Mi-Ock Mun

Optical Properties of Solid Pentacene

We have prepared pentacene films on Si, Al2O3, and glass substrates by thermal evaporation and have investigated their optical properties at room temperature over a wide range of frequencies from infrared to ultraviolet (5 meV to 6.5 eV). A series of optical phonon modes and electronic transitions have been observed. The internal vibrational modes in the infrared region match well their molecular counterparts but the electronic transitions show substantial changes from those of a free pentacene molecule. The HOMO–LUMO gap energy of the pentacene films deposited on Al2O3 and glass substrates is 1.85 eV.

Synthesis and superconducting properties of HgBa2Ca2Cu3O8 +x

The influence of precursor composition, nominal mercury vapour pressure (PHg) and reaction temperature (t) on the formation of HgBa2Ca2Cu3O8 +x have been studied. By reducing the empty space inside the quartz tube, we could increase PHg higher than 200 atm and obtain samples containing only HgBa2Ca2Cu3O8 +x as the superconducting phase, with Tc(onset)= 134 K and Tc(zero)= 131 K.

A freeze-drying approach to the preparation of HgBa2Ca2Cu3O8+x superconductor

A freeze-dried mixture of metal nitrates was used to prepare the BaCaCuO precursor for the HgBa2Ca2Cu3O8+x (Hg-1223) superconducting phase. From this study we found that a highly homogeneous precursor is essential to increase the fraction of the Hg-1223 phase and to reduce the impurity phases. The quality of the samples prepared by using this method is highly improved in comparison with samples prepared by the conventional method of thermal decomposition of the nitrates.

Mixed-state magnetoresistance of c-axis-oriented MgB2 thin films

Abstract We investigated the mixed-state magnetoresistance of high quality c -axis-oriented MgB 2 thin films while varying the magnetic field and the current density. The temperature dependence of the magnetoresistance was well described by a vortex-glass theory for the low dissipative part of the resistivity and by a thermal fluctuation theory near the transition temperature. Interestingly, the observed thermally activated flux-flow region was very narrow or did not exist whereas, compared to conventional superconductors, the vortex-liquid phase was relatively broad in the H – T phase diagram. From the Werthamer–Helfand–Hohenberg theory, we obtained H c2 (0)=25.4 T, which was consistent with the value measured using a direct method.

Conductivity fluctuation in HBCCO superconductor

Abstract We have analyzed the measured excess conductivity of a HBCCO high- T c superconductor as a function of temperature, which is attributed to the fluctuation of superconducting order parameter above T c 0 . This excess conductivity is consistent with the two dimensional Aslamazov-Larkin mean-field theory based on the Ginzburg-Landau theory. Applying the Lawrence-Doniach theory, we found that the interlayer coupling stregth is very small, and that a 2D to 3D crossover is not found down to ( T - T c 0 )/ T c 0 = 0.015. This two-dimensional fluctuation seems to be intrinsic to this highly anisotropic layered superconductor.

Normal-State Optical Response Functions of MgB2 Superconductor

The reflectivity of superconducting MgB2 (Tc = 39 K) has been measured on a randomly oriented thin film at room temperature over a wide-range of frequencies, 20 ≤ ω < 100000 cm−1. The conductivity shows highly metallic behavior but cannot be explained with a simple Drude model alone. The electronic contribution is analyzed by a generalized Drude model. The scattering rate 1/τ(ω) and the mass renormalization ratio m*(ω)/m = 1 + λ (ω) exhibit clear frequency dependence. The electron–phonon coupling strength is estimated to be λ ∼ 1.5 ± 0.5 while the plasma frequency ωp is 2.4 eV.

Determination of the precise upper critical field of YNi2B2C superconductor

Abstract The paramagnetic susceptibility of YNi 2 B 2 C single crystal in a high magnetic field is found to be about 2×10 −5 cm 3 /mol, which is one order of magnitude smaller than the previous reports. The paramagnetic behavior is caused by the sample impurity, not by the intrinsic magnetism of the YNi 2 B 2 C superconductor and shows simple Curie–Weiss behavior. In this case, an upper critical field of the YNi 2 B 2 C can be easily obtained by extracting the paramagnetic contribution. The upper critical field shows an anomalous behavior, the steep rising at the low temperature region.

Thermoelectric power of a single-phase HgBa2Ca2Cu3Oy superconductor

An almost single-phase Hg-1223 superconductor has been prepared and its thermoelectric power (TEP) has been measured as a function of temperature. The sign of the TEP of this material is positive; the TEP shows a sharp increase just above the transition temperature and then decreases slowly; and the zero-crossing of the TEP coincides with the resistive transition temperature, Tc. Two models were applied to explain the anomalous TEP data. One model is based on the modified narrow-band Hubbard system with the additional Fermion contribution and the other is the phenomenological mixed-state model where the localized bound pairs and the delocalized carriers coexist in the normal state. Although both models can fit the semiconductor-like temperature dependence, the latter model takes into account the sharp increase just above the transition temperature.

Infrared and optical properties of single-crystal YNi2B2C superconductor

Abstract We have determined the optical conductivity and other key spectral functions of single-crystal YNi 2 B 2 C for the spectral range of 5 meV – 4.1 ev via infrared reflectivity measurements and spectroscopic ellipsometry. Polished crystals and polished-and-annealed crystals were compared to understand the effect of annealing on the optical response. The conductivity spectrum of YNi 2 B 2 C exhibits metallic electronic response with the screened plasma frequency of 3.76 eV. The free-electron response is not well described by the Drude model alone. A number of interband transitions also appear beyond 1 eV. Their energies and JDOS critical-point types are analyzed on the basis of selection rules and results of band-structure calculations.

THE MIXED STATE PROPERTIES OF SUPERCONDUCTING YNI2B2C

Abstract YNi 2 B 2 C intermetallic superconductor has the fluctuation magnetoconductance showing the 3D scaling behavior. In the mixed state, due to this fluctuation effect, the vortices form a solid phase at low temperature and make a liquid phase near T c similar to high- T c materials. Moreover, the vortex solid becomes a lattice at low fields and the vortex glass transition is found for high fields of H ⩾2 T while the liquid phase is narrow.