arXiv subjects

Xiaolong Feng

Publications and source records attributed to Xiaolong Feng.

At least 19 recordsLinked to original sources

arXiv2026

Electronic correlations shape the low-energy optical response of the kagome antiferromagnets Mn$_3$Sn and Mn$_3$Ge

Using optical spectroscopy and density functional theory, we provide evidence that electron-electron interactions strongly affect the properties of the kagome antiferromagnet Mn$_3$Sn, since its low-energy optical interband transitions arise exclusively from the correlation-modified electronic band structure. Mn$_3$Sn possesses an optical effective mass about three times larger than that of its isostructural analog Mn$_3$Ge. The DFT+$U$ treatment in the Lichtenstein formulation, with an on-site Coulomb repulsion $U = 4$~eV and Hund's coupling $J = 0.25$~eV, accounts for the optical transitions in Mn$_3$Sn, whereas those in Mn$_3$Ge are already reproduced without Hubbard corrections ($U, J = 0$). The near-isotropic electronic structure of Mn$_3$Sn reveals a three-dimensional metal whose electronic response is shaped prominently by the Mn $d$-states. The apparent linear regime in $σ_1(ω)$ observed in Mn$_3$Sn and Mn$_3$Ge arises from several overlapping interband transitions and therefore should not be interpreted as the optical signature of the Weyl cones. The optical response does not alter with magnetic field up to 17 T, consistent with strong free-carrier screening. Our findings establish a comprehensive-correlated picture of Mn$_3$Sn and Mn$_3$Ge that offers a template for other correlated topological metals.

cond-mat.str-el↗

arXiv2026

Quantum Geometric Origin of Nonlinear Current Induced Orbital Magnetization

Electric generation of magnetization is a focus of condensed matter research, and has recently been advanced into the nonlinear regime. However, due to the nonlocal nature of orbital magnetism, how to properly formulate nonlinear current-induced orbital magnetization remains a fundamental challenge. Here, we develop the proper theory for this effect. This is based on the microscopic derivation of field-corrected orbital magnetic moment of a Bloch electron, a critical missing piece in the present theory. We show that the quantum geometric origin of this phenomenon lies in both the anomalous orbital polarizability and the Berry-connection polarizability, which often provide competing contributions. Combining our theory with first-principles calculations, we predict significant, experimentally accessible nonlinear orbital magnetization generated in strained bilayer graphene, monolayer 1T' $\mathrm{MoS_2}$ and $\mathrm{MoTe_2}$. Remarkably, nonlinear orbital magnetization can dominate over its spin counterpart in materials with topological band features, irrespective of the spin-orbit coupling strength.

cond-mat.mes-hall↗

arXiv2026

Efficient spin-pumping and spin transport across epitaxial Mn$_3$Sn(0001) noncollinear antiferromagnet/permalloy interfaces

The generation and control of spin currents are crucial for advancing next-generation spintronic technologies. These technologies depend on materials capable of efficiently sourcing and interconverting spin and charge currents, while overcoming some limitations associated with conventional ferromagnets and heavy metals. Kagome topological antiferromagnetic Weyl semimetals, such as Mn3Sn, present unique advantages owing to their distinct magnetic order and significant Berry curvature-driven transport phenomena. In this study, we systematically investigate spin current generation and spin-to-charge conversion phenomena in epitaxial (0001)-oriented Mn$_3$Sn thin films. Our findings reveal a spin Hall angle of 0.9$\%$ and a nearly isotropic in-plane spin Hall conductivity of ~44.4 ($\hbar$/e) $Ω^{-1}$.cm$^{-1}$ at room temperature, originating from a combination of intrinsic and extrinsic contributions, as discussed in light of first-principle calculations. Furthermore, in Mn$_3$Sn(0001)/Ni$_{81}$Fe$_{19}$ heterostructures, we observe a high spin-mixing conductance of 28.52 nm$^{-2}$ and an interfacial spin-transparency of approximately 72$\%$. Notably, we also find that the spin diffusion length in Mn$_3$Sn(0001) epitaxial films exceeds 15 nm at room temperature. Our results highlight the potential, and limitations, of the topological Weyl noncollinear antiferromagnet Mn$_3$Sn as an efficient material for spin transport and conversion in prospective spintronic applications.

