황정식 교수

Polarized resonant Raman and optical spectroscopy of aligned single-wall carbon nanotubes show that the optical transitions are strongly polarized along the nanotubes axis. This behavior is consistent with recent electronic structure calculations.

Abstract. We report on the ab-plane reflectance of the high temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212). Samples spanning the doping range from under doped with Tc = 67 K(UD), to optimally doped with Tc =96 K (OPT), to over doped with Tc =60 K (OD) were measured from room temperature down to the superconducting state. The measured reflectance data were analyzed to extract the optical conductivity and the real and imaginary parts of the free carrier optical selfenergy. We get an estimate

We report the optical properties of three dioxythiophene-based conjugated polymers: poly(3,4-ethylenedioxythiophene) poly(3,4-propylenedioxythiophene), and poly(3,4-(2,2-dimethylpropylenedioxy)thiophene). Films of ca. 200 nm thickness of these polymers were prepared on indium-tin-oxide coated glass substrates using a potentiostatic electropolymerization method. The reflectance and transmittance of the samples were measured over a broad energy range from the midinfrared through the ultraviolet. T

We demonstrate that an Eliashberg inversion of the optical self-energy, based on maximum-entropy considerations, can be used to extract in numerical form the bosonic excitation spectra of high-transition-temperature superconductors. In ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ we explicitly show that the bosonic mode that dominates the self-energy at low temperatures and small energies directly evolves out of a balanced transfer of spect

We report the synthesis and characterization of a two-dimensional (2D) MX₂Y₂-type (M = metal, X, Y = N, S, O, and X ≠ Y) copper 1,3,5-triamino-2,4,6-benzenetriol metal-organic framework (Cu₃(TABTO)₂-MOF). The role of oxygen in the synthesis of this MOF was investigated. Copper metal is formed along with the MOF when the synthesis is done in argon as suggested by XRD. When the reaction was exposed to air with vigorous stirring, copper metal was not obse

The $a$-axis optical properties of a detwinned single crystal of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.50}$ in the ortho-II phase (Ortho-II Y123, ${T}_{c}=59\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) were determined from reflectance data over a wide frequency range $(70--42\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$ for nine temperature values between 28 and $295\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Above $200\phantom{\rule{0.3

We report on optical properties of reduced ${\text{BaTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ at different doping levels including insulating and metallic samples. In all the samples, including metallic one, we observe structural phase transitions from the changes in the infrared-active phonon modes. Metallic ground state is confirmed by the Drude-type low-frequency optical reflectance. Similar to ${\text{SrTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ we find that the midinfrared-absorption ba

The real part of the in-plane optical self-energy data in underdoped Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} (Bi-2212) and ortho II YBa2Cu3O6.5 contains new and important information on the pseudogap. Using a theoretical model approach, a major new finding is that states lost below the pseudogap Delta_{pg} are accompanied by a pileup of states just above this energy. The pileup along with a sharp mode in the bosonic spectral function leads to an unusually rapid increase in the optical scattering rate as

Abstract Although large exciton binding energies of typically 0.6–1.0 eV are observed for monolayer transition metal dichalcogenides (TMDs) owing to strong Coulomb interaction, multilayered TMDs yield relatively low exciton binding energies owing to increased dielectric screening. Recently, the ideal carrier-multiplication threshold energy of twice the bandgap has been realized in multilayered semiconducting 2H-MoTe 2 with a conversion efficiency of 99%, which suggests strong Coulomb interaction

We present infrared $ab$-plane conductivity data for the layered cobaltate ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$ at three different doping levels ($x=0.25$, 0.50, and 0.75). The Drude weight increases monotonically with hole doping, $1\ensuremath{-}x$. At the lowest hole doping level $x=0.75$ the system resembles the normal state of underdoped cuprate superconductors with a scattering rate that varies linearly with frequency and temperature and there is an onset of scattering by a boson

Using optical spectroscopy with a derivative technique, we find for the high Tc cuprate Bi2Sr2CaCu2O8+delta (Bi-2212) evidence for a new high energy scale at 900 meV beyond the two previously well-known ones at roughly 50 and 400 meV. The intermediate scale at 400 meV has recently been seen in angle-resolved photoemission spectroscopy experiments along the nodal direction as a large kink. In YBa2Cu3O6.50, the three energy scales are shifted to lower energy relative to Bi-2212 and we observe the

We use optical spectroscopy to investigate the excitations responsible for the structure in the optical self-energy of thin epitaxial films of La(1.83)Sr(0.17)CuO(4). Using Eliashberg's formalism to invert the optical spectra we extract the electron-boson spectral function and find that at low temperature it has a two component structure closely matching the spin excitation spectrum recently measured by magnetic neutron scattering. We contrast the temperature evolution of the spectral density an

Abstract We investigated the temperature-dependent optical properties of methylammonium lead iodide (MAPbI 3 : CH 3 NH 3 PbI 3 ) and bromide (MAPbBr 3 : CH 3 NH 3 PbBr 3 ) single crystals using optical spectroscopy and photoluminescence (PL) techniques. We obtained the optical conductivity from the measured reflectance spectra of the single crystals at various temperatures using a Kramers–Kronig relation and measured their PL spectra at selected temperatures. We observed structural phase transit

We use 300 K reflectance data to investigate the normal-state electrodynamics of the high-temperature superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ over a wide range of doping levels. The data show that at this temperature the free carriers are coupled to a continuous spectrum of fluctuations. Assuming the marginal Fermi-liquid (MFL) form as a first approximation for the fluctuation spectrum, the doping-dependent coupling constant $\en