Edge superconductivity in Nb thin film microbridges revealed by electric transport measurements and visualized by scanning laser microscopy

Abstract

The resistance R versus perpendicular external magnetic field H was measured for superconducting Nb thin film microbridges with and without microholes (antidots, ADs). Well below the transition temperature, integral R(H) measurements of the resistive transition to the normal state on the plain bridge show two distinct regions, which can be identified as bulk and edge superconductivity, respectively. The latter case appears when bulk superconductivity becomes suppressed at the upper critical field H c2 and below the critical field of edge superconductivity H c3 ≈ 1.7 H c2 . The presence of additional edges in the AD bridge leads to a different shape of the R(H) curves. We used low-temperature scanning laser microscopy (LTSLM) to visualize the current distribution in the plain and AD bridges upon sweeping H. While the plain bridge shows a dominant LTSLM signal at its edges for H > H c2 the AD bridge also gives a signal from the inner parts of the bridge due to the additional edge states around the ADs. LTSLM reveals an asymmetry in the current distribution between the left and right edges, which confirms theoretical predictions. Furthermore, the experimental results are in good agreement with our numerical simulations (based on the time-dependent Ginzburg-Landau model), yielding the spatial distribution of the order parameter and current density for different bias currents and H values.

Surface superconductivity as the primary cause of broadening of superconducting transition in Nb films at high magnetic fields

“…In perfectly uniform films SSC should not occur at perpendicular field orientation [1,16]. Yet, SSC in perpendicular fields has been directly visualized by scanning laser microscopy for similar films [21] and also reported for some layered supercondcutors [22] and sintered polycrystalline MgB 2 samples [23]. Presumably it is the polycrystallinity of our films that allows SSC at grain boundaries even in perpendicular fields.…”

Onset of surface superconductivity beyond the Saint-James–de Gennes limit

“…It was revealed that the H c3 /H c2 ratio is not a universal number but rather a complicated function that exhibits pronounced dependence on both T and the mean-free time τ mediated by disorder scattering. Niobium became essentially a paradigmatic material to study various phenomena associated with surface superconductivity and multiple experiments continue to follow [14][15][16][17][18][19][20]. Naturally over time experimental efforts on surface superconductivity branched into different directions.…”

A high-resolution combined scanning laser and widefield polarizing microscope for imaging at temperatures from 4 K to 300 K

“…It was shown by Dieckmann et al 69 that this technique, lowtemperature scanning laser microscopy (LTSLM), delivers results that are equivalent to LTSEM. LTSLM has, among others, been applied to research on superconductors 32,34,70,71 , the quantum Hall effect 72 , spintronics 73,74 and spin-caloritronics 75 . The LTSPM can be operated in LTSLM mode to gain information on the electric trans- port properties of a sample.…”