arXiv subjects

Xiaohan Yao

Publications and source records attributed to Xiaohan Yao.

17 recordsLinked to original sources

arXiv2025

Engineering the Magnetocaloric Effect in Nd$T_4$B

We present a comprehensive study of the magnetocaloric effect (MCE) in the Nd$T_4$B system where $T$ = Fe, Co, and Ni. These compounds are ferromagnetic kagome materials with tunable ordering temperatures, transition width, and magnetic moments depending on the choice of transition metal. Thus, they are good candidates for investigating the MCE. We characterize the MCE using standard metrics and construct ternary phase diagrams as functions of Fe, Co, and Ni concentrations. Using these phase diagrams, we engineer the composition NdFe$_{1.15}$Co$_{0.46}$Ni$_{2.39}$B to maximize the MCE. Interestingly, the Nd$T_4$B system shows a notable entropy change over a wide temperature range ($\sim$10 to 650 K), and particular compositions have notable MCEs spanning hundreds of Kelvin, making this a suitable system to study for technologies used in a wide range of temperatures. In a few cases, we observe a two-peak MCE. These two transitions, releasing comparable entropy, provide an interesting platform to study for applications in multi-stage cooling.

cond-mat.mtrl-sci↗

arXiv2025

Anisotropic suppression of the phononic thermal conductivity by magnetic field in SmAlSi

We report the thermal and electrical conductivity data for the magnetic Weyl semimetal SmAlSi measured in a magnetic field (B) with two different orientations. In one case, B was applied perpendicular to the heat or charge current, in the other they were parallel. For both configurations, the magnetic field affects the magnetic structure identically as B is always parallel to the equivalent tetragonal axis. Our results indicate that phonon heat transport in response to the magnetic field exhibits strong anisotropy at low temperature: it appears to be independent of B in the perpendicular configuration but is strongly suppressed in the parallel configuration. Understanding this unusual behaviour can lead to designing better materials for thermoelectricity or directional heat switches.

cond-mat.str-el↗

arXiv2024

Magnetic excitations and interactions in the Weyl ferrimagnet NdAlSi

Weyl fermions can arise from time-reversal symmetry-breaking magnetism, but their impact on magnetic order is a source of ongoing research. Using high-precision neutron diffraction and spectroscopy, we present a comprehensive exploration of the magnetic structure and excitation spectrum of Weyl semimetal and helical magnet NdAlSi. We use Luttinger-Tisza, classical mean-field, and random-phase approximation techniques to model the dispersive crystal field excitons. We find extended-ranged and sign-changing interactions, suggesting a coupling between conduction electrons and the local moments. We demonstrate that low-symmetry anisotropic Dzyaloshinskii-Moriya interactions, in contrast with higher-symmetry interactions enabled by Weyl fermions, play an important role in stabilizing the complex spin spiral ground state of NdAlSi. Our work provides a first detailed view of microscopic interactions in a Weyl magnet, and constrains the role of Weyl electrons and their chirality on the spiral magnetism.

cond-mat.str-el↗

arXiv2024

Two types of colossal magnetoresistance with distinct mechanisms in Eu5In2As6

Recent reports of colossal negative magnetoresistance (CMR) in a few magnetic semimetals and semiconductors have attracted attention, because these materials are devoid of the conventional mechanisms of CMR such as mixed valence, double exchange interaction, and Jahn-Teller distortion. New mechanisms have thus been proposed, including topological band structure, ferromagnetic clusters, orbital currents, and charge ordering. The CMR in these compounds has been reported in two forms: either a resistivity peak or a resistivity upturn suppressed by a magnetic field. Here we reveal both types of CMR in a single antiferromagnetic semiconductor Eu5In2As6. Using the transport and thermodynamic measurements, we demonstrate that the peak-type CMR is likely due to the percolation of magnetic polarons with increasing magnetic field, while the upturn-type CMR is proposed to result from the melting of a charge order under the magnetic field. We argue that similar mechanisms operate in other compounds, offering a unifying framework to understand CMR in seemingly different materials.

cond-mat.str-el↗

arXiv2024

Characterization of the Magnetocaloric Effect in RMn6Sn6 including High Entropy Alloys

We present a comprehensive study of the magnetocaloric effect (MCE) in a family of kagome magnets with formula RMn6Sn6 (R=Tb, Ho, Er, and Lu). These materials have a small rare-earth content and tunable magnetic ordering, hence they provide a venue to study the fundamentals of the MCE. We examine the effect of different types of order (ferrimagnetic and antiferromagnetic) and the presence of a metamagnetic transition on the MCE. We extend the study to high-entropy rare-earth alloys of the family. Our results suggest several guidelines for enhancing the MCE in tunable magnetic materials with a small rare-earth content.

