arXiv subjects

Chunli Huang

Publications and source records attributed to Chunli Huang.

At least 19 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.

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arXiv2026

Signatures of a ferro-Josephson effect in twisted graphene

When a spin-polarized current is driven across a magnetic domain wall, the resulting spin-transfer torque may, beyond a critical threshold, set the wall's moments into precession. This precession modulates the Berry curvature experienced by electrons traversing the wall, producing an electromotive force that is topological in nature and proportional to the precession frequency, mapping precisely onto the DC Josephson effect and leading to the name ferro-Josephson effect. We report signatures consistent with this effect in a twisted graphene van der Waals heterostructure, where spin and valley textures are linked by exchange, Hund's coupling, and spin-orbit interactions. Tuned to fillings where the isospin degeneracy is spontaneously broken, the samples develop a sharp peak in the longitudinal resistance within a fraction of a millitesla of $B_\parallel=0$---a peak that disappears as the current is reduced toward zero. In differential resistance the feature resolves into sharp resonances that disperse with $B_\parallel$ on microtesla and picoampere scales. We argue that these arise from the current-driven precession of spin-domain-wall moments, in competition with the in-plane anisotropy set by a minuscule applied field, and that they establish nonlinear transport as a sensitive probe of isospin domain-wall dynamics at energy scales far below $k_BT$.

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arXiv2026

Metastable magnetic domains and the anomalous $B_\parallel=0$ resistance peak in twisted double bilayer graphene

In graphene moir\'es, valley polarization gives rise to orbital magnetism, manifested as an anomalous Hall effect and resulting in Barkhausen jumps in longitudinal resistance when changing domain configurations modify quasiparticle scattering. Beyond a simple picture of polarized domains, however, spin and valley textures within and between the domains are less well understood, as is the effect of these textures on transport. In the valley-polarized quarter-metal state of twisted double bilayer graphene, a sharp and metastable peak in longitudinal resistance often appears at zero in-plane magnetic field, whose microscopic origin has yet to be identified. Here, we show that this peak depends on the configuration of domains of orbital magnetism, which is itself set by the gate-voltage trajectory used to enter the ordered state and by the magnetic field --- particularly the in-plane component --- present during that trajectory. The sensitivity of the effect to in-plane magnetic field components points to spin, linked to valley polarization through spin-orbit coupling, as the key degree of freedom in both the domain formation and the resistance peak.

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arXiv2026

Multicomponent Magnetic Domain Walls in Rhombohedral Graphene

Spatial textures of magnetic order, such as domain walls and skyrmions, are fundamental objects in magnetism. In rhombohedral multilayer graphene, magnetic order involves spin and valley degrees of freedom, opening the possibility of qualitatively new spatial textures. Here, we explore this possibility through a microscopic study of a one-dimensional domain wall in the valley-imbalanced quarter-metal phase of rhombohedral graphene. We uncover two different classes of domain walls. One resembles a conventional magnetic domain wall, locally rotating between the two bulk states, whereas the other is intrinsically multicomponent and explores states that are not occupied in either bulk domain. Which texture is realized is controlled by the competition between intervalley Hund's coupling and spin-orbit coupling, and we identify experimental signatures to distinguish them. We further show that, in a superconducting junction formed across the wall, the superconducting phase difference couples directly to the intervalley-coherent phase of the texture. Precession of this internal phase can therefore generate a voltage across the junction. Our theory shows that rhombohedral graphene indeed has magnetic textures beyond conventional magnet and that their dynamics can couple to superconducting transport.

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arXiv2026

Retarded interaction between opposite chiral edges in anomalous Hall crystals

An anomalous Hall crystal combines spontaneous electronic crystallization with a Chern insulating gap, supporting both chiral edge modes and low-energy electronic phonons. We show that this coexistence produces a distinct dynamical effect from ordinary Chern insulators: transverse bulk phonons can mediate a retarded interaction between counterpropagating chiral edge modes on opposite sides of the sample, realizing a Luttinger-liquid variant with delayed inter-edge coupling. Using microscopic time-dependent Hartree--Fock calculations for rhombohedral pentalayer graphene, we find that lowering the carrier density softens the long-wavelength transverse phonon mode. Near this instability regime, the resulting boundary-projected phonon continuum inevitably overlaps with the edge dispersion, thereby enabling their coupling. A smoking-gun probe is a nonlocal measurement: a drive applied to one edge can induce a response on the other edge, delayed by the transverse phonon time of flight across the sample.

