arXiv subjects
Publications and source records attributed to Kazumasa Miyake.
We investigate properties below T_c of odd-frequency pairing which is realized by antiferromagnetic critical spin fluctuations or spin wave modes. It is shown that Δ(ε_n) becomes maximum at finite ε_n, and Δ(πT) becomes maximum at finite T. Implications of the present results to the experimental results of CeCu_2Si_2 and CeRhIn_5 are given.
A new microscopic model of the charge Kondo effect is proposed, and is studied on the basis of a numerical renormalization group calculations. It is shown that the charge Kondo effect is caused by a pair hopping interaction between the conduction band and the localized band. It is found that the formation of a charge Kondo-Yosida singlet due to its mechanism corresponds to the appearance of valence skipping phenomenon, and that the charge Kondo-Yosida singlet and the spin Kondo-Yosida singlet state coexist for a tuned parameter set, because the charge and spin degrees of freedom are separated in this model. Then, it is also found that the Sommerfeld coefficient is enhanced by the competition between these two singlet states.
Upper critical field, H_c2, in quasi-1D superconductors is investigated by the weak coupling renormalization group technique. It is shown that H_c2 greatly exceeds not only the Pauli limit, but also the conventional paramagnetic limit of the Flude-Ferrell-Larkin-Ovchinnikov (FFLO) state. This increase is mainly due to quasi-1D fluctuations effect as triggered by interference between unconventional superconductivity and density-wave instabilities. Our results give a novel viewpoint on the large H_c2 observed in TMTSF-salts in terms of a d-wave FFLO state that is predicted to be verified by the H_c2 measurements under pressure.
We discuss the ground state of Benzene dianion (Bz$^{2-}$) on the basis of the numerical diagonalization method of an effective model of $π$ orbitals. It is found that the ground state can be the spin singlet state, and the exchange coupling between LUMOs can be antiferromagnetic.
Transition metal-phtahalocyanine(Pc) compound, TPP[Fe(Pc)(CN)$_2$]$_2$, which is one of molecular conductors of charge transfer type with 3/4-filled conduction band consisting of LUMO of Pc molecules, shows characteristic features in transport and magnetic properties resulting from localized magnetic moments $S=1/2$ associated with Fe$^{+3}$ atoms. We construct an effective tight-binding model of this system and study the mechanism of exchange interaction, $J$, between $d$ and $π$ electrons based on both second order perturbation of transfer integrals between $d$ and $π$ orbitals and numerical diagonalization. It is found that there is no hybridization between $d$-orbitals and LUMO of $π$-orbitals and then super-exchange interaction in the Anderson model does not exist. Instead, processes associated with Hund's rule both on $d$ and $π$ orbitals, which may be called "the double Hund's exchange mechanism", turn out to play important roles and the sign of resultant $J$ can be either ferromagnetic or antiferromagnetic depending on model parameters because of competition among various processes. By taking account of magnetic anisotropy due to spin-orbit interactions and comparing with experimental results, it is indicated that $J$ is antiferromagnetic and of the order of 100K.
A new universality class of quantum criticality emerging in itinerant electron systems with strong local electron correlations is discussed. The quantum criticality of a Ce- or Yb-valence transition gives us a unified explanation for unconventional criticality commonly observed in heavy fermion metals such as YbRh2Si2 and β-YbAlB4, YbCu5-xAlx, and CeIrIn5. The key origin is due to the locality of the critical valence fluctuation mode emerging near the quantum critical end point of the first-order valence transition, which is caused by strong electron correlations for f electrons. Wider relevance of this new criticality and important future measurements to uncover its origin are also discussed.
The roles of critical valence fluctuations of Ce and Yb are discussed as a key origin of several anomalies observed in Ce- and Yb-based heavy fermion systems. Recent development of the theory has revealed that a magnetic field is an efficient control parameter to induce the critical end point of the first-order valence transition. Metamagnetism and non-Fermi liquid behavior caused by this mechanism are discussed by comparing favorably with CeIrIn5, YbAgCu4, and YbIr2Zn20. The interplay of the magnetic order and valence fluctuations offers a key concept for understanding Ce- and Yb-based systems. It is shown that suppression of the magnetic order by enhanced valence fluctuations gives rise to the coincidence of the magnetic-transition point and valence-crossover point at absolute zero as a function of pressure or magnetic field. The interplay is shown to resolve the outstanding puzzle in CeRhIn5 in a unified way. The broader applicability of this newly clarified mechanism is discussed by surveying promising materials such as YbAuCu4, beta-YbAlB4, and YbRh2Si2.
