arXiv subjects

Anwei Zhang

Publications and source records attributed to Anwei Zhang.

14 recordsLinked to original sources

arXiv2026

Nonlinear Ohmic electromagnetic response

We systematically investigate nonlinear Ohmic responses in second-harmonic generation and bilinear magnetoelectric effects within the Matsubara Green's function formalism. The optical nonlinear Ohmic conductivity is shown to consist of a nonlinear Drude-like part and an intrinsic term determined by the fully symmetrized normalized quantum metric dipole. Notably, we predict a previously unrecognized intrinsic Ohmic conductivity arising from band geometry in the bilinear magnetoelectric response, which exhibits transverse behavior similar to its optical counterpart. Using a two-dimensional Dirac model, we demonstrate that this geometrically induced nonlinear Ohmic response is observable in materials with high Fermi velocity and narrow band gaps. Our work provides a systematic quantum field-theoretic framework for describing nonlinear Ohmic transport in condensed matter systems.

cond-mat.mes-hall↗

arXiv2025

Quantum theory of nonlinear electromagnetic response

In recent years, the investigation of nonlinear electromagnetic responses has received significant attention due to its potential for elucidating the quantum properties of matter. Although remarkable progress has been achieved in developing quantum theories of nonlinear responses to electric field, a comprehensive quantum theory framework that systematically addresses nonlinear responses to both electric and magnetic fields has yet to be thoroughly discussed. Here, we present a systematic quantum theory of nonlinear electromagnetic response using the Matsubara Green's function approach, which explicitly incorporates the wave vector dependence of external electromagnetic fields. We provide diagrammatic representation and reveal the general properties of transport coefficients. We apply our theory to second-order responses, deriving the nonlinear Hall effects and magneto-nonlinear Hall effects in both time-reversal symmetric and time-reversal breaking systems. These effects stem from diverse quantum geometric quantities. Additionally, we analyze the contributions arising from the Zeeman interaction. Our work presents a unified quantum theory of nonlinear electromagnetic response, paving the way for further exploration of novel phenomena in this field.

cond-mat.mes-hall↗

arXiv2023

Magneto-nonlinear Hall effect in time-reversal breaking system

Magneto-nonlinear Hall effect is known to be intrinsic and requires time-reversal symmetry. Here we show that a new type of magneto-nonlinear Hall effect can occur in the time-reversal breaking materials within the second-order response to in-plane electric and vertical magnetic fields. Such a Hall response is generated by the oscillation of the electromagnetic field and has a quantum origin arising from a geometric quantity associated with the Berry curvature and band velocity. We demonstrate that the massive Dirac model of LaAlO3/LaNiO3/LaAlO3 quantum well can be used to detect this Hall effect. Our work widens the theory of the Hall effect in the time-reversal breaking system by proposing a new kind of nonlinear electromagnetic response.

cond-mat.mes-hall↗

arXiv2022

Flat bands in Network Superstructures of Atomic Chains

We investigate the origin of the ubiquitous existence of flat bands in the network superstructures of atomic chains, where one-dimensional(1D) atomic chains array periodically. While there can be many ways to connect those chains, we consider two representative ways of linking them, the dot-type and triangle-type links. Then, we construct a variety of superstructures, such as the square, rectangular, and honeycomb network superstructures with dot-type links and the honeycomb superstructure with triangle-type links. These links provide the wavefunctions with an opportunity to have destructive interference, which stabilizes the compact localized state(CLS). The CLS is a localized eigenstate whose amplitudes are finite only inside a finite region and guarantees the existence of a flat band. In the network superstructures, there exist multiple flat bands proportional to the number of atoms of each chain, and the corresponding eigenenergies can be found from the stability condition of the compact localized state. Finally, we demonstrate that the finite bandwidth of the nearly flat bands of the network superstructures arising from the next-nearest-neighbor hopping processes can be suppressed by increasing the length of the chains consisting of the superstructures.

cond-mat.str-el↗

arXiv2022

Geometric Origin of Intrinsic Spin Hall Effect in an Inhomogeneous Electric Field

In recent years, the spin Hall effect has received great attention because of its potential application in spintronics and quantum information processing and storage. However, this effect is usually studied under the external homogeneous electric field. Understanding how the inhomogeneous electric field affects the spin Hall effect is still lacking. Here, we investigate a two-dimensional two-band time-reversal symmetric system and give an expression for the intrinsic spin Hall conductivity in the presence of the inhomogeneous electric field, which is shown to be expressed through gauge-invariant geometric quantities. On the other hand, when people get physical intuition on transport phenomena from the wave packet, one issue appears. It is shown that the conductivity obtained from the conventional wave packet approach cannot be fully consistent with the one predicted by the Kubo-Greenwood formula. Here, we attempt to solve this problem.

cond-mat.mes-hall↗

arXiv2021

Revealing Chern number from quantum metric

Chern number is usually characterized by Berry curvature. Here, by investigating the Dirac model of even-dimensional Chern insulator, we give the general relation between Berry curvature and quantum metric, which indicates that the Chern number can be encoded in quantum metric as well as the surface area of the Brillouin zone on the hypersphere embedded in Euclidean parameter space. We find that there is a corresponding relationship between the quantum metric and the metric on such hypersphere. We show the geometrical property of quantum metric. Besides, we give a protocol to measure the quantum metric in the degenerate system.

quant-ph↗

arXiv2021

Relating the topology of Dirac Hamiltonians to quantum geometry: When the quantum metric dictates Chern numbers and winding numbers

