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It is often claimed that among the strongest evidence for preformed-pair physics in the cuprates are the experimentally observed large values for the diamagnetic susceptibility and Nernst coefficient. These findings are most apparent in the underdoped regime, where a pseudogap is also evident. While the conventional (Gaussian) fluctuation picture has been applied to address these results, this preformed-pair approach omits the crucial effects of a pseudogap. In this paper we remedy this omission by computing the diamagnetic susceptibility and Nernst coefficient in the presence of a normal state gap. We find a large diamagnetic response for a range of temperatures much higher than the transition temperature. In particular, we report semi-quantitative agreement with the measured diamagnetic susceptibility onset temperatures, over the entire range of hole dopings. Notable is the fact that at the lower critical doping of the superconducting dome, where the transition temperature vanishes and the pseudogap onset temperature remains large, the onset temperature for both diamagnetic and transverse thermoelectric transport coefficients tends to zero. Due to the importance attributed to the cuprate diamagnetic susceptibility and Nernst coefficient, this work helps to clarify the extent to which pairing fluctuations are a component of the cuprate pseudogap.

The effect of fluctuations on the nuclear magnetic resonance (NMR) relaxation rate, $W$, is studied in a complete phase diagram of a 2D superconductor above the upper critical field line $H_{c2}(T)$ . In the region of relatively high temperatures and low magnetic fields, the relaxation rate $W$ is determined by two competing effects. The first one is its decrease in result of suppression of quasi-particle density of states (DOS) due to formation of fluctuation Cooper pairs (FCP). The second one is a specific, purely quantum, relaxation process of the Maki-Thompson (MT) type, which for low field leads to an increase of the relaxation rate. The latter describes particular fluctuation processes involving self-pairing of a single electron on self-intersecting trajectories of a size up to phase-breaking length $l_{\phi }$ which becomes possible due to an electron spin-flip scattering event at a nucleus. As a result, different scenarios with either growth or decrease of the NMR relaxation rate are possible upon approaching the normal metal - type-II superconductor transition. The character of fluctuations changes along the line $H_{c2}$ from the thermal long-wavelength type in weak magnetic fields to the clusters of rotating FCP in fields comparable to $H_{c2}$. We find that below the well-defined temperature $T^*_0\approx 0.6T_{c0}$, the MT process becomes ineffective even in absence of intrinsic pair-breaking. The small scale of FCP rotations ($\xi_{xy}$) in so high fields impedes formation of long (, Comment: 12 pages, 7 figures

Thermoelectric energy conversion is a direct but low-efficiency process, which precludes the development of long-awaited wide-scale applications. As a breakthrough permitting a drastic performance increase is seemingly out of reach, we fully reconsider the problem of thermoelectric coupling enhancement. The corner stone of our approach is the observation that heat engines are particularly efficient when their operation involves a phase transition of their working fluid. We derive and compute the thermoelastic coefficients of various systems, including Bose and Fermi gases, and fluctuation Cooper pairs. Combination of these coefficients yields the definition of the thermodynamic figure of merit, the divergence of which at $finite$ temperature indicates that conditions are fulfilled for the best possible use of the thermoelectric working fluid. Here, this situation occurs in the fluctuation regime only, as a consequence of the increased compressibility of the working fluid near its phase transition. Our results and analysis clearly show that efforts in the field of thermoelectricity can now be productively directed towards systems where electronic phase transitions are possible.

