Fluctuation-induced Diamagnetism Research Articles

Measurement of fluctuation-induced diamagnetism in BSCCO-2212 single crystals using magneto-optics

Fluctuation-induced diamagnetism in BSCCO single crystals with different doping levels (i.e., different oxygen stoichiometry) was measured using magneto-optics (MO) in a wide frequency range up to several MHz. A shift in Tc onset, measured by MO, of up to 6K towards higher temperatures was observed at high frequencies in those samples which are far from the optimally doped condition. An explanation of the observed effect in terms of the phase fluctuations of the superconducting order parameter is proposed.

Superconducting Fluctuating Diamagnetism Versus Precursor Diamagnetism in Heterogeneous Superconductors

The fluctuation-induced diamagnetism (FD) above the transition temperature Tc is studied through the isothermal field-dependence of the diamagnetic magnetization Mdia. While for T ≈ Tc and H→0 one has –Mdia ∝ (H)1/2,on increasing field an upturn occurs and \(|M_{{\rm dia}} |\) begins to decrease, in correspondence to field-induced quenching of the fluctuating pairs. For BCS superconductors (SC) as MgB2 the upturn field Hup is in the range 100–800 Oe, while in optimally doped high-temperature SC(HTcSC) Hup is expected in the range of several Tesla, because of the small coherence length. At variance, in non-homogeneous HTcSC (under- or overdoped) or in the presence of impurities causing diffuse transition, strong increase of Mdia and Hup reduced by order of magnitudes are observed. The reasons of these behaviors are discussed in the report. In particular the difference between the FD in heterogeneous HTcSC (related to SF) and the precursor diamagnetism for diffuse transitions such as in Al-doped MgB2 (unrelated to SF) is emphasized and it is shown how the temperature dependence of Hup discriminates between the two effects.

Fluctuation induced diamagnetism in La 1.9Sr 0.1CuO 4 superconductors well inside the high-reduced temperature and the finite magnetic field regimes

By using two grain oriented La 1.9Sr 0.1CuO 4 (LaSCO) samples, with masses as big as 48 and 110 mg, the fluctuation induced diamagnetism (FD) above the superconducting transition temperature, T c0, has been measured in the reduced temperature region bounded by 3×10 −2≲ ϵ≲0.5 and under reduced magnetic fields to within 8×10 −3≲ h≲0.2. Here ϵ≡ln( T/ T c0) and h≡ H/ H c2(0), where T c0 and H c2(0) are, respectively, the mean field normal–superconducting transition temperature at zero applied magnetic field and the upper critical magnetic field amplitude. These measurements deeply penetrate, then, in both the Schmidt and Schmid (or zero magnetic field)-limit ( h/ ϵ≪1) and the Prange regime ( h/ ϵ≳1). For the first time in a high-temperature cuprate superconductor (HTSC), these FD data in both fluctuation regimes cover all the reduced temperatures not too close to T co, including the high-reduced temperature region ( ϵ≳0.1), where the contribution of the short-wavelength fluctuations may be particularly important. Then, the existing FD calculations on the grounds of the Gaussian–Ginzburg–Landau (GGL) approach for 2D-layered superconductors are extended to the short-wavelength regime by introducing two possible cutoff conditions: The conventional kinetic energy (or momentum) cutoff and the total energy cutoff, this last taking also into account the quantum localization energy contribution associated with the shrinkage of the superconducting wavefunction. The differences between both cutoff conditions are expected to be appreciable mainly in the high-reduced temperature region, where the superconducting coherence length, ξ( T), becomes of the order of ξ(0), the coherence length extrapolated to T=0 K. The analyses of our experimental data on the grounds of these phenomenological approaches show that the different FD regimes may be explained in terms of the GGL theory for layered superconductors only under a total energy cutoff. In contrast, when regularized through a momentum cutoff condition the GGL approach fails to explain the high-reduced temperature region in both fluctuation regimes (Schmidt and Schmid-limit and Prange regime). The corresponding cutoff amplitude is found to be of the order of 0.6, in excellent agreement with our previous FD results obtained in the finite magnetic field regime but at low-reduced temperatures ( ϵ≲0.1). This cutoff amplitude is defined in units of ℏ 2/2m ∗ξ 2 ab(0) , where m ∗ is the effective mass of the superconducting carriers, ℏ is the reduced Planck constant and ξ ab (0) is the in-plane superconducting coherence length amplitude. These results strongly support, then, the adequacy of the mean field like GGL approach, regularized through a total energy cutoff, to explain the thermal fluctuations in HTSC at all the temperatures not too close to T c0 (above ϵ≃3×10 −2), including the high-reduced temperature region (above typically ϵ≃0.1). These results also show that, in contrast with the low-reduced temperature superconductors (LTSC), due to the shortness of their superconducting coherence length amplitudes the FD in the HTSC is not appreciably affected by non-local electrodynamic effects, at least for reduced magnetic fields up to 0.2. In addition, our present findings suggest that the presence of a normal-state pseudogap in underdoped LaSCO superconductors does not appreciably affect the behaviour of the fluctuation induced diamagnetism in all the accessible reduced temperatures above the superconducting transition.

