Anomalous enhancement of electronic heat capacity in the organic conductorsκ−(BEDT−TTF)4Hg3−δX8(X=Br,Cl)

Abstract

Peculiar low-temperature electronic properties induced by the filling change from a half-filled band realized in $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}{\mathrm{Hg}}_{2.89}{\mathrm{Br}}_{8}$ and $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}{\mathrm{Hg}}_{2.78}{\mathrm{Cl}}_{8}$, which belong to the well-known $\ensuremath{\kappa}$-type family of organic conductors, were studied by thermodynamic measurements. We observed unusual behaviors both in the lattice and the electronic heat capacities. The former is associated with the dimensional crossover in the acoustic phonons, which is attributable to the peculiarity of mercury chains in the anion layers. The electronic heat capacity represented by the $\ensuremath{\gamma}$ term at low temperatures is enhanced up to $52--55\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.2em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}1}$ [for the formula ${(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Hg}}_{1.5\ensuremath{-}1∕2\ensuremath{\delta}}{X}_{4}$] due to anomalous growth of the antiferromagnetic spin fluctuations in these salts.