arXiv subjects

Chumin Wang

Publications and source records attributed to Chumin Wang.

8 recordsLinked to original sources

arXiv2026

Analytical solution of the Eliashberg equations for strong-coupling superconductivity in hydrides

The recent discovery of room-temperature superconductivity in hydrogen-rich materials under extreme pressures has renewed interest in phonon-mediated pairing mechanisms described by the Eliashberg theory, which generalizes the BCS framework by incorporating phonon retardation effects. In this article, we present an analytical solution to the isotropic Eliashberg equations for the superconducting critical temperature, formulated within the Debye model. This solution exhibits good agreement with fully selfconsistent numerical calculations across both weak- and strong-coupling regimes, correctly reproducing the exponential and square-root dependences on the electron-phonon coupling parameter. We further apply the solution to YH6, benchmarking against experiment and the McMillan-Allen-Dynes formula. More broadly, across a diverse set of hydride superconductors, the predicted critical temperature shows scaling consistency with ab initio calculations and experimental data.

cond-mat.supr-con↗

arXiv2025

Self-consistent solution of Eliashberg equations for metal hydride superconductors

In recent years, the quest for high critical-temperature superconductors has increasingly focused on metal and molecular hydrides, which have demonstrated potential for superconductivity at or near room temperature under extremely high pressures. Such hydrides were first proposed by N. W. Ashcroft in 1968, because hydrogen-rich materials possess elevated vibrational frequencies due to the low atomic mass of hydrogen. This article presents a self-consistent solution to the Eliashberg equations for analysing superconductivity in hydrides, contrasting with the commonly used McMillan-Allen-Dynes parameterized formula. We also analyse effects of the electron-phonon spectral function and the broadening parameters arising from phonon lifetime and sample imperfections on the superconducting critical temperature. Finally, both theoretical approaches are applied to a typical metal hydride superconductor, and the reliability of self-consistent solutions is validated against the experimentally measured critical temperature.

cond-mat.supr-con↗

arXiv2023

Superconductivity in correlated carbon nanotubes under pressure: A Bogoliubov-de Gennes study

In contrast to most microscopic theories of superconductivity based on the reciprocal space, the Bogoliubov-de Gennes (BdG) formalism provides a real-space alternative for addressing inhomogeneous systems. In this article, we study the superconducting states in correlated single-walled carbon nanotubes (SWNTs) with curvature and spin-orbit corrections, as well as the inter-tube interaction through a connecting molecule using an attractive Hubbard model. The results reveal a close relationship between the on-site superconducting gap and the single-electron local density of states. For the limiting case of independent large-diameter nanotubes, the BdG equations can be reduced to the standard Bardeen-Cooper-Schrieffer one with analytical solutions. Moreover, an optimal separation between nanotubes is found, which leads to a maximal superconducting critical temperature. This finding has a remarkable accordance with the experimental data obtained from Buckypapers built of boron doped SWNTs under external pressure.

cond-mat.supr-con↗

arXiv2018

Theory for strained graphene beyond the Cauchy-Born rule

The low-energy electronic properties of strained graphene are usually obtained by transforming the bond vectors according to the Cauchy-Born rule. In this work, we derive a new effective Dirac Hamiltonian by assuming a more general transformation rule for the bond vectors under uniform strain, which takes into account the strain-induced relative displacement between the two sublattices of graphene. Our analytical results show that the consideration of such relative displacement yields a qualitatively different Fermi velocity with respect to previous reports. Furthermore, from the derived Hamiltonian, we analyze effects of this relative displacement on the local density of states and the optical conductivity, as well as the implications on the scanning tunneling spectroscopy, including external magnetic field, and optical transmittance experiments of strained graphene.

cond-mat.mes-hall↗

arXiv2018

Resonant thermoelectric transport in atomic chains with Fano defects

Atomic clusters attached to a low-dimensional system, called Fano defects, produce rich wave interferences. In this work, we analytically found an enhanced thermoelectric figure-of-merit (ZT) in periodic atomic chains with Fano defects, compared to those without such defects. We further study self-assembled DNA-like systems with periodic and quasiperiodically placed Fano defects by using a real-space renormalization method developed for the Kubo-Greenwood formula, in which tight-binding and Born models are respectively used for the electric and lattice thermal conductivities. The results reveal that the quasiperiodicity could be another ZT-improving factor, whose long-range disorder inhibits low-frequency acoustic phonons insensitive to local defects.

cond-mat.mes-hall↗

arXiv2018

Fingerprints of a position-dependent Fermi velocity on scanning tunnelling spectra of strained graphene

Nonuniform strain in graphene induces a position dependence of the Fermi velocity, as recently demonstrated by scanning tunnelling spectroscopy experiments. In this work, we study the effects of a position-dependent Fermi velocity on the local density of states (LDOS) of strained graphene, without and with the presence of a uniform magnetic field. The variation of LDOS obtained from tight-binding calculations is successfully explained by analytical expressions derived within the Dirac approach. These expressions also rectify a rough Fermi velocity substitution used in the literature that neglects the strain-induced anisotropy. The reported analytical results could be useful for understanding the nonuniform strain effects on scanning tunnelling spectra of graphene, as well as when it is exposed to an external magnetic field.

cond-mat.mes-hall↗

arXiv2017

Magneto-optical conductivity of anisotropic two-dimensional Dirac-Weyl materials

In the presence of an external magnetic field, the optical response of two-dimensional materials, whose charge carriers behave as massless Dirac fermions with arbitrary anisotropic Fermi velocity, is investigated. Using Kubo formalism, we obtain the magneto-optical conductivity tensor for these materials, which allows to address the magneto-optical response of anisotropic Dirac fermions from the well known magneto-optical conductivity of isotropic Dirac fermions. As an application, we analyse the combined effects of strain-induced anisotropy and magnetic field on the transmittance, as well as on the Faraday rotation, of linearly polarized light after passing strained graphene. The reported analytical expressions can be a useful tool to predict the absorption and the Faraday angle of strained graphene under magnetic field. Finally, our study is extended to anisotropic two-dimensional materials with Dirac fermions of arbitrary pseudospin.

cond-mat.mes-hall↗

arXiv2017

Low-energy theory for strained graphene: an approach up to second-order in the strain tensor

An analytical study of low-energy electronic excited states in an uniformly strained graphene is carried out up to second-order in the strain tensor. We report an new effective Dirac Hamiltonian with an anisotropic Fermi velocity tensor, which reveals the graphene trigonal symmetry being absent in low-energy theories to first-order in the strain tensor. In particular, we demonstrate the dependence of the Dirac-cone elliptical deformation on the stretching direction respect to graphene lattice orientation. We further analytically calculate the optical conductivity tensor of strained graphene and its transmittance for a linearly polarized light with normal incidence. Finally, the obtained analytical expression of the Dirac point shift allows a better determination and understanding of pseudomagnetic fields induced by nonuniform strains.

cond-mat.mes-hall↗