The electrical transport properties of $[{\mathrm{BaCuO}}_{x}{]}_{2}/[{\mathrm{CaCuO}}_{2}{]}_{n}(\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}n)$ underdoped high-temperature superconducting superlattices grown by pulsed laser deposition have been investigated. Starting from the optimally doped $\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}2$ superlattice, having three ${\mathrm{CuO}}_{2}$ planes and ${T}_{c}$ around 80 K, we have systematically increased the number n up to 15 moving toward the underdoped region and hence decreasing ${T}_{c}.$ For $ng11$ the artificial structures are no longer superconducting, as expected, for a uniformly distributed charge carrier density inside the conducting block layer. The sheet resistance of such artificial structures $(n\ensuremath{\approx}11)$ turns out to be quite temperature independent and close to the two-dimensional quantum resistance 26 k\ensuremath{\Omega}. A further increase of the number of ${\mathrm{CuO}}_{2}$ planes results in an insulator-type dependence of $R(T)$ in the wide range of temperatures from room temperature to 1 K. The value of the sheet resistance separating the superconducting and the insulating regimes supports the fermionic scenario of the superconductor-insulator transition in these systems.

Magnetization and ${}^{63}\mathrm{Cu}$ nuclear magnetic resonance-nuclear quadrupole resonance relaxation measurements are used to study the superconducting fluctuations in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (YBCO) oriented powders. In optimally doped YBCO the fluctuating negative magnetization ${M}_{\mathrm{fl}}(H,T)$ is rather well described by an anisotropic Ginzburg-Landau (GL) functional and the curves ${M}_{\mathrm{fl}}/\sqrt{H}$ cross at ${T}_{c}.$ In underdoped YBCO, instead, over a wide temperature range an anomalous diamagnetism is observed, stronger than in the optimally doped compound by about an order of magnitude. The field and temperature dependence of ${M}_{\mathrm{fl}}$ cannot be described either by an anisotropic GL functional or on the basis of scaling arguments. The anomalous diamagnetism is more pronounced in samples with a defined order in the Cu(1)O chains. The ${}^{63}\mathrm{Cu}(2)$ relaxation rate shows little, if any, field dependence in the vicinity of the transition temperature ${T}_{c}(H=0).$ It is argued how the results in the underdoped compounds can be accounted for by the presence of charge inhomogeneities, favored by chains ordering.

The analysis of tunneling experiments showing the pseudogap-type behavior is carried out based on the idea of the renormalization of density of states due to the interelectron interaction in the Cooper channel (superconducting fluctuations contribution in tunneling current). It is demonstrated that the observed kink of the zero-bias conductance $G(0,T)$ of Y-Ba-Cu-O/Pb junctions in the vicinity of ${T}_{c}$ can be explained in terms of fluctuation theory in a quite wide range of temperatures above ${T}_{c},$ using the values of microscopic parameters of the Y-Ba-Cu-O electron spectrum taken from independent experiments. The approach proposed also permits to explain qualitatively the shape of the tunneling anomalies in $G(V,T)$ and gives a correct estimate for the pseudogap position and amplitude observed in the experiments on Bi-Sr-Ca-Cu-O junctions.

${(\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{u}\mathrm{O}}_{2}{)}_{2}{/(\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{u}\mathrm{O}}_{2}{)}_{n}$ superconducting superlattices have been grown by pulsed laser deposition for $n$ ranging from 1 to 6. The critical temperature was found to depend strongly on the growth conditions. A maximum ${T}_{c}$ value of about 80 K (zero resistance temperature) was found for superlattices grown at high oxygen pressure ${(P}_{{\mathrm{O}}_{2}}\ensuremath{\simeq}0.8\mathrm{mbar})$ and relatively high temperature ${(T}_{g}\ensuremath{\simeq}600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}).$ The dependence of ${T}_{c}$ on the number $n$ of ${\mathrm{CaCuO}}_{2}$ layers (for superlattices grown exactly in the same conditions) was investigated. ${T}_{c}$ versus $n$ showed the expected behavior with a maximum ${T}_{c}$ value of 80 K for $n=2--3.$ However, such value is somewhat lower than that expected for a cuprate structure containing 3--4 ${\mathrm{CuO}}_{2}$ planes and having an optimum carrier concentration. Two possible explanations for this effect can be envisaged. The first one deals with the chemistry of the ``charge reservoir'' block, the second one with the special role of interelectron interaction in these highly disordered artificial structures. Finally, it is shown that, taking into account the degree of disorder, a shift of ${T}_{c}$ of about 20 K relative to the bulk value ${T}_{{c}_{0}},$ consistent with the microscopic parameters of these materials, can be foreseen.