We investigate the magnetic field dependence of the physical properties of f^2-configuration systems with a crystalline-electric field (CEF) singlet ground state, which gives rise to a non- Fermi liquid (NFL) fixed point due to the competition between the Kondo-Yosida singlet and CEF singlet states. On the basis of the numerical renormalization group method, we find that the magnetic field breaks this NFL fixed point via two mechanisms: one causing the polarization of f-electrons and the other giving the "channel" anisotropy. These two mechanisms induce a difference in the magnetic field dependence of the characteristic temperature T_F^{*}(H), the crossover temperature from NFL to Fermi-liquid behavior. While the polarization of f-electrons gives T_F^{*}(H) \propto H^x (x\sim2.0), the "channel" anisotropy gives the H-independent T_F^{*}(H). These two mechanisms cross over continuously at approximately the crossover magnetic field H_c, where an anomalous H-dependence of T_F^{*}(H) appears. Such T_F^{*}(H) well reproduces the NFL behaviors observed in Th_{1-x}U_xRu_2Si_2. We also find that the H-dependence of the resistivity and the magnetic susceptibility are in good agreement with the experimental results of this material. These results suggest that the NFL behaviors observed in Th_{1-x}U_xRu_2Si_2 can be understood if this material is located in the CEF singlet side near the critical phase boundary between the two singlet states.
Influence of quantum critical point (QCP) of first-order valence transition (FOVT) on Ce- and Yb-based heavy fermions is discussed as a key origin of anomalies such as non-Fermi liquid, metamagnetism, and unconventional superconductivity. Even in intermediate-valence materials, the QCP of the FOVT is shown to be induced by applying a magnetic field, which creates a new characteristic energy distinct from the Kondo temperature. It is stressed that the key concept is closeness to the QCP of the FOVT by pointing out that the proximity of the QCP explains sharp contrast between X=Ag and X=Cd in YbXCu4, field-induced valence crossover in X=Au, and field-induced first order transition as well as non-Fermi-liquid critical behaviours in CeIrIn5. The valence fluctuations can be a key origin of unresolved phenomena in this family of materials.
We investigate the influence of critical spin fluctuations on electromagnetic responses in the odd-frequency superconductivity. It is shown that the Meissner kernel of the odd-frequency superconductivity is strongly reduced by the critical spin fluctuation or the massless spin wave mode in the antiferromagnetic phase. These results imply that the superfluid density is reduced, and the London penetration depth is lengthened for the odd-frequency pairing. It is also shown that the zero field Flude-Ferrell-Larkin-Ovchinnikov pairing is spontaneously realized both for even- and odd-frequency in the case of sufficiently strong coupling with low lying spin-modes.
A possibility of the odd-frequency pairing in the strong-coupling electron-phonon systems is discussed. Using the Holstein-Hubbard model, we demonstrate that the anomalously soft Einstein mode with the frequency $ω_{\rm E}\llω_{c}$ ($ω_{c}$ is the order of the renormalized bandwidth) mediates the s-wave odd-frequency triplet pairing against the ordinary even-frequency singlet pairing. It is necessary for the emergence of the odd-frequency pairing that the pairing interaction is strongly retarded as well as the strong coupling, since the pairing interaction for the odd-frequency pairing is effective only in the diagonal scattering channel, $(ω_{n},-ω_{n})\to(ω_{n'},-ω_{n'})$ with $ω_{n'}=ω_{n}\gtrsim ω_{\rm E}$. Namely, the odd-frequency superconductivity is realized in the opposite limit of the original BCS theory. The Ginzburg-Landau analysis in the strong-coupling region shows that the specific-heat discontinuity and the slope of the temperature dependence of the superfluid density can be quite small as compared with the BCS values, depending on the ratio of the transition temperature $T_{c}$ and $ω_{c}$.
Since the first theoretical proposal by Berezinskii, an odd-frequency superconductivity has encountered the fundamental problems on its thermodynamic stability and rigidity of a homogenous state accompanied by unphysical Meissner effect. Recently, Solenov {\it et al}. [Phys. Rev. B {\bf 79} (2009) 132502.] have asserted that the path-integral formulation gets rid of the difficulties leading to a stable homogenous phase with an ordinary Meissner effect. Here, we show that it is crucial to choose the appropriate saddle-point solution that minimizes the effective free energy, which was assumed {\it implicitly} in the work by Solenov and co-workers. We exhibit the path-integral framework for the odd-frequency superconductivity with general type of pairings, including an argument on the retarded functions via the analytic continuation to the real axis.