Quantum geometry has emerged as a central and ubiquitous concept in quantum sciences, with direct consequences on quantum metrology and many-body quantum physics. In this context, two fundamental geometric quantities are known to play complementary roles: the Fubini-Study metric, which introduces a notion of distance between quantum states defined over a parameter space, and the Berry curvature associated with Berry-phase effects and topological band structures. In fact, recent studies have revealed direct relations between these two important quantities, suggesting that topological properties can, in special cases, be deduced from the quantum metric. In this work, we establish general and exact relations between the quantum metric and the topological invariants of generic Dirac Hamiltonians. In particular, we demonstrate that topological indices (Chern numbers or winding numbers) are bounded by the quantum volume determined by the quantum metric. Our theoretical framework, which builds on the Clifford algebra of Dirac matrices, is applicable to topological insulators and semimetals of arbitrary spatial dimensions, with or without chiral symmetry. This work clarifies the role of the Fubini-Study metric in topological states of matter, suggesting unexplored topological responses and metrological applications in a broad class of quantum-engineered systems.

cond-mat.mes-hall↗

arXiv2020

Additivity of Röntgen term and recoil-induced correction to spontaneous emission rate

For a moving atom, the spontaneous emission rate is modified due to the contributions from two factors, the Röntgen interaction term and the recoil effect induced by the emitted photon. Here we investigate the emission rate of a uniformly moving atom near a perfectly conducting plate and obtain the corrections induced by these two factors. We find that the corrections individually induced by the Röntgen term and the recoil effect can be simply added and result in the total correction to the decay rate. Moreover, it is shown that the Röntgen term gives positive correction, while the recoil effect induces negative correction. Our work paves the way towards the future studies of the light-matter interaction for the moving particle in quantum optics.

quant-ph↗

arXiv2019

Frozen condition of quantum coherence for atoms on a stationary trajectory

We consider two co-moving atoms on a stationary trajectory and develop a formalism to characterize the properties of such atoms. We give a criterion under which quantum coherence (QC) is frozen to a nonzero value and show that the frozen condition (FC) is not so sensitive to the initial condition of state. We introduce the concept super- and subradiant spontaneous excitation rates which plays an equivalent role as conventional collective emission rates in the evolution of quantum coherence. We also give the general relationship between the quantities characterizing properties of atoms in thermal bath and show that the enhanced quantum coherence and subradiant state can be gained from initial state.

quant-ph↗

arXiv2019

Tunable Topologically-protected Super- and Subradiant Boundary States in One-Dimensional Atomic Arrays

Single-photon super- and subradiance are important for the quantum memory and quantum information. We investigate one-dimensional atomic arrays under the spatially periodic magnetic field with a tunable phase, which provides a distinctive physics aspect of revealing exotic two-dimensional topological phenomena with a synthetic dimension. A butterfly-like nontrivial bandstructure associated with the non-Hermitian physics involving strong long-range interactions has been discovered. It leads to pairs of topologically-protected edge states, which exhibit the robust super- or subradiance behavior, localized at the boundaries of the atomic arrays. This work opens an avenue of exploring an interacting quantum optical platform with synthetic dimensions pointing to potential implications for quantum sensing as well as the super-resolution imaging.

quant-ph↗

arXiv2017

Thermal Casimir Effect in Kerr Space-time

We investigate the thermal Casimir effect of a massless scalar field for two parallel plates moving in the equatorial orbit in Kerr space-time. Under the assumption that the typical cavity size is much smaller than the orbital radius, proposed by Sorge, we deduce the analytical expression of the renormalized free energy in this curved space-time. We also get the analytical representation for the renormalized internal energy, and find that there is a thermal correction to the Casimir energy, which depends on the proper temperature and the proper geometrical parameters of the plates. The asymptotic behavior of the Casimir free energy, entropy and internal energy at low temperature is also investigated.

quant-ph↗

arXiv2017

Theoretical analysis of Casimir and thermal Casimir effect in stationary space-time

We investigate Casimir effect as well as thermal Casimir effect for a pair of parallel perfectly plates placed in general stationary space-time background. It is found that the Casimir energy is influenced by the 00-component of metric and the corresponding quantity in dragging frame. We give a scheme to renormalize thermal correction to free energy in curved space-time. It is shown that the thermal corrections to Casimir thermodynamic quantities not only depend on the proper temperature and proper geometrical parameters of the plates, but also on the determinant of space-time metric.

quant-ph↗

arXiv2017

Protection of entanglement between two two-level atoms

The dynamical evolution of entanglement between two polarizable two-level atoms in weak interaction with electromagnetic vacuum fluctuations is investigated. We find that, for initial Bell state $ψ^{\pm}$, the decay rate of entanglement between atoms is just the superradiant spontaneous emission rate, which depends not only on the spontaneous emission rate of atom but also on the modulation of the spontaneous emission rate due to the presence of another atom. It is shown that, with the presence of a boundary, the entanglement between transversely polarizable atoms can be protected for a very long time. It is pointed out that when the two atoms in initial Bell state $ψ^{-}$ are put close enough , the entanglement between them can also be protected.

quant-ph↗

arXiv2016

The formalism for energy changing rate of an accelerated atom coupled with electromagnetic vacuum fluctuations

The structure of the rate of variation of the atomic energy for an arbitrary stationary motion of the atom in interaction with a quantum electromagnetic field is investigated. Our main purpose is to rewrite the formalism in Ref. \cite{zz} and to deduce the general expressions of the Einstein A coefficients of an atom on an arbitrary stationary trajectory. The total rate of change of the energy and Einstein coefficients of the atom near a plate with finite temperature or acceleration are also investigated.

quant-ph↗