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

Abstract

The internal magnetic field distribution in a mixed state of a cuprate superconductor, Ca${}_{2-x}$Na${}_x$CuO${}_2$Cl${}_2$ ($T_{\mathrm{c}}\simeq 28.5$ K, near the optimal doping), was measured by muon spin rotation ($\hat{I}¼$SR) technique up to 60 kOe. The $\hat{I}¼$SR linewidth $\hat{I}›(B)$ which exhibits excess broadening at higher fields ($B>5$ kOe) due to field-induced magnetism (FIM), is described by a relation, $\hat{I}›(B)\propto \sqrt{B}$. 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 $T_{\mathrm{c}}$, as inferred from the trace of FIM observed up to $\sim 80$ K, and develops continuously without a singularity at $T_{\mathrm{c}}$ as the temperature decreases.