arXiv subjects

Weiliang Yao

Publications and source records attributed to Weiliang Yao.

At least 19 recordsLinked to original sources

arXiv2026

$d$-spacing distributions as a probe of nematoelastic response in iron-based superconductors

Electronic nematicity in iron-based superconductors (FeSCs) couples bilinearly to orthorhombic strain, allowing nematic correlations to appear in the lattice response. Here we use neutron Larmor diffraction to measure the temperature-dependent distribution of relative $d$ spacings in electron-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, hole-doped Ba$_{0.83}$K$_{0.17}$Fe$_2$As$_2$, FeSe, and Fe$_{1.07}$Te. In Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ crystals without intentionally applied uniaxial stress, the in-plane distribution width, $\varepsilon_{\rm FWHM}$, increases on cooling in the tetragonal phase and can be described phenomenologically by a Curie--Weiss-like form. The fitted scale $T^*$ decreases with Co doping and evolves similarly to the nematic phase diagram inferred from elastoresistance, although the two experiments probe different response functions. Related broadening in Ba$_{0.83}$K$_{0.17}$Fe$_2$As$_2$ and FeSe supports extending this interpretation beyond electron-doped BaFe$_2$As$_2$. By contrast, Fe$_{1.07}$Te shows no extended Curie--Weiss-like regime without applied stress, whereas uniaxial pressure produces a strongly anisotropic broadening that can contain contributions from both the field-biased lattice response and inhomogeneous loading. A mean-field model with bilinear nematoelastic coupling and spatially varying symmetry-breaking stress explains the Curie--Weiss-like broadening in terms of the renormalized orthorhombic compliance. Neutron Larmor diffraction therefore provides a bulk-sensitive probe of nematic-related lattice broadening that complements electronic and elastic measurements.

cond-mat.supr-con↗

arXiv2026

Winding feature and thermal evolution of the Dirac magnons in CrI$_3$

Two-dimensional honeycomb lattice ferromagnet chromium tri-iodide (CrI$_3$) has attracted tremendous interest because it retains ferromagnetism down to the monolayer limit and hosts intriguing topological magnons. As a prototypical van der Waals magnet, CrI$_3$ provides an ideal platform for exploring the interplay between reduced dimensionality, magnetic order, and nontrivial spin excitations. Here, using inelastic neutron scattering together with improved sample quality, we uncover the magnon winding feature around the $K$-point of the hexagonal Brillouin zone, a key signature of Dirac magnons. In addition, we find that the magnon energy follows a $T^2$-renormalization behavior at elevated temperatures, consistent with magnon-magnon interactions. These results provide previously missing information on the magnon spectrum of CrI$_3$ and further consolidate the topological nature of its spin excitations.

cond-mat.mtrl-sci↗

arXiv2026

Uniaxial strain tuned magnetism of the altermagnet candidate h-FeS

Altermagnets are collinear magnetic materials with 'alter'nating local crystalline environments, characterized by joint spin and crystalline symmetries that enable ferromagnetic-like transport properties but with vanishing net magnetization. Hexagonal FeS (h-FeS) is a recently identified altermagnet candidate that shows a spontaneous anomalous Hall effect (AHE) accompanied by a tiny net magnetization. Here, we show that both the spontaneous AHE and magnetization can be effectively suppressed by an in-plane compressive strain. Since neutron diffraction measurements show that the applied uniaxial strain only modifies the in-plane domain population but does not affect the in-plane magnetic structure, the major effect of the applied strain is to tune the small $c$-axis ferromagnetic moment. Our results demonstrate a strong correlation between the tiny net magnetization and the spontaneous AHE in h-FeS, and show that uniaxial strain provides an effective knob to tune both properties in this altermagnet candidate for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Signatures of three-state Potts nematicity in spin excitations of the van der Waals antiferromagnet FePSe$_3$

