arXiv subjects

Kwang-Yong Choi

Publications and source records attributed to Kwang-Yong Choi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Spin waves in Na$_2$Co$_2$TeO$_6$ studied by high-frequency/high-field ESR: Successes and failures of the triple-$\mathbf{q}$ model

The Kitaev candidate material Na$_2$Co$_2$TeO$_6$ is proposed to be proximate to a quantum spin liquid state but a suitable spin model and the nature of its ground states are still under debate. Our high-frequency/high-field electron spin resonance spectroscopy studies of Na$_2$Co$_2$TeO$_6$ single-crystals under in-plane and out-of-plane magnetic fields elucidate the ground state by investigating its low-energy spin wave excitations. Several excitation modes are observed in the low-field phase and in the phases induced by $B\parallel a^*$. In addition, the spectra exhibit a frequency-independent feature at the phase boundary connected to the putative quantum phase transition. For magnetic fields applied along the $c$ axis, the observation of three distinct spin wave modes in the antiferromagnetic (AFM) ground state reveals a previously unresolved splitting of the zero-field excitation gap into $Δ= 211\,$GHz and $Δ_2 = 237\,$GHz. The softening of one of these modes evidences a field-induced phase transition at $B_{\rm c1} = 4.7\,$T, which is corroborated by a clear anomaly in the isothermal magnetization. Spin wave calculations based on the extended Heisenberg-Kitaev model exclude a zigzag ground state of the AFM phase. A triple-q spin configuration correctly predicts two spin wave modes, but fails to reproduce the softening mode. Our analysis shows that the triple-q ground state model of Na$_2$Co$_2$TeO$_6$ is incomplete and suggests the relevance of interlayer interactions.

cond-mat.str-el↗

arXiv2025

Manipulation of anisotropic Zhang-Rice exciton in van der Waals antiferromagnets NiPS3-xSex by anion substitution

Spin-entangled excitons have emerged as intriguing quasi-particle excitations in van der Waals magnets. Among them, the recently observed Zhang-Rice (ZR) exciton in NiPS3 has garnered significant research interest due to its strong correlation with magnetic ordering and its exceptionally long-lived coherence. Herein, we present our in-depth temperature- and polarization-dependent photoluminescence (PL) study of anion-substituted NiPS3-xSex (x=0.008,0.03,0.06 and 0.09 ) to explore the nature and dynamics of the ZR exciton. Our results reveal that, similar to the cation substitution, a small percentage of anion substitution effectively destroys and modulates the ZR exciton, as evidenced by the emergence of a weaker, lower-energy PL peak in addition to the primary ZR peak. The primary and secondary PL peaks exhibit the same anisotropic polarization but differ in their peak energy shift and intensity evolution with Se substitution, suggesting varying charge transfers of p-orbitals. Notably, the ZR exciton undergoes rapid thermal destabilization at much lower temperatures than two-magnon excitations, highlighting that p-orbital inhomogeneity beyond the magnetic ordering structure is a decisive factor in driving its thermal quenching.

cond-mat.mtrl-sci↗

arXiv2025

Two-stage evolution of magnetic correlations in spiral spin liquid material, Ca$_{10}$Cr$_{7}$O$_{28}$

We present an X-band and tunable high-frequency/high-field electron spin resonance (HF-ESR) study of single-crystalline Ca$_{10}$Cr$_{7}$O$_{28}$, which constitutes alternating antiferromagnetic and ferromagnetic kagome bilayers. At high temperatures, a phonon-assisted relaxation process is evoked to account for the pronounced increase of the linewidth in an exchange-narrowing regime ($k_{\rm B}T\gg J$). In contrast, at low temperatures ($k_{\rm B}T\lesssim J$), a power-law behavior in line narrowing is observed. Our data reveal two distinct power-law regimes for the linewidth which crossover at $T^*\approx 7.5$~K. Notably, the intriguing evolution of the ESR linewidth in this alternating kagome bilayer system with opposite sign of exchange interactions highlights distinct spin dynamics compared to those in a uniform kagome antiferromagnet.

cond-mat.str-el↗

arXiv2024

Signatures of Floquet Engineering in the proximal Kitaev Quantum Spin Liquid H$_3$LiIr$_2$O$_6$ by tr-RIXS

