Shot noise in tunneling experiments reflects the Poissonian nature of charge transport, with the noise power directly proportional to the current and the effective charge of the tunneling carriers. This relation provides a powerful spectroscopic tool to probe electron pairing phenomena in superconductors [1, 2]. In this talk, I will report on two distinct applications of local shot-noise measurements using scanning tunneling microscopy to explore superconducting states.
First, I will present our shot-noise measurements on the cuprate high-Tc superconductors, Bi₂Sr₂Ca₂Cu₁O_8+p, where a pseudogap state is observed. The origin of the pseudogap has long been debated, with hypotheses involving either precursor electron pairing or competing local orders. Our local noise measurements reveal that the pseudogap energy is associated with electron pairing [3], up to more than 70meV.
Second, I will present our results of multiple Andreev reflections (MAR). MAR processes in superconductor–insulator–superconductor (SIS) junctions involve successive Andreev reflections, resulting in the transfer of an effective charge ne [4]. We succeeded to measure MAR noise of an SIS junction on a Pb(111) surface by using a superconducting tip.

References
[1] Y. M. Blanter and M. Büttiker, Shot noise in mesoscopic conductors. Physics Reports, 336, 1 (2000).
[2] K. M. Bastiaans, D Cho, et al., Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above Tc, Science 374, 608 (2021).
[3] J. Niu, M. O. Larrazabal, et al., Equivalence of pseudogap and pairing energy in a cuprate high-temperature superconductor, arXiv:2409.15928.
[4] T. M. Klapwijk, G. E. Blonder, and M. Tinkham, Physica (Amsterdam) 109B & 110B, 1657 (1982).