arXiv subjects

S. D. Wilson

Publications and source records attributed to S. D. Wilson.

At least 19 recordsLinked to original sources

arXiv2026

Unified pressure and field response across distinct charge-order regimes in Ti-doped CsV$_3$Sb$_5$

Understanding the phase diagram of kagome superconductors from a microscopic perspective is crucial for clarifying the interplay between charge order and superconductivity. Ti-doped CsV$_{3}$Sb$_{5}$ exhibits a nonmonotonic temperature-doping phase diagram in which both $T_{\rm c}$ and the charge-order temperature initially decrease with doping, followed by a crossover from long-range to short-range charge order and a subsequent increase in $T_{\rm c}$. Here, we report a muon spin rotation ($\mu$SR) study of Ti-doped CsV$_{3}$Sb$_{5}$ at two representative compositions: underdoped (Ti$_{0.05}$-CVS) and optimally doped (Ti$_{0.22}$-CVS). Using zero-field, high-field, and high-pressure $\mu$SR, we find spontaneous time-reversal-symmetry (TRS) breaking in the normal state of both compositions, strongly enhanced by an applied magnetic field and associated with long-range and short-range charge-order correlations, respectively. In the superconducting state, both samples exhibit anisotropic nodeless pairing with low superfluid density. Hydrostatic pressure substantially enhances both $T_{\rm c}$ and the superfluid density (by $\sim$2.5), revealing a linear correlation between them and pointing to unconventional pairing. Above $\sim$1 GPa, a crossover from anisotropic to isotropic nodeless pairing is observed. Despite the different nature of charge order in the two doping regimes, the superconducting responses are remarkably similar, suggesting that the competition between superconductivity and charge order occurs on a local scale, largely independent of the long-range coherence of the charge-ordered state.

cond-mat.supr-con↗

arXiv2026

Influence of controlled disorder on the dipolar spin ice state of Ho-based pyrochlores

Pyrochlore magnets of the form $R_2B_2$O$_7$, in which rare-earth ions on the $R$-site form a three-dimensional network of corner-sharing tetrahedra, provide a canonical setting for geometrical frustration. Ho-based pyrochlores host a dipolar spin-ice ground state, characterized by Ising moments constrained by the ice rules and elementary excitations analogous to magnetic monopoles. Here we examine how controlled chemical disorder influences this state by introducing site mixing on the non-magnetic $B$-site in two compounds. Ho$_2$GaSbO$_7$ contains only Ga$^{3+}$/Sb$^{5+}$ charge disorder, whereas Ho$_2$ScSbO$_7$ exhibits both charge and substantial size disorder arising from the large ionic-radius mismatch between Sc$^{3+}$ and Sb$^{5+}$. Although both materials retain the pyrochlore structure, neutron scattering measurements reveal a reduced correlation length for the $R/B$-site cation ordering and enhanced local structural distortions in Ho$_2$ScSbO$_7$. Despite these structural differences, bulk thermodynamic measurements and magnetic diffuse scattering demonstrate that both systems exhibit the defining signatures of a dipolar spin-ice state. Low-energy inelastic neutron spectroscopy further uncovers broad magnetic excitations that develop within the dipolar spin-ice regime, a feature absent in pristine Ho pyrochlores and indicative of disorder-induced splitting of the non-Kramers ground-state doublet. Together, these results show that controlled disorder generates tunable transverse-field-driven quantum fluctuations in Ho-based pyrochlores, although the dipolar spin-ice state is remarkably robust to this disorder.

cond-mat.str-el↗

arXiv2025

Interplay of $d$- and $p$-states in RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ flat-band kagome metals

Shifting the Fermi level of the celebrated $AM_3X_5$ (135) compounds into proximity of flat bands strongly enhances electronic correlations and severely affects the formation of density waves and superconductivity. Our broadband infrared spectroscopy measurements of RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ combined with density-functional band-structure calculations reveal that the correlated Ti $d$-states are intricately coupled with the Bi $p$-states that form a tilted Dirac crossing. Electron-phonon coupling manifests itself in the strong damping of itinerant carriers and in the anomalous shape of the phonon line in RbTi$_3$Bi$_5$. An anomaly in these spectral features around 150 K can be paralleled to the onset of nematicity detected by low-temperature probes. Our findings show that the materials with low band filling open unexplored directions in the physics of kagome metals and involve electronic states of different nature strongly coupled with lattice dynamics.