We present the first circularly polarized Floquet engineering time-resolved Resonant Inelastic X-ray Scattering (tr-RIXS) experiment in H$_3$LiIr$_2$O$_6$, an iridium-based Kitaev system. Our calculations and experimental results are consistent with the modification of the low energy magnetic excitations in H$_3$LiIr$_2$O$_6$ only during illumination by the laser pulse, consistent with the Floquet engineering of the exchange interactions. However, the penetration length mismatch between the X-ray probe and laser pump and the intrinsic complexity of Kitaev magnets prevented us from unequivocally extracting towards which ground H$_3$LiIr$_2$O$_6$ was driven. We outline possible solutions to these challenges for Floquet stabilization and observation of the Kitaev Quantum Spin Liquid limit by RIXS.

cond-mat.str-el↗

arXiv2024

Magnetism and field-induced effects in the S = 5/2 honeycomb lattice antiferromagnet FeP3SiO11

Quantum magnets based on honeycomb lattices with low-coordination number offer a viable ground to realize exotic emergent quantum excitations and phenomena arising from the interplay between competing magnetic interactions, spin correlations, and spatial anisotropy. However, unlike their low-spin analogues, high-spin honeycomb lattice antiferromagnets have remained comparatively less explored in the context of capturing the classical analogs of quantum phenomena. Herein, we report the crystal structure, magnetic susceptibility, specific heat, and electron spin resonance (ESR), complemented by ab initio density functional theory (DFT) calculations on polycrystalline samples of FeP3SiO11 wherein the Fe3+ ions decorate a nearly-perfect S = 5/2 honeycomb lattice without any site disorder among constituent atoms. Above 150 K, an antiferromagnetic Weiss temperature of - 12 K is observed consistent with DFT calculations, which suggest the presence of strong intra-planar nearest-neighbor and weaker inter-planar further nearest-neighbor exchange interactions. An anomaly at TN = 3.5 K in specific heat and magnetic susceptibility reveals the presence of a long-range ordered ground state in zero field. Above TN, ESR evidences short-range spin correlations and unsaturated magnetic entropy, while below TN unconventional excitations are seen via power-law specific heat. A spin-flop transition is observed in an applied field of Hc1 = 0.2 T. At higher applied fields, TN is gradually suppressed down to zero at Hc2 = 5.6 T with a 2D critical exponent 0.255. Above Hc2, a broad maximum in specific heat due to gapped magnon excitations indicates the emergence of an interesting nearly-polarized state dressed by a disordered state in the honeycomb lattice antiferromagnet FeP3SiO11.

cond-mat.str-el↗

arXiv2024

Emergence of flat bands and ferromagnetic fluctuations via orbital-selective electron correlations in Mn-based kagome metal

Kagome lattice has been actively studied for the possible realization of frustration-induced two-dimensional flat bands and a number of correlation-induced phases. Currently, the search for kagome systems with a nearly dispersionless flat band close to the Fermi level is ongoing. Here, by combining theoretical and experimental tools, we present Sc$_3$Mn$_3$Al$_7$Si$_5$ as a novel realization of correlation-induced almost-flat bands in the kagome lattice in the vicinity of the Fermi level. Our magnetic susceptibility, $^{27}$Al nuclear magnetic resonance, transport, and optical conductivity measurements provide signatures of a correlated metallic phase with tantalizing ferromagnetic instability. Our dynamical mean-field calculations suggest that such ferromagnetic instability observed originates from the formation of nearly flat dispersions close to the Fermi level, where electron correlations induce strong orbital-selective renormalization and manifestation of the kagome-frustrated bands. In addition, a significant negative magnetoresistance signal is observed, which can be attributed to the suppression of flat-band-induced ferromagnetic fluctuation, which further supports the formation of flat bands in this compound. These findings broaden a new prospect to harness correlated topological phases via multiorbital correlations in 3$d$-based kagome systems.

cond-mat.str-el↗

arXiv2024

Interplay between magnetic and lattice excitations and emergent multiple phase transitions in MnPSe3-xSx

The intricate interplay between spin and lattice degrees of freedom in two-dimensional magnetic materials plays a pivotal role in modifying their magnetic characteristics, engendering hybrid quasiparticles, and implementing functional devices. Herein, we present our comprehensive and in-depth investigations on magnetic and lattice excitations of MnPSe3-xSx (x = 0, 0.5, and 1.5) alloys, utilizing temperature- and polarization-dependent Raman scattering. Our experimental results reveal the occurrence of multiple phase transitions, evidenced by notable changes in phonon self-energy and the appearance or splitting of phonon modes. These emergent phases are tied to the development of long and short-range spin-spin correlations, as well as to spin reorientations or magnetic instabilities. Our analysis of two-magnon excitations as a function of temperature and composition showcases their hybridization with phonons whose degree weakens with increasing x. Moreover, the suppression of spin-dependent phonon intensity in chemically most-disordered MnPSe3-xSx (x = 1.5) suggests that chalcogen substitution offers a control knob of tuning spin and phonon dynamics by modulating concurrently superexchange pathways and a degree of trigonal distortions.

