arXiv subjects

Xiuquan Zhou

Publications and source records attributed to Xiuquan Zhou.

11 recordsLinked to original sources

arXiv2025

Evolution from Topological Dirac Metal to Flat-band-Induced Antiferromagnet in Layered KxNi4S2 (0<=x<=1)

Condensed matter systems with coexisting Dirac cones and flat bands, and a switchable control between them within a single system, are desirable but remarkably uncommon. Here we report a layered quantum material system, KxNi4S2 (0 <= x <= 1), that simultaneously hosts both characteristics without involving typical Kagome/honeycomb lattices. Enabled by a topochemical K-deintercalation process, the Fermi surface can be fine-tuned continuously over a wide range of energies. Consequently, a non-magnetic Dirac-metal state with a topological nontrivial Z2 index of 1;(000), supported by first-principles calculations and high mobility up to 1471 cm2V-1s-1, is observed on the K-rich x = 1 side, whereas a flat-band induced antiferromagnetic state with TN up to 10.1 K emerges as K-content approaches 0. The KxNi4S2 system offers a versatile platform for exploring emerging phenomena and underscores a viable pathway for in-situ control of quantum materials dominated by Dirac cones, flat bands, and their interplay.

cond-mat.mtrl-sci↗

arXiv2024

Tunable Anomalous Hall Effect in a Kagome Ferromagnetic Weyl Semimetal

Emerging from the intricate interplay of topology and magnetism, the giant anomalous Hall effect (AHE) is the most known topological property of the recently discovered kagome ferromagnetic Weyl semimetal Co_3Sn_2S_2 with the magnetic Co atoms arranged on a kagome lattice. Here we report that the AHE in Co_3Sn_2S_2 can be fine-tuned by an applied magnetic field orientated within ~2 degrees of the kagome plane, while beyond this regime, it stays unchanged. Particularly, it can vanish in magnetic fields parallel to the kagome plane and even decrease in magnetic fields collinear with the spin direction. This tunable AHE can be attributed to local spin switching enabled by the geometrical frustration of the magnetic kagome lattice, revealing that spins in a kagome ferromagnet change their switching behavior as the magnetic field approaches the kagome plane. Our results also suggest a versatile way to tune the properties of a kagome magnet.

cond-mat.mtrl-sci↗

arXiv2023

Extended Kohler's Rule of Magnetoresistance in TaCo$_2$Te$_2$

TaCo$_2$Te$_2$ is recently reported to be an air-stable, high mobility Van der Waals material with probable magnetic order. Here we investigate the scaling behavior of its magnetoresistance. We measured both the longitudinal ($ρ_{xx}$) and Hall ($ρ_{xy}$) magnetoresistivities of TaCo$_2$Te$_2$ crystals in magnetic fields parallel to the c-axis and found that the magnetoresistance violates the Kohler's rule $MR \sim f[H/ρ_0]$ while obeying the extended Kohler's rule $MR \sim f[H/(n_Tρ_0)]$, where $MR \sim [ρ_{xx}(H)-ρ_0]/ρ_0$, $H$ is the magnetic field, $n_T$ is a thermal factor, $ρ_{xx}(H)$ and $ρ_0$ are the resistivities at $H$ and zero field, respectively. While deviating from those of the densities of electrons ($n_e$) and holes ($n_h$) obtained from the two-band model analysis of the magnetoconductivities, the temperature dependence of $n_T$ is close to that of the Hall carrier densities $n_H$ calculated from the slopes of $ρ_{xy}(H)$ curves at low magnetic fields, providing a new way to obtain the thermal factor in the extended Kohler's rule.

cond-mat.mtrl-sci↗

arXiv2022

Unreliability of two-band model analysis of magnetoresistivities in unveiling temperature-driven Lifshitz transition

Recently, anomalies in the temperature dependences of the carrier density and/or mobility derived from analysis of the magnetoresistivities using the conventional two-band model have been used to unveil intriguing temperature-induced Lifshitz transitions in various materials. For instance, two temperature-driven Lifshitz transitions were inferred to exist in the Dirac nodal-line semimetal ZrSiSe, based on two-band model analysis of the Hall magnetoconductivities where the second band exhibits a change in the carrier type from holes to electrons when the temperature decreases below T = 106 K and a dip is observed in the mobility versus temperature curve at T = 80 K. Here, we revisit the experiments and two-band model analysis on ZrSiSe. We show that the anomalies in the second band may be spurious, because the first band dominates the Hall magnetoconductivities at T > 80 K, making the carrier type and mobility obtained for the second band from the two-band model analysis unreliable. That is, care must be taken in interpreting these anomalies as evidences for temperature-driven Lifshitz transitions. Our skepticism on the existence of such phase transitions in ZrSiSe is further supported by the validation of the Kohler's rule for magnetoresistances at temperatures below 180 K. This work showcases potential issues in interpreting anomalies in the temperature dependence of the carrier density and mobility derived from the analysis of magnetoconductivities or magnetoresistivities using the conventional two-band model.

