The upper critical field H c2 ͑T͒ of Bi 2 Sr 2 CaCu 2 O 8 crystals with T c . 92 K has been measured from the out-of-plane resistivity in magnetic fields ͑H Ќ ab͒ up to 15 T. By the use of the empirical procedure the H c2 ͑T ͒ curve has been extrapolated up to H c2 Ӎ 230 T and T ͞T c Ӎ 0.35 which is independent of the choice of the R͞R N ratio. We found that H c2 ͑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 T c . Our results together with the heat capacity measurements provide an evidence for the possibility of 2e Bose liquid ground state of high-T c oxides. proposed an explanation for this phenomenon based on the Bose-Einstein condensation (BEC) of charged bosons. However, in the highest T c cuprates, the inplane superconducting transition is known to display pronounced broadening in a magnetic field, with the top of the transition having a much weaker field dependence than in the region near the bottom [6][7][8]. This together with the high values of H c2 have made an experimental determination of H c2 very difficult in materials with T c . 60 K, with the consequence that widely varying values of H c2 ͑0͒ have been estimated based on different models. The out-of-plane resistive transition is known to show a different behavior in a magnetic field. An increasingly pronounced maximum (peak) developed below T c and shifted to lower temperature rather than broadened with increasing field [9]. By modeling the c-axis conduction with a stack of Josephson junctions in series, Gray and Kim [10] have obtained an unusually large energy gap with large fluctuations. Consequently, no attempts have been made so far to determine H c2 ͑T ͒ from these observations. We report in this Letter the out-of-plane resistivity of Bi 2 Sr 2 CaCu 2 O 8 (BSCCO-2212) crystals measured in perpendicular fields up to 15 T ͑H k I k c͒. We propose a procedure for extrapolating the values of the resistive upper critical field H c2 ͑T ͒ which is independent of the background normal resistance and the choice of R͞R N values over a wide temperature range. In contrast to the predictions of conventional theory, this H c2 ͑T͒ shows a negative curvature just below T c followed by a positive curvature at lower temperatures with no sign of saturation down to 0.35T c . We have also determined the irreversibility field H irr ͑T ͒ ͑k c͒. This has a qualitatively different temperature dependence. We propose a quantitative explanation of H c2 ͑T ͒ based on the theory of BEC in the magnetic field [5].BSCCO-2212 single crystals have been grown by the self-flux method during solid-state reaction and have ϳ͑7 10͒% Y substitution for Ca [11]. For the present measurements, three homogeneous single crystals have been selected with T c0 between 92 and 95 K and in-plane dimensions 870 3 270, 340 3 150, and 800 3 350 mm 2 . Figure 1(A) shows the typical out-of-plane resistive transitions in magnetic fields up to 15 T ͑H k I k c͒. In a...

We calculate the Nernst signal in disordered conductors with the chemical potential near the mobility edge. The Nernst effect originates from the interference of itinerant and localized-carrier contributions to the thermomagnetic transport. It reveals a strong temperature and magnetic field dependence, which describes quantitatively the anomalous Nernst signal in high-Tc cuprates.

The resistive upper critical field, Hc2(T ) of cuprates, superconducting spin-ladders, and organic (TMTSF)2X systems is shown to follow a universal nonlinear dependence Hc2 ∝ (Tc − T ) 3/2 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. 74.70.Kn