SR study of ferromagnetic fluctuations in heavily overdoped Bi-2201 cuprates

High-temperature superconductivity in cuprates is known to emerge by hole doping into the antiferromagnetic (AF) Mott insulators. Superconducting (SC) transition temperature Tc indicates the maximum in the optimally doped regime and the superconductivity disappears with heavy overdoping. Theoretical studies in this decade1,2) have suggested that the decrease in Tc with hole doping in the overdoped (OD) regime is related to the development of ferromagnetic (FM) fluctuations. In fact, it has been reported from zero-field (ZF) SR, ab-plane electrical resistivity ab and magnetization measurements that the FM fluctuations are observed in non-SC heavily overdoped (HOD) La2-xSrxCuO4. However, because the FM fluctuations have been observed only in La2-xSrxCuO4 thus far, the universality of the FM fluctuations in cuprates is still unclear. To investigate the universality of the FM fluctuations, we performed ab, magnetization, and measurements on Bi-2201 cuprates.4) It was found that the temperature dependence of ab is proportional to T4/3 in the OD and HOD regimes in a wide temperature range between 2-310 K. This dependence is a characteristic of metals with two-dimensional FM fluctuations according to the self-consistent renormalization (SCR) theory.5) The magnetization curves in the HOD regime show a tendency to be saturated in high magnetic fields below 20 K. This is a precursor phenomenon to the formation of a possible FM order. Transverse-field (TF) measurements on HOD Bi-2201 also show that muon spin relaxation rate increases with decreasing temperature below 100 K and that the enhancement of is more pronounced in 9.5 T than in 3 T. These results suggest that the spin correlation is enhanced by the application of a magnetic field, which is consistent with the existence of FM fluctuations. To further investigate the temperature and doping dependences of the FM fluctuations in Bi-2201, we performed ZFSR measurements on non-SC HOD (Tc < 2 K) and SC OD (Tc = 13 K) Bi1.76Pb0.35Sr1.89CuO single crystals (Bi-2201[A]) using a MiniCryo and a 3He cryostat at RIKEN-RAL. The hole concentration per Cu of p was estimated from the thermoelectric power at 290 K. Figure 1 shows the temperature dependence of in the single crystals of non-SC HOD (p = 0.274) and SC OD (p = 0 . 2 3 6 ) B i 2 2 0 1 [ A ] . T h e r e s u l t s o f H O D Bi1.71Pb0.32Sr1.97CuO (Bi-2201[B]) with p = 0.264 at J-PARC, Japan, are also shown. In the HOD Bi-2201[A]