arXiv subjects

Chunqiang Xu

Publications and source records attributed to Chunqiang Xu.

At least 19 recordsLinked to original sources

arXiv2026

Emergent magnetism, heavy electrons and pressure-induced reentrant superconductivity in the iron substituted 4d transition-metal sulfides

Superconductivity emerging from or in the vicinity of magnetic states is generally considered to be mediated by spin fluctuations and thus lies beyond the scope of conventional electron-phonon coupled BCS framework. Here we report the emergence of novel ferromagnetism in the d-electron rhodium sulfide Rh17S15 superconductor, characterized by an enhanced Sommerfeld coefficient γ arising from the flat topological band and many-body correlations. We further demonstrate that the ferromagnetism can be tuned via Fe substitution at the Rh sites, leading to a spin glass ground state induced by the competing ferromagnetic and antiferromagnetic exchange interactions. Fe doping results in a further enhancement of both the γ and electron effective masses. At a doping level of x = 0.67 in Rh17-xFexS15, γ reaches 312 mJ mol-1K-2, second only to the well-documented d-electron heavy-fermion material LiV2O4. Furthermore, upon applying pressure, superconductivity is first suppressed; under high pressures, however, we observe the reentrant superconductivity in both pristine and Fe-doped samples. Our results not only demonstrate the unusual magnetic states and possible heavy-fermion features in these frustration-free, d-based superconductors, but also suggest that the superconductivity in this system is likely mediated by the intrinsic spin fluctuations and may thus be unconventional.

cond-mat.supr-con↗

arXiv2026

Observation of Magnetic-Anisotropy Crossover and High-Temperature Skyrmions in the Dirac Magnet Fe3Ge with a Distorted Kagome Lattice

Topological materials that simultaneously host robust high-temperature skyrmions and nontrivial electronic band structures have attracted tremendous interest owing to their distinctive advantages for both fundamental research and prospective technological applications. Here, we report the observation of robust skyrmions in the Dirac kagome magnet Fe3Ge, which exhibits a high Curie temperature of ~ 650 K. At room temperature, Fe3Ge shows a large intrinsic anomalous Hall conductivity of ~ 380 Ω-1cm-1, originating from its nontrivial electronic band topology. Systematic magnetization measurements reveal a spin reorientation transition at ~ 375 K, indicating a crossover from easy-plane to easy-axis magnetic anisotropy. Below the spin reorientation temperature, a large topological Hall effect is observed, arising from microscopic noncoplanar spin structures. Lorentz transmission electron microscopy shows that mesoscopic skyrmions are stabilized in the easy-axis magnetic anisotropy regime and persist over an exceptionally wide temperature window of 375-650 K, far exceeding that of most previously reported skyrmion-hosting materials. These results establish Fe3Ge as a promising platform for exploring diverse topological properties, with strong potential for advancing future high-temperature spintronic applications, ranging from next-generation information storage to logic computing devices.

cond-mat.mtrl-sci↗

arXiv2026

Enhancement of metallicity by Na doping in La$_3$Ni$_2$O$_{7+δ}$

The observation of high-$T_c$ superconductivity in bilayer nickelate La$_3$Ni$_2$O$_7$ under high pressure provides a new venue for exploring novel unconventional superconductors and elucidating the mechanism of high-$T_c$ superconductivity. Subsequently, numerous chemical substitution studies have been reported, aiming to stabilize superconductivity at ambient pressure, or significantly reduce the pressure threshold required for its occurrence. Here, we report the comprehensive study on sodium (Na) doping in the Ruddlesden-Popper nickelate La$_3$Ni$_2$O$_{7+δ}$, where Na$^+$ substitutes for La$^{3+}$ at the A-site with varying doping concentrations. The structural, thermal, magnetic, and electronic transport properties of as-synthesized polycrystalline samples were systematically investigated. X-ray diffraction (XRD) analysis reveals that Na doping induces a structural transition from the '327' Amam phase to the '4310' Bmab phase when $x\geq0.075$, which is further corroborated by thermogravimetric analysis (TGA) measurements. Substitution of La$^{3+}$ with Na$^+$ gives rise to a gradual expansion of the '327' phase lattice. Meanwhile, resistivity measurements indicate that the density wave (DW) transition is marginally suppressed and metallicity is significantly enhanced. Upon the application of pressure, DW transition can be further suppressed, whereas the low-$T$ insulating behaviors remain insensitive to pressure. These results offer critical insights into the roles of elemental substitution and charge carrier doping in steering the competing electronic phases in layered nickelates.

