Magnetotransport and the upper critical magnetic field in MgB2

Magnetotransport measurements are presented on polycrystalline MgB2 samples. The resistive upper critical magnetic field reveals a temperature dependence with a positive curvature from Tc = 39.3 K down to about 20 K, then changes to a slightly negative curvature reaching 25 T at 1.5 K. The 25- Tesla upper critical field is much higher than what is known so far on polycrystals of MgB2 but it is in agreement with recent data obtained on epitaxial MgB2 films. The deviation of Bc2(T) from standard BCS might be due to the proposed two-gap superconductivity in this compound. The observed quadratic normal-state magnetoresistance with validity of Kohler's rule can be ascribed to classical trajectory effects in the low-field limit.Comment: 6 pages, incl. 3 figure

Superconductivity at 11.3 K induced by cobalt doping in CeOFeAs

Pure phases of a new oxyarsenide superconductor of the nominal composition CeOFe0.9Co0.1As was successfully synthesized by solid state reaction in sealed silica ampoules at 1180 C. It crystallizes in the layered tetragonal ZrCuSiAs type structure (sp gp P4/nmm) with lattice parameter of a = 3.9918(5) angstrom and c = 8.603(1) angstrom. A sharp superconducting transition is observed at 11.31 K with an upper critical field of 45.22 T at ambient pressure. The superconducting transition temperature is drastically lowered (~ 4.5, 4.9 K) on increasing the concentration (x = 0.15, 0.2) of cobalt

Dissipative current in SIFS Josephson junctions

We investigate superconductor/insulator/ferromagnet/superconductor (SIFS) tunnel Josephson junctions in the dirty limit, using the quasiclassical theory. We consider the case of a strong tunnel barrier such that the left S layer and the right FS bilayer are decoupled. We calculate quantitatively the density of states (DOS) in the FS bilayer for arbitrary length of the ferromagnetic layer, using a self-consistent numerical method. We compare these results with a known analytical DOS approximation, which is valid when the ferromagnetic layer is long enough. Finally we calculate quantitatively the current-voltage characteristics of a SIFS junction.Comment: Proceedings of the Vortex VI conference, to be published in Physica

Superconductivity in the Correlated Pyrochlore Cd_2Re_2O_7

We report the observation of superconductivity in high-quality Cd$_2$Re$_2$O$_7$ single crystals with room-temperature pyrochlore structure. Resistivity and ac susceptibility measurements establish an onset transition temperature T$_c^{onset}$ = 1.47 K with transition width $\Delta$T$_c$ = 0.25 K. In applied magnetic field, the resistive transition shows a type-II character, with an approximately linear temperature-dependence of the upper critical field H$_{c2}$. The bulk nature of the superconductivity is confirmed by the specific heat jump with $\Delta$C = 37.9 mJ/mol-K. Using the $\gamma$ value extracted from normal-state specific heat data, we obtain $\Delta$C/$\gamma$T$_c$ = 1.29, close to the weak coupling BCS value. In the normal state, a negative Hall coefficient below 100 K suggests electron-like conduction in this material. The resistivity exhibits a quadratic T-dependence between 2 and 60 K, i.e., $\rho =\rho_0$+AT$^2$, indicative of Fermi-liquid behavior. The values of the Kadowaki-Woods ratio A/$\gamma^2$ and the Wilson ratio are comparable to that for strongly correlated materials.Comment: 4 pages, 5 figure

Superconductivity Induced by Bond Breaking in the Triangular Lattice of IrTe2

IrTe2, a layered compound with a triangular iridium lattice, exhibits a structural phase transition at approximately 250 K. This transition is characterized by the formation of Ir-Ir bonds along the b-axis. We found that the breaking of Ir-Ir bonds that occurs in Ir1-xPtxTe2 results in the appearance of a structural critical point in the T = 0 limit at xc = 0.035. Although both IrTe2 and PtTe2 are paramagnetic metals, superconductivity at Tc = 3.1 K is induced by the bond breaking in a narrow range of x > xc in Ir1-xPtxTe2. This result indicates that structural fluctuations can be involved in the emergence of superconductivity.Comment: 10 pages, 4 figure