In two-dimensional (2D) nearly square-lattice quantum materials, electron correlations can induce an electronic nematic phase with twofold rotational ($C_2$) symmetry that profoundly impacts their properties. For 2D materials with threefold rotational ($C_3$) symmetry, such as the honeycomb lattice, a vestigial three-state Potts nematic order has been observed in the van der Waals antiferromagnet (AFM) FePSe$_3$ via optical and thermodynamic methods under uniaxial strain. Here, we use neutron scattering to study the magnetic order and spin excitations of FePSe$_3$ under uniaxial strain. In the AFM ordered state, we find that $\sim$0.6% tensile strain significantly suppresses one zigzag domain and promotes the other two, lowering the AFM order and spin waves to $C_2$ symmetry. The broken $C_3$ symmetry in spin excitations persists slightly above $T_{\rm{N}}\approx 108.6$ K, where the zigzag AFM order is absent. Our results thus provide direct evidence of magnetoelastic coupling and suggest that the three-state Potts nematicity in paramagnetic spin excitations arises from the vestigial order associated with the low-temperature zigzag AFM order.

cond-mat.str-el↗

arXiv2025

Direct probe of magnetic field effects on phonons by ultrasound propagation in a quasi-two-dimensional honeycomb magnet Na$_2$Co$_2$TeO$_6$

We study the phonon behavior of a Co-based honeycomb frustrated magnet Na$_2$Co$_2$TeO$_6$ under magnetic field applied perpendicular to the honeycomb plane. The temperature and field dependence of the sound velocity and sound attenuation unveil prominent spin-lattice coupling in this material, promoting ultrasound as a sensitive probe for magnetic properties. An out-of-plane ferrimagnetic order is determined below the Néel temperature $T_N=27$~K. A comprehensive analysis of our data further supports a triple-Q ground state of Na$_2$Co$_2$TeO$_6$. Furthermore, the ultrasound data were systematically compared to the thermal transport results from literature, to unveil the importance of phononic contribution to the observed transport behaviors.

cond-mat.str-el↗

arXiv2025

Anomalous Electrical Transport in the Kagome Magnet YbFe$_6$Ge$_6$

Two-dimensional (2D) kagome metals offer a unique platform for exploring electron correlation phenomena derived from quantum many-body effects. Here, we report a combined study of electrical magnetotransport and neutron scattering on YbFe$_6$Ge$_6$, where the Fe moments in the 2D kagome layers exhibit an $A$-type collinear antiferromagnetic order below $T_{\rm{N}} \approx 500$ K. Interactions between the Fe ions in the layers and the localized Yb magnetic ions in between reorient the $c$-axis aligned Fe moments to the kagome plane below $T_{\rm{SR}} \approx 63$ K. Our magnetotransport measurements show an intriguing anomalous Hall effect (AHE) that emerges in the spin-reorientated collinear state, accompanied by the closing of the spin anisotropy gap as revealed from inelastic neutron scattering. The gapless spin excitations and the Yb-Fe interaction are able to support a dynamic scalar spin chirality, which explains the observed AHE. Therefore, our study demonstrates spin fluctuations may provide an additional scattering channel for the conduction electrons and give rise to AHE even in a collinear antiferromagnet.

cond-mat.str-el↗

arXiv2025

Universal dynamics of a pair condensate

Pair condensates appear in multiple branches of physics, always introducing exotic phenomena. The pair condensate in quantum magnetism is the spin nematic, whose static (quadrupolar) order is difficult to access, favoring dynamical probes. Here, we perform high-resolution neutron spectroscopy to obtain direct evidence for the presence of two spin-nematic phases induced in the triangular-lattice antiferromagnet Na2BaNi(PO4)2 by controlling the applied magnetic field. By combining precise experiments with quantitative theoretical and numerical analysis, we identify universal dynamics arising from the pair condensate. We show explicitly how the gapless Goldstone mode influences the dispersion and induces Cherenkov-like velocity-selective decay of the gapped single-quasiparticle band. These common spectral features shed new light on spin-nematic dynamics and underline the universal phenomenology shared by pair condensates across different physical systems.

cond-mat.str-el↗

arXiv2024

Anomalous Proximitized Transport in Metal/Quantum Magnet Heterostructure $\rm{Bi_{2}Ir_{2}O_{7}/Yb_{2}Ti_{2}O_{7}}$

