Fluctuation Induced Diamagnetism in the Zero Magnetic Field Limit in a Low Temperature Superconducting Alloy

Abstract

By using a Pb-18 at. % In alloy, the fluctuation induced diamagnetism was measured in the zero magnetic field limit, never observed until now in a low-T(C) superconductor. This allows us to disentangle the dynamic and the nonlocal electrodynamic effects from the short-wavelength fluctuation effects. The latter may be explained on the grounds of the Gaussian-Ginzburg-Landau approach by introducing a total energy cutoff in the fluctuation spectrum, which strongly suggests the existence of a well-defined temperature in the normal state above which all fluctuating modes vanish. This conclusion may also have implications when describing the superconducting state formation of the high-T(C) cuprates.

Breakdown of the superconducting fluctuations by a magnetic field

“…To make easier the comparison with the existing theories for Dw (see below) these data are normalized by Txð0Þ, where xð0Þ is the corresponding GL coherence length amplitude (see Table 1). As may be clearly seen, the data for the different alloys collapse into a unique curve, which indicates that the SCF effects on Dw are independent of the amount non-magnetic impurities, and also of the value of k. At very low reduced magnetic fields (i.e., for ho), Dw=Txð0Þ is h-independent, and in excellent agreement with the prediction of the GL theory in the Gaussian approximation (see below) [4]. However, for higher h-values the effect of the thermal fluctuations is strongly reduced, to end up by disappearing at h$1.…”

“…From those expressions, the -value above which the SCF should disappear is C ¼ 0:55. This quantum confinement effect may be formally introduced in the GL theory in the h5 limit through a total-energy cutoff [4], leading to the expression…”

Breakdown by a magnetic field of the superconducting fluctuations in the normal state: a simple phenomenological explanation

“…(1) is fulfilled by a smaller number of Landau levels: For instance, for (x(0)/x 0 ) 2 Z0.55, 3Z0.03 and hZ0.25 the higher n-value allowed by Eq. (1) would be nx0.5, which must be rounded w50% to be consistent with the discreteness of n. Note that, in fact, the final DM expressions proposed in [11] do not predict any vanishing of the fluctuation magnetization for magnetic fields above h C x1.1. To solve such difficulties, we introduce a total-energy-dependent weighting function, WðE nk z Þ, pondering the contribution of each fluctuating mode in the freeenergy statistical sum.…”

“…and wave vector parallel to the field k z , this constraint may be written as: where the energies are expressed in units of Z 2 =2m à x 2 ð0Þ, and Z and m * are, respectively, the Planck constant and the effective mass of the Cooper pairs. Note that this inequality was already introduced in previous GGL calculations of DM [11] for low magnetic fields by cutting off the statistical sums over the Landau-level index. The Landau index n was there considered as a continuous variable and Eq.…”

Effect of proton irradiation on the fluctuation-induced magnetoconductivity of FeSe1−xTexthin films

“…This approach includes a cutoff in the energy of the fluctuation modes, which extends its applicability to high reduced temperatures [35]. Such a cutoff scheme has been probed in other iron-based superconductors [20,27], but also in high-T c cuprates [36], and low-T c alloys [37]. It has also been recently taken into account in very recent theoretical works about possible multiband effects on the fluctuation diamagnetism [38].…”