We report the measurement of magnetoresistance in a c-axis-oriented MgB2 thin film while varying the magnetic field and temperature. The scaling functions for the critical fluctuations were employed to analyze the excess conductance of these thin films. The fluctuating magnetoresistance, R(T,H), followed the 3D scaling suggested by Ullah and Dorsey.

We investigated the mixed-state magnetoresistance of high quality c-axis-oriented MgB2 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 Hc2(0)=25.4 T, which was consistent with the value measured using a direct method.

We report critical fluctuations of magnetoconductance in a MgB2 superconductor prepared under a high pressure of 3 GPa. The temperature dependence of the excess magnetoconductance, Deltasigma(T, H), follows the three-dimensional scaling function for the critical fluctuations proposed by Ullah and Dorsey. This feature is inferred to originate from the strongly linked nature of the intergrains in the MgB2 compound.

We have studied the optical properties of single crystals of RNi2B2C (R = Y, Lu, Er, Ho) intermetallic superconductors over a wide range of energies from 50 cm(-1) to 100000 cm(-1) (6 meV to 12.4 eV) at room temperature by performing normal-incidence reflectivity measurements. The optical spectral functions of the four compounds show similar properties irrespective of the type of rare-earth ion or its magnetic moment. The optical conductivity of YNi2B2C at room temperature is reasonably well described by the simple Drude model for the electronic contribution and by six Lorentzian peaks representing interband transitions.

We have identified the optical phonon modes of high-quality superconducting infinite-layer compounds Sr0.9Ln0.1CuO2 (Ln=La and Sm, Tc=43 K for both) from their infrared reflectivity spectra obtained with a Fourier-transform infrared spectrometer. The La-doped compound exhibited only four (2A2u+2Eu) out of the five (2A2u+3Eu) infrared-active phonon modes predicted by a group theoretical analysis, with one Eu mode screened by free carriers. The Sm-doped compound exhibited all five modes. We propose the displacement pattern for each mode based on reported lattice dynamics calculations and through comparison with the phonon modes of other single-layer high-Tc cuprates.

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.

We report Raman and infrared measurements of optical phonons in superconducting Sm1.85Ce0.15CuO4 single crystals (T-c = 15 K). All cone-center phonon modes predicted by a factor-group analysis have been identified, except for the low-frequency Eg mode involving Sm(Ce) atoms. Two additional c-axis modes with A(1g) symmetry were found at 131 cm(-1) and 591 cm(-1) in the Ramman spectra. We identify the 134 cm(-1) mode as the lowest-frequency infrared-active c-axis Az, mode, mixed into the Raman spectra due to disorder via a breakdown of the usual exclusive selection rule. The 591 cm(-1) Raman mode is found to be closely connected with an extra infrared made observed at the same frequency. Our measurements on Sm2-xCexCuO4 (x = 0.0 - 0.18) ceramics show, for the first time, that the strong Raman peak at 591 cm(-1) appears only in the narrow range of Ce-doping, 0.1 less than or equal to x less than or equal to 0.17, which nearly coincides with the Ce-doping range where the Sm2-xCexCuO4 system is superconducting at low temperatures.

The mixed state of intermetallic superconductor YNi2B2C, like that of high-T, materials, is found to be separated into vortex solid and vortex liquid phases due to its large fluctuations. In addition, the vortex solid phase changes from a vortex lattice at low fields to a vortex glass at high fields. A very sharp resistive transition with a kink and an S-shaped I - V curve are observed in the vortex lattice transition region, H ≤ 1 T. At high fields, H ≥ 2 T, a scaling law holds for the vortex glass transition in R -- T and I - V curves, with field-independent critical exponents, v = 1.23 f 0.02 and z = 5.46 ± 0.32.

The paramagnetic susceptibility of YNi2B2C single crystal in a high magnetic field is found to be about 2×10−5 cm3/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 YNi2B2C superconductor and shows simple Curie–Weiss behavior. In this case, an upper critical field of the YNi2B2C 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.

We present the comprehensive description of the magnetic flux noise of the Bi2Sr2CaCu2Ox high T-c superconductor. The effect of the dissipation of flux motions on the noise power spectrum was investigated below the transition temperature. The noise power is proportional to the magnetic field in the flux creep region. The exponent of the spectrum deceases for increasing magnetic field. And it has a corner frequency which becomes lower when stronger the fields. These are related to the self-organized critical(SOC) properties of the flux creep noise.

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.

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.

The influence of precursor composition, nominal mercury vapour pressure (P(Hg)) 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 P(Hg) higher than 200 atm and obtain samples containing only HgBa2Ca2Cu3O8+x as the superconducting phase, with T(c) (onset) = 134 K and T(c) (zero) = 131 K.

We have measured the reversible magnetization M(H, T) for grain-aligned HgBa2Ca2Cu3O8+δ. Our data exhibit a clear vortex fluctuation about 6 K below the superconducting onset temperature. The derived in-plane magnetic penetration depth λab(0) is 1700 Å which is similar to those of other high-Tc superconductors. In addition, the zero-temperature GL coherence length ξab (0) is 22±5 Å, which implies GL parameter ϰ=77±13. This ϰ is much greater than that of HgBa2CuO6, but similar to that of YBa2Cu3O7.

Isothermal magnetization of grain-aligned HgBa2Ca2Cu3O8 as a function of the applied magnetic field parallel to the c-axis was measured. The curves were analyzed by using the extended critical state model which includes the combined effects of the surface barrier and the magnetization due to pinning. It is shown that the effect of the surface barrier in this mercury-based superconductor is significant especially at the high-temperature region.

By using the modified version of Dutta, Dimon, and Horn's model, we have analyzed the noise power spectral density, S(v)(f), of Bi2Sr2CaCu2Ox single crystal. The important function for this model, i.e., the distribution function of activation energy D(U0), was best fitted by the Gaussian distribution function when current induced vortex unbinding is dominant at the I-V curves. From this analysis, we found that the pinning potential and hopping rate strongly depend on applied current since this current changes the slope of this potential. We found that this noise power originates from the hopping of the current induced free vortices or the field induced free vortices.

We produced the HgBa2Ca2Cu3O8+x (Hg-1223) superconductors by changing the composition, mercury vapor pressure and reaction temperature. We found that the high mercury pressure is essential for the formation of the Hg-1223 phase by preventing the decomposition of HgO at low temperature. This sample was characterized by the X-ray diffraction, Auger electron microprobe, AC susceptibility and resistivity measurement.

We present an analysis of the measured excess conductivity that results from the fluctuations of the superconducting order parameter for a single crystal of the Bi2Sr2CaCu2Ox high-T(c) superconductor. The measured excess conductivity in the temperature range 84-245 K is best fitted by the two-dimensional (2D) Aslamazov-Larkin theory. By using this theory, the 2D characteristic length was obtained to be 12.6 angstrom, which is within a physically acceptable range. We find that the interlayer coupling strength predicted by the Lawrence-Doniach theory leads to too small a value of the c-direction coherence length. This seems to be an intrinsic property of this superconductor due to its high anisotropy. Further, the indirect Maki-Thompson effect is found to be negligible. Thus we claim that the excess conductivity of the Bi2Sr2CaCu2Ox single crystal is solely caused by the two-dimensional thermal fluctuations of the order parameter with the absence of the 2D-3D crossover and that the effect of fluctuating Cooper pairs in association with pair breakers is negligible in considering the excess conductivity.