arXiv subjects

Xuezhi Chen

Publications and source records attributed to Xuezhi Chen.

10 recordsLinked to original sources

arXiv2026

Termination-Dependent Surface States and Magnetic Fingerprints of Chiral Helimagnet Cr1/3TaS2

Chiral helimagnets based on intercalated transition-metal dichalcogenides, characterized by nano-scale spin ordering, provide a powerful route to engineer chiral spin textures (e.g. the topologically protected magnetic solitons) and emergent electronic functionality at reduced dimensions, where surface and interface states often dominate device operation. However, despite growing interest, direct experimental studies of termination-dependent surface electronic structures and their temperature-driven magnetic evolution remain largely unexplored, hindering a microscopic understanding of the electronic states that is crucial for the development of low-dimensional spintronic devices. Here, for the first time, taking Cr1/3TaS2 as a representative example, we systematically investigate the termination-dependent surface electronic states of the chiral helimagnets and uncover their distinct temperature evolution across the magnetic transition (TC~142K) by combining high-resolution ARPES with a micro-focused beam and surface-state-resolved first-principles calculations. The TaS2-terminated surface hosts folded monolayer-like TaS2 bands under the $\sqrt3\times\sqrt3$ superlattice potential and a shallow triangular electron pocket at the superlattice $\bar K$ point arising from Cr-Ta orbital hybridization. In contrast, the Cr-terminated surface exhibits reconstructed hole pockets with pronounced magnetic band splitting. This splitting disappears above TC and closely follows the chiral helimagnetic order parameter, providing a direct spectroscopic fingerprint of chiral helimagnetic order. In addition, multiple ultranarrow Cr-d-derived surface flat bands are resolved. These findings establish Cr1/3TaS2 as a model system in which surface electronic states are strongly coupled to chiral magnetism, opening new opportunities for chiral spintronic and valleytronic micro/nanodevices.

cond-mat.mtrl-sci↗

arXiv2025

Observation of Orbital-Selective Dual Modulations in an Anisotropic Antiferromagnetic Kagome Metal TbTi3Bi4

Orbital selectivity is pivotal in dictating the phase diagrams of multiorbital systems, with prominent examples including the orbital-selective Mott phase and superconductivity, etc. The intercalation of anisotropic layers represents an effective method for enhancing orbital selectivity and, thereby shaping the low-energy physics of multiorbital systems. Despite its potential, related experimental studies remain limited. In this work, we systematically examine the interplay between orbital selectivity and magnetism in the newly discovered anisotropic kagome TbTi3Bi4 single crystal, and report a unidirectional, orbital-selective band reconstruction within the antiferromagnetic (AFM) state. By combining soft X-ray and vacuum ultraviolet angle-resolved photoemission spectroscopy (ARPES) measurements with orbital-resolved density functional theory (DFT) calculations, we identify that the band reconstruction is a manifestation of the AFM order, driven by a 1/3 nesting instability of the intercalated Tb 5dxz orbitals. Such an orbital-selective modulation leads the unusual momentum-dependent band folding and the emergence of symmetry-protected Dirac cones only at the M1 point. More importantly, the discovery of orbital-selective 3 x 1 AFM order offers crucial insights into the underlying mechanism of the fractional magnetization plateau in this Kagome AFM metal. Our findings not only underscore the essential role of both conducting and localized electrons in determining the magnetic orders of LnTi3Bi4 (Ln = Lanthanide) kagome metals but also offer a pathway for manipulating magnetism through selective control of anisotropic electronic structures.

cond-mat.str-el↗

arXiv2024

Two-point polynomial patterns in subsets of positive density in $\mathbb{R}^n$

Let $γ(t)=(P_1(t),\ldots,P_n(t))$ where $P_i$ is a real polynomial with zero constant term for each $1\leq i\leq n$. We will show the existence of the configuration $\{x,x+γ(t)\}$ in sets of positive density $ε$ in $[0,1]^n$ with a gap estimate $t\geq δ(ε)$ when $P_i$'s are arbitrary, and in $[0,N]^n$ with a gap estimate $t\geq δ(ε)N^n$ when $P_i$'s are of distinct degrees where $δ(ε)=\exp\left(-\exp\left(cε^{-4}\right)\right)$ and $c$ only depends on $γ$. To prove these two results, decay estimates of certain oscillatory integral operators and Bourgain's reduction are primarily utilised. For the first result, dimension-reducing arguments are also required to handle the linear dependency. For the second one, we will prove a stronger result instead, since then an anisotropic rescaling is allowed in the proof to eliminate the dependence of the decay estimate on $N$. And as a byproduct, using the strategy token to prove the latter case, we extend the corner-type Roth theorem previously proven by the first author and Guo.

math.CA↗

arXiv2024

Orbital-selective effect of spin reorientation on the Dirac fermions in a non-charge-ordered kagome ferromagnet Fe$_3$Ge

Kagome magnets provide a fascinating platform for the realization of correlated topological quantum phases under various magnetic ground states. However, the effect of the magnetic spin configurations on the characteristic electronic structure of the kagome lattice layer remains elusive. Here, utilizing angle-resolved photoemission spectroscopy and density functional theory calculations, we report the spectroscopic evidence for the spin-reorientation effect of a kagome ferromagnet Fe$_3$Ge, which is composed solely of kagome planes. As the Fe moments cant from the $c$ axis into the $ab$ plane upon cooling, the two kinds of kagome-derived Dirac fermions respond quite differently. The one with less-dispersive bands ($k_z$ $\sim$ 0) containing the $3d_{z^2}$ orbitals evolves from gapped into nearly gapless, while the other with linear dispersions ($k_z$ $\sim$ $π$) embracing the $3d_{xz}$/$3d_{yz}$ components remains intact, suggesting that the effect of spin reorientation on the Dirac fermions has an orbital selectivity. Moreover, we demonstrate that there is no signature of charge order formation in Fe$_3$Ge, contrasting with its sibling compound FeGe, a newly established charge-density-wave kagome magnet.

cond-mat.str-el↗

arXiv2024

$L^p$ maximal estimates for Weyl sums with $k\ge3$ on $\mathbb{T}$

In this paper, we study the $L^p$ maximal estimates for the Weyl sums $\sum_{n=1}^{N}e^{2πi(nx + n^{k}t)}$ with higher-order $k\ge3$ on $\mathbb{T}$, and obtain the positive and negative results. Especially for the case $k=3$, our result is sharp up to the endpoint. The main idea is to investigate the structure of the set where large values of Weyl sums are achieved by making use of the rational approximation and the refined estimate for the exponential sums.

math.NT↗

arXiv2023

Coexistence of near-EF flat band and van Hove singularity in a two-phase superconductor

In quantum many-body systems, particularly, the ones with large near-EF density states, like flat bands or van Hove singularity (VHS), electron correlations often give rise to rich phase diagrams with multiple coexisting/competing orders occurring at similar energy scales. The recently discovered locally noncentrosymmetric heavy fermion superconductor CeRh2As2 has stimulated extensive attention due to its unusual H-T phase diagram, consisting of two-phase superconductivity, antiferromagnetic order, and possible quadrupole-density wave orders. However, despite its great importance, the near-EF electronic structure remains experimentally elusive. Here, we provide this key information by combining soft X-ray and vacuum ultraviolet (VUV) angle-resolved photoemission spectroscopy measurements and atom-resolved DFT+U calculations. With bulk-sensitive soft X-rays, we reveal quasi-2D hole- and 3D electron- pockets with a pronounced nesting feature. Most importantly, we observe a symmetry-protected fourfold VHS coexisting with the Ce 4f flat bands near the EF, which, to the best of our knowledge, has never been reported before. Such a rare coexistence is expected to lead to a large density of states at the zone edge, enhancement in electron correlations, and a large upper critical field of the odd-parity superconducting phase. Uniquely, it will also result in a new type of f-VHS hybridization that alters the order and fine electronic structure of the symmetry-protected VHS and flat bands. These peculiarities offer important dimensions for understanding the reported rich phase diagram and are discussed as an origin of superconductivity with two phases. Our findings not only provide key insights into the nature of multiple phases in CeRh$_2$As$_2$, but also open up new prospects for exploring the novelties of many-body systems with f-VHS hybridization.

cond-mat.str-el↗

arXiv2023

A polynomial Roth theorem for corners in $\mathbb{R}^2$ and a related bilinear singular integral operator

We prove a quantitative Roth-type theorem for polynomial corners in $\mathbb{R}^2$. Let $P_1$ and $P_2$ be two linearly independent polynomials with zero constant term. We show that any measurable subset of $[0,1]^2$ with positive measure contains three points $(x,y)$, $(x+P_1(t),y)$, $(x,y+P_2(t))$ with a gap estimate on $t$. We also prove boundedness results for a variant of the triangular Hilbert transform involving two polynomials and its associated maximal function. These results extend some earlier work of Christ, Durcik and Roos. The key of the proof is to establish certain smoothing inequalities involving two polynomials. To accomplish that we give sublevel set estimates with general polynomials, explicit exponents and simplified proofs.

math.CA↗

arXiv2020

Two bipolynomial Roth theorems in $\mathbb{R}$

We give two Roth theorems, related to the nonlinear configuration $x$, $x+P_1(t)$, $x+P_2(t)$ involving two polynomials, for sets in $\mathbb{R}$ of positive density and of fractional dimensions. The proof uses Fourier analysis.

math.CA↗

arXiv2005

A novel route to lower the dielectric loss in CaCu$_{3}$Ti$_{4}$O$_{12}$ ceramics

Based upon a widely-accepted internal-barrier-layer-capacitance (IBLC) model, the dielectric loss in the giant-dielectric-constant CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO) material is expected to decrease as the conductivity of the CCTO grains/subgrains increases. In this letter, we report that this idea was successfully realized through the La-for-Ca substitution in the CCTO ceramics. The impedance spectroscopy analysis gives the evidence.

cond-mat.mtrl-sci↗

arXiv2004

Proton incorporations and superconductivity in a cobalt oxyhydrate

We report the evidence of proton incorporations in a newly-discovered cobalt oxyhydrate superconductor. During the hydration process for Na$_{0.32}$CoO$_{2}$ by the direct reaction with water liquid, it was shown that substantial NaOH was gradually liberated, indicating that H$^{+}$ is incorporated into the hydrated compound. Combined with the thermogravimetric analysis, the chemical composition of the typical sample is Na$_{0.22}$H$_{0.1}$CoO$_{2}\cdot 0.85$H$_{2}$O, which shows bulk superconductivity at 4.4 K.

cond-mat.supr-con↗