332 research outputs found
Cooperative Effect of Coulomb Interaction and Electron-Phonon Coupling on the Heavy Fermion State in the Two-Orbital Periodic Anderson Model
We investigate the two-orbital periodic Anderson model, where the local
orbital fluctuations of f-electrons couple with a two-fold degenerate
Jahn-Teller phonon, by using the dynamical mean-field theory. It is found that
the heavy fermion state caused by the Coulomb interaction between f-electrons U
is largely enhanced due to the electron-phonon coupling g, in contrast to the
case with the single-orbital periodic Anderson model where the effects of U and
g compete to each other. In the heavy fermion state for large and g, both
the orbital and lattice fluctuations are enhanced, while the charge (valence)
and spin fluctuations are suppressed. In the strong coupling regime, a sharp
soft phonon mode with a large spectral weight is observed for small U, while a
broad soft phonon mode with a small spectral weight is observed for large U.
The cooperative effect of U and g for half-filling with two f-electrons per
atom is more pronounced than that for quarter-filling with .Comment: 8 pages, 11 figures, accepted for publication in JPS
Heavy-Electron Formation and Bipolaronic Transition in the Anharmonic Holstein Model
The emergence of the bipolaronic phase and the formation of the
heavy-electron state in the anharmonic Holstein model are investigated using
the dynamical mean-field theory in combination with the exact diagonalization
method. For a weak anharmonicity, it is confirmed that the first-order
polaron-bipolaron transition occurs from the observation of a discontinuity in
the behavior of several physical quantities. When the anharmonicity is
gradually increased, the polaron-bipolaron transition temperature is reduced as
well as the critical values of the electron-phonon coupling constant for
polaron-bipolaron transition. For a strong anharmonicity, the polaron-bipolaron
transition eventually changes to a crossover behavior. The effect of
anharmonicity on the formation of the heavy-electron state near the
polaron-bipolaron transition and the crossover region is discussed in detail.Comment: 11 pages, 13 figure
Kondo Effect in an Electron System with Dynamical Jahn-Teller Impurity
We investigate how Kondo phenomenon occurs in the Anderson model dynamically
coupled with local Jahn-Teller phonons. It is found that the total angular
moment composed of electron pseudo-spin and phonon angular moments is screened
by conduction electrons. Namely, phonon degrees of freedom essentially
contribute to the formation of singlet ground state. A characteristic
temperature of the Kondo effect due to dynamical Jahn-Teller phonons is
explained by an effective - Hamiltonian with anisotropic exchange
interaction obtained from the Jahn-Teller-Anderson model in a non-adiabatic
region.Comment: 5 pages, 3 figure
First Order Bipolaronic Transition at Finite Temperature in the Holstein Model
We investigate the Holstein model by using the dynamical mean-field theory
combined with the exact diagonalization method. Below a critical temperature
Tcr, a coexistence of the polaronic and the bipolaronic solutions is found for
the same value of the electron-phonon coupling $ in the range gc1(T)<g<gc2(T).
In the coexistence region, the system shows a first order phase transition from
the bipolaronic to the polaronic states as T decreases at T=Tp(<Tcr), where the
double occupancy and the lattice fluctuation together with the anharmonicity of
the effective ion potential change discontinuously without any symmetry
breaking. The obtained bipolaronic transition seems to be consistent with the
rattling transition in the beta-pyrochlore oxide KOs2O6.Comment: 5 pages, 5 figures, J. Phys. Soc. Jpn. 79 (2010) 09370
Local Heavy Quasiparticle in Four-Level Kondo Model
An impurity four-level Kondo model, in which an ion is tunneling among
4-stable points and interacting with surrounding conduction electrons, is
investigated using both perturbative and numerical renormalization group
methods. The results of numerical renormalization group studies show that it is
possible to construct the ground state wavefunction including the excited ion
states if we take into account the interaction between the conduction electrons
and the ion. The resultant effective mass of quasiparticles is moderately
enhanced. This result offers a good explanation for the enhanced and
magnetically robust Sommerfeld coefficient observed in SmOsSb, some
other filled-skutterudites, and clathrate compounds.Comment: 9 pages, 7 figures. Added references and "Note added
Strong-Coupling Theory of Rattling-Induced Superconductivity
In order to clarify the mechanism of the enhancement of superconducting
transition temperature due to anharmonic local oscillation of a
guest ion in a cage composed of host atoms, i.e., {\it rattling}, we analyze
the anharmonic Holstein model by applying the Migdal-Eliashberg theory. From
the evaluation of the normal-state electron-phonon coupling constant, it is