Fluctuations of quantum spins play a crucial role in the emergence of exotic magnetic phases and excitations. The lack of the charge degree of freedom in insulating quantum magnets, however, precludes such fluctuations from mediating electronic transport. Here we show that the quantum fluctuations of a localized frustrated magnet induce strong proximitized charge transport of the conduction electrons in a synthetic heterostructure comprising an epitaxial $\rm{Bi_{2}Ir_{2}O_{7}}$ ultrathin film on the single crystal of $\rm{Yb_{2}Ti_{2}O_{7}}$. The proximity effects are evidenced by the scaling behavior of the $\rm{Bi_{2}Ir_{2}O_{7}}$ resistance in correspondance with the dynamic scaling of the dynamic spin correlation function of $\rm{Yb_{2}Ti_{2}O_{7}}$, which is a result of quantum fluctuations near a multi-phase quantum critical point. The proximitized transport in $\rm{Bi_{2}Ir_{2}O_{7}}$ can be effectively tuned by magnetic field through suppressing the quantum spin fluctuations as well as inducing transitions via magnetic anisotropy in $\rm{Yb_{2}Ti_{2}O_{7}}$. Our work establishes a new pathway for harnessing quantum spin fluctuations in magnetic insulators with electric transport, offering exciting prospects for potential applications in the realm of quantum spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Absence of Acoustic Phonon Anomaly in a Kagome Metal with Short-ranged Structural Modulation

Kagome lattice $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) superconductors without magnetism from vanadium $d$-electrons are intriguing because they have a novel charge density wave (CDW) order around 90 K and display superconductivity at $\sim$3 K that competes with the CDW order. Recently, CsCr$_3$Sb$_5$, isostructural to $A$V$_3$Sb$_5$, was found to have concurrent structural and magnetic phase transition at $T^{\ast}\approx$ 55 K that can be suppressed by pressure to induce superconductivity [Liu \textit{et al.}, \href{https://doi.org/10.1038/s41586-024-07761-x}{Nature \textbf{632}, 1032 (2024)}]. Here, we use elastic and inelastic X-ray scattering to study the microscopic origin of the structural transition in CsCr$_3$Sb$_5$. Although our elastic measurements confirm the 4$\times$1$\times$1 superlattice order below $T^{\ast}$, its underlying correlation is rather short-ranged. Moreover, our inelastic measurements at the superlattice wavevectors around (3, 0, 0) find no evidence of a significant acoustic phonon anomaly below $T^{\ast}$, similar to the case of $A$V$_3$Sb$_5$. The absence of acoustic phonon anomaly indicates a weak electron-phonon coupling in CsCr$_3$Sb$_5$, suggesting that the structural transition is likely associated with an unconventional CDW order.

cond-mat.str-el↗

arXiv2024

Conventional Superconductivity in the Doped Kagome Superconductor Cs(V0.86Ta0.14)3Sb5 from Vortex Lattice Studies

A hallmark of unconventional superconductors is their complex electronic phase diagrams where "intertwined orders" of charge-spin-lattice degrees of freedom compete and coexist as in copper oxides and iron pnictides. While the electronic phase diagram of kagome lattice superconductor such as CsV3Sb5 also exhibits complex behavior involving coexisting and competing charge density wave order and superconductivity, much is unclear about the microscopic origin of superconductivity. Here, we study the vortex lattice (VL) in superconducting state of Cs(V0.86Ta0.14)3Sb5, where the Ta-doping suppresses charge order and enhances superconductivity. Using small-angle neutron scattering, a strictly bulk probe, we show that the VL exhibits a strikingly conventional behavior. This includes a triangular VL with a period consistent with 2e-pairing, a field dependent scattering intensity that follows a London model, and a temperature dependence consistent with a uniform superconducting gap expected for s-wave pairing. These results suggest that optimal bulk superconductivity in Cs(V1-xTax)3Sb5 arises from a conventional Bardeen-Cooper-Schrieffer electron-lattice coupling, different from spin fluctuation mediated unconventional copper and iron based superconductors.

cond-mat.supr-con↗

arXiv2023

Persistent spin dynamics in magnetically ordered honeycomb cobalt oxides

In the quest to find quantum spin liquids, layered cobalt oxides Na2Co2TeO6 and Na3Co2SbO6 have been proposed as promising candidates for approximating the Kitaev honeycomb model. Yet, their suitability has been thrown into question due to observed long-range magnetic order at low temperatures and indications of easy-plane, rather than Kitaev-type, spin anisotropy. Here we use muon spin relaxation to reveal an unexpected picture: contrary to the anticipated static nature of the long-range order, the systems show prevalent spin dynamics with spatially uneven distribution and varied correlation times. This underlines that the magnetic ground states cannot be solely described by the long-range order, suggesting a significant role of quantum fluctuations. Our findings not only shed new light on the complex physics of these systems but also underscore the need for a refined approach in the search for realizable quantum spin liquids.

