arXiv subjects

Fang Xie

Publications and source records attributed to Fang Xie.

At least 37 records · Page 2Linked to original sources

arXiv2025

Topological Kondo semimetals emulated in hetero-bilayer transition metal dichalcogenides

The moiré structure of AB-stacked $\rm{MoTe_2/WSe_2}$ represents a natural platform to realize Kondo lattice models, due to the discrepancy of the bandwidth between the individual layers. Here, we study this system at the commensurate filling of $ν_{\rm tot}=2$. Our focus is on the $1+1$ filling setting of $ν_{\rm Mo} =ν_{\rm W}=1$, which enables a Kondo lattice description. We find a Kondo semimetal due to the sizable intra-orbital hopping among the electrons in the $\rm{MoTe_2}$ layer. The Kondo-driven (emergent) flat band is naturally pinned to the Fermi energy. When combined with the inherent topology of the electronic structure, a topological Kondo semimetal phase ensues. We calculate the valley Hall response and, due to the breaking of inversion symmetry, also identify a spontaneous Hall effect. There is a Berry curvature dodecapole, which leads to a fourth-order spontaneous Hall effect in the perturbative regime of the electric field that is further amplified in the non-perturbative regime. As such, the system provides a tunable setting to simulate topological Kondo semimetals. Finally, we discuss the pathways that connect the physics realized here to the Weyl Kondo semimetals and their proximate phases that have been advanced in recent years in topological Kondo lattice models and materials.

cond-mat.str-el↗

arXiv2025

Electron correlations in the kagome flat band metal $\rm CsCr_3Sb_5$

Kagome metals offer a unique platform for investigating robust electron-correlation effects because of their lattice geometry, flat bands and multi-orbital nature. In the cases with active flat bands, recent theoretical studies have pointed to a rich phase diagram that contains not only electronic orders but also quantum criticality. Very recently, $\rm CsCr_3Sb_5$ has emerged as a strong candidate for exploring such new physics. Here, using effective tight-binding models obtained from ab initio calculations, we study the effects of electronic correlations and symmetries on the electronic structure of $\rm CsCr_3 Sb_5$. The effective tight-binding model and Fermi surface comprise multiple Cr-$d$ orbitals and Sb-$p$ orbitals. The introduction of Hubbard-Kanamori interactions leads to orbital-selective band renormalization dominated by the $d_{xz}$ band, concurrently producing emergent flat bands very close to the Fermi level. Our analysis sets the stage for further investigations into the electronic properties of $\rm CsCr_3Sb_5$, including electronic orders, quantum criticality and unconventional superconductivity, which promise to shed much new light into the electronic materials with frustrated lattices and bring about new connections with the correlation physics of a variety of strongly correlated systems.

cond-mat.str-el↗

arXiv2025

Kondo-lattice phenomenology of twisted bilayer WSe$_2$ from compact molecular orbitals of topological bands

The discovery of superconductivity and correlated electronic phases in twisted bilayer WSe$_2$ (Xia et al., Nature 2024; Guo et al., Nature 2025) has generated considerable excitement. Accompanying the superconductivity and a correlated insulator phase is the Kondo-lattice-like phenomenology in transport properties. Here we consider how such phenomenology can develop when the combination of the active bands are topological. We advance a unique construction of compact molecular orbitals through a partial Wannierization that is symmetry preserving. The resulting Anderson lattice model provides the basis for a microscopic understanding of the experimental observation, including the involved energy scales. Our approach may apply to a broad range of settings where topology and correlations interplay.

cond-mat.str-el↗

arXiv2025

Resolving the Kagome Origin of the Strange Metallicity in Ni$_3$In

Strong correlations promote singular properties such as strange metallicity, which shows considerable commonality across quantum materials platforms. Understanding the mechanism for such emerging universality is an outstanding challenge, given that the underlying degrees of freedom can be complex and varied. Progress may be made in flat band systems, especially kagome and other frustrated-lattice metals with active flat bands. These systems show strange metal behavior that bears a striking resemblance to what happens in heavy-fermion metals. Here, in scanning tunneling spectroscopy of kagome metal Ni$_3$In, we find a zero-bias peak-dip structure whose variation with magnetic field and temperature tracks the evolution of the strange metal properties. We identify the origin of the peak as compact molecular orbitals formed by destructive interference over the kagome sites, resulting in emergent $f$-shell-like localized moments. Using quasi-particle interference, we visualize their interaction with the Dirac light bands. We thus unveil the essential microscopic ingredients of the $d$-electron-based kagome metals that, while distinct from the atomic orbitals of the $f$-electron-based heavy fermion materials, are responsible for a shared phenomenology between the two types of systems. Our findings provide a new window to uncover and interconnect the essential and yet diverse microscopic building blocks in disparate families of quantum materials that drive a convergence towards a universal understanding in the regime of amplified quantum fluctuations.

