arXiv subjects

Junren Shi

Publications and source records attributed to Junren Shi.

At least 19 recordsLinked to original sources

arXiv2026

Ward identities and orbital magnetization in current density functional theory

We revisit the derivation of the orbital magnetization formula for periodic crystals in current density functional theory (CDFT)[1]. Our new derivation computes the linear response of the energy density to a periodic magnetic field in the long-wavelength limit. We unveil a Ward identity which connects the current vertex to the derivative of the Kohn-Sham self-energy. The result of Ref.[1] is confirmed: the orbital magnetization of the interacting solid can be computed exactly (in principle) from the self-consistent eigenfunctions and eigenvalues of the Kohn-Sham equation of CDFT.

cond-mat.mtrl-sci↗

arXiv2026

Kinetic instability and superconductivity in Li$_2$AuH$_6$ and Li$_2$AgH$_6$ at ambient pressure

Li$_2$AuH$_6$ and Li$_2$AgH$_6$ have been proposed as promising candidates for high-temperature superconductors under ambient pressure. While previous studies confirm the dynamic stability of these two thermodynamically unstable systems, their kinetic stability against quantum and thermal fluctuations remains to be verified. In this work, we use path integral molecular dynamics simulations to examine the kinetic stability of Li$_2$AuH$_6$ and Li$_2$AgH$_6$ under ambient pressure. We find both compounds are kinetically unstable. Li$_2$AgH$_6$ undergoes lattice collapse, whereas Li$_2$AuH$_6$ retains a stable fluorite-type Li-Au sublattice, but hydrogen atoms partially dimerize into molecules and diffuse within the host lattice. Using the stochastic path-integral approach, which is a nonperturbative approach applicable to systems with diffusive atoms, we investigate the superconductivity of Li$_2$AuH$_6$ in this state. We predict a superconducting transition temperature of 22 K, well below earlier predictions, due to the low density of states at the Fermi level caused by the collapse of hydrogen sublattice and hydrogen dimerization.

cond-mat.supr-con↗

arXiv2025

Hierarchical Spatio-Temporal Attention Network with Adaptive Risk-Aware Decision for Forward Collision Warning in Complex Scenarios

Forward Collision Warning systems are crucial for vehicle safety and autonomous driving, yet current methods often fail to balance precise multi-agent interaction modeling with real-time decision adaptability, evidenced by the high computational cost for edge deployment and the unreliability stemming from simplified interaction models.To overcome these dual challenges-computational complexity and modeling insufficiency-along with the high false alarm rates of traditional static-threshold warnings, this paper introduces an integrated FCW framework that pairs a Hierarchical Spatio-Temporal Attention Network with a Dynamic Risk Threshold Adjustment algorithm. HSTAN employs a decoupled architecture (Graph Attention Network for spatial, cascaded GRU with self-attention for temporal) to achieve superior performance and efficiency, requiring only 12.3 ms inference time (73% faster than Transformer methods) and reducing the Average Displacement Error (ADE) to 0.73m (42.2% better than Social_LSTM) on the NGSIM dataset. Furthermore, Conformalized Quantile Regression enhances reliability by generating prediction intervals (91.3% coverage at 90% confidence), which the DTRA module then converts into timely warnings via a physics-informed risk potential function and an adaptive threshold mechanism inspired by statistical process control.Tested across multi-scenario datasets, the complete system demonstrates high efficacy, achieving an F1 score of 0.912, a low false alarm rate of 8.2%, and an ample warning lead time of 2.8 seconds, validating the framework's superior performance and practical deployment feasibility in complex environments.

