Field-induced magnetism in the high-$T_c$ superconductor Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ with flat CuO$_2$ planes

TL;DR

This study investigates the magnetic field effects in a high-$T_c$ cuprate superconductor using muon spin rotation, revealing field-induced magnetism and vortex dynamics that extend above the critical temperature.

Contribution

It provides the first detailed measurement of field-induced magnetism in Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ and links it to quasiparticle excitations and vortex fluctuations.

Findings

Excess broadening of $ ext{muon}$ linewidth at high fields indicates field-induced magnetism.

$ ext{Lambda}(B)$ scales as $ ext{sqrt}(B)$, suggesting orbital current influence.

Vortex fluctuations slow down above $T_c$, observable up to 80 K.

Abstract

The internal magnetic field distribution in a mixed state of a cuprate superconductor, Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ ($T_{\rm c}\simeq28.5$ K, near the optimal doping), was measured by muon spin rotation ($\mu$SR) technique up to 60 kOe. The $\mu$SR linewidth $\Lambda(B)$ which exhibits excess broadening at higher fields ($B>5$ kOe) due to field-induced magnetism (FIM), is described by a relation, $\Lambda(B)\propto\sqrt{B}$. This suggests that the orbital current and associated quasiparticle excitation plays predominant roles in stabilizing the quasistatic correlation. Moreover, a slowing down of the vortex fluctuation sets in well above $T_{\rm c}$, as inferred from the trace of FIM observed up to $\sim80$ K, and develops continuously without a singularity at $T_{\rm c}$ as the temperature decreases.