nl7b03659_si_001.pdf (801.46 kB)
Is There a Lower Size Limit for Superconductivity?
journal contribution
posted on 2017-10-05, 00:00 authored by Subhrangsu Sarkar, Nilesh Kulkarni, Ruta Kulkarni, Krishnamohan Thekkepat, Umesh Waghmare, Pushan AyyubThe ultimate lower size limit for
superconducting order to exist
is set by the “Anderson criterion”arising from
quantum confinementthat appears to be remarkably accurate
and universal. We show that carefully grown, phase-pure, nanocrystalline bcc-Ta remains superconducting (with ordering temperature, TC ≈ 0.9 K) down to sizes 40% below the
conventional estimate of the Anderson limit of 4.0 nm. Further, both
the TC and the critical magnetic field
exhibit an unusual, nonmonotonic size dependence, which we explain
in terms of a complex interplay of quantum size effects, surface phonon
softening, and lattice expansion. A quantitative estimation of TC within first-principles density functional
theory shows that even a moderate lattice expansion allows superconductivity
in Ta to persist down to sizes much lower than the conventional Anderson
limit, which can be traced to anomalous softening of a phonon due
to its coupling with electrons. This appears to indicate the possibility
of bypassing the Anderson criterion by suitable crystal engineering
and obtaining superconductivity at arbitrarily small sizes, an obviously
exciting prospect for futuristic quantum technologies. We take a critical
look at how the lattice expansion modifies the Anderson limit, an
issue of fundamental interest to the study of nanoscale superconductivity.