cond-mat.str-el↗

arXiv2023

Phonon thermal transport shaped by strong spin-phonon scattering in a Kitaev material Na$_2$Co$_2$TeO$_6$

The recent report of a half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ has sparked a strong debate on whether it is generated by Majorana fermion edge currents or whether other more conventional mechanisms involving magnons or phonons are at its origin. A more direct evidence for Majorana fermions which could be expected to arise from a contribution to the longitudinal heat conductivity $κ_{xx}$ at $T\rightarrow0$ is elusive due to a very complex magnetic field dependence of $κ_{xx}$. Here, we report very low temperature (below 1~K) thermal conductivity ($κ$) of another candidate Kitaev material, Na$_2$Co$_2$TeO$_6$. The application of a magnetic field along different principal axes of the crystal reveals a strong directional-dependent magnetic-field ($\bf B$) impact on $κ$. We show that no evidence for mobile quasiparticles except phonons can be concluded at any field from 0~T to the field polarized state. In particular, severely scattered phonon transport is observed across the $B-T$ phase diagram, which is attributed to prominent magnetic fluctuations. Cascades of phase transitions are uncovered for all $\bf B$ directions by probing the strength of magnetic fluctuations via a precise record of $κ$($B$). Our results thus rule out recent proposals for itinerant magnetic excitations in Na$_2$Co$_2$TeO$_6$, and emphasise the importance of discriminating true spin liquid transport properties from scattered phonons in candidate materials.

cond-mat.str-el↗

arXiv2023

Continuous spin excitations in the three-dimensional frustrated magnet K2Ni2(SO4)3

Continuous spin excitations are widely recognized as one of the hallmarks of novel spin states in quantum magnets, such as quantum spin liquids (QSLs). Here, we report the observation of such kind of excitations in K2Ni2(SO4)3, which consists of two sets of intersected spin-1 Ni2+ trillium lattices. Our inelastic neutron scattering measurement on single crystals clearly shows a dominant excitation continuum, which exhibits a distinct temperature-dependent behavior from that of spin waves, and is rooted in strong quantum spin fluctuations. Further using the self-consistent-gaussian-approximation method, we determined the fourth- and fifth-nearest neighbor exchange interactions are dominant. These two bonds together form a unique three-dimensional network of corner-sharing tetrahedra, which we name as ''hyper-trillium'' lattice. Our results provide direct evidence for the existence of QSL features in K2Ni2(SO4)3 and highlight the potential for the hyper-trillium lattice to host frustrated quantum magnetism.

cond-mat.str-el↗

arXiv2023

Phononic-magnetic dichotomy of the thermal Hall effect in the Kitaev-Heisenberg candidate material Na$_2$Co$_2$TeO$_6$

Majorana fermions as emergent excitations of the Kitaev quantum spin liquid ground state constitute a promising concept in fault tolerant quantum computation. Experimentally, the recently reported topological half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ seems to confirm the Majorana nature of the material's magnetic excitations. It has been argued, however, that the thermal Hall signal in $α$-RuCl$_3$ rather stems from phonons or topological magnons than from Majorana fermions. Here we investigate the thermal Hall effect of the closely related Kitaev quantum material Na$_2$Co$_2$TeO$_6$, and we show that the thermal Hall signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the transversal heat conductivity $κ_{xy}$ carries clear signatures of the phononic $κ_{xx}$, but changes sign upon entering the low-temperature, magnetically ordered phase. We systematically resolve the two components by considering the detailed temperature and field dependence of both $κ_{xy}$ and $κ_{xx}$. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in a Kitaev topological quantum spin liquid requires to disentangle phonon vs. magnetic contributions where the latter include potentially fractionalized excitations such as the expected Majorana fermions.

