arXiv subjects

Young-June Kim

Publications and source records attributed to Young-June Kim.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic fields in monoclinic $\alpha$-RuCl$_3$ reveal rhombohedral inclusions underlying apparent oscillations

The majority of research on $\alpha$-RuCl$_3$ has focused on applying in-plane magnetic fields to suppress antiferromagnetic order and induce a quantum spin liquid (QSL). However, this effort has been complicated by the materials temperature-dependent crystal structure and sensitivity to strain-induced stacking disorder, making interpretation of field-induced phenomena contentious. The crystal structure of $\alpha$-RuCl$_3$ has recently been clarified as a function of temperature and sample size, motivating a reassessment of its magnetic properties and connection to proposed spin-liquid signatures. Here, we show that the monoclinic structure can be isolated in nanogram-scale crystals, enabling the study of Kitaev physics in a new regime. We focus on a structurally well-defined monoclinic crystal at low temperature and perform high-resolution magnetotropic susceptibility measurements in several crystal planes. Mapping the AFM phase boundary versus temperature, field, and orientation, we find the monoclinic phase diagram closely resembles rhombohedral crystals but is systematically shifted to higher transition temperatures and critical fields. For $B \parallel a$, we observe a two-step suppression of AFM order, indicating an intermediate ordered phase analogous to the ZZ2 phase reported in rhombohedral samples. Our results show that transitions previously observed beyond the AFM regime under in-plane fields arise from multiple shifted AFM phase boundaries associated with monoclinic inclusions, rather than non-magnetic phases. These findings indicate that features attributed to a QSL are instead due to an incomplete transition from the high-temperature monoclinic to the low-temperature rhombohedral structure. They also highlight the role of structural symmetry and sample homogeneity in interpreting field-induced phenomena in $\alpha$-RuCl$_3$ and related two-dimensional quantum magnets.

cond-mat.str-el↗

arXiv2026

Sample thickness dependence of structural and magnetic properties in $\alpha$-RuCl$_3$

The layered transition metal trihalide $\alpha$-RuCl$_3$ has been studied extensively in recent years as a promising candidate for a proximate Kitaev quantum spin liquid state. In high quality samples, a complete structural transition from room-temperature C2/m to low-temperature R$\bar{3}$ is consistently observed, with a single magnetic transition to antiferromagnetic ordering at $\sim$7K. However, magnetic and physical properties have been shown to depend heavily on both sample size and sample quality, with small and damaged samples exhibiting incomplete structural transitions and multiple magnetic anomalies. Although large high quality samples have been well studied, an understanding of the features attributed to low quality or small sample size is limited. Here, we probe the structural and magnetic transitions of $\alpha$-RuCl$_3$ single crystal samples via magnetic susceptibility through a range of thickness, manipulated through careful mechanical exfoliation. We present a non-destructive protocol for exfoliating crystals and show success to 30 $\mu$m, where sample quality is observed to improve with successive cleaving. Higher temperature magnetic features at 10 K/12 K are found to emerge through cleaving, both with and without induced sample damage. In both cases, we link these additional magnetic features to a persistence of C2/m structure to the low-temperature regime.

cond-mat.mtrl-sci↗

arXiv2025

Phonon Hall Viscosity and the Intrinsic Thermal Hall Effect of $\alpha$-RuCl$_3$

The thermal Hall effect has been observed in a wide variety of magnetic insulators, yet its origins remains controversial. While some studies attribute the effect to intrinsic mechanism, such as heat carriers with Berry curvature, others propose extrinsic mechanisms, such as heat carriers scattering off crystal defects. Even the nature of the heat carriers is unknown: magnons, phonons, and fractionalized spin excitations have all been proposed. Resolving these issues is essential for the study of quantum spin liquids, and particularly for $\alpha$-RuCl$_3$, where a quantized thermal Hall effect has been attributed to Majorana edge modes. Here, we use ultrasonic measurements of the acoustic Faraday effect to demonstrate that the phonons in $\alpha$-RuCl$_3$ have Hall viscosity -- a non-dissipative viscosity that rotates phonon polarizations and deflects phonon heat currents. We show that phonon Hall viscosity produces an intrinsic thermal Hall effect that quantitatively accounts for a significant fraction of the measured thermal Hall effect in $\alpha$-RuCl$_3$: the thermal Hall effect in $\alpha$-RuCl$_3$ is due to phonons \textit{and} it is intrinsic. More broadly, we demonstrate that the acoustic Faraday effect is a powerful tool for detecting phonon Hall viscosity and the associated phonon Berry curvature, offering a new way to uncover and study exotic states of matter that elude conventional experiments.

