arXiv subjects

Yu-Seong Seo

Publications and source records attributed to Yu-Seong Seo.

At least 19 recordsLinked to original sources

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

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↗

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

Temperature-dependent $f$-electron evolution in CeCoIn$_5$ via a comparative infrared study with LaCoIn$_5$

We investigated CeCoIn$_5$ and LaCoIn$_5$ single crystals, which have the same HoCoGa$_5$-type tetragonal crystal structure, using infrared spectroscopy. However, while CeCoIn$_5$ has 4$f$ electrons, LaCoIn$_5$ does not. By comparing these two material systems, we extracted the temperature-dependent electronic evolution of the $f$ electrons of CeCoIn$_5$. We observed that the differences caused by the $f$ electrons are more obvious in low-energy optical spectra at low temperatures. We introduced a complex optical resistivity and obtained a magnetic optical resistivity from the difference in the optical resistivity spectra of the two material systems. From the temperature-dependent average magnetic resistivity, we found that the onset temperature of the Kondo effect is much higher than the known onset temperature of Kondo scattering ($\simeq$ 200 K) of CeCoIn$_5$. Based on momentum-dependent hybridization, the periodic Anderson model, and a maximum entropy approach, we obtained the hybridization gap distribution function of CeCoIn$_5$ and found that the resulting gap distribution function of CeCoIn$_5$ was mainly composed of two (small and large) components (or gaps). We assigned the small and large gaps to the in-plane and out-of-plane hybridization gaps, respectively. We expect that our results will provide useful information for understanding the temperature-dependent electronic evolution of $f$-electron systems near Fermi level.

cond-mat.str-el↗

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

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↗

arXiv2021

Defect engineering of magnetic ground state in EuTiO$_3$ epitaxial thin films

Atomistic defect engineering through the pulsed laser epitaxy of perovskite transition metal oxides offers facile control of their emergent opto-electromagnetic and energy properties. Among the various perovskite oxides, EuTiO3 exhibits a strong coupling between the lattice, electronic, and magnetic degrees of freedom. This coupling is highly susceptible to atomistic defects. In this study, we investigated the magnetic phase of EuTiO$_3$ epitaxial thin films via systematic defect engineering. A magnetic phase transition from an antiferromagnet to a ferromagnet was observed when the unit cell volume of EuTiO3 expanded due to the introduction of Eu-O vacancies. Optical spectroscopy and density functional theory calculations show that the change in the electronic structure as the ferromagnetic phase emerges can be attributed to the weakened Eu-Ti-Eu super-exchange interaction and the developed ferromagnetic Eu-O-Eu interaction. Facile defect engineering in EuTiO$_3$ thin films facilitates understanding and tailoring of their magnetic ground state.

cond-mat.mtrl-sci↗

arXiv2021

Color of Copper/Copper oxide

Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. We demonstrate coherent propagation of an oxidation front in single-crystal Cu thin film to achieve a full-color spectrum for Cu by precisely controlling its oxide-layer thickness. Grain boundary-free and atomically flat films prepared by atomic-sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temperature gradient. Color tuning of nearly full-color RGB indices is realized by precise control of oxide-layer thickness; our samples covered ~50.4% of the sRGB color space. The color of copper/copper oxide is realized by the reconstruction of the quantitative yield color from oxide pigment (complex dielectric functions of Cu2O) and light-layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. We further demonstrate laser-oxide lithography with micron-scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic-order-driven metal-insulator transitions in the quasi-one-dimensional spin-ladder compounds BaFe$_2$S$_3$ and BaFe$_2$Se$_3$

The quasi-one-dimensional spin ladder compounds, BaFe$_2$S$_3$ and BaFe$_2$Se$_3$, are investigated by infrared spectroscopy and density functional theory (DFT) calculations. We observe strong anisotropic electronic properties and an optical gap in the leg direction that is gradually filled above the antiferromagnetic (afm) ordering temperature, turning the systems into a metallic phase. Combining the optical data with the DFT calculations we associate the optical gap feature with the $p$-$d$ transition that appears only in the afm ordered state. Hence, the insulating ground state along the leg direction is attributed to Slater physics rather than Mott-type correlations.