cond-mat.mtrl-sci↗

arXiv2023

Engineering Anomalously Large Electron Transport in Topological Semimetals

Anomalous transport of topological semimetals has generated significant interest for applications in optoelectronics, nanoscale devices, and interconnects. Understanding the origin of novel transport is crucial to engineering the desired material properties, yet their orders of magnitude higher transport than single-particle mobilities remain unexplained. This work demonstrates the dramatic mobility enhancements result from phonons primarily returning momentum to electrons due to phonon-electron dominating over phonon-phonon scattering. Proving this idea, proposed by Peierls in 1932, requires tuning electron and phonon dispersions without changing symmetry, topology, or disorder. This is achieved by combining de Haas - van Alphen (dHvA), electron transport, Raman scattering, and first-principles calculations in the topological semimetals MX$_2$ (M=Nb, Ta and X=Ge, Si). Replacing Ge with Si brings the transport mobilities from an order magnitude larger than single particle ones to nearly balanced. This occurs without changing the crystal structure or topology and with small differences in disorder or Fermi surface. Simultaneously, Raman scattering and first-principles calculations establish phonon-electron dominated scattering only in the MGe$_2$ compounds. Thus, this study proves that phonon-drag is crucial to the transport properties of topological semimetals and provides insight to further engineer these materials.

cond-mat.mtrl-sci↗

arXiv2023

Gravitational anomaly in the ferrimagnetic topological Weyl semimetal NdAlSi

Quantum anomalies are the breakdowns of classical conservation laws that occur in quantum-field theory description of a physical system. They appear in relativistic field theories of chiral fermions and are expected to lead to anomalous transport properties in Weyl semimetals. This includes a chiral anomaly, which is a violation of the chiral current conservation that takes place when a Weyl semimetal is subjected to parallel electric and magnetic fields. A charge pumping between Weyl points of opposite chirality causes the chiral magnetic effect that has been extensively studied with electrical transport. On the other hand, if the thermal gradient, instead of the electrical field, is applied along the magnetic field, then as a consequence of the gravitational (also called the thermal chiral) anomaly an energy pumping occurs within a pair of Weyl cones. As a result, this is expected to generate anomalous heat current contributing to the thermal conductivity. We report an increase of both the magneto-electric and magneto-thermal conductivities in quasi-classical regime of the magnetic Weyl semimetal NdAlSi. Our work also shows that the anomalous electric and heat currents, which occur due to the chiral magnetic effect and gravitational anomalies respectively, are still linked by a 170 years old relation called the Wiedemann-Franz law.

cond-mat.mes-hall↗

arXiv2023

Observation of Fermi arcs and Weyl nodes in a non-centrosymmetric magnetic Weyl semimetal

Weyl semimetal (WSM), a novel state of quantum matter, hosts Weyl fermions as emergent quasiparticles resulting from the breaking of either inversion or time-reversal symmetry. Magnetic WSMs that arise from broken time-reversal symmetry provide an exceptional platform to understand the interplay between magnetic order and Weyl physics, but few WSMs have been realized. Here, we identify CeAlSi as a new non-centrosymmetric magnetic WSM via angle-resolved photoemission spectroscopy (ARPES) and first-principles, density-functional theory based calculations. Our surface-sensitive vacuum ultraviolet ARPES data confirms the presence of surface Fermi arcs as, the smoking gun evidence for the existence of the Weyl semimetallic state in CeAlSi. We also observe bulk Weyl cones in CeAlSi using bulk-sensitive soft-X-ray ARPES measurements. In addition, Ce 4f at bands are found near the Fermi level, indicating that CeAlSi is a unique platform for investigating exotic quantum phenomena resulting from the interaction of topology, magnetism and electronic correlations.

cond-mat.mes-hall↗

arXiv2023

Introducing the monoclinic polymorph of the Kitaev magnet Na$_{2}$Co$_{2}$TeO$_{6}$

Recent theoretical studies have suggested that the low-energy Hamiltonian of honeycomb cobaltate systems could be dominated by anisotropic Kitaev interactions. Motivated by the theory, a honeycomb layered material Na$_{2}$Co$_{2}$TeO$_{6}$ with a hexagonal unit cell has been studied and found to exhibit antiferromagnetic (AFM) ordering at 27 K with two spin reorientation transitions at 15 and 5 K. Here we report a monoclinic polymorph of Na$_{2}$Co$_{2}$TeO$_{6}$, also with honeycomb layered structure but with a single AFM transition at 9.6 K and without spin reorientation transitions at lower temperatures. Using neutron diffraction, we identify an in-plane zigzag AFM order in the ground-state with the spins canted out of the honeycomb planes and ferromagnetically coupled between them. The zigzag order is suppressed by a magnetic field of 6 T.

