arXiv subjects

Xiao-Wei Zhang

Publications and source records attributed to Xiao-Wei Zhang.

25 records · Page 2Linked to original sources

arXiv2023

Kekulé Moiré Superlattices

Moiré superlattices from stacks of van der Waals materials offer an exciting arena in the fields of condensed matter physics and materials science. Typically, these moiré superlattices consist of materials with identical or similar structures, and the long moiré period arises from a small twist angle or lattice mismatch. In this article, we discuss that long moiré period appears in a new moiré system by stacking two dissimilar van der Waals layers with large lattice mismatch, resulting in couplings between moiré bands from remote valleys in the momentum space. In this system, the first layer is reconstructed using a $\sqrt{3}$ by $\sqrt{3}$ supercell that resembles the Kekulé distortion in graphene, and such reconstruction becomes nearly commensurate with the second layer. This Kekulé moiré superlattice is realized in heterostructures of transition metal dichalcogenides and metal phosphorus trichalcogenides such as MoTe$_2$/MnPSe$_3$. By first-principles calculations, we demonstrate that the antiferromagnetic MnPSe$_3$ strongly couples the otherwise degenerate Kramers' valleys of MoTe$_2$, resulting in valley pseudospin textures that depend on Néel vector direction, stacking geometry, and external fields. With one hole per moiré supercell, we predict that the system can become a Chern insulator, of which the topology is tunable by external fields.

cond-mat.mes-hall↗

arXiv2022

Development of a compact high-resolution absolute gravity gradiometer based on atom interferometers

We present a compact high-resolution gravity gradiometer based on dual Rb-85 atom interferometers using stimulated Raman transitions. A baseline L=44.5 cm and an interrogation time T=130 ms are realized in a sensor head with volume of less than 100 liters. Experimental parameters are optimized to improve the short-term sensitivity while a rejection algorithm relying on inversion of the Raman wave vector is implemented to improve the long-term stability. After an averaging time of 17000 s, a phase resolution of 104 μrad is achieved, which corresponds to a gravity gradient resolution of 0.86 E. As far as we know, this is the sub-E atom gravity gradiometer with the highest level of compactness to date. After the evaluation and correction of system errors induced by light shift, residual Zeeman shift, Coriolis effect and self-attraction effect, the instrument serves as an absolute gravity gradiometer and with it the local gravity gradient is measured to be 3114 (53) E.

physics.atom-ph↗

arXiv2022

Non-perturbative $ab$ $initio$ approach for calculating the electrical conductivity of a liquid metal

We propose a non-perturbative $ab$ $initio$ approach to calculate the electrical conductivity of a liquid metal. Our approach is based on the Kubo formula and the theory of electron-phonon coupling (EPC), and unlike the conventional empirical approach based on the Kubo-Greenwood formula, fully takes into account the effect of coupling between electrons and moving ions. We show that the electrical conductivity at high temperature is determined by an EPC parameter $λ_{\mathrm{tr}}$, which can be inferred, non-perturbatively, from the correlation of electron scattering matrices induced by ions. The latter can be evaluated in a molecular dynamics simulation. Based on the density-functional theory and pseudopotential methods, we implement the approach in an $ab$ $initio$ manner. We apply it to liquid sodium and obtain results in good agreement with experiments. This approach is efficient and based on a rigorous theory, suitable for applying to general metallic liquid systems.

cond-mat.mtrl-sci↗

arXiv2021

First-principles estimation of the superconducting transition temperature of a metallic hydrogen liquid

We present a first-principles implementation of the stochastic path-integral approach proposed by Liu et al. [H. Liu, Y. Yuan, D. Liu, X.-Z. Li, and J. Shi, Phys. Rev. Research 2, 013340 (2020)] for estimating the superconducting transition temperature ($T_c$) of a liquid. The implementation is based on the all-electron projector-augmented-wave (PAW) method. We generalize Liu et al.'s formalism to accommodate the pseudo-description of electron states in the PAW method. A formula for constructing the overlap operator of the PAW method is proposed to eliminate errors due to the incompleteness of a pseudo-basis set. We apply the implementation to estimate $T_c$'s of metallic hydrogen liquids. It confirms Liu et al.'s prediction that metallic hydrogen can form a superconducting liquid.

cond-mat.supr-con↗

arXiv2021

Phonon-mediated exciton relaxation in two-dimensional semiconductors: selection rules and relaxation pathways

Exciton-phonon coupling (ExPC) is crucial for energy relaxation in semiconductors, yet the first-principles calculation of such coupling remains challenging, especially for low-dimensional systems. Here, an accurate algorithm for calculating ExPC is developed and applied in exciton relaxation problems in monolayer WSe2. Considering the interplay between the exciton wave functions and electron-phonon coupling (EPC) matrix elements, we find that ExPC shows distinct selection rules from the ones of EPC. By employing the Wannier exciton model, we generalize these selection rules, which state that the angular quantum numbers of the exciton must match the winding numbers of the EPC matrix elements for the ExPC to be allowed. To verify our theory and algorithm, we calculate inter-valley exciton relaxation pathways, which agrees well with a recent experiment.

cond-mat.mtrl-sci↗

arXiv2020

The influence of high-energy local orbitals and electron-phonon interactions on the band gaps and optical spectra of hexagonal boron nitride

We report $ab$ $initio$ band diagram and optical absorption spectra of hexagonal boron nitride ($h$-BN), focusing on unravelling how the completeness of basis set for $GW$ calculations and how electron-phonon interactions (EPIs) impact on them. The completeness of basis set, an issue which was seldom discussed in previous optical spectra calculations of $h$-BN, is found crucial in providing converged quasiparticle band gaps. In the comparison among three different codes, we demonstrate that by including high-energy local orbitals in the all-electron linearized augmented plane waves based $GW$ calculations, the quasiparticle direct and fundamental indirect band gaps are widened by $\sim$0.2 eV, giving values of 6.81 eV and 6.25 eV respectively at the $GW_0$ level. EPIs, on the other hand, reduce them to 6.62 eV and 6.03 eV respectively at 0 K, and 6.60 eV and 5.98 eV respectively at 300 K. With clamped crystal structure, the first peak of the absorption spectrum is at 6.07 eV, originating from the direct exciton contributed by electron transitions around $K$ in the Brillouin zone. After including the EPIs-renormalized quasiparticles in the Bethe-Salpeter equation, the exciton-phonon coupling shifts the first peak to 5.83 eV at 300 K, lower than the experimental value of $\sim$6.00 eV. This accuracy is acceptable to an $ab$ $initio$ description of excited states with no fitting parameter.

cond-mat.mtrl-sci↗

arXiv2006

Proof of Security of a High-Capacity Quantum Key Distribution Protocol

We prove the security of a high-capacity quantum key distribution protocol over noisy channels. By using entanglement purification protocol, we construct a modified version of the protocol in which we separate it into two consecutive stages. We prove their securities respectively and hence the security of the whole protocol.

quant-ph↗