Y. Haga

A change of the Fermi surface in UGe2 across the critical pressure

We carried out the de Haas-van Alphen (dHvA) experiment under pressure for a ferromagnet UGe2. The dHvA frequency of a main dHvA branch named α, that corresponds to a majority up-spin band 40-hole Fermi surface, decreases monotonously with increasing pressure, but in the pressure range from pc* (1.2 GPa) to pc (1.5 GPa) the dHvA signal disappears completely, where pc and pc* correspond to critical pressures for a Curie temperature TC and the second phase transition temperature T* ( pc we observed new dHvA branches with large cyclotron masses of 19-64m0, which correspond to main Fermi surfaces in the paramagnetic state.

NON-CENTROSYMMETRIC HEAVY FERMION SUPERCONDUCTIVITY IN CeCoGe3

We measured the electrical resistivity under pressure for an antiferromagnet CeCoGe3 without inversion symmetry in the tetragonal crystal structure. The Neel temperature TN1 = 21 K at ambient pressure decreases monotonically with increasing pressure, and becomes zero at about 5.5 GPa. Superconductivity appears above 4.3 GPa, with a superconduting transition temperature Tsc = 0.72 K and a large upper critical field Hc2(0) = 7 T at 5.6 GPa. The large upper critical field Hc2(0)= 7 T exceeds the Pauli limitting field Hp (≃ 1.86Tsc)=1.3 T as in CePt3Si, CeRhSi3 and CeIrSi3. The large slope of Hc2 at Tsc, -dHc2/dT = 18T/K, at 5.6 GPa indicates the heavy-fermion superconductivity in CeCoGe3.

Long-period, longitudinal spin density modulation in an itinerant 5f magnetic compound UCu2Si2

The magnetic structure of UCu2Si2 has been studied by means of neutron scattering. We have observed the first- and very weak third-order incommensurate satellite reflections for 100 K<T<106 K with the propagation q = [0 0 δ], δ = 0.116 at T = 101 K, indicating nearly sinusoidal magnetic modulation with long periodicity of Λ = 85.7 A. The intensity of the first-order satellite reflections is consistent with a moment direction parallel to the c-axis. The second-order satellite due to lattice modulation accompanied by a charge density wave could not be detected within our experimental sensitivity. The long-period magnetic modulation in UCu2Si2 cannot be explained in terms of an ANNNI model based on frustrated antiferromagnetic short-range interactions. The nearly sinusoidal long-period modulation is evidence for a spin density wave state as a consequence of itinerant behaviour of the 5f electrons in UCu2Si2. The ferromagnetic ground state and the very weak upper critical field for the incommensurate phase are consistent with the long periodicity.

A change of the Fermi surface across the metamagnetic transition under pressure in UGe2

We have performed the de Haas-van Alphen (dHvA) and ac-susceptibility experiments on a ferromagnet UGe2 under pressure for the field along the easy a-axis. The p-H phase diagram was determined, indicating three kinds of phase: paramagnetic and weakly and strongly polarized phases. The detected dHvA frequencies as well as the cyclotron masses are found to be very different between the weakly and strongly polarized states. A change of the Fermi surface occurs when the weakly polarized phase is changed into the strongly polarized phase with increasing magnetic field.

Shubnikov-de Haas effect study of cylindrical Fermi surfaces in UAs2

Shubnikov-de Haas effect measurements were performed on a high-quality whisker single crystal of UAs2 . Complementary electrical resistivity and specific heat experiments were also carried out. Five of the seven detected branches exhibit angular dependences clearly indicating cylindrical Fermi surfaces. The effective cyclotron masses were determined to range from 0.33 to 4.5 m 0 , which yielded an electronic specific heat coefficient of 10 mJ K-2 mol-1 . This is in very good agreement with the value 11.7 mJ K-2 mol-1 determined directly by the low-temperature specific heat measurement. The two-dimensional Fermi surfaces are formed in a strongly flattened magnetic Brillouin zone and the conduction electrons are confined mainly in the U planes, indicating an important contribution of itinerant 5f electrons.

Studies of superconductivity and magnetism in UPt3 by μ+SR

Abstract We have performed μ+SR studies on a single-crystal UPt3 by implanting spin-polarized μ+ parallel to the c-axis in a zero magnetic field from 10 K down to 20.8 mK in order to study magnetism and superconductivity. Down to 20.8 mK, zero-field μ+SR spectra have not exhibited a significant change; thus, neither a spontaneous magnetization in the B phase nor a precursor of a long-range magnetic ordering has been observed down to 20.8 mK.

Magnetic and electronic properties in CeTSi3 and CeTGe3 (T: Transition metal)

We investigated the magnetic properties of CeTSi3CeTSi3 (T: Ru, Os, Co, Rh, Ir, Pd and Pt) and CeTGe3CeTGe3 (T: Co, Rh and Ir) by measuring their electrical resistivity and magnetic susceptibility. CeRuSi3CeRuSi3, CeOsSi3CeOsSi3 and CeCoSi3CeCoSi3 do not order magnetically, with a large Kondo temperature of about 200K. The other compounds order antiferromagnetically, and are very similar to each other in their magnetic and electronic properties, which is related to a large crystalline electric field (CEF) splitting energy of the 4f4f electron, about 500K in CeIrSi3CeIrSi3.

Electronic band structure and Fermi surface of URu2Si2 studied by high-resolution angle-resolved photoemission spectroscopy

Abstract We have studied the electronic band structure and the Fermi surface of paramagnetic URu 2 Si 2 with high-resolution angle-resolved photoemission spectroscopy. It was found that Ru 4d bands form a main body of the valence band and exhibit a remarkable energy dispersion in qualitatively good agreement with the band structure calculation. In addition to the dispersive Ru 4d bands, we found a less dispersive band near the Fermi level, which is assigned to the U 5f-Ru 4d hybridized band forming a hole-like Fermi surface at Z point in the Brillouin zone.

Magnetic and electronic properties in CeTSi3CeTSi3 and CeTGe3CeTGe3 (T: transition metal)

We investigated the magnetic properties of CeTSi3CeTSi3 (T: Ru, Os, Co, Rh, Ir, Pd and Pt) and CeTGe3CeTGe3 (T: Co, Rh and Ir) by measuring their electrical resistivity and magnetic susceptibility. CeRuSi3CeRuSi3, CeOsSi3CeOsSi3 and CeCoSi3CeCoSi3 do not order magnetically, with a large Kondo temperature of about 200K. The other compounds order antiferromagnetically, and are very similar to each other in their magnetic and electronic properties, which is related to a large crystalline electric field (CEF) splitting energy of the 4f4f electron, about 500K in CeIrSi3CeIrSi3.

de Haas–van Alphen effect of the heavy-fermion superconductor URu2Si2

Abstract We have measured the de Haas–van Alphen (dHvA) effect of URu2Si2. One kind of the dHvA branch, which corresponds to an almost spherical Fermi surface, has been detected both in the normal and super-conducting mixed states. The dHvA frequency in the mixed state is unchanged from the normal state. The cyclotron effective mass is clearly found to decrease with decreasing the magnetic field below Hc2 and the corresponding Dingle temperature increases with decreasing the field.