Evidence is presented of superconducting fluctuations in the {sup 63}Cu NQR-NMR relaxation rate in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, as obtained from a careful comparison of measurements carried out in the absence and in the presence of a field parallel to the {ital c} axis. It is shown that the field causes a reduction of the relaxation rate {ital W} in a range of about 10 K above {ital T}{sub {ital c}}. This effect is related to the suppression by the magnetic field of the phase-sensitive positive Maki-Thompson contribution while the negative contribution from the DOS fluctuations is almost field independent. Furthermore, it is argued how the fluctuation effects on {ital W} can be used to discuss the pairing state symmetry, at variance with the insensitivity of the transport measurements. It is pointed out that the existence of the Maki-Thompson contribution to {ital W} evidences an {ital s}-wave symmetry component for the pairing in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. {copyright} {ital 1996 The American Physical Society.}

The measurements of the out-of-plane component of resistivity {rho}{sub {ital c}}({ital T},{ital B}) in magnetic field in high-quality epitaxial Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{ital x}} films are presented. We found that {rho}{sub {ital c}}({ital T},{ital B}) exhibits a well-pronounced peak at the edge of the superconducting transition. Magnetic field causes both a sizeable increase of the relative magnitude of the peak and a strong shift of its position to lower temperatures. We demonstrate that zero-field data and data for weak magnetic fields {ital B}{le}1 T are well described within the fluctuation theory taking into account Gaussian approximation in fluctuation behavior. Quantitative fit of the data in the framework of this theory allowed us to extract the effective anisotropy parameter, the quasiparticle relaxation time and the phase pair-breaking relaxation time. We discuss the reasons of the discrepancy between theory and experiment for stronger magnetic fields. {copyright} {ital 1996 The American Physical Society.}

Current-voltage characteristics of 2212-phase epitaxial ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$ films have been measured in a temperature region about 8 K wide immediately below ${\mathit{T}}_{\mathit{c}}$. It is shown that the power law V=A ${\mathit{I}}^{\mathit{a}}$ valid for two-dimensional (2D) systems does not correctly describe the data, and interlayer coupling must be taken into account. The 2D approximation still remains valid at high currents, so that a crossover from 2D to 3D behavior is observed as the current is reduced. Furthermore, annealing treatments in redox atmospheres change the interlayer coupling, therefore leading to an increase of the critical current density and a decrease of the anisotropy factor with increasing oxygen content in the film. These effects can be quantitatively evaluated fitting the I-V curves with a theory due to Jensen and Minnhagen valid for quasi-2D systems. Not too far from ${\mathit{T}}_{\mathit{c}}$, however, an additional dissipation appears besides the Jensen-Minnhagen contribution in the low-voltage region. We propose to explain this in terms of fluctuations of the phase of the order parameter. The calculated effect is in reasonable agreement with the experimental data. As the oxygen content in the sample is varied the relative weight of the two contributions changes in qualitative agreement with the proposed fluctuation origin of the additional dissipation at low voltages.

