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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 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.

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The electric transport of the charged particles in a spin texture was investigated in a strongly underdoped YBa2Cu3O6.25 single crystal in order to identify the characteristic electrical transport mechanism. The in-plane resistivity revealed three different regimes of charge transport: a chiral 2D VRH regime up to 55 K with a characteristic temperature Td ≈ 12,400 K, an impurity band conduction regime above 55 K, and a metallic-like regime beyond 170 K. The out-of-plane resistivity has only one crossover at 115 K, but the conduction mechanisms controlling the two regimes are not clear.

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 present a novel approach to the analysis of the normal state in-plane $$\sigma _{ab} $$ and out-of-plane σc conductivities of anisotropic layered crystals such as oxygen deficient YBa 2 Cu 3 O x . It can be shown that the resistive anisotropy is determined by the ratio of the phase coherence lengths in the respective directions; i.e., $$\sigma _{ab} /\sigma _c = \ell _{ab}^2 /\ell _c^2 $$ . From the idea that at all doping levels and temperatures T the out-of-plane transport in these crystals is incoherent, follows that $$\ell _c $$ is T-independent, equal to the spacing $$\ell _0 $$ between the neighboring bilayers. Thus, the T-dependence of $$\ell _{ab} $$ is given by the measured anisotropy, and $$\sigma _{ab} (\ell _{ab} )$$ dependence is obtained by plotting $$\sigma _{ab} {\text{ }}vs{\text{ }}\ell = {\text{ (}}\sigma _{ab} /\sigma _c )^{1/2} \ell _0 $$ .The analysis of several single crystals of YBa 2 Cu 3 O x (6.35 < x < 6.93) shows that for all of them $$\sigma _{ab} (\ell ) $$ is described by a universal dependence $$\sigma _{ab} /\overline \sigma = f(\ell /\overline \ell ) $$ with doping dependent parameters $$\overline \sigma {\text{ }}and{\text{ }}\overline \ell $$ .

We report measurements of out-of-plane (ρ c ) and in-plane (ρab) normal-state resistivities of single crystals of insulating PrBa2Cu3O 7-δ and strongly underdoped oxygen deficient YBa2Cu3O 6.41 using a flux transformer method. In the superconducting specimens, the onset of superconductivity was suppressed by a magnetic field of 9 T. We have found that the anisotropy ρc/ρab of these samples increases monotonically at low temperatures with no signs of saturation. The temperature dependence of ρc/ρab for YBa2Cu3O6.41 is well described by ρc/ρab=a +bT-2/3, but over a smaller temperature range than for insulating PrBa2Cu3O 7-δ. Both the absence of saturation of ρc/ρab and its T-2/3 dependence indicate two-dimensional conduction. This means that the average in-plane hopping distance of the localized charge carriers increases with decreasing T according to Mott's [Formula: see text] law, while the elementary step in the c-direction remains T independent, equal to the spacing between the bilayers.

In-plane $({\ensuremath{\rho}}_{\mathrm{ab}})$ and out-of-plane $({\ensuremath{\rho}}_{c})$ normal state resistivities of insulating ${\mathrm{PrBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and superconducting ${\mathrm{Y}}_{0.47}{\mathrm{Pr}}_{0.53}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ $({T}_{c}\ensuremath{\approx}18\mathrm{K})$ crystals have been measured using a ``flux transformer'' method. In the insulating sample we have found that ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{\mathrm{ab}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}a+bT}^{\ensuremath{-}2/3}$ over two decades of temperature, which indicates two-dimensional localization. By suppressing the onset of superconductivity with a magnetic field, we have observed that low ${T}_{c}$ ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ is transformed into a 2D insulator with transport properties similar to those of ${\mathrm{PrBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$.

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.