cond-mat.str-el↗

arXiv2018

Oxygen vacancy-induced structural evolution of SrFeO$_{3-x}$ epitaxial thin film from brownmillerite to perovskite

We investigated SrFeO$_{3-x}$ thin films on a SrTiO$_3$ (001) substrate prepared via pulsed laser epitaxy using an optical spectroscopy technique. The oxygen vacancy level ($x$) was controlled by post-annealing processes at different oxygen partial pressures. We achieved a brownmillerite(BM) structure at $x =$ 0.5 and observed the evolution of the crystal structure from BM into perovskite(PV) as the oxygen concentration increased. We observed the evolution of infrared-active phonons with respect to the oxygen concentration, which was closely related to the structural evolution observed via X-ray diffraction. We identified the phonons using the shell-model calculation. Furthermore, we studied temperature-dependent behaviors of the phonon modes of three representative samples: PV, and two BMs (BM$_{\mathrm{oop}}$ and BM$_{\mathrm{ip}}$) with different orientations of the oxygen vacancy channel. In the BM$_{\mathrm{oop}}$ sample, we observed a phonon mode, which exhibited an unusual red-shift with decreasing temperature; this behavior may have been due to the apical oxygen instability in the FeO$_6$ octahedron. Our results provide important information regarding the ionic conduction mechanism in SrFeO$_{3-x}$ material systems.

cond-mat.mtrl-sci↗

arXiv2017

Temperature-dependent evolutions of excitonic superfluid plasma frequency in a srong excitonic insulator candidate, Ta$_2$NiSe$_5$

We investigate an interesting anisotropic van der Waals material, Ta$_{2}$NiSe$_{5}$, using optical spectroscopy. Ta$_{2}$NiSe$_{5}$ has been known as one of the few excitonic insulators proposed over 50 years ago. Ta$_{2}$NiSe$_{5}$ has quasi-one dimensional chains along the $a$-axis. We have obtained anisotropic optical properties of a single crystal Ta$_{2}$NiSe$_{5}$ along the $a$- and $c$-axes. The measured $a$- and $c$-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the $a$-axis optical conductivity, a sharp peak near 3050 cm$^{-1}$ at 9 K, with a well-defined optical gap ($Δ^{EI} \simeq$ 1800 cm$^{-1}$) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around $\sim$325 K($T_c^{EI}$). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap ($Δ^{EI}(T)/Δ^{EI}(0)$) is $1-(T/T_c^{EI})^2$. The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta$_{2}$NiSe$_{5}$ has been determined from measured optical data. Our findings may be useful for future research on excitonic insulators.

cond-mat.mtrl-sci↗

arXiv2017

Revisiting optical properties of MgB$_{2}$ with a high-quality sample prepared by a HPCVD method

We investigated a high-quality MgB$_{2}$ thin film with a thickness of $\sim$1000 nm on an Al$_{2}$O$_{3}$ substrate using optical spectroscopy. We measured the reflectance spectra of the film at various temperatures both below, and above, the superconducting transition temperature, $T_c$ $\simeq$ 40 K. An earlier study showed that when the sample surface is exposed to air the optical properties of the surface change immediately, however, the saturated change is negligibly small in the far-infrared region. The optical conductivity spectrum in the normal state shows two (narrow and broad) Drude modes, with the narrow Drude mode being dominant in the low frequency region below 1000 cm$^{-1}$. Our study, which uses a good-quality sample, provides more reliable data on the optical properties of MgB$_2$, in a similar spectral range. The optical data is analyzed further using an extended Drude model, and the electron-phonon spectral density function, $α^2F(ω)$, is extracted. The spectral density function $α^2F(ω)$ features two peaks: a small one near 114 cm$^{-1}$, and a strong peak around the 550 cm$^{-1}$ where the B-B bond stretching phonon exists. Our data in the superconducting state does not show the expected energy shift of the onset of scattering associated with the $α^2F(ω)$ peaks.

cond-mat.supr-con↗

arXiv2013

Ab-initio studies on the phonons of BaTiO3 polytypes: pressure dependences with a hybrid functional

