Thermodynamic Features of the Magnetic Ordering in Low-Dimensional Mott Insulators

We are interested in the thermodynamic features around the Néel temperature of several organic systems consisting of BEDT-TTF molecules and their counter ions. They are known as a kind of Mott insulators, since one electron is localized on each dimmer consisting of BEDT-TTF. Due to the layered structure of donors and anions, the behavior of the 2D magnetic systems is expected. From the heat capacity measurements of β′-(BEDT-TTF)ICl2 and κ-(BEDT-TTF)2Cu(N(CN)2)Cl, we could not detect sharp thermal anomalies, if the reduced entropy around the Néel temperature due to the two-dimensional character is taken into account. It may be reasonable to take into account the quantum character in the Mott insulating states. The reduced entropy around the Néel temperature seems to be the common features of Mott insulators with low-dimensional structure. The frustration behavior of a Mott insulating salt with triangular structure is also reported.

(by S. Yamashita & Y. Nakazawa)

	Fig. 1. Heat capacity of β′-(BEDT-TTF)ICl2. An anti-ferromagnetic transition is suggested at 22 K by NMR spectra and magnetic susceptibility measurement.
	Fig. 2. Heat capacity of κ-(BEDT-TTF)2Cu(N(CN)2)Cl. We have failed to detect any thermal anomaly around 27 K, at which a kind of Néel ordering is reported by NMR experiment.
	Fig. 3. Heat capacity of κ-(BEDT-TTF)2Cu2(CN)3. The lattice heat capacity is very similar to the same type salt κ-(BEDT-TTF)2Cu(NCS)2. However in the aκ-(BEDT-TTF)2Cu2(CN)3, a broad but reproducible thermal anomaly was detected around 5 K.

The Problems Related to the Residual Electron Density of States Existing in the Superconducting State of Organic Superconductors

Focusing on the low-temperature thermodynamic properties, especially those on the superconducting states, we have performed systematic heat capacity measurements through the thermal relaxation calorimeters for several κ-type salts and κ-(BEDT-TTF)2Cu[N(CN)2]Br salts of which donor molecules are partially substituted by BEDSe-TTF. We have observed that the electronic heat capacity coefficient γ obtained by applying magnetic fields varies drastically even in the superconductive phase, if one approaches gradually from the metallic region to the Mott boundary. We also pay attention to the residual γ* value for these materials. This term represents the normal electron density of states remaining in the superconducting state. The γ*/γ increases gradually in the region where the bulk Tc decreases from 10 K and continues to increase with decreasing Tc, where the Brinkmann-Rice enhancement is observable. The growth of Fermi-liquid manner seems to give a worse influence for both Tc and the volume fraction of superconductivity.

(by Y. Nakazawa)

Fig. 1. The conceptual phase diagram of electronic states of dimerized (BEDT-TTF)2X salt. The horizontal axis corresponding to the pressure, which effectively controls the U/W ratio. The behavior of the low-temperature heat-capacity coefficient (γ and γ*)of κ-(BEDT-TTF)2X type organic superconductors is also shown with the phase diagram. The increase of γ/γ* ratio at the metallic region demonstrates that normal electrons are coexisting in the superconducting phase.

Fig. 2. The electronic heat capacity of κ-(BEDT-TTF)2X salts. The horizontal axis is divided by Tc. The shape of the CpT–1 peak is different between 10 K class salts and 4-5 K class salts.

Heat Capacities of p-type Icosahedral Quasicrystals Zn-Mg-RE (RE = Y, Dy, Ho and Er)

Heat capacity Cp(T) measurements have been performed between 0.4 K and 300 K by relaxation method for p-type Zn-Mg-RE (RE = Y, Dy, Ho and Er) icosahedral quasicrystals. The Cp(T) for the Dy, Ho and Er compounds reveal large magnetic contributions below 50 K, which are estimated by comparing a proper lattice reference, and the evolution of magnetic entropies with temperature is analyzed.

(by H. Takakura & D. Kanki)

	Fig. 1. Molar heat capacities of p-Zn-Mg-RE (RE = Y, Dy, Ho and Er) measured by relaxation method.		Fig. 2. Magnetic specific heat contribution Cmag of p-Zn-Mg-RE (RE = Dy, Ho and Er).
	Fig. 3. Magnetic entropies Smag of p-Zn-Mg-RE (RE = Dy, Ho and Er). The levels indicate the entropies expected for the respective free RE3+ ion.

Six-Dimensional Electron Densities of a p-type Icosahedral Quasicrystal Zn-Mg-Ho

We report the phase reconstructed six-dimensional (6D) electron densities of a p-type icosahedral quasicrystal Zn-Mg-Ho by the low density elimination method. The densities exhibit three large occupation domains centered at (0,0,0,0,0,0), (1,1,1,1,1,1)/2, and (1,0,0,0,0,0)/2 in the 6D primitive unit cell. It is shown that a Bergman-type icosahedral atomic cluster characterizes the quasicrystal structure.

(by H. Takakura)

	Fig. 1. 2D cut of reconstructed 6D electron densities containing a five-fold axis in both the physical (r∥) and complementary (r⊥) space directions. The inner rectangle with thick lines indicates the unit cell. The V: (0,0,0,0,0,0), B:(1,1,1,1,1,1)/2, and E(1,0,0,0,0,0)/2 indicate the special points in the unit cell. Successive icosahedral shells can be found from the positions indicated by the letters inside circles (a), (b), (c).
	Fig. 2. 2D cut of reconstructed 6D electron densities containing a threefold axis in both the physical (r∥) and complementary (r⊥) space directions. Successive dodecahedral shells can be found from the positions indicated by the letters inside circles (d), (e). The meaning of the other symbols is the same as in Fig. 1.
	Fig. 3. Successive atom shells in ascending order that found by the analysis of the reconstructed 6D electron densities of the p-Zn-Mg-Ho. Atom species and atom numbers forming each atom shell together with the radius of the shell are given underneath each shell.