Fluctuation-induced diamagnetism above the superconducting transition inMgB2

The behavior of the superconducting fluctuations in the normal state in ${\mathrm{MgB}}_{2}$ is studied through measurements of the fluctuation-induced diamagnetism (FD). These experiments show that the superconducting fluctuations in this compound are three dimensional in nature. Moreover, for reduced magnetic fields up to 0.5 the FD is not appreciably affected by nonlocal electrodynamic effects. This last FD behavior is in striking contrast with the one always observed, at equivalent reduced magnetic fields, in all other clean low-temperature metallic superconductors studied until now, but it is similar to the one found in cuprate superconductors. A sharp decrease of the fluctuation effects is observed around $\ensuremath{\varepsilon}=\mathrm{ln}{(T/T}_{C})\ensuremath{\approx}0.6,$ in agreement with recent results obtained in other low-${T}_{C}$ and high-${T}_{C}$ superconductors. This last result confirms the need of a total-energy cutoff condition to extend the applicability of the Gaussian-Ginzburg-Landau approach to the high reduced-temperature region.

Fluctuation-induced diamagnetism in bulk isotropicsuperconductors at high reduced temperatures

The short-wavelength effects on the fluctuation-induced diamagnetism(FD) in bulk isotropic three-dimensional (3D) superconductors aretaken into account by introducing in the Gaussian-Ginzburg-Landauapproach different cut-off conditions. These calculations, whichextend to the 3D case our previous results on layeredsuperconductors, are then used to briefly analyse the FD datameasured for the low-temperature superconducting alloyPb-8 at.% In. These analyses confirm the adequacy of atotal-energy cut-off for explaining, for low-temperature 3Dsuperconductors also, the thermal fluctuation effects in thehigh-reduced-temperature region. These results thus provide furthersupport to the recent proposal that, due to the localization energy,the size of the effective fluctuations cannot be appreciably smallerthan the superconducting coherence length amplitude extrapolated toT = 0 K.

First observation for a cuprate superconductor of fluctuation-induced diamagnetism well inside the finite-magnetic-field regime

For the first time for a cuprate superconductor, measurements performed above T(c) in high quality grain aligned La1.9Sr0.1CuO4 samples have allowed the observation of the thermal fluctuation induced diamagnetism well inside the finite-magnetic-field fluctuation regime. These results may be explained in terms of the Gaussian Ginzburg-Landau approach for layered superconductors, but only if the finite field contributions are estimated by taking off the short-wavelength fluctuations.

High-field magnetization ofHgBa2Ca2Cu3O8+δ:Fluctuations, scaling, and the crossing point

Fluctuation-induced diamagnetism is observed well above ${T}_{c}$ in high-field torque magnetization measurements on the cuprate superconductor ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{8+\ensuremath{\delta}}.$ Universal scaling behavior anticipated by fluctuation theories is not observed in the high-field, high-temperature regime. A crossing point, where M reaches a field-independent value at a temperature which characterizes the interlayer separation, disappears in the fluctuation regime above $\ensuremath{\sim}{H}_{c2},$ suggesting that deviations from the lowest Landau level description of Ginzburg-Landau theory appear at high fields.

