Electronic structure of insulating salts of the $\kappa -{(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})}_{2}X$ family studied by low-temperature specific-heat measurements

The low-temperature specific heat of $\kappa$-${\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2$X ($X=\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl} \mathrm{and} \mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$) and $\kappa -{(d_8, \mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$ have been studied down to about 0.85 K and under magnetic fields up to 8 T. The $\kappa$-${\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2$Cu[N${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_2$]Cl and $\kappa -{(d_8, \mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$ compounds do not have any finite $\gamma$ value, which indicates they are of an insulating nature with nonvanishing charge gap at low temperatures. The $X=\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$ salt shows no distinct peak around the antiferromagnetic transition at 26-27 K, which suggests a two-dimensional short-range magnetic order far above the three-dimensional ordering temperature typical of a high-$T_c$ cuprate.