arXiv subjects

Xiaoyuan Zhou

Publications and source records attributed to Xiaoyuan Zhou.

31 records · Page 2Linked to original sources

arXiv2023

Quantum metric-induced nonlinear transport in a topological antiferromagnet

The Berry curvature and quantum metric are the imaginary part and real part, respectively, of the quantum geometric tensor which characterizes the topology of quantum states. The former is known to generate a zoo of important discoveries such as quantum Hall effect and anomalous Hall effect (AHE), while the consequences of the quantum metric have rarely been probed by transport. In this work, we observed quantum metric induced nonlinear transport, including both nonlinear AHE and diode-like nonreciprocal longitudinal response, in thin films of a topological antiferromagnet, MnBi$_2$Te$_4$. Our observation reveals that the transverse and longitudinal nonlinear conductivities reverse signs when reversing the antiferromagnetic order, diminish above the Néel temperature, and are insensitive to disorder scattering, thus verifying their origin in the band structure topology. They also flip signs between electron and hole-doped regions, in agreement with theoretical calculations. Our work provides a pathway to probe the quantum metric through nonlinear transport and to design magnetic nonlinear devices.

cond-mat.mes-hall↗

arXiv2023

Spin-phonon scattering-induced low thermal conductivity in a van der Waals layered ferromagnet Cr$_2$Si$_2$Te$_6$

Layered van der Waals (vdW) magnets are prominent playgrounds for developing magnetoelectric, magneto-optic and spintronic devices. In spintronics, particularly in spincaloritronic applications, low thermal conductivity ($κ$) is highly desired. Here, by combining thermal transport measurements with density functional theory calculations, we demonstrate low $κ$ down to 1 W m$^{-1}$ K$^{-1}$ in a typical vdW ferromagnet Cr$_2$Si$_2$Te$_6$. In the paramagnetic state, development of magnetic fluctuations way above $T_\mathrm{c}=$ 33 K strongly reduces $κ$ via spin-phonon scattering, leading to low $κ\sim$ 1 W m$^{-1}$ K$^{-1}$ over a wide temperature range, in comparable to that of amorphous silica. In the magnetically ordered state, emergence of resonant magnon-phonon scattering limits $κ$ below $\sim$ 2 W m$^{-1}$ K$^{-1}$, which would be three times larger if magnetic scatterings were absent. Application of magnetic fields strongly suppresses the spin-phonon scattering, giving rise to large enhancements of $κ$. Our calculations well capture these complex behaviours of $κ$ by taking the temperature- and magnetic-field-dependent spin-phonon scattering into account. Realization of low $κ$ which is easily tunable by magnetic fields in Cr$_2$Si$_2$Te$_6$, may further promote spincaloritronic applications of vdW magnets. Our theoretical approach may also provide a generic understanding of spin-phonon scattering, which appears to play important roles in various systems.

cond-mat.mtrl-sci↗

arXiv2023

Charge fluctuations above $T_\mathrm{CDW}$ revealed by glasslike thermal transport in kagome metals $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs)

We present heat capacity, electrical and thermal transport measurements of kagome metals $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs). In all three compounds, development of short-range charge fluctuations above the charge density wave (CDW) transition temperature $T_\mathrm{CDW}$ strongly scatters phonons via electron-phonon coupling, leading to glasslike phonon heat transport, i.e., phonon thermal conductivity decreases weakly upon cooling. Once the long-range charge order sets in below $T_\mathrm{CDW}$, short-range charge fluctuations are quenched, and the typical Umklapp scattering dominated phonon heat transport is recovered. The charge-fluctuations-induced glasslike phonon thermal conductivity implies sizable electron-phonon coupling in $A$V$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2023

Anomalous Nernst effect in a ferrimagnetic nodal-line semiconductor Mn$_3$Si$_2$Te$_6$

In the ferrimagnetic nodal-line semiconductor Mn$_3$Si$_2$Te$_6$, colossal magnetoresistance (CMR) arises below $T_\mathrm{c}=78$ K due to the interplay of magnetism and topological nodal-line fermiology. The Berry curvature associated with the topological nodal-line is expected to produce an anomalous Nernst effect. Here, we present sizable anomalous Nernst signal in Mn$_3$Si$_2$Te$_6$ below $T_\mathrm{c}$. In the low-magnetic-field region where CMR is most apparent, the scaling ratio between the Nernst signal and magnetization is significantly enhanced compared to that in conventional magnetic materials. The enhanced Nernst effect and CMR likely share the same mechanisms, which are closely linked to the nodal-line topology.