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arXiv2026

Multi-Knob Switchable Chiral Superconductivity Quartet in Rhombohedral Graphene

Chiral superconductors break orbital time-reversal symmetry and may host topological quasiparticles with non-Abelian statistics. In rhombohedral graphene, superconductivity develops from a spin-valley-polarized quarter-metal (QM) parent state and features unique magnetic hysteresis of resistance that indicates orbital time-reversal-symmetry-breaking. Exploring and controlling the full spin-valley flavors of such superconductivity could enable novel superconducting and topological devices, but have remained unexplored. Here we report transport measurements on rhombohedral hexalayer graphene (R6G), which reveal a new superconducting state (SCH) that is induced by an out-of-plane magnetic field, in addition to chiral superconductivity (CSC) similar to those observed in thinner layers. This SCH state emerges above 0.8 T, persists up to 1.6 T and can be switched on/off by magnetic field $H_\perp$, carrier density $n$, and gate displacement field $D$. Quantum oscillations and anomalous Hall measurements show that SCH stems from a field-induced quarter-metal (QM$'$) parent phase, which carries orbital magnetization opposite to that of the zero-field QM. Across the full $(n, D, H_\perp)$ parameter space, superconductivity can be realized from all four spin-valley isospin flavors, establishing a switchable chiral-superconductor quartet in R6G. We interpret the parent-state switching as arising from competition between a Kane-Mele-like spin-valley splitting and magnetic-field coupling to spin-valley-dependent magnetic moments. Our work establishes rhombohedral graphene as a multi-knob platform for different isospin-polarized superconductivities, which enables programmable superconducting networks with possible Majorana modes along domain walls.

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arXiv2026

Orbital Magnetization from Uniform and Periodic Magnetic Fields

Magnetization is thermodynamically defined as the derivative of the grand potential with respect to a uniform magnetic field. However, a uniform magnetic field makes the kinetic momentum operators noncommuting and Landau-quantizes the electron motion. This changes the zero-field momentum-space to Landau-levels and raises a fundamental question: how can the thermodynamic response to a uniform field be reproduced by a linear-response calculation carried out in the momentum space of the zero-field problem? We address this question analytically in a quantum Hall ferromagnet that allows the orbital magnetization $M$ to be computed in a closed form. We first compute $M$ from the local Hartree--Fock projector response to a periodic magnetic field with zero net flux. We then compute $M$ from the derivative of the grand potential with respect to a uniform magnetic field along the St\v{r}eda line. The two approaches give the same result, even though the first keeps the Hilbert space fixed while the second changes the Landau-level degeneracy. Their agreement suggests that we should view orbital magnetization as the energy associated with the spectral flow that gives rise to the St\v{r}eda formula. Our work provides a tutorial introduction to orbital magnetization and its relation to the St\v{r}eda formula.

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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.

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arXiv2026

Microscopic origin of orbital magnetization in chiral superconductors

Chiral superconductivity is a time-reversal-symmetry-breaking superconducting phase that has attracted broad interest as a potential platform for topological quantum computation. A fundamental consequence of this symmetry breaking is orbital magnetization, yet a clear microscopic formulation of this quantity has remained elusive. This difficulty arises because Bogoliubov quasiparticles do not carry a definite electric charge, precluding a simple interpretation of orbital magnetization in terms of circulating quasiparticle currents. Moreover, superconductivity and ferromagnetism rarely coexist, and in the few materials where they do (e.g. uranium-based compounds), strong spin-orbit coupling obscures the orbital contribution to the magnetization. The recent report of chiral superconductivity in rhombohedral multilayer graphene, which has negligible spin-orbit coupling, therefore provides a unique opportunity to develop and test a microscopic theory of orbital magnetization in chiral superconductors. Here we develop such a theory, unifying the interband coherence effects underlying normal-state orbital magnetization with the intrinsic orbital moments of the Cooper-pair condensate. Applying our theory to rhombohedral tetralayer graphene, we find that the onset of superconductivity can either enhance or suppress the normal-state orbital magnetization, depending on the bandstructure. We further identify a generalized clapping mode with a gap set by the sublattice winding form factor. This collective mode is unique to chiral superconductors and contributes to the orbital magnetization through its role in dressing the photon vertex. Our theory resolves a long-standing conceptual difficulty in defining orbital magnetization in superconducting systems, and measurements of the orbital magnetization relative to the quarter-metal phase would provide a direct experimental test.

