Linear-field dependence of the normal-state in-plane magnetoresistance of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$

The transverse and longitudinal in-plane magnetoresistances in the normal state of superconducting ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ single crystals have been measured. At low temperatures, both of them were found to be positive with a linear magnetic-field dependence above a threshold field, a result not expected from electronic band theory. We argue that such behavior is a manifestation of a novel coherent state characterized by a spin pseudogap in the quasiparticle excitation spectrum in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$