Osamu Narikiyo

Itinerant-localized duality picture of spin fluctuations in high-Tc cuprates. I: Nested spin-fluctuation theory

The mode-mode coupling theory of spin fluctuations in the normal state of high- T c cuprates is developed on the basis of the itinerant-localized duality picture of technically nested two-dimensional metals. This offers a unified description of the anomalies observed in the normal state of high- T c cuprates. The so-called “spin-gap” behaviors of the resistivity, the specific heat, the uniform spin susceptibility, and the optical conductivity are characterized by the energy scale T * of the crossover between the itinerant and the localized spin fluctuations, while those of the Hall coefficient, the thermo-electric power, the longitudinal NMR relaxation rate, and the neutron scattering intensity, are characterized by different energy scales less than T * . The integrated weight of the nested spin fluctuation decreases when decreasing the temperature or approaching the perfect nesting, leading to the “spin-gap” behavior.

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.

Two-dimensional Hubbard model at low electron density

The two dimensional Hubbard model with repulsive interaction is studied in the t -matrix approximation, which is the valid approximation in low electron density. The particle-particle correlation function and the t -matrix are obtained analytically. By use of these analytical results it is shown that the t -matrix and hence the self-energy have no singularities in the upper half plane of the complex energy variable. The phase angle δ(2 k F + q , ω), with k F being the Fermi wave vector, in the limit of q =0 and ω=0 is found to depend strongly on the ratio ω/ q and is not always zero. The case of the attractive interaction is also briefly studied.

Enhanced Impurity Scattering due to Quantum Critical Fluctuations : Perturbational Approach

It is shown on the basis of the lowest order perturbation expansion with respect to critical fluctuations that the critical fluctuations give rise to an enhancement of the potential scattering of non-magnetic impurities. This qualitatively accounts for the enhancement of the resistivity due to impurities which has been observed in variety of systems near the quantum critical point, while the higher order processes happen to give much larger enhancement as seen from the Ward identity arguments. The cases with dynamical critical exponent z =2 and z =3 are discussed explicitly.

Aspect of Spin-Charge Separation Due to Antiferromagnetic Spin Fluctuations in Technically Nested 2D Metals

It is shown that the charge susceptibility in nearly half-filled two-dimensional (2D) metals, with technically nested Fermi surface, shows anomaly at the wave vector different from that for the spin susceptibility at low temperatures. Namely charge degrees of freedom behave there as if their “Fermi surface” corresponded to that of “holes” created in the Mott insulator. Such anomaly is caused by the Aslamazov-Larkin-type contribution of 2D antiferromagnetic spin fluctuations. This phenomenon gives a possible clue to resolve the paradox of normal state properties of high- T c cuprates starting from the Fermi-liquid fixed point.

PERTURBATION THEORY AND SINGULARITIES IN TWO-DIMENSIONAL HUBBARD MODEL AT LOW ELECTRON DENSITY

The two-dimensional Hubbard model has been studied in the region of low electron density based on the t -matrix approximation. This can take proper account of the singular behaviors of the forward scattering whose importance was indicated by Anderson. By performing first the angular integrations in the irreducible particle-particle correlation function, we can prove analytically that all of the poles of the t -matrix lie on the real axis. Hence the t -matrix and then the self-energy of the retarded Green function, Σ( k , z ), are analytic in the upper half-plane of the energy variable, z . This, together with Im Σ( k F , e+i δ)∝e 2 ln |e|, leads to the conclusion that the electrons described by the two-dimensional Hubbard model are Fermi liquid, at least in the region of low electron density.

Proximity Induced Meissner Effect in Dirty Normal Metals

Temperature dependences of the proximity induced Meissner diamagnetization in dirty normal metals are calculated by use of the Usadel equation. Besides the effect of the normal impurity scattering, that of the magnetic one is investigated. Especially the Kondo effect is examined based on the Anderson Hamiltonian employing a scheme that interpolates magnetic and non-magnetic behaviors, and a crossover between these two has been described.

Itinerant-Localized Duality Picture of Spin Fluctuations in High-Tc Cuprates. II. A Unified Phenomenology.

We present a phenomenology of spin fluctuations in the normal metallic state of high- T c cuprates where the dynamical spin susceptibility resulting from the mode-mode couplings of the itinerant fermions and the localized spins consists of two parts: the coherent and the incoherent ones. The long-standing paradoxes on the experiments of the neutron scattering and the NMR, concerning the magnetic correlation length and the longitudinal and the transverse NMR relaxation rates, are resolved and the nature of the spin gap for these quantities is clarified.

Vibration of the Dimer on Si(001) Surface Excited by STM Current

The vibration of the dimer excited by STM current on Si(001) surface is investigated. We describe this system by the Hamiltonian which has the electron–vibration coupling term as the key ingredient. In order to characterize the transition rates induced by STM current between vibrational states we have introduced the effective temperature of the vibration which differs from the temperature of the substrate. The behavior of the effective temperature depends on the substrate temperature and STM current in highly nonlinear manner and qualitatively changes around 50 K of the substrate temperature. At lower temperatures, the effective temperature strongly deviates from the substrate temperature and reaches a few hundred Kelvin for the typical values of STM current. At higher substrate temperatures, the effective temperature reduces to the substrate temperature. On the basis of these behaviors of the effective temperature, we solve the puzzle of the symmetric–asymmetric crossover in dimer images of STM observati...

A theory of uniform spin susceptibility around the antiferromagnetic quantum critical point

The uniform spin susceptibility χ around the quantum critical point is discussed on the basis of the mode-mode coupling theory of the antiferromagnetic (AF) spin fluctuations. The temperature dependence of χ around the quantum critical point reflects the magnetic-field dependence of the free energy due to that of the spin fluctuations. The results are χ∝1-β 2 t 1/4 for the conventional class of AF fluctuations with the dynamical exponent z =2, and χ∝1-β 3 t 1/3 for a new class of AF fluctuations with z =3, where t is the reduced temperature and β 2 and β 3 are numerical constants. For Ce 7 Ni 3 , the behavior of χ around the quantum critical point is possibly explaned by this theory with the conventional class of AF fluctuations with z =2. The latter type of AF fluctuations with z =3 is possible when the system contains even number of magnetic ions in the unit cell such as CeCu 6- x Au x ( x =0.1), because the AF ordering vector can coincide with one of the reciprocal lattice vectors.