arXiv subjects

Jungseek Hwang

Publications and source records attributed to Jungseek Hwang.

At least 19 recordsLinked to original sources

arXiv2025

Universal Fabrication of Graphene/Perovskite Oxide Hybrid Heterostructures

Hybrid heterostructures composed of graphene and perovskite oxides provide a promising platform for exploiting synergetic interfacial functionalities. Conventional fabrication methods of the hybrid heterostructures rely on transferring graphene grown on metallic substrates-- a process that is time-consuming, labor-intensive, and prone to introducing numerous defects. In this study, we present a universal, catalyst-free method for the direct growth of graphene on insulating substrates by employing three different perovskite oxide substrates (SrTiO$_3$, LaAlO$_3$, and (La$_{0.18}$Sr$_{0.82}$)(Al$_{0.59}$Ta$_{0.41}$)O$_3$) using atmospheric chemical vapor deposition. Comprehensive characterization via Raman spectroscopy, X-ray spectroscopy, scanning probe microscopy, and electron microscopy confirmed the formation of a uniform, continuous monolayer graphene on all substrates. We identified that growth temperature critically governs graphene quality, as excessive active species may lead to secondary nucleation and the formation of multilayer graphene. Notably, all substrates shared the same optimal growth conditions. Low-temperature Raman spectroscopy and scanning tunneling microscopy of the graphene/SrTiO$_3$ hybrid heterostructure revealed cooperative phenomena, including substrate-induced lattice-phonon and electron-phonon coupling. Our work establishes a reproducible, transfer-free fabrication route for graphene/perovskite oxide hybrid heterostructures and provides empirical support for the universal growth of graphene on insulating substrates.

cond-mat.mtrl-sci↗

arXiv2025

Stray light in 3D porous nanostructures of single crystalline copper film

In the design of optical devices and components, geometric structures and optical properties of materials, such as absorption, refraction, reflection, diffraction, scattering, and trapping, have been utilized. Finding the ideal material with certain optical and geometric characteristics is essential for a customized application. Here, we fabricated unoxidizable achromatic copper films (ACFs) on Al2O3 substrates utilizing an atomic sputtering epitaxy apparatus. ACFs are made up of two regions vertically: a comparatively flat layer region and a three-dimensional (3D) porous nanostructured region on top of the flat region. The measured specular reflectance displayed low-pass filter behaviour with a sharp cutoff frequency in the infrared spectrum. Furthermore, the measured diffusive reflectance spectra showed light-trapping behaviour in the spectral region above the cutoff frequency, where there are no known absorption mechanisms, such as phonons and interband transitions. A focused ion beam scanning electron microscope was utilized to study the thin film's nanostructured region through 3D tomographic analysis in order to comprehend the phenomena that were observed. This work will shed fresh light on the design and optimization of optical filters and light-trapping employing porous nanostructured metallic thin films.

physics.optics↗

arXiv2025

Effect of C additives with 0.5% in weight on structural, optical and superconducting properties of Ta-Nb-Hf-Zr-Ti high entropy alloy films

We investigated the superconducting (SC) properties of Ta-Nb-Hf-Zr-Ti high-entropy alloy (HEA) thin films with 0.5% weight C additives. The C additives stabilize the structural properties and enhance the SC critical properties, including $μ_0$Hc$_2$ (13.45 T) and Tc (7.5 K). The reflectance of the C-added HEA film is enhanced in the low-energy region, resulting in a higher optical conductivity, which is consistent with the lower electrical resistivity. In addition, we observed SC vortices in the C-added HEA film using magnetic force microscopy. The magnetic penetration depths ($λ$) of the pure HEA and C-added HEA films were estimated from their Meissner force curves by comparing them with those of a reference Nb film. At 4.2 K, the λ of the C-added film is 360 nm, shorter than that of the pure HEA film (560 nm), indicating stronger superconductivity against an applied magnetic field.

