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

We present NMR measurements on alpha-(BEDT-TTF)(2)KHg(SCN)(4), in which the six inner carbon sites of BEDT-TTF are labeled with the C-13 isotope, at l ow temperatures and in magnetic fields up to 28.8 T. We find, based on micr oscopic measurements which probe the center of the primary molecular orbita l, 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 full y metallic, but still partially gapped, even to 28.8 T. We obtain a BCS-lik e relationship between the transition temperature and the energy gap, as de rived from the spin relaxation measurements using a simple model. Further, the C-13 NMR spectrum is relatively insensitive to crossing the phase bound ary, a result that does not support the notion that the ground state is a c onventional spin density wave ground state. (C) 1999 Published by Elsevier Science Ltd.