Fluctuation-induced diamagnetism below the critical temperature in high-Tcsuperconductors

We have calculated the fluctuation-induced diamagnetism (\ensuremath{\Delta}\ensuremath{\chi}) of the high-${T}_{c}$ superconductors below the critical temperature ${T}_{c}$ and in the weak-field region. The theoretical formulation estimates the relation between \ensuremath{\Delta}\ensuremath{\chi} and the reduced temperature.

In-plane paraconductivity and fluctuation-induced magnetoconductivity in biperiodic layered superconductors: Application to YBa2Cu3O7- delta.

The existing mean-field-like calculations of the different direct and indirect order parameter fluctuation (OPF) contributions to the in-plane paraconductivity \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{ab}}$ and fluctuation-induced magnetoconductivity \ensuremath{\Delta}\ensuremath{\sigma}\ifmmode \tilde{}\else \~{}\fi{}${\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}}_{\mathit{ab}}$ are extended here to layered superconductors with two different interlayer distances and different strengths of the tunneling couplings between adjacent layers (the so-called bilayered, or biperiodic layered, superconductors). The calculations are performed for magnetic fields H in the weak limit, applied perpendicular to the superconducting layers, and at temperatures near but above the H=0 mean-field transition temperature, ${\mathit{T}}_{\mathit{c}0}$. We obtain final explicit expressions and find that the effects of the layer biperiodicity may be summarized through an effective number ${\mathit{N}}_{\mathit{e}}$ of independent fluctuating superconducting layers per unit cell length, already encountered also in our recent calculations of the fluctuation-induced diamagnetism \ensuremath{\Delta}${\mathrm{\ensuremath{\chi}}}_{\mathit{ab}}$ in biperiodic layered superconductors. Our study includes some limiting cases of the indirect contributions associated with the density of states (DOS) fluctuations, which have been recently proposed for \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{ab}}$ and \ensuremath{\Delta}\ensuremath{\sigma}\ifmmode \tilde{}\else \~{}\fi{}${\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}}_{\mathit{ab}}$ for single periodic layered superconductors. As an application, we use then our theoretical results to analyze the paraconductivity and the fluctuation-induced magnetoconductivity recently measured in the a direction (nonaffected by the presence of CuO chains) of untwinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals. This analysis shows that the approaches based on the conventional Lawrence-Doniach (i.e., single layered) model cannot explain simultaneously and quantitatively the intrinsic \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{a}}$, \ensuremath{\Delta}\ensuremath{\sigma}\ifmmode \tilde{}\else \~{}\fi{}${\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}}_{\mathit{a}}$, and \ensuremath{\Delta}${\mathrm{\ensuremath{\chi}}}_{\mathit{ab}}$ in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals, even when the DOS contributions are considered. In contrast, when the double periodicity of this layered superconductor is taken into account, it is possible to explain consistently and at a quantitative level all such experimental data in the reduced temperature region above ${\mathit{T}}_{\mathit{c}0}$ bounded by, approximately, 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ and ${10}^{\mathrm{\ensuremath{-}}1}$, which is expected to correspond to the mean-field region without high temperature and nonlocal effects. In the resulting biperiodic scenario, the indirect (i.e., Maki-Thompson and DOS) contributions to the in-plane \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{a}}$ and \ensuremath{\Delta}\ensuremath{\sigma}\ifmmode \tilde{}\else \~{}\fi{}${\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}}_{\mathit{a}}$ are negligible, confirming our earlier findings which suggested unconventional, pair-breaking, wave pairing. Moreover, the direct OPF effects have a dimensionality (two-dimensional--three-dimensional) crossover in the mean-field region. Our results strongly suggest also that to understand at a quantitative level the OPF effects on any in-plane or bulk physical observable in layered superconductors with various superconducting layers per unit cell length, it is crucial to take into account the influence of such a multiperiodicity. \textcopyright{} 1996 The American Physical Society.