cond-mat.str-el↗

arXiv2025

Impact of charge-density-wave pattern on the superconducting gap in V-based kagome superconductors

Kagome metals $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) provide a compelling platform to explore the interplay between superconductivity (SC) and charge-density-wave (CDW) orders. While distinct CDW orders have been identified in K/RbV$_3$Sb$_5$ versus CsV$_3$Sb$_5$, their influence on the SC order parameter remains unresolved. Here, we investigate low-energy quasiparticle excitations in $A$V$_3$Sb$_5$, uncovering a striking difference in SC gap anisotropy: K/RbV$_3$Sb$_5$ exhibit fully gapped, nearly isotropic $s$-wave states, in contrast to the strongly anisotropic SC gap in CsV$_3$Sb$_5$. Contrary to previous vortex-state studies suggesting nodal SC in K/RbV$_3$Sb$_5$, our Meissner-state measurements in high-quality crystals demonstrate fully gapped states with reduced anisotropy compared to CsV$_3$Sb$_5$. Impurity scattering introduced via electron irradiation in K/RbV$_3$Sb$_5$ has a minimal impact on low-energy excitations, and it induces an increase in the SC transition temperature $T_{\rm c}$, consistent with more isotropic $s$-wave SC competing with CDW order. Our theoretical analysis attributes the observed SC gap anisotropy differences to distinct CDW modulation patterns: the star-of-David structure unique to CsV$_3$Sb$_5$ preserves van Hove singularities near the Fermi level, promoting anisotropic $s$-wave SC with enhanced $T_{\rm c}$ via bond-order fluctuations. These findings establish a systematic framework for understanding the interplay between SC and CDW orders in $A$V$_3$Sb$_5$, driven by electron correlations.

cond-mat.supr-con↗

arXiv2025

High-pressure floating zone crystal growth of Sr$_2$IrO$_4$

Here we demonstrate the floating zone crystal growth of the $J_\mathrm{eff}=1/2$ Mott insulator Sr$_2$IrO$_4$. Historically, the growth of iridates from a ternary melt has been precluded by the extreme vapor pressure of the metal oxide species and the difficulty of maintaining the correct oxidation state of Ir at high temperatures. Here, we show that the application of a high-pressure oxygen growth environment stabilizes the Sr$_2$IrO$_4$ phase, leading to the first demonstration of cm$^{3}$-scale crystals. In contrast to the conventional SrCl$_2$ flux growth method, where poor control over disorder leads to strong sample dependence, the high-pressure floating zone growth enables active control over the homogeneity of the melt. Crystals grown via this technique possess qualitatively similar properties to those grown via flux, with a relatively sharp onset of antiferromagnetic order observed in temperature-dependent magnetization. Further, we demonstrate that by tuning the mixing rate of the melt, we are able to grow natively hole-doped Sr$_2$Ir$_{1-y}$O$_4$, which exhibits a strongly modified magnetic and electronic response.

cond-mat.str-el↗

arXiv2025

Impact of tiny Fermi pockets with extremely high mobility on the Hall anomaly in the kagome metal CsV$_3$Sb$_5$

The kagome metal CsV$_3$Sb$_5$ exhibits an unusual charge-density-wave (CDW) order, where the emergence of loop current order that breaks time-reversal symmetry (TRS) has been proposed. A key feature of this CDW phase is a non-monotonic Hall effect at low fields, often attributed to TRS breaking. However, its origin remains unclear. Here, we conduct comprehensive magnetotransport measurements on CsV$_3$Sb$_5$ and, through mobility spectrum analysis, identify the formation of tiny Fermi pockets with extremely high mobility below the CDW transition. Furthermore, electron irradiation experiments reveal that the non-monotonic Hall effect is significantly suppressed in samples with reduced mobility, despite no substantial change in the electronic structure. These results indicate that the non-monotonic Hall effect originates from these tiny Fermi pockets with high mobility carriers rather than anomalous Hall mechanisms, providing new insights into understanding the Hall anomaly in this kagome system.