cond-mat.mtrl-sci↗

arXiv2026

An AI-driven robotic system for two-dimensional hetero-assemblies

Nanomaterials stacked on-demand, such as rotationally assembled two-dimensional (2D) van der Waals (vdW) layered compounds, provides a versatile platform for quantum simulation and the exploration of exotic electronic phases. Currently, however, such nanoassemblies remain largely confined to inefficiency, manually operated process, limiting their potential for probing emergent physical phenomena. There is a pressing need in the field for high-precision, automated assembling techniques, especially for the scalable fabrication of 2D twistronic heterostructures. Here, we present an intelligent automation system dedicated to the fabrication of van der Waals stacks, following the state-of-the-art protocol for dry transfer of exfoliated 2D materials. The system further employs metadata generated from each automated stacking procedure to perform reinforcement learning, thereby continuously bettering its performances. As a concrete demonstration, we fabricate twisted bilayer graphene (TBLG) -- known for its challenging preparation -- and exhibit its unconventional superconductivity near the magic angle. Our work may pave the way for high-throughput fabrication of low-dimensional nanomaterials including twistronic heterostructures, where integrating data mining and artificial intelligence can accelerate the discovery of novel physical phenomena.

cond-mat.mes-hall↗

arXiv2026

Bond-density-wave orders induced by geometric frustration in the kagome metal CeRu3Si2

Geometric frustration gives rise to vast manifolds of degenerate ground states and competing orders in spin and charge systems. Typically, classical ground states are governed by a local ``zero-sum constraint" that relieves frustrated antiferromagnetic interactions or Coulomb repulsion. To date, the paradigm of geometric frustration has yielded a rich landscape of emergent phases, from spin ices and quantum spin liquids to charge glasses. However, an analogous phase rooted in chemical bonding has yet to be firmly demonstrated. Here we report the discovery of bond-density-wave orders induced by geometric frustration in the kagome metal CeRu$_3$Si$_2$ above room temperature. Through synchrotron X-ray diffraction, real-space transmission electron microscopy, and model calculations, we observe two distinct long-period superlattices with harmonic and anharmonic structural modulations. Crucially, interlayer bonds between kagome planes modulate in a sublattice-selective manner to fulfill the zero-sum constraint on the kagome lattice. We demonstrate the potential of kagome metals to host complex bond-ordered states constrained by geometric frustration and establish chemical bonding as a distinct pathway to frustration physics in quantum materials even above room temperature.

cond-mat.str-el↗

arXiv2026

Anisotropic anomalous Hall effect in distorted kagome GdTi3Bi4

Topological kagome magnets offer a rich landscape for exploring the intricate interplay of quantum interactions among geometry, topology, spin, and correlation. GdTi3Bi4 crystallizes in layered Ti based kagome nets intertwined with zigzag Gd chains along the a axis and orders antiferromagnetically below 15 K. Here, we present the temperature and field dependent electrical transport of GdTi3Bi4 in different directions. The material exhibits anomalous Hall conductivity (AHC) of 410 S cm-1 at 2 K for B parallel c and it is completely absent for B parallel a, despite the similar magnetization observed in both orientations. This behavior is quite contradictory, as anomalous Hall effect (AHE) typically scales with the magnetization. Through first principles calculations, it is demonstrated that in the presence of time reversal symmetry broken by the Gd 4f sublattice and spin orbit coupling, the magnetization direction controls the orbital mixing in the Ti t2g bands, relocating Berry curvature hot spots and producing the observed orientation selective AHC. The results establish GdTi3Bi4 as platform for investigating new avenues of AHE, such as directional AHE, and thus shed new light on the intricate coupling between magnetic and electronic structures, paving the way for exploring novel quantum phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Orbital-selective band modifications in a charge-ordered kagome metal LuNb$_6$Sn$_6$

The origin of the charge order in kagome lattice materials has attracted great interest due to the unique electronic structure features connected to kagome networks and the interplay between electron and lattice degrees of freedom. Recently, compounds with composition $Ln$Nb$_6$Sn$_6$ ($Ln$ = Ce-Nd, Sm, Gd-Tm, Lu, Y) appear as a new family of kagome metals, structurally analogous to $R$V$_6$Sn$_6$ ($R$ = Sc, Y, or rare earth) systems. Among them, LuNb$_6$Sn$_6$ emerges as a novel material hosting charge density wave (CDW) with a $\sqrt{3}$ $\times$ $\sqrt{3}$ $\times$ $3$ wave vector, akin to that in ScV$_6$Sn$_6$. Here, we employ high-resolution angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and density functional theory calculations to systematically investigate the electronic properties of LuNb$_6$Sn$_6$. Our observation reveals the characteristic band structures of the "166" kagome system. A charge instability driven by Fermi surface nesting is decisively ruled out through an analysis of the interactions between van Hove singularities. Across the CDW transition, we observe orbital-selective band modifications, with noticeable evolutions of Lu 5$d$ and Sn 5$p$ electrons, while Nb 4$d$ electrons exhibit minimal change, suggesting that the Lu and Sn sites other than the Nb kagome lattice play a key role in the formation of CDW. Our findings substantiate a universal lattice-driven CDW mechanism rather than a charge-instability-driven one in the "166" kagome compounds, making it a distinct material class compared to other charge-ordered kagome systems, such as $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) and FeGe.

