Microscopic evidence for a partially gapped density wave state in α-(BEDT–TTF) 2KHg(SCN) 4 in high magnetic fields

Abstract

We present NMR measurements on α-(BEDT–TTF) 2KHg(SCN) 4, in which the six inner carbon sites of BEDT–TTF are labeled with the 13C isotope, at low temperatures and in magnetic fields up to 28.8 T. We find, based on microscopic measurements which probe the center of the primary molecular orbital, that the density wave ground state of this system persists up to fields well above the so-called “kink field”, a hysteretic transition observed in transport near 23 T below 6 K. The main implication of this result is that in the low temperature limit, the ground state of this material is not fully metallic, but still partially gapped, even to 28.8 T. We obtain a BCS-like relationship between the transition temperature and the energy gap, as derived from the spin relaxation measurements using a simple model. Further, the 13C NMR spectrum is relatively insensitive to crossing the phase boundary, a result that does not support the notion that the ground state is a conventional spin density wave ground state.