Superconducting diamagnetic fluctuations in MgB 2

The fluctuating diamagnetic magnetization M fl at a 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 Mfl 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 2M fl /H 1/2 T is found to be field dependent. In the limit of evanescent field the behavior 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 nonlocal 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. Thermodynamical fluctuations on approaching the superconducting ~SC! transition temperature from above yield the formation of evanescent SC droplets causing a bulk diamagnetic magnetization 2M fl . This fluctuating diamagnetism is strongly enhanced in high-temperature superconductors with respect to the conventional SC’s because of the hightemperature 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 also be detected in these systems by means of superconducting quantum interference device ~SQUID! magnetization measurements. Early data 2 for M fl at constant field as a function of temperature in zero-dimensional limit ~aluminum particles of size less than 1000 A! 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. 3 From the measurements of M fl vs T of Gollub et al. 4 one can deduce the occurrence of an upturn in the field dependence of M fl : for H!Hup the diamagnetic magnetization increases with H while for H*Hup the field tends to suppress the fluctuating magnetization. The upturn field Hup can be approximately related to the Ginzburg-Landau ~GL! coherence length j(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, to the authors’ 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 nonisotropic systems. 5,6 The new superconductor MgB 2 , 7 although being of ‘‘conventional’’ BCS character has two characteristics that can be expected to enhance SF and therefore the value of M fl at T5Tc(0): the high value of the transition temperature and, as we will deduce later on, an anisotropic spectrum of the fluctuations, similarly to cuprate superconductors. This paper deals with a study of FD in MgB2 by means of high-resolution SQUID magnetization measurements, with a successful detection of the magnetization curves 2M fl vs H in the temperature range Tc(0)&T&Tc(0)10.5 K. From the experimental findings we have been able to prove that M fl and its field dependence are close to the one predicted by theories based on the extension of the GL approach to include short-wavelength fluctuations of the order parameter and nonlocal electrodynamic effects. It is also deduced that MgB2 has an anisotropic spectrum of fluctuations, with remarkable enhancement of the FD.