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arXiv2025

Orbital Magnetization and Streda Formula of Interacting Electrons in the Mean-field Approximation

We study the magnetic-field response of interacting electron systems within mean-field theory using perturbation theory. We show that the linear response of the mean-field density-matrix to a weak magnetic field is purely geometric: it depends only on wavefunction derivatives, the Berry connections linking the occupied and unoccupied subspaces, and does not explicitly depend on the interaction potential and the quasiparticle dispersion. This leads to compact, gauge-invariant projector expressions for both the St\v{r}eda formula and the formula for orbital magnetization. Our calculation explicitly elucidates the role of exchange and self-consistency in defining current vertices for orbital magnetization calculations. Our work establishes a direct connection between mean-field theory, quantum geometry and the non-interacting topological band theory.

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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.

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arXiv2025

Emblems of pair density waves: dual identity of topological defects and their transport signatures

The pair density wave (PDW) exemplifies intertwined orders in strongly correlated systems. A recent discovery of superconductivity in a quarter-metal state offers the first experimental system where a pure PDW without uniform superconductivity is suspected, offering a unique opportunity to examine the consequences of intertwined orders. A pure two-dimensional PDW supports an unusual fractional excitation as its topological defect (TD). A TD simultaneously winds the phase of the Cooper pair and distorts the amplitude modulation -- a dual role reflecting its intertwined character. As a vortex, a TD carries fractional vorticity of $\frac{1}{3} h/2e$, whose movement would cause resistance. As a crystalline defect, a TD can be sourced by charge disorder in the system. We show that experimentally observed resistive switching can originate from mobile TDs, while a small magnetic field will restore zero resistance by blocking their motion. The resulting resistive state exhibits extreme anisotropy and a Hall response, with the Hall angle determined by the angle between the current and the TD's Burgers vector. These features will serve as confirmation of the dual identity of topological defects as emblems of PDW order.

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arXiv2025

Elementary Excitations, Melting Temperature, and Correlation Energy in Wigner Crystals

We present a fully quantum-mechanical study of the energy--momentum dispersion of running waves, spin-conserving neutral excitations, and spin-reversal neutral excitations in a spin-polarized two-dimensional Wigner crystal (WC). Our results show that the collective modes---plasmon and transverse sound---closely follow classical predictions even at surprisingly low values of $r_s\sim8$. Furthermore, by extracting the shear modulus from the transverse sound speed, we find that quantum-mechanical effects enhance the shear modulus at high densities, leading to a (Kosterlitz--Thouless--Halperin--Nelson--Young) melting temperature that exceeds the classical prediction. In addition, we apply the quasi-boson approximation to compute the correlation energy of the 2D WC based on its neutral excitation spectrum. While this approach underestimates the absolute correlation energy compared to quantum Monte Carlo results, it successfully captures the overall trend. We further investigate the stability of the metallic Wigner crystal (MWC), a partially melted crystalline state that retains a Fermi surface, and find that it is unstable toward density fluctuations, with the instability concentrated at wave vectors set by the Fermi pocket size. These findings establish a robust quantum-mechanical foundation for understanding elementary excitations in Wigner crystals within low-dimensional electron systems and provide valuable theoretical insights for future experimental studies.

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arXiv2025

Momentum-Space AC Josephson Effect and Intervalley Coherence in Multilayer Graphene

Electron transport driven by the phase coherence and interference of quantum many-body wavefunctions is a fascinating phenomenon with potential technological significance. Superconductivity, for example, enables dissipationless transport through macroscopic phase twisting. Similarly, in charge-density waves, once the phase degree of freedom-representing the collective position of electrons relative to the lattice-is depinned, it generates characteristic broadband noise and intriguing AC-DC interference patterns. In this work, we point out a phase-coherent dynamics in the intervalley coherent (IVC) state, also known as the bond-ordered or Kekul\'e distorted state, frequently reported in rhombohedral multilayer graphene. Under a static magnetic field, the IVC state responds with an oscillating intervalley current, which in turn causes oscillating orbital magnetization, thereby inducing a detectable AC Hall effect. This mechanism mirrors the AC Josephson effect observed in superconductors but now happening in momentum space. In this analogy, the static magnetic field acts as the DC voltage, while the oscillating intervalley current assumes the role of the AC Josephson current. We present detailed microscopic calculations for all the parameters of the phase-number free-energy in rhombohedral trilayer graphene, predicting an orbital magnetization oscillation frequency of approximately 12 GHz at 0.1 Tesla. We comment on this phase-coherent dynamics in other 2D materials like twisted homobilayer transition metal dichalcogenides.

