arXiv subjects

B. J. Kim

Publications and source records attributed to B. J. Kim.

At least 19 recordsLinked to original sources

arXiv2026

Sublattice-resolved coherent phonon dynamics in charge density waves

Phonons govern fundamental material properties and play a central role in various electronic phase transitions. Coherent driving of specific phonon modes enables on-demand phase control, motivating sublattice-resolved identification of real-space phonon motions. Yet experimentally resolving these motions remains challenging, limiting precise phonon-based control. Here, we introduce a dynamical protocol to track element-resolved phonon dynamics in the charge density wave material EuTe4, in which the dominant Te-sublattice charge order is accompanied by a previously unreported Eu-sublattice component. We leverage the elemental selectivity of time-resolved resonant X-ray scattering to reveal three coherent phonon modes with distinct sublattice character, thereby disentangling Eu- and Te-dominated lattice dynamics, in good agreement with theoretical calculations of the phonon eigenvectors. This time-domain approach, which surpasses the energy-resolution limits of conventional frequency-domain inelastic scattering, provides a broadly applicable framework for decomposing coherent phonons in multi-element materials, which is crucial for the targeted control of phases of matter.

cond-mat.mtrl-sci↗

arXiv2026

Finite-momentum inter-orbital superconductivity driven by chiral charge-density-wave quantum criticality beyond the BCS regime

Superconductivity emerging near charge-density-wave (CDW) quantum critical points often defies a conventional BCS description, particularly in multi-orbital systems with small and orbitally distinct Fermi surfaces. In TiSe$_2$, superconductivity appears under pressure near the suppression of a chiral CDW, yet its microscopic origin has remained unresolved. Here we show that the chiral CDW quantum criticality in TiSe$_2$ originates from a fluctuation-induced intertwining of charge-order and phonon modes that are symmetry incompatible at the Brillouin-zone center but become mixable at the CDW ordering wave vector. This resolution of symmetry frustration enables a single continuous chiral CDW transition and strongly enhances collective fluctuations near criticality. We demonstrate that these critical chiral CDW fluctuations drive a finite-center-of-mass-momentum inter-orbital pairing instability fundamentally different from BCS superconductivity. Because electrons near the $Γ$ and $L$ points occupy small $p$- and $d$-orbital Fermi pockets connected only by the CDW ordering vector, the inter-orbital pair susceptibility does not develop a Cooper logarithm. As a result, superconductivity is governed by an interaction-driven pairing mechanism rather than by the density of states. Using a symmetry-constrained low-energy theory and a random-phase-approximation analysis, we show that the fluctuation-enhanced pairing interaction is maximized near the chiral CDW quantum critical point, giving rise to a dome-shaped superconducting phase. A group-theoretical analysis further identifies an orbital-selective $s$-wave pairing symmetry as the most likely superconducting state.

cond-mat.str-el↗

arXiv2025

Bidirectional ultrafast control of charge density waves via phase competition

The intricate competition between coexisting charge density waves (CDWs) can lead to rich phenomena, offering unique opportunities for phase manipulation through electromagnetic stimuli. Leveraging time-resolved X-ray diffraction, we demonstrate ultrafast control of a CDW in EuTe$_4$ upon optical excitation. At low excitation intensities, the amplitude of one of the coexisting CDW orders increases at the expense of the competing CDW, whereas at high intensities, it exhibits a nonmonotonic temporal evolution characterized by both enhancement and reduction. This transient bidirectional controllability, tunable by adjusting photo-excitation intensity, arises from the interplay between optical quenching and phase-competition-induced enhancement. Our findings, supported by phenomenological time-dependent Ginzburg-Landau theory simulations, not only clarify the relationship between the two CDWs in EuTe$_4$, but also highlight the versatility of optical control over order parameters enabled by phase competition.