cond-mat.supr-con↗

arXiv2026

Ferroelectricity in Atomically Thin Metallic TaNiTe$_5$ with Ultrahigh Carrier Density

Ferroelectric metals, characterized by the coexistence of ferroelectricity and metallic conductivity, present a fundamental challenge due to the screening effect of free charge carriers on the long-range electric dipole order. Existing strategies to circumvent this obstacle include employing two-dimensional (2D) crystals, where reduced dimensionality and low carrier densities suppress screening, or designing materials of van der Waals (vdW) superlattice with spatially separated and decoupled conductive and nearly insulating ferroelectric layers. Here, we report an alternative paradigm in TaNiTe5, where an ultrahigh carrier density coexists with an out-of-plane ferroelectric order within the same surface monolayer. Using piezoresponse force microscopy (PFM), we observed robust ferroelectric behavior in TaNiTe5 down to single-unit-cell thickness (~1.3 nm) at room temperature. Scanning transmission electron microscopy (STEM) gives structural evidence that the ferroelectricity might originate from the vertical displacement of outmost Te atoms on the surface, breaking the inversion symmetry. Concurrently, electrical transport measurements reveal a metallic state with a carrier density on the order of 10$^{15}$ cm$^{-2}$ (or 10$^{22}$ cm$^{-3}$) -- comparable to that of Copper (Cu). Our findings establish a unique platform for exploring the interplay between ferroelectricity and an ultrahigh density of mobile carriers in the 2D limit.

cond-mat.mtrl-sci↗

arXiv2026

Multigap nodeless superconductivity in Dirac semimetal PdTe

PdTe has recently been reported to be a type-II Dirac semimetal while a bulk nodal and surface nodeless superconductivity (SC) has been claimed to coexist. In this work, we applied point-contact spectroscopy (PCS) method to systematically study the superconducting gap in PdTe single crystals with a SC transition temperature $T_{c}=4.3$ K. The obtained differential conductance curves show a common deviation from a single-gap superconducting behavior and can be better fitted by a two-gap Blonder-Tinkham-Klapwijk model, suggesting the larger gap $Δ_{L}$ with $2Δ_{L}$=3.7 $k_{B}T_{c}$ and the smaller gap $Δ_S$ yielding $2Δ_{S}$=1.1-2.2 $k_{B}T_{c}$ with a weak interband scattering. The variations of conductance spectra among different contacts are proposed to be caused by the anisotropy of Fermi surface topology associated with different gaps.

cond-mat.supr-con↗

arXiv2025

Large Anomalous and Topological Hall Effect and Nernst Effect in a Dirac Kagome Magnet Fe3Ge

The search for kagome magnets with unconventional magnetic and electronic properties has gained significant attention in recent years. We report the magnetic, electronic, and thermoelectric properties of Fe3Ge single crystals, where the Fe atoms form a slightly distorted kagome lattice. Fe3Ge exhibits a large anomalous Hall effect and anomalous Nernst effect. The anomalous transverse thermoelectric conductivity reaches about 4.6 A m^-1 K^-1, exceeding values reported for conventional ferromagnets and most topological ferromagnets. First-principles calculations indicate that these transport responses are primarily governed by intrinsic mechanisms, highlighting the dominant role of Berry curvature arising from massive Dirac gaps in momentum space. In addition, we observe a topological Hall resistivity of about 0.9 microOhm cm and a topological Nernst coefficient of 1.2 microvolt K^-1, which are attributed to the Berry phase associated with field-induced scalar spin chirality. These findings demonstrate the combined influence of Berry phases in both momentum and real space, establishing Fe3Ge as a promising candidate for room-temperature transverse thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2025

