Muon spin relaxation study of superconducting Bi₂Sr_2−xLa_xCuO_6+δ

We have performed transverse-field (TF) and zero-field (ZF) μSR measurements of Bi₂Sr_2−xLa_xCuO_6+δ (Bi2201) systems with x=0.2, 0.4, 0.6, and 1.0, using ceramic specimens with modest c-axis alignment and single-crystal specimens. The absence of static magnetic order has been confirmed in underdoped (x=0.6) and optimally doped (x=0.4) systems at T=2 K, while only a very weak signature towards static magnetism has been found at T=2 K in the x=1.0 system, which is a lightly hole-doped nonsuperconducting insulator. In the superconducting (x=0.6, 0.4, and 0.2) systems, the relaxation rate σ in TF-μSR, proportional to n_s∕m^* (superconducting carrier density and effective mass), followed a general trend found in other cuprate systems in a plot of T_c vs n_s∕m^*(T→0). Assuming the in-plane effective mass m^* for Bi2201 to be comparable to three to four times the bare electron mass m_e as found in La_2−xSr_xCuO₄ (LSCO) and YBa₂Cu₃O_7−δ (YBCO) systems, we obtain n_s∼0.15–0.2 per Cu for the x=0.4 Bi2201 system. This carrier density is much smaller than the Hall number n_Hall∼10 per Cu obtained at T<1.6 K in high magnetic fields (40–60 T) along the c axis applied to suppress superconductivity. The present results of the superfluid density (n_s∕m^*) in Bi2201 are compared with those from other cuprate systems, including YBCO systems with very much reduced T_c<20 K studied by microwave, H_c1, and inductance methods. Additional muon-spin-relaxation (μSR) measurements have been performed on a single-crystal specimen of Bi2201 (x=0.4) in a high transverse magnetic field of 5 T parallel to the c axis, in order to search for the field-induced muon spin relaxation recently found in LSCO and some other high-temperature superconducting cuprate (HTSC) systems well above T_c. The nearly temperature-independent and very small relaxation rate observed in Bi2201 above T_c rules out a hypothesis that the field-induced relaxation is directly proportional to the magnitude of the Nernst coefficient, which is a measure of the strength of dynamic superconductivity. We also describe a procedure for angular averaging of σ in μSR measurements using ceramic specimens with modest alignment of c-axis orientations, together with the neutron-scattering results obtained for determining the orientation distribution of microcrystallites in the present ceramic specimens.