High-resolution spectroscopy of proximity superconductivity in finite-size quantized surface states
arxiv(2024)
摘要
Adding superconducting (SC) electron pairing via the proximity effect to
pristinely non-superconducting materials can lead to a variety of interesting
physical phenomena. Particular interest has recently focused on inducing SC
into two-dimensional surface states (SSs), potentially also combined with
non-trivial topology. We study the mechanism of proximity-induced SC into the
Shockley-type SSs of the noble metals Ag(111) and Cu(111) grown on the
elemental SC Nb(110) using scanning tunneling spectroscopy. The tunneling
spectra exhibit an intriguing multitude of sharp states at low energies. Their
appearance can be explained by Andreev bound states (ABS) formed by the weakly
proximitized SSs subject to lateral finite-size confinement. We study
systematically how the proximity gap in the bulk states of both Ag(111) and
Cu(111) persists up to island thicknesses of several times the bulk coherence
length of Nb. We find that even for thick islands, the SSs acquire a gap, with
the gap size for Cu being consistently larger than for Ag. Based on this, we
argue that the SC in the SS is not provided through direct overlap of the SS
wavefunction with the SC host but can be understood to be mediated by step
edges inducing electronic coupling to the bulk. Our work provides important
input for the microscopic understanding of induced superconductivity in
heterostructures and its spectral manifestation. Moreover, it lays the
foundation for more complex SC heterostructures based on noble metals.
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