황준석 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

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

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

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

Conjugated polymers can be reversibly doped by electrochemical means. This doping introduces new subband-gap optical absorption bands in the polymer while decreasing the band-gap absorption. To study this behavior, we have prepared an electrochemical cell allowing in situ measurements of the optical properties of the polymer. The cell consists of a thin polymer film deposited on gold-coated Mylar behind which is another polymer that serves as a counterelectrode. An infrared transparent window pr

We investigate the optical properties of bromine-intercalated highly orientated pyrolytic graphite (Br-HOPG) and provide an interpretation of the data. We observe absorption features below 620 meV which are absent in the absorption spectrum of graphite. Comparing our results with those of theoretical studies on graphite, single- and bilayer graphene, as well as recent optical studies of multilayer graphene, we conclude that Br-HOPG contains the signatures of ultrapure bilayer graphene, ultrapure