Normal state of extremely anisotropic superconducting cuprates as revealed by magnetotransport

High magnetic-field studies of cuprate superconductors revealed a non-BCS temperature dependence of the upper critical field Hc2(T) determined resistively by several groups. These determinations caused some doubts on the grounds of both the contrasting effect of the magnetic field on the in-plane and out-of-plane resistances reported for large Bi2212 samples and the large Nernst signal well above Tc. Here we present both ρab(B) and ρc(B) of tiny Bi2212 crystals in magnetic fields up to 50 T. None of our measurements revealed a situation when on the field increase ρc reaches its maximum while ρab remains very small if not zero. The resistive Hc2(T) estimated from ρab(B) and ρc(B) are approximately the same. Our results support any theory of cuprates that describes the state above the resistive phase transition as perfectly normal with a zero off-diagonal order parameter

Universal upper critical field of unconventional superconductors

The resistive upper critical field, Hc2(T) of cuprates, superconducting spin-ladders, and organic (TMTSF)2X systems is shown to follow a universal nonlinear temperature dependence in a wide range near Tc, while its low-temperature behaviour depends on the chemical formula and sample quality. Hc2(T) is ascribed to the Bose-Einstein condensation field of preformed pairs. The universality originates from the scaling arguments. Exceeding the Pauli paramagnetic limit is explained. Controversy in the determination of Hc2(T) from the kinetic and thermodynamic measurements is resolved in the framework of the charged Bose-gas model with impurity scattering

Resistive upper critical field of high- Tc single crystals of Bi2Sr2CaCu2O8

The upper critical field Hc2(T) of Bi2Sr2CaCu2O8 crystals with Tc>92 K has been measured from the out-of-plane resistivity in magnetic fields (H ⊥ ab) up to 15 T. By the use of the empirical procedure the Hc2(T) curve has been extrapolated up to Hc2≃230 T and T/Tc≃0.35 which is independent of the choice of the R/RN ratio. We found that Hc2(T) does not follow the conventional theory with or without fluctuations but is consistent with the prediction based on the Bose-Einstein condensation of charged bosons formed above Tc. Our results together with the heat capacity measurements provide an evidence for the possibility of 2e Bose liquid ground state of high-Tc oxides

Resistive upper critical field of high- Tc single crystals of Bi2Sr2CaCu2O8

The upper critical field Hc2(T) of Bi2Sr2CaCu2O8 crystals with Tc>92 K has been measured from the out-of-plane resistivity in magnetic fields (H ⊥ ab) up to 15 T. By the use of the empirical procedure the Hc2(T) curve has been extrapolated up to Hc2≃230 T and T/Tc≃0.35 which is independent of the choice of the R/RN ratio. We found that Hc2(T) does not follow the conventional theory with or without fluctuations but is consistent with the prediction based on the Bose-Einstein condensation of charged bosons formed above Tc. Our results together with the heat capacity measurements provide an evidence for the possibility of 2e Bose liquid ground state of high-Tc oxides

'Giant' normal state magnetoresistances of Bi2Sr2CaCu2O8+.

Magnetoresistance (MR) of Bi-2212 single crystals with T$_{c}$ $\approx 87-92 K$ is studied in pulsed magnetic fields up to 50T along the c-axis in a wide temperature range. The negative out-of-plane and the positive in-plane MRs are measured in the normal state. Both MRs have similar magnitudes, exceeding any orbital contribution by two orders in magnitude. These are explained as a result of the magnetic pair-breaking of preformed pairs. Resistive upper critical fields H$_{c2}$(T) determined from the in-- and out-of-plane MRs are about the same. They show non-BCS temperature dependences compatible with the Bose-Einstein condensation field of preformed charged bosons