466 research outputs found
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
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
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
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
Enhanced Kondo Effect in an Electron System Dynamically Coupled with Local Optical Phonon
We discuss Kondo behavior of a conduction electron system coupled with local
optical phonon by analyzing the Anderson-Holstein model with the use of a
numerical renormalization group (NRG) method. There appear three typical
regions due to the balance between Coulomb interaction and
phonon-mediated attraction . For , we
observe the standard Kondo effect concerning spin degree of freedom. Since the
Coulomb interaction is effectively reduced as , the
Kondo temperature is increased when is increased. On
the other hand, for , there occurs the Kondo effect
concerning charge degree of freedom, since vacant and double occupied states
play roles of pseudo-spins. Note that in this case, is decreased
with the increase of . Namely, should be maximized for
. Then, we analyze in detail the Kondo behavior
at , which is found to be explained by the polaron
Anderson model with reduced hybridization of polaron and residual repulsive
interaction among polarons. By comparing the NRG results of the polaron
Anderson model with those of the original Anderson-Holstein model, we clarify
the Kondo behavior in the competing region of .Comment: 8 pages, 8 figure
Novel mutations in TARDBP (TDP-43) in patients with familial amyotrophic lateral sclerosis.
The TAR DNA-binding protein 43 (TDP-43) has been identified as the major disease protein in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitin inclusions (FTLD-U), defining a novel class of neurodegenerative conditions: the TDP-43 proteinopathies. The first pathogenic mutations in the gene encoding TDP-43 (TARDBP) were recently reported in familial and sporadic ALS patients, supporting a direct role for TDP-43 in neurodegeneration. In this study, we report the identification and functional analyses of two novel and one known mutation in TARDBP that we identified as a result of extensive mutation analyses in a cohort of 296 patients with variable neurodegenerative diseases associated with TDP-43 histopathology. Three different heterozygous missense mutations in exon 6 of TARDBP (p.M337V, p.N345K, and p.I383V) were identified in the analysis of 92 familial ALS patients (3.3%), while no mutations were detected in 24 patients with sporadic ALS or 180 patients with other TDP-43-positive neurodegenerative diseases. The presence of p.M337V, p.N345K, and p.I383V was excluded in 825 controls and 652 additional sporadic ALS patients. All three mutations affect highly conserved amino acid residues in the C-terminal part of TDP-43 known to be involved in protein-protein interactions. Biochemical analysis of TDP-43 in ALS patient cell lines revealed a substantial increase in caspase cleaved fragments, including the approximately 25 kDa fragment, compared to control cell lines. Our findings support TARDBP mutations as a cause of ALS. Based on the specific C-terminal location of the mutations and the accumulation of a smaller C-terminal fragment, we speculate that TARDBP mutations may cause a toxic gain of function through novel protein interactions or intracellular accumulation of TDP-43 fragments leading to apoptosis
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
Step-Wise Computational Synthesis of Fullerene C60 derivatives. 1.Fluorinated Fullerenes C60F2k
The reactions of fullerene C60 with atomic fluorine have been studied by
unrestricted broken spin-symmetry Hartree-Fock (UBS HF) approach implemented in
semiempirical codes based on AM1 technique. The calculations were focused on a
sequential addition of fluorine atom to the fullerene cage following indication
of the cage atom highest chemical susceptibility that is calculated at each
step. The effectively-non-paired-electron concept of the fullerene atoms
chemical susceptibility lays the foundation of the suggested computational
synthesis. The obtained results are analyzed from energetic, symmetry, and the
composition abundance viewpoints. A good fitting of the data to experimental
findings proves a creative role of the suggested synthesis methodology.Comment: 33 pages, 11 figures, 2 tables, 2 chart
Symptoms and quality of life in late stage Parkinson syndromes: a longitudinal community study of predictive factors
BACKGROUND
Palliative care is increasingly offered earlier in the cancer trajectory but rarely in Idiopathic Parkinson's Disease(IPD), Progressive Supranuclear Palsy(PSP) or Multiple System Atrophy(MSA). There is little longitudinal data of people with late stage disease to understand levels of need. We aimed to determine how symptoms and quality of life of these patients change over time; and what demographic and clinical factors predicted changes.
METHODS
We recruited 82 patients into a longitudinal study, consenting patients with a diagnosis of IPD, MSA or PSP, stages 3-5 Hoehn and Yahr(H&Y). At baseline and then on up to 3 occasions over one year, we collected self-reported demographic, clinical, symptom, palliative and quality of life data, using Parkinson's specific and generic validated scales, including the Palliative care Outcome Scale (POS). We tested for predictors using multivariable analysis, adjusting for confounders.
FINDINGS
Over two thirds of patients had severe disability, over one third being wheelchair-bound/bedridden. Symptoms were highly prevalent in all conditions - mean (SD) of 10.6(4.0) symptoms. More than 50% of the MSA and PSP patients died over the year. Over the year, half of the patients showed either an upward (worsening, 24/60) or fluctuant (8/60) trajectory for POS and symptoms. The strongest predictors of higher levels of symptoms at the end of follow-up were initial scores on POS (AOR 1.30; 95%CI:1.05-1.60) and being male (AOR 5.18; 95% CI 1.17 to 22.92), both were more predictive than initial H&Y scores.
INTERPRETATION
The findings point to profound and complex mix of non-motor and motor symptoms in patients with late stage IPD, MSA and PSP. Symptoms are not resolved and half of the patients deteriorate. Palliative problems are predictive of future symptoms, suggesting that an early palliative assessment might help screen for those in need of earlier intervention
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