cond-mat.str-el↗

arXiv2023

Stripe Helical Magnetism and Two Regimes of Anomalous Hall Effect in NdAlGe

We report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate Ising spin density wave ($T_{\text{inc}}=6.8$ K) with a small helical spin canting of 3$^\circ$ and a long-wavelength of $\sim$ 35 nm, which transitions to a commensurate ferrimagnetic state below $T_{\text{com}}=5.1$ K. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 nm and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are respectively governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our study suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAl$X$, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.

cond-mat.mtrl-sci↗

arXiv2023

Extreme sensitivity of the magnetic ground-state to halide composition in FeCl$_{3-x}$Br$_x$

Mixed halide chemistry has recently been utilized to tune the intrinsic magnetic properties of transition-metal halides $-$ one of the largest families of magnetic van der Waals materials. Prior studies have shown that the strength of exchange interactions, hence the critical temperature, can be tuned smoothly with halide composition for a given ground-state. Here we show that the ground-state itself can be altered by a small change of halide composition leading to a quantum phase transition in FeCl$_{3-x}$Br$_x$. Specifically, we find a three-fold jump in the Néel temperature and a sign change in the Weiss temperature at $x= 0.08$ corresponding to only $3\%$ bromine doping. Using neutron scattering, we reveal a change of the ground-state from spiral order in FeCl$_3$ to A-type antiferromagnetic order in FeBr$_3$. Using first-principles calculations, we show that a delicate balance between nearest and next-nearest neighbor interactions is responsible for such a transition. These results support the proximity of FeCl$_3$ to a spiral spin liquid state, in which competing interactions and nearly degenerate magnetic $k$-vectors may cause large changes in response to small perturbations.

cond-mat.mtrl-sci↗

arXiv2022

Visualizing bulk and edge photocurrent flow in anisotropic Weyl semimetals

Materials that rectify light into current in their bulk are desired for optoelectronic applications. In inversion-breaking Weyl semimetals, bulk photocurrents may arise due to nonlinear optical processes that are enhanced near the Weyl nodes. However, the photoresponse of these materials is commonly studied by scanning photocurrent microscopy (SPCM), which convolves the effects of photocurrent generation and collection. Here, we directly image the photocurrent flow inside the type-II Weyl semimetals WTe2 and TaIrTe4 using high-sensitivity quantum magnetometry with nitrogen-vacancy center spins. We elucidate an unknown mechanism for bulk photocurrent generation termed the anisotropic photothermoelectric effect (APTE), where unequal thermopowers along different crystal axes drive intricate circulations of photocurrent around the photoexcitation. Using simultaneous SPCM and magnetic imaging at the sample's interior and edges, we visualize how the APTE stimulates the long-range photocurrent collected in our Weyl semimetal devices through the Shockley-Ramo theorem. Our results highlight an overlooked, but widely relevant source of current flow and inspire novel photodetectors using homogeneous materials with anisotropy.

cond-mat.mtrl-sci↗

arXiv2022

Anisotropy of the magnetic and transport properties in EuZn$_2$As$_2$

Several recent studies have shown that the anisotropy in the magnetic structure of \ECA\ plays a significant role in stabilizing the Weyl nodes. To investigate the relationship between magnetic anisotropy and Weyl physics, we present a comparative study between EuZn$_2$As$_2$ and EuCd$_2$As$_2$ that are isostructural but with different magnetic anisotropy. We performed structural analysis, electronic transport, and magnetization experiments on millimeter-sized single crystals of EuZn$_2$As$_2$, and compared the results to those of EuCd$_2$As$_2$. By combining the first principle calculations and neutron diffraction experiment, we identify the magnetic ground state of EuZn$_2$As$_2$ as A-type antiferromagnetic order with a transition temperature ($T_\mathrm{N}$ = 19.6 K) twice that of EuCd$_2$As$_2$. Like EuCd$_2$As$_2$, the negative magnetoresistance of EuZn$_2$As$_2$ is observed after suppressing the resistivity peak at $T_\mathrm{N}$ with increasing fields. However, the anisotropy in both transport and magnetization are much reduced in EuZn$_2$As$_2$. The difference could be ascribed to the weaker spin-orbit coupling, more localized $d$-orbitals, and a larger contribution from the Eu $s$-orbitals in the zinc compound, as suggested by the electronic band calculations. The same band structure effect could be also responsible for the observation of a smaller non-linear anomalous Hall effect in EuZn$_2$As$_2$ compared to EuCd$_2$As$_2$.