cond-mat.str-el↗

arXiv2025

Correlated flat-band physics in a bilayer kagome metal based on compact molecular orbitals

Flat bands, when located close to the Fermi energy, can considerably enhance the influence of electron correlations on the low energy physics in kagome and other frustrated-lattice metals. A major challenge in describing the interaction effects in such bulk materials is that the flat band is often intermixed with a large number of other bands. Here we show that the recently introduced notion of compact molecular orbitals (CMOs) enable a path forward in describing the dominant effect of the Coulomb interactions in spite of the complexity of the bandstructure. Our materials-based analysis allows for the understanding of the scanning-tunneling-microscopy experiment [J. C. Souza et al., preprint (2024)] of the bilayer kagome metal Ni$_3$In in terms of the CMO notion. From the resulting CMO, an effective Anderson lattice model can be set up. This CMO-based approach enables the calculation of correlation effects that is difficult to do based on the atomic orbitals. Furthermore, it suggests an enriched phase diagram for the strange metal physics of the kagome metal, which can be tested by future experiments. We discuss the implications of our results for the general correlation physics of flat band systems and beyond.

cond-mat.str-el↗

arXiv2025

Metallic quantum criticality enabled by flat bands in a kagome lattice

Strange metals arise in a variety of platforms for strongly correlated electrons, ranging from the cuprates, heavy fermions to flat band systems. Motivated by recent experiments in kagome metals, we study a Hubbard model on a kagome lattice whose noninteracting limit contains flat bands. A Kondo lattice description is constructed, in which the correlation effects are captured by symmetry preserving and exponentially localized molecular orbitals. These compact molecular orbitals represent the local degrees of freedom that emerge from topological flat bands. We identify a quantum critical point at which quasiparticles are lost and strange metallicity emerges. Our theoretical work opens up a new route for realizing beyond-Landau quantum criticality, as well as the associated strange metallicity and emergent quantum phases.

cond-mat.str-el↗

arXiv2025

Superconductivity in twisted WSe$_2$ from topology-induced quantum fluctuations

Recently, superconductivity has been observed in twisted WSe$_2$ moiré structures (Xia et al., Nature 2024; Guo et al., Nature 2025). Its transition temperature is high, reaching a few percent of the Fermi temperature scale. Here, we advance a mechanism for superconductivity based on the notion that electronic topology enables quantum fluctuations in a suitable regime of intermediate correlations. In this regime, the Coulomb interaction requires that an active topological flat band and nearby wider bands are considered together. Compact molecular orbitals arise, which give rise to quantum fluctuations through topology-dictated hybridization with the other molecular orbitals. The hybridization competes with the active flat band's natural tendency towards static electronic ordering, thereby weakening the latter; we link this effect with certain salient observations by experiments. Furthermore, the competition yields a quantum critical regime where quasiparticles are lost. The corresponding quantum critical fluctuations drive superconductivity. Broader implications and new connections among correlated materials platforms are discussed.

cond-mat.str-el↗

arXiv2025

Local and Non-local Entanglement Witnesses of Fermi Liquid

There is a growing interest both in utilizing entanglement means to characterize many-body systems and in uncovering their entanglement depth. Motivated by recent findings that the spin quantum Fisher information witnesses amplified multipartite entanglement of strange metals and characterizes their loss of quasiparticles, we study the quantum Fisher information in various cases of Fermi liquid. We show that local operators generically do not witness any multipartite entanglement in a Fermi liquid, but non-local many-body operators do. Our results point to novel experimental means to detect the entanglement depth of metallic fermionic systems and, in general, open a new avenue to the emerging exploration of entanglement in quantum materials.

cond-mat.str-el↗

arXiv2024

Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film

Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X=Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe d_xy+d_(x^2-y^2 ) spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems.

