김기훈 교수

Infrared reflectivity spectra were measured for a polycrystalline ${\mathrm{La}}_{0.7}$${\mathrm{Ca}}_{0.3}$Mn${\mathrm{O}}_{3}$ sample. The internal phonon modes, i.e., the bending and stretching modes, of the Mn${\mathrm{O}}_{6}$ octahedra show significant frequency shifts near the Curie temperature ${T}_{C}$. The frequency shifts of the internal phonon modes were found to have a linear relationship with the excess lattice expansion above ${T}_{C}$, which deviates from a prediction of the Gr\"

We measured thermal conductivity $\ensuremath{\kappa}$, thermoelectric power $S$, and electric conductivity $\ensuremath{\sigma}$ of ${\mathrm{La}}_{5/8\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ca}}_{3/8}{\mathrm{MnO}}_{3}$, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of $\ensuremath{\kappa}$, $S$, and $\ensuremath{\sigma}$ with temperature $(T)$ and $x$ agrees well with the effective medium theories for binary metal-insula

Temperature dependent optical conductivity spectra of a ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ (LCMO) sample were measured. In the metallic regime at very low temperatures, they clearly showed two types of absorption features, i.e., a sharp Drude peak and a broad midinfrared absorption band, which could be explained as coherent and incoherent bands of a large lattice polaron. Electrodynamic analyses suggest that the elementary excitation in LCMO should be in a strong coupling

Transparent conducting oxides (TCOs) and transparent oxide semiconductors (TOSs) have become necessary materials for a variety of applications in the information and energy technologies, ranging from transparent electrodes to active electronics components. Perovskite barium stannate (BaSnO 3 ), a new TCO or TOS system, is a potential platform for realizing optoelectronic devices and observing novel electronic quantum states due to its high electron mobility, excellent thermal stability, high tra

A comprehensive transport study, as a function of temperature and continuous magnetic fields of up to 45 T, reveals that URu2Si2 possesses all the essential hallmarks of quantum criticality at fields around 37+/-1 T. The formation of multiple phases at low temperatures at and around the quantum critical point suggests the existence of competing order parameters.

2H-TaSe2 has been one of unique transition metal dichalcogenides exhibiting several phase transitions due to a delicate balance among competing electronic ground states. An unusual metallic state at high-T is sequentially followed by an incommensurate charge density wave (ICDW) state at ≈122 K and a commensurate charge density wave (CCDW) state at ≈90 K, and superconductivity at TC ~ 0.14 K. Upon systematic intercalation of Pd ions into TaSe2, we find that CCDW order is destabilized more rapidly

Abstract CuCrP 2 S 6 , a van der Waals magnet having stacked layers of 2D honeycomb lattice made of CuS 3 triangles and CrS 6 octahedra, exhibits an A‐type antiferromagnetic order with the Néel temperature ( T N ) = 32 K. Upon in‐plane magnetic field ( H ) being applied below T N , H ‐induced modulation of the c *‐axis electric polarization (Δ P c* ) is found at fields lower than the saturation field µ 0 H S = 6.1 T, at which a forced ferromagnetic alignment sets in. Based on the symmetry analys

Optical spectroscopy was used to investigate the optical gap ( 2 Delta) due to charge ordering (CO) and related pseudogap developments with x and temperature ( T) in La(1-x)Ca(x)MnO(3) ( 0.48< or =x< or =0.67). Surprisingly, we found 2 Delta/k(B)T(CO) is as large as 30 for x approximately 0.5, and decreases rapidly with increasing x. Simultaneously, the optical pseudogap, possibly starting from T* far above T(CO) becomes drastically enhanced near x = 0.5, producing non-BCS T-dependence of 2 Delt

We performed temperature dependent x-ray linear dichroism (XLD) experiments on an iron pnictide system, Ba(Fe(1-x)Co(x))2As2 with x=0.00, 0.05, 0.08, and 0.10 to experimentally verify the existence of orbital ordering (OO). Substantial XLD was observed in polarization dependent x-ray absorption spectra of Fe L edges. By exploiting the difference in the temperature dependent behaviors, OO, and structure contributions to XLD could be clearly separated. The observed OO signal indicates different oc

Bulk magnetite (Fe 3 O 4 ), the loadstone used in magnetic compasses, has been known to exhibit magnetoelectric (ME) properties below ∼10 K; however, corresponding ME effects in Fe 3 O 4 nanoparticles have been enigmatic. We investigate quantitatively the ME coupling of spherical Fe 3 O 4 nanoparticles with uniform diameters ( d ) from 3 to 15 nm embedded in an insulating host, using a sensitive ME susceptometer. The intrinsic ME susceptibility (MES) of the Fe 3 O 4 nanoparticles is measured, ex

We report the observation of multiferroicity in a clinopyroxene NaFeGe(2)O(6) polycrystal from the investigation of its electrical and magnetic properties. Following the previously known first magnetic transition at T(N1) = 13 K, a second magnetic transition appears at T(N2) = 11.8 K in the temperature dependence of the magnetization. A ferroelectric polarization starts to develop clearly at T(N2) rather than T(N1) and its magnitude increases up to ~13 μC m(-2) at 5 K, supporting the idea that t

Infrared (IR) reflectivity spectra were measured in temperatures between 160 K and 290 K for polycrystalline ${\mathrm{La}}_{0.7}$${\mathrm{Ca}}_{0.3}$${\mathrm{MnO}}_{3}$ (LCMO) samples. Though all the samples showed nearly the same reflectivities, their dc resistivities were varied by more than one order of magnitude. Their resistivities predicted by the Hagen-Rubens relation from the IR spectra were much lower than the values from dc measurements, but they were very close to those of epitaxia

We report a discovery of superconductivity with Tc = 5.5 K in Nb2PdxSe5, in which one-dimensional (1D) Nb-Se chains existing along the b-direction hybridize each other to form the conducting b-c* plane. The magnetic susceptibility and specific heat data in both single- and poly-crystals show evidence of bulk superconductivity. The resistivity, Hall coefficient, and magneto-resistance data all indicate the presence of an energy scale T* = ~ 50 K, which becomes systematically lowered under hydrost