Extreme enhancement of superconductivity in epitaxial aluminum near the monolayer limit

Abstract

BCS theory has been widely successful at describing elemental bulk superconductors. Yet, as the length scales of such superconductors approach the atomic limit, dimensionality as well as the environment of the superconductor can lead to drastically different and unpredictable superconducting behavior. Here, we report a threefold enhancement of the superconducting critical temperature and gap size in ultrathin epitaxial Al films on Si(111), when approaching the 2D limit, based on high-resolution scanning tunneling microscopy/spectroscopy (STM/STS) measurements. Using spatially resolved spectroscopy, we characterize the vortex structure in the presence of a strong Zeeman field and find evidence of a paramagnetic Meissner effect originating from odd-frequency pairing contributions. These results illustrate two notable influences of reduced dimensionality on a BCS superconductor and present a platform to study BCS superconductivity in large magnetic fields.

Doubling the superconducting transition temperature of ultraclean wafer-scale aluminum nanofilms

“…Finally, we discuss recent in situ scanning tunneling microscopy studies of ultraclean Al films grown on Si(111) [21]. In this work, a threefold enhanced T c (3.3 ± 0.1 K) was reported.…”

“…In our work, the presence of a thin overlaying AlO x layer when the Al film is exposed to air during ex situ processing does not hinder the enhanced superconductivity. Moreover, Si intermixing and Si doping cannot be the origins of the enhanced T c in the work of van Weerdenburg et al [21]. Hence, enhanced superconductivity is an intrinsic property of ultrathin Al films, provided the normal-state square resistance is much lower than the quantum resistance h/(4e 2 ).…”

The Paramagnetic Meissner Effect (PME) in Metallic Superconductors

“…Very recently, van Weerdenburg [175] showed that in ultrathin Al films (8.5 ML film) a paramagnetic contribution from odd s-wave superconductivity gives rise to an enhanced magnetic penetration depth and an enhanced vortex size due to the Meservey-Tedrow-Fulde (MTF) effect [176]. The vortex size was measured in this paper via dI/dU-maps in STM/STS, and no magnetization data were reported.…”

“…Future work on PME will consider very thin films of metallic superconductors, where significant progress has been made to fabricate uniform atomic monolayer films of Pb, Nb, Al, V, and MgB 2 [175,[178][179][180][181][182]. Giant PME may appear from non-monotonic vortex interactions as predicted in [174].…”