Pseudogap in underdoped cuprate seen in longitudinal magnetoresistance

We report the results of in-plane magnetotransport study of slightly underdoped cuprate La1.85Sr0.15CuO4 (LSCO15) with Ni impurity. Increasing Ni content y causes a sharp drop in longitudinal magnetoresistance (LMR) in LSCO15 to broaden and move towards higher temperatures. Temperature T-mLMR(y) of this local maximum in LMR coincides with temperature T-dev(y), below which ideal resistivity from the parallel-resistor model deviates from its T-2-dependence. A direct comparison with the hole doping evolution of pseudogap (PG) in La2-xSrxCuO4 (LSCO), possible through the mobile-carrier concentration extracted from the thermopower measurements, allows to equate both characteristic temperatures T-mLMR congruent to T-dev with PG opening temperature T *. The rate of PG closing by magnetic field parallel to the CuO2 plane, in measurements up to 9 T, is consistent with spin-paramagnetic effect in this configuration and yields PG closing field B-pc close to the second critical field B-c2 predicted for superconducting gap with the help of Werthamer-Helfand-Hohenberg theory. The field anisotropy of B-pc suggests that orbital degrees of freedom also play a role in PG formation.