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We investigated the anisotropy dependence of the characteristic fields of the second peak magnetization, specifically, the onset field Bon and the peak field Bsp, in Y1−xPrxBa2Cu3O7−δ single crystals. The empirical dependence arising from data analysis suggests a logarithmic dependence of both these fields on the anisotropy factor γ which disagrees with the universal scaling law γ 2 B = F ( t ) proposed previously. We attribute the faster decrease of the two fields at high anisotropy to the dramatic changes in the magnetic state of the system which starts to display a noticeable paramagnetic contribution at high Pr-doping, hence high anisotropy. The incipient phase separation could also reduce the elastic properties of the flux line lattice which determines onset of the second peak magnetization.

We give a brief overview of transport and magnetic relaxation measurements in the mixed state of strongly underdoped Y 1−χ Pr χ Ba 2 Cu 3 O 7−δ crystals. Two phenomena are especially interesting from the point of view of both fundamental physics of “vortex matter” and power applications of high- T c superconductors. First is the transition from thermally activated flux creep to temperature independent quantum flux creep which is observed in both transport and magnetic relaxation measurements at temperatures T ≤ 5 K . Flux transformer measurements indicate that the crossover to quantum creep is preceded by a coupling transition. Second, in the mixed state below the coupling transition, the dissipation is non-ohmic, and we speculate that, as a result, the current distribution may be unstable with respect to self-channeling, resulting in the formation of thin current-carrying layers.

We report the observation of anomalies in the irreversibility line H ( T ∗ ) and resistive upper critical field H c2 ( T ) for two different samples of the electron-doped superconductor Sm 1.85 Ce 0.15 CuO 4− y , a specimen consisting of magnetically aligned single-crystal grains and a single crystal. The irreversibility line conforms to a power a law dependence in the H-T plane with an exponent that changes at T ∗ T c ≅0.5 and H ≅1 kOe m ≅1.8 in low fields to m ≅4.2 in high fields, for applied magnetic fields parallel to the c -axis. These observations may indicate two different temperature and/or field dependences for the pinning potential. A single value of m =1.6 is observed for the exponent of the irreversibility line for H ⊥ c in the field and temperature range studied. The resistively determined H c2 ( T ) data taken with H ‖ c show the same power law behavior, with a change in the value of the exponent at T T c ≅0.5 and H ≅10 kOe from m =1.6 in low fields to m =2.4 in high fields. For H ⊥ c , the H c2 ( T ) data are characterized by a single exponent m =1.4 in the field and temperature range studied.

A phenomenological model is proposed to account for a recently observed crossover in the temperature dependence of the irreversibility line of high temperature superconductors from a (1 - T/Tc) 3 2 to a (1 - T/Tc)3 dependence. The model, which distinguishes between weak correlations and strong correlations in the flux line lattice, indicates that the crossover should occur at magnetic fields corresponding to a flux line spacing of about λ/4, λ being the penetration depth transverse to the field.

We report on the static and dynamic magnetic behavior of a Sm 1.85 Ce 0.15 CuO 4− y single crystal for fields ranging from 20 Oe to 6 kOe. The irreversibility line, extracted from static magnetization versus temperature measurements in different magnetic fields H , shows power law behavior with an exponent that changes at ∼ 1 koe from 1.7 for fields H ⩽1 kOe to 3.5 for H ⩾ 1 kOe. The magnetic relaxation study shows that for values of the applied field greater than 600 Oe, an extrapolation of the normalized relaxation rate S n to zero temperature gives a non-zero value. Thismay be indicative of nonthermally activated relaxation processes. For 1 kOe, S n versus temperature exhibits two peaks, a possible indication of the presence of two pinning mechanisms. A distribution of activation energies for the pinning barriers was inferred to explain the experimental observations. The distribution functions were calculated from magnetic relaxation data taken in different applied magnetic fields.

Electrical resistivity and magnetic susceptibility measurements on the new electron-doped Nd 2− x Ce x Cu 1− z In z O 4− y superconductors are reported. Replacing 3% of Cu 2+ with In 3+ in the non-superconducting compound Nd 1.88 Ce 0.12 CuO 4− y yields superconductivity with an onset superconducting critical temperature T c ≈23 K . The combined substitution of Ce 4+ and In 3+ with x + z in the range 0.15 to 0.17 affects T c in a similar way as the substitution of Ce 4+ alone (z = 0) where electrons are supplied from outside the CuO 2 planes. Furthermore, for x + z = 0.15 (i.e., constant electron concentration), we find that T c remains relatively constant (∽24 K), while Δ T c increases with increasing z from 4 K at z = 0 to 19.4 K at z = 0.04.

We present two algorithms for the calculation of the resistivity tensor from current/voltage measurements for the two different contact configurations: “flux transformer” and “ I ‖ c ”. The application of this method is illustrated on data for an Y 0.78 Pr 0.22 Ba 2 Cu 3 O 7− δ single crystal.

Hall effect measurements on single crystal specimens of Y 1−x Pr x Ba 2 Cu 3 O 7−δ (0 ≤ x ≤ 1) and YBa 2 Cu 3 O y (6.35 ≤ y ≤ 7) revealed a “negative Hall anomaly” below T c , in the mixed state. For both systems, the depth Δ of the negative Hall signal scales with T/T c , while the maximum value of Δ decreases monotonically with decreasing T c and vanishes at T c ≈ 68 K. These results show that the sign anomaly is not consistent with models that invoke pinning to explain this effect, and suggest that the negative anomaly could have an intrinsic origin. Hall effect measurements in the normal state of both systems show that the cotangent of the Hall angle can be described by cotθ H = AT 2 + B, where A increases linearly with decreasing T c for both systems, while B increases linearly with decreasing T c for Y 1−x Pr x Ba 2 Cu 3 O 7−δ and is constant within experimental error for YBa 2 Cu 3 O y .

The effect of pressure on high- T c superconductors was studied by Josephson absorption as a new and very accurate method for determination of the critical temperature. The uncertainty in T c was less than 0.06 K. We propose our method as a new standard of T c measurements. The pressure derivative d T c /d p equals 1.06 K GPa -1 for YBa 2 Cu 3 O 7−λ and 2.20 K GPa -1 for Tl-Ca-Ba-Cu-O ceramics. Absence of thermal hysteresis was observed. The change of microwave absorption under hydrostatic pressure was also investigated.