Superconductivity in Pb cluster assembled systems with different degrees of coagulation

Superconducting properties are observed and investigated in Pb nanogranular systems prepared by deposition of clusters produced in a laser vaporization cluster source. Different morphologies were achieved by controlling the degree of coagulation via the substrate temperature and the magnetic response of these systems was studied. Deposition on substrates at temperatures above $200°\text{C}$ results in an ensemble of weakly coupled islands showing superconducting confinement effects. Cluster deposition on cooled substrates limits the coagulation and results in cluster assembled thin films with strong intergrain coupling. This method provides a unique way to produce systems with very high flux pinning leading to avalanche effects.

Figure 1

Size distribution of Pb clusters in the beam, inferred from the time-of-flight mass spectrum.

Figure 2

(Color online) (a) $2\times 2\mu {\text{m}}^2$ AFM image of sample PbA deposited at $T_{\text{subs}}=600°\text{C}$. The color scale spans 100 nm. (b) Section of AFM image of sample PbA.

Figure 3

(Color online) (a) $2\times 2\mu {\text{m}}^2$ AFM image of sample PbB deposited at $T_{\text{subs}}=-70°\text{C}$. The color scale spans 10 nm. (b) Section of AFM image of sample PbB taken across an artificial scratch on the sample.

Figure 4

Phase boundary of sample PbA: nanoscale islands produced through cluster deposition at $T_{\text{subs}}=600°\text{C}$. The experimental data are fitted with a square-root dependence $\left(\propto \sqrt{1-T/T_c}\right)$. For comparison the bulk Pb phase boundary is also included in the figure.

Figure 5

Magnetization as a function of the magnetic field for sample PbA at $T=4\text{K}$.

Figure 6

Phase boundary of sample PbB. The linear behavior corresponds to the dirty type II nature of the film.

Figure 7

(a) Magnetization loop of sample PbB at 4.5, 5, and 6 K. The magnetization of an MBE reference film at 4.5 K is also shown in comparison. (b) Detail of the magnetization as a function of field at $T=3.5\text{K}$ for a Pb cluster assembled sample produced at low substrate temperature showing flux avalanches.

Figure 8

$H\text{−}T$ diagram marking the region where vortex avalanches occur in PbB in comparison with a Pb film with artificial pinning sites (data from Ref. 44) and a continuous reference Pb film grown by MBE.