# Specific Heat Signature of the Berezinskii-Kosterlitz-Thouless Transition in Ultrathin Superconducting Films

## TL;DR

This study measures the specific heat of ultrathin superconducting films to identify signatures of the Berezinskii-Kosterlitz-Thouless transition, revealing BKT-like behavior in the 2D limit.

## Contribution

It provides experimental evidence of BKT-like specific heat signatures in ultrathin superconducting films, bridging the gap between BCS and BKT theories.

## Key findings

- Thick films show a BCS-like specific heat jump at Tc.
- Ultrathin films exhibit BKT-like specific heat behavior.
- Observation supports BCS-BKT crossover in 2D superconductors.

## Abstract

The Berezinskii-Kosterlitz-Thouless (BKT) transition is expected to have a clear signature on the specific heat. The singularity at the transition temperature $T_{BKT}$ is predicted to be immeasurable, and a broad non-universal peak is expected at $T>T_{BKT}$. Up to date this has not been observed in two-dimensional superconductors. We use a unique highly sensitive technique to measure the specific heat of ultrathin Pb films. We find that thick films exhibit a specific heat jump at $T_C$ that is consistent with BCS theory. As the film thickness is reduced below the superconducting coherence length and the systems enters the 2D limit the specific heat reveals BKT-like behavior. We discuss these observations in the framework of the continuous BCS-BKT crossover as a function of film thickness.