Ni-Driven Evolution of Pseudogap and Superconducting Gap in Underdoped Cuprate

Abstract

In the presented study, Ni impurity is used to probe pseudogap and superconducting gap in the underdoped cuprate La_1.85Sr_0.15CuO₄.  The longitudinal  magnetoresistance  measurements reveal the presence of local maximum at the same temperature T_max,  below which  zero-field resistivity in  the parallel-resistor model  deviates  from its T²-dependence. Tracking the systematic evolution with Ni content y allows equating T_max with pseudogap temperature T^*. The rate of pseudogap closing by the magnetic field is consistent with the spin-paramagnetic effect, which is also the main pair-breaking process destroying superconductivity in the same field configuration. The thermal energy scale of pseudogap k_BT^* for y=0 is equal to the relevant magnetic Zeeman energy scale. With increasing y, both energy scales separate from each other. The increased Stoner factor is partially responsible for the enhancement of the Pauli effect and the decrease of the Zeeman limiting field with increased y.