399 research outputs found
Calculating critical temperatures of superconductivity from a renormalized Hamiltonian
It is shown that one can obtain quantitatively accurate values for the
superconducting critical temperature within a Hamiltonian framework. This is
possible if one uses a renormalized Hamiltonian that contains an attractive
electron-electron interaction and renormalized single particle energies. It can
be obtained by similarity renormalization or using flow equations for
Hamiltonians. We calculate the critical temperature as a function of the
coupling using the standard BCS-theory. For small coupling we rederive the
McMillan formula for Tc. We compare our results with Eliashberg theory and with
experimental data from various materials. The theoretical results agree with
the experimental data within 10%. Renormalization theory of Hamiltonians
provides a promising way to investigate electron-phonon interactions in
strongly correlated systems.Comment: 6 pages, LaTeX, using EuroPhys.sty, one eps figure included, accepted
for publication in Europhys. Let
Superlight small bipolarons from realistic long-range Coulomb and Fr\"ohlich interactions
We report analytical and numerical results on the two-particle states of the
polaronic t-Jp model derived recently with realistic Coulomb and
electron-phonon (Frohlich) interactions in doped polar insulators. Eigenstates
and eigenvalues are calculated for two different geometries. Our results show
that the ground state is a bipolaronic singlet, made up of two polarons. The
bipolaron size increases with increasing ratio of the polaron hopping integral
t to the exchange interaction Jp but remains small compared to the system size
in the whole range 0<t/Jp<1. Furthermore, the model exhibits a phase transition
to a superconducting state with a critical temperature well in excess of 100K.
In the range t/Jp<1, there are distinct charge and spin gaps opening in the
density of states, specific heat, and magnetic susceptibility well above Tc.Comment: Calculation section and discussion of gap have been updated. Revised
calculations now enhance the predicted T_c in our model to over 200 K at
large hoppin
Phonon deficit effect and solid state refrigerators based on superconducting tunnel junctions
Thin film devices have the advantage of being extremely compact, operate in a
continuous mode, dissipate little power, and can easily be integrated in
cryogenic detectors. Motivated by such possibilities, we investigate the phonon
deficit effect in thin film (superconductor--insulator--superconductor)
and tunnel junctions. Under certain circumstances, the phonon absorption
spectra of such tunnel junctions have spectral windows of phonon
absorption/emission. We propose to use phonon filters to select the phonon
absorbtion windows and thus to enhance the cooling effect. Membranes attached
to such tunnel junctions can be cooled in this way more effectively. We discuss
a particular superlattice design of corresponding phonon filters.Comment: 8 pages 7 figure
Quantum kinetic approach for studying thermal transport in the presence of electron-electron interactions and disorder
A user friendly scheme based on the quantum kinetic equation is developed for
studying thermal transport phenomena in the presence of interactions and
disorder. We demonstrate that this scheme is suitable for both a systematic
perturbative calculation as well as a general analysis. We believe that we
present an adequate alternative to the Kubo formula, which for the thermal
transport is rather cumbersome.Comment: 30 pages, 16 figure
Renormalization Group Approach to Strong-Coupled Superconductors
We develop an asymptotically exact renormalization group (RG) approach that
treats electron-electron and electron-phonon interactions on equal footing. The
approach allows an unbiased study of the instabilities of Fermi liquids without
the assumption of a broken symmetry. We apply our method to the problem of
strongly coupled superconductors and find the temperature T* below which the
high-temperature Fermi liquid state becomes unstable towards Cooper pairing. We
show that T* is the same as the critical temperature Tc obtained in
Eliashberg's strong coupling theory starting from the low-temperature
superconducting phase. We also show that Migdal's theorem is implicit in our
approach. Finally, our results lead to a novel way to calculate numerically,
from microscopic parameters, the transition temperature of superconductors.Comment: 6 pages, 3 figures, expanded presentation, final versio
Umklapp scattering of pairs in BCS superconductivity theory
The BCS theory of superconductivity is extended to recognize pairing of
electrons by both normal and umklapp scattering. Application of the variational
approach shows that coexistence of normal and umklapp scattering frustrates
superconductivity.Comment: 9 pages, 5 figures. to be published in Journal of Physics: Condensed
Matte
Effect of electron-phonon interaction range on lattice polaron dynamics: a continuous-time quantum Monte Carlo study
We present the numerically exact ground state energy, effective mass, and
isotope exponents of a one-dimensional lattice polaron, valid for any range of
electron-phonon interaction, applying a new continuous-time Quantum Monte Carlo
(QMC) technique in a wide range of coupling strength and adiabatic ratio. The
QMC method is free from any systematic finite-size and finite-time-step errors.
We compare our numerically exact results with analytical weak-coupling theory
and with the strong-coupling expansion. We show that the exact
results agree well with the canonical Fr\"ohlich and Holstein-Lang-Firsov
theories in the weak and strong coupling limits, respectively, for any range of
interaction. We find a strong dependence of the polaron dynamics on the range