It is shown that the temperature dependence of the irreversibility line B(T*) of the ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.97}$ (0\ensuremath{\le}x\ensuremath{\le}0.55) system, inferred from magnetoresistance measurements, can be described by a Lindemann-type model of a vortex-solid\char21{}vortex-fluid phase transition triggered by vortex fluctuations. In this model, the previously observed transition in B(T*) from a power-law temperature dependence near ${\mathit{T}}_{\mathit{c}}$ to a more rapid dependence below T*/${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}0.6 can be accounted for in terms of a crossover from three-dimensional (3D) to two-dimensional (2D) vortex fluctuations. For different x values, a lower limit for the anisotropy ratio \ensuremath{\gamma} and an upper limit for the crossover magnetic induction ${\mathit{B}}_{\mathrm{cr}}$ above which 2D vortex fluctuations dominate were determined. \textcopyright{} 1996 The American Physical Society.

The maximum value [Delta] of the negative Hall resistivity [rho][sub [ital xy]] of Y[sub 1[minus][ital x]]Pr[sub [ital x]]Ba[sub 2]Cu[sub 3]O[sub 7[minus][delta]] (0[le][ital x][approx lt]0.13) and YBa[sub 2]Cu[sub 3]O[sub [ital y]] (6.73[approx lt][ital y][le]6.84) single crystals scales with [ital T]/[ital T][sub [ital c]], exhibiting a maximum [Delta][sub max] for [ital T]/[ital T][sub [ital c]][approx]0.96 and vanishing for [ital T]/[ital T][sub [ital c]][approx lt]0.87. [Delta][sub max] scales with [ital T][sub [ital c]] and decreases monotonically with decreasing [ital T][sub [ital c]] to zero for [ital T][sub [ital c]][approx lt]70 K. The Hall conductivity [sigma][sub [ital xy]] vs [ital B] data have been analyzed using the expression [sigma][sub [ital xy]]=[ital C][sub 1]/[ital B]+[ital C][sub 2][ital B], where the two terms are associated with motion of the magnetic vortices and the quasiparticles in the vortex cores, respectively, yielding [ital C][sub 1] and [ital C][sub 2] as functions of [ital x], [ital y], and [ital T].

Using a magnetic Frederiks transition technique, we measure the temperature and concentration dependences of splay K 1, twist K 2, and bend K 3 elastic constants for the lyotropic chromonic liquid crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water. K 1 and K 3 are comparable to each other and are an order of magnitude higher than K 2. At higher concentrations and lower temperatures, K 1 and the ratios K 1/K 3 and K 1/K 2 increase, which is attributed to elongation of self-assembled lyotropic chromonic liquid crystal aggregates, a feature not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of a fixed length.

Investigations of Y1−x M x Ba2Cu3O7−δ (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofT c withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y1−x Pr x Ba2Cu3O7−δ single crystals reveal aT 2 dependence of the cotangent of the Hall angle in the normal state and a “negative Hall anomaly” belowT c in the superconducting state, in agreement with recent reports. Our research shows that the depth, Δ, of the negative Hall signal scales withT/T c and that the maximum value of Δ decreases linearly withx and vanishes atx≈0.24. Magnetoresistance measurements on Y1−x Pr x Ba2Cu3O7−δ single crystals indicate that the irreversibility lineH(T *) obeys a universal scaling relation characterized by anm=3/2 power law nearT c, with a crossover to a more rapid temperature dependence of belowT/T c ≈0.6, similar to that observed for polycrystalline specimens.

Magnetic relaxation measurements in a magnetic field of 5 kOe at temperatures 5 K \ensuremath{\le}T\ensuremath{\le}90 K have been made on the system ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ for 0\ensuremath{\le}x\ensuremath{\le}0.4. The measurements reveal two peaks in the normalized logarithmic decay of the magnetization S as a function of temperature for the full range of Pr concentrations studied. The occurrence of the second peak in S could be a reflection of the softening of the flux lattice on approaching the melting line. Both the pinning energy at 27 K and the intragranular critical current density at 10 K exhibit maxima at x\ensuremath{\approxeq}0.1--0.2, with values that are about twice the corresponding values at x=0.