We report the first principles investigations on the phonons of three polytypes of BaTiO3 (BTO): paraelectric (PE) cubic Pm-3m and two ferroelectric (FE) phases, tetragonal P4mm and rhombohedral R3m. The phonon frequencies calculated using various exchange-correlation functionals, including density functional theory, Hartree-Fock approximation, and their hybrids were reviewed. The pressure-induced interplays between the modes form individual phases were explored by calculating the phonon modes as a function of pressure, P from -15 to 230 GPa. The pressure-sensitive modes of the FE phases showed softening and converged to the modes of the PE phase at pressures below ~ 10 GPa. These results on the FE phases can be interpreted as phonon-precursors for a change in symmetry from low- to high-symmetry and partly as a theoretical explanation for the pressure-induced mode-coupling behaviors reported by Sood et al. [Phys. Rev. B 51, 8892 (1995)]. As pressure is applied further beyond ~ 50 GPa to the cubic PE phase, the lowest F1u mode softens again and diverges into two separate modes of tetragonal FE P4mm at above ~ 150 GPa. These phonon-branching behaviors at high pressures provide a clear re-confirmation of the re-entrant ferroelectricity predicted in [Phys. Rev. Lett. 95, 196804 (2005); Phys. Rev. B 74, 180101 (2006); ibid. 85, 054108 (2012)]. The high-pressure-re-entrant FE polarization was not found in the rhombohedral structure. Instead, the centosymmetric R-3m phase was favored at above ~ 30 GPa. The phonon modes calculated for the phonon-propagation vectors in the high-symmetry directions show that the Pm-3m phase exhibits polar instability at the Γpoint and non-polar instability at the X, M, and R points under high pressure.

cond-mat.mtrl-sci↗

arXiv2013

Determination of the Local Symmetry and the Multiferroic-ferromagnetic Crossover in Ni3-xCoxV2O8 by using Raman Scattering Spectroscopy

Comprehensive vibrational studies on the multiferroic-to-normal-ferromagnetic crossover in isostructural Ni3-xCoxV2O8 (NCVO, x = 0 - 3.0) are performed using Raman scattering spectroscopy. The systematically red-shifted phonon modes are discussed in terms of the mode Grüneisen parameters, and are interpreted as a chemical pressure effect. In addition, we present evidence that the local symmetry is broken in the multiferroic phase.

cond-mat.mtrl-sci↗

arXiv2013

First-principles Investigations on Polytypes of BaTiO3: Hybrid Calculations and Pressure Dependences

We report our first-principles investigations on three polytypes of BaTiO3 (BTO): a paraelectric phase with cubic Pm-3m structure and two ferroelectric (FE) phases with tetragonal P4mm and rhombohedral R3m structures. We compared the structural and the electrical properties of BTO obtained by using various approaches: e.g., the Hartree-Fock (HF) theory, the density functional theory (DFT) with the local density approximation (LDA) or with the two generalized gradient approximations (two GGAs: PWGGA and PBE), and three hybrid functionals of the HF and the DFT (B3LYP, B3PW, and PBE0). For the P4mm structure, the two GGAs and the hybrid functionals reproduced the cell volumes, but slightly overestimated the c/a ratio. The hybrid functionals provided accurate predictions for the experimental energy gaps, but slightly underestimated the experimental dielectric constants. The calculated dielectric constants were inversely proportional to the c/a ratios for the P4mm structure (or the cH/aH ratio for the R3m structure), irrespective of the functional choice. Also, the over-estimated polarization could be ascribed to a super-tetragonality in the GGA/hybrid functionals. The pressure dependences for the cell parameters, fractional atomic displacements, energy gaps, dielectric constants, and FE polarizations were calculated by using the B3PW hybrid functional. As pressure was increased, the polarization decreased monotonically until it reached zero at a critical pressure of ~ 20 GPa for both the P4mm and the R3m structures. Anomalous behaviors were also observed in the atomic movements and the polarizations for the P4mm structure: (Omitted due to length..) Such behaviors of the polarizations, together with super-tetragonality/super-trigonality, are discussed.

cond-mat.mtrl-sci↗