cond-mat.str-el↗

arXiv2025

Structural Phase Separation and Enhanced Superconductivity in La1.875Ba0.125CuO4 under Uniaxial Strain

Strain engineering has attracted significant attention in recent years due to its capability in tuning lattice and electronic structures of quantum materials. Using moderate uniaxial compressive strain, we induce structural phase separation in the low-temperature phase of x=1/8 La2-xBaxCuO4 (LBCO) single crystals. These structures are low temperature tetragonal (LTT), low temperature less orthorhombic (LTLO), and a plastically deformed nano-domain structure (PDNS), comprised of few-nanometer-sized orthorhombic domains within an amorphous matrix. These three structures exhibit distinct superconducting behaviors. The volume fraction of the LTT structure is suppressed with increasing strain, while its superconducting transition temperature increases and broadens. The LTLO structure exhibits a sharp superconducting transition above 32 K, which increases up to ~36 K at maximum strain. The PDNS phase exhibits a very broad superconducting transition and persists even after removing the strain. Our study illustrates the sensitivity of superconductivity to the structure of the LBCO sample near its stripe instability.

cond-mat.supr-con↗

arXiv2025

Resonant inelastic X-ray scattering investigation of Hund's and spin-orbit coupling in $5d^2$ double perovskites

B site ordered $5d^2$ double perovskites ($\mathrm{A_2BB'O_6,\ B'}=5d^2)$ display a remarkable range of physical properties upon variation of the chosen B and $\mathrm{B'}$ site ions. This sensitivity to chemical substitution reflects the delicate balance and profound impact of strong electronic correlation and spin-orbit coupling in such systems. We present rhenium $L_2$ and $L_3$ resonant inelastic X-ray scattering (RIXS) measurements of two such physically dissimilar materials, Mott-insulating $\mathrm{Ba_2YReO_6}$ and semiconducting $\mathrm{Sr_2CrReO_6}$. Despite these differences, our RIXS results reveal similar energy scales of Hund's ($J_H$) and spin-orbit coupling ($\zeta$) in the two materials, with both systems firmly in the intermediate Hund's coupling regime where $J_H/\zeta\sim 1$. However, there are clear differences in their RIXS spectra. The conductive character of $\mathrm{Sr_2CrReO_6}$ broadens and obfuscates the atomic transitions within an electron-hole continuum, while the insulating character of $\mathrm{Ba_2YReO_6}$ results in sharp atomic excitations. This contrast in their RIXS spectra despite their similar energy scales reflects a difference in the itinerancy-promoting hopping integral and illustrates the impact of the local crystal environment in double perovskites. Finally, $L_2$ and $L_3$ edge analyses of the atomic excitations in $\mathrm{Ba_2YReO_6}$ reveal that the ordering of the low lying excited states is inverted compared to previous reports, such that the appropriate energy scales of Hund's and spin-orbit coupling are significantly modified. We present exact diagonalization calculations of the RIXS spectra at both edges which show good agreement with our results for new energy scales of $\zeta=0.290(5)$ eV and $J_H=0.38(2)$ eV ($J_H/\zeta=1.30(5)$).

cond-mat.str-el↗

arXiv2024

Exploring Quantum Materials with Resonant Inelastic X-Ray Scattering

Understanding quantum materials -- solids in which quantum-mechanical interactions among constituent electrons yield a great variety of novel emergent phenomena -- is a forefront challenge in modern condensed matter physics. This goal has driven the invention and refinement of several experimental methods, which can spectroscopically determine the elementary excitations and correlation functions that determine material properties. This Perspectives article focuses on the future experimental and theoretical trends of resonant inelastic x-ray scattering (RIXS), which is a remarkably versatile and rapidly growing technique for probing different charge, lattice, spin, and orbital excitations in quantum materials. We provide a forward-looking introduction to RIXS and outline how this technique is poised to deepen our insight into the nature of quantum materials and their emergent electronic phenomena.