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}}}$.

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.

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.

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.

It has been suggested that the maximum magnitude of colossal magnetoresistance occurs in mixed-valent manganites with a tolerance factor $t=0.96$ [Zhou, Archibald, and Goodenough, Nature (London) 381, 770 (1996)]. However, at $t\ensuremath{\approx}0.96$ most manganites have relatively low values of the metal-insulator transition temperature ${T}_{MI}\phantom{\rule{0.3em}{0ex}}(\ensuremath{\sim}60\text{--}150\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Here, we report that a 50 \AA{} ${\mathrm{La}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{MnO}}_{3}$ thin film with $t=0.96$ grown on a (100) ${\mathrm{SrTiO}}_{3}$ substrate has a metal-insulator transition above room temperature, which represents a doubling of ${T}_{MI}$ compared with its value in the bulk material. We show that this spectacular increase of ${T}_{MI}$ is a result of the epitaxially compressive strain-induced reduction of the Jahn-Teller distortion.

We report measurements of the in-plane (ρ a b ) and out-of-plane (ρ c ) resistivities on a single crystal of the half-doped bilayer manganite LaSr 2 Mn 2 O 7 . In the temperature T range 220 to 300 K, the resistive anisotropy ρ c /ρ a b =A + B/T (A and B constants), which provides evidence for the variable-range-hopping conduction in the presence of a Coulomb gap. This hopping mechanism also accounts for the quadratic magnetic field H and sin 2 φ dependences of the negative magnetoresistivity ln[ρ i (T,H,φ)/ρ i (T,H=0)] (i=ab,c), where φ is the in-plane angle between the magnetic field and the current.

We report anisotropic resistivity measurements on a ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ single crystal over a temperature T range from 2 to 400 K and in magnetic-fields H up to 14 T. For $Tg~218\mathrm{K},$ the temperature dependence of the zero-field in-plane resistivity ${\ensuremath{\rho}}_{\mathrm{ab}}(T)$ obeys the adiabatic small polaron hopping mechanism, while the out-of-plane resistivity ${\ensuremath{\rho}}_{c}(T)$ can be ascribed by an Arrhenius law with the same activation energy. Considering the magnetic character of the polarons and the close correlation between resistivity and magnetization, we developed a model which allows the determination of ${\ensuremath{\rho}}_{ab,c}(H,T).$ The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites.

Hall effect measurements on single-crystal specimens of Y 1 − x Pr x Ba 2 Cu 3 O 7 − δ (0 ≤ x ≤ 1) reveal a “negative Hall anomaly” below T c , in the superconducting state. The depth Δ the negative Hall signal scales with T / T c and the maximum value of Δ decreases linearly with x and vanishes at x ≈ 0.24. 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 show that the cotangent of the Hall angle can be described by cot Θ H = AT 2 + B with both A and B increasing linearly with decreasing T c , in agreement with recent reports on Y 1 − x Pr x Ba 2 Cu 3 O 7 − δ thin films.

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.

The upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) for the magnetic field applied parallel to the c axis of a ${\mathrm{Sm}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ single crystal has been determined from an analysis of magnetoresistance data in terms of the Ginzburg-Landau fluctuation theory for layered superconductors. The fluctuations are predominantly two dimensions (2D) away from ${\mathit{T}}_{\mathit{c}}$(H) with a crossover to 3D as ${\mathit{T}}_{\mathit{c}}$(H) is approached. The ${\mathit{H}}_{\mathit{c}2}$(T) curve shows negative curvature and is in excellent agreement with the Werthamer, Helfand, Hohenberg, and Maki theory for conventional superconductors without paramagnetic limiting effects. The values of the initial slope (${\mathit{dH}}_{\mathit{c}2}$/dT${)}_{\mathit{T}\mathit{c}}$=-7.38 kOe/K, coherence length \ensuremath{\xi}(0)=48.4 \AA{}, and orbital critical field ${\mathit{H}}_{\mathit{c}2}$(0)=97.6 kOe, derived from this analysis, are in good agreement with the values we previously obtained from fits of the Hao-Clem variational model to reversible magnetization data below ${\mathit{T}}_{\mathit{c}}$.