found that the strong coupling state is developed, when the bottom of a
potential for the guest ion becomes wide and flat. Then, is
enhanced with the increase of the anharmonicity in the potential, although
is rather decreased when the potential becomes a double-well type
due to very strong anharmonicity. From these results, we propose a scenario of
anharmonicity-controlled strong-coupling tendency for superconductivity induced
by rattling. We briefly discuss possible relevance of the present scenario with
superconductivity in -pyrochlore oxides.Comment: 8 pages, 6 figure
The hydrogen and helium lines of the symbiotic binary Z And during its brightening at the end of 2002
High resolution observations in the region of the lines Halpha, He II 4686
and Hgamma of the spectrum of the symbiotic binary Z And were performed during
its small-amplitude brightening at the end of 2002. The profiles of the
hydrogen lines were double-peaked. These profiles give a reason to suppose that
the lines can be emitted mainly by an optically thin accretion disc. The Halpha
line is strongly contaminated by the emission of the envelope, therefore for
consideration of accretion disc properties we use the Hgamma line. The Halpha
line had broad wings which are supposed to be determined mostly from radiation
damping but high velocity stellar wind from the compact object in the system
can also contribute to their appearance. The Hgamma line had a broad emission
component which is assumed to be emitted mainly from the inner part of the
accretion disc. The line He II 4686 had a broad emission component too, but it
is supposed to appear in a region of a high velocity stellar wind. The outer
radius of the accretion disc can be calculated from the shift between the
peaks. Assuming, that the orbit inclination can ranges from 47 to
76, we estimate the outer radius as 20 - 50 R_sun. The behaviour of the
observed lines can be considered in the framework of the model proposed for
interpretation of the line spectrum during the major 2000 - 2002 brightening of
this binary.Comment: 19 pages, 5 figures. Accepted for publication in Astronomy Report
Enhanced insulin sensitivity associated with provision of mono and polyunsaturated fatty acids in skeletal muscle cells involves counter modulation of PP2A
International audienceAims/Hypothesis: Reduced skeletal muscle insulin sensitivity is a feature associated with sustained exposure to excess saturated fatty acids (SFA), whereas mono and polyunsaturated fatty acids (MUFA and PUFA) not only improve insulin sensitivity but blunt SFA-induced insulin resistance. The mechanisms by which MUFAs and PUFAs institute these favourable changes remain unclear, but may involve stimulating insulin signalling by counter-modulation/repression of protein phosphatase 2A (PP2A). This study investigated the effects of oleic acid (OA; a MUFA), linoleic acid (LOA; a PUFA) and palmitate (PA; a SFA) in cultured myotubes and determined whether changes in insulin signalling can be attributed to PP2A regulation. Principal Findings: We treated cultured skeletal myotubes with unsaturated and saturated fatty acids and evaluated insulin signalling, phosphorylation and methylation status of the catalytic subunit of PP2A. Unlike PA, sustained incubation of rat or human myotubes with OA or LOA significantly enhanced Akt-and ERK1/2-directed insulin signalling. This was not due to heightened upstream IRS1 or PI3K signalling nor to changes in expression of proteins involved in proximal insulin signalling, but was associated with reduced dephosphorylation/inactivation of Akt and ERK1/2. Consistent with this, PA reduced PP2Ac demethylation and tyrosine 307 phosphorylation-events associated with PP2A activation. In contrast, OA and LOA strongly opposed these PA-induced changes in PP2Ac thus exerting a repressive effect on PP2A.Conclusions/Interpretation: Beneficial gains in insulin sensitivity and the ability of unsaturated fatty acids to oppose palmitate-induced insulin resistance in muscle cells may partly be accounted for by counter-modulation of PP2A
Kondo Effect of a Magnetic Ion Vibrating in a Harmonic Potential
To discuss Kondo effects of a magnetic ion vibrating in the sea of conduction
electrons, a generalized Anderson model is derived. The model includes a new
channel of hybridization associated with phonon emission or absorption. In the
simplest case of the localized electron orbital with the s-wave symmetry,
hybridization with p-waves becomes possible. Interesting interplay among the
conventional s-wave Kondo effect and the p-wave one and the Yu-Anderson type
Kondo effect is found and the ground state phase diagram is determined by using
the numerical renormalization group method. Two different types of stable fixed
points are identified and the two-channel Kondo fixed points are generically
realized on the boundary.Comment: 15 pages, 17 figures, J. Phys. Soc. Jpn. 80 (2011) No.6 to be
publishe
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