cond-mat.str-el↗

arXiv2022

Magnetic ground state of the Kitaev Na$_2$Co$_2$TeO$_6$ spin liquid candidate

As a candidate Kitaev material, Na$_2$Co$_2$TeO$_6$ exhibits intriguing magnetism on a honeycomb lattice that is believed to be $C_3$-symmetric. Here we report a neutron diffraction study of high quality single crystals under $a$-axis magnetic fields. Our data support the less common notion of a magnetic ground state that corresponds to a triple-$\mathbf{q}$ magnetic structure with $C_3$ symmetry, rather than the multi-domain zigzag structure typically assumed in prototype Kitaev spin liquid candidates. In particular, we find that the field is unable to repopulate the supposed zigzag domains, where the only alternative explanation is that the domains are strongly pinned by hitherto unidentified structural reasons. If the triple-$\mathbf{q}$ structure is correct then this requires reevaluation of many candidate Kitaev materials. We also find that fields beyond about 10 Tesla suppress the long range antiferromagnetic order, allowing new magnetic behavior to emerge different from that expected for a spin liquid.

cond-mat.str-el↗

arXiv2022

Methods pressure control apparatus for lithium metal battery

Lithium (Li) metal anodes are essential for developing next-generation high-energy-density batteries. However, Li dendrite/whisker formation caused short-circuiting issue and short cycle life have prevented lithium metal from being viably used in rechargeable batteries. Numerous works have been done to study how to regulate the Li growth in electrochemical cycling by using external stacking forces. While it is widely agreed that stack pressure positively affects the lithium plating/stripping process, the optimized pressure range provided by different works varies greatly because of the difference in the pressure control setup. In this work, a pressure control apparatus is designed for Li metal batteries with liquid and solid-state electrolytes (SSE). With considerations of minimizing cell to cell variation, a reusable split cell and pressure load cell are made for testing electrochemical cells with high precision pressure control. The capability of the designed setup is demonstrated by studying the pressure effect on the Li plating/stripping process.

cond-mat.mtrl-sci↗

arXiv2022

Excitations in the ordered and paramagnetic states of honeycomb magnet Na2Co2TeO6

Na2Co2TeO6 is a proposed approximate Kitaev magnet, yet its actual magnetic interactions are elusive due to a lack of knowledge on the full excitation spectrum. Here, using inelastic neutron scattering and single crystals, we determine the system's temperature-dependent magnetic excitations over the entire Brillouin zone. Without committing to specific models, we unveil a distinct signature of the third-nearest-neighbor coupling in the spin waves, which signifies the associated distance as an emerging "soft link" in the ordered state. The presence of at least six non-overlapping spin-wave branches is at odds with all models proposed to date. Above the ordering temperature, persisting dynamic correlations can be described by equal-time magnetic structure factors of a hexagonal cluster, which reveal the leading instabilities. Our result sets definitive constraint on theoretical models for Na2Co2TeO6 and provides new insight for the materialization of the Kitaev model.

cond-mat.str-el↗

arXiv2021

Bridging Nano and Micro-scale X-ray Tomography for Battery Research by Leveraging Artificial Intelligence

X-ray Computed Tomography (X-ray CT) is a well-known non-destructive imaging technique where contrast originates from the materials' absorption coefficients. Novel battery characterization studies on increasingly challenging samples have been enabled by the rapid development of both synchrotron and laboratory-scale imaging systems as well as innovative analysis techniques. Furthermore, the recent development of laboratory nano-scale CT (NanoCT) systems has pushed the limits of battery material imaging towards voxel sizes previously achievable only using synchrotron facilities. Such systems are now able to reach spatial resolutions down to 50 nm. Given the non-destructive nature of CT, in-situ and operando studies have emerged as powerful methods to quantify morphological parameters, such as tortuosity factor, porosity, surface area, and volume expansion during battery operation or cycling. Combined with powerful Artificial Intelligence (AI)/Machine Learning (ML) analysis techniques, extracted 3D tomograms and battery-specific morphological parameters enable the development of predictive physics-based models that can provide valuable insights for battery engineering. These models can predict the impact of the electrode microstructure on cell performances or analyze the influence of material heterogeneities on electrochemical responses. In this work, we review the increasing role of X-ray CT experimentation in the battery field, discuss the incorporation of AI/ML in analysis, and provide a perspective on how the combination of multi-scale CT imaging techniques can expand the development of predictive multiscale battery behavioral models.

cond-mat.mtrl-sci↗