Low field magnetic response of the granular superconductor La1.8Sr0.2CuO4

Abstract

The properties of the low excitation field magnetic response of the granular high temperature (HTc) superconductor La1.8Sr0.2CuO4 have been analyzed at low temperatures. The response of the Josephson currents has been extracted from the data. It is shown that intergrain current response is fully irreversible, producing shielding response, but do not carry Meissner magnetization. Analysis of the data shows that the system of Josephson currents freezes into a glassy state even in the absense of external magnetic field, which is argued to be a consequence of the d-wave nature of superconductivity in La1.8Sr0.2CuO4. The macroscopic diamagnetic response to very weak variations of the magnetic field is shown to be strongly irreversible but still qualitatively different from any previously known kind of the critical-state behaviour in superconductors. A phenomenological description of these data is given in terms of a newly proposed "fractal" model of irreversibility in superconductors.

Nonlinear Seebeck effect in a model granular superconductor

“…The so-called 3D model of Josephson junction arrays (which is often used to simulate a thermodynamic be-havior of a real granular superconductor) is based on the well-known tunneling Hamiltonian (see, e.g., [8][9][10][11][12][13] )…”

“…To discuss a true ∇T induced thermophase effect only, in what follows we completely ignore the effects due to a nonzero applied magnetic field (by putting H = 0 in Eq. ( 6)) as well as rather important in granular superconductors "self-field" effects (see 12,13 for discussion of this problem) and assume that in equilibrium (initial) state (with ∇T = 0) < j s >≡ 0, implying thus φ ij (0) ≡ 0. The latter condition in fact coincides with a current density conservation requirement at zero temperature 9 .…”

Electric field dependence of the thermal conductivity of a granular superconductor: Giant field-induced effects predicted

“…Finally, taking as an example granular aluminum films with phonon dominated heat transport [15] (with κ ph (T ) ≃ 2 × 10 −7 W/mK at T = T J ≃ 0.2T c ), let us estimate the absolute value of the predicted here zero-field electronic contribution κ e (T ) ≡ κ L (T, 0) at T = 0.2T c . Recalling that within our model the scattering of normal electrons is due to the presence of mutual inductance between the adjacent grains L 0 , and assuming that [13] L 0 ≃ µ 0 d ≃ 4π × 10 −12 H and V ≃ N d 2 l with l ≃ 0.5mm (l is a film's thickness), we obtain κ e (T = 0.2T c ) ≃ β L (0) × 10 −7 W/mK for a rough estimate of the electronic contribution to the discussed here inductance-driven effect. Correspondingly, we get κ e (0.2T c )/κ ph (0.2T c ) ≃ β L (0)/2 for the ratio, where β L (0) = 2πI c (0)L 0 /Φ 0 (for example, β L (0) ≃ 4 for I c (0) = 10 −4 A).…”

“…Namely, the smaller is the normal resistance between grains R n (or the better is the quality of the sample) the higher is the temperature at which the peak is developed. As a matter of fact, the peak temperature T p is related to the so-called phase-locking temperature T J (which marks the establishment of phase coherence between the adjacent grains in the array and always lies below a single grain superconducting temperature T c ) which is usually defined via an average (per grain) Josephson coupling energy as [13] J(T J , r n ) = k B T J . In particular, for T ≃ T c , it can be shown analytically that T J (r n ) indeed increases with r n as T J (r n )/T c ≃ r n /(1 + r n ).…”