cond-mat.str-el↗

arXiv2022

Phononic Obstructed Atomic Insulators with Robust Corner Modes

Higher-order topological insulators (HOTIs) are described by symmetric exponentially decayed Wannier functions at some $necessary$ unoccupied Wyckoff positions and classified as obstructed atomic insulators (OAIs) in the topological quantum chemistry (TQC) theory. The boundary states in HOTIs reported so far are often fragile, manifested as strongly depending on crystalline symmetries and cleavage terminations in the disk or cylinder geometry. Here, using the TQC theory, we present an intuitive argument about the connection between the obstructed Wannier charge centers of OAIs and the emergence of robust corner states in two-dimensional systems. Based on first-principles calculations and Real Space Invariant theory, we extend the concept of OAIs to phonon systems and thereby predict that the robust corner states can be realized in the phonon spectra of $MX_3$ ($M$=Bi, Sb, As, Sc, Y; $X$=I, Br, Cl) monolayers. The phonon corner modes in different shapes of nano-disks are investigated, and their robustness facilitates the detection in experiments and further applications. This work suggests a promising avenue to explore more attractive features of higher-order band topology.

cond-mat.mtrl-sci↗

arXiv2022

Multiband effects in thermoelectric and electrical transport properties of kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs)

We studied the effects of multiband electronic structure on the thermoelectric and electrical transport properties in the normal state of kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs). In all three members, the multiband nature is manifested by sign changes in the temperature dependence of the Seebeck and Hall resistivity, together with sublinear response of the isothermal Nernst and Hall effects to external magnetic fields in the charge ordered state. Moreover, ambipolar transport effects appear ubiquitously in all three systems, giving rise to sizable Nernst signal. Finally, possible origins of the sign reversal in the temperature dependence of the Hall effect are discussed.

cond-mat.supr-con↗

arXiv2022

Non-reciprocal charge transport in an intrinsic magnetic topological insulator MnBi2Te4

Symmetries, quantum geometries and electronic correlations are among the most important ingredients of condensed matters, and lead to nontrivial phenomena in experiments, for example, non-reciprocal charge transport. Here we report the non-reciprocal charge transport in the intrinsic magnetic topological insulator MnBi2Te4. The current direction relevant resistance is observed at chiral edges, which is magnetically switchable, edge position sensitive and septuple layer number controllable. Applying gate voltage can effectively tune the non-reciprocal resistance. The observation and manipulation of non-reciprocal charge transport indicate the fundamental role of chirality in charge transport of MnBi2Te4, and pave ways to develop van der Waals spintronic devices by chirality engineering.

cond-mat.mes-hall↗

arXiv2022

Nonlinear Hall effect with non-centrosymmetric topological phase in ZrTe$_5$

The non-centrosymmetric topological material has attracted intense attention due to its superior characters as compared to the centrosymmetric one. On one side, the topological phase coming from global geometric properties of the quantum wave function remains unchanged, on the other side, abundant exotic phenomena are predicted to be merely emerged in non-centrosymmetric ones, due to the redistribution of local quantum geometry. Whereas, probing the local quantum geometry in non-centrosymmetric topological material remains challenging. Here, we report a non-centrosymmetric topological phase in ZrTe$_5$, probed by the nonlinear Hall (NLH) effect. The angle-resolved and temperature-dependent NLH measurement reveals the inversion and ab-plane mirror symmetries breaking under 30 K, consistent with our theoretical calculation. Our findings identify a new non-centrosymmetric phase of ZrTe$_5$ and provide a platform to probe and control local quantum geometry via crystal symmetries.

cond-mat.mtrl-sci↗

arXiv2022

Photo-Induced Ultrafast Symmetry Switch in SnSe

Layered tin selenide (SnSe) has recently emerged as a high-performance thermoelectric material with the current record for the figure of merit (ZT) observed in the high-temperature Cmcm phase. So far, access of the Cmcm phase has been mainly obtained via thermal equilibrium methods based on sample heating or application of external pressure, thus restricting the current understanding only to ground-state conditions. Here, we investigate the ultrafast carrier and phononic dynamics in SnSe. Our results demonstrate that optical excitations can transiently switch the point-group symmetry of the crystal from Pnma to Cmcm at room temperature in a few hundreds of femtoseconds with an ultralow threshold for the excitation carrier density. This non-equilibrium Cmcm phase is found to be driven by the displacive excitation of coherent Ag phonons and, given the absence of low-energy thermal phonons, exists in SnSe with the status of 'cold lattice with hot carriers'. Our findings provide important insight for understanding non-equilibrium thermoelectric properties of SnSe.

