arXiv subjects

Dongjoon Song

Publications and source records attributed to Dongjoon Song.

At least 19 recordsLinked to original sources

arXiv2026

Prominent Dimensional Effects on High-order Harmonic Generation in Strongly Correlated Electron Systems

Dimensionality strongly affects elementary excitations in correlated quantum materials, yet its impact on extreme nonlinear optical responses remains largely unexplored. Here, we combine high-harmonic generation (HHG) experiments on quasi-one-dimensional SrCuO$_2$ and quasi-two-dimensional Pr$_2$CuO$_4$ with nonequilibrium simulations of Hubbard models. We find a pronounced dimensional contrast: SrCuO$_2$ exhibits a robust plateau-like high-harmonic spectrum with weak temperature dependence, whereas Pr$_2$CuO$_4$ shows a monotonic decrease in harmonic yield and strong thermal suppression, especially at higher harmonics. The simulations qualitatively reproduce these trends and identify dimensionality-dependent doublon--holon dephasing, governed by spin--charge coupling, as their microscopic origin. These results establish dimensionality and spin--charge coupling as key control parameters for extreme nonlinear optical responses in correlated insulators.

cond-mat.str-el↗

arXiv2026

Machine Learning Guided Discovery of Corundum High Entropy Oxides

Early thinking in the field of high entropy oxides (HEOs) emphasized their likely abundance, with combinatorial arguments hinting at a myriad of new materials. The experimental reality has proven more challenging: the stability of HEOs cannot be straightforwardly predicted based on ionic radii, lattice geometry, and charge-balancing considerations alone. In this work, we employ machine learning interatomic potentials (MLIPs) to predict the synthesizability of HEOs of the form $A_2$O$_3$ derived from a selection of trivalent cations. From nearly 500 possible compositions, we identify 16 promising candidates for experimental validation with solid-state and combustion synthesis. We discover three new HEOs in the corundum structure, including (Al,Cr,Fe,Rh,Sc)$_2$O$_3$, and one novel cation-ordered phase, (Al,Fe,Ga,Sc)$_2$O$_3$. By far the most common synthesis outcome was a mixture of competing phases, sometimes involving redox reactions. Our results also reveal profound synthesis method dependence for the final product, where qualitatively equivalent outcomes between the two synthesis methods were only observed for 3 of the 16 tested compositions. We conclude that the occurrence rate of HEOs is far rarer than initially believed and that machine learning approaches can effectively guide us to the "needle in the haystack".

cond-mat.mtrl-sci↗

arXiv2025

Emergence of low-energy spin waves in superconducting electron-doped cuprates

In order to fully utilize the technological potential of unconventional superconductors, an enhanced understanding of the superconducting mechanism is necessary. In the best performing superconductors, the cuprates, superconductivity is intimately linked with magnetism, although the details of this coupling remain elusive. In search of clarity in the magnetism-superconductivity relationship, we focus on the electron-doped cuprate $\mathrm{Nd}_{1.85}\mathrm{Ce}_{0.15}\mathrm{CuO}_{4-\delta}$ (NCCO). NCCO has an antiferromagnetic ground state when synthesized, and only becomes superconducting after a reductive annealing process. This makes NCCO an ideal template to study how the magnetism differs in the superconducting and non-superconducting state, while keeping the material template as constant as possible. Using neutron spectroscopy, we reveal that the as-grown crystal exhibits a large spin pseudogap in the magnetic fluctuation spectrum. Upon annealing, defects that are introduced by the commonly employed synthesis method are removed and the spin pseudogap is significantly reduced. While the spin pseudogap in the annealed sample is likely an effect of superconductivity, we argue that the spin pseudogap in the as-grown sample is caused by the absence of long-wavelength spin waves. The defects in as-grown NCCO thus play the dual role of suppressing both superconductivity and low-energy spin waves, highlighting a potential connection between these two phenomena.

