Vortex entropy and superconducting fluctuations in ultrathin underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ superconductor

TL;DR

This study investigates vortex entropy in ultrathin underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ superconductors, revealing how vortex entropy decays with decreasing $T_c$ and providing insights into superconducting fluctuations in two dimensions.

Contribution

It introduces combined magneto-transport and Nernst effect measurements on ultrathin flakes to analyze vortex entropy and superconducting fluctuations in underdoped cuprates.

Findings

Vortex entropy decays exponentially with $T_c$ and superfluid stiffness.

Superfluid phase stiffness scales linearly with $T_c$ in ultrathin samples.

Gaussian fluctuations explain the Nernst signal above $T_c$.

Abstract

Vortices in superconductors can help identify emergent phenomena but certain fundamental aspects of vortices, such as their entropy, remain poorly understood. Here, we study the vortex entropy in underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ by measuring both magneto-resistivity and Nernst effect on ultrathin flakes ($\le$2 unit-cell). We extract the London penetration depth from the magneto-transport measurements on samples with different doping levels. It reveals that the superfluid phase stiffness $\rho_s$ scales linearly with the superconducting transition temperature $T_c$, down to the extremely underdoped case. On the same batch of ultrathin flakes, we measure the Nernst effect via on-chip thermometry. Together, we obtain the vortex entropy and find that it decays exponentially with $T_c$ or $\rho_s$. We further analyze the Nernst signal above $T_c$ in the framework of Gaussian superconducting fluctuations. The combination of electrical and thermoelectric measurements in the two-dimensional limit provides fresh insight into high temperature superconductivity.