We study a magnetic field effect on the Non-Fermi Liquid (NFL) which arises around the quantum critical point (QCP) due to the competition between the f^2-crystalline-electric-field singlet and the Kondo-Yosida singlet states by using the numerical renormalization ground method. We show the characteristic temperature T_F^*, corresponding to a peak of a specific heat, is not affected by the magnetic field up to H_z^* which is determined by the distance from the QCP or characteristic energy scales of each singlet states. As a result, in the vicinity of QCP, there are parameter regions where the NFL is robust against the magnetic field, at an observable temperature range T > T_F^*, up to H_z^* which is far larger than T_F^* and less than min(T_{K2}, $Delta).
It is shown that unconventional critical phenomena commonly observed in paramagnetic metals YbRh2Si2, YbRh2(Si0.95Ge0.05)2, and beta-YbAlB4 is naturally explained by the quantum criticality of Yb-valence fluctuations. We construct the mode coupling theory taking account of local correlation effects of f electrons and find that unconventional criticality is caused by the locality of the valence fluctuation mode. We show that measured low-temperature anomalies such as divergence of uniform spin susceptibility $χT^{-ζ)$ with $ζ~0.6$ giving rise to a huge enhancement of the Wilson ratio and the emergence of T-linear resistivity are explained in a unified way.
The drastic changes of Fermi surfaces as well as transport anomalies near P=Pc~2.35 GPa in CeRhIn5 are explained theoretically from the viewpoint of sharp valence change of Ce. It is pointed out that the key mechanism is the interplay of magnetic order and Ce-valence fluctuations. It is shown that the antiferromagnetic state with "small" Fermi surfaces changes to the paramagnetic state with "large" Fermi surfaces with huge enhancement of effective mass of electrons with keeping finite c-f hybridization. This naturally explains the de Haas-van Alphen measurement and also the transport anomalies of T-linear resistivity emerging simultaneously with the residual resistivity peak at P=Pc in CeRhIn5.
We investigate f-electron charge susceptibility in a two-impurity Anderson model on the basis of Wilson's numerical renormalization group method. The f-electron charge susceptibility diverges logarithmically at the critical point of this model when conduction-electron bands exhibit particle-hole symmetry. Although the critical point disappears without the particle-hole symmetry, the f-electron charge fluctuation is much more enhanced near the crossover regime between the Kondo-Yosida singlet and intersite spin-singlet states than that in the single-impurity case. This result shows that charge fluctuations are enhanced owing to the competition between intersite and Kondo-Yosida spin singlets. A possible scenario for the enhanced residual resistivity near the region where the Kondo temperature becomes comparable with the Néel temperatures under pressure in some heavy-fermion compounds is proposed.
Theoretical studies on the possible origin of room temperature ferromagnetism (ferromagnetic once crystallized) in the molecular transition metal complex, V(TCNE)_x (1.5<x<2) have been carried out. For this family, there have been no definite understanding of crystal structure so far because of sample quality, though the effective valence of V is known to be close to +2. Proposing a new crystal structure for the stoichiometric case of x=2, where the valence of each TCNE molecule is -1 and resistivity shows insulating behavior, exchange interaction among d-electrons on adjacent V atoms has been estimated based on the cluster with 3 vanadium atoms and one TCNE molecule. It turns out that Hund's coupling among d orbitals within the same V atoms and antiferromagnetic coupling between d oribitals and LUMO of TCNE (bridging V atoms) due to hybridization result in overall ferromagnetism (to be precise, ferrimagnetism). This view based on localized electrons is supplemented by the band picture, which indicates the existence of a flat band expected to lead to ferromagnetism as well consistent with the localized view. The off-stoichiometric cases (x<2), which still show ferromagnetism but semiconducting transport properties, have been analyzed as due to Anderson localization.
The mechanism of drastic change of Fermi surfaces as well as transport anomalies near P=Pc=2.35 GPa in CeRhIn5 is explained theoretically. The key mechanism is pointed out to be the interplay of magnetic order and Ce-valence fluctuations. We show that the antiferromagnetic state with "small" Fermi surfaces changes to the paramagnetic state with "large" Fermi surfaces with huge enhancement of effective mass of electrons with keeping finite c-f hybridization. This explains the drastic change of the de Haas-van Alphen signals. Furthermore, it is also consistent with the emergence of T-linear resistivity simultaneous with the residual resistivity peak at P=Pc in CeRhIn5.