arXiv subjects

Jingkai Quan

Publications and source records attributed to Jingkai Quan.

6 recordsLinked to original sources

arXiv2026

Symmetry Rules for Cavity Materials Engineering with Linearly Polarized Vacuum Fields

Cavity materials engineering, aiming to manipulate material properties by coupling to vacuum fluctuations inside a cavity, is a rapidly advancing field. Despite significant progress, most studies to date have focused on specific materials and cavity configurations. Here, through a comprehensive group-theoretical analysis, we establish general symmetry rules for cavity materials engineering with linearly polarized cavity photon modes. By analyzing the symmetry of the effective photon-free quantum-electrodynamics Hamiltonian, we provide a complete classification of the symmetry-breaking patterns induced by cavity modes for all crystallographic point groups. The power of this framework is then demonstrated by quantum-electrodynamical density functional theory calculations. In particular, we explain the distinct cavity-induced lifting of band degeneracies in cubic BaTiO$_3$ for different cavity mode configurations, and the cavity-modified infrared and Raman spectra of monolayer MoS$_2$ due to symmetry breaking. Our results highlight the central role of symmetry in cavity materials engineering and provide general guidelines for future studies in this field.

cond-mat.mtrl-sci↗

arXiv2026

Macroscopic Polarization and Magnetization from Cavity Vacuum Fluctuations

Cavity light-matter interaction has recently emerged as a new avenue for manipulating material properties without driving fields. Here, we demonstrate that cavity vacuum fluctuations can induce macroscopic polarization (magnetization), even in materials that lack spontaneous polarization (net magnetization) in free space. Starting from the effective photon-free quantum-electrodynamics Hamiltonian, we identify all crystallographic (magnetic) point groups that allow such cavity-induced responses. We derive the form of the corresponding response tensors based on symmetry analysis, whose elements can be obtained by quantum electrodynamical density functional theory (QEDFT) calculations. As representative examples, we show that the cavity-induced polarization in $\alpha$-quartz can be continuously controlled by rotating the cavity. For antiferromagnetic Mn$_3$Sn, we demonstrate that cavity-induced symmetry breaking generates an out-of-plane magnetization, accompanied by an anomalous Hall conductivity component that is forbidden outside the cavity. Our work establishes symmetry as a guiding principle for cavity materials engineering and provides a route for controlling polarization and magnetization through quantum vacuum fluctuations, i.e., cavity materials engineering.

cond-mat.mtrl-sci↗

arXiv2025

Efficient Band Structure Unfolding with Atom-centered Orbitals: General Theory and Application

Band structure unfolding is a key technique for analyzing and simplifying the electronic band structure of large, internally distorted supercells that break the primitive cell's translational symmetry. In this work, we present an efficient band unfolding method for atomic orbital (AO) basis sets that explicitly accounts for both the non-orthogonality of atomic orbitals and their atom-centered nature. Unlike existing approaches that typically rely on a plane-wave representation of the (semi-)valence states, we here derive analytical expressions that recasts the primitive cell translational operator and the associated Bloch-functions in the supercell AO basis. In turn, this enables the accurate and efficient unfolding of conduction, valence, and core states in all-electron codes, as demonstrated by our implementation in the all-electron ab initio simulation package FHI-aims, which employs numeric atom-centered orbitals. We explicitly demonstrate the capability of running large-scale unfolding calculations for systems with thousands of atoms and showcase the importance of this technique for computing temperature-dependent spectral functions in strongly anharmonic materials using CuI as example.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast dynamics of local charge order in a THz-induced metastable quantum state

Controlling quantum materials with ultrafast light pulses enables access to transient and metastable states that are inaccessible under equilibrium conditions. Yet their local dynamics remain poorly understood due to the challenge of resolving ultrafast processes with angstrom-scale spatial resolution. Here, we use terahertz scanning tunnelling microscopy (THz-STM) to probe coherent collective dynamics within a THz-induced metastable state in the layered charge density wave (CDW) material 1T-TaS2. Following ultrafast photoexcitation, we locally resolve coherent oscillations of the CDW amplitude mode at 2.5 THz together with two previously unreported modes at 1.3 THz and 0.7 THz. Comparison with phonon calculations identifies these as interlayer breathing and shear vibrations that are sensitive to the stacking configuration. These coherent dynamics are observed within a THz-induced metastable state that exhibits long-lived and spatially inhomogeneous modifications of the local density of states within the insulating gap, while higher THz fields drive a local redistribution and disordering of Star-of-David clusters near defects and domain boundaries. Our results suggest that the THz-induced metastable state involves a modification of the local interlayer stacking configuration, and demonstrate the role of interlayer degrees of freedom in the ultrafast dynamics of light-induced phases in layered quantum materials.

cond-mat.mes-hall↗

arXiv2024

Temperature-dependent Electronic Spectral Functions from Band-Structure Unfolding

The electronic band structure, describing the periodic dependence of electronic quantum states on lattice momentum in reciprocal space, is a fundamental concept in solid-state physics. However, it's only well-defined for static nuclei. To account for thermodynamic effects, this concept must be generalized by introducing the temperature-dependent spectral function, which characterizes the finite-width distributions of electronic quantum states at each reciprocal vector. Many-body perturbation theory can compute spectral functions and associated observables, but it approximates the dynamics of nuclei and its coupling to the electrons using the harmonic approximation and linear-order electron-phonon coupling elements, respectively. These approximations may fail at elevated temperatures or for mobile atoms. To avoid inaccuracies, the electronic spectral function can be obtained non-perturbatively, capturing higher-order couplings between electrons and vibrational degrees of freedom. This process involves recovering the representation of supercell bands in the first Brillouin zone of the primitive cell, a process known as unfolding. In this contribution, we describe the implementation of the band-structure unfolding technique in the electronic-structure theory package FHI-aims and the updates made since its original development.

cond-mat.mtrl-sci↗

arXiv2024

Carrier Mobility of Strongly Anharmonic Materials from First Principles

First-principle approaches for phonon-limited electronic transport are typically based on many-body perturbation theory and transport equations. With that, they rely on the validity of the quasi-particle picture for electrons and phonons, which is known to fail in strongly anharmonic systems. In this work, we demonstrated the relevance of effects beyond the quasi-particle picture by combining ab initio molecular dynamics and the Kubo-Greenwood (KG) formalism to establish a non-perturbative, stochastic method to calculate carrier mobilities while accounting for all orders of anharmonic and electron-vibrational couplings. In particular, we propose and exploit several numerical strategies that overcome the notoriously slow convergence of the KG formalism for both electronic and nuclear degree of freedom in crystalline solids. The capability of this method is demonstrated by calculating the temperature-dependent electron mobility of the strongly anharmonic oxide perovskites SrTiO3 and BaTiO3 across a wide range of temperatures. We show that the temperature-dependence of the mobility is largely driven by anharmonic, higher-order coupling effects and rationalize these trends in terms of the non-perturbative electronic spectral functions.

cond-mat.mtrl-sci↗