Yoshifumi Onishi

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.

Resonance state localized around nonmagnetic impurity with unitarity scattering in two-dimensional d-wave superconductor

Effects of nonmagnetic impurities with unitarity scattering in two-dimensional d -wave superconductors are investigated by the numerical diagonalization of finite-size systems of up to 35×35 sites. It is shown that a localized state with almost zero excitation energy is formed around an impurity site if it is isolated. However, its localized state is extended in real space along the direction of the node of the d -wave gap, so that the localized states of adjacent impurities easily interfere with each other forming a kind of impurity band, i.e., resonance states around the zero energy. It is also pointed out that there exists a strong association between the resonance density of states due to impurity scattering in the unitarity limit and the so-called zero-bias anomaly of tunneling conductance.

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.

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.

Speaker adaptation by hierarchical EigenVoice

We propose a novel speaker adaptation method, hierarchical EigenVoice (HEV). This method extends the eigenvoice through clustering the Gaussian components of HMMs into a hierarchical tree structure. It enables one to autonomously control a number of adaptation parameters (model complexity) depending on the amount of adaptation utterances from a new speaker. The experimental results of Japanese large vocabulary continuous speech recognition confirmed the significant performance increase in all range of the adaptation utterance amounts compared with the conventional speaker adaptation methods.

Paramagnon-Mediated d-Wave Pairing in Two-Dimensional Isotropic Fermi Liquid

It is studied which type of superfluidity is induced by the paramagnon-mediated interaction in a two-dimensional isotropic Fermion system. The stability of various types of Cooper pairing is invest...

Anomaly detection of motors with feature emphasis using only normal sounds

This paper proposes an anomaly detection method for sound signals observed from motors in operation without using abnormal signals. It is based on feature emphasis and effectively detects anomalies that appear in a small subset of features. To emphasize the features, the method optimally estimates the contribution rates of various features to the dissimilarity score between an observed signal and the distribution of normal signals. We report here our evaluation of the method using sound data observed from PCs and fans in operation. The evaluation demonstrates that the proposed method emphasizes a small subset of narrow frequency ranges of sounds and that it achieves an error reduction rate of up to 76%.

Novel type of pairing in quasi-one-dimensional superconductivity

Abstract A new type of unconventional superconductivity develops in one-dimensional (1D) and quasi-one-dimensional (Q1D) metals mediated both by charge and spin fluctuations. The transition temperature T c is higher for the singlet pairing than that for the triplet in general, but only by a few times. Therefore, the triplet pairing can be stabilized under moderately large Zeeman magnetic fields. The gap function inevitably changes its sign in the radial direction near the Fermi surface; especially in purely 1D case, the gap vanishes just at the Fermi points. The effect of the nonmagnetic impurity scattering in the unitarity limit is also considered.