cond-mat.supr-con↗

arXiv2025

Dark states of electrons in a quantum system with two pairs of sublattices

A quantum state of matter that is forbidden to interact with photons and is therefore undetectable by spectroscopic means is called a dark state. This basic concept can be applied to condensed matter where it suggests that a whole band of quantum states could be undetectable across a full Brillouin zone. Here we report the discovery of such condensed matter dark states in palladium diselenide as a model system that has two pairs of sublattices in the primitive cell. By using angle-resolved photoemission spectroscopy, we find valence bands that are practically unobservable over the whole Brillouin zone at any photon energy, polarisation, and scattering plane. Our model shows that two pairs of sublattices located at half-translation positions and related by multiple glide-mirror symmetries make their relative quantum phases polarised into only four kinds, three of which become dark due to double destructive interference. This mechanism is generic to other systems with two pairs of sublattices, and we show how the phenomena observed in cuprates, lead-halide perovskites, and density wave systems can be resolved by the mechanism of dark states. Our results suggest that the sublattice degree of freedom, which has been overlooked so far, should be considered in the study of correlated phenomena and optoelectronic characteristics.

cond-mat.supr-con↗

arXiv2025

Reconstruction of the occupied and unoccupied electronic states driven by quantum charge fluctuations in electron doped cuprate superconductors

The origin of electron-boson interactions is central to understanding high-$T_c$ superconductivity in cuprates. While phonons and magnetic fluctuations are widely considered as candidates for mediating electron pairing, the role of charge fluctuations -- one of the fundamental electronic degrees of freedom -- remains unclear. Here, we investigate the electronic structure of the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$ using angle-resolved photoemission spectroscopy and angle-resolved inverse photoemission spectroscopy, which reveal the occupied and unoccupied states, respectively. We identify emergent spectral features on both occupied and unoccupied states that are consistent with excitations driven by quantum charge fluctuations. The results obtained in this study offer direct experimental insight into charge fluctuations in cuprates, thereby paving the way towards clarifying their fine electronic structure and the mechanism of high-$T_c$ superconductivity.

cond-mat.str-el↗

arXiv2023

Spontaneous breaking of mirror symmetry beyond critical doping in Pb-Bi2212

Identifying ordered phases and their underlying symmetries is the first and most important step toward understanding the mechanism of high-temperature superconductivity; critical behaviors of ordered phases are expected to be correlated with superconductivity. Efforts to find such ordered phases have been focused on symmetry breaking in the pseudogap region while the Fermi liquid-like metal region beyond the so-called critical doping $p_{c}$ has been regarded as a trivial disordered state. Here, we used rotational anisotropy second harmonic generation and uncovered a broken mirror symmetry in the Fermi liquid-like phase in (Bi,Pb)$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ with $p = 0.205 > p_{c}$. By tracking the temperature evolution of the symmetry-breaking response, we verify an order parameter-like behavior with the onset temperature $T_{up}$ at which the strange metal to Fermi liquid-like-metal crossover takes place. Complementary angle-resolved photoemission study showed that the quasiparticle coherence between $\mathrm{CuO_{2}}$ bilayers is enhanced in proportion to the symmetry-breaking response as a function of temperature, indicating that the change in metallicity and symmetry breaking are linked. These observations contradict the conventional quantum disordered scenario for over-critical-doped cuprates and provide new insight into the nature of the quantum critical point in cuprates.

cond-mat.supr-con↗

arXiv2023

Linear scaling relationship of Néel temperature and dominant magnons in pyrochlore ruthenates

We present a systematic Raman spectroscopy study on a series of pyrochlore ruthenates, a system which is not yet clearly settled on its magnetic origin and structure. Apart from the Raman-active phonon modes, new peaks that appear in the energy range of 15 - 35 meV below the Néel temperature are assigned as one-magnon modes. The temperature evolution of one-magnon modes displays no significant thermal dependence in mode frequencies while the intensities decrease monotonically. Remarkably, one-magnons from all compounds show similar characteristics with a single dominant peak at lower energy and weaker side peaks at a couple of meV higher energy. Most importantly, we uncover a striking proportionality between the dominant magnon mode energies and the Néel temperature values. Our results suggest the Ru ions may have similar or the same magnetic phase in all pyrochlore ruthenates of our study. We have thus found an avenue for directly tuning the magnetic exchange interaction by the selection of the $A$-site ion.

cond-mat.str-el↗

arXiv2023

Correlation effects obtained from optical spectra of Fe-pnictides using an extended Drude-Lorentz model analysis