H-T phase diagram and the nature of Vortex-glass phase in a quasi two-dimensional superconductor: Sn metal layer sandwiched between graphene sheets

The magnetic properties of a quasi two-dimensional superconductor, Sn-metal graphite (MG), are studied using DC and AC magnetic susceptibility. Sn-MG has a unique layered structure where Sn metal layer is sandwiched between adjacent graphene sheets. This compound undergoes a superconducting transition at $T_{c}$ = 3.75 K at $H$ = 0. The $H$-$T$ diagram of Sn-MG is similar to that of a quasi two-dimensional superconductors. The phase boundaries of vortex liquid, vortex glass, and vortex lattice phase merge into a multicritical point located at $T^{*}$ = 3.4 K and $H^{*}$ = 40 Oe. There are two irreversibility lines denoted by $H_{gl}$ (de Almeida-Thouless type) and $H_{gl^{\prime}}$ (Gabay-Toulouse type), intersecting at $T_{0}^{\prime}$= 2.5 K and $H_{0}^{\prime}$ = 160 Oe. The nature of slow dynamic and nonlinearity of the vortex glass phase is studied.Comment: 24 pages, 13 figures; Physica C (2003), in pres

Superconducting Properties under Magnetic Field in Na0.35_{0.35}CoO2⋅1.3_{2}{\cdot}1.3H2_{2}O Single Crystal

We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na$_{0.35}$CoO$_{2}{\cdot}1.3$H$_{2}$O single crystal. The temperature dependence of the resistivity shows metallic behavior from room temperature to the superconducting transition temperature $T_{c}$ of 4.5 K. Sharp resistive transition, zero resistivity and almost perfect superconducting volume fraction below $T_{c}$ indicate the good quality and the bulk superconductivity of the single crystal. The upper critical field $H_{c2}$ and the coherence length $\xi$ are obtained from the resistive transitions in magnetic field parallel to the c-axis and the $ab$-plane. The anisotropy of $\xi$, $\xi_{ab} / \xi_{c} =$ 12 nm/1.3 nm $\simeq$ 9.2, suggests that this material is considered to be an anisotropic three dimensional superconductor. In the field parallel to the $ab$-plane, $H_{c2}$ seems to be suppressed to the value of Pauli paramagnetic limit. It may indicate the spin singlet superconductivity in the cobalt oxide.Comment: 4 pages, 4 figure

Large upper critical field in non-centrosymmetric superconductor Y2C3

We determine the upper critical field $\mu_0 H_{c2}(T_c)$ of non-centrosymmetric superconductor $Y_2 C_3$ using two distinct methods: the bulk magnetization M(T) and the tunnel-diode oscillator (TDO) based impedance measurements. It is found that the upper critical field reaches a value of 30T at zero temperature which is above the weak-coupling Pauli paramagnetic limit. We argue that the observation of such a large $\mu_0 H_{c2}(0)$ in $Y_2 C_3$ could be attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of broken inversion symmetry.Comment: 4 pages, 3 figures, accepted by J. Phys. Chem. Solid

Hall Coefficient of Equilibrium Supercurrents Flowing inside Superconductors

We study augmented quasiclassical equations of superconductivity with the Lorentz force, which is missing from the standard Ginzburg-Landau and Eilenberger equations. It is shown that the magnetic Lorentz force on equilibrium supercurrents induces finite charge distribution and the resulting electric field to balance the Lorentz force. An analytic expression is obtained for the corresponding Hall coefficient of clean type-II superconductors with simultaneously incorporating the Fermi-surface and gap anisotropies. It has the same sign and magnitude at zero temperature as the normal state for an arbitrary pairing, having no temperature dependence specifically for the s-wave pairing. The gap anisotropy may bring a considerable temperature dependence in the Hall coefficient and can lead to its sign change as a function of temperature, as exemplified for a model d-wave pairing with a two-dimensional Fermi surface. The sign change may be observed in some high-$T_{c}$ superconductors.Comment: 7 pages, 3 figure