cond-mat.str-el↗

arXiv2024

Orbital Origin of Intrinsic Planar Hall Effect

Recent experiments reported an antisymmetric planar Hall effect, where the Hall current is odd in the in-plane magnetic field and scales linearly with both electric and magnetic fields applied. Existing theories rely exclusively on a spin origin, which requires spin-orbit coupling to take effect. Here, we develop a general theory for the intrinsic planar Hall effect (IPHE), highlighting a previously unknown orbital mechanism and connecting it to a band geometric quantity -- the anomalous orbital polarizability (AOP). Importantly, the orbital mechanism does not request spin-orbit coupling, so sizable IPHE can occur and is dominated by orbital contribution in systems with weak spin-orbit coupling. Combined with first-principles calculations, we demonstrate our theory with quantitative evaluation for bulk materials $\mathrm{TaSb_{2}}$, $\mathrm{NbAs_{2}}$, and $\mathrm{SrAs_{3}}$. We further show that AOP and its associated orbital IPHE can be greatly enhanced at topological band crossings, offering a new way to probe topological materials.

cond-mat.mes-hall↗

arXiv2024

Tuning charge density wave of kagome metal ScV6Sn6

Compounds with a kagome lattice exhibit intriguing properties and the charge density wave (CDW) adds an additional layer of interest to research on them. In this study, we investigate the temperature and magnetic field dependent electrical properties under a chemical substitution and hydrostatic pressure of ScV6Sn6, a non-magnetic charge density wave (CDW) compound. Substituting 5 % Cr at the V site or applying 1.5 GPa of pressure shifts the CDW to 50 K from 92 K. This shift is attributed to the movement of the imaginary phonon band, as revealed by the phonon dispersion relation. The longitudinal and Hall resistivities respond differently under these stimuli. The magnetoresistance (MR) maintains its quasilinear behavior under pressure, but it becomes quadratic after Cr substitution. The anomalous Hall-like behavior of the parent compound persists up to the respective CDW transition under pressure, after which it sharply declines. In contrast, the longitudinal and Hall resistivities of Cr substituted compounds follow a two-band model and originates from the multi carrier effect. These results clearly highlight the role of phonon contributions in the CDW transition and call for further investigation into the origin of the anomalous Hall-like behavior in the parent compound.

cond-mat.mtrl-sci↗

arXiv2024

Quantum oscillations revealing topological band in kagome metal ScV6Sn6

Compounds with kagome lattice structure are known to exhibit Dirac cones, flat bands, and van Hove singularities, which host numerous versatile quantum phenomena. Inspired by these intriguing properties, we investigate the temperature and magnetic field dependent electrical transports along with the theoretical calculations of ScV6Sn6, a nonmagnetic charge density wave (CDW) compound. At low temperatures, the compound exhibits Shubnikov-de Haas quantum oscillations, which help to design the Fermi surface (FS) topology. This analysis reveals the existence of several small FSs in the Brillouin zone, combined with a large FS. Among them, the FS possessing Dirac band is a non-trivial and generates a non-zero Berry phase. In addition, the compound also shows the anomalous Hall-like behaviour up to the CDW with the CDW phase, ScV6Sn6 presents a unique material example of the versatile HfFe6Ge6 family and provides various promising opportunities to explore the series further.

cond-mat.mtrl-sci↗

arXiv2023

Pb$_9$Cu(PO4)$_6$(OH)$_2$: Phonon bands, Localized Flat Band Magnetism, Models, and Chemical Analysis

In a series of recent reports, doped lead apatite (LK-99) has been proposed as a candidate ambient temperature and pressure superconductor. However, from both an experimental and theoretical perspective, these claims are largely unsubstantiated. To this end, our synthesis and subsequent analysis of an LK-99 sample reveals a multiphase material that does not exhibit high-temperature superconductivity. We study the structure of this phase with single-crystal X-ray diffraction (SXRD) and find a structure consistent with doped $\text{Pb}_{10}(\text{PO}_4)_6(\text{OH})_2$. However, the material is transparent which rules out a superconducting nature. From ab initio defect formation energy calculations, we find that the material likely hosts $\text{OH}^-$ anions, rather than divalent $\text{O}^{2-}$ anions, within the hexagonal channels and that Cu substitution is highly thermodynamically disfavored. Phonon spectra on the equilibrium structures reveal numerous unstable phonon modes. Together, these calculations suggest it is doubtful that Cu enters the structure in meaningful concentrations, despite initial attempts to model LK-99 in this way. However for the sake of completeness, we perform ab initio calculations of the topology, quantum geometry, and Wannier function localization in the Cu-dominated flat bands of four separate doped structures. In all cases, we find they are atomically localized by irreps, Wilson loops, and the Fubini-Study metric. It is unlikely that such bands can support strong superfluidity, and instead are susceptible to ferromagnetism (or out-of-plane antiferromagnetism) at low temperatures, which we find in ab initio studies. In sum, $\text{Pb}_{9}\text{Cu}(\text{PO}_4)_6(\text{OH})_2$ could more likely be a magnet, rather than an ambient temperature and pressure superconductor.

