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Measurements of the current-voltage characteristics ($I\ensuremath{-}V$) were performed on ${\mathrm{Ba}(\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$ single crystals with doping level $0.044\ensuremath{\le}x\ensuremath{\le}0.100$. An unconventional increase in the flux-flow resistivity ${\ensuremath{\rho}}_{\mathrm{ff}}$ with decreasing magnetic field $H$ was observed across this doping range. Such an abnormal field dependence of ${\ensuremath{\rho}}_{\mathrm{ff}}$ is in contrast with the linear ${\ensuremath{\rho}}_{\mathrm{ff}}(H)$ of conventional type-II superconductors, but similar to the behavior recently observed in the heavy-fermion superconductor ${\mathrm{CeCoIn}}_{5}$. A significantly enhanced ${\ensuremath{\rho}}_{\mathrm{ff}}$ was found for the $x=0.06$ single crystals, implying a strong single-particle energy dissipation around the vortex cores. At different temperatures and fields and for a given doping concentration, the normalized ${\ensuremath{\rho}}_{\mathrm{ff}}$ scales with normalized field and temperature. The doping level dependence of the scaling parameters strongly suggests that the abnormal upturn in ${\ensuremath{\rho}}_{\mathrm{ff}}$ is likely related to the enhancement of spin fluctuations around the vortex cores of the samples with $x\ensuremath{\approx}0.06$.

We investigated the relaxation of the irreversible magnetization of YBa2Cu3O72d , Pb2Sr2Y0.53Ca0.47Cu3O8, and Bi2Sr2CaCu2O81d single crystals with significant quenched disorder in the region of the second magnetization peak. It was found that for an applied magnetic field between the onset field and the peak field the relevant current-density dependence of the activation energy exhibits a sudden change, which can be interpreted as a crossover from elastic to plastic vortex creep. The evolution of this change with magnetic field illustrates the increase of the collective pinning barrier between the onset field and the peak field. The observed increase of the collective pinning barrier is limited by the plastic barrier at the peak field. This appears to be a general behavior, and may have important consequences on the interpretation of the thermally induced vortex phase transition at high magnetic fields.

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.

The large ($10^2 - 10^5$) and strongly temperature dependent resistive anisotropy $\eta = (\sigma_{ab}/\sigma_c)^{1/2}$ of cuprates perhaps holds the key to understanding their normal state in-plane $\sigma_{ab}$ and out-of-plane $\sigma_{c}$ conductivities. It can be shown that $\eta$ is determined by the ratio of the phase coherence lengths $\ell_i$ in the respective directions: $\sigma_{ab}/\sigma_c = \ell_{ab}^2/\ell_{c}^2$. In layered crystals in which the out-of-plane transport is incoherent, $\ell_{c}$ is fixed, equal to the interlayer spacing. As a result, the T-dependence of $\eta$ is determined by that of $\ell_{ab}$, and vice versa, the in-plane phase coherence length can be obtained directly by measuring the resistive anisotropy. We present data for hole-doped $YBa_2Cu_3O_y$ ($6.3 < y < 6.9$) and $Y_{1-x}Pr_xBa_2Cu_3O_{7-\delta }$ ($0 < x \leq 0.55$) and show that $\sigma_{ab}$ of crystals with different doping levels can be well described by a two parameter universal function of the in-plane phase coherence length. In the electron-doped $Nd_{2-x}Ce_{x}CuO_{4-y}$, the dependence $\sigma_{ab}(\eta)$ indicates a crossover from incoherent to coherent transport in the c-direction., Comment: 5 pages, 2 figures

We report measurements of the magnetoresistance of ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\ifmmode\pm\else\textpm\fi{}\mathrm{\ensuremath{\delta}}}$ epitaxial thin films with varying oxygen content in magnetic fields H applied parallel (H\ensuremath{\parallel}c) and perpendicular (H\ensuremath{\perp}c) to the tetragonal c axis. We have observed critical scaling of the electrical resistivity that is consistent with a vortex-glass transition for a film with an optimum superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ of \ensuremath{\approxeq}22 K and H\ensuremath{\parallel}c . The values of the zero-temperature upper critical field ${\mathit{H}}_{{\mathit{c}}_{2}}$(0)=80 kOe and the in-plane zero-temperature coherence length ${\ensuremath{\xi}}_{\mathit{ab}}$(0)=64 \AA{} were obtained from an analysis of the fluctuation conductivity. For an overoxygenated film with ${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}10 K, an anomaly develops with increasing field for H\ensuremath{\parallel}c and T\ensuremath{\lesssim}2 K that is characterized by a minimum in the temperature dependence of the resistivity followed by a second resistive transition at a lower temperature, which is nearly independent of H. This behavior is similar to that previously observed in ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals and may be associated with the magnetic ordering of the ${\mathrm{Nd}}^{3+}$ ions. \textcopyright{} 1996 The American Physical Society.

