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-(BEDT-TTF) 4 Hg 2.89 Br 8 is an ambient-pressure superconductor with a transition temperature (T c ) value of 4.3 K. This salt can be considered as an electron system that is naturally carrier doped into a Mott insulator by a slight excalation of the Hg ions. In order to obtain information about the nature of the metallic and superconducting states in this doping system, we performed electrical resistivity measurements as functions of temperature and pressure. We revealed that T c showed an anomalous two-peak structure along the pressure axis. In addition, we found that the power of the temperature in the low-temperature normal-state resistivity correlated with the T c diagram. We concluded that an observable critical boundary existed between the non-Fermi-liquid and Fermi-liquid states in this system.

We studied the normal-state properties of an organic superconductor, -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, through the investigation of the Hall effect as functions of temperature and pressure. At low pressures, the Hall coefficient shows an apparent increase with lowering temperature and a following peak structure. In this pressure range, the Hall effect possibly correlates with the spin fluctuation. At high pressures, we observed a drastic decrease and sign change in the Hall coefficient. With the dissimilarity with the behavior of similar compounds such as -(BEDT-TTF) 2 Cu(NCS) 2 and -(BEDT-TTF) 2 Cu(N[CN] 2 )Cl, we concluded that this behavior originates from a superstructure below 200 K.KEYWORDS: organic superconductor, Hall effect, superstructure, Fermi surface, high pressure It is generally known that the Hall effect depends on the Fermi surface topology and carrier density in conducting systems. The metallic material that has an uncomplicated Fermi surface and one kind of carrier is the simplest example for the test of this relation. In this case, the Hall coefficient, R H , has the value connected to the relation R H = 1/ne, where n is the carrier density and e is the electron charge. Under this condition, R H does not depend on temperature as long as the Fermi energy is much higher than the measured temperatures.Organic conductors are known to have a simple Fermi surface created by the overlap of molecular orbitals. However, the temperature-dependent Hall coefficient is often observed. K. Murata et al. studied the Hall effect of -(BEDT-TTF) 2 I 3 and -(BEDT-TTF) 2 Cu(NCS) 2 (hereafter referred to as -NCS) as functions of temperature and pressure 1) , where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene. They found that R H increased with decreasing temperature at ambient and low pressures and a further increase in pressure suppressed such behaviors.-and -(BEDT-TTF) 2 X salts with the monovalent anion, X, have a dimeric molecular structure in the conducting layers. By virtue of the dimerization, the 1/2-hole system originally introduced to each BEDT-TTF by the anion can be regarded as a half-filled system. In addition, it is considered that the electron-electron correlation is enhanced by this dimerization.2)The Hall effect in such highly correlated electron systems is an intriguing issue. Many inorganic correlated systems such as V 2 O 3 , high-T C cuprate, and heavy fermion systems often show anomalous transport properties including temperature-dependent R H . In this sense, a high-pressure study of the Hall effect in an organic correlated system is worthwhile since the correlation effect is easily suppressed by applying pressure especially in organics. In this respect, we studied the Hall effect of one of the organic superconductors, -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br (hereafter referred to as -Br), under pressures in addition to Murata's works. The Hall effect of this material has not been studied so far although it is well known to be a highest-T C ambient-pressure superconductor. We found that R H of th...

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