cond-mat.mtrl-sci↗

arXiv2021

Angle dependent field-driven reorientation transitions in uniaxial antiferromagnet MnBi$_2$Te$_4$ single crystal

MnBi$_2$Te$_4$, a two-dimensional magnetic topological insulator with a uniaxial antiferromagnetic structure, is an ideal platform to realize quantum anomalous Hall effect. However, the strength of magnetic interactions is not clear yet. We performed systematic studies on the magnetization and angle dependent magnetotransport of MnBi$_2$Te$_4$ single crystal. The results show that the direction of the magnetic field has significant effects on the critical field values and magnetic structure of this compound, which leads to different magnetotransport behaviors. The field-driven reorientation transitions can be utilized to estimate the AFM interlayer exchange interaction coupling and uniaxial magnetic anisotropy D. The obtained Hamiltonian can well explain the experimental data by Monte Carlo simulations. Our comprehensive studies on the field-driven magnetic transitions phenomenon in MnBi$_2$Te$_4$ provide a general approach for other topological systems with antiferromagnetism.

cond-mat.mtrl-sci↗

arXiv2021

Magneto-Seebeck effect and ambipolar Nernst effect in CsV$_3$Sb$_5$ superconductor

We present a study of Seebeck and Nernst effect in combination with magnetoresistance and Hall measurements of the Kagome superconductor CsV$_3$Sb$_5$. Sizable magneto-Seebeck signal appears once the charge density wave (CDW) order sets in below $T_{CDW}$=94 K. The Nernst signal peaks at a lower temperature $T^*$=35 K, crossing which the Hall coefficient switches sign, which we attribute to the ambipolar transport of compensated bands due to the multi-band nature of CsV$_3$Sb$_5$. Sublinear Nernst signal as a function of magnetic field, together with large anomalous Nernst effect (ANE) also emerge inside the CDW phase, despite the absence of long-range magnetic order. These findings suggest that, the transport properties are dominated by small pockets with multi-band profile, and that the unusual band topology also plays an import role in the CDW state of CsV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2020

Copper ion dynamics and phase segregation in Cu-rich tetrahedrite: an NMR study

$^{63}$Cu NMR measurements are reported for the Cu-rich phase of \ch{Cu_{12+x}Sb4S13} ($x \lesssim 2$) and compared to \ch{Cu12Sb4S13}. We identify the NMR signatures of the phase segregation into Cu-poor ($x \approx 0$) and Cu-rich ($x \lesssim 2$) phases, with the metal-insulator transition observed in \ch{Cu12Sb4S13} suppressed in the Cu-rich phase. Based on NMR $T_1$ and $T_2$ measurements, the results demonstrate Cu-ion hopping below room temperature with an activation energy of $\sim$150 meV for the Cu-rich phase, consistent with superionic behavior. The NMR results also demonstrate the effects of Cu-ion mobility in the \ch{Cu12Sb4S13} phase, but with a larger activation barrier. We identify a small difference in NMR Knight shift for the metallic phase of \ch{Cu12Sb4S13}, compared to the Cu-rich phase, and when compared to DFT calculations the results indicate a mix of hyperfine contributions to the metallic shift.

cond-mat.mtrl-sci↗

arXiv2017

Topological Dirac line nodes and superconductivity coexist in SnSe at high pressure

Due to fundamental interest and potential applications in quantum computation, tremendous efforts have been invested to study topological superconductivity. However, bulk topological superconductivity seems to be difficult to realize and its mechanism is still elusive. Several possible routes to induce topological superconductivity have been proposed, including proximity efforts, doping or pressurizing a topological insulator or semimetal. Among them, the pressurizing is considered to be a clean way to tune the electronic structures. Here we report the discovery of a pressure-induced topological and superconducting phase of SnSe, a material which is highly focused recently due to its superior thermoelectric properties. In situ high-pressure electrical transport and synchrotron X-ray diffraction measurements show that the superconductivity emerges along with the formation of a CsCl-type structural symmetry of SnSe above around 27 GPa, with a maximum critical temperature of 3.2 K at 39 GPa. Based on ab initio calculations, this CsCl-type SnSe is predicted to be a Dirac line nodes (DLN) semimetal in the absence of spin-orbit coupling, whose DLN states are protected by the coexistence of time-reversal and inversion symmetries. These results make CsCl-type SnSe an interesting model platform with simple crystal symmetry to study the interplay of topological physics and superconductivity.

cond-mat.mtrl-sci↗