# Zeeman-limited Superconductivity in Crystalline Al Films

## TL;DR

This study investigates how ultra-thin crystalline aluminum films transition from superconducting to normal states under magnetic fields, revealing agreement with theory in thicker films and anomalies in thinner ones.

## Contribution

It provides the first detailed phase diagram of Zeeman-limited superconductivity in ultra-thin Al films, highlighting deviations from homogeneous BCS theory in very thin samples.

## Key findings

- Thicker films (>20 ML) align well with quasi-classical Zeeman transition theory.
- Thinner films (<20 ML) show anomalous phase diagrams not explained by homogeneous BCS.
- Critical field measurements extend down to 50 mK across the tricritical point.

## Abstract

We report the evolution of the Zeeman-mediated superconducting phase diagram (PD) in ultra-thin crystalline Al films. Parallel critical field measurements, down to 50 mK, were made across the superconducting tricritical point of films ranging in thickness from 7 ML to 30 ML. The resulting phase boundaries were compared with the quasi-classical theory of a Zeeman-mediated transition between a homogeneous BCS condensate and a spin polarized Fermi liquid. Films thicker than $\sim$20 ML showed good agreement with theory, but thinner films exhibited an anomalous PD that cannot be reconciled within a homogeneous BCS framework.