arXiv subjects

Guopeng Xu

Publications and source records attributed to Guopeng Xu.

5 recordsLinked to original sources

arXiv2026

Singular Weak-Field Thermodynamics of 2D Superconductors

In a bulk 3D type-II superconductor, the lower critical field at which an isolated vortex becomes thermodynamically favorable is a size-independent material property. We show that the situation is different in 2D superconductors: the larger the superconductor, the weaker the field needed to create its first vortex. The lower critical field in 2D is always size-dependent. For a disk of area $\mathcal A$, the lower critical field $B_v(\mathcal A)$ scales as $\mathcal A^{-1}\ln(\mathcal A/\mathcal A_0)$ in the weak-screening regime and as $\mathcal A^{-1/2}$ in the strong-screening regime. We derive these results from an analytically tractable microscopic model that admits many-body wavefunctions for both the uniform and singly quantized vortex states in a magnetic field, and incorporate screening by coupling their long-distance 2D supercurrents to 3D Maxwell equations. These results motivate organizing the weak-field ground-state of a 2D superconductor in the $(1/\mathcal A,B)$ plane. The origin represents the zero-field thermodynamic limit and it is singular. Approaching the origin along the $B$ axis leads to an increasingly dilute vortex lattice, whereas approaching along the $1/\mathcal A$ axis yields the uniform vortex-free state. Our theory shows that every trajectory carrying fixed finite flux ultimately approaches the vortex-free state in the thermodynamic limit and provides a firm microscopic foundation for the weak-field thermodynamics of 2D superconductors.

cond-mat.supr-con↗

arXiv2026

Two-Body Solution and Instabilities along Streda Lines in Moire Flat Bands

Moire minibands in twisted homobilayer semiconductors can, under suitable approximations, be modeled as a pair of Landau levels with opposite Chern numbers. This provides a minimal model for searching novel topological states in a time-reversal-symmetric Hamiltonian. In this work, we investigate the effects of an external magnetic field in this model. We study the many-body ground state in the density-magnetic-field (n-B) plane along the dn/dB = \pm1/Phi0 Streda line with Hartree-Fock approximation. Away from charge neutrality, we find the Chern-insulating (incompressible) state is very robust while towards charge neutrality, we find a transition from incompressible phase to compressible phase as the interaction strength kappa decreases. Using time-dependent mean-field theory, we further analyze spin-flip excitations and find that the incompressible state along Streda line toward charge neutrality becomes unstable even at large kappa when magnetic field is sufficiently large. Finally, we solve the two-body problem in a given Landau level exactly where the two particles experience unequal magnetic fields using a new basis called center-of-charge basis. This basis allows any isotropic interaction to be parameterized by a single quantum number, the relative angular momentum, thereby extending the Haldane pseudopotentials to the unequal-magnetic-fields case. As the difference of the two magnetic fields varies, these pseudopotentials show a sequence of level crossings, leading to non-monotonic structure of pseudopotentials that is absent in ordinary Landau level systems. Our formulation provides a useful starting point for studying weak-field physics in moire flat bands, where magnetic Bloch-state basis becomes computationally impossible due to the large basis sizes.

cond-mat.str-el↗

arXiv2025

Localized Excitons and Landau-Level Mixing in Time-Reversal Symmetric Pairs of Chern Bands

