Magnetic phase diagram of the layered superconductor Bi{2+x}Sr{2-x}CuO{6+\delta} (Bi2201) with Tc~7 K

TL;DR

This study maps the magnetic phase diagram of Bi2201 superconductor, revealing phase fluctuations, critical scaling behavior, and fluctuation effects above the transition temperature using magnetization measurements.

Contribution

It provides the first detailed phase diagram of Bi2201 with analysis of phase fluctuations and fluctuation effects extending the Ginzburg-Landau theory.

Findings

Deviation of the exponent m from 1/2 indicates phase fluctuations.

The phase diagram lacks upward curvature and is below the Ta(H) line.

Estimated Hc2(0) is twice the value from the phase diagram.

Abstract

We report on magnetization measurements performed on a single crystal of Bi{2+x}Sr{2-x}CuO{6+\delta} with Tc\sim7 K for H||c-axis. The isofield M(T) curves show a large reversible region, with a pronounced rounding effect as M approaches zero which prevents the determination of Tc(H). Deviations from the linear behavior of magnetisation near Tc(H) are studied through the asymptotic behavior of $\sqrt{-M(T)}\sim(T-Ta)^m$, where m is an exponent and Ta an apparent temperature transition. Values of m deviates from the expected mean field value, 1/2, suggesting the importance of phase fluctuations. Resulting values of Ta, interpreted as the onset of phase correlations, decrease as field increases showing an upward curvature. Values of Tc(H) are obtained through a two-dimensional critical scaling analysis obeyed by many M(T) curves. The resulting phase diagram do not show upward curvature and lies below the Ta(H) line. The value of H{c2}(0) estimated from the initial slope $H_{c2}/dT is twice the value suggested by the phase diagram. Amplitude fluctuations above Ta(H) are explained in terms of a Ginzburg-Landau approach extended to high reduced temperatures and magnetic fields by the introduction of a total-energy cutoff in the fluctuation spectrum.