Dimensional crossover and incipient quantum size effects in superconducting niobium nanofilms

Abstract

Superconducting and normal state properties of Niobium nanofilms have been systematically investigated as a function of film thickness, on different substrates. The width of the superconducting-to-normal transition for all films is remarkably narrow, confirming their high quality. The superconducting critical current density exhibits a pronounced maximum for thickness around 25 nm, marking the 3D-to-2D crossover. The magnetic penetration depth shows a sizeable enhancement for the thinnest films. Additional amplification effects of the superconducting properties have been obtained with sapphire substrates or squeezing the lateral size of the nanofilms. For thickness close to 20 nm we measured a doubled perpendicular critical magnetic field compared to its large thickness value, indicating shortening of the correlation length and the formation of small Cooper pairs. Our data analysis indicates an exciting interplay between quantum-size and proximity effects together with strong-coupling effects and the importance of disorder in the thinnest films, placing these nanofilms close to the BCS-BEC crossover regime.

Substrate-Induced Proximity Effect in Superconducting Niobium Nanofilms

“…In this approach α = d N N N (0)/N S (0) is an effective thickness of the conductive layer at the interface; T C0 = 9.22 K is the bulk T c of Nb and T D = 277 K is the Debye temperature. The quantities N N (0) and N S (0) are the density of states in the normal (N ) and superconducting (S) layers, respectively and here assumed to be equal [12].…”

“…In this work we report an extended experimental investigation of the superconducting to normal state transition in Nb nanofilms, with a thickness in the range 9 nm to 80 nm, deposited on SiO 2 and Al 2 O 3 substrates. A detailed analysis of several superconducting properties of these Nb nanofilms has been reported in [12]. The main microscopic parameters characterizing the normal and the superconducting state of these films were investigated based on recent models.…”

Tunable Vortex Dynamics in Proximity Junction Arrays: A Possible Accurate and Sensitive 2D THz Detector

“…In this region the normal-metal coherence length increases until it becomes comparable to the island spacing. Then, inter-island phase coherence begins to emerge and finally reaching T 2 the system undergoes a transition to a fully superconducting state [7,11,12]. The dynamic resistance of the PAD was measured at 4.2 K, being in the fully superconducting state, as a function of the applied magnetic field and current (see Figure 2).…”

Angular Dependence of Copper Surface Damage Induced by an Intense Coherent THz Radiation Beam

“…The THz electromagnetic spectral range, typically referred to frequencies between 100 GHz and 10 THz, is fundamental for the spectroscopic analysis of many condensed matter systems [10][11][12][13][14][15][16], low dimensional semiconductors and superconducting materials with different electronic properties [17][18][19][20]. This frequency range is, in fact, resonant with molecular and phonon excitations and with free electrons in metals [21].…”