cond-mat.str-el↗

arXiv2025

Dynamics of a jointly commensurate moir\'e charge density wave

The advent of two-dimensional moir\'e systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moir\'e superstructure formed by two coexisting charge density waves (CDWs) with slightly mismatched wavevectors has been realized. These incommensurate CDWs can collectively exhibit commensurability, resulting in the jointly commensurate CDW (JC-CDW) and establishing a new paradigm for controlling moir\'e potential and periodicity. Achieving such functionality, however, hinges on a key open question: how do the amplitude, phase coherence, and periodicity of this order respond to external perturbations? Here, we address this question using a suite of time- and momentum-resolved diffraction and spectroscopic techniques to probe light-induced CDW dynamics in EuTe$_4$. Our time-resolved diffraction measurements distinguish the instantaneous quenching of the JC-CDW amplitude, as verified by time-resolved photoemission spectroscopy, from the much slower evolution of phase fluctuations. Furthermore, while the JC-CDW wavevector remains locked along the CDW direction upon photoexcitation, indicating a preserved moir\'e periodicity, the correlation length of JC-CDW shows an exclusive reduction perpendicular to its wavevector, unveiling the formation of previously unexplored shear-type defects. Together, this multimodal methodology reconstructs the spatiotemporal evolution of the JC-CDW upon excitation. These findings not only highlight the remarkable robustness of JC-CDWs out of equilibrium, but also provide insight into optical manipulation and engineering of moir\'e quantum materials through defect control.

cond-mat.str-el↗

arXiv2025

Intertwined Orders in a Quantum-Entangled Metal

Entanglement underpins quantum information processing and computing, yet its experimental quantification in complex, many-body condensed matter systems remains a considerable challenge. Here, we reveal a highly entangled electronic phase proximate to a quantum metal-insulator transition, identified by resonant inelastic x-ray scattering interferometry. This approach reveals that entanglement across atomic sites generates characteristic interference patterns, which our model accurately reproduces, enabling extraction of a full entanglement spectrum and resolution of the underlying quantum states. Our analysis of the pyrochlore iridate Nd2Ir2O7 demonstrates that the system undergoes pronounced quantum fluctuations in its spin, orbital and charge degrees of freedom, even in the presence of a long-range 'all-in-all-out' antiferromagnetic order. Importantly, the observed entanglement signatures facilitate the coexistence of multiple exotic symmetry-breaking orders. Complementary investigations using Raman spectroscopy corroborate the presence of these hidden orders and their emergent excitations. In particular, we observe a two-magnon-bound state below the lowest single-magnon excitation energy, which, together with split phonon modes, provides strong evidence for cubic symmetry-breaking orders of magnetic origin juxtaposed with the all-in-all-out order. Our work thus establishes a direct link between quantum entanglement and emergent unconventional orders, opening new avenues for investigating quantum materials.

cond-mat.str-el↗

arXiv2025

Interplay of canted antiferromagnetism and nematic order in Mott insulating Sr2Ir1-xRhxO4

Sr2IrO4 is one of the prime candidates for realizing exotic quantum spin orders owing to the subtle combination of spin-orbit coupling and electron correlation. Sensitive local magnetization measurement can serve as a powerful tool to study these kinds of systems with multiple competing spin orders since the comprehensive study of the spatially-varying magnetic responses provide crucial information of their energetics. Here, using sensitive magneto-optical Kerr effect measurements and spin Hamiltonian model calculations, we show that Sr2IrO4 has non-trivial domain structures which cannot be understood by conventional antiferromagnetism. This unconventional magnetic response exhibits broken symmetry along the Ir-O-Ir bond direction and is enhanced upon spin-flip transition or Rh-doping. Our analysis, based on possible stacking patterns of spins, shows that introduction of an additional rotational-symmetry breaking is essential to describe the magnetic behavior of Sr2Ir1-xRhxO4, providing strong evidence for a nematic hidden order phase in this highly correlated spin-orbit Mott insulator.

cond-mat.mtrl-sci↗

arXiv2025

Quantum interference and occupation control in high harmonic generation from monolayer $WS_2$

Two-dimensional hexagonal materials such as transition metal dichalcogenides exhibit valley degrees of freedom, offering fascinating potential for valley-based quantum computing and optoelectronics. In nonlinear optics, the K and K' valleys provide excitation resonances that can be used for ultrafast control of excitons, Bloch oscillations, and Floquet physics. Under intense laser fields, however, the role of coherent carrier dynamics away from the K/K' valleys is largely unexplored. In this study, we observe quantum interferences in high harmonic generation from monolayer $WS_2$ as laser fields drive electrons from the valleys across the full Brillouin zone. In the perturbative regime, interband resonances at the valleys enhance high harmonic generation through multi-photon excitations. In the strong-field regime, the high harmonic spectrum is sensitively controlled by light-driven quantum interferences between the interband valley resonances and intraband currents originating from electrons occupying various points in the Brillouin zone, also away from K/K' valleys such as $Γ$ and M. Our experimental observations are in strong agreement with quantum simulations, validating their interpretation. This work proposes new routes for harnessing laser-driven quantum interference in two-dimensional hexagonal systems and all-optical techniques to occupy and read-out electronic structures in the full Brillouin zone via strong-field nonlinear optics, advancing quantum technologies.

