Dependence of magnetic penetration depth on the thickness of superconducting Nb thin films

Abstract

In this paper we present the results of a systematic study on the magnetic field penetration depth of superconducting niobium thin films. The films of thicknesses ranging from 8 to 300 nm were deposited on a Si substrate by dc magnetron sputtering. The values of the penetration depth ͑0͒ were obtained from the measurements of the effective microwave surface impedance by employing a sapphire resonator technique. Additionally, for the films of thickness smaller than 20 nm, the absolute values of ͑0͒ were determined by a microwave transmission method. We found that the reduction of the film thickness below 50 nm leads to a significant increase of the magnetic field penetration depth from about 80 nm for 300 nm thick film up to 230 nm for a 8 nm thick film. The dependence of the penetration depth on film thickness is described well by taking into account the experimental dependences of the critical temperature and residual resistivity on the thickness of the niobium films. Structural disordering of the films and suppression of superconductivity due to the proximity effect are considered as mechanisms responsible for the increase of the penetration depth in ultrathin films.

Observation of Anomalous Meissner Screening in Cu/Nb and Cu/Nb/Co Thin Films

“…For the Nb single layers we find a small Meissner expulsion with a field penetration depth of 270 nm (I) and 160 nm (II) respectively. These value are much larger than the typical 30 nm found for clean Nb systems [9][10][11] but are not inconsistent with results on dirty systems (similar to those considered here) where the penetration length increases with decreasing mean free path and values up to 230 nm have been observed [12,13]. For the Cu/Nb bilayers we find an enhancement of the Meissner expulsion, which we verify by theory.…”

Magnetic field resilient superconducting fractal resonators for coupling to free spins

“…This number is to be compared against the expected scaling for the most simple geometry: a single strip of superconducting thin film. For a strip, the vortex entry field for in-plane orientation can be estimated as 20 B jj S % / 0 =2 ffiffi ffi 2 p pk L n; substituting typical values 21 for the London penetration depth k L % 90 nm and coherence length n % 50 nm, we arrive at B jj S % 50 mT. This is close to the observed onset ($40 mT) of vortex dissipation in Fig.…”

Entangled Andreev pairs and collective excitations in nanoscale superconductors

“…Thus, C can be obtained from the typical charging energy for an oxide barrier tunnel junction E c S ∼ 1 μeV × μm 2 and L can be estimated as μ 0 l 2 /d, where l is the field penetration depth 16 . The actual value of l for a Nb film is strongly dependent on its thickness, degree of disorder and it is also influenced by the properties of the nonsuperconducting substrate on which it is deposited 17 . Reported values range between 100 nm and 1 μm for d ∼ 10-100 nm (refs 17,18).…”