We introduce an analysis model, an extended Drude-Lorentz model, and apply it to Fe-pnictide systems to extract their electron-boson spectral density functions (or correlation spectra). The extended Drude-Lorentz model consists of an extended Drude mode for describing correlated charge carriers and Lorentz modes for interband transitions. The extended Drude mode can be obtained by a reverse process starting from the electron-boson spectral density function and extending to the optical self-energy and, eventually, to the optical conductivity. Using the extended Drude-Lorentz model, we obtained the electron-boson spectral density functions of K-doped BaFe$_2$As$_2$ (Ba-122) at four different doping levels. We discuss the doping-dependent properties of the electron-boson spectral density function of K-doped Ba-122. We also can include pseudogap effects in the model using this approach. Therefore, this approach is very helpful for understanding and analyzing measured optical spectra of strongly correlation electron systems, including high-temperature superconductors (cuprates and Fe-pnictides).

cond-mat.str-el↗

arXiv2023

Doping-dependent superconducting physical quantities of K-doped BaFe$_2$As$_2$ obtained through infrared spectroscopy

We investigated four single crystals of K-doped BaFe$_2$As$_2$ (Ba-122), Ba$_{1-x}$K$_x$Fe$_2$As$_2$ with $x$ = 0.29, 0.36, 0.40, and 0.51, using infrared spectroscopy. We explored a wide variety of doping levels, from under- to overdoped. We obtained the superfluid plasma frequencies ($Ω_{\mathrm{sp}}$) and corresponding London penetration depths ($λ_{\mathrm{L}}$) from the measured optical conductivity spectra. We also extracted the electron-boson spectral density (EBSD) functions using a two-parallel charge transport channel approach in the superconducting (SC) state. From the extracted EBSD functions, the maximum SC transition temperatures ($T_c^{\mathrm{Max}}$) were determined using a generalized McMillan formula and the SC coherence lengths ($ξ_{\mathrm{SC}}$) were calculated using the timescales encoded in the EBSD functions and reported Fermi velocities. We identified some similarities and differences in the doping-dependent SC quantities between the K-doped Ba-122 and the hole-doped cuprates. We expect that the various SC quantities obtained across the wide doping range will provide helpful information for establishing the microscopic pairing mechanism in Fe-pnictide superconductors.

cond-mat.supr-con↗

arXiv2023

Collective magnetic Higgs excitation in a pyrochlore ruthenate

The emergence of scalar Higgs-type amplitude modes in systems where symmetry is spontaneously broken has been a highly successful, paradigmatic description of phase transitions, with implications ranging from high-energy particle physics to low-energy condensed matter systems. Here, we uncover two successive high temperature phase transitions in the pyrochlore magnet Nd$_2$Ru$_2$O$_7$ at $T_{\mathrm{N}} = 147$ K and $T^* = 97$ K, that lead to giant phonon instabilities and culminate in the emergence of a highly coherent excitation. This coherent excitation, distinct from other phonons and from conventional magnetic modes, stabilizes at a low energy of 3 meV. We assign it to a collective Higgs-type amplitude mode, that involves bond energy modulations of the Ru$_4$ tetrahedra. Its striking two-fold symmetry, incompatible with the underlying crystal structure, highlights the possibility of multiple entangled broken symmetries.

cond-mat.str-el↗

arXiv2023

Thermal decoupling in high-$T_c$ cuprate superconductors

In unconventional high-$T_c$ cuprate superconductors, the intricate interplay between the non-ergodic bad metal and the strange metal state has remained enigmatic. Herein, we unravel this mystery using ab initio molecular dynamics simulations and the temperature-dependent effective potential method. Our investigation, centered on YBa$_2$Cu$_3$O$_7$ , provides the first simulation report on the $B_{1g}$ phonon anomaly, unveiling thermal decoupling induced by the bond weakening between the Ba atom and CuO$_2$ plane. This decoupling emerges as a pivotal underpinning behind several puzzling phenomena in high-$T_c$ superconductivity. Our results indicate that the effective temperature on the BaO plane deviates from that of the CuO$_2$ plane at low temperatures. Furthermore, we delineate the correlation between thermal decoupling and the Planckian dissipation, rigorously and quantitatively revealing a connection between linear-$T$ resistivity, Uemura relation, and superconducting domes, which are known to be the most important unsolved mysteries of high-$T_c$ superconductivity. The suppressed isotope effect ($\boldsymbol{\alpha}$ $\approx$ 0.02) in cuprates is also quantitatively explained from thermal decoupling. Our discoveries offer a revolutionary perspective on high-$T_c$ superconductivity, suggesting the potential for a transformative shift in our comprehension. Furthermore, they suggest that the autonomous emergence of low-temperature layers within materials has the potential to revolutionize industrial thermal management challenges.