cond-mat.mtrl-sci↗

arXiv2024

Novel quantum spin liquid ground state in the trimer rhodate Ba$_4$NbRh$_3$O$_{12}$

Frustrated magnets offer a plethora of exotic magnetic ground states, including quantum spin liquids (QSLs), in which enhanced quantum fluctuations prevent a long-range magnetic ordering of the strongly correlated spins down to lowest temperature. Here we have investigated the trimer based mixed valence hexagonal rhodate Ba$_4$NbRh$_3$O$_{12}$ using a combination of dc and ac magnetization, electrical resistivity, specific heat, and muon spin rotation/relaxation ($μ$SR) measurements. Despite the substantial antiferromagnetic exchange interactions, as evident from the Weiss temperature ($θ_{\mathrm{W}}\sim -35$ to -45 K), among the Rh-local moments, neither long-range magnetic ordering nor spin-freezing is observed down to at least 50 mK, in ac-susceptibility, specific heat and ZF-$μ$SR measurements (down to 0.26 K). We ascribe the absence of any magnetic transition to enhanced quantum fluctuations as a result of geometrical frustration arising out of the edge-sharing equilateral Rh-triangular network in the structure. Our longitudinal-field $μ$SR result evidences persistent spin fluctuations down to 0.26~K, thus stabilizing a dynamic QSL ground state in Ba$_4$NbRh$_3$O$_{12}$. Furthermore, the magnetic specific heat ($C_{\mathrm{m}}$) data at low-$T$ reveal a significant $T$-linear contribution plus a quadratic $T$-dependence. A $T$-linear behavior is evocative of gapless spin excitations, while the $T^2$-term of $C_{\mathrm{m}}$ may indicate the Dirac QSL phenomenology of the spinon excitations with a linear dispersion.

cond-mat.str-el↗

arXiv2024

Magnetism and spin dynamics of an S=3/2 frustrated trillium lattice antiferromagnet K2CrTi(PO4)3

Competing magnetic interactions, frustration driven quantum fluctuations, and spin correlations offer an ideal route for the experimental realization of emergent quantum phenomena and exotic quasi particle excitations in 3D frustrated magnets. In this context, trillium lattice, wherein magnetic ions decorate a three-dimensional chiral network of corner-shared equilateral triangular motifs, provides a viable ground. Herein, we present the crystal structure, magnetic susceptibility, specific heat, electron spin-resonance, muSR results on the polycrystalline samples of K2CrTi(PO4)3 wherein the Cr3+ ions form a perfect trillium lattice without any detectable anti-site disorder. The Curie-Weiss fit of the magnetic susceptibility data above 100 K yields a Curie-Weiss temperature of -23 K, which indicates the presence of dominant antiferromagnetic interactions between Cr3+ (S=3/2) moments. The specific heat measurements reveal the occurrence of two consecutive phase transitions, at temperatures TL = 4.3 K and TH = 8 K, corresponding to two different magnetic phases and it unveils the existence of short-range spin correlations above the ordering temperature TH. The power-law behavior of ESR linewidth suggests the persistence of short-range spin correlations over a relatively wide critical region in agreement with the specific heat results. The muSR resultsprovide concrete evidence of two different phases corresponding to two transitions, coupled with thecritical slowing down of spin fluctuations above TL and persistent spin dynamics below TL, consistent with the thermodynamic results. Moreover, the muSR results reveal the coexistence of static and dynamic local magnetic fields below TL, signifying the presence of complex magnetic phases owing to the entwining of spin correlations and competing magnetic interactions in this three-dimensional frustrated magnet.

cond-mat.str-el↗

arXiv2023

One-ninth magnetization plateau stabilized by spin entanglement in a kagome antiferromagnet

The spin-1/2 antiferromagnetic Heisenberg model on a Kagome lattice is geometrically frustrated, which is expected to promote the formation of many-body quantum entangled states. The most sought-after among these is the quantum spin liquid phase, but magnetic analogs of liquid, solid, and supersolid phases may also occur, producing fractional plateaus in the magnetization. Here, we investigate the experimental realization of these predicted phases in the Kagome material YCu3(OD)6+xBr3-x (x=0.5). By combining thermodynamic and Raman spectroscopic techniques, we provide evidence for fractionalized spinon excitations and observe the emergence of a 1/9 magnetization plateau. These observations establish YCu3(OD)6+xBr3-x as a model material for exploring the 1/9 plateau phase.