In-Plane Magnetic Anisotropy and Large topological Hall Effect in Self-Intercalated Ferromagnet Cr1.61Te2

Self-intercalated chromium tellurides Cr1+xTe2 have garnered growing attention due to their high-temperature ferromagnetism, tunable spin structures and air stability, all of which are vital for versatile applications in next-generation memory and information technology. Here, we report strong magnetic anisotropy and a large topological Hall effect (THE) in self-intercalated Cr1.61Te2 single crystals, which are both highly desirable properties for future spintronic applications. Our results demonstrate that Cr1.61Te2 is a soft ferromagnet with strong in-plane magnetic anisotropy. Remarkably, distinct THE behaviors are observed in different temperature regimes, reflecting the intricate spin structures and competing exchange interactions. More interestingly, a large topological Hall resistivity, induced by microscopic non-coplanar spin structures, emerges in the temperature range 70-240 K, reaching a maximum value of 0.93 μΩ cm at 150 K. Moreover, a sign-reversed and weak THE is observed at low temperatures below ~70 K, indicating the emergence of an additional topological spin structure with opposite topological charges. This work not only offers valuable insights into the correlation between magnetocrystalline anisotropy and topological phenomena in Cr1+xTe2 systems, but also provides a robust platform for engineering the evolution of complex spin textures that can be leveraged in diverse spintronic device applications.

cond-mat.mtrl-sci↗

arXiv2025

Pressure-driven superconductivity in the topological insulator GeBi4Te7

The van der Waals, pseudo-binary chalcogenides (ACh)m(Pn2Ch3)n (A = Ge, Mn, Pb, etc.; Pn = Sb or Bi; Ch = Te, Se) have recently been reported to host a vast landscape of topological phases of matter, including the quantum anomalous Hall state and topological axion state with quantized magnetoelectric effect. A subgroup in this series, like MnSb4Te7 and GeSb4Te7, can be driven to a superconducting state by applying a physical pressure, making them viable candidates to realize so-called topological superconductivity. However, the role of magnetic fluctuations in this pressure-induced superconductivity remains unclear. Here, we report the pressure-induced multiple superconducting phases in the nonmagnetic GeBi4Te7, accompanied by corresponding structural transitions evidenced from the high-pressure Raman scattering. In comparison with other members in this family, we find the superconducting transition temperature of the nonmagnetic subgroup is significantly higher than their magnetic homologues, possibly hinting at the detrimental role played by the magnetic fluctuations in the superconductivity formation, at least in this pseudo-binary chalcogenide family.

cond-mat.supr-con↗

arXiv2024

Thermal Hall effect in a van der Waals ferromagnet CrI3

CrI3 is a prototypical van der Waals ferromagnet with a magnetic honeycomb lattice. Previous inelastic neutron scattering studies have suggested topological nature of its magnetic excitations with a magnon gap at the Dirac points, which are anticipated to give rise to magnon thermal Hall effect. Here we report thermal transport properties of CrI3 and show that the long-sought thermal Hall signal anticipated for topological magnons is fairly small. In contrast, we find that CrI3 exhibits an appreciable anomalous thermal Hall signal at lower temperature which may arise from magnon-phonon hybridization or magnon-phonon scattering. These findings are anticipated to stimulate further neutron scattering studies on CrI3 single crystal, which can shed light not only on the intrinsic nature of magnetic excitations but also on the magnon-phonon interaction.

