Abstract:We report the evolution of the magnetic penetration depth (λ) and superconducting energy gap (∆) in epitaxial NbN films with thickness (d) varying between 51-3nm. With decrease in film thickness T c and ∆(0) monotonically decreases, whereas λ(0) monotonically increases. Our results show that while the values of ∆(0) and λ(0) are well described by BardeenCooper-Schrieffer (BCS) theory, at elevated temperatures, films with d≤6.5nm show sudden drop in superfluid density associated with the Kosterlitz-Thouless-Berezinski (KTB) transition.We discuss the implication of these results on the time response of superconducting bolometers made out of ultrathin NbN films. * films with d>20nm, the thickness was measured using a stylus profilometer while for thinner films it was estimated from the time of deposition. λ was measured using a "two coil" mutual inductance technique operating at 60kHz. The main advantage of this technique is that it allows measurement of the absolute value of λ over the entire temperature range up to T c without any prior assumption about the temperature dependence of λ. In this technique, a 8mm diameter thin superconducting film is sandwitched between a quadrupole primary coil and a dipole secondary coil ( figure 1(a)). This technique operates on the principle that the thin superconducting film will partially shield the secondary coil from the magnetic field produced by the primary, the degree of shielding being dependent on λ. The mutual inductance between the primary and the secondary coil is measured as a function of temperature by passing a small a.c. excitation current (1mA) through the primary and measuring the in-phase and out-of-phase induced voltage in the secondary using a lock-in amplifier. λ is determined by evaluating the mutual inductance for different values of λ by numerically solving the Maxwell equations and comparing the measured value with the theoretically calculated value 10 . The quadrupole configuration of the primary coil ensures a fast radial decay of the magnetic field such that edge effects are minimized 11 . The excitation field was kept very low (~7mOe) and the cryostat was shielded from the earth's magnetic field using a mu-metal shield. ∆(Τ) was measured using a home built low temperature scanning tunneling microscope (STM) on freshly prepared NbN films using Pt-Ir tip. The show the tunneling spectra at various temperatures. ∆ is extracted by fitting these spectra to the tunneling equation,where,is the lifetime broadened BCS density of states. While the broadening parameter, ( ), formally incorporated 12 to take into account the lifetime (τ) of the quasiparticle, phenomenologically incorporates all sources of non-thermal broadening in the BCS DOS. We observe that the temperature dependence of ∆(Τ) (Fig. 2(c)-closely follows the BCS curve 13 within experimental accuracy.Figure 3(a) shows the temperature variation of λ −2 (Τ)∝n s (T) (where n s is the superfluid density). λ increases from 275nm to 529nm as the thickness decreases from 51nm to 3nm. The val...

We report the evolution of superconducting properties as a function of disorder in homogeneously disordered epitaxial NbN thin films grown on (100) MgO substrates, studied through a combination of electrical transport, Hall Effect and tunneling measurements. The thickness of all our films are >50nm much larger than the coherence length ξ 0~5 nm. The effective disorder in different films encompasses a large range, with the Ioffe-Regel parameter varying in the range k F l~1. 38-8.77. Tunneling measurements on films with different disorder reveals that for films with large disorder the bulk superconducting transition temperature (T c ) is not associated with a vanishing of the superconducting energy gap, but rather a large broadening of the superconducting density of states. Our results provide strong evidence of the loss of superconductivity via phase-fluctuations in a disordered s-wave superconductor. 1

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