Kazumasa Miyake

Huge Enhancement of Impurity Scattering due to Critical Valence Fluctuations in a Ce-Based Heavy Electron System

On the basis of the Ward–Pitaevskii identity, the residual resistivity ρ 0 is shown to exhibit huge enhancement around the quantum critical point of valence transition in Ce-based heavy electron systems. This explains the sharp peak of ρ 0 observed in CeCu 2 Ge 2 under a pressure of P ∼16 GPa where the superconducting transition temperature also exhibits a sharp peak.

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.

Orbital Dependence in Kondo Effect Enlarged by Hund's-Rule Coupling

The effect of the Hunds-rule coupling on the Kondo effect is studied in connection with the nature of renormalized quasiparticles. Applying the Wilson numerical renormalization-group method, we investigate a system where two localized electrons at impurity site hybridize with conduction electrons with different strength. It is shown that two characteristic energy scales, T K1 and T K2 , ( T K1 > T K2 ) exist irrespective of the existence of the Hunds-rule coupling due to the orbital dependence of hybridizations. As the Hunds-rule coupling is switched on and increased, the ratio between two characteristic energy scales is much more enhanced with both being decreased. We find widely a region of parameters in which T K2 cannot be seen in the temperature range accessible by experiments. Practically, the system shows a new type of itinerant-localized duality where one of two localized electrons forms local Fermi liquid with conduction electrons and the other remains as a localized spin. The case of antiferr...

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.

A route to small Drude weight in metals with nested spin fluctuations and enhanced impurity scattering associated with quantum critical phenomena

Abstract It is discussed that the small Drude weight D observed in high- T c cuprates is compatible with the small renormalization amplitude z and not enhanced effective mass m ∗ of quasiparticles even in the framework of Fermi liquid theory. In particular, such a situation is expected if the Fermi surface of quasiparticle is nested as expected near the magnetic metal–insulator transition point. It is also discussed that the quantum critical fluctuations give rise to enhancement of impurity potential through the vertex correction of scattering leading to enhancement of “residual resistivity”. Two cases, physically interesting, are discussed: (1) non-magnetic impurity scattering near the ferromagnetic critical point and (2) non-magnetic impurity near the charge density critical point. The latter case has some relevance to the reduction of superconducting transition temperature of high- T c cuprates in the underdoped region.

Anomalous Fermi Liquid Effects in Two-Dimensional Hubbard Model near Half-Filling

The Landau parameters F 0 s , F 0 a , F 1 s and F 1 a in the two-dimensional Hubbard model are calculated by the second order perturbation method of Abrikosov-Khalatnikov. The following points are revealed as the half-filling is approached: (1) F 0 a changes its sign from negative to positive, if the Coulomb interaction U is strong enough, i.e., (U/tgtrsim 2.0 ). This means that the spin density oscillations, zero spin-sound, can propagate there; (2) F 1 s changes its sign from positive to negative, so that we can understand, on the basis of the anisotropic Fermi liquid theory, the behavior of the Drude weight whose magnitude decreases as the half-filling is approached even if there were no mass enhancement; (3) From the results of F 0 s and F 0 a , it is shown that both the charge and spin susceptibility decrease rapidly close to the half-filling. The latter is regarded as the so-called spin-gap behavior.

Deformation of fermi surface due to many-body effect: renormalization group approach

Abstract Perturbational renormalization group (RG) is used to study the renormalization of the Fermi surface (FS) of the t–t′ Hubbard model. We construct and solve numerically the RG equations for t, t′ and U, and found that FS is deformed towards nesting at half-filling. Phase boundary of the antiferromagnetic state and the paramagnetic state in U/t–t′/t plane is determined exactly in the weak coupling region.

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...

New varieties of order parameter symmetry in quasi-one-dimensional superconductors

Abstract The symmetries of the superconducting order parameter in quasi-one-dimensional systems are investigated by the renormalization group method in a model characterized by backward scattering g 1 and forward scattering g 2 . Various types of pairing can be realized for different sets of ( g 1 , g 2 ). In the case of g 1 = g 2 (Hubbard model), for example, the transition temperature of p -like triplet becomes higher than that of d -like singlet.

Possible existence of zero spin-sound in two-dimensional Hubbard model

Abstract The Landau parameter Fa0 in two-dimensional Hubbard model is calculated by the second-order perturbation method. It is shown that Fa0 changes its sign from negative to positive as the filling is increased to the half-filling for the interaction U/t≳2.0. This means that the spin density waves propagation, zero spin-sound, is possible in the system described by Hubbard model near half-filling. It is shown that the spin susceptibility decreases for U/t≳2.0 when the half-filling is approached exhibiting the so-called spin-gap behavior.