physics.optics↗

arXiv2024

Using $k$-means to sort spectra: electronic order mapping from scanning tunneling spectroscopy measurements

Hyperspectral imaging techniques have a unique ability to probe the inhomogeneity of material properties whether driven by compositional variation or other forms of phase segregation. In the doped cuprates, iridates, and related materials, scanning tunneling microscopy/spectroscopy (STM/STS) measurements have found the emergence of pseudogap 'puddles' from the macroscopically Mott insulating phase with increased doping. However, categorizing this hyperspectral data by electronic order is not trivial, and has often been done with ad hoc methods. In this paper we demonstrate the utility of $k$-means, a simple and easy-to-use unsupervised clustering method, as a tool for classifying heterogeneous scanning tunneling spectroscopy data by electronic order for Rh-doped Sr$_2$IrO$_{4}$, a cuprate-like material. Applied to STM data acquired within the Mott phase, $k$-means successfully identified areas of Mott order and of pseudogap order. The unsupervised nature of $k$-means limits avenues for bias, and provides clustered spectral shapes without a priori knowledge of the physics. Additionally, we demonstrate successful use of $k$-means as a preprocessing tool to constrain phenomenological function fitting. Clustering the data allows us to reduce the fitting parameter space, limiting over-fitting. We suggest $k$-means as a fast, simple model for processing hyperspectral data on materials of mixed electronic order.

cond-mat.str-el↗

arXiv2023

Origin of chirality in transition-metal dichalcogenides

Chirality is a ubiquitous phenomenon in which a symmetry between left- and right-handed objects is broken, examples in nature ranging from subatomic particles and molecules to living organisms. In particle physics, the weak force is responsible for the symmetry breaking and parity violation in beta decay, but in condensed matter systems interactions that lead to chirality remain poorly understood. Here, we unravel the mechanism of chiral charge density wave formation in the transition-metal dichalcogenide 1T-TiSe2. Using representation analysis, we show that charge density modulations and ionic displacements, which transform as a continuous scalar field and a vector field on a discrete lattice, respectively, follow different irreducible representations of the space group, despite the fact that they propagate with the same wave-vectors and are strongly coupled to each other. This charge-lattice symmetry frustration is resolved by further breaking of all symmetries not common to both sectors through induced lattice distortions, thus leading to chirality. Our theory is verified using Raman spectroscopy and inelastic x-ray scattering, which reveal that all but translation symmetries are broken at a level not resolved by state-of-the-art diffraction techniques.

cond-mat.str-el↗

arXiv2023

Quantum spin nematic phase in a square-lattice iridate

Spin nematic (SN) is a magnetic analog of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid. Particularly intriguing is a valence-bond SN, in which spins are quantum entangled to form a multi-polar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here, we establish a SN phase in the square-lattice iridate Sr$_2$IrO$_4$, which approximately realizes a pseudospin one-half Heisenberg antiferromagnet (AF) in the strong spin-orbit coupling limit. Upon cooling, the transition into the SN phase at T$_C$ $\approx$ 263 K is marked by a divergence in the static spin quadrupole susceptibility extracted from our Raman spectra, and concomitant emergence of a collective mode associated with the spontaneous breaking of rotational symmetries. The quadrupolar order persists in the antiferromagnetic (AF) phase below T$_N$ $\approx$ 230 K, and becomes directly observable through its interference with the AF order in resonant x-ray diffraction, which allows us to uniquely determine its spatial structure. Further, we find using resonant inelastic x-ray scattering a complete breakdown of coherent magnon excitations at short-wavelength scales, suggesting a resonating-valence-bond-like quantum entanglement in the AF state. Taken together, our results reveal a quantum order underlying the Néel AF that is widely believed to be intimately connected to the mechanism of high temperature superconductivity (HTSC).

