This article reviews and discusses the thermodynamic properties of dimer-Mott-type molecular superconductive compounds with (BEDT-TTF)₂X composition, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes counter-anions, respectively. We focus mainly on the features occurring in the κ-type structure in which the d-wave superconductive phase appears depending on the Coulomb repulsion U and the bandwidth W, which is tunable by external and chemical pressures. First, we report the high-pressure ac (alternating current) calorimetry technique and experimental system constructed to measure single-crystal samples of molecule-based compounds to derive low-temperature thermodynamic parameters. Using extremely small resistance chips as a heater and a thermometer allows four-terminal detection of an accurate temperature and its oscillation in the sample part with sufficient sensitivity. From the analyses of the temperature dependence of the ac heat capacity of κ-(BEDT-TTF)₂Cu(NCS)₂ under external pressures, we discuss the changes in the peak shape of the thermal anomaly at the superconductive transition temperature T_c at various external pressures p. The rather sharp peak in C_pT⁻¹ at T_c = 9.1 K with a strong coupling character at ambient pressure is gradually reduced to weaker coupling as the pressure increases to 0.45 GPa concomitant with suppression of the transition temperature. This feature is compared with the systematic argument of the chemical–pressure effect on the basis of thermal anomalies around the superconductive transition of κ-(BEDT-TTF)₂X compounds and other previously studied typical dimer-Mott 2:1 compounds. Finally, the discussion is extended to the chemical pressure effect on the normal state electronic heat capacity coefficient γ obtained by applying magnetic fields higher than H_c2 and the residual γ*, which remains in the superconductive state due to the induced electron density of states around the node structure. From the overall arguments with respect to both chemical and external pressures, we suggest that a crossover of the electronic state inside the superconductive phase occurs and the coupling strength of electron pairs varies from the electron correlation region near the metal-insulator boundary to the band picture region.

We investigate low-temperature electronic properties of the nondimeric organic superconductor ${\beta }^{\prime \prime }\text{−}(\mathrm{ET}{)}_4[({\mathrm{H}}_3\mathrm{O})\mathrm{Ga}({\mathrm{C}}_2{\mathrm{O}}_4{)}_3]\mathrm{Ph}{\mathrm{NO}}_2$. By examining ultrasonic properties, charge disproportionation (CD) without magnetic field dependence is detected below $T_{\mathrm{CD}}\sim 8\mathrm{K}$ just above the superconducting critical temperature $T_c\sim 6\mathrm{K}$. From quantum oscillations in high fields, we find variation in the Fermi surface and mass enhancement induced by the CD. Heat capacity studies elucidate that the superconducting gap function is fully gapped in the Fermi surface, but anisotropic with fourfold symmetry. We point out that the pairing mechanism of the superconductivity is possibly dominated by charge fluctuations.

Systematic variation of the low-temperature heat transport properties of the quasi-two-dimensional quantum spin liquid (QSL) candidate ${\beta }^{\prime }\text{−}{\mathrm{EtMe}}_3\mathrm{Sb}\left[\mathrm{Pd}\right({\mathrm{dmit})}_2{]}_2$ and its analog compounds showing antiferromagnetic (AFM) ordering is discussed using thermal conductivity $\kappa$ measurements. All compounds with the QSL ground state show a monotonic decrease in $\kappa$ with decreasing temperature, which is a typical feature of glass or amorphous materials, caused by their low-energy phonon structures. In contrast, the AFM compounds exhibit a peak structure of $\kappa$ around 10 K, which is the characteristic temperature dependence of normal crystals with coherent phonons. Our results indicate that the anomalous glassy behavior in ${\beta }^{\prime }\text{−}{\mathrm{EtMe}}_3\mathrm{Sb}\left[\mathrm{Pd}\right({\mathrm{dmit})}_2{]}_2$ is an intrinsic and universal feature of the organic QSL phase and is not caused by extrinsic factors, such as the quality of the samples and the microcracks occurring during the cooling process. We propose a scenario for the origin of the glassy behavior of QSL compounds in which the charge fluctuation within the Pd(${\mathrm{dmit})}_2$ dimers changes the phonon properties from crystalline to glasslike.