Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above T _c

Abstract

Measuring the effective charge At low enough temperatures, superconductors are capable of conducting electricity without any resistance because of the formation of so-called Cooper pairs of electrons. Cooper pairs typically form at the same critical temperature at which superconductivity sets in. In certain materials, they are thought to form above that temperature, but showing this property directly in an experiment is tricky. Bastiaans et al . used tunneling noise spectroscopy to measure the effective charge of current carriers in the disordered superconductor titanium nitride. As expected, below the critical temperature, the effective charge was equal to two electron charges. However, this behavior persisted above the critical temperature, indicating that electron pairs exist in that regime. —JS

Resistance fluctuation spectroscopy of the superconducting transition in epitaxial TiN films

“…Still, many studies reveal a much larger RT broadening, than what is predicted by existing theories [13]. Such discrepancies can be attributed either to a built-in inhomogeneity [14] or, as recent studies of low-dimensional superconductors suggest, to more fundamental effects such as the existence of preformed Cooper pairs above T c [15,16]. To obtain a deeper insight into fluctuation mechanisms, we go beyond the static limit and focus on the fluctuation dynamics.…”

Shot-noise measurements of single-atom junctions

“…[2]). For instance, shot-noise measurements have provided evidence of fractional charges in the quantum Hall effect [3][4][5] and Cooper-pair transport in superconductors [6][7][8]. It may further yield information of the scattering processes at Kondo impurities [9][10][11][12][13][14][15][16][17].…”

“…Alongside mesoscopic measurements, molecules and atomic-scale structures have also been studied [28][29][30][31][32][33][34], mainly by means of mechanically controlled break junctions. More recently a few experimental groups demonstrated local shot noise measurements using a scanning tunneling microscope (STM) [7,8,14,17,[35][36][37][38][39][40][41][42].…”

Towards strong evidence of Majorana bound state by a combined measurement of tunneling conductance and current noise

“…Alternatively, the combined measurement of local tunneling spectroscopy and current noise is more attractive and practical as no modification of the existing devices is re-quired. Instead, it is fueled by the development of highprecision current noise measurement [50][51][52][53]. One anticipates the observation of ZBCPs accompanied by an integer Fano factor (the ratio between tunneling current and its noise) of 1 in the large-bias limit [54].…”