cs.LG↗

arXiv2025

Generalizing the composite fermion theory for fractional Chern insulators

We propose a generalized composite fermion (CF) theory for fractional Chern insulators (FCIs) by adapting the quantum mechanics approach of CFs. The theoretical framework naturally produces an effective CF Hamiltonian and a wavefunction ansatz, and the Bloch band characteristics of FCIs determine effective scalar and vector potentials experienced by CFs. Our analysis clarifies the construction of CF wavefunctions and state counting in CF phase space, which is subject to a density-of-states correction for filling factors $|\nu| \neq 1/2$. We apply the theory to study the $\nu=-2/3$ FCI state of the twisted bilayer MoTe$_2$ system, modeling it as either a $1/3$-filled electron band or a $2/3$-filled hole band. While both CF models exhibit trends and features consistent with exact diagonalization results, the electron-based model shows better agreement. Furthermore, we find that the FCI phase transition coincides with a topological phase transition in unoccupied CF $\Lambda$-bands.

cond-mat.str-el↗

arXiv2025

Metallic Contact Contributions in Thermal Hall Conductivity Measurements

We investigate the influence of metallic contacts on thermal Hall measurements. By analyzing typical measurement setups, we show that heat currents bypassing through metallic contacts could generate non-negligible thermal Hall signals. We find that contributions from metallic contacts with thicknesses on the order of 10$^{-2}$ of sample widths can approximately replicate experimental observations across different materials in both temperature dependence and magnitude, assuming silver contacts with a conductivity of $10^{8}~\mathrm{S/m}$. Our analysis underscores the need to minimize metallic contact effects in thermal Hall measurements, which can be achieved by optimizing measurement configurations.

cond-mat.mtrl-sci↗

arXiv2025

Anharmonicity and Coulomb pseudopotential effects on superconductivity in YH$_6$ and YH$_9$

Anharmonic effects are widely believed to be the primary cause of the overestimation of superconducting transition temperatures of yttrium hydrides YH$_6$ and YH$_9$ in theoretical predictions. However, prior studies indicate that anharmonicity alone may be insufficient to account for this discrepancy. In this work, we employ the stochastic path-integral approach to investigate the quantum and anharmonic effects of ions in yttrium hydrides. Our calculations reveal significant corrections to the electron-phonon coupling parameters and an increase in the average phonon frequency compared to density functional perturbation theory, aligning closely with results from the stochastic self-consistent harmonic approximation. We find that properly taking into account the renormalization of the Coulomb pseudopotential due to the frequency cutoff, which is often overlooked in previous calculations, is critical to predicting transition temperatures consistent with experimental values for both YH$_6$ and YH$_9$. This indicates that, with this correction, anharmonic effects are sufficient to explain the discrepancies between experimental and theoretical results.

cond-mat.supr-con↗

arXiv2025

Intrinsic thermal Hall effect of optical phonons enhanced by discrete rotational symmetry

We investigate the intrinsic thermal Hall conductivity contributed by optical phonons in a cubic system. The discrete rotational symmetry of the system splits the degeneracy of transverse modes across most regions of wave-vector space, except along a few high-symmetry lines. Consequently, in the presence of an external magnetic field, phonon Berry curvatures become sharply peaked near these high-symmetry lines. We find that the singular distribution of the Berry curvature induces an intrinsic thermal Hall conductivity that is significantly enhanced compared to an isotropic system. It exhibits a nonlinear $B\ln B$ dependence on the magnetic field $B$ and a non-monotonic temperature dependence. At elevated temperatures, it reverses sign and approaches a non-vanishing value asymptotically. Our analysis indicates that the behavior results from competition between contributions from Berry curvatures near different high-symmetry lines.

cond-mat.mes-hall↗

arXiv2024

Enhancing superconducting transition temperature of lanthanum superhydride by increasing hydrogen vacancy concentration

Various clathrate superhydride superconductors have been found to possess hydrogen deficiency in experimental samples, while their impacts on superconductivity are often neglected. In this study, we investigate the superconductivity of lanthanum superhydride with hydrogen deficiency (LaH$_{10-\delta}$) from first principles using path-integral approaches. Under the effects of thermal and quantum fluctuations, hydrogen vacancies are found to diffuse within the system, leading to modifications in ion vibrations, electronic structure and electron-phonon coupling. These changes result in a non-monotonic dependence of superconducting transition temperature ($T_c$) on the vacancy concentration ($\delta$). By comparing the experimental and theoretical equations of state, we suggest that $\delta$ varies across samples under different pressures. This explains the positive pressure dependence of $T_c$ in experiments below 150 GPa. Remarkably, within this pressure range, we find that $T_c$ could be further raised by increasing $\delta$.

