Antiferromagnetic fluctuations and proton Schottky heat capacity in doped organic conductor κ-(BEDT-TTF)4Hg2.78Cl8

Low temperature heat capacity measurements of hole-doped organic conducting compound of κ-(BEDT-TTF)4Hg2.78Cl8 were performed by the ac calorimetry and the thermal relaxation technique. It was observed that low temperature electronic heat capacity at 0 T can be fitted by a kind of spin fluctuations model for two-dimensional (2D) itinerant electrons. The enhancement of electronic heat capacity is originated from these magnetic fluctuations occur at extremely low temperature region. We also observed that the low temperature heat capacity under magnetic fields show large additional contribution explained by the two-level Schottky model of protons located on the edge of BEDT-TTF molecules. The strong spin fluctuations peculiar for 2D electrons in this hole-doped organic compound influence the relaxation process of proton spins to get thermal equilibrium state. This is the first example among organic conducting systems where magnetic fluctuations of itinerant electrons affect on the relaxation of proton spins. Similar tendency was observed in another hole-doped system of κ-(BEDT-TTF)4Hg2.89Br8 of which magnitude is reduced down to 20-25% of that of κ-(BEDT-TTF)4Hg2.78Cl8.