Observations of Pauli paramagnetic effects on the flux line lattice in CeCoIn₅

Abstract

From small-angle neutron scattering studies of the flux line lattice (FLL) in\nCeCoIn5, with magnetic field applied parallel to the crystal c-axis, we obtain\nthe field- and temperature-dependence of the FLL form factor, which is a\nmeasure of the spatial variation of the field in the mixed state. We extend our\nearlier work [A.D. Bianchi et al. 2008 Science 319, 177] to temperatures up to\n1250 mK. Over the entire temperature range, paramagnetism in the flux line\ncores results in an increase of the form factor with field. Near H_c2 the form\nfactor decreases again, and our results indicate that this fall-off extends\noutside the proposed FFLO region. Instead, we attribute the decrease to a\nparamagnetic suppression of Cooper pairing. At higher temperatures, a gradual\ncrossover towards more conventional mixed state behavior is observed.\n