Low temperature vortex liquid in $\rm La_{2-x}Sr_xCuO_4$

TL;DR

This study reveals that in lightly-doped La_{2-x}Sr_xCuO_4, a quantum vortex liquid phase persists at very low temperatures over a wide range of magnetic fields, challenging traditional views of the superconducting transition.

Contribution

It provides experimental evidence of a quantum vortex liquid phase in LSCO, showing superconductivity is lost via quantum fluctuations at a critical doping level.

Findings

Superconductivity is lost at critical doping x_c due to quantum phase fluctuations.

Vortex solid-to-liquid transition occurs at a magnetic field H_m much lower than H_{c2}.

Quantum vortex liquid occupies a large part of the phase diagram at low temperatures.

Abstract

In the cuprates, the lightly-doped region is of major interest because superconductivity, antiferromagnetism, and the pseudogap state \cite{Timusk,Lee,Anderson} come together near a critical doping value $x_c$. These states are deeply influenced by phase fluctuations \cite{Emery} which lead to a vortex-liquid state that surrounds the superconducting region \cite{WangPRB01,WangPRB06}. However, many questions \cite{Doniach,Fisher,FisherLee,Tesanovic,Sachdev} related to the nature of the transition and vortex-liquid state at very low tempera- tures $T$ remain open because the diamagnetic signal is difficult to resolve in this region. Here, we report torque magnetometry results on $\rm La_{2-x}Sr_xCuO_4$ (LSCO) which show that superconductivity is lost at $x_c$ by quantum phase fluctuations. We find that, in a magnetic field $H$, the vortex solid-to-liquid transition occurs at field $H_m$ much lower than the depairing field $H_{c2}$. The vortex liquid exists in the large field interval $H_m \ll H_{c2}$, even in the limit $T\to$0. The resulting phase diagram reveals the large fraction of the $x$-$H$ plane occupied by the quantum vortex liquid.