cond-mat.supr-con↗

arXiv2023

Light-induced coherent interlayer transport in stripe-ordered ${\rm La}_{1.6-x}{\rm Nd}_{0.4}{\rm Sr}_{x}{\rm CuO}_{4}$

We have investigated the photoexcited transient responses of stripe-ordered phase in a cuprate superconductor, ${\rm La}_{1.6-x}{\rm Nd}_{0.4}{\rm Sr}_{x}{\rm CuO}_{4}~(x = 0.12)$ using optical-pump terahertz (THz)-probe spectroscopy. Upon the near-infrared photoexcitation with the electric field polarized along the $c$-axis, a clear plasma edge appears in the THz reflection spectrum along the $c$-axis with its position nearly coinciding with the Josephson plasma resonance of similarly doped ${\rm La}_{2-x}{\rm Sr}_{x}{\rm CuO}_{4}~(x = 0.125)$ in the low-temperature superconducting phase. The appearance of light-induced plasma edge sustains up to the onset temperature of the charge-stripe order, indicating the inherent interplay between the light-induced phase and the charge-stripe order. The optical conductivity spectrum of the light-induced state is mostly reproduced by the Drude model with a scattering rate as small as a few meV, and its imaginary part does not exhibit $1/ω$-divergence behavior in any temporal region after the photoexcitation. We discuss the possible origin of the observed coherent interlayer transport behavior as manifested by the narrow Drude response in the THz reflectivity along the $c$-axis.

cond-mat.supr-con↗

arXiv2023

Stripe Symmetry of Short-range Charge Density Waves in Cuprate Superconductors

The omnipresence of charge density waves (CDWs) across almost all cuprate families underpins a common organizing principle. However, a longstanding debate of whether its spatial symmetry is stripe or checkerboard remains unresolved. While CDWs in lanthanum- and yttrium-based cuprates possess a stripe symmetry, distinguishing these two scenarios has been challenging for the short-range CDW in bismuth-based cuprates. Here, we employed high-resolution resonant inelastic x-ray scattering to uncover the spatial symmetry of the CDW in Bi$_2$Sr$_{2-x}$La$_{x}$CuO$_{6+δ}$. Across a wide range of doping and temperature, anisotropic CDW peaks with elliptical shapes were found in reciprocal space. Based on Fourier transform analysis of real-space models, we interpret the results as evidence of unidirectional charge stripes, hosted by mutually 90$^\circ$-rotated anisotropic domains. Our work paves the way for a unified symmetry and microscopic description of CDW order in cuprates.

cond-mat.str-el↗

arXiv2022

Identification of a Critical Doping for Charge Order Phenomena in Bi-2212 Cuprates via RIXS

Identifying quantum critical points (QCPs) and their associated fluctuations may hold the key to unraveling the unusual electronic phenomena observed in cuprate superconductors. Recently, signatures of quantum fluctuations associated with charge order (CO) have been inferred from the anomalous enhancement of CO excitations that accompany the reduction of the CO order parameter in the superconducting state. To gain more insight about the interplay between CO and superconductivity, here we investigate the doping dependence of this phenomenon throughout the Bi-2212 cuprate phase diagram using resonant inelastic x-ray scattering (RIXS) at the Cu L3- edge. As doping increases, the CO wavevector decreases, saturating at a commensurate value of 0.25 r.l.u. beyond a characteristic doping pc, where the correlation length becomes shorter than the apparent periodicity (4a0). Such behavior is indicative of the fluctuating nature of the CO; and the proliferation of CO excitations in the superconducting state also appears strongest at pc, consistent with expected behavior at a CO QCP. Intriguingly, pc appears to be near optimal doping, where the superconducting transition temperature Tc is maximal.

cond-mat.supr-con↗

arXiv2022

Electron-hole symmetry in quasiparticle spectral weight of cuprates observed via infrared and photoemission spectroscopy

