Magnetic phase diagram of the layered superconductor Bi2+xSr2−xCuO6+δ (Bi2201) with Tc ≈ 7 K

Abstract

We report on magnetization measurements performed on a single crystal of Bi2+xSr2−xCuO6+δ with Tc ≈ 7 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 magnetization near Tc(H) are studied through the asymptotic behavior of , where m is an exponent and Ta an apparent temperature transition. Values of m deviate 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 diagrams do not show upward curvature and lie below the Ta(H) line. The value of Hc2(0) estimated from the initial slope dHc2/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.