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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.

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arXiv2024

Magnetism in the Dilute Electron Gas of Rhombohedral Multilayer Graphene

Lightly-doped rhombohedral multilayer graphene has recently emerged as one of the most promising material platforms for exploring electronic phases driven by strong Coulomb interactions and non-trivial band topology. This review highlights recent advancements in experimental techniques that deepen our understanding of the electronic properties of these systems, especially through the application of weak-field magnetic oscillations for studying phase transitions and Fermiology. Theoretically, we advocate modeling these systems using an electron gas framework, influenced primarily by two major energy scales: the long-range Coulomb potential and band energy. The interplay between these energies drives transitions between paramagnetic and ferromagnetic states, while smaller energy scales like spin-orbit coupling and sublattice-valley-dependent interactions at the atomic lattice scale shape the (magnetic anisotropic energy) differences between distinct symmetry-broken states. We provide first-principles estimates of lattice-scale coupling constants for Bernal bilayer graphene under strong displacement field, identifying the on-site inter-valley scattering repulsion, with a strength of $g_{\perp \perp}=269\text{meV nm}^2$ as the most significant short-range interaction. The mean-field phase diagram is analyzed and compared with experimental phase diagrams. New results on spin and valley paramagnons are presented, highlighting enhanced paramagnetic susceptibility at finite wavevectors and predicting valley and spin density-wave instabilities. The interplay between superconductivity and magnetism, particularly under the influence of spin-orbit coupling, is critically assessed. The review concludes with a summary of key findings and potential directions for future research.

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arXiv2024

Competition of Exchange and Correlation Energies in Two-Dimensional $N$-component Electron Gas Ferromagnetism

Motivated by recent observations of symmtry broken phases in lightly-doped multilayer graphene, we investigate magnetic phase transitions in a generalized electron gas model with four-component electron spin. This model simplifies the problem with a parabolic dispersion band, abstracting away the details of the graphene band structure to focus solely on the effects of the Coulomb interaction. We report four findings: 1) In the Hartree-Fock approximation, we observe that the paramagnetic state undergoes a sequence of density-driven, first-order phase transitions, progressively depopulating electrons from each spin component until achieving complete polarization within a very narrow density window where $1.2<r_s<2$ ($r_s$ being the electron gas parameter). 2) Further incorporating the correlation energy via the Bohm-Pines random-phase approximation shows that the cascade of transitions obtained within Hartree-Fock approximation is replaced by a single ferromagnetic phase transition at $r_s = 6.12$. 3) The disappearance of cascade is due to the correlation energy difference between the four-component paramagnetic state and symmetry-broken phases, which is nearly an order of magnitude more negative than the corresponding Hartree-Fock energy difference for $1.2 < r_s < 2$. 4) The transition from the paramagnetic state to the fully polarized state at $r_s=6.12$ is governed by the balance between exchange and correlation energies, a competition that cannot be captured by mean-field approximations to models featuring effective (density-dependent) delta-function interactions, such as the Stoner model. We use the insights from our model to comment on the phase diagram of multilayer graphene electron gas.

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arXiv2024

Superpolarized Electron-Hole Liquid and Multiferroicity in Multilayer Graphene

We introduce a many-body state termed superpolarized electron-hole liquid to explain the multiferroic properties observed in a recent experiment on rhombohedral pentalayer graphene by Han et al. [Nature 623, 41-47, 2023] . Superpolarization refers to a state where electrons and holes are fully polarized in opposite directions within the extended spin-valley space, resulting in a polarization per charge that exceeds the saturated value of one. This state exhibits multiferroic order, characterized by spontaneous spin, layer, and valley polarization. We demonstrate the independent control of valley polarization and orbital magnetization in this state through the application of electric-displacement field and weak magnetic fields. The large magnetoelectric effect observed in the experiment is attributed to the substantial Berry curvature concentrated near the band edge. The concept of superpolarization can be probed experimentally through magnetic oscillation and local current distribution measurements to determine the area of the Fermi surfaces and their magnetic moments.

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