Nano-Nb Superconducting 3D Josephson Junction Arrays in Cu-Normal Matrix: Superconducting Phase Coherence and Quantum Fluctuations Properties
ROGÉRIO LÚCIO DE ALMEIDA, J. Albino Aguiar, C. A. C. Passos, S. K. Sharma, J. A. H. Coaquira
Abstract
We investigate the superconducting behavior, phase coherence and quantum fluctuation properties of three-dimensional arrays of nanometric Nb Josephson junctions embedded in a normal Cu matrix (Cu–x wt% Nb, x = 3, 5, 15 and 20), with emphasis on the Cu10wt%Nb and Cu20wt%Nb compositions. Magnetic properties were measured by dc magnetization as a function of temperature and field in a SQUID magnetometer and by ac susceptibility. Transport properties, including magnetoresistance, V(I) curves and the critical current Ic(H,T), were measured at fields up to 90 kOe. A micro-Hall sensor with ten transverse voltage bridges (5.0 × 0.2 × 0.01 mm³) was used to calibrate the magnetic field actually applied to the sample. Magnetization and transport reveal a double superconducting transition that depends on the applied field and current: Tc(H), with Tc(0)=8.2 K, associated with the intragranular (volumetric) response of the Nb grains, and a lower TcJ(H), associated with Josephson coupling between grains through the Cu matrix. The response combines that of bulk Nb with signatures of weakly coupled grains, consistent with a collection of superconducting grains in a non-superconducting matrix. The characteristic fields Hc1=400.03 Oe and Hc2 = 15 kOe (grains), and Hc1J = 99.93 Oe and Hc2J=199.25 Oe (intergranular coupling), were determined. The critical current Ic(H,T) shows a central maximum at low fields, followed by a long tail and small oscillations of period ΔH≈24 Oe, attributed to quantum interference between coupled grains. At higher fields, magnetic flux gradients destroy phase coherence and randomize the phase θ of the order parameter ψ = |ψ|eiθ, suppressing the oscillations. Magnetic and transport results are correlated and described by an effective-medium theory for superconducting grains in a normal matrix under magnetic field and current, supporting a three-dimensional Josephson junction array in a Cu matrix.