cond-mat.mtrl-sci↗

arXiv2021

Extended Kohler$^,$s Rule of Magnetoresistance

A notable phenomenon in topological semimetals is the violation of Kohler$^,$s rule, which dictates that the magnetoresistance $MR$ obeys a scaling behavior of $MR = f(H/ρ_0$), where $MR = [ρ_H-ρ_0]/ρ_0$ and $H$ is the magnetic field, with $ρ_H$ and $ρ_0$ being the resistivity at $H$ and zero field, respectively. Here we report a violation originating from thermally-induced change in the carrier density. We find that the magnetoresistance of the Weyl semimetal, TaP, follows an extended Kohler$^,$s rule $MR = f[H/(n_Tρ_0)]$, with $n_T$ describing the temperature dependence of the carrier density. We show that $n_T$ is associated with the Fermi level and the dispersion relation of the semimetal, providing a new way to reveal information on the electronic bandstructure. We offer a fundamental understanding of the violation and validity of Kohler$^,$s rule in terms of different temperature-responses of $n_T$. We apply our extended Kohler$^,$s rule to BaFe$_2$(As$_{1-x}$P$_x$)$_2$ to settle a long-standing debate on the scaling behavior of the normal-state magnetoresistance of a superconductor, namely, $MR$ ~ $tan^2θ_H$, where $θ_H$ is the Hall angle. We further validate the extended Kohler$^,$s rule and demonstrate its generality in a semiconductor, InSb, where the temperature-dependent carrier density can be reliably determined both theoretically and experimentally.

cond-mat.mtrl-sci↗

arXiv2019

Long range magnetic order in hydroxide layer doped (Li$_{1-x-y}$Fe$_{x}$Mn$_{y}$OD)FeSe

The (Li$_{1-x}$Fe$_{x}$OH)FeSe superconductor has been suspected to exhibit long-range magnetic ordering due to Fe substitution in the LiOH layer. However, no direct observation such as magnetic reflection from neutron diffraction has be reported. Here, we use a chemical design strategy to manipulate the doping level of transition metals in the LiOH layer to tune the magnetic properties of the (Li$_{1-x-y}$Fe$_{x}$Mn$_{y}$OD)FeSe system. We find Mn doping exclusively replaces Li in the hydroxide layer resulting in enhanced magnetization in the (Li$_{0.876}$Fe$_{0.062}$Mn$_{0.062}$OD)FeSe superconductor without significantly altering the superconducting behavior as resolved by magnetic susceptibility and electrical/thermal transport measurements. As a result, long-range magnetic ordering was observed below 12 K with neutron diffraction measurements. This work has implications for the design of magnetic superconductors for the fundamental understanding of superconductivity and magnetism in the iron chalcogenide system as well as exploitation as functional materials for next generation devices.

cond-mat.supr-con↗

arXiv2018

Frustrated magnetism in tetragonal CoSe, analogue to superconducting FeSe

Recently synthesized metastable tetragonal CoSe, isostructural to the FeSe superconductor, offers a new avenue for investigating systems in close proximity to the iron-based superconductors. We present magnetic and transport property measurements on powders and single crystals of CoSe. High field magnetic susceptibility measurements indicate a suppression of the previously reported 10 K ferromagnetic transition with the magnetic susceptibility exhibiting time-dependence below the proposed transition. Dynamic scaling analysis of the time-dependence yields a critical relaxation time of $τ^{*} = 0.064 \pm 0.008 $ s which in turn yields an activation energy of $E_{a}^{*}$ = 14.84 $\pm$ 0.59 K and an ideal glass temperature $T_{0}^{*}$ = 8.91 $\pm$ 0.09 K from Vogel-Fulcher analysis. No transition is observed in resistivity and specific heat measurements, but both measurements indicate that CoSe is metallic. These results are interpreted on the basis of CoSe exhibiting frustrated magnetic ordering arising from competing magnetic interactions. Arrott analysis of single crystal magnetic susceptibility has indicated that the magnetic moments lie in the $ab$-plane so frustration may arise from intralayer magnetic fluctuations as well as interlayer coupling. The results have implications for understanding the superconductivity in the iron chalcogenide systems as well as utilizing CoSe as a host for chemical and physical manipulation to tune and explore emergent phenomena within an expanding new class of transition metal chalcogenides.

cond-mat.supr-con↗

arXiv2017

Superconductivity and magnetism in Iron Sulfides Intercalated by Metal Hydroxides

Inspired by naturally occurring sulfide minerals, we present a new family of iron-based superconductors. A metastable form of FeS known as the mineral mackinawite forms two-dimensional sheets that can be readily intercalated by various cationic guest species. Under hydrothermal conditions using alkali metal hydroxides, we prepare three different cation and metal hydroxide-intercalated FeS phases including (Li$_{1-x}$Fe$_x$OH)FeS, [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS, and K$_x$Fe$_{2-y}$S$_2$. Upon successful intercalation of the FeS layer, the superconducting critical temperature $T_c$ of mackinawite is enhanced from 5 K to 8 K for the (Li$_{1-x}$Fe$_x$OH)$^{δ+}$ intercalate. Layered heterostructures of [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS resemble the natural mineral tochilinite, which contains an iron square lattice interleaved with a hexagonal hydroxide lattice. Whilst heterostructured [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS displays long-range magnetic ordering near 15 K, K$_x$Fe$_{2-y}$S$_2$ displays short range antiferromagnetism.