cond-mat.str-el↗

arXiv2024

Fermi surface of magnetic kagome compound GdV6Sn6 investigated using de Haas van Alphen Oscillations

The shape of the Fermi surface, and the cyclotron effective mass of the kagome magnet GdV6Sn6 charge carriers are investigated using de Haas van Alphen (dHvA) oscillations measurements and electronic band structure calculations. The temperature and angle-dependent torque magnetometry measurements revealed at least nine different frequencies ranging from ~10 T up to ~9000 T. These frequencies correspond to extremal areas of the Fermi surface ranging from ~0.2 % up to 50% of the first Brillouin zone, qualitatively consistent with the electronic band structure calculations. The angle dependent dHvA oscillation frequencies indicate that the smaller pockets of the Fermi surface have almost 3D character whereas the bigger pockets of the Fermi surface are mostly two-dimensional. We also find evidence of the presence of light (0.28(1) m0) as well as heavy (2.37(18) m0) charge carriers through the analysis of the temperature dependence of dominant frequencies. The comparison of the observed frequencies with the electronic band structure calculations indicates that the heavy masses correspond to saddle-point-like features of electronic band structure at the M point. The observation of the multiple low frequencies and the calculated contributions from various bands to such low frequencies prevent the estimation of topological nature of bands containing lighter fermions. In conclusion, our work reveals the features of a Fermi surface containing enhanced mass fermions originated from saddle points in the electronic band structure at the M point, which is inherent to kagome lattices.

cond-mat.mtrl-sci↗

arXiv2024

Depth-dependent study of time-reversal symmetry-breaking in the kagome superconductor $A$V$_{3}$Sb$_{5}$

The breaking of time-reversal symmetry (TRS) in the normal state of kagome superconductors $A$V$_{3}$Sb$_{5}$ stands out as a significant feature. Yet the extent to which this effect can be tuned remains uncertain, a crucial aspect to grasp in light of the varying details of TRS breaking observed through different techniques. Here, we employ the unique low-energy muon spin rotation technique combined with local field numerical analysis to study the TRS breaking response as a function of depth from the surface in single crystals of RbV$_{3}$Sb$_{5}$ with charge order and Cs(V$_{0.86}$Ta$_{0.14}$)$_{3}$Sb$_{5}$ without charge order. In the bulk (i.e., > 33 nm from the surface) of RbV$_{3}$Sb$_{5}$, we have detected a notable increase in the internal magnetic field width experienced by the muon ensemble. This increase occurs only within the charge ordered state. Intriguingly, the muon spin relaxation rate is significantly enhanced near the surface (i.e., < 33 nm from the surface) of RbV$_{3}$Sb$_{5}$, and this effect commences at temperatures significantly higher than the onset of charge order. Conversely, in Cs(V$_{0.86}$Ta$_{0.14}$)$_{3}$Sb$_{5}$, we do not observe a similar enhancement in the internal field width, neither in the bulk nor near the surface. These observations indicate a strong connection between charge order and TRS breaking on one hand, and on the other hand, suggest that TRS breaking can occur prior to long-range charge order. This research offers compelling evidence for depth-dependent magnetism in $A$V$_{3}$Sb$_{5}$ superconductors in the presence of charge order. Such findings are likely to elucidate the intricate microscopic mechanisms that underpin the TRS breaking phenomena in these materials.

cond-mat.supr-con↗

arXiv2023

Evidence for an odd-parity nematic phase above the charge density wave transition in kagome metal CsV$_3$Sb$_5$