cond-mat.supr-con↗

arXiv2023

Kagome Materials I: SG 191, ScV$_6$Sn$_6$. Flat Phonon Soft Modes and Unconventional CDW Formation: Microscopic and Effective Theory

Kagome Materials with flat bands exhibit wildly different physical properties depending on symmetry group, and electron number. For the case of ScV$_6$Sn$_6$ in space group 191, we investigate the existence of a charge density wave (CDW) at vector $\bar{K}=(\frac{1}{3},\frac{1}{3},\frac{1}{3})$ and its relationship with the phonon behavior. The experimental findings reveal a $\sim$95K CDW without nesting/peaks in the electron susceptibility at $\bar{K}$. Notably, ScV$_6$Sn$_6$ exhibits a collapsed phonon mode at $H=(\frac{1}{3},\frac{1}{3},\frac{1}{2})$ and an imaginary flat phonon band in the vicinity of $H$. The soft phonon is attributed to triangular Sn ($Sn^T$) mirror-even vibrations along the $z$-direction. We develop a simple force constant model to describe the entire soft phonon dispersion. By employing a new (Gaussian) approximation of the hopping parameter, we demonstrate the renormalization of the phonon frequency and the consequent collapse of the $H$ phonon. Additionally, we propose an effective model with two order parameters (OPs) to explain the appearance of the CDW at $\bar{K}$, which competes with the collapsed phonon at $H$. Through comparisons with experimental data, we show that the $H$ OP undergoes a second-order phase transition while exhibiting substantial fluctuations, ultimately inducing the first-order transition of the $\bar{K}$ OP. Furthermore, we extend our analysis to the similar compound YV$_6$Sn$_6$, which lacks a CDW phase, attributing this difference to the participation of the heavier Y atom in the out-of-plane phonon behavior. Our comprehensive study not only elucidates the CDW in ScV$_6$Sn$_6$ but also presents a significant advancement in modeling complex electronic systems, fostering collaborations between ab-initio simulations and analytical approaches.

cond-mat.str-el↗

arXiv2022

Plasmons in a two-dimensional nonsymmorphic nodal-line semimetal

Recent experiments have established a type of nonsymmorphic symmetry protected nodal lines in the family of two-dimensional (2D) composition tunable materials NbSi$_x$Te$_2$. Here, we theoretically study the plasmonic properties of such nonsymmorphic nodal-line semimetals. We show that the nonsymmorphic character endows the plasmons with extremely strong anisotropy. There exist both intraband and interband plasmon branches. The intraband branch is gapless and has a $q^{1/2}$ dispersion. It is most dispersive and is independent of carrier density in direction normal to the nodal line, whereas along the nodal line, its dispersion is largely suppressed and its frequency scales linearly with carrier density. The interband branches are gapped and their long wavelength limits are connected with van Hove singularities of the band structure. We find that the single particle excitations are strongly suppressed in such systems, which decreases the Landau damping of plasmons. These characters are further verified by first-principles calculations on 2D NbSi$_x$Te$_2$. Interesting features in static screening of charged impurity are also discussed. Our result reveals characteristic plasmons in a class of nonsymmorphic topological semimetals and offers guidance for its experimental detection and possible applications.

cond-mat.mes-hall↗

arXiv2022

Time-Reversal-Even Nonlinear Current Induced Spin Polarization

We propose a time-reversal-even spin generation in second order of electric fields, which dominates the current induced spin polarization in a wide class of centrosymmetric nonmagnetic materials, and leads to a novel nonlinear spin-orbit torque in magnets. We reveal a quantum origin of this effect from the momentum space dipole of the anomalous spin polarizability. First-principles calculations predict sizable spin generations in several nonmagnetic hcp metals, in monolayer TiTe$_{2}$, and in ferromagnetic monolayer MnSe$_{2}$, which can be detected in experiment. Our work opens up the broad vista of nonlinear spintronics in both nonmagnetic and magnetic systems.

