81 research outputs found
Dynamical Mean-Field Theory of Electron-Phonon Interactions in Correlated Systems: Application to Isotope Effects on Electronic Properties
We use a recently developed formalism (combining an adiabatic expansion and
dynamical mean-field theory) to obtain expressions for isotope effects on
electronic properties in correlated systems. As an example we calculate the
isotope effect on electron effective mass for the Holstein model as a function
of electron-phonon interaction strength and doping. Our systematic expansion
generates diagrams neglected in previous studies, which turn out to give the
dominant contributions. The isotope effect is small unless the system is near a
lattice instability. We compare this to experiment.Comment: 6 pages, 4 figures; added discussion of isotope effect away from half
fillin
Condensation energy in strongly coupled superconductors
We consider the condensation energy in superconductors where the pairing is
electronic in origin and is mediated by a collective bosonic mode.
We use magnetically-mediated superconductivity as an example, and show that
for large spin-fermion couplings, the physics is qualitatively different from
the BCS theory as the condensation energy results from the feedback on spin
excitations, while the electronic contribution to the condensation energy is
positive due to an ``undressing'' feedback on the fermions. The same feedback
effect accounts for the gain of the kinetic energy at strong couplings.Comment: 4 pages, revtex 4, 3 eps figure
Non-linear feedback effects in coupled Boson-Fermion systems
We address ourselves to a class of systems composed of two coupled subsystems
without any intra-subsystem interaction: itinerant Fermions and localized
Bosons on a lattice. Switching on an interaction between the two subsystems
leads to feedback effects which result in a rich dynamical structure in both of
them. Such feedback features are studied on the basis of the flow equation
technique - an infinite series of infinitesimal unitary transformations - which
leads to a gradual elimination of the inter-subsystem interaction. As a result
the two subsystems get decoupled but their renormalized kinetic energies become
mutually dependent on each other. Choosing for the inter - subsystem
interaction a charge exchange term (the Boson-Fermion model) the initially
localized Bosons acquire itinerancy through their dependence on the
renormalized Fermion dispersion. This latter evolves from a free particle
dispersion into one showing a pseudogap structure near the chemical potential.
Upon lowering the temperature both subsystems simultaneously enter a
macroscopic coherent quantum state. The Bosons become superfluid, exhibiting a
soundwave like dispersion while the Fermions develop a true gap in their
dispersion. The essential physical features described by this technique are
already contained in the renormalization of the kinetic terms in the respective
Hamiltonians of the two subsystems. The extra interaction terms resulting in
the process of iteration only strengthen this physics. We compare the results
with previous calculations based on selfconsistent perturbative approaches.Comment: 14 pages, 16 figures, accepted for publication in Phys. Rev.
Jahn-Teller polarons and their superconductivity in a molecular conductor
We present a theoretical study of a possibility of superconductivity in a
three dimensional molecular conductor in which the interaction between
electrons in doubly degenerate molecular orbitals and an {\em intra}molecular
vibration mode is large enough to lead to the formation of
Jahn-Teller small polarons. We argue that the effective polaron-polaron
interaction can be attractive for material parameters realizable in molecular
conductors. This interaction is the source of superconductivity in our model.
On analyzing superconducting instability in the weak and strong coupling
regimes of this attractive interaction, we find that superconducting transition
temperatures up to 100 K are achievable in molecular conductors within this
mechanism. We also find, for two particles per molecular site, a novel Mott
insulating state in which a polaron singlet occupies one of the doubly
degenerate orbitals on each site. Relevance of this study in the search for new
molecular superconductors is pointed out.Comment: Submitted to Phys. Rev.