cond-mat.str-el↗

arXiv2022

Gate-tunable quantum pathways of high harmonic generation in graphene

Under strong laser fields, electrons in solids radiate high-harmonic fields by travelling through quantum pathways in Bloch bands in the sub-laser-cycle timescales. Understanding these pathways in the momentum space through the high-harmonic radiation can enable an all-optical ultrafast probe to observe coherent lightwave-driven processes and measure electronic structures as recently demonstrated for semiconductors. However, such demonstration has been largely limited for semimetals because the absence of the bandgap hinders an experimental characterization of the exact pathways. In this study, by combining electrostatic control of chemical potentials with HHG measurement, we resolve quantum pathways of massless Dirac fermions in graphene under strong laser fields. Electrical modulation of HHG reveals quantum interference between the multi-photon interband excitation channels. As the light-matter interaction deviates beyond the perturbative regime, elliptically polarized laser fields efficiently drive massless Dirac fermions via an intricate coupling between the interband and intraband transitions, which is corroborated by our theoretical calculations. Our findings pave the way for strong-laser-field tomography of Dirac electrons in various quantum semimetals and their ultrafast electronics with a gate control.

cond-mat.mes-hall↗

arXiv2022

Single crystal growth of iridates without platinum impurities

Iridates have attracted much interest in the last decade for their novel magnetism emerging in the limit of strong spin-orbit coupling and possible unconventional superconductivity. A standard for growing iridate single crystals has been the flux method using platinum crucibles. Here, we show that this widely used method compromises the sample quality by inclusion of platinum impurities. We find that Sr2IrO4 single crystals grown in iridium crucibles show remarkable differences from those grown in platinum crucibles in their sample characterizations using Raman spectroscopy, resistivity, magnetization, optical third harmonic generation, resonant X-ray diffraction, and resonant inelastic X-ray scattering measurements. In particular, we show that several peaks of sizable intensities disappear in the Raman spectra of samples free of platinum impurities, and a significantly larger activation energy is extracted from the resistivity data compared to previously reported values. Furthermore, we find no evidence of the previously reported glide symmetry breaking structural distortions and confirm the I41/acd space group of the lattice symmetry. Although the platinum impurities are not apparent in the magnetic properties and thus went unnoticed in the stoichiometric insulating phase for a long time, their effects can be much more detrimental to transport properties in chemically doped compounds. Therefore, our result suggests using growth methods that avoid platinum impurities for an investigation of intrinsic physical properties of iridates, and possible superconducting phases.

cond-mat.mtrl-sci↗

arXiv2022

Metrology of Band Topology via Resonant Inelastic X-ray Scattering

Topology is a central notion in the classification of band insulators and characterization of entangled many-body quantum states. In some cases, it manifests as quantized observables such as quantum Hall conductance. However, being inherently a global property depending on the entirety of the system, its direct measurement has remained elusive to local experimental probes in many cases. Here, we demonstrate that various topological band indices can be directly probed by resonant inelastic x-ray scattering. Specifically, we show that the crystalline symmetry eigenvalues at the high-symmetry momentum points, which determine the band topology, leads to distinct scattering intensity for particular momentum and energy. Our approach can be explicitly demonstrated in several examples such as 1D Su-Schrieffer-Heeger chain, 2D quadrupole insulator, 3D topological band insulator, and chiral hinge insulator. Our result establishes an incisive bulk probe for the measurement of band topology.

cond-mat.str-el↗

arXiv2022

Sr$_2$IrO$_4$/Sr$_3$Ir$_2$O$_7$ superlattice for a model 2D quantum Heisenberg antiferromagnet

Spin-orbit entangled pseudospins hold promise for a wide array of exotic magnetism ranging from a Heisenberg antiferromagnet to a Kitaev spin liquid depending on the lattice and bonding geometry, but many of the host materials suffer from lattice distortions and deviate from idealized models in part due to inherent strong pseudospin-lattice coupling. Here, we report on the synthesis of a magnetic superlattice comprising the single ($n$=1) and the double ($n$=2) layer members of the Ruddlesden-Popper series iridates Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ alternating along the $c$-axis, and provide a comprehensive study of its lattice and magnetic structures using scanning transmission electron microscopy, resonant elastic and inelastic x-ray scattering, third harmonic generation measurements and Raman spectroscopy. The superlattice is free of the structural distortions reported for the parent phases and has a higher point group symmetry, while preserving the magnetic orders and pseudospin dynamics inherited from the parent phases, featuring two magnetic transitions with two symmetry-distinct orders. We infer weaker pseudospin-lattice coupling from the analysis of Raman spectra and attribute it to frustrated magnetic-elastic couplings. Thus, the superlattice expresses a near ideal network of effective spin-one-half moments on a square lattice.

