K Miyake

Superconductivity of Ce-based heavy fermions under pressure: Valence fluctuation mediated pairing associated with valence instability of Ce

Abstract It is argued that the coincidence of peaks of the residual resistivity and the superconducting transition temperature observed in CeCu2Ge2 under pressure can be understood as a phenomenon of enhanced valence fluctuations associated with rapid valence change which is signaled by rapid decrease of the coefficient A of T2-term of the resistivity. A type of Cooper pairing predicted by this scenario can be different from that mediated by the antiferromagnetic spin fluctuations. The superconductivity observed in isostructural compounds, CeCu2Si2 and CeNi2Ge2, under pressure should be understood in parallel.

Thermal properties of metamagnetic transition in heavy-fermion systems

Abstract In order to investigate the metamagnetic anomaly (MA) observed in several heavy-fermion compounds, field dependence of specific heat C(B) and magnetocaloric effect have been measured in the prototypical 4f-electron compound CeRu2Si2, and compared to that of a 5f-electron metamagnet UCoAl. In the C/T versus B curves, an unexpected double-peak structure is found in CeRu2Si2, suggesting that MA is closely related to a sharp peak in the density of states of the hybridized quasiparticle bands. On the other hand, for UCoAl, only a small decrease in C/T is observed, although both compounds exhibit similar bulk magnetic properties.

Electronic States of the Antiferromagnet UGa3

We have grown a single crystal with the residual resistivity ratio of 81, and studied its electronic properties of the cubic antiferromagnetic compound UGa 3 . From the transverse magnetoresistance...

Sharp valence transition caused by f–c Coulomb interaction in an extended periodic Anderson model

Abstract An extended periodic Anderson model with Uff=∞ and Ufc, f–c Coulomb interaction, is investigated by the variational Monte Carlo (VMC) method. In order to parameterize the effect of charge transfer between f- and c-electron states caused by Ufc, a projection operator ∏iσσ′[1−(1−h)niσfniσ′c] is introduced in the variational function together with the conventional Gutzwiller projection operator ∏i[1−ni↑fni↓f]. It is shown that the valence of f-electrons decreases rapidly as the level of f-electrons ef is increased, if Ufc is large enough comparable to half of the bandwidth of conduction electrons. The result implies a possibility of valence fluctuation mechanism as an interpretation of the coincidence of peaks of the residual resistivity and superconducting transition temperature observed in CeCu2Ge2 under pressure.

Magnetic and Electrical Properties in a Dense Kondo Compound PrSn3.

We measured the specific heat, thermal expansion coefficient, magnetization, electrical resistivity, magnetoresistance and de Haas-van Alphen (dHvA) effect in PrSn 3 . The dense Kondo effect was confirmed from a negative magnetoresistance and the presence of heavy electrons with the cyclotron effective mass of 8–9 m 0 for the main Fermi surface. The magnetic phase diagram was also clarified by the high-field magnetization measurement.

Fermi Surface Properties in Sr2RuO4

We measured the de Haas-van Alphen (dHvA) effect, magnetoresistance and specific heat to clarify the two-dimensional Fermi surface properties of Sr 2 RuO 4 . From the angular dependence of the dHvA amplitude, we determined the Yamaji angle for three quasi-cylindrical Fermi surfaces α, β and γ to be 30.6° for the Fermi surface α, about 30° for β and 15.3° for γ. The cyclotron masses of sum and difference oscillatory components, m * β±α and m * γ±α , are found to be highly different from m * β + m * α and m * γ + m * α , respectively, for any field direction. The dHvA oscillation due to sums and differences is not due to the magnetic interaction but to the chemical potential oscillation for quasi-two-dimensional Fermi surfaces. The specific heat data are discussed from a viewpoint of a characteristic energy gap in a possible p -wave superconductor Sr 2 RuO 4 .

Fermi Surface Properties of USi3

We succeeded in growing a single crystal of USi 3 with the cubic crystal structure and measured the transverse magnetoresistance and de Haas-van Alphen(dHvA) oscillation. Fifteen dHvA branches were observed, ranging from 7.37×10 6 to 2.33×10 8 Oe. These branches are in good agreement with theoretical ones based on 5 f -itinerant FLAPW band calculations. Cyclotron masses, which are in the range from 0.39 to 4.17 m 0 , are approximately the same as the corresponding band masses. These results indicate that the itinerant 5 f -electrons form wide bands. The magnetoresistance results confirmed the existence of open orbits, which can be well explained by the band 10-electron Fermi surface.

Unconventional superconductivity with a radial-node gap in quasi-one-dimensional metals

It is shown that a new type of superconductivity is possible in one-dimensional (1D) and quasi-one-dimensional (Q1D) metals where the pairing interaction is mediated by both spin and charge fluctuations. The gap function inevitably changes sign in the radial direction near the Fermi surface; it vanishes at the Fermi points for 1D, and for Q1D it has line nodes on the Fermi surface as in usual anisotropic pairings. The transition temperature Tc is generally higher for the singlet pairing than for the triplet, but only by a few times. This implies that Tc for the triplet pairing can exceed that of the singlet under a moderately large Zeeman magnetic field, i.e., field-induced triplet pairing. This feature seems to explain the anomalous behaviours of Hc2 and the Knight shift observed in (TMTSF)2PF6.

Pseudogap and superconducting fluctuations in Bi2Sr2CaCu2O8+y probed by 63Cu NMR

Abstract We report temperature dependence of the Gaussian component of spin-echo decay rate (1/T2G) in single crystals Bi2Sr2CaCu2O8+y(Bi2212). We found that 1/T2G starts to decrease at a certain temperature, T c ∗ , which is higher than Tc and lower than the pseudogap temperature, T ∗ , suggesting the formation of local Cooper pairing without global phase coherency. Based on our NMR results, we develop the phase diagram in the Bi2212 system which includes three crossover temperatures, T mK , T ∗ and T c ∗ .

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.