We performed an optical spectroscopy study on single crystals of Pr$_{0.85}$LaCe$_{0.15}$CuO$_{4-δ}$ (PLCCO) to revisit the electron-hole asymmetry, which has been understood as a fundamental property of cuprates. Four differently annealed samples - as-grown, reduced, optimally oxygenated, and over-oxygenated samples - were prepared, which have superconducting transition temperatures, $T_c$ = 0, 15, 24, and 18 K, respectively. We observed that low-energy quasiparticle spectral weights of all the PLCCO samples are significantly small in comparison with those of other electron-doped cuprate families. Instead, they are rather close to those of hole-doped counterpart La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). Accordingly, estimated effective carrier numbers per Cu atom ($N_{\mathrm{eff}}$/Cu) of superconducting samples are also considerably small, despite their relatively high critical temperatures. Complementary photoemission study reveals that the low-energy quasiparticle spectral weight of PLCCO is much smaller than that of Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-δ}$ (NCCO), consistent with the optical results. Our observations demonstrate that PLCCO provides the electron-hole symmetry in quasiparticle spectral weight, and highlight the importance of Cu3$d$-O2$p$ hybridization to understand the low-energy spectral weight transfer in doped cuprates.

cond-mat.str-el↗

arXiv2022

Kondo interaction in FeTe and its potential role in the magnetic order

Finding d-electron heavy fermion (HF) states has been an important topic as the diversity in d-electron materials can lead to many exotic Kondo effect-related phenomena or new states of matter such as correlation-driven topological Kondo insulator or cooperation between long-range magnetism and Kondo lattice behavior. Yet, obtaining direct spectroscopic evidence for a d-electron HF system has been elusive to date. Here, we report the observation of Kondo lattice behavior in an antiferromagnetic metal, FeTe, via angle-resolved photoemission spectroscopy (ARPES) and transport properties measurements. The Kondo lattice behavior is represented by the emergence of a sharp quasiparticle at low temperatures. The transport property measurements confirm the low-temperature Fermi liquid behavior and reveal successive coherent-incoherent crossover upon increasing temperature. We interpret the Kondo lattice behavior as a result of hybridization between localized Fe 3dxy and itinerant Te 5pz orbitals. Our interpretation is further evidenced by Fano-type tunneling spectra which accompany a hybridization gap. Our observations strongly suggest unusual cooperation between Kondo lattice behavior and long-range magnetic order.

cond-mat.str-el↗

arXiv2021

Deep learning-based statistical noise reduction for multidimensional spectral data

In spectroscopic experiments, data acquisition in multi-dimensional phase space may require long acquisition time, owing to the large phase space volume to be covered. In such case, the limited time available for data acquisition can be a serious constraint for experiments in which multidimensional spectral data are acquired. Here, taking angle-resolved photoemission spectroscopy (ARPES) as an example, we demonstrate a denoising method that utilizes deep learning as an intelligent way to overcome the constraint. With readily available ARPES data and random generation of training data set, we successfully trained the denoising neural network without overfitting. The denoising neural network can remove the noise in the data while preserving its intrinsic information. We show that the denoising neural network allows us to perform similar level of second-derivative and line shape analysis on data taken with two orders of magnitude less acquisition time. The importance of our method lies in its applicability to any multidimensional spectral data that are susceptible to statistical noise.

cs.LG↗

arXiv2021

$B_{\rm 1g}$ phonon anomaly driven by Fermi surface instability at intermediate temperature in YBa$_2$Cu$_3$O$_{7-δ}$

We performed temperature- and doping-dependent high-resolution Raman spectroscopy experiments on YBa$_2$Cu$_3$O$_{7-δ}$ to study $B$$_{\rm 1g}$ phonons. The temperature dependence of the real part of the phonon self-energy shows a distinct kink at $T=T_{\rm B1g}$ above $T$$_{\rm c}$ due to softening, in addition to the one due to the onset of the superconductivity. $T$$_{\rm B1g}$ is clearly different from the pseudogap temperature with a maximum in the underdoped region. The region between $T$$_{\rm B1g}$ and $T$$_{\rm c}$ resembles that of superconducting fluctuation or charge density wave order. While the true origin of the $B$$_{\rm 1g}$ phonon softening is not known, we can attribute it to a gap on the Fermi surface due to an electronic order. Our results may reveal the role of the $B$$_{\rm 1g}$ phonon not only in the superconducting state but also in the intertwined orders in multilayer copper oxide high-$T$$_{\rm c}$ superconductors.

cond-mat.supr-con↗