Fluctuation-induced diamagnetism in biperiodic layered superconductors in the weak magnetic field regime

By using a generalization of the Lawrence-Doniach free-energy functional, we calculate in this work the fluctuation-induced diamagnetism above the superconducting transition, Δ χ, in a layered superconductor having two different interlayer distances, d 1 and d 2, in the superconducting layer periodicity length, s= d 1+ d 2, and two different Josephson-coupling strengths, γ 1 and γ 2, between adjacent superconducting layers. The calculations are performed in the weak magnetic field limit. We include in our treatment the possibility of an unconventional superconducting pairing by considering in each plane a superconducting wave function of g complex components. For the first time, Δ χ in a multiperiodic layered superconductor is obtained explicitly, as a function of the coherence length amplitudes, of g and of γ 1 and γ 2. Our results show that by introducing an effective number, N e, of independent fluctuating layers in the layer periodicity length, it is possible to express Δ χ in terms of the fluctuation-induced diamagnetism for one-single periodicity layered superconductor, with the same periodicity length s= d 1+ d 2. As an example of their interest, these theoretical results for the superconducting-order parameter fluctuations above T c are used to quantitatively analyze the existing experimental Δ χ( T) data in the mean-field0like region, measured in the weak magnetic field limit in single crystals of two different copper oxide systems having two Josephson coupled superconducting CuO 2 planes in s: The Y 1 Ba 2 Cu 3 O 7− δ and the Bi 2Sr 2Ca 1Cu 2O 8 compounds.

Measurements of fluctuation-induced diamagnetism above the superconducting transition in YBa2Cu3O7- delta single crystals in low magnetic fields: Comparison with paraconductivity.

We report detailed experimental results on the magnetic susceptibility of high-quality ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals for the applied magnetic field H parallel, ${\mathrm{\ensuremath{\chi}}}_{\mathit{a}\mathit{b}}$, and perpendicular, ${\mathrm{\ensuremath{\chi}}}_{\mathit{c}}$, to the c direction. The data were obtained from 5 to 300 K, and with ${\mathrm{\ensuremath{\mu}}}_{0}$H\ensuremath{\le}0.6 T, for which ${\mathrm{\ensuremath{\chi}}}_{\mathit{a}\mathit{b}}$ and ${\mathrm{\ensuremath{\chi}}}_{\mathit{c}}$ above the superconducting transition correspond to the so-called weak magnetic-field limit, even for reduced temperatures, \ensuremath{\varepsilon}\ensuremath{\equiv}(T-${\mathit{T}}_{\mathit{C}\mathit{O}}$)/${\mathit{T}}_{\mathit{C}\mathit{O}}$, of the order of \ensuremath{\varepsilon}=2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$, where ${\mathit{T}}_{\mathit{C}\mathit{O}}$ is the mean-field-like normal-superconducting transition temperature. For T>${\mathit{T}}_{\mathit{C}\mathit{O}}$, ${\mathrm{\ensuremath{\chi}}}_{\mathit{c}}$ is found to be temperature independent and its average value for the samples measured here is (2.7\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$. In contrast, ${\mathrm{\ensuremath{\chi}}}_{\mathit{a}\mathit{b}}$(T) is found to be temperature independent, and equal to (5.2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$, only above T-${\mathit{T}}_{\mathit{C}\mathit{O}}$\ensuremath{\approxeq}25 K and up to the highest temperature studied here (300 K). Between ${\mathit{T}}_{\mathit{C}\mathit{O}}$ and T-${\mathit{T}}_{\mathit{C}\mathit{O}}$\ensuremath{\approxeq}25 K, ${\mathrm{\ensuremath{\chi}}}_{\mathit{a}\mathit{b}}$(T) is appreciably rounded. The excess diamagnetism above but near ${\mathit{T}}_{\mathit{C}\mathit{O}}$, for H parallel to the c axis, \ensuremath{\Delta}${\mathrm{\ensuremath{\chi}}}_{\mathit{a}\mathit{b}}$, is analyzed in terms of the existing approaches for independent Gaussian fluctuations of the superconducting-order-parameter amplitude in layered materials and also compared with the paraconductivity measured in the same samples.The so-called mean-field-like region will correspond to, approximately, ${10}^{\mathrm{\ensuremath{-}}2}$\ensuremath{\lesssim}\ensuremath{\varepsilon}\ensuremath{\lesssim}0.15. Beyond the lower \ensuremath{\varepsilon} limit, closer to ${\mathit{T}}_{\mathit{C}\mathit{O}}$, both the paraconductivity and \ensuremath{\Delta}${\mathrm{\ensuremath{\chi}}}_{\mathit{a}\mathit{b}}$(\ensuremath{\varepsilon}) separate from the mean-field region behavior. Although so close to the transition we cannot exclude the influence of small stoichiometric inhomogeneities, these data for \ensuremath{\varepsilon}\ensuremath{\lesssim}${10}^{\mathrm{\ensuremath{-}}2}$ may be explained on the grounds of the three-dimensional XY model, with a critical exponent of x=-1/3 for the paraconductivity and x=-2/3 for the fluctuation-induced diamagnetism. A scenario compatible with the experimental results is characterized by the values of the correlation length amplitude in the ab plane and in the c direction of, respectively, ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0)=(1.1\ifmmode\pm\else\textpm\fi{}0.2) nm and ${\ensuremath{\xi}}_{\mathit{c}}$(0)=(0.12\ifmmode\pm\else\textpm\fi{}0.02) nm, two Josephson coupled superconducting planes in the unit-cell length, and a complex order parameter with two real components, i.e., conventional $^{1}$${\mathit{s}}_{0}$-wave pairing or one-complex-component unconventional pairing.