cond-mat.mes-hall↗

arXiv2024

Electronic properties, correlated topology and Green's function zeros

There is extensive current interest about electronic topology in correlated settings. In strongly correlated systems, contours of Green's function zeros may develop in frequency-momentum space, and their role in correlated topology has increasingly been recognized. However, whether and how the zeros contribute to electronic properties is a matter of uncertainty. Here we address the issue in an exactly solvable model for Mott insulator. We show that the Green's function zeros contribute to several physically measurable correlation functions, in a way that does not run into inconsistencies. In particular, the physical properties remain robust to chemical potential variations up to the Mott gap as it should be based on general considerations. Our work sets the stage for further understandings on the rich interplay among topology, symmetry and strong correlations.

cond-mat.str-el↗

arXiv2024

Chern Bands' Optimally Localized Wannier Functions and Fractional Chern Insulators

Recent development on fractional Chern insulators and proximate phases call for a real space representation of isolated Chern bands. Here we propose a new method for a general construction of optimally localized Wannier functions from such Chern bands. We do so through an optimal gauge choice of the Bloch states of a Chern band with the singularity placed at any desired position in momentum space. We apply this method to construct the optimally localized Wannier functions for kagome lattice, and use it to identify channels of interactions that are favorable to the development of fractional Chern insulators. Implications of the approach for the interplay between correlations and topology in broader contexts are discussed.

cond-mat.mes-hall↗

arXiv2024

Symmetry constraints and spectral crossing in a Mott insulator with Green's function zeros

Lattice symmetries are central to the characterization of electronic topology. Recently, it was shown that Green's function eigenvectors form a representation of the space group. This formulation has allowed the identification of gapless topological states even when quasiparticles are absent. Here we demonstrate the profundity of the framework in the extreme case, when interactions lead to a Mott insulator, through a solvable model with long-range interactions. We find that both Mott poles and zeros are subject to the symmetry constraints, and relate the symmetry-enforced spectral crossings to degeneracies of the original non-interacting eigenstates. Our results lead to new understandings of topological quantum materials and highlight the utility of interacting Green's functions toward their symmetry-based design.

cond-mat.str-el↗

arXiv2024

Emergent flat band and topological Kondo semimetal driven by orbital-selective correlations

Flat electronic bands are expected to show proportionally enhanced electron correlations, which may generate a plethora of novel quantum phases and unusual low-energy excitations. They are increasingly being pursued in $d$-electron-based systems with crystalline lattices that feature destructive electronic interference, where they are often topological. Such flat bands, though, are generically located far away from the Fermi energy, which limits their capacity to partake in the low-energy physics. Here we show that electron correlations produce emergent flat bands that are pinned to the Fermi energy. We demonstrate this effect within a Hubbard model, in the regime described by Wannier orbitals where an effective Kondo description arises through orbital-selective Mott correlations. Moreover, the correlation effect cooperates with symmetry constraints to produce a topological Kondo semimetal. Our results motivate a novel design principle for Weyl Kondo semimetals in a new setting, viz. $d$-electron-based materials on suitable crystal lattices, and uncover interconnections among seemingly disparate systems that may inspire fresh understandings and realizations of correlated topological effects in quantum materials and beyond.

cond-mat.str-el↗

arXiv2024

Kondo effect and its destruction in hetero-bilayer transition metal dichalcogenides

Moiré structures, along with line-graph-based $d$-electron systems, represent a setting to realize flat bands. One form of the associated strong correlation physics is the Kondo effect. Here, we address the recently observed Kondo-driven heavy fermion state and its destruction in AB-stacked hetero-bilayer transition metal dichalcogenides, which can be controlled by the gate voltages. By studying an effective interacting Hamiltonian using the slave spin approach, we obtained a phase diagram with the total filling factor and the displacement field strength as the tunable parameters. In an extended range of the tunable displacement field, our numerical results show that the relative filling of the $d$ orbital, which is associated with the highest moiré band from the $\rm MoTe_2$ layer, is enforced to be $ν_d \approx 1$ by the interaction. This agrees with the experimental observation. We also argue that the observed high coherence temperature scale could be explained by the non-negligible bandwidth of the $d$ orbital. Our results set the stage to address the amplified quantum fluctuations that the Kondo effect may produce in these structures and new regimes that the systems open up for Kondo-destruction quantum criticality.

