Universal vortex-state Hall conductivity of YBa2Cu3O7 single crystals with differing correlated disorder

The vortex-state Hall conductivity ([sigma][sub]xy) of YBa2Cu3O7 single crystals in the anomalous-sign-reversal region is found to be independent of the density and orientation of the correlated disorder. After the anisotropic-to-isotropic scaling transformation is carried out, a universal scaled Hall conductivity [sigma][bar][sub]xy is obtained as a function of the reduced temperature (T/T[sub]c) and scaled magnetic field strength (H[bar]) for five samples with different densities and orientation of controlled defects. The transport scattering times {tau], derived from applying the model given by Feigel'man et al (Feigel'man M V, Geshkenbein V B, Larkin A I and Vinokur V M 1995 Pis. Zh. Eksp. Teor. Fiz. 62 811 (Engl. Transl. 1995 JETP Lett. 62 835)) to the universal Hall conductivity [sigma bar](T/T[sub]c, H[bar]), are consistent in magnitude with those derived from other measurements for quasiparticle scattering, and are much smaller than the thermal relaxation time of vortex displacement and than the vortex–defect interaction time. Our experimental results and analyses therefore suggest that the anomalous sign reversal in the vortex-state Hall conductivity is associated with the intrinsic properties of type-II superconductors, rather than extrinsic disorder effects

Current-driven vortex dynamics in untwinned superconducting single crystals

Current-driven vortex dynamics of type-II superconductors in the weak-pinning limit is investigated by quantitatively studying the current-dependent vortex dissipation of an untwinned YBa2Cu3O7 single crystal. For applied current densities (J) substantially larger than the critical current density (Jc), non-linear resistive peaks appear below the thermodynamic first-order vortex-lattice melting transition temperature (Tm), in contrast to the resistive hysteresis in the low-current limit (J < Jc). These resistive peaks are quantitatively analysed in terms of the current-driven coherent and plastic motion of vortex bundles in the vortex-solid phase, and the non-linear current - voltage characteristics are found to be consistent with the collective flux-creep model. The effects of high-density random point defects on the vortex dynamics are also investigated via proton irradiation of the same single crystal. Neither resistive hysteresis at low currents nor peak effects at high currents are found after the irradiation. Furthermore, the current-voltage characteristics within the instrumental resolution become completely ohmic over a wide range of currents and temperatures, despite theoretical predictions of much larger Jc-values for the given experimental variables. This finding suggests that the vortex-glass phase, a theoretically proposed low-temperature vortex state which is stabilized by point disorder and has a vanishing resistivity, may become unstable under applied currents significantly smaller than the theoretically predicted Jc. More investigation appears necessary in order to resolve this puzzling issue

Phase diagram of heavy fermion systems

The Meccano of heavy fermion systems is shown on different cases going from anomalous monochalcogenides to cerium intermetallic compounds with special focus on the ideal case of the CeRu2Si2 series. Discussion is made in the frame of the interplay between valence, electronic structure (Fermi surface), and magnetism. The nice tools given by the temperature, the pressure, and the magnetic field allow to explore different ground states as well as the slow downhill ''race'' before reaching a Fermi liquid finish line at very low temperature. Experimentally, the Gr"uneisen parameter i.e. the ratio of the thermal expansion by the specific heat is a coloured magic number; its temperature, pressure, and magnetic field dependence is a deep disclosure of competing hierarchies and the conversion of this adaptive matter to external responses.Comment: submitted ICM200

Thermodynamics of a Heavy Ion-Irradiated Superconductor: the Zero-Field Transition

Specific heat measurements show that the introduction of amorphous columnar defects considerably affects the transition from the normal to the superconducting state in zero magnetic field. Experimental results are compared to numerical simulations of the 3D XY model for both the pure system and the system containing random columnar disorder. The numerics reproduce the salient features of experiment, showing in particular that the specific heat peak changes from cusp-like to smoothly rounded when columnar defects are added. By considering the specific heat critical exponent alpha, we argue that such behavior is consistent with recent numerical work [Vestergren et al., PRB 70, 054508 (2004)] showing that the introduction of columnar defects changes the universality class of the transition.Comment: 4 pages, 2 figure

Gadolinium scandium germanide, Gd2Sc3Ge4

Gd2Sc3Ge4 adopts the ortho­rhom­bic Pu5Rh4-type structure. The crystal structure contains six sites in the asymmetric unit: two sites are statistically occupied by rare-earth atoms with Gd:Sc ratios of 0.967 (4):0.033 (4) and 0.031 (3):0.969 (3), one site (.m. symmetry) is occupied by Sc atoms, and three distinct sites (two of which with .m. symmetry) are occupied by Ge atoms. The rare-earth atoms form two-dimensional slabs with Ge atoms occupying the trigonal-prismatic voids

Mobility gap in intermediate valent TmSe

The infrared optical conductivity of intermediate valence compound TmSe reveals clear signatures for hybridization of light $d$- and heavy f-electronic states with m* ~ 1.6 m_0 and m* ~ 16 m_0, respectively. At moderate and high temperatures, the metal-like character of the heavy carriers dominate the low-frequency response while at low temperatures (T_N < T < 100 K) a gap-like feature is observed in the conductivity spectra below 10 meV which is assigned to be a mobility gap due to localization of electrons on local Kondo singlets, rather than a hybridization gap in the density of states

Effects of Controlled Defects on the Vortex-Solid Melting Transition of Y-Ba-Cu-O Single Crystals

We report systematic studies of the dc transport properties in proton-irradiated Y-Ba-Cu-O single crystals. We find that the onset of vortex dissipation in moderately irradiated samples is associated with the occurrence of a second-order vortex-solid melting transition. In addition, the decreasing zero-field transition temperature and increasing critical current density with the increasing defects reveal the effects of disorder on reducing the electron mean-free-path and on increasing the pinning density

Do columnar defects produce bulk pinning?

From magneto-optical imaging performed on heavy-ion irradiated YBaCuO single crystals, it is found that at fields and temperatures where strong single vortex pinning by individual irradiation-induced amorphous columnar defects is to be expected, vortex motion is limited by the nucleation of vortex kinks at the specimen surface rather than by half-loop nucleation in the bulk. In the material bulk, vortex motion occurs through (easy) kink sliding. Depinning in the bulk determines the screening current only at fields comparable to or larger than the matching field, at which the majority of moving vortices is not trapped by an ion track.Comment: 5 pages, 5 figures, submitted to Physical Review Letter