-dimensional(1D) non-Hermitian topological superconductors. The universal physical properties of non-Hermitian topological superconductors are revealed based on the theory. According to the particle-hole symmetry, there exist reciprocal particle and hole loops of generalized Brillouin zone (GBZ). The critical point of quantum phase transition, where the energy gap closes, appears when the particle and hole loops intersect and their values of GBZ satisfy |β| = 1. If the non-Hermitian system has skin modes, these modes should be Z2 style, i.e., the corresponding eigenstates of particle and hole localize at opposite ends of an open chain, respectively. The non-Bloch band theory is applied to two examples, non-Hermitian p- and s-wave topological superconductors. Topological phase transitions occur at β_{c}= \pm 1 in the two systems. In terms of Majorana Pfaffian, a Z2 non-Bloch topological invariant is defined to establish the non-Hermitian bulk-boundary correspondence in non-Hermitian superconductors.

cond-mat.supr-con↗

arXiv2020

Non-Hermitian Bulk-Boundary Correspondence in Periodically Driven System

Bulk-boundary correspondence, connecting the bulk topology and the edge states, is an essential principle of the topological phases. However, the bulk-boundary correspondence is broken down in general non-Hermitian systems. In this paper, we construct one-dimensional non-Hermitian Su-Schrieffer-Heeger model with periodic driving that exhibits non-Hermitian skin effect: all the eigenstates are localized at the boundary of the systems, whether the bulk states or the zero and the $π$ modes. To capture the topological properties, the non-Bloch winding numbers are defined by the non-Bloch periodized evolution operators based on the generalized Brillouin zone. Furthermore, the non-Hermitian bulk-boundary correspondence is established: the non-Bloch winding numbers ($W_{0,π}$) characterize the edge states with quasienergies $ε=0, π$. In our non-Hermitian system, a novel phenomenon can emerge that the robust edge states can appear even when the Floquet bands are topological trivial with zero non-Bloch band invariant, which is defined in terms of the non-Bloch effective Hamiltonian. We also show that the relation between the non-Bloch winding numbers ($W_{0,π}$) and the non-Bloch band invariant ($\mathcal{W}$): $\mathcal{W}= W_{0}- W_π$.

cond-mat.mes-hall↗

arXiv2019

Non-Hermitian Weyl Semimetals: Non-Hermitian Skin Effect and non-Bloch Bulk-Boundary Correspondence

We investigate novel features of three dimensional non-Hermitian Weyl semimetals, paying special attention to its unconventional bulk-boundary correspondence. We use the non-Bloch Chern numbers as the tool to obtain the topological phase diagram, which is also confirmed by the energy spectra from our numerical results. It is shown that, in sharp contrast to Hermitian systems, the conventional (Bloch) bulk-boundary correspondence breaks down in non-Hermitian topological semimetals, which is caused by the non-Hermitian skin effect. We establish the non-Bloch bulk-boundary correspondence for non-Hermitian Weyl semimetals: the Fermi-arc edge modes are determined by the non-Bloch Chern number of the bulk bands. Moreover, these Fermi-arc edge modes can manifest as the unidirectional edge motion, and their signatures are consistent with the non-Bloch bulk-boundary correspondence. Our work establish the non-Bloch bulk-boundary correspondence for non-Hermitian topological semimetals.

cond-mat.mes-hall↗

arXiv2018

Dynamical instability towards finite-momentum pairing in quenched BCS superconducting phases

In this work we numerically investigate the fate of the Bardeen-Cooper-Schrieffer (BCS) pairing in the presence of quenched phase under Peierls substitution using time-dependent real space and momentum space Bogoliubov-de Gennes equation methods and Anderson pseudospin representation method. This kind of phase imprint can be realized by modulating electric field in ultracold atoms and illumining of THz optical pump pulse in solids with conventional and unconventional superconductors. In the case of weak phase imprint, the BCS pairing is stable; while in the strong phase imprint, instability towards finite-momentum pairing is allowed, in which the real space and momentum space methods yield different results. In the pulsed gauge potential, we find that this instability will not happen even with much stronger vector potential. We also show that the uniform and staggered gauge potentials yield different behaviors. While the staggered potential induces transition from the BCS pairing to over-damped phase, the uniform gauge may enhance the pairing and will not induce to the over-damped phase. These result may shade light on the realization of finite momentum pairing, such as Fulde-Ferrell-Larkin-Ovchinnikov phase with dynamical modulation.

cond-mat.quant-gas↗

arXiv2017

Type-I and type-II topological nodal superconductors with $s$-wave interaction

Topological nodal superconductors are generally realized based on unconventional pairings. In this work, we propose a minimal model to realize these topological nodal phases with only $s$-wave interaction. In our model the linear and quadratic spin-orbit couplings along the two directions break the isotropy in momentum space and introduce effective unconventional pairings on the Fermi surface. This model may support different nodal superconducting phases characterized by either winding number in BDI class or Pfaffian in D class at the particle-hole invariant axes. In the vicinity of the nodal points the effective Hamiltonian can be described by either type-I or type-II Dirac equation; and the crossover between these two nodal points can be driven by external Zeeman fields. We show that these nodal phases are robust against weak disorders, thus are possible to be realized in experiments with real materials. The smoking-gun evidences to verify these phases based on scanning tunneling spectroscopy are also briefly discussed.

cond-mat.supr-con↗

arXiv2014

Floquet Topological Superfluid and Majorana Zero Modes in Two-Dimensional Periodically Driven Fermi Systems

We propose a simple approach to realize two-dimensional Floquet topological superfluid by periodically tuning the depth of square optical lattice potentials. We show that the periodic driving can induce topological phase transitions between trivial superfluid and Floquet topological superfluid. For this systems we verify the anomalous bulk-boundary correspondence, namely that the robust chiral Floquet edge states can appear even when the winding number of all the bulk Floquet bands is zero. We establish the existence of two Floquet Majorana zero modes separated in the quasienergy space, with $\varepsilon_{0,π}=0,π/T$ at the topological defects.

cond-mat.quant-gas↗

arXiv2014

Floquet Quantum Spin Hall Insulator in Cold Atomic Systems

For cold atomic systems, varying the optical lattice potential periodically provides a general and simple way to drive the system into phases with nontrivial topology. Besides its simplicity, this driving approach, compared to the usual driving approach by exerting an external electromagnetic field to the static system, has the merit that it does not break the original static system's time-reversal symmetry at any given time. Based on this approach, we find that a trivial insulator with time-reversal symmetry can be driven into a Floquet quantum spin Hall insulator. This novel state of matter can stably host one or two pair of gapless helical states on the same boundary, which suggests this state is not a simple analog of the quantum spin Hall insulator. The effect of a time-reversal-symmetry-breaking periodic perturbation, the stability of the novel states, and this new driving approach to a system without time-reversal symmetry are discussed.

cond-mat.quant-gas↗

arXiv2013

The Pairing of Spin-orbit Coupled Fermi Gas in Optical Lattice

We investigate Rashba spin-orbit coupled Fermi gases in square optical lattice by using the determinant quantum Monte Carlo (DQMC) simulations which is free of the sign-problem. We show that the Berezinskii-Kosterlitz-Thoules phase transition temperature is firstly enhanced and then suppressed by spin-orbit coupling in the strong attraction region. In the intermediate attraction region, spin-orbit coupling always suppresses the transition temperature. We also show that the spin susceptibility becomes anisotropic and retains finite values at zero temperature.

cond-mat.quant-gas↗