The quest for fascinating quantum states arising from the interplay between correlation, frustration, and topology is at the forefront of condensed-matter physics. Recently discovered nonmagnetic kagome metals $A$V${_3}$Sb${_5}$ ($A=$ K, Cs, Rb) with charge density wave (CDW) and superconducting instabilities may host such exotic states. Here we report that an odd electronic nematic state emerges above the CDW transition temperature ($T_{\rm CDW}=94$ K) in CsV${_3}$Sb${_5}$. High-resolution torque measurements reveal a distinct twofold in-plane magnetic anisotropy that breaks the crystal rotational symmetry below $T^*\approx130$ K. However, no relevant anomalies are detected in the elastoresistance data near $T^*$, which excludes the even-parity ferro-orbital nematicity often found in other superconductors. Moreover, in the temperature range between $T_{\rm CDW}$ and $T^*$, conical rotations of magnetic field yield a distinct first-order phase transition, indicative of time-reversal symmetry breaking. These results provide thermodynamic evidence for the emergence of an odd-parity nematic order, implying that an exotic loop-current state precedes the CDW in CsV$_3$Sb$_5$.

cond-mat.str-el↗

arXiv2023

Spin-orbital correlations from complex orbital order in MgV$_{2}$O$_{4}$

MgV$_{2}$O$_{4}$ is a spinel based on magnetic V$^{3+}$ ions which host both spin ($S=1$) and orbital ($l_{eff}=1$) moments. Owing to the underlying pyrochlore coordination of the magnetic sites, the spins in MgV$_{2}$O$_{4}$ only antiferromagnetically order once the frustrating interactions imposed by the $Fd\overline{3}m$ lattice are broken through an orbitally-driven structural distortion at T$_{S}$ $\simeq$ 60 K. Consequently, a Néel transition occurs at T$_{N}$ $\simeq$ 40 K. Low temperature spatial ordering of the electronic orbitals is fundamental to both the structural and magnetic properties, however considerable discussion on whether it can be described by complex or real orbital ordering is ambiguous. We apply neutron spectroscopy to resolve the nature of the orbital ground state and characterize hysteretic spin-orbital correlations using x-ray and neutron diffraction. Neutron spectroscopy finds multiple excitation bands and we parameterize these in terms of a multi-level (or excitonic) theory based on the orbitally degenerate ground state. Meaningful for the orbital ground state, we report an "optical-like" mode at high energies that we attribute to a crystal-field-like excitation from the spin-orbital $j_{eff}$=2 ground state manifold to an excited $j_{eff}$=1 energy level. We parameterize the magnetic excitations in terms of a Hamiltonian with spin-orbit coupling and local crystalline electric field distortions resulting from deviations from perfect octahedra surrounding the V$^{3+}$ ions. We suggest that this provides compelling evidence for complex orbital order in MgV$_{2}$O$_{4}$. We then apply the consequences of this model to understand hysteretic effects in the magnetic diffuse scattering where we propose that MgV$_{2}$O$_{4}$ displays a high temperature orbital memory of the low temperature spin order.

cond-mat.str-el↗

arXiv2022

A light-induced Weyl semiconductor-to-metal transition mediated by Peierls instability

Elemental tellurium is a strongly spin-orbit coupled Peierls-distorted semiconductor whose band structure features topologically protected Weyl nodes. Using time-dependent density functional theory calculations, we show that impulsive optical excitation can be used to transiently control the amplitude of the Peierls distortion, realizing a mechanism to switch tellurium between three states: Weyl semiconductor, Weyl metal and non-Weyl metal. Further, we present experimental evidence of this inverse-Peierls distortion using time-resolved optical second harmonic generation measurements. These results provide a pathway to multifunctional ultrafast Weyl devices and introduce Peierls systems as viable hosts of light-induced topological transitions.

