We confirmed bulk-superconductivity of a ferromagnet UGe 2 by the specific heat measurement, together with the measurements of the electrical resistivity and ac susceptibility, in a pressure range from p = 1.0 to 1.5 GPa, where the Curie temperature T C (= 22-36 K) is still high, but another characteristic temperature T * is close to zero. In this pressure range, the heavy fermion state is found to be formed at low temperatures.Cerium and uranium compounds indicate a variety of phenomena including magnetic and quadrupolar ordering, heavy fermion and anisotropic superconductivity [1]. In these compounds, the RKKY interaction and the Kondo effect compete with each other. The former interaction enhances the long-range magnetic order, while the latter effect quenches the magnetic moments of localized f electrons. Most of the cerium and uranium compounds order magnetically, where the former interaction overcomes the latter effect. When the magnetic ordering temperature is low enough or close to zero, the heavy fermion state is formed at low temperatures. The conduction electrons in the heavy fermion state are highly different from bare electrons. They are interacting electrons, moving slowly in the crystal, which correspond to a large effective mass m * or a large electronic specific heat coefficient γ .When pressure p is applied to the cerium compounds with antiferromagnetic ordering such as CeIn 3 and CePd 2 Si 2 , the Néel tempereture T N shifts to lower temperatures, and the magnetic quantum critical point corresponding to the extrapolation T N → 0 is reached at p = p c [2]. Superconductivity appears around p c . Correspondingly, the heavy fermion state is formed as p approaches p c . This seems to be a general feature, although the sample quality is essentially important for the appearance of superconductivity. This is because superconductivity is most likely to be magnetically-mediated or of a non-s-wave type and then the breaking of Cooperpairs is mainly due to impurities and crystal defects.

We performed the DC-magnetization and neutron scattering experiments under pressure P for a pressure-induced superconductor UGe2. We found that the magnetic moment is enhanced at a characteristic temperature T * in the ferromagnetic state, where T * is smaller than a Curie temperature TC. This enhancement becomes remarkable in the vicinity of P * C = 1.20 GPa, where T * becomes 0 K and the superconducting transition temperature TSC shows a maximum. The characteristic temperature T * , which decreases with increasing pressure, also depends on the magnetic field. KEYWORDS: UGe 2 , Ferromagnetism, SuperconductivityUGe 2 is known as a metallic ferromagnet with a Curie temperature of T C = 52 K at ambient pressure.1) The magnetization is extremely anisotropic, with the easy a-axis in the orthorhombic crystal structure. The ordered moment is 1.4 µ B /U well below T C .2) Application of pressure suppresses T C , and finally T C becomes zero at the critical pressure P C of 1.6-1.7 GPa. 3)Recently superconductivity was observed under pressure in UGe 2 .4-6) Superconductivity appears in the pressure range from 1.0 to 1.6 GPa. In other words the present superconductivity disappears at P > P C , namely in the paramagnetic state. The superconducting transition temperature T SC shows a maximum of 0.8 K around 1.2 GPa where the ferromagnetic state is still stable with T C =32K.4-6) The heat capacity anomaly due to the superconducting transition was found at 1.13 GPa, which indicates the bulk property of superconductivity.6) From the elastic neutron scattering experiment under pressure, the ferromagnetic moment of about 1.0 µ B /U was found even in the superconducting state.5) It is thus concluded that superconductivity and ferromagnetism coexist below T SC .In UGe 2 another characteristic anomaly exists at T * in the ferromagnetic state below T C . 3,5,6) This anomaly, which was observed on the resistivity, becomes sharper with applying pressure.@Application of the pressure also suppresses T * , and finally T * becomes 0 K at the another critical pressure P * C = 1.2 GPa, which is smaller than P C . The superconducting transition temperature T SC shows a maximum around P * C .5, 6) @In the pressure just above P * C , the characteristic transition T * is * E-mail:tateiwa@djebel.mp.es.osaka-u.ac.jp induced by the magnetic field and then the upper critical field H C2 is enhanced. 5) Similar enhancement of H c2 has been confirmed in our resistance measurement.7) Therefore it is natural to consider that the critical fluctuation related to the transition at T * may be a driving force for the superconducting pairing in UGe 2 . The previous elastic neutron scattering experiment under high pressure also revealed that the magnetic scattering intensity was slightly enhanced below T * at 0.89 GPa. 5) Cambridge and Grenoble groups suggested the existence of the CDW/SDW state below T * because the nesting was expected from the topology of the Fermi surface. 4,5,8,9) In this study, the magnetic properties under high pressure were investigate...

The electrical resistivity measurements in the magnetic field are carried out on the pressureinduced superconductor UGe2. The superconductivity is observed from 1.06 to 1.44 GPa. The upper critical field of HC2 is anisotropic where HC2(T ) exhibits positive curvature for H // b and c-axis. The characteristic enhancement of HC2 is reconfirmed for H // a-axis. In the temperature and field dependence of resistivity at P > PC where the ferromagnetic ordering disappears, it is observed that the application of the external field along the a-axis increases the coefficient of Fermi liquid behavior AT 2 correspondingly to the metamagnetic transition.

In the references of our paper we omitted to mention the studies of the Grenoble group (Huxley et al 2001) where a similar relation between T* and TSC under high pressure and more detailed results of Hc2 are reported. Huxley A et al 2001 Phys. Rev. B 144519

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