We revise the long-studied problem of fluctuation conductivity (FC) in disordered two-dimensional superconductors placed in a perpendicular magnetic field by finally deriving the complete solution in the temperature-magnetic field phase diagram. The obtained expressions allow both to perform straightforward (numerical) calculation of the FC surface $\ensuremath{\delta}{\ensuremath{\sigma}}_{xx}^{(\mathrm{tot})}(T,H)$ and to get asymptotic expressions in all its qualitatively different domains. This surface becomes in particular nontrivial at low temperatures, where it is trough-shaped with $\ensuremath{\delta}{\ensuremath{\sigma}}_{xx}^{(\mathrm{tot})}(T,H)l0$. In this region, close to the quantum-phase transition, $\ensuremath{\delta}{\ensuremath{\sigma}}_{xx}^{(\mathrm{tot})}(T,H=\mathrm{const})$ is nonmonotonic, in agreement with experimental findings. We reanalyzed and present comparisons to several experimental measurements. Based on our results we derive a qualitative picture of superconducting fluctuations close to ${H}_{\mathrm{c}2}(0)$ and $T=0$ where fluctuation Cooper pairs rotate with cyclotron frequency ${\ensuremath{\omega}}_{c}\ensuremath{\sim}{\ensuremath{\Delta}}_{\mathrm{BCS}}^{\ensuremath{-}1}$ and Larmor radius $\ensuremath{\sim}$${\ensuremath{\xi}}_{\mathrm{BCS}}$, forming some kind of quantum liquid with long coherence length ${\ensuremath{\xi}}_{\mathit{QF}}\ensuremath{\gg}{\ensuremath{\xi}}_{\mathrm{BCS}}$ and slow relaxation (${\ensuremath{\tau}}_{\mathit{QF}}\ensuremath{\gg}\ensuremath{\hbar}{\ensuremath{\Delta}}_{\mathrm{BCS}}^{\ensuremath{-}1}$).

The observation of a large Nernst signal in cuprates above the superconducting transition temperature has attracted much attention. A potential explanation is that it originates from superconducting fluctuations. Although the Nernst signal is indeed consistent with Gaussian fluctuations for overdoped cuprates, Gaussian theory fails to describe the temperature dependence seen for underdoped cuprates. Here, we consider the vertex correction to Gaussian theory resulting from the pseudogap. This yields a Nernst signal in good agreement with the data.

Superconducting fluctuations in strongly anisotropic planar superconductors substantially affect the resistivity above the transition temperature. In a wide temperature range above T(c), the main effect of these fluctuations is to create a virtual gap in the electronic spectrum, causing an increase in resistivity. As a result the resistivity shows a maximum close to the transition temperature. Closer to the transition temperature the correction to the resistivity becomes negative and grows.as 1/(T - T(c))2, WhiCh is followed by the usual (T - T(c))1/2 law still closer to the transition temperature. This observation may help to understand the unusual behavior of the resistivity in the c direction in high-T(c) cuprates.

The experimentally observed increase of the transversal resistance near the edge of the transition in Bi 2 Sr 2 CaCu 2 O 8+δ epitaxial film is analyzed on the basis of a recent theoretical proposal on its fluctuation origin [L. Ioffe, A. I. Larkin, A. A. Varlamov, and L. Yu, Phys. Rev. B (to be published). It is shown that the excess resistance in a wide range of temperatures increases logarithmically as the superconducting transition temperature is approached from above, in agreement with the theoretical consideration. These results are also compared with the available data on YBa 2 Cu 3 O 7 compounds

The problem of a correct definition of the charged carrier effective mass in superfluid helium is revised. It is shown that the effective mass of such a quasiparticle can be introduced without Atkins's idea about the solidification of liquid He{sup 4} in the close vicinity of an ion (the so-called ''snowball'' model). Moreover, in addition to the generalization of Atkins's model, the charged carrier effective mass formation is considered within the framework of the two-fluid scenario. The physical reasons of the normal-fluid contribution divergency and the way of the corresponding regularization procedure are discussed. Agreement between the theory and the available experimental data is found in a wide range of temperatures.

A theory of fluctuations in two-band superconductor ${\mathrm{MgB}}_{2}$ is developed. Since the standard Ginzburg-Landau (GL) approach fails in description of its properties, we generalize it based on the microscopic theory of a two-band superconductor. Calculating the microscopic fluctuation propagator, we build up the nonlocal two-band GL functional and the corresponding time-dependent GL equations. This allows us to calculate the main fluctuation observables such as fluctuation specific heat and conductivity.