cond-mat.str-el↗

arXiv2022

Optical study of RbV$_3$Sb$_5$: Multiple density-wave gaps and phonon anomalies

Temperature-dependent reflectivity studies on the non-magnetic kagome metal RbV$_3$Sb$_5$ in a broad energy range (50 cm$^{-1}$ $-$ 20000 cm$^{-1}$, equivalent to 6 meV $-$ 2.5 eV) down to 10 K are reported. Below $T_{\rm CDW}=102$ K, the optical spectra demonstrate a prominent spectral-weight transfer from low to higher energies as the fingerprint of the charge-density wave (CDW) formation with the opening of a partial gap. A detailed analysis reveals two energy scales of, respectively, $\sim$ 800 cm$^{-1}$ (100 meV) and 360 cm$^{-1}$ (45 meV), the latter visible below 50 K only. Additionally, two modes at, respectively, 160 cm$^{-1}$ (20 meV) and 430 cm$^{-1}$ (53 meV) can be traced both above and below $T_{\rm CDW}$. They show strong anomalies already above $T_{\rm CDW}$ with a further renormalization across the transition, suggesting the importance of the electron-phonon coupling in RbV$_3$Sb$_5$ in both normal and CDW states. While the 160 cm$^{-1}$ mode can be attributed to the E$_{1u}$ phonon, the 430 cm$^{-1}$ mode could not be reproduced in our phonon calculations. The antiresonance nature of this mode suggests a nontrivial electron-phonon coupling in RbV$_3$Sb$_5$. A distinct localization peak observed at all temperatures signals damped electron dynamics, whereas the reduced Drude spectral weight manifests moderate deviations from the band picture in RbV$_3$Sb$_5$.

cond-mat.str-el↗

arXiv2022

Bulk evidence of anisotropic $s$-wave pairing with no sign change in the kagome superconductor CsV$_3$Sb$_5$

The recently discovered kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) possess a unique band structure with van Hove singularities and Dirac dispersions, in which unusual charge-density-wave (CDW) orders with time-reversal and rotational symmetry breaking have been reported. One of the most crucial unresolved issues is identifying the symmetry of the superconductivity that develops inside the CDW phase. Theory predicts a variety of unconventional superconducting symmetries, including exotic states with chiral and topological properties accompanied by a sign-changing superconducting gap. Experimentally, however, the phase information on the superconducting gap in $A$V$_3$Sb$_5$ is still lacking. Here we report the electron irradiation effects in CsV$_3$Sb$_5$ using introduced impurities as a phase-sensitive probe of superconductivity. Our magnetic penetration depth measurements reveal that with increasing impurities, a highly anisotropic fully-gapped state changes gradually to an isotropic full-gap state without passing through a nodal state. Furthermore, transport measurements under high pressure show that the double superconducting dome in the pressure-temperature phase diagram survives against sufficient impurities. These results are strong bulk evidence that CsV$_3$Sb$_5$ is a non-chiral, anisotropic $s$-wave superconductor with no sign change both at ambient and high pressure, which provides a clue to understanding the relationship between CDW and superconductivity in kagome superconductors.

cond-mat.supr-con↗

arXiv2022

Optical detection of charge-density-wave instability in the non-magnetic kagome metal KV$_3$Sb$_5$

Coexisting density-wave and superconducting states along with the large anomalous Hall effect in the absence of local magnetism remain intriguing and enigmatic features of the AV$_3$Sb$_5$ kagome metals (A = K, Rb, Cs). Here, we demonstrate via optical spectroscopy and density-functional calculations that low-energy dynamics of KV$_3$Sb$_5$ is characterized by unconventional localized carriers, which are strongly renormalized across the density-wave transition and indicative of electronic correlations. Strong phonon anomalies are prominent not only below the density-wave transition, but also at high temperatures, suggesting an intricate interplay of phonons with the underlying electronic structure. We further propose the star-of-David and tri-hexagon (inverse star-of-David) configurations for the density-wave order in KV$_3$Sb$_5$. These configurations are strongly reminiscent of $p$-wave states expected in the Hubbard model on the kagome lattice at the filling level of the van Hove singularity. The proximity to this regime should have intriguing and far-reaching implications for the physics of KV$_3$Sb$_5$ and related materials.

cond-mat.str-el↗

arXiv2022

Spin dynamics in the skyrmion-host lacunar spinel GaV4S8

In the lacunar spinel GaV4S8, the interplay of spin, charge, and orbital degrees of freedom produces a rich phase diagram that includes an unusual Neel-type skyrmion phase composed of molecular spins. To provide insight into the interactions underlying this complex phase diagram, we study the spin excitations in GaV4S8 through inelastic neutron scattering measurements on polycrystalline and single-crystal samples. Using linear spin-wave theory, we describe the spin-wave excitations using a model where V4 clusters decorate an FCC lattice. The effective cluster model includes a ferromagnetic interaction and a weaker antisymmetric Dzyaloshinskii-Moriya (DM) interaction between the neighboring molecular spins. Our work clarifies the spin interactions in GaV4S8 and supports the picture of interacting molecular clusters.