cond-mat.supr-con↗

arXiv2025

Quantum criticality in cuprate superconductors revealed by optical conductivity measurement

The ubiquitous temperature ($T$)-linear behaviour of the transport scattering rate in the normal state of strongly correlated electron systems is called strange metallicity \cite{zaanen:2004,phillips:2022,hartnoll:2022,chowdhury:2022,yuan:2022}. Although strange metallicity is crucial to understanding superconductivity in correlated electron systems, its origin remains elusive to date \cite{hussey:2023}. Here, we present the doping-, temperature-, and frequency ($ω$)-dependent transport properties of overdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ in a wide doping range of 0.183 to 0.231. We observe that the optical scattering rate and effective mass exhibit an $ω/T$ scaling behaviour at a critical doping of $p_{c} \simeq$ 0.231. Away from the critical doping, the $ω/T$ scaling behaviour is destroyed below a doping-dependent crossover temperature $T_Δ(p) \sim |p-p_{c}|^{0.24}$. Furthermore, the optical coherence mode (OCM) observed within the superconducting dome rapidly broadens and eventually disappears as the critical doping is approached. The emergence of the $ω/T$ scaling behaviour of the transport scattering rate and broadening of the OCM near the critical doping strongly suggests that strange metallic behaviour is caused by quantum critical fluctuations. Our results provide compelling spectroscopic evidence for quantum criticality in cuprate superconductors.

cond-mat.supr-con↗

arXiv2025

Hybridization gap and $f$-electron effect evolutions with Cd- and Sn-doping in CeCoIn$_5$ via infrared spectroscopy

We investigated hole (Cd)- and electron (Sn)-doped CeCoIn$_5$ (CeCo(In$_{1-x}T_x$)$_5$ ($T$ = Cd or Sn)) using infrared spectroscopy. Doping-dependent hybridization gap distribution functions were obtained from the optical conductivity spectra based on the periodic Anderson model formalism. The hybridization gap distribution exhibits two components: in-plane and out-of-plane hybridization gaps. The doping-dependent evolution of the two gaps indicated that the out-of-plane gap was more sensitive to doping. Furthermore, the magnetic optical resistivity exhibited a doping-dependent evolution of the $f$-electron amplitude. The two dopant types exhibited different physical properties depending on the level of doping. The Sn dopant increases the $f$-electron amplitude, whereas the Cd dopant does not affect the $f$-electron amplitude. Doping-dependent effective mass is peaked at pure (or undoped) CeCoIn$_5$. Our spectroscopic results may help understand the doping-dependent electronic evolution of one of the canonical heavy fermion systems, CeCoIn$_5$.

cond-mat.str-el↗

arXiv2025

Roles of Fe-ion irradiation on MgB$_2$ thin films: Structural, superconducting, and optical properties

The effects of Fe-ion irradiation on the crystal structure and superconducting properties of MgB$_2$ thin films were investigated. Pristine samples were prepared using hybrid physical-chemical vapor deposition (HPCVD), and ion irradiation was performed at three different doses of 5 x 10$^{13}$, 1 x 10$^{14}$, and 2 x 10$^{14}$ ions/cm$^2$. The measured temperature-dependent resistivity showed that as the irradiation dose increased from pristine to most irradiated, the superconducting critical temperature, $T_c$, significantly decreased from 38.33 to 3.02 K. The crystal structures of the films were investigated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements. The results showed that the higher the dose, the greater the change in crystal structure, such as the lattice constant and bond length. This suggests that the destruction of the crystal structure at higher doses leads to the degradation of superconductivity in the irradiated MgB$_2$ thin films. Raman spectroscopy showed that the electron-phonon coupling constant decreased with increasing irradiation dose, which was directly related to the reduction of $T_c$ in the samples. The optical conductivity indicates that the charge-carrier density of the $σ$-band plays an important role in the superconductivity of ion-irradiated MgB$_2$.