cond-mat.supr-con↗

arXiv2024

Non-quasiconvex dispersion of composite fermions and the fermionic Haffnian state in the first-excited Landau level

It has long been puzzling that fractional quantum Hall states in the first excited Landau level (1LL) often differ significantly from their counterparts in the lowest Landau level. We show that the dispersion of composite fermions (CFs) is a deterministic factor driving the distinction. We find that CFs with two quantized vortices in the 1LL have a non-quasiconvex dispersion. Consequently, in the filling fraction $7/3$, CFs occupy the second $\Lambda$-level instead of the first. The corresponding ground state wave function, based on the CF wave function ansatz, is identified to be the fermionic Haffnian wave function rather than the Laughlin wave function. The conclusion is supported by numerical evidence from exact diagonalizations in both disk and spherical geometries. Furthermore, we show that the dispersion becomes quasiconvex in wide quantum wells or for CFs with four quantized vortices in the filling fraction $11/5$, coinciding with observations that the distinction between the Landau levels disappears under these circumstances.

cond-mat.mes-hall↗

arXiv2023

Quantum mechanics of composite fermions

We establish the quantum mechanics of composite fermions based on the dipole picture initially proposed by Read. It comprises three complimentary components: a wave equation for determining the wave functions of a composite fermion in ideal fractional quantum Hall states and when subjected to external perturbations, a wave function ansatz for mapping a many-body wave function of composite fermions to a physical wave function of electrons, and a microscopic approach for determining the effective Hamiltonian of the composite fermion. The wave equation resembles the ordinary Schr\"{o}dinger equation but has drift velocity corrections which are not present in the Halperin-Lee-Read theory. The wave-function ansatz constructs a physical wave function of electrons by projecting a state of composite fermions onto a half-filled bosonic Laughlin state of vortices. Remarkably, Jain's wave function ansatz can be reinterpreted as the new ansatz in an alternative wave-function representation of composite fermions. The dipole model and the effective Hamiltonian can be derived from the microscopic model of interacting electrons confined in a Landau level, with parameters fully determined. In this framework, we can construct the physical wave function of a fractional quantum Hall state deductively by solving the wave equation and applying the wave function ansatz, based on the effective Hamiltonian derived from first principles, rather than relying on intuitions or educated guesses. For ideal fractional quantum Hall states in the lowest Landau level, the approach yields physical wave functions identical to those prescribed by the standard theory of composite fermions. We further demonstrate that the reformulated theory of composite fermions can be easily generalized for flat Chern bands.

cond-mat.mes-hall↗

arXiv2023

Coexistence of Superconductivity and Superionicity in Li$_2$MgH$_{16}$

We study superconductivity in the superionic phase of the clathrate hydride Li$_2$MgH$_{16}$, where hydrogen ions diffuse among the lattice formed by lithium and magnesium ions. By employing the stochastic path-integral approach, we non-perturbatively take into account the effects of quantum diffusion and anharmonic vibrations. Our calculations reveal strong electron-ion coupling ($\lambda(0)$ = 3.7) and a high superconducting transition temperature ($T_c$) of 277 K under 260 GPa, at which the material is still superionic. $T_c$ is significantly suppressed compared with the result $T_c$ = 473 K obtained from the conventional approach based on the harmonic approximation. Our study, based on a first-principles approach applicable to superionic systems, indicates that the superconductivity and superionicity can coexist in Li$_2$MgH$_{16}$.

cond-mat.supr-con↗

arXiv2022

Quantum anomalous Hall insulator of composite fermions in twisted bilayer graphene