We study Landau-level mixing in a time-reversal-symmetric Hamiltonian composed of two sets of Landau levels with opposite magnetic field, relevant to moir\'e minibands in twisted homobilayer transition-metal dichalcogenides in the adiabatic limit, where electrons in opposite valleys have flat Chern bands with opposite Chern numbers. Strong spin-orbit coupling polarizes spins in opposite directions in opposite valleys, separating Coulomb interactions into like-spin ($V^{\uparrow\uparrow}$) and opposite-spin ($V^{\uparrow\downarrow}$). Using degenerate perturbation theory, we compute Landau-level mixing corrections to $V^{\uparrow\uparrow}$ and $V^{\uparrow\downarrow}$ for different filling fractions. In the lowest Landau level, screening exhibits an even-odd effect: $V^{\uparrow\uparrow}$ is reduced more strongly than $V^{\uparrow\downarrow}$ in even-$m$ angular momentum Haldane pseudopotential and less strongly in odd-$m$ angular momentum ones. In the first Landau level, the short-range part ($m=0,1$) of $V^{\uparrow\downarrow}$ is reduced comparably to $V^{\uparrow\uparrow}$, while the strongest spin anisotropy appears in the $m=2$ pseudopotential. These novel short-range spin correlations have important implications for candidate correlated phases of fractional quantum spin Hall insulators. A distinctive feature of this time-reversal-symmetric Hamiltonian, absent in conventional quantum Hall systems, is that spin-flip excitations form localized quasiparticles. We compute their excitation spectrum and predict a non-monotonic dependence of the ordering temperature of Chern ferromagnetism in MoTe$_2$ on the Landau-level mixing parameter.

cond-mat.str-el↗

arXiv2024

Influence of the Dirac Sea on Phase Transitions in Monolayer Graphene under Strong Magnetic Fields

Recent scanning tunneling microscopy experiments have found Kekul\'e-Distorted (KD) ordering in graphene subjected to strong magnetic fields, a departure from the antiferromagnetic (AF) state identified in earlier transport experiments on double-encapsulated devices with larger dielectric screening constant $\epsilon$. This variation suggests that the magnetic anisotropic energy is sensitive to dielectric screening constant. To calculate the magnetic anisotropic energy without resorting to perturbation theory, we adopted a two-step approach. First, we derived the bare valley-sublattice dependent interaction coupling constants from microscopic calculations and account for the leading logarithmic divergences arising from quantum fluctuations by solving renormalization group flow equations in the absence of magnetic field from the carbon lattice scale up to the much larger magnetic length. Subsequently, we used these renormalized coupling constants to perform non-perturbative, self-consistent Hartree-Fock calculations. Our results demonstrate that the ground state at neutrality ($\nu=0$) transitions from a AF state to a spin-singlet KD state when dielectric screening and magnetic fields become small, consistent with experimental observations. For filling fraction $\nu=\pm1$, we predict a transitions from spin-polarized charge-density wave states to spin-polarized KD state when dielectric screening and magnetic fields become small. Our self-consistent Hartree-Fock calculations, which encompass a large number of Landau levels, reveal that the magnetic anisotropic energy receives substantial contributions from the Dirac sea when $\epsilon$ is small. Our work provides insights into how the Dirac sea, which contributes to one electron per graphene unit cell, affects the small magnetic anisotropic energy in graphene.

cond-mat.mes-hall↗

arXiv2024

Landau-Level Mixing and SU(4) Symmetry Breaking in Graphene

Recent scanning tunneling microscopy experiments on graphene at charge neutrality under strong magnetic fields have uncovered a ground state characterized by Kekulé distortion (KD). In contrast, non-local spin and charge transport experiments in double-encapsulated graphene, which has a higher dielectric constant, have identified an antiferromagnetic (AF) ground state. We propose a mechanism to reconcile these conflicting observations, by showing that Landau-level mixing can drive a transition from AF to KD with the reduction of the dielectric screening. Our conclusion is drawn from studying the effect of Landau-level mixing on the lattice-scale, valley-dependent interactions to leading order in graphene's fine structure constant $κ= e^2/(\hbar v_F ε)$. This analysis provides three key insights: 1) Valley-dependent interactions remain predominantly short-range with the $m=0$ Haldane pseudopotential being at least an order of magnitude greater than the others, affirming the validity of delta-function approximation for these interactions. 2) The phase transition between the AF and KD states is driven by the microscopic process in the double-exchange Feynman diagram. 3) The magnitudes of the coupling constants are significantly boosted by remote Landau levels. Our model also provides a theoretical basis for numerical studies of fractional quantum Hall states in graphene.

cond-mat.mes-hall↗