4 research outputs found
Why shot noise does not generally detect pairing in mesoscopic superconducting tunnel junctions
The shot noise in tunneling experiments reflects the Poissonian nature of the
tunneling process. The shot noise power is proportional to both the magnitude
of the current and the effective charge of the carrier. Shot-noise spectroscopy
thus enables - in principle - to determine the effective charge q of the charge
carriers that tunnel. This can be used to detect electron pairing in
superconductors: in the normal state, the noise corresponds to single electron
tunneling (q = 1e), while in the paired state, the noise corresponds to q = 2e,
because of Andreev reflections. Here, we use a newly developed amplifier to
reveal that in typical mesoscopic superconducting junctions, the shot noise
does not reflect the signatures of pairing and instead stays at a level
corresponding to q = 1e. We show that transparency can control the shot noise
and this q = 1e is due to the large number of tunneling channels with each
having very low transparency. At such transparencies, the shot noise in the
junction resembles that of a metallic instead of a superconducting tunnel
junction. Our results indicate that in typical mesoscopic superconducting
junctions one should expect q = 1e noise, and lead to design guidelines for
junctions that allow the detection of electron pairing
Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above T c
The idea that preformed Cooper pairs could exist in a superconductor at temperatures higher than its zero-resistance critical temperature (Tc) has been explored for unconventional, interfacial, and disordered superconductors, but direct experimental evidence is lacking. We used scanning tunneling noise spectroscopy to show that preformed Cooper pairs exist up to temperatures much higher than Tc in the disordered superconductor titanium nitride by observing an enhancement in the shot noise that is equivalent to a change of the effective charge from one to two electron charges. We further show that the spectroscopic gap fills up rather than closes with increasing temperature. Our results demonstrate the existence of a state above Tc that, much like an ordinary metal, has no (pseudo)gap but carries charge through paired electrons