cond-mat.str-el↗

arXiv2021

Decoupling of static and dynamic criticality in a driven Mott insulator

Dynamically driven interacting quantum many-body systems have the potential to exhibit properties that defy the laws of equilibrium statistical mechanics. A widely studied model is the impulsively driven antiferromagnetic Mott insulator, which is predicted to realize exotic transient phenomena including dynamical phase transitions into thermally forbidden states and highly non-thermal magnon distributions. However such far-from-equilibrium regimes, where conventional time-dependent Ginzburg-Landau descriptions fail, are experimentally challenging to prepare and to probe especially in solid state systems. Here we use a combination of time-resolved second harmonic optical polarimetry and coherent magnon spectroscopy to interrogate $n$-type photo-doping induced ultrafast magnetic order parameter dynamics in the Mott insulator Sr$_2$IrO$_4$. We uncover an unusual far-from-equilibrium critical regime in which the divergences of the magnetic correlation length and relaxation time are decoupled. This violation of conventional thermal critical behavior arises from the interplay of photo-doping and non-thermal magnon population induced demagnetization effects. Our findings, embodied in a non-equilibrium "phase diagram", provide a blueprint for engineering the out-of-equilibrium properties of quantum matter, with potential applications to terahertz spintronics technologies.

cond-mat.str-el↗

arXiv2021

Geometry of the charge density wave in kagom${é}$ metal AV$_{3}$Sb$_{5}$

Kagom${é}$ lattice is a fertile platform for topological and intertwined electronic excitations. Recently, experimental evidence of an unconventional charge density wave (CDW) is observed in a Z2 kagom${é}$ metal AV$_{3}$Sb$_{5}$ (A= K, Cs, Rb). This observation triggers wide interests on the interplay between frustrated crystal structure and Fermi surface instabilities. Here we analyze the lattice effect and its impact on CDW in AV$_{3}$Sb$_{5}$. Based on published experimental data, we show that the CDW induced structural distortions is consistent with the theoretically predicted inverse star-of-David pattern, which preserves the $D_{6h}$ symmetry in the kagom${é}$ plane but breaks the sixfold rotational symmetry of the crystal due to the phase shift between kagom${é}$ layers. The coupling between the lattice and electronic degrees of freedom yields a weak first order structural transition without continuous change of lattice dynamics. Our result emphasizes the fundamental role of lattice geometry in proper understanding of unconventional electronic orders in AV$_{3}$Sb$_{5}$.

cond-mat.str-el↗

arXiv2021

Mapping the structural, magnetic and electronic behavior of (Eu$_{1-x}$Ca$_{x}$)$_{2}$Ir$_{2}$O$_{7}$ across a metal-insulator transition

In this study, we employ bulk electronic properties characterization and x-ray scattering/spectroscopy techniques to map the structural, magnetic and electronic properties of (Eu$_{1-x}$Ca$_{x}$)$_{2}$Ir$_{2}$O$_{7}$ as a function of Ca-doping. As expected, the metal-insulator transition temperature, $T_{MIT}$, decreases with Ca-doping until a metallic state is realized down to 2 K. In contrast, $T_{AFM}$ becomes decoupled from the MIT and (likely short-range) AFM order persists into the metallic regime. This decoupling is understood as a result of the onset of an electronically phase separated state, the occurrence of which seemingly depends on both synthesis method and rare earth site magnetism. PDF analysis suggests that electronic phase separation occurs without accompanying chemical phase segregation or changes in the short-range crystallographic symmetry while synchrotron x-ray diffraction confirms that there is no change in the long-range crystallographic symmetry. X-ray absorption measurements confirm the $J_{eff}$ = 1/2 character of (Eu$_{1-x}$Ca$_{x}$)$_{2}$Ir$_{2}$O$_{7}$. Surprisingly these measurements also indicate a net electron doping, rather than the expected hole doping, indicating a compensatory mechanism. Lastly, XMCD measurements show a weak Ir magnetic polarization that is largely unaffected by Ca-doping.

cond-mat.str-el↗