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].

Measurements of the irreversibility line H(${\mathit{T}}^{\mathrm{*}}$) of ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.97}$, a system in which the superconducting transition ${\mathit{T}}_{\mathit{c}}$ can be varied from 0 to 92 K, show that H(${\mathit{T}}^{\mathrm{*}}$) obeys a universal scaling relation characterized by an m=3/2 power law near ${\mathit{T}}_{\mathit{c}}$, with a crossover to a more rapid temperature dependence below T/${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}0.6. The applicability of the scaling formula to other high ${\mathit{T}}_{\mathit{c}}$ superconductors is tested, with the results suggesting that both the crossover phenomenon and the power law temperature dependence are ubiquitous, scale-invariant properties of high temperature superconductors.

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.

We have measured the low temperature magnetic susceptibility of two series of f.c.c. FeCrNi alloys: one series contained varied amounts of interstitial carbon and nitrogen; the other contained varied amounts of manganese. Characteristic of antiferromagnets, all specimens displayed a distinct peak in magnetic susceptibility as a function of temperature. Above the transition temperature the behavior of all specimens could be well described by a modified Curie-Weiss law x= x o C T+θ In the first series the position of the susceptibility maximum decreased from 47.9 ± 0.5 to 35.0 ± 0.5 K as the carbon-plus-nitrogen content increased from 0.080 to 0.359 wt. %. In the second series the position of the susceptibility maximum increased from 46 ± 0.5 to 86.5 ± 0.5 K as the manganese content was increased from 1.02 to 5.80 wt.%. A linear relationship was found to predict the Neel temperature from the alloy composition with an r.m.s. deviation of 1 K.

This paper reports on the magnetic field dependence of the transport critical current and dc magnetic susceptibility measurements on YBa2Cu3O(7-x) superconductors formed by melt-solid reactions at 950 C between Ba-Cu-O (or Tb-Ba-cu-O) and solid nonstoichiometric Y-Ba-Cu oxide. Four-probe dc critical current measurements at 77, 64, and 4.2 K show strong depression of the critical current density with increasing magnetic field, in agreement with a model of weakly linked superconducting regions. Diamagnetic shielding and Meissner flux expulsion measurements in the temperature range 10-300 K show about one-third volume fraction of perfect superconductivity. Both shielding and flux expulsion were observed to be approximately temperature independent below 60 K, indicating strong coupling between the grains throughout the entire volume below this temperature.

The low-temperature (5 K T T N . The dc susceptibility, χ( T ), did not exhibit a simple Curie-Weiss dependence. Above T N , atemperature independent component χ 0 was observed, i.e., χ( T ) = χ 0 + C / (itT + θ. T N was systematically influenced by the lattice parameter, a , decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ B obtained from neutron scattering was lower than the ≈ 1 μ B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction ( J 1 ) and ferromagnetic second-nearest-neighbors interaction ( J 2 ) indicate that | J 2 / J 1 |≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.

In the Ising mean-field model of a spin-glass it is predicted that the onset of strong irreversibility and the susceptibility cusp coincide and that the critical exponent of the order parameter is unity. Both of these behaviors have been observed in our measurements on the alloy Fe65Mn21Cr14, near 280 K. In contrast, recently published measurements on Fe69Mn26Cr5 exibited two distinct changes of magnetic behavior below the transition temperature as predicted in the Heisenberg mean-field model. In view of these observations, we believe that these two materials represent the extreme cases of mean-field theory.