The weak localization correction to the conductivity of a granular metal is calculated using the diagrammatic technique in the reciprocal grain lattice representation. The properties of this correction are very similar to that one in disordered metal, with the replacement of the electron mean free path $\ell $ by the grain diameter $d$ and the dimensionless conductance $g$ by the tunnelling dimensionless conductance $g_{T}$. In particular, we demonstrate that at zero temperature no conducting phase can exist for dimensions $D\leq 2$. We also analyze the WL correction to magnetoconductivity in the weak field limit., Comment: 4 pages, 3 figures; minor corrections added

High-resolution isothermal magnetization curves above the transition temperature ${T}_{c}$ in a single crystal of superconducting underdoped cuprate ${\mathrm{La}}_{1.9}{\mathrm{Sr}}_{0.1}{\mathrm{CuO}}_{4}$ ${(T}_{c}=26\mathrm{K})$ are reported. For magnetic field $Hg~1\mathrm{kOe}$ the diamagnetic magnetization is rather well described by the Ginzburg-Landau theory in finite fields, as observed in previous works. On the contrary, in the low-field range and for T within ${T}_{c}+2\mathrm{K},$ evidence is given for precursor diamagnetism of different character, with magnetization curves displaying an upturn in the field dependence and magnetic-history dependent effects below $T\ensuremath{\simeq}27\mathrm{K}.$ These findings are the magnetic counterpart of the observation by scanning superconducting quantum interference device microscopy of regions precursor of bulk superconductivity. The interpretation of the experimental data in terms of phase fluctuations of local order parameter is shown to account for most of the aspects of the precursor diamagnetism, which appears to be a general phenomenon in cuprates.

Within the time-dependent Ginzburg-Landau theory, the effects of the superconducting fluctuations on the transport properties above the critical temperature are characterized by a nonzero imaginary part of the relaxation rate y of the order parameter. Here, we evaluate Im y for an anisotropic dispersion relation typical of the high-T c cuprate superconductors (HTS's), characterized by a proximity to an electronic topological transition (ETT). We find that Im y abruptly changes sign at the ETT as a function of doping, in agreement with the universal behavior of the HTS's. We also find that an increase of the in-plane anisotropy, as is given by a nonzero value of the next-nearest to nearest hopping ratio r=t'/t, increases the value of ‖Im γ‖ close to the ETT, as well as its singular behavior at low temperature, therefore enhancing the effect of superconducting fluctuations. Such a result is in qualitative agreement with the available data for the excess Hall conductivity for several cuprates and cuprate superlattices.

Superconduting quantum interference device magnetization measurements in oriented powders of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y},$ with x ranging from $0$ to $0.2,$ for $y\ensuremath{\approx}6.1$ and 6.97, are performed in order to study the doping dependence of the fluctuating diamagnetism above the superconducting transition temperature ${T}_{c}.$ While for optimally doped compounds the diamagnetic susceptibility and the magnetization curves $\ensuremath{-}{M}_{\mathrm{fl}}(T=\mathrm{const})$ vs H are rather well justified on the basis of an anisotropic Ginzburg-Landau (GL) functional, in underdoped and overdoped regimes an anomalous diamagnetism is observed, with a large enhancement with respect to the GL scenario. Furthermore, the shape of magnetization curves differs strongly from the one derived in that scheme. The anomalies are discussed in terms of phase fluctuations of the order parameter in a layered system of vortices and with the assumption of charge inhomogeneities inducing local, non-percolating, superconducting regions with ${T}_{c}^{(\mathrm{loc})}$ higher than the resistive transition temperature ${T}_{c}.$ The susceptibility displays an activated temperature behavior, a marked characteristic of the vortex-antivortex description, while the history-dependent magnetization, with relaxation after zero-field cooling, is consistent with the hypothesis of superconducting droplets in the normal state. Thus the theoretical picture consistently accounts for most experimental findings.