cond-mat.supr-con↗

arXiv2025

Optical properties of RCd3P3 (R: Ce or La) compounds: Insulator-metal transition induced by displacement of atoms in the unit cell

We examined the electronic structures and optical properties of single crystals of RCd3P3 (R = Ce or La). Our first-principles analysis indicates that CeCd3P3 and LaCd3P3 exhibit semiconductor characteristics with narrow energy gaps of approximately 0.51 and 0.70 eV, respectively. Notably, a slight displacement of the Cd and P atoms within the unit cell significantly transforms the electronic structure from insulating to metallic state. Optical spectroscopy of both compounds reveals a metallic state with a low charge carrier density, suggesting a finite density of states at the Fermi level. A comparison between the theoretical electronic structures and experimental optical properties elucidates the observed metallic behavior. Additionally, the notable modification of the infrared-active phonons strongly indicates a structural phase transition in these compounds. Our findings also suggest that CeCd3P3 serves as a suitable platform for investigating the photoinduced Kondo effect due to its metallic ground state with limited charge carriers.

cond-mat.mtrl-sci↗

arXiv2025

Optical properties of popular dielectric substrate materials in a wide spectral range from far-infrared to ultraviolet

We investigated the optical properties of 13 different dielectric materials (slide glass, quartz, Al2O3 (c-cut), DyScO3 (110), KTaO3 (001), LaAlO3 (001), (LaAlO3)0.3-(Sr2AlTaO6)0.7 (001) (LSAT), MgF2 (100), MgO (100), SiC, SrTiO3 (001), TbScO3 (110), and TiO2). The single-bounce reflectance spectra of the bulk samples were measured using Fourier transform infrared (FTIR) and monochromatic spectrometers across a wide spectral range, from far infrared to ultraviolet (80-50,000 cm-1). Using the Kramers-Kronig analysis, we obtained the optical conductivity and dielectric function of the dielectric materials from their measured reflectance spectra. Moreover, we measured the transmittance spectra of the materials to obtain their bandgaps. We fitted the measured reflectance spectra using the Lorentz model to obtain phononic structures. Each dielectric material exhibits unique phononic structures and optical bandgaps, associated with the composition and crystal structure of the material. The observed optical properties of these dielectric materials provide valuable information for the optical analysis of thin films grown on them.

cond-mat.mtrl-sci↗

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

Planckian behavior of cuprates at the pseudogap critical point simulated via flat electron-boson spectral density

Planckian behavior has been recently observed in La1.76Sr0.24CuO4 at the pseudogap critical point. The Planckian behavior takes place in an intriguing quantum metallic state at a quantum critical point. Here, the Planckian behavior was simulated with an energy-independent (or flat) and weakly temperature-dependent electron-boson spectral density (EBSD) function by using a generalized Allen's (Shulga's) formula. We obtained various optical quantities from the flat EBSD function, such as the optical scattering rate, the optical effective mass, and the optical conductivity. These quantities are well fitted with the recently observed Planckian behavior. Fermi-liquid behavior was also simulated with an energy-linear and temperature-independent EBSD function. The EBSD functions agree well with the overall doping- and temperature-dependent trends of the EBSD function obtained from the optically measured spectra of cuprate systems, which can be crucial for understanding the microscopic electron-pairing mechanism for high-Tc superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2024

An inversion problem for optical spectrum data via physics-guided machine learning

We propose the regularized recurrent inference machine (rRIM), a novel machine-learning approach to solve the challenging problem of deriving the pairing glue function from measured optical spectra. The rRIM incorporates physical principles into both training and inference and affords noise robustness, flexibility with out-of-distribution data, and reduced data requirements. It effectively obtains reliable pairing glue functions from experimental optical spectra and yields promising solutions for similar inverse problems of the Fredholm integral equation of the first kind.

physics.data-an↗

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

Fermi liquid-like behaviour of cuprates in the pseudogap phase simulated via T-dependent electron-boson spectral density