Abstract We theoretically study the realization of quantum anomalous Hall insulator (QAHI) of composite fermions (CFs) in the twisted bilayer graphene (TBG) system. We show that the moiré pattern in TBG is not only able to provide a commensurate moiré superlattice, but also a tunable effective periodic potential necessary for the realization, without the need of imposing an additional superstructure as in the conventional GaAs system. These make the TBG an ideal platform for realizing the QAHI of CFs. We establish the phase diagram with respect to tunable experimental parameters based on the Dirac CF theory. We find that the topological property of the system depends critically on the orbital magnetic susceptibility of CFs, which is not specified in the pristine Dirac CF theory. The experimental realization of QAHI of CFs would be helpful for unveiling the magnetic property of CFs and clarifying the issue.

cond-mat.mes-hall↗

arXiv2022

Stochastic path-integral approach for predicting the superconducting temperatures of anharmonic solids

We develop a stochastic path-integral approach for predicting the superconducting transition temperatures of anharmonic solids. By defining generalized Bloch basis, we generalize the formalism of the stochastic path-integral approach, which is originally developed for liquid systems. We implement the formalism for ab initio calculations using the projector augmented-wave method, and apply the implementation to estimate the superconducting transition temperatures of metallic deuterium and hydrogen sulfide. For metallic deuterium, which is approximately harmonic, our result coincides well with that obtained from the standard approach based on the harmonic approximation and the density functional perturbation theory. For hydrogen sulfide, we find that anharmonicity strongly suppresses the predicted superconducting transition temperature. Compared to the self-consistent harmonic approximation approach, our approach yields a transition temperature closer to the experimentally observed one.

cond-mat.supr-con↗

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

Emergence of Dirac Composite Fermions: Dipole Picture

Composite fermions (CFs) are the particles underlying the novel phenomena observed in partially filled Landau levels. Both microscopic wave functions and semi-classical dynamics suggest that a CF is a dipole consisting of an electron and a double $2h/e$ quantum vortex, and its motion is subject to a Berry curvature that is uniformly distributed in the momentum space. Based on the picture, we study the electromagnetic response of composite fermions. We find that the response in the long-wavelength limit has a form identical to that of the Dirac CF theory. To obtain the result, we show that the Berry curvature contributes a half-quantized Hall conductance which, notably, is independent of the filling factor of a Landau level and not altered by the presence of impurities. The latter is because CFs undergo no side-jumps when scattered by quenched impurities in a Landau-level with the particle-hole symmetry. The remainder of the response is from an effective system that has the same Fermi wavevector, effective density, Berry phase, and therefore long-wavelength response to electromagnetic fields as a Dirac CF system. By interpreting the half-quantized Hall conductance as a contribution from a redefined vacuum, we can explicitly show the emergence of a Dirac CF effective description from the dipole picture. We further determine corrections due to electric quadrupoles and magnetic moments of CFs and show deviations from the Dirac CF theory when moving away from the long wavelength limit.

cond-mat.mes-hall↗

arXiv2020

Berry phase of the composite Fermi-liquid

We derive the definition of the Berry phase for the adiabatic transport of a composite fermion (CF) in a half-filled composite Fermi-liquid (CFL). It is found to be different from that adopted in previous investigations by Geraedts et al. For the standard CFL wave function, we analytically show that the Berry curvature is uniformly distributed in the momentum space. For the Jain-Kamilla wave function, we numerically show that its Berry curvature has a continuous distribution inside the Fermi sea and vanishes outside. We conclude that the CF with respect to both the microscopic wave-functions is not a massless Dirac particle.

cond-mat.str-el↗

arXiv2020

Asymmetry of the Geometrical Resonances of Composite Fermions

We propose an experiment to test the uniform-Berry-curvature picture of composite fermions. We show that the asymmetry of geometrical resonances observed in a periodically modulated composite fermion system can be explained with the uniform-Berry-curvature picture. Moreover, we show that an alternative way of modulating the system, i.e., modulating the external magnetic field, will induce an asymmetry opposite to that of the usual periodic grating modulation which effectively modulates the Chern-Simons field. The experiment can serve as a critical test of the uniform-Berry-curvature picture, and probe the dipole structure of composite fermions proposed by Read.

cond-mat.mes-hall↗