In our study of three YBa2 Cu3 oxides prepared with different oxygen anneals, we observed that the temperature derivative of the magnetization or susceptibility χ(T) of these high‐Tc superconductors has a maximum at approximately the transition temperature, Tc . This temperature as estimated by the peak value of (∂χ/∂T)H , ranged between 55 and 65 K. The most oxygen‐poor sample showed the lowest Tc and the most oxygen‐rich sample showed the highest Tc . In the subcritical region, a novel irreversibility is observed in the field‐cooled warm‐up and cool‐down behaviors.

We have shown that the power law behavior of H versus T*, attributed to a de Almeida-Thouless criticality in a high temperature superconductor, may be due to shielding effects and critical currents. We calculate a hitherto unrecognized scaling of T* with J0 and specimen size. Verification of these predictions will favor the conventional Type II model over the glass model for these systems.

The shape of the normalized temperature derivative of the dc susceptibility provides useful information about superconducting materials. We propose that the position of the derivative peak be defined as the transition temperature ${T}_{c}$ and the width of this peak define its sharpness. The second derivative also helps the analysis. This method of parametrization is easily generalizable for other temperature-dependent properties such as the resistivity.

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.

The shape of the normalized temperature derivative of the dc susceptibility provides useful information about superconducting materials. We propose that the position of the derivative peak be defined as the transition temperature Tc and the width of this peak define its sharpness.

We suggest a model---reentrant elastic softening---that achieves three useful results. First, and principally, it reconciles existing sound-velocity--elastic-constant measurements with thermodynamics. Second, it leads to Debye characteristic temperatures that agree with those from specific-heat and phonon density-of-states determinations. Third, it links elastic-constant--temperature behavior in Y-Ba-Cu-O and La-Sr-Cu-O. The model predicts a superconducting-state elastic stiffness lower than the normal state.

On the basis of our reentrant-softening model, sound-velocity results in La1.85Sr0.15CuO4 and YBa2Cu3O7−x are shown to be neither dissimilar nor inconsistent with thermodynamic requirements. In both materials, as temperature decreases, the lattice softens just above Tc. Below Tc, this softening is offset by increased stiffness associated with the developing superconducting phase. This model agrees with results from other physical-property measurements, and it predicts that elastic stiffness is higher in the normal state than in the superconducting state.

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.

For more than 40 years, there has been a continuous and heated debate in literature, with regard to the difficult relationship between the three components of the accounting profession: research, practice and teaching. With few exceptions, there is a large consensus of opinions that there is at least a gap between accounting research and practice, if not a schism. In bridging the gap, a better communication between the two parties seems to be the best solution and the professional bodies, the most proper moderator. Under these circumstances, starting from the identified results of previous research, we investigated the mutual relationships within the research-practice-teaching triangle, aiming at identifying and assessing the obstacles that determine and maintain the gap between the accounting academia and practice in the Western part of Romania. With regard to the research-practice line, our results were not significantly different from the ones of other authors. We found that the accounting profession in Romania is characterized by a disruption between the interests of academics and practitioners, determined by a lack of communication between the two parties. Our personal contribution to the debate consists however in a thorough analysis of the role of university education in narrowing the existing gap between academia and practice.

We have carried out dc magnetization (x = 0, and 0.05) and /sup 57/Fe Moessbauer spectroscopy (x = 0.05) measurements of Ho/sub 1/Ba/sub 2/(Cu/sub 1/minus/x/Fe/sub x/)/sub 3/O/sub 7/minus/y/ between room temperature (RT) and 4.2K. Based on x-ray diffraction results all samples are single phase, and either orthorhombic (x = 0) or tetragonal (x = 0.05). Magnetization measurements determined the onset T/sub c/ to be either 90K (orthorhombic) or 48K (tetragonal). Moessbauer spectra between RT and 20K are nearly identical, each consisting of three superposed quadrupole doublets. As the temperature is lowered below about 10K, the Moessbauer spectra exhibit paramagnetic relaxation. The relaxation time increases by about two orders of magnitude as the temperature decreases from 10K to 4.2K. No magnetic ordering of the Fe moments was observed. 9 refs., 4 figs.