cond-mat.str-el↗

arXiv2024

Effect of Ti-doping on the dimer transition in Lithium Ruthenate

We carried out a comprehensive crystal structure characterization of Ti-doped lithium ruthenate (Li$_2$Ti$_x$Ru$_{1-x}$O$_3$), to investigate the effect of Ti-doping on the structural phase transition. Experimental tools sensitive to the average structure (X-ray diffraction), as well as those sensitive to local structure (Extended X-ray Absorption Fine Structure, EXAFS; pair distribution function, PDF) are used. We observed non-monotonic dependence of the structural transition temperature on the Ti-doping level. At low doping, the transition temperature slightly increases with doping, while at high doping, the temperature decreases significantly with doping. We note two important observations from our studies. First, Ti K-edge EXAFS data shows persistent Ti-Ru dimerization even with substantial Ti doping. Second, we were able to use the PDF data to estimate the dimer correlation length above the transition temperature, which would correspond to the size of the proposed local `dimer clusters' formed by Ru-Ru and Ti-Ru neighbours. The dimer correlation length is found to be around 10~\AA, which remains robust regardless of doping. Our study therefore suggests that Ti$^{4+}$ with its $d^0$ electronic configuration is a special type of dopant when replacing Ru.

cond-mat.mtrl-sci↗

arXiv2024

High-Tc superconductor candidates proposed by machine learning

We cast the relation between the chemical composition of a solid-state material and its superconducting critical temperature (Tc) as a statistical learning problem with reduced complexity. Training of query-aware similarity-based ridge regression models on experimental SuperCon data achieve average Tc prediction errors of ~5 K for unseen out-of-sample materials. Two models were trained with one excluding high pressure data in training ("ambient" model) and a second also including high pressure data ("implicit" model). Subsequent utilization of the approach to scan ~153k materials in the Materials Project enables the ranking of candidates by Tc while accounting for thermodynamic stability and small band gap. The ambient model is used to predict stable top three high-Tc candidate materials that include those with large band gaps of LiCuF4 (316 K), Ag2H12S(NO)4 (316 K), and Na2H6PtO6 (315 K). Filtering these candidates for those with small band gaps correspondingly yields LiCuF4 (316 K), Cu2P2O7 (311 K), and Cu3P2H2O9 (307 K).

cond-mat.supr-con↗

arXiv2024

Field-dependent Magnons in a Honeycomb Antiferromagnet CoTiO$_3$

We report field-dependent high-resolution inelastic neutron scattering (INS) measurements on the honeycomb lattice magnet, CoTiO$_3$, to study the evolution of its magnon excitations across a spin reorientation transition driven by an in-plane magnetic field. By carrying out elastic neutron scattering in a magnetic field, we show that the sample transitions from a collinear antiferromagnetic state with multiple magnetic domains at a low field to a mono-domain state with a canted magnetic structure at a high field. Concurrent with this transition, we observed significant changes in both the energy and the width of the zone center magnon peak. The observed width change is argued to be consistent with an unusual zero-field state with extended domain walls. On the other hand, the magnon spectra near the $\mathbf{K}$ point of the Brillouin zone boundary are found to be largely insensitive to the changes in the ordered moment directions and the domain configuration. We argue that this observation is difficult to explain within the framework of the bond-dependent model proposed in a recent INS study [Elliot \textit{et\,al}, Nat. Commun., \textbf{12}, 3936 (2021)]. Our study therefore calls for alternative explanations for the observed $\mathbf{K}$-point gap in CoTiO$_3$.

cond-mat.str-el↗

arXiv2024

Re-investigation of Moment Direction in a Kitaev Material $\alpha$-RuCl$_{3}$

We report X-ray diffraction and resonant elastic X-ray scattering (REXS) studies on two $\alpha$-RuCl$_{3}$ crystals with distinct magnetic transition temperatures: T$_{N}$=7.3K and 6.5K. We find that the sample with T$_{N}$=6.5K exhibits a high degree of structural twinning at low temperature, whereas the T$_{N}$=7.3K sample primarily comprises a single domain of R$\bar{3}$. Notwithstanding, both samples exhibit an identical zigzag magnetic structure, with magnetic moments pointing away from the honeycomb plane by $\alpha=31(2)^{\circ}$. We argue that the identical ordered moment directions in these samples suggest that the intralayer magnetic Hamiltonian remains mostly unchanged regardless of T$_{N}$.

cond-mat.str-el↗

arXiv2024

Static and fluctuating zigzag order, and possible signatures of Kitaev physics, in torque measurements of ${\alpha}$-RuCl${_3}$