We report a systematic study of the double resistive superconducting transition in carefully prepared polycrystalline samples of the electron-doped superconducting series ${\mathrm{Sm}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ for 0.12\ensuremath{\le}x\ensuremath{\le}0.20. From high-resolution x-ray-diffraction measurements, the solubility limit of Ce in this series is slightly greater than 0.16. An intrinsic double resistive transition present in all superconducting compounds is attributed to the granularity of these polycrystalline samples. There is a partial transition when the grains become superconducting at a temperature ${\mathit{T}}_{\mathit{c}1}$, and coupling between grains at a lower temperature ${\mathit{T}}_{\mathit{c}2}$ apparently completes the transition. Only the transition associated with coupling between grains is observed in magnetic-susceptibility measurements due to inhomogeneous grains and large penetration depths. From compositional dependences of resistive and magnetic measurements, the superconducting volume fraction increases linearly with increasing dopant concentration up to the solubility limit, while the best coupling between grains is at a dopant concentration of 0.15. Thus, there is a subtle relationship between doping and coupling in this series. Also, arguments for a true thermodynamic phase transition at ${\mathit{T}}_{\mathit{c}2}$ are given.

The anisotropic superconducting-state thermodynamic parameters of the electron-doped compound ${\mathrm{Sm}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ have been derived from reversible dc magnetization M(H,T) measurements on magnetically aligned single-crystal particles of the compound for magnetic fields applied perpendicular and parallel to the ab plane. A recent model developed by Hao et al., which describes M(H,T) over the entire field region between ${\mathit{H}}_{\mathit{c}1}$ and ${\mathit{H}}_{\mathit{c}2}$, was used to analyze the M(H,T) data. The analysis of the measurements provides a self-consistent test of the model and resolves a controversy by showing that the anisotropy of ${\mathit{H}}_{\mathit{c}2}$(T) of the electron-doped compound ${\mathrm{Sm}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ is comparable to that of the hole-doped compound ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.

Although previous resistive measurements of the superconducting transition under applied magnetic field on a Sm1.85Ce0.15CuO4-y single crystal did not show much dissipation effects, magnetic relaxation data on another superconducting single crystal yield a small flux pinning energy, of the order of 20 meV at T = 6 K when H⌈c, similar to hole-doped superconductors. In the electron-doped compounds, the normal-state resistivity, which exhibits substantial anisotropy, does not follow a linear temperature dependence, but rather a T2 law up to about 200 K, consistent with Fermi-liquid theory for electron-electron scattering. In superconducting Nd2-xCexCuO4-y single crystals, both in-plane and out-of- plane resistivities exhibit metallic behavior with T2 dependence.

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.

A direct method for determining the magnetic field Hfcs at which a superconductor enters the fully critical state (i.e., the field at which flux first penetrates throughout the entire sample) is described, and a formula for calculating the critical current density JcH from the value of Hfcs is presented. The behavior of JcH(x) is compared to the variation of the fluxoid pinning energy U, inferred from magnetic relaxation measurements in the Y1−xPrxBa2Cu3O7−δ system.

As a consequence of the high values of the superconducting transition temperature T c, the short coherence lengths ξ, large penetration depths λ, and the considerable anisotropy of the superconducting properties of the layered high-T c cuprates, the role of thermal fluctuations is enhanced compared to the conventional superconductors. The interplay of thermal activation, intervortex correlations, and the interaction of vortices with pinning centers, which are all of the same order in high-T c systems, gives rise to a rich variety of fluxoid phases in the mixed state, which have been predicted and partially verified by experiment [1].

Stable and reproducible superconductivity has been unambiguously observed in a new Tl-Ba-Cu-O system containing no group-IIIB elements. Resistance in these rare-earth--free oxides starts to drop sharply above 90 K and reaches zero at 81 K. Meissner flux expulsion and diamagnetic shielding show bulk superconductivity. Further substitutions of elements for this system may lead to even higher-temperature superconductors.

Some of the most intriguing behavior of a superconducting medium pertain to its magnetic properties. An ideal plasma is just as highly conducting as a superconductor, but magnetically these are totally different systems. Indeed the statement, that it may be more accurate to regard a superconductor as a perfect diamagnet than a perfect conductor, has been attributed to John Bardeen. This pivotal role of magnetism makes it essential to understand the magnetic properties of superconductors.