cond-mat.str-el↗

arXiv2021

Evidence of a coupled electron-phonon liquid in NbGe$_2$

Whereas electron-phonon scattering typically relaxes the electron's momentum in metals, a perpetual exchange of momentum between phonons and electrons conserves total momentum and can lead to a coupled electron-phonon liquid with unique transport properties. This theoretical idea was proposed decades ago and has been revisited recently, but the experimental signatures of an electron-phonon liquid have been rarely reported. We present evidence of such a behavior in a transition metal ditetrelide, NbGe$_2$, from three different experiments. First, quantum oscillations reveal an enhanced quasiparticle mass, which is unexpected in NbGe$_2$ due to weak electron-electron correlations, hence pointing at electron-phonon interactions. Second, resistivity measurements exhibit a discrepancy between the experimental data and calculated curves within a standard Fermi liquid theory. Third, Raman scattering shows anomalous temperature dependence of the phonon linewidths which fits an empirical model based on phonon-electron coupling. We discuss structural factors, such as chiral symmetry, short metallic bonds, and a low-symmetry coordination environment as potential sources of coupled electron-phonon liquids.

cond-mat.mtrl-sci↗

arXiv2021

Colossal Magnetoresistance without Mixed Valence in a Layered Phosphide Crystal

Materials with strong magnetoresistive responses are the backbone of spintronic technology, magnetic sensors, and hard drives. Among them, manganese oxides with a mixed valence and a cubic perovskite structure stand out due to their colossal magnetoresistance (CMR). A double exchange interaction underlies the CMR in manganates, whereby charge transport is enhanced when the spins on neighboring Mn3+ and Mn4+ ions are parallel. Prior efforts to find different materials or mechanisms for CMR resulted in a much smaller effect. Here we show an enormous CMR at low temperatures in EuCd2P2 without manganese, oxygen, mixed valence, or cubic perovskite structure. EuCd2P2 has a layered trigonal lattice and exhibits antiferromagnetic ordering at 11 K. The magnitude of CMR (104 percent) in as-grown crystals of EuCd2P2 rivals the magnitude in optimized thin films of manganates. Our magnetization, transport, and synchrotron X-ray data suggest that strong magnetic fluctuations are responsible for this phenomenon. The realization of CMR at low temperatures without heterovalency leads to a new regime for materials and technologies related to antiferromagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2019

Observation of Fano resonance in photoluminescence of a two-dimensional magnetic semiconductor

Quantum interference gives rise to the asymmetric Fano resonance line shape when the final states of an electronic transition follows within a continuum of states and a discrete state, which has significant applications in optical switching and sensing. The resonant optical phenomena associated with Fano resonance have been observed by absorption spectra, Raman spectra, transmission spectra, etc., but have rarely been reported in photoluminescence (PL) spectroscopy. In this work, we performed spectroscopic studies on layered chromium thiophosphate (CrPS4), a promising ternary antiferromagnetic semiconductor with PL in near-infrared wavelength region and observed Fano resonance when CrPS4 experiences phase transition into the antiferromagnetic state below Néel temperature (38 K). The photoluminescence of the continuum states results from the d band transitions localized at Cr3+ ions, while the discrete state reaches saturation at high excitation power and can be enhanced by the external magnetic field, suggesting it is formed by an impurity level from extra atomic phosphorus. Our findings provide insights into the electronic transitions of CrPS4 and their connection to its intrinsic magnetic properties.

cond-mat.mtrl-sci↗

arXiv2019

Scaling-up atomically thin coplanar semiconductor-metal circuitry via phase engineered chemical assembly

Two-dimensional (2D) layered semiconductors, with their ultimate atomic thickness, have shown promise to scale down transistors for modern integrated circuitry. However, the electrical contacts that connect these materials with external bulky metals are usually unsatisfactory, which limits the transistor performance. Recently, contacting 2D semiconductors using coplanar 2D conductors has shown promise in reducing the problematic high resistance contacts. However, many of these methods are not ideal for scaled production. Here, we report on the large-scale, spatially controlled chemical assembly of the integrated 2H-MoTe2 field-effect transistors (FETs) with coplanar metallic 1T' MoTe2 contacts via phase engineered approaches. We demonstrate that the heterophase FETs exhibit ohmic contact behavior with low contact resistance, resulting from the coplanar seamless contact between 2H and 1T' MoTe2 confirmed by transmission electron microscopy characterizations. The average mobility of the heterophase FETs was measured to be as high as 23 cm2 V-1 s-1 (comparable with those of exfoliated single crystals), due to the large 2H MoTe2 single-crystalline domain (486μm). By developing a patterned growth method, we realize the 1T' MoTe2 gated heterophase FET array whose components of channel, gate, and contacts are all 2D materials. Finally, we transfer the heterophase device array onto a flexible substrate and demonstrate the near-infrared photoresponse with high photoresponsivity (~1.02 A/W). Our study provides a basis for the large-scale application of phase-engineered coplanar MoTe2 semiconductors-meter structure in advanced electronics and optoelectronics.

cond-mat.mtrl-sci↗