We investigated the temperature- and frequency-dependent optical scattering rates in the pseudogap phase of cuprates using model pseudogap and electron-boson spectral density (EBSD) functions. We obtained the scattering rates at various temperatures below and above a given pseudogap temperature using a generalized Allen's (or Sharapov's) formula, which has been used to analyse the measured optical spectra of correlated electron systems with a non-constant density of states at finite temperatures. The pseudogap and EBSD functions should be temperature dependent to simulate the Fermi liquid-like behaviour of underdoped cuprate systems observed in optical studies. Therefore, the observed Fermi liquid-like behaviour can be understood by considering the combined contribution from the T-dependent EBSD function and the T-dependent pseudogap. We also obtained the optical conductivity spectra from the optical scattering rates and analyzed them to investigate intriguing electronic properties. We expect that our results will aid in understanding the Fermi liquid-like optical response in the pseudogap phase and in revealing the microscopic pairing mechanism for superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2023

Bosonic spectrum of a correlated multiband system, BaFe1.80Co0.20As2, obtained via infrared spectroscopy

We investigated a single crystal BaFe(2-x)CoxAs2 (Co-doped BaFe2As2: Co-doped Ba122) with x = 0.20 using infrared spectroscopy. We obtained the bosonic spectrum from the measured spectrum using an extended Drude-Lorentz model for the normal state and a two-parallel-channel approach for the superconducting (SC) state, based on the generalized Allen formula. The coupling constant, maximum SC transition temperature, SC coherence length, and upper critical field were extracted from the bosonic spectrum. The superfluid plasma frequency and the London penetration depth were obtained from the optical conductivity. We compared the physical quantities of Co-doped Ba122 and K-doped Ba122 and found some interesting differences. Our results may be helpful for understanding superconductivity in doped Ba122 systems and may provide useful information on doped Ba122 systems for their applications.

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↗

arXiv2021

Electron-boson spectral density functions of cuprates obtained from optical spectra via machine learning

The electron-boson spectral density (or glue) function can be obtained from measured optical scattering rate by solving a generalized Allen formula, which relates the two quantities with an integral equation and is an inversion problem. Thus far, numerical approaches, such as the maximum entropy method (MEM) and the least squares fitting method, have been applied for solving the generalized Allen formula. Here, we developed a new method to obtain the glue functions from the optical scattering rate using a machine learning approach (MLA). We found that the MLA is more robust against random noise compared with the MEM. We applied the new developed MLA to experimentally measured optical scattering rates and obtained reliable glue functions in terms of their shapes including the amplitudes. We expect that the MLA can be a useful and rapid method for solving other inversion problems, which may contain random noise.

cond-mat.str-el↗

arXiv2021

Evolution of electronic structure of Ru-doped single-crystal iridiates, Sr$_2$Ir$_{1-x}$Ru$_x$O$_4$

We investigated Ru-doped single-crystal 5$d$ iridiates, Sr$_2$Ir$_{1-x}$Ru$_x$O$_{4}$, at three different doping concentrations ($x =$ 0.01, 0.07 and 0.10) using optical spectroscopy. The undoped pristine compound (Sr$_2$IrO$_{4}$) is known as a novel $J_{eff}$ = 1/2 Mott insulator. Remarkably, the optical conductivity spectra of all three samples exhibited the insulating behavior, although we observed weak Drude components in the optical conductivity spectra down to the lowest temperature of 30 K. The charge-carrier densities of the Ru-doped iridiates estimated from the Drude components are significantly smaller than the expected values estimated from the nominal Ru-doping concentrations. Herein, we provide temperature- and doping-dependent electronic structure evolution of Ru-doped iridiates. We expect that our results will be useful for understanding the intriguing Ru-doping-dependent properties of 5$d$ iridiate Sr$_2$IrO$_{4}$.

cond-mat.str-el↗