We have measured magnetic torque on a $T_N=7$ K single crystal of $\alpha$-RuCl$_3$, as a function of the field angle in the $ab$-plane, focusing on temperatures between 2 and 20 K and fields from 0 to 9 T. We find a number of features, many of which can be classified by their angular periodicity. The sample shows an oscillation with a period of 180$^\circ$ (i.e.\ two-fold periodicity) and within the magnetically ordered zigzag phase there is a 60$^\circ$ period (i.e.\ six-fold) sawtooth pattern, which can be explained by reorientation of the zigzag domains as the crystal rotates in the applied field. We argue that the six-fold sawtooth and the two-fold sinusoidal signals arise from distinct regions of the crystal. Suppressing the zigzag order with an applied field above $\sim8$ T at low temperature, a six-fold {\sl sinusoidal} signal remains, suggesting that there is fluctuating zigzag order in the putative field-induced quantum spin liquid state. Finally, in testing theoretical results which predict a torque response with divergent slope across C$_2$-preserving $b$-axes (B1-axis), we find no features like that predicted for Ising topological order. Instead we find features at low temperatures and fields just above the zigzag phase across the non-C$_2$-preserving $b$-axes (B2-axes). Interpretation of this feature is complicated by the development of other similar signatures nearby at slightly lower fields, and by clear enhancement with thermal cycling. Additionally, we contrast the torque response of $T_N \sim$ 7 K and 14 K samples.

cond-mat.str-el↗

arXiv2023

Structural Transition and Magnetic Anisotropy in $α$-RuCl$_{3}$

We report X-ray diffraction and magnetic susceptibility studies of the structural phase transition in $α$-RuCl$_{3}$. By utilizing a single crystal sample with predominantly single twin domain, we show that $α$-RuCl$_{3}$ goes from high-temperature C2/m structure to a rhombohedral structure with R$\bar{3}$ symmetry at low temperature. While the defining feature of the structural transition is changing the stacking direction from the monoclinic a-axis to the b-axis, bond-anisotropy disappears when the structural change occurs, indicating that the local $C_3$ symmetry is restored within the honeycomb layer. The symmetry change is corroborated by the vanishing magnetic anisotropy in the low-temperature structure. Our study demonstrates that magnetic interaction is extremely sensitive to structural details in $α$-RuCl$_{3}$, which could explain the sample dependence found in this material.

cond-mat.str-el↗

arXiv2023

Spin-orbit-lattice entangled state in A$_2$MgReO$_6$ (A = Ca, Sr, Ba) revealed by resonant inelastic X-ray scattering

The $5d^1$ ordered double perovskites present an exotic playground for studying novel multi-polar physics due to large spin-orbit coupling. We present Re L3 edge resonant inelastic X-ray scattering (RIXS) results that reveal the presence of the dynamic Jahn-Teller effect in the A$_2$MgReO$_6$ (A = Ca, Sr, Ba) family of $5d^1$ double perovskites. The spin-orbit excitations in these materials show a strongly asymmetric lineshape and exhibit substantial temperature dependence, indicating that they are dressed with lattice vibrations. Our experimental results are explained quantitatively through a RIXS calculation based on a spin-orbit-lattice entangled electronic ground state with the dynamic Jahn-Teller effect taken into consideration. We find that the spin-orbit-lattice entangled state is robust against magnetic and structural phase transitions as well as against significant static Jahn-Teller distortions. Our results illustrate the importance of including vibronic coupling for a complete description of the ground state physics of $5d^1$ double perovskites. Usage: Secondary publications and information retrieval purposes.

cond-mat.str-el↗

arXiv2023

Non-local features of the spin-orbit exciton in Kitaev materials

A comparative resonant inelastic x-ray scattering (RIXS) study of three well-known Kitaev materials is presented: $α$-Li$_2$IrO$_3$, Na$_2$IrO$_3$, and $α$-RuCl$_3$. Despite similar low-energy physics, these materials show distinct electronic properties, such as the large difference in the size of the charge gap. The RIXS spectra of the spin-orbit exciton for these materials show remarkably similar three-peak features, including sharp low energy peak (peak A) as well as transitions between $j_{\text{eff}}=1/2$ and $j_{\text{eff}}=3/2$ states. Comparison of experimental spectra with cluster calculations reveals that the observed three-peak structure reflects the significant role that non-local physics plays in the electronic structure of these materials. In particular, the low-energy peak A arises from a holon-doublon pair rather than a conventional particle-hole exciton as proposed earlier. Our study suggests that while spin-orbit assisted Mott insulator is still the best description for these materials, electron itinerancy cannot be ignored when formulating low-energy Hamiltonian of these materials.