cond-mat.str-el↗

arXiv2022

Planar Thermal Hall Effects in Kitaev Spin Liquid Candidate Na2Co2TeO6

We investigate both the longitudinal thermal conductivity ($κ_{xx}$) and the planar thermal Hall conductivity ($κ_{xy}$) in the Kitaev spin liquid candidate of Co-based honeycomb antiferromagnet Na$_2$Co$_2$TeO$_6$ in a magnetic field ($B$) applied along the $a$ and $a^*$ axes. A finite $κ_{xy}$ is resolved for both field directions in the antiferromagnetic (AFM) phase below the Néel temperature of 27 K. The temperature dependence of $κ_{xy}/T$ shows the emergence of topological bosonic excitations. In addition, the field dependence of $κ_{xy}$ shows sign reversals at the critical fields in the AFM phase, suggesting the changes in the Chern number distribution of the topological magnons. Remarkably, a finite $κ_{xy}$ is observed in $B \parallel a^*$ between the first-order transition field in the AFM phase and the saturation field, which is prohibited in a disordered state by the two-fold rotation symmetry around the $a^*$ axis of the honeycomb lattice, showing the presence of a magnetically ordered state that breaks the two-fold rotation symmetry. Our results demonstrate the presence of topological magnons in this compound in the whole field range below the saturation field.

cond-mat.str-el↗

arXiv2022

Emergent nematicity and intrinsic vs. extrinsic electronic scattering processes in the kagome metal CsV$_3$Sb$_5$

Fermi surface fluctuations and lattice instabilities in the 2D metallic kagome superconductor CsV$_3$Sb$_5$ are elucidated via polarization-resolved Raman spectroscopy. The presence of a weak electronic continuum in high-quality samples marks the cross-over into the charge-density-wave (CDW) ordered phase, while impurity-rich samples promote strong defect-induced electronic scattering processes that affect the coherence of the CDW phase. CDW-induced phonon anomalies appear below $T_{\mathrm{CDW}}$, with emergent $C2$ symmetry for one of the CDW amplitude modes, alluding to nematicity. In conjunction with symmetry-breaking lattice distortions, a kink-like hardening of the A$_{1g}$ phonon energy at $T_{\mathrm{CDW}}$ signifies a concerted interplay of electronic correlations and electron-phonon coupling in the exotic CDW order.

cond-mat.str-el↗

arXiv2022

Weak-coupling to strong-coupling quantum criticality crossover in a Kitaev quantum spin liquid $α$-RuCl$_3$

We report an unprecedented quantum criticality crossover representing two different universal scaling behaviors in a Kitaev quantum magnetic material $α$-RuCl$_3$. $α$-RuCl$_3$ presents both a symmetry breaking antiferromagnetic order and a long-range entangled topological order of a quantum spin liquid, and thus could be a candidate system for a new universality class involving deconfined fractionalized excitations of the local Z$_2$ fluxes and itinerant Majorana fermions. Theoretical analyses on the inelastic neutron scattering and specific heat results demonstrate that Wilson-Fisher-Yukawa-type 'conventional' weak-coupling quantum criticality in high energy scales crosses over to heavy-fermion-type 'local' strong-coupling one in low energy scales. Our findings provide deep insight on how the quantum criticality evolves in fermion-boson coupled topological systems with different types of deconfined fermions.

cond-mat.str-el↗

arXiv2021

Persistence of Ising-like easy-axis spin correlations in the paramagnetic state of the spin-1 chain compound NiTe$_2$O$_5$

A $^{125}$Te nuclear magnetic resonance (NMR) study was carried out in the paramagnetic state of the recently discovered quasi-one-dimensional spin-1 chain compound NiTe$_2$O$_5$. We observed that the $^{125}$Te NMR spectrum splits into two in a magnetic field applied along the $c$ axis. Based on the strong temperature variation of the relative intensity ratio of the split lines, we infer that the line splitting arises from the two sublattice susceptibilities induced in opposite directions along the chains. In great support of this interpretation, a quantitative analysis of the spin-lattice relaxation rate $T_1^{-1}$ and the Knight shift data unravels dominant transverse spin fluctuations. We conclude that Ising-like uniaxial spin correlations persist up to surprisingly high temperatures compared to the exchange energy scales. Spin-charge coupling mechanism via a self-doping effect may be important.

