The fluctuating diamagnetic magnetization M fl at constant field H as a function of temperature and the isothermal magnetization M fl vs H are measured in MgB2, above the superconducting transition temperature. The expressions for M fl in randomly oriented powders are derived in the Gaussian approximation of local Ginzburg-Landau theory and used for the analysis of the data. The scaled magnetization −M fl /H 1/2 T is found to be field dependent. In the limit of evanescent field the behaviour for Gaussian fluctuations is obeyed while for H > ∼ 100 Oe the field tends to suppress the fluctuating pairs, with a field dependence of M fl close to the one expected when short wavelength fluctuations and non-local electrodynamic effects are taken into account. Our data, besides providing the isothermal magnetization curves for T > Tc(0) in a BCS-type superconductor such as MgB2, evidence an enhancement of the fluctuating diamagnetism which is related to the occurrence in this new superconductor of an anisotropic spectrum of the superconducting fluctuations. numbers: 74.40.+k, 74.20.De Thermodynamical fluctuations on approaching the superconducting (SC) transition temperature from above yield the formation of evanescent SC droplets causing a bulk diamagnetic magnetization −M fl . This fluctuating diamagnetism is strongly enhanced in high temperature superconductors with respect to conventional SC's because of the high temperature range and of the anisotropy of the cuprates [1]. In spite of the difficulty of evidencing the superconducting fluctuations (SF) in low-temperature conventional BCS superconductors, the fluctuating diamagnetism (FD) can be detected also in these systems by means of SQUID magnetization measurements. Early data for M fl at constant field as a function of temperature in zero dimensional limit (aluminum particles of size less than 1000Å) and in metals compounds, evidenced the rounding of the transition due to SF and the effect of the magnetic field in quenching the fluctuating Cooper pairs [2]. From the measurements of M fl vs T of Gollub et al. PACS[3] one can deduce the occurrence of an upturn in the field dependence of M fl : for H ≪ H up the diamagnetic magnetization increases with H while for H > ∼ H up the field tends to suppress the fluctuating magnetization. The upturn field H up can be approximately related to the Ginzburg-Landau (GL) coherence length ξ(T ) (see later on). Most likely in view of the small value of M fl in conventional BCS superconductors, isothermal magnetization curves M fl vs H have not been studied in detail, in the authors's knowledge. The relevance of the field dependence of M fl (T, H) for the study of FD has been recently stressed in the framework of a Gaussian GL approach for non-isotropic systems [4,5]. The new superconductor MgB 2 [6], although being of "conventional" BCS character has two characteristics that can be expected to enhance SF and therefore the value of M fl at T = T c (0): the high value of the transition temperature and, as we will deduce later on, a...

A detailed theoretical analysis of the Gaussian fluctuations of the order parameter in layered superconductors is performed within the Ginzburg-Landau (GL) theory. The available results for the Gaussian fluctuations are systematized and a number of novel formulae for the fluctuation magnetization, nonlinear magnetic susceptibility, heat capacity and high-frequency conductivity in layered superconductors are derived. We propose several new prescriptions: how to determine the life-time constant of fluctuation Cooper pairs τ 0 , the in-plane coherence length ξ ab (0), the energy cutoff parameter ε ✄ = c 2 /2m ab ξ 2 ab (0), and the Ginzburg number ǫ Gi . It is demonstrated, for example, how the spectroscopy of the life-time can be used to verify the existence of depairing mechanisms in layered cuprates. The ultraviolet regularization of the GL free energy is carried out by means of the well-known from the field theory ζ-function method. We further show that the archetype of the latter method has its origin in the century of enlightenment and the novel result is that the fluctuation part of the thermodynamic variables of the layered superconductors can be expressed in terms of the Euler Γ-function and its derivatives. Universal scaling curves for the magnetic field dependence of the paraconductivity σ ab (Tc, B), fluctuation magnetization M (Tc, B) and the heat capacity C(Tc, B) are found for the quasi-2D superconductors at Tc and further related to the form-factor of the Cooper pairs.

We study the effect of a strong electric field on the fluctuation conductivity within the timedependent Ginzburg-Landau theory for the case of arbitrary dimension. Our results are based on the analytical derivation of the velocity distribution law for the fluctuation Cooper pairs, from the Boltzmann equation. Special attention is drawn to the case of small nonlinearity of conductivity, which can be investigated experimentally. We obtain a general relation between the nonlinear conductivity and the temperature derivative of the linear Aslamazov-Larkin conductivity, applicable to any superconductor. For the important case of layered superconductors we derive an analogous relation between the small nonlinear correction for the conductivity and the fluctuational magnetoconductivity. On the basis of these relations we provide new experimental methods for determining both the lifetime constant of metastable Cooper pairs above Tc and the coherence length. A systematic investigation of the 3rd harmonic of the electric field generated by a harmonic current can serve as an alternative method for the examination of the metastable Cooper-pair relaxation time.