cond-mat.str-el↗

arXiv2023

Tuning charge density wave order and structure via uniaxial stress in a stripe-ordered cuprate superconductor

Unidirectional spin and charge density wave order in the cuprates is known to compete with superconductivity. In the stripe order (La,M)$_2$CuO$_4$ family of cuprates, spin and charge order occur as unidirectional order that can be stabilized by symmetry breaking structural distortions, such as the low temperature tetragonal (LTT) phase. Here we examine the interplay between structure and the formation of charge density wave (CDW) order in the LTT phase of La$_{1.475}$Nd$_{0.4}$Sr$_{0.125}$CuO$_4$ by applying uniaxial stress to distort the structure and influence the formation of CDW order. Using resonant soft x-ray scattering to measure both the CDW order and (0 0 1) structural-nematic Bragg peaks, we find that the application of uniaxial stress along the Cu-O bond direction suppresses the (0 0 1) peak and has the net effect of reducing CDW order, but does so only for CDW order propagating parallel to the applied stress. We connect these observations to previous work showing an enhanced superconducting transition temperature under uniaxial stress; providing insight into how CDW, superconductivity, nematicity, and structure are related and can be tuned relative to one another in cuprates.

cond-mat.str-el↗

arXiv2023

Stacking disorder in $α$-RuCl$_3$ via x-ray three-dimensional difference pair distribution function analysis

The van der Waals layered magnet $α$-RuCl$_3$ offers tantalizing prospects for the realization of Majorana quasiparticles. Efforts to understand this are, however, hampered by inconsistent magnetic and thermal transport properties likely coming from the formation of structural disorder during crystal growth, postgrowth processing, or upon cooling through the first order structural transition. Here, we investigate structural disorder in $α$-RuCl$_3$ using x-ray diffuse scattering and three-dimensional difference pair distribution function (3D-$Δ$PDF) analysis. We develop a quantitative model that describes disorder in $α$-RuCl$_3$ in terms of rotational twinning and intermixing of the high and low-temperature structural layer stacking. This disorder may be important to consider when investigating the detailed magnetic and electronic properties of this widely studied material.

cond-mat.str-el↗

arXiv2023

Investigation of Cu-site disorder in undoped and doped BiCuSeO

We carried out X-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS) studies to investigate the origin of the low lattice thermal conductivity in BiCuSeO, and the role of silver (Ag) doping in doped samples. BiCuSeO is a promising thermoelectric material with high thermoelectric efficiency, which is significantly enhanced by doping either single Ag dopants or dual dopants (Pb and Ag). We verified that the thermal displacement parameters associated with copper (Cu) are significantly large in undoped BiCuSeO. Ag dopant, which replaces Cu, was also found to have similarly large thermal displacement parameters, that remain large down to low temperatures. Our results point towards significant disorder on Cu-site in both undoped and doped BiCuSeO, which is retained by Ag dopant on replacing Cu. The disorder is observed to be localized on the Cu-site and seems to be independent of other atoms in the crystal structure. Our observation of the disorder, which could be either static or quasi-static, is consistent with a rattling mode scenario.

cond-mat.mtrl-sci↗

arXiv2023

Oscillations in the magnetothermal conductivity of $\boldsymbolα$-RuCl$_3$: Evidence of transition anomalies

The 2D layered insulator $α$-RuCl$_3$ is a candidate material for a quantum spin-liquid state, which may be realized when a magnetic field suppresses the antiferromagnetic order present at low temperature. Oscillations in the field dependence of the thermal conductivity, observed for an in-plane magnetic field $B$ up to a critical field $B^{\star}$, have been attributed to exotic charge-neutral fermions, viewed as evidence of a quantum spin-liquid state between the critical field $B_{c}$ $\simeq 7$ T at which the antiferromagnetic phase ends and $B^{\star}$. Here we report measurements of the thermal conductivity of $α$-RuCl$_3$ as a function of magnetic field up to 15 T applied in two distinct in-plane directions: parallel and perpendicular to the Ru-Ru bond. We find that the number of oscillations between $B_{c}$ and $B^{\star}$ is the same for the two field directions even though the field interval between $B_{c}$ and $B^{\star}$ is different. In other words, the period of the oscillations is controlled by the transition fields $B_{c}$ and $B^{\star}$. We conclude that these are not true oscillations -- coming from putative fermions in a spin-liquid state -- but anomalies associated with a sequence of magnetic transitions.

cond-mat.str-el↗