cond-mat.str-el↗

arXiv2021

Thermally populated versus field-induced triplon bound states in the Shastry-Sutherland lattice SrCu$_2$(BO$_3$)$_2$

The Shastry-Sutherland compound SrCu$_2$(BO$_3$)$_2$ constituting orthogonally coupled dimers harbors a $S=0$ singlet ground state. The confluence of strong interdimer interaction and frustration engenders a spectrum of low-energy excitations including localized triplons as well as singlet and triplet bound states. Their dynamics are controlled by an external magnetic field and temperature. Here, we employ high-field Raman spectroscopy to map the field and temperature evolution of such bosonic composite quasiparticles on approaching the 1/8 magnetization plateau. Our study unveils that the magnetic field and thermal fluctuations show remarkably similar effects in melting the singlet bound states, but are disparate in their effects on the fine spectral shapes. This, together with the anti-crossing of two singlet bound states in the intermediate field $B=10-16$ T, is discussed in terms of the correlated dynamics of frustrated, interacting bosons.

cond-mat.str-el↗

arXiv2021

Dimer Crystallization Induced by Elemental Substitution in the Honeycomb Lattice of Ru1-xOsxCl3

Substitution effects of Os for Ru in α-RuCl3 are investigated in a wide composition range of 0 =< x =< 0.67 in Ru1-xOsxCl3 by X-ray and electron diffraction, magnetic susceptibility, heat capacity, and Raman spectroscopy measurements. Apart from the Kitaev physics with antiferromagnetic interactions increasing with x, a rich phase diagram is obtained, which includes an antiferromagnetic long-range order below 12 K for x =< 0.15, a dome-shaped spin-singlet dimer phase below 130 K for 0.15 =< x =< 0.40, and a magnetic short-range order for x > 0.40. A dimerization as similarly observed in α-RuCl3 under high pressure occurs in the spin-singlet phase. It is suggested that Ru-Os pairs in the solid solutions tend to form dimers with short bonds and trigger the first-order transition in the presence of pseudo-threefold rotational symmetry for dimerization around a substituted Os atom only at low substitutions. This is a rare example of molecular orbital crystallization induced by elemental substitution in a highly disordered system. The short-range order at high substitutions may be related to a random-singlet state stabilized by bond disorder in the honeycomb net.

cond-mat.mtrl-sci↗

arXiv2021

Unusual spin pseudogap behavior in the spin web lattice Cu$_3$TeO$_6$ probed by $^{125}$Te nuclear magnetic resonance

We present a $^{125}$Te nuclear magnetic resonance (NMR) study in the three-dimensional spin web lattice Cu$_3$TeO$_6$, which harbors topological magnons. The $^{125}$Te NMR spectra and the Knight shift $\mathcal{K}$ as a function of temperature show a drastic change at $T_\text{S}\sim 40$ K much lower than the Néel ordering temperature $T_\text{N}\sim 61$ K, providing evidence for the first-order structural phase transition within the magnetically ordered state. Most remarkably, the temperature dependence of the spin-lattice relaxation rate $T_1^{-1}$ unravels spin-gap-like magnetic excitations, which sharply sets in at $T^*\sim 75$ K, the temperature well above $T_\text{N}$. The spin gap behavior may be understood by weakly dispersive optical magnon branches of high-energy spin excitations originating from the unique corner-sharing Cu hexagon spin-1/2 network with low coordination number.

cond-mat.str-el↗

arXiv2020

Observation of a random singlet state in a diluted Kitaev honeycomb material

We report a $^{35}$Cl nuclear magnetic resonance (NMR) study of the diluted Kitaev material $α$-Ru$_{1-x}$Ir$_x$Cl$_3$ ($x=0.1$ and $0.2$) where non-magnetic Ir$^{3+}$ dopants substitute Ru$^{3+}$ ions. Upon dilution, the $^{35}$Cl spectra exhibit unusual large magnetic inhomogeneity, which sets in at temperatures below the Kitaev exchange energy scale. At the same time, the $^{35}$Cl spin-lattice relaxation rate $T_1^{-1}$ as a function of dilution and magnetic field unravels a critical doping of $x_c\approx 0.22$, towards which both the field-induced spin gap and the zero-field magnetic ordering are simultaneously suppressed, while novel gapless low-energy spin excitations dominate the relaxation process. These NMR findings point to the stabilization of a random singlet phase in $α$-Ru$_{1-x}$Ir$_x$Cl$_3$, arising from the interplay of dilution and exchange frustration in the quantum limit.

cond-mat.str-el↗