cond-mat.str-el↗

arXiv2021

Absence of temperature-dependent phonon anomalies in Sr2IrO4 and Sr3Ir2O7

Following previous works reporting an anomalous behavior of several zone-center optical phonons across the magnetic transition of square lattice iridates Sr2IrO4 and Sr3Ir2O7, we have investigated the lattice dynamics as a function of momentum in these materials by means of high-resolution inelastic x-ray scattering (IXS). The observed phonon energies and scattering intensities across the Brillouin zone are in excellent agreement with ab-initio lattice dynamical calculations based on non-magnetic density-functional-perturbation theory (DFPT). Our results do not evidence any renormalization of the phonons at finite momentum across the magnetic transition of Sr2IrO4. The only anomalous behavior was detected for the in-plane polarized longitudinal-acoustic phonon branch in Sr3Ir2O7, which anomalously softens towards low temperatures and might be related to anisotropic negative thermal expansion in this compound. No anomalies related to potential charge ordering were observed.

cond-mat.str-el↗

arXiv2021

Ultrafast renormalization of the onsite Coulomb repulsion in a cuprate superconductor

Ultrafast lasers are an increasingly important tool to control and stabilize emergent phases in quantum materials. Among a variety of possible excitation protocols, a particularly intriguing route is the direct light-engineering of microscopic electronic parameters, such as the electron hopping and the local Coulomb repulsion (Hubbard $U$). In this work, we use time-resolved x-ray absorption spectroscopy to demonstrate the light-induced renormalization of the Hubbard $U$ in a cuprate superconductor, La$_{1.905}$Ba$_{0.095}$CuO$_4$. We show that intense femtosecond laser pulses induce a substantial redshift of the upper Hubbard band, while leaving the Zhang-Rice singlet energy unaffected. By comparing the experimental data to time-dependent spectra of single- and three-band Hubbard models, we assign this effect to a $\sim140$ meV reduction of the onsite Coulomb repulsion on the copper sites. Our demonstration of a dynamical Hubbard $U$ renormalization in a copper oxide paves the way to a novel strategy for the manipulation of superconductivity, magnetism, as well as to the realization of other long-range-ordered phases in light-driven quantum materials.

cond-mat.str-el↗

arXiv2021

Direct observation of excitonic instability in Ta2NiSe5

Coulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed 'excitonic insulator'. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of Tw ~241 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at Tc =325 K. Concurrently, a B2g optical phonon becomes heavily damped to the extent that its trace is almost invisible around Tc, which manifests a strong electron-phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our result unambiguously reveals the electronic origin of the phase transition.

cond-mat.str-el↗

arXiv2021

Optical magnons with dominant bond-directional exchange interactions in a honeycomb lattice iridate $α$-Li$_{2}$IrO$_{3}$

We have used resonant inelastic x-ray scattering to reveal optical magnons in a honeycomb lattice iridate $α$-Li$_{2}$IrO$_{3}$. The spectrum in the energy region 20-25 meV exhibits momentum dependence, of which energy is highest at the location of the magnetic Bragg peak, ($\textit{h}, \textit{k}$) = ($\pm$0.32, 0), and lowered toward (0, 0) and ($\pm$1, 0). We compare our data with a linear spin-wave theory based on a generic nearest-neighbor spin model. We find that a dominant bond-directional Kitaev interaction of order 20 meV is required to explain the energy scale observed in our study. The observed excitations are understood as stemming from optical magnon modes whose intensity is modulated by a structure factor, resulting in the apparent momentum dependence. We also observed diffuse magnetic scattering arising from the short-range magnetic correlation well above $\textit{T}_{N}$. In contrast to Na$_{2}$IrO$_{3}$, this diffuse scattering lacks the $C_3$ rotational symmetry of the honeycomb lattice, suggesting that the bond anisotropy is far from negligible in $α$-Li$_{2}$IrO$_{3}$.

cond-mat.str-el↗