Measurements of the fluctuation-induced magnetoconductivity in the a-direction of an untwinned Y 1Ba 2Cu 3O 7−δ single crystal in the weak magnetic field limit

Measurements of the fluctuation-induced magnetoconductivity in the a-direction of an untwinned Y 1Ba 2Cu 3O 7−δ single crystal, not affected by the presence of CuO chains, are presented. The measurements were performed with the applied magnetic field, H, perpendicular to the ab-planes, and with H≤1 T, which corresponds to the weak magnetic field limit even for reduced temperatures, ϵ, of the order of 10 −2. In the mean-field region, i.e., for 10 −2⪅ϵ⪅10 −1, these data may be explained in terms of direct thermal order-parameter fluctuations, in agreement with our previous paraconductivity and fluctuation-induced dia-magnetism measurements in the same sample. These results suggest that the pair-breaking parameter, δ, is less than 5×10 −2 at T=100 K. This upper limit not only leads to the absence of appreciable Maki-Thompson effects in cooper oxide superconductors (HTSC's), but also has implications in other central open HTSC problems. By using the BCS-like expressions to relate δ and the Cooper-pair relaxation time, τ φ, we obtain, at T=100 K, τ φ⪅10 -−15 s in the clean limit and τφ⪅10 −14 s in the dirty limit. This last value is comparable with the expected electronic-scattering time.

Measurements of the paraconductivity in the a-direction of untwinned Y 1Ba 2Cu 3O 7−δ single crystals

We report detailed experimental results of the paraconductivity in the a-direction, Δσ a , of two high-quality Y 1 ba 2 Cu 3 O 7- δ single crystals having a large (∼ 0.3 mm 2) untwinned region in their centers, and with transition temperatures of 90.0 K and 90.8 K. The exceptional sharpness of their resistive transitions (their upper half widths are less than 0.15 K) allows us to obtain quantitative information of Δσ a (ϵ), which is not affected by the CuO chains, up to reduced temperatures, ϵ, of the order of 10 -3. The general scenario compatible with our experiments consists of the assumption of (1): A mean field-like region (MFR) up to ϵ≈2×10 -2, where Δσ a (ϵ) may be explained in terms of the Lawrence-Doniach-like (LD) approach by taking into account the existence of two Josephson-coupled CuO 2 superconducting planes per unit cell. The characteristic superconducting length amplitudes in this MFR are ς ab (0) = (1.1±0.2) nm, and ς c (0) = (0.12±0.02) nm. (2) In agreement with our earlier qualitative resul ts, below ϵ≈2×10 -2 the measured ϱ a ( T) separates from the mean-field behavior. Although so close to the transition we cannot exclude the influence of small stoichiometric inhomogeneities, these Δσ a (ϵ) data for ϵ⪅2×10 -2 may be explained, also on a quantitative level, on the grounds of the 3D-XY model, with a critical exponent of x = - 1 3 for Δσ a (ϵ), and x = - 2 3 for fluctuation-induced diamagnetism. This scenario correponds to one complex component order parameter, i.e., conventional 1s 0-wave pairing or one complex component unconventional pairing.