cond-mat.mtrl-sci↗

arXiv2023

Pressure-tuned superconductivity in the Dirac semimetal PdTe

We report an unusual evolution of superconductivity (SC) in the three-dimensional Dirac semimetal PdTe with increasing pressure up to $\sim$50 GPa. The compressed PdTe exhibits a sudden reversal in the superconducting transition temperature $T\mathrm{_c}$, from an initial decrease with pressure to an increase above a critical pressure $P\mathrm{_c} \approx$ 15 GPa, showing a V-shaped feature composed of SC-I and SC-II phases due to a structural phase transition through $P\mathrm{_c}$. Subsequently, the $T\mathrm{_c}$ goes into a plateau around 2.5 K when the pressure is higher than $\sim$32 GPa. In addition, we find the variations of carrier concentrations and mobilities also manifest a similar trend on the pressure response as the $T\mathrm{_c}$, and the normal-state electronic properties change from the electron-dominated single-band model to two-carrier model after the structural phase transition, implying the close correlation between electronic properties and two SC phases. Our findings establish the SC of PdTe is highly tunable under varying pressures.

cond-mat.supr-con↗

arXiv2023

Multigap nodeless superconductivity in the topological semimetal PdTe

Recently PdTe was identified as a spin-orbit coupled topological Dirac semimetal and was claimed to exhibit both bulk-nodal and surface-nodeless superconducting gaps. Here we report the ultralow-temperature thermal conductivity measurements on PdTe single crystals with $T_c$ = 4.5 K to investigate its superconducting gap structure. It is found that the residual linear term $κ_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of $κ_0(H)/T$ exhibits an $\sf S$-shaped curve. These results suggest that PdTe has multiple nodeless superconducting gaps, which is at odds with the claimed bulk-nodal gap. The reason for the discrepancy is likely that previous angle-resolved photoemission spectroscopy measurements were only performed down to 2 K and cannot observe the smaller nodeless gap. The fully gapped superconducting state in PdTe is compatible with it being a topological superconductor candidate.

cond-mat.supr-con↗

arXiv2023

Thermal Hall effect in a van der Waals triangular magnet FeCl2

Thermal transport is a pivotal probe for studying low-energy, charge-neutral quasi-particles in insulating magnets. In this Letter, we report an observation of large magneto-thermal conductivity and thermal Hall effect (THE) in a van der Waals antiferromagnet FeCl2. The magneto-thermal conductivity reaches over ~700%, indicating strong magnon-phonon coupling. Furthermore, we find an appreciable thermal Hall signal which changes sign concurrently with the spin-flip transition from the antiferromagnetic state to the polarized ferromagnetic state. Our theoretical calculations suggest that, in addition to the Berry curvature induced at the anticrossing points of the hybridized magnon and acoustic phonon modes of FeCl2, other mechanisms are needed to account for the magnitude of the observed THE.

cond-mat.mtrl-sci↗

arXiv2022

de Haas-van Alphen effect and the first-principles study of the possible topological stannide Cu$_3$Sn

The quest for quantum materials with diverse symmetry-protected topological states has been the focus of recent research interest, primarily due to their fascinating physical properties and the potential technological utility. In this work, we report on the magnetotransport, de Haas-van Alphen (dHvA) oscillations, and the first-principles calculations of the stannide Cu$_3$Sn that is isostructural with the recently reported topological semimetal Ag$_3$Sn. The magnetoresistance was found to vary quasi-linearly in field. Clear dHvA oscillations were observed under a field as low as 1 Tesla at 2 K, with three major oscillation frequencies $F_α$=8.74 T, $F_β$=150.19 T and $F_γ$=229.66 T and extremely small effective masses. The analysis of dHvA quantum oscillations revealed a possible nonzero Berry phase, suggestive of the nontrivial band topology. The corroborating evidence for the nontrivial electronic topology also comes from the first-principles calculations which yield a nonzero $\mathbb{Z}_2$ topological index. These results collectively suggest that Cu$_3$Sn, in analogy to its homologue Ag$_3$Sn, may be another intermetallic stannide hosting topological Dirac fermions.

cond-mat.mtrl-sci↗

arXiv2022

Exchange-biased topological transverse thermoelectric effects in a Kagome ferrimagnet

