Proximity-effect-induced superconductivity at millikelvin temperatures

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A systematic study is reported of the proximity effect in superconductivity of thick Cu layers (4≤dn≤105 µm) in contact with Nb or Nb-Ti (46≤Ds≤1041 µm) from measurements of the ac susceptibility and of the thermal conductivity. The induced superconducting properties of Cu are studied as a function of temperature (7 mK≤T≤1 K), field (0.4 mG≤H≤ 10 G), and electronic mean free path (0.2≤l≤14 µm). The Meissner screening length in Cu increases faster than the coherence length with decreasing temperature and eventually saturates, making the Cu fully superconducting. In this case, superconductivity can be destroyed sharply at a breakdown fieldHb; the transition atHb is hysteretic. The data are in agreement with numerical solutions of the de Gennes/Ginzburg-Landau theory, as demonstrated by the dependence of the Meissner screening length and of the breakdown field on temperature, field, and electronic mean free path. The data indicate that observation of a reduction in thermal conductivity requires a substantially larger induced pair potential than is necessary even for total field shielding. The concentration of Cooper pairs in Cu in contact to Nb is larger than in Cu in contact to Nb-Ti, even for Nb-Ti/Cu samples with a larger electron mean free path in the Cu part. No measurable superconducting field screening could be induced in the investigated temperature range in Pd (26≤dn≤72 µm) in contact to Nb/Cu or to Nb (Ds≈5dn). This is explained by the fact that the coherence length is at least an order of magnitude smaller in Pd than in Cu. Evaporating a layer of Fe on Cu in the Nb/Cu samples results in a strong depression of the proximity effect.