Anomalous specific heat jump in the heavy fermion superconductor CeCoIn
We study the anomalously large specific heat jump and its systematic change
with pressure in CeCoIn superconductor. Starting with the general free
energy functional of the superconductor for a coupled electron boson system, we
derived the analytic result of the specific heat jump of the strong coupling
superconductivity occurring in the coupled electron boson system. Then using
the two component spin-fermion model we calculate the specific heat coefficient
both for the normal and superconducting states and show a good
agreement with the experiment of CeCoIn. Our result also clearly
demonstrated that the specific heat coefficient of a coupled electron
boson system can be freely interpreted as a renormalization either of the
electronic or of the bosonic degrees of freedom.Comment: 5 pages, 2 figure
Hybridization-induced superconductivity from the electron repulsion on a tetramer lattice having a disconnected Fermi surface
Plaquette lattices with each unit cell containing multiple atoms are good
candidates for disconnected Fermi surfaces, which are shown by Kuroki and Arita
to be favorable for spin-flucutation mediated superconductivity from electron
repulsion. Here we find an interesting example in a tetramer lattice where the
structure within each unit cell dominates the nodal structure of the gap
function. We trace its reason to the way in which a Cooper pair is formed
across the hybridized molecular orbitals, where we still end up with a T_c much
higher than usual.Comment: 4 pages, 6 figure
Mixed symmetry superconductivity in two-dimensional Fermi liquids
We consider a 2D isotropic Fermi liquid with attraction in both and
channels and examine the possibility of a superconducting state with mixed
and symmetry of the gap function. We show that both in the weak coupling
limit and at strong coupling, a mixed symmetry state is realized in a
certain range of interaction. Phase transitions between the mixed and the pure
symmetry states are second order. We also show that there is no stable mixed
symmetry state at any coupling.Comment: 3 figures attached in uuencoded gzipped file
The Ising-Kondo lattice with transverse field: an f-moment Hamiltonian for URu2Si2?
We study the phase diagram of the Ising-Kondo lattice with transverse
magnetic field as a possible model for the weak-moment heavy-fermion compound
URu2Si2, in terms of two low-lying f singlets in which the uranium moment is
coupled by on-site exchange to the conduction electron spins. In the mean-field
approximation for an extended range of parameters, we show that the conduction
electron magnetization responds logarithmically to f-moment formation, that the
ordered moment in the antiferromagnetic state is anomalously small, and that
the Neel temperature is of the order observed. The model gives a qualitatively
correct temperature-dependence, but not magnitude, of the specific heat. The
majority of the specific heat jump at the Neel temperature arises from the
formation of a spin gap in the conduction electron spectrum. We also discuss
the single-impurity version of the model and speculate on ways to increase the
specific heat coefficient. In the limits of small bandwidth and of small
Ising-Kondo coupling, we find that the model corresponds to anisotropic
Heisenberg and Hubbard models respectively.Comment: 20 pages RevTeX including 5 figures (1 in LaTeX, 4 in uuencoded EPS),
Received by Phys. Rev. B 19 April 199
Semiclassical action based on dynamical mean-field theory describing electrons interacting with local lattice fluctuations
We extend a recently introduced semiclassical approach to calculating the
influence of local lattice fluctuations on electronic properties of metals and
metallic molecular crystals. The effective action of electrons in degenerate
orbital states coupling to Jahn-Teller distortions is derived, employing
dynamical mean-field theory and adiabatic expansions. We improve on previous
numerical treatments of the semiclassical action and present for the
simplifying Holstein model results for the finite temperature optical
conductivity at electron-phonon coupling strengths from weak to strong.
Significant transfer of spectral weight from high to low frequencies is
obtained on isotope substitution in the Fermi-liquid to polaron crossover
regime.Comment: 10 pages, 7 figure
Structural and superconducting transition in selenium under high pressures
First-principles calculations are performed for electronic structures of two
high pressure phases of solid selenium, -Po and bcc.
Our calculation reproduces well the pressure-induced phase transition from
-Po to bcc observed in selenium.
The calculated transition pressure is 30 GPa lower than the observed one, but
the calculated pressure dependence of the lattice parameters agrees fairly well
with the observations in a wide range of pressure.
We estimate the superconducting transition temperature of both
the -Po and the bcc phases by calculating the phonon dispersion and the
electron-phonon interaction on the basis of density-functional perturbation
theory.
The calculated shows a characteristic pressure dependence, i.e.
it is rather pressure independent in the -Po phase, shows a
discontinuous jump at the transition from -Po to bcc, and then decreases
rapidly with increasing pressure in the bcc phase.Comment: 8 pages, 11 figure
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