arXiv subjects

Xu-Cheng Wang

Publications and source records attributed to Xu-Cheng Wang.

4 recordsLinked to original sources

arXiv2026

Quantum geometric localization length and localization criticality in an ideally flat Chern band

We propose that the localization length in an isolated, ideally flat Chern band is set by quantum geometry. We explore the corresponding localization transition and its critical scaling by applying transfer matrix calculations in the maximally localized hybrid Wannier basis, whose spatial spread is exactly characterized by a quantum geometric length. Remarkably, upon tuning the quantum metric of the Chern band, we observe a crossover from a universal regime controlled by the Dirac fixed point to a non-universal regime with continuously varying critical exponents. Within the universal regime, the localization length exhibits a pronounced linear dependence on the quantum geometric length, supporting its quantum geometric nature. These findings provide a novel quantum geometric perspective on the localization in quantum Hall systems such as twisted moiré superlattices, and shed new light on the long-standing controversy over the criticality of the integer quantum Hall transition.

cond-mat.str-el↗

arXiv2026

Emergent magnetic pseudogap from phase fluctuations and hierarchy of scales in two-dimensional superconductors

Preformed pairs and phase fluctuations are believed to play a vital role in predicting the charge pseudogap in the normal state of two-dimensional superconductors. In this work, we extend this idea and further identify the emergent magnetic pseudogap from pure phase fluctuations without invoking any competing order. We examine the NMR relaxation rate $1/T_1T$ by evaluating the bubble contribution and leading-order vertex correction within perturbation theory. It is found that the magnetic pseudogap, manifesting as a smooth suppression of $1/T_1T$ in the normal state, is characterized by a temperature scale $T_\text{mPG}$ distinct from the superconducting gap $Δ_\text{SC}$ and transition temperature $T_c$. The onset scales of both charge and magnetic pseudogap are dominated by the competition of BKT correlation length $ξ(T)$ and BCS coherence length $ξ_\text{BCS}$. Moreover, the vertex correction is shown to be irrelevant for $d$-wave pairing, while it becomes prominent in $s$-wave systems and drives a coherent enhancement of $1/T_1T$ at lower temperatures just above $T_c$. We attribute this normal-state enhancement of $1/T_1T$ to the diverging coherence peak at the $s$-wave superconducting gap edge, which shares the same spirit as the celebrated Hebel-Slichter peak in the BCS theory. Analogous to the coherent Hebel-Slichter peak, regularization by Fermi-liquid-like scatterings is important and is characterized by a scattering length $\ell$. The normal-state coherent enhancement of $1/T_1T$ is hence described by the competition of $ξ(T)$ and $\ell$, through which the coherence scale $T_\text{coh}$ is determined. As a result, the complete evolution of $1/T_1T$ is understood quantitatively in a unified picture as the interplay among hierarchy of scales $ξ(T)$, $ξ_\text{BCS}$ and $\ell$.

cond-mat.supr-con↗

arXiv2026

The interplay of phase fluctuations and nodal quasiparticles: ubiquitous Fermi arcs in two-dimensional d-wave superconductors

We propose that the pseudogap and Fermi arcs can universally emerge due to thermal (static) phase fluctuations in the normal state of 2D nodal superconductors. By considering a minimal phenomenological model with spatially fluctuating superconducting pairings, we theoretically investigate the role of superconducting phase fluctuations in generic 2D superconductors with disorder-average technique. It is shown for nodal d-wave superconductors that phase fluctuations mediate the scattering of d-wave quasiparticles, smearing out the nodal quasiparticle gap and further leading to pseudogap and Fermi arcs. Moreover, the evolution of Fermi arcs is quantitatively described by two emergent characteristic length scales of the system: one is the finite superconducting correlation length $ξ(T)$, and another the nodal BCS coherence length $ξ_\text{BCS}(k)$. To support our theoretical findings, we numerically report the observation of Fermi arcs in a Hubbard-like model, proposed originally by X. Y. Xu and T. Grover in Phys. Rev. Lett. $\textbf{126}$, 217002 (2021), with sign-problem-free determinant quantum Monte Carlo (DQMC) calculations. As far as we noticed, it is the first time in a correlated model that phase-fluctuating Fermi arcs are identified with unbiased simulations. The numerical results for the scattering rate $Γ_\text{pf}$ of Cooper pairs exhibit excellent agreements with our theoretical predictions, where $Γ_\text{pf}$ is expected to scale linearly with the inverse superconducting correlation length $ξ(T)^{-1}$. This convergence of theory and numerics thereby strongly validates the universal connection between phase fluctuations and Fermi arcs in 2D nodal superconductors.

cond-mat.supr-con↗

arXiv2023

Phase fluctuations in two-dimensional superconductors and pseudogap phenomenon

We study the phase fluctuations in the normal state of generic two-dimensional superconducting systems with s-wave pairing. The effect of phase fluctuations of the pairing fields can be dealt with perturbatively using disorder averaging, after we treat the local superconducting order parameter as a static disordered background. It is then confirmed that the phase fluctuations above the two-dimensional Berezinskii-Kosterlitz-Thouless transition lead to a significant broadening of the single-particle spectrum, giving birth to the pseudogap phenomenon. Quantitatively, the broadening of spectral weights near the BCS gap is characterized by the ratio of the superconducting coherence length and the spatial correlation length of the superconducting pairing order parameter. Our results are tested on the fermionic attractive-U Hubbard model on the square lattice, using the unbiased determinant quantum Monte Carlo method and stochastic analytic continuation.

cond-mat.supr-con↗