cond-mat.mes-hall↗

arXiv2022

Periodic Clifford symmetry algebras on flux lattices

Real Clifford algebras play a fundamental role in the eight real Altland-Zirnbauer symmetry classes and the classification tables of topological phases. Here, we present another elegant realization of real Clifford algebras in the $d$-dimensional spinless rectangular lattices with $π$ flux per plaquette. Due to the $T$-invariant flux configuration, real Clifford algebras are realized as projective symmetry algebras of lattice symmetries. Remarkably, $d$ mod $8$ exactly corresponds to the eight Morita equivalence classes of real Clifford algebras with eightfold Bott periodicity, resembling the eight real Altland-Zirnbauer classes. The representation theory of Clifford algebras determines the degree of degeneracy of band structures, both at generic $k$ points and at high-symmetry points of the Brillouin zone. Particularly, we demonstrate that the large degeneracy at high-symmetry points offers a rich resource for forming novel topological states by various dimerization patterns, including a $3$D higher-order semimetal state with double-charged bulk nodal loops and hinge modes, a $4$D nodal surface semimetal with $3$D surface solid-ball zero modes, and $4$D Möbius topological insulators with a eightfold surface nodal point or a fourfold surface nodal ring. Our theory can be experimentally realized in artificial crystals by their engineerable $\mathbb{Z}_2$ gauge fields and capability to simulate higher dimensional systems.

cond-mat.mes-hall↗

arXiv2022

Intrinsic Nonlinear Planar Hall Effect

We propose an intrinsic nonlinear planar Hall effect, which is of band geometric origin, independent of scattering, and scales with the second order of electric field and first order of magnetic field. We show that this effect is less symmetry constrained compared to other nonlinear transport effects and is supported in a large class of nonmagnetic polar and chiral crystals. Its characteristic angular dependence provides an effective way to control the nonlinear output. Combined with first-principles calculations, we evaluate this effect in the Janus monolayer MoSSe and report experimentally measurable results. Our work reveals an intrinsic transport effect, which offers a new tool for material characterization and a new mechanism for nonlinear device application.

cond-mat.mes-hall↗

arXiv2022

Observation of Dimension-Crossover of a Tunable 1D Dirac Fermion in Topological Semimetal NbSi$_x$Te$_2$

Condensed matter systems in low dimensions exhibit emergent physics that does not exist in three dimensions. When electrons are confined to one dimension (1D), some significant electronic states appear, such as charge density wave, spin-charge separations and Su-Schrieffer-Heeger (SSH) topological state. However, a clear understanding of how the 1D electronic properties connects with topology is currently lacking. Here we systematically investigated the characteristic 1D Dirac fermion electronic structure originated from the metallic NbTe$_2$ chains on the surface of the composition-tunable layered compound NbSi$_x$Te$_2$ ($x$ = 0.40 and 0.43) using angle-resolved photoemission spectroscopy. We found the Dirac fermion forms a Dirac nodal line structure protected by the combined $\widetilde{\mathcal{M}}{\rm_y}$ and time-reversal symmetry T and proves the NbSi$_x$Te$_2$ system as a topological semimetal, in consistent with the ab-initio calculations. As $x$ decreases, the interaction between adjacent NbTe2 chains increases and Dirac fermion goes through a dimension-crossover from 1D to 2D, as evidenced by the variation of its Fermi surface and Fermi velocity across the Brillouin zone in consistence with a Dirac SSH model. Our findings demonstrate a tunable 1D Dirac electron system, which offers a versatile platform for the exploration of intriguing 1D physics and device applications.

cond-mat.mtrl-sci↗

arXiv2022

Berry connection polarizability tensor and third-order Hall effect

One big achievement in modern condensed matter physics is the recognition of the importance of various band geometric quantities in physical effects. As prominent examples, Berry curvature and the Berry curvature dipole are connected to the linear and the second-order Hall effects, respectively. Here, we show that the Berry connection polarizability (BCP) tensor, as another intrinsic band geometric quantity, plays a key role in the third-order Hall effect. Based on the extended semiclassical formalism, we develop a theory for the third-order charge transport and derive explicit formulas for the third-order conductivity. Our theory is applied to the two-dimensional (2D) Dirac model to investigate the essential features of the BCP and the third-order Hall response. We further demonstrate the combination of our theory with the first-principles calculations to study a concrete material system, the monolayer FeSe. Our work establishes a foundation for the study of third-order transport effects, and reveals the third-order Hall effect as a tool for characterizing a large class of materials and for probing the BCP in band structure.

cond-mat.mes-hall↗