Anisotropic susceptibility in the normal state and superconducting fluctuation-induced diamagnetism of single-crystal La2− xSraCuO4

Magnetic susceptibility of single-crystal La 2−xSr xCuO 4 with various x was measured with a SQUID magnetometer. The susceptibility with magnetic field ( H) perpendicular to the CuO 2 plane is 1.5≈ 3 times as large as that with H parallel to the CuO 2 plane, whereas they have similar temperature dependence. The field-dependent susceptibility in the vicinity of T c was also studied. From these data, T c ( H), i.e., the field dependence of T c for H normal to the CuO 2 plane, was determined by a new method. The coherence length at 0 K in the CuO 2 plane estimated from T c ( H) is about 30 Å for the samples of x = 0.12, 0.14 and 0.19

On fluctuation-induced diamagnetism of a superconductive glass

Fluctuation-induced diamagnetism (FID) above the critical temperature is considered via the superconductive glass (SG) model. The model predicts a crossover between two-and three-dimensional behaviours of FID. It is shown that for temperatures well above T c a two-dimensional analog of the Aslamazov-Larkin law is realized, while in the region near T c FID reduces to a three-dimensional form of the XY-like model of a granular superconductor.

Oxygen-deficiency dependent magnetism of YBa 2Cu 3O 7−x: EPR and static susceptibility

Magnetism of YBa 2Cu 3O 7−x with various oxygen deficiency has been investigated by static susceptibility and electron paramagnetic resonance (EPR) measurements. The susceptibility of the samples in the orthorhombic phase (0 < x < 0.44) was found to vary systematically with the oxygen deficiency, without any low-temperature upturn. Near T c, superconducting fluctuation-induced diamagnetism was observed. Its temperature dependence revealed a three-dimensional nature of the fluctuations for a sample with x = 0 and a crossover from three- to two-dimensional behaviors for deficient samples. EPR signal from the electrons free from superconducting condensation was detected. The anomalous temperature dependence of spin susceptibility for this signal is discussed in connection with the electron localization induced by the randomness of the oxygen nonstoichiometry.

Fluctuation-induced diamagnetism above Tc in Nb/Si superconductive multilayers

Fluctuation-induced diamagnetism above the critical temperature was observed in Nb/Si superconductive multilayers, using an extremely sensitive superconducting quantum magnetometer. Samples were prepared in an ultrahigh-vacuum deposition system. When the Si layers were thick, two-dimensional behavior was observed and when the Si layers were thin, three-dimensional behavior was observed. An intermediate sample shows a crossover between the two- and three-dimensional behaviors. The experimental results were in good agreement with the theory of Gerhardts [Phys. Rev. B 9 (1974) 2945].

Dimensionality of the Superconductivity in YBa2Cu3O7-δ

Superconducting fluctuation-induced diamagnetism was observed in YBa 2 Cu 3 O 7-δ with different oxygen deficiencies. In a slightly deficient sample, the fluctuation is small and exhibits the temperature dependence of a three-dimensional nature. With increasing defects, however, the fluctuations were found to be enhanced and exhibit a crossover from three- to two-dimensional behaviors at higher temperatures. The dimensionality, which is controlled by the interlayer coupling between the CuO 2 planes, is very sensitive to oxygen deficiency in the CuO chains, even when the critical temperature remains around 90 K.

Fluctuation-Induced Diamagnetism above Tc in Nb/Si Multilayers

The fluctuation-induced diamagnetism above the superconducting critical temperature Tc was investigated in detail in the thin films of Nb/Si superconductive multilayers. Samples were prepared in an ultrahigh vacuum by electron-beam deposition method. When the Si layers were thick, two-dimensional behavior was observed and when they were thin, three-dimensional one was observed. An intermediate sample showed a crossover between the two- and three-dimensional behaviors. The experimental results were in good agreement with the theory of Gerhardts (Phys. Rev. B9 (1974) 2945).

Fluctuation-induced diamagnetism in type II superconductors above H c2

The fluctuation-induced diamagnetism in strong magnetic fields in type II superconductors is studied theoretically. It is shown that the fluctuation-induced diamagnetism at low temperatures (i.e., Tvery-much-less-thanT/sub c0/) is described in terms of a universal function of (B-H/sub c2/(T))/H/sub s/(T) for both dirty and pure superconductors, where H/sub s/(T), the scaling field, is proportional to (T/T/sub c0/)H/sub c2/(O).