cond-mat.str-el↗

arXiv2023

Topological Diagnosis of Strongly Correlated Electron Systems

The intersection of electronic topology and strong correlations offers a rich platform to discover exotic quantum phases of matter and unusual materials. An overarching challenge that impedes the discovery is how to diagnose topology in strongly correlated settings, as exemplified by Mott insulators. Here, we develop a general framework to address this outstanding question and illustrate its power in the case of Mott insulators. The concept of Green's function Berry curvature -- which is frequency dependent -- is introduced. We apply this notion in a system that contains symmetry-protected nodes in its noninteracting bandstructure; strong correlations drive the system into a Mott insulating state, creating contours in frequency-momentum space where the Green's function vanishes. The Green's function Berry flux of such zeros is found to be quantized, and is as such direct probe of the system's topology. Our framework allows for a comprehensive search of strongly correlated topological materials with Green's function topology.

cond-mat.str-el↗

arXiv2023

Phase diagram of twisted bilayer graphene at filling factor $ν=\pm3$

We study the correlated insulating phases of twisted bilayer graphene (TBG) in the absence of lattice strain at integer filling $ν=\pm3$. Using the self-consistent Hartree-Fock method on a particle-hole symmetric model and allowing translation symmetry breaking terms, we obtain the phase diagram with respect to the ratio of $AA$ interlayer hopping $(w_0)$ and $AB$ interlayer hopping $(w_1)$. When the interlayer hopping ratio is close to the chiral limit ($w_0/w_1 \lesssim 0.5$), a quantum anomalous Hall state with Chern number $ν_c = \pm 1$ can be observed consistent with previous studies. Around the realistic value $w_0/w_1 \approx 0.8$, we find a spin and valley polarized, translation symmetry breaking, state with $C_{2z}T$ symmetry, a charge gap and a doubling of the moiré unit cell, dubbed the $C_{2z}T$ stripe phase. The real space total charge distribution of this $C_{2z}T$ stripe phase in the flat band limit does not have modulation between different moiré unit cells, although the charge density in each layer is modulated, and the translation symmetry is strongly broken. Other symmetries, including $C_{2z}$, $C_{2x}$ and particle-hole symmetry $P$, and the topology of the $C_{2z}T$ stripe phase are also discussed in detail. We observed braiding and annihilation of the Dirac nodes by continuously turning on the order parameter to its fully self-consistent value, and provide a detailed explanation of the mechanism for the charge gap opening despite preserving $C_{2z}T$ and valley $U(1)$ symmetries. In the transition region between the quantum anomalous Hall phase and the $C_{2z}T$ stripe phase, we find an additional competing state with comparable energy corresponding to a phase with a tripling of the moiré unit cell.

cond-mat.str-el↗

arXiv2023

Deep learning reveals the common spectrum underlying multiple brain disorders in youth and elders from brain functional networks

Brain disorders in the early and late life of humans potentially share pathological alterations in brain functions. However, the key evidence from neuroimaging data for pathological commonness remains unrevealed. To explore this hypothesis, we build a deep learning model, using multi-site functional magnetic resonance imaging data (N=4,410, 6 sites), for classifying 5 different brain disorders from healthy controls, with a set of common features. Our model achieves 62.6(1.9)% overall classification accuracy on data from the 6 investigated sites and detects a set of commonly affected functional subnetworks at different spatial scales, including default mode, executive control, visual, and limbic networks. In the deep-layer feature representation for individual data, we observe young and aging patients with disorders are continuously distributed, which is in line with the clinical concept of the "spectrum of disorders". The revealed spectrum underlying early- and late-life brain disorders promotes the understanding of disorder comorbidities in the lifespan.

cs.AI↗

arXiv2022

Distribution Estimation of Contaminated Data via DNN-based MoM-GANs

This paper studies the distribution estimation of contaminated data by the MoM-GAN method, which combines generative adversarial net (GAN) and median-of-mean (MoM) estimation. We use a deep neural network (DNN) with a ReLU activation function to model the generator and discriminator of the GAN. Theoretically, we derive a non-asymptotic error bound for the DNN-based MoM-GAN estimator measured by integral probability metrics with the $b$-smoothness Hölder class. The error bound decreases essentially as $n^{-b/p}\vee n^{-1/2}$, where $n$ and $p$ are the sample size and the dimension of input data. We give an algorithm for the MoM-GAN method and implement it through two real applications. The numerical results show that the MoM-GAN outperforms other competitive methods when dealing with contaminated data.

stat.ML↗