The fluctuation-related diamagnetism above the superconducting transition temperature T-c in neutron irradiated and in Al-doped MgB2 is studied by means of high-resolution isothermal measurements of the diamagnetic contribution to the magnetization, M-dia=M-dia(H,T=const). In both the neutron irradiated and the Al-doped compounds, T-c decreases on increasing the fluence and the Al amount, respectively. The magnetic field dependence of M-dia is apparently similar in both types of compounds: in the limit H -> 0, -M-dia goes as H-n (with n in between 1/2 and 1), while by increasing the field above a given value H-up, an upturn in the field dependence occurs and parallel to M-dia parallel to decreases. From the temperature behaviors of H-up, it is proved that the origin of the precursor diamagnetism is quite different in neutron irradiated and in Al-doped MgB2. In the latter, the magnetization curves reflect the precursor diamagnetism typical of heterogeneous systems and unrelated to superconducting fluctuations, due to site dependence of the transition temperature. At variance, neutron irradiated MgB2 displays different properties. The fluence-dependent transition temperature is practically site independent, the superconducting fluctuations and the related diamagnetism basically retaining the features of the pure (unirradiated) MgB2. Upon irradiation, the anisotropy parameter involved in the fluctuation spectrum decreases. Correspondingly, also the upturn field H-up decreases, consistent with a less anisotropic coherence length in strongly irradiated compounds. The implications of these experimental findings on the disorders induced by heterovalent substitutions and by neutron irradiation in MgB2 are discussed.
Superconducting fluctuating diamagnetism in neutron irradiated MgB2 in relation to precursor diamagnetism in Al-doped MgB2
Lascialfari A;
2008
Abstract
The fluctuation-related diamagnetism above the superconducting transition temperature T-c in neutron irradiated and in Al-doped MgB2 is studied by means of high-resolution isothermal measurements of the diamagnetic contribution to the magnetization, M-dia=M-dia(H,T=const). In both the neutron irradiated and the Al-doped compounds, T-c decreases on increasing the fluence and the Al amount, respectively. The magnetic field dependence of M-dia is apparently similar in both types of compounds: in the limit H -> 0, -M-dia goes as H-n (with n in between 1/2 and 1), while by increasing the field above a given value H-up, an upturn in the field dependence occurs and parallel to M-dia parallel to decreases. From the temperature behaviors of H-up, it is proved that the origin of the precursor diamagnetism is quite different in neutron irradiated and in Al-doped MgB2. In the latter, the magnetization curves reflect the precursor diamagnetism typical of heterogeneous systems and unrelated to superconducting fluctuations, due to site dependence of the transition temperature. At variance, neutron irradiated MgB2 displays different properties. The fluence-dependent transition temperature is practically site independent, the superconducting fluctuations and the related diamagnetism basically retaining the features of the pure (unirradiated) MgB2. Upon irradiation, the anisotropy parameter involved in the fluctuation spectrum decreases. Correspondingly, also the upturn field H-up decreases, consistent with a less anisotropic coherence length in strongly irradiated compounds. The implications of these experimental findings on the disorders induced by heterovalent substitutions and by neutron irradiation in MgB2 are discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
