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We demonstrate that an organic Mott insulator, 0 -(BEDT-TTF) 2 ICl 2 , is metallized by application of extremely high pressure by means of the cubic anvil press. When the metallic state is stabilized, superconductivity with the highest transition-temperature (T c ) among the organic systems appears. The lower limit of the pressure at which the superconducting transition manifests itself is approximately 7.0 GPa and T c reaches a maximum value, 14.2 K (onset), at 8.2 GPa. The pressure-temperature phase diagram is constructed and discussed in terms of the comparison with those of other organic systems.

We present a systematic study of the dc-resistivity, Hall effect, and magnetoresistance in the normal state of quasi two-dimensional (2D) heavy fermion superconductors CeMIn 5 (M: Rh and Co) under pressure. Here the electronic system evolves with pressure from an antiferromagnetic (AF) metal, through a highly unconventional non-Fermi liquid, and finally into a Fermi-liquid state. The novelty of these materials is best highlighted when compared with LaMIn 5 , a system with similar electronic structures, which shows a nearly temperature independent Hall coefficient and a magnetoresisitance which is well described by the classical Kohler's rule. In sharp contrast, in CeMIn 5 , the amplitude of the Hall coefficient increases dramatically with decreasing temperature, reaching at low temperatures a value significantly larger than 1=ne, where n is the carrier number. Furthermore, the magnetoresistance is characterized by T-and H-dependence which clearly violate Kohler's rule. We found that the cotangent of the Hall angle cot  H varies as T 2 , and the magnetoresistance is well scaled by the Hall angle as Á xx = xx / tan 2  H . These non-Fermi liquid properties in the electron transport are remarkably pronounced when the AF fluctuations are enhanced in the vicinity of the quantum critical point. We lay particular emphasis on the striking resemblance of these anomalous magnetotransport with those of the high-T c cuprates. We argue that features commonly observed in quasi 2D heavy fermion and cuprates very likely capture universal features of strongly correlated electron systems in the presence of strong AF fluctuations, holding the promise of bridging our understanding of heavy fermion systems and high-T c cuprates.

We present the results of electrical resistivity, ac specific heat, magnetic susceptibility, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) of the ternary iron arsenide EuFe2As2 single crystal under pressure. Applying pressure leads to a continuous suppression of the antiferromagnetism associated with Fe moments and the antiferromagnetic transition temperature becomes zero in the vicinity of a critical pressure Pc ~2.5-2.7 GPa. Pressure-induced re-entrant superconductivity, which is highly sensitive to the homogeneity of the pressure, only appears in the narrow pressure region in the vicinity of Pc due to the competition between superconductivity and the antiferromagnetic ordering of Eu2+ moments. The antiferromagnetic state of Eu2+ moments changes to the ferromagnetic state above 6 GPa. We also found that the ferromagnetic order is suppressed with further increasing pressure, which is connected with a valence change of Eu ions.Comment: 7 pages, 7 figures, accepted for publication in Phys. Rev.

Transport, thermal and magnetic measurements have been carried out on (Pr1-ySmy)1-xCaxCoO3. The system exhibits a structural phase transition accompanied by the spin state change from the intermediate spin (IS) state to the low spin (LS) state with decreasing temperature T. We have constructed a T-y phase diagram for x=0.3 and T-x ones for y=0.2 and 0.3. By analyzing their magnetic susceptibilities, the number of Co ions excited to IS state (or the electron number in the eg orbitals), nIS, are roughly estimated. With increasing y or with decreasing x, nIS decreases, and the phase transition changes gradually to the (IS-LS) crossover-like one. We discuss on the possible role of the Pr atoms in realizing the transition.Comment: 13 pages, 12 figures, Submitted to J. phys. Soc. Jp

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We have measured the electrical resistivity of a single crystal of YbCo 2 Zn 20 at pressures up to 2.37 GPa and at temperatures from 50 mK to 300 K. Above a critical pressure P c ($ 1 GPa), we have found a resistivity anomaly at T M ($ 0:15 K at 1 GPa) that increases with the pressure. At the ambient pressure, the system shows a nonmagnetic ground state described by the Fermi-liquid model. The T 2 coefficient of the electrical resistivity A strongly increases with the pressure upon approaching P c . However, in the vicinity of P c , the temperature dependence of the resistivity deviates from the Fermiliquid description. These observations suggest that the application of hydrostatic pressure induces a magnetically ordered state for P ! P c and T T M .In intermetallic compounds, including Ce and Yb, the hybridization between the 4f and itinerant conduction-band electrons induces the instability of magnetic moments and charge configurations. In recent years, one of the most interesting topics is the ground state properties of heavy fermion metals located at or close to a magnetic quantum critical point (QCP). 1) The application of external pressure is one of the important tools for controlling the electronic configurations as well as chemical pressure. In the case of heavy fermion Ce compounds exhibiting antiferromagnetic order, such as CeIn 3 and CePd 2 Si 2 , 2) the magnetic order is suppressed by applying pressure. Interestingly, unconventional superconductivity appears in the vicinity of the QCP at which the magnetic ordering temperature is decreased to zero.So far, pressure-induced magnetic transitions have been observed in some Yb-based compounds. [3][4][5][6][7] The key point here is that Yb ions fluctuate between the nonmagnetic Yb 2þ (J ¼ 0) and the magnetic Yb 3þ (J ¼ 7=2) states. Since the ionic volume of the magnetic Yb 3þ state is smaller than that of the nonmagnetic Yb 2þ one, applying pressure stabilizes the magnetic Yb 3þ configuration and induces the appearance of a magnetically ordered state in contrast to the Ce case. However, the critical pressure for these compounds is as high as or higher than 6 GPa, which prevents us from understanding the physics in the vicinity of the magnetic QCP because of difficulties in high-pressure experiments.The series of compounds YbT 2 Zn 20 (T = Fe, Co, Ru, Rh, Or, Ir) belongs to a new heavy fermion system crystallizing in the cubic CeCr 2 Al 20 structure. 8,9) All these compounds show an enhanced Sommerfeld coefficient of the specific heat exceeding 400 mJ/(mol K 2 ). The high-temperature magnetic susceptibility of these compounds follows the Curie-Weiss law with the effective moments close to the value for the free Yb 3þ ion ( eff ¼ 4:54 B ), although there is no indication of magnetic order down to 20 mK. In the YbT 2 Zn 20 family, YbCo 2 Zn 20 exhibits some notable features as follows. The low temperature electrical resistivity and specific heat can be described by the formulas ¼ 0 þ AT 2 and C=T ¼ constant, as expected from the Fermiliquid behavior; the value...

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