cond-mat.supr-con↗

arXiv2016

Strong anisotropy in nearly ideal-tetrahedral superconducting FeS single crystals

We report the novel preparation of single crystals of tetragonal iron sulfide, FeS, which exhibits a nearly ideal tetrahedral geometry with S--Fe--S bond angles of 110.2(2) $^\circ$ and 108.1(2) $^\circ$. Grown via hydrothermal de-intercalation of K${_x}$Fe${_{2-y}}$S${_2}$ crystals under basic and reducing conditions, the silver, plate-like crystals of FeS remain stable up to 200 $^\circ$C under air and 250 $^\circ$C under inert conditions, even though the mineral "mackinawite" (FeS) is known to be metastable. FeS single crystals exhibit a superconducting state below $T_c=4$ K as determined by electrical resistivity, magnetic susceptibility, and heat capacity measurements, confirming the presence of a bulk superconducting state. Normal state measurements yield an electronic specific heat of 5~mJ/mol-K$^2$, and paramagnetic, metallic behavior with a low residual resistivity of 250~$μΩ\cdot$cm. Magnetoresistance measurements performed as a function of magnetic field angle tilted toward both transverse and longitudinal orientations with respect to the applied current reveal remarkable two-dimensional behavior. This is paralleled in the superconducting state, which exhibits the largest known upper critical field $H_{c2}$ anisotropy of all iron-based superconductors, with $H_{c2}^{||ab}(0) / H_{c2}^{||c}(0)=$(2.75~T)/(0.275~T)=10. Comparisons to theoretical models for 2D and anisotropic-3D superconductors, however, suggest that FeS is the latter case with a large effective mass anisotropy. We place FeS in context to other closely related iron-based superconductors and discuss the role of structural parameters such as anion height on superconductivity.

cond-mat.supr-con↗

arXiv2015

The preparation and phase diagrams of (${^{7}}$Li${_{1-x}}$Fe${_{x}}$OD)FeSe and (Li${_{1-x}}$Fe${_{x}}$OH)FeSe superconductors

We report the phase diagram for the superconducting system (${^{7}}$Li${_{1-x}}$Fe${_{x}}$OD)FeSe and contrast it with that of (Li${_{1-x}}$Fe${_{x}}$OH)FeSe both in single crystal and powder forms. Samples were prepared via hydrothermal methods and characterized with laboratory and synchrotron X-ray diffraction, high-resolution neutron powder diffraction (NPD), and high intensity NPD. We find a correlation between the tetragonality of the unit cell parameters and the critical temperature, $T_{c}$, which is indicative of the effects of charge doping on the lattice and formation of iron vacancies in the FeSe layer. We observe no appreciable isotope effect on the maximum $T_{c}$ in substituting H by by D. The NPD measurements definitively rule out an antiferromagnetic ordering in the non-superconducting (Li${_{1-x}}$Fe${_{x}}$OD)FeSe samples below 120 K, which has been reported in non-superconducting (Li${_{1-x}}$Fe${_{x}}$OH)FeSe.$^{1}$ A likely explanation for the observed antiferromagnetic transition in (Li${_{1-x}}$Fe${_{x}}$OH)FeSe samples is the formation of impurities during their preparation such as Fe${_{3}}$O${_{4}}$ and LixFeO2, which express a charge ordering transition known as the Verwey transition near 120 K. The concentration of these oxide impurities is found to be dependent on the concentration of the lithium hydroxide reagent and the use of H${_{2}}$O vs. D${_{2}}$O as the solvent during synthesis. We also describe the reaction conditions that lead to some of our superconducting samples to exhibit ferromagnetism below $T_{c}$.

cond-mat.supr-con↗

arXiv2015

Neutron investigation of the magnetic scattering in an iron-based ferromagnetic superconductor

Neutron diffraction and small angle scattering experiments have been carried out on the double-isotopic polycrystalline sample (7Li0.82Fe0.18OD)FeSe. Profile refinements of the diffraction data establish the composition and reveal an essentially single phase material with lattice parameters of a= 3.7827 Å and c= 9.1277 Å at 4 K, in the ferromagnetic-superconductor regime, with a bulk superconducting transition of TC = 18 K. Small angle neutron scattering (SANS) measurements in zero applied field reveal the onset of ferromagnetic order below TF ~ 12.5 K, with a wave vector and temperature dependence consistent with an inhomogeneous ferromagnet of spontaneous vortices or domains in a mixed state. No oscillatory long range ordered magnetic state is observed. Field dependent measurements establish a separate component of magnetic scattering from the vortex lattice, which occurs at the expected wave vector. The temperature dependence of the vortex scattering does not indicate any contribution from the ferromagnetism, consistent with diffraction data that indicate that the ordered ferromagnetic moment is quite small.

cond-mat.supr-con↗