Kagome metal TbMn6Sn6 was recently discovered to be a ferrimagnetic topological Dirac material by scanning tunneling microscopy/spectroscopy measurements. Here, we report the observation of large anomalous Nernst effect and anomalous thermal Hall effect in this compound. The anomalous transverse transport is consistent with the Berry curvature contribution from the massive Dirac gaps in the 3D momentum space as demonstrated by our first-principles calculations. Furthermore, the transverse thermoelectric transport exhibits asymmetry with respect to the applied magnetic field, i.e., an exchange-bias behavior. Together, these features place TbMn6Sn6 as a promising system for the outstanding thermoelectric performance based on anomalous Nernst effect.

cond-mat.mtrl-sci↗

arXiv2022

Time-Reversal Symmetry Breaking Superconductivity in Three-Dimensional Dirac Semimetallic Silicides

Superconductors with broken time-reversal symmetry represent arguably one of the most promising venues for realizing highly sought-after topological superconductivity that is vital to fault-tolerant quantum computation. Here, by using extensive muon-spin relaxation and rotation measurements, we report that the isostructural silicide superconductors (Ta, Nb)OsSi spontaneously break time-reversal symmetry at the superconducting transition while surprisingly showing a fully-gapped superconductivity characteristic of conventional superconductors. The first-principles calculations show that (Ta, Nb)OsSi are three-dimensional Dirac semimetals protected by nonsymmorphic symmetries. Taking advantage of the exceptional low symmetry crystal structure of these materials, we have performed detailed theoretical calculations to establish that the superconducting ground state for both (Ta, Nb)OsSi is most likely a nonunitary triplet state.

cond-mat.supr-con↗

arXiv2022

Exploring Native Atomic Defects in NiTe2

Nickel ditelluride (NiTe2), a new discovered type-II Dirac semimetal whose Dirac node lies close to its Fermi level, is expected to exhibit exotic phenomena including Lifshitz transition and superconductivity. As we know, defects are inevitable for transition metal dichalcogenides and have significant impacts on the optical and electronic properties. However, the systematic study of defects in NiTe2 is still lack. Here, by using high-resolution scanning tunneling microscopy combined with first-principles calculations, the point defects including the vacancy, intercalation and antisite defects in NiTe2 are systematically investigated. We identified five main types native defects and revealed that the growth condition could affect the type of native defects. By controlling the ratio of ingredient during synthesis, the types of point defects are expected to be manipulated, especially antisite defects. Additionally, we find native defects could slightly dope the topological surface states. Our results provide a facile way to manipulate defects for future optimizing the electronic properties of NiTe2 and other related materials.

cond-mat.mtrl-sci↗

arXiv2021

Anomalous Thermal Hall Effect in an Insulating van der Waals Magnet VI3

Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile playground for the discovery and exploration of physical phenomena and new physics. In this Letter, we report the observation of an anomalous thermal Hall effect (THE) with \k{appa}_xy ~ 1x10^(-2) W K^(-1) m^(-1) in an insulating van der Waals ferromagnet VI3. The thermal Hall signal persists in the absence of an external magnetic field and flips sign upon the switching of the magnetization. In combination with theoretical calculations, we show that VI3 exhibits a dual nature of the THE, i.e., dominated by topological magnons hosted by the ferromagnetic honeycomb lattice at higher temperatures and by phonons induced by the magnon-phonon coupling at lower temperatures. Our results not only position VI3 as the first ferromagnetic system to investigate both anomalous magnon and phonon THEs, but also render it as a potential platform for spintronics/magnonics applications.

cond-mat.mtrl-sci↗

arXiv2021

Coexistence of Ferroelectric-like Polarization and Dirac-like Surface State in TaNiTe5

By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), piezoresponse force microscopy (PFM) and first-principles calculations, we have studied the low-energy band structure, atomic structure and charge polarization on the surface of a topological semimetal candidate TaNiTe5. Dirac-like surface states were observed on the (010) surface by ARPES, consistent with the first-principles calculations. On the other hand, PFM reveals a switchable ferroelectric-like polarization on the same surface. We propose that the noncentrosymmetric surface reconstruction observed by STM could be the origin of the observed ferroelectric-like state in this novel material. Our findings provide a new platform with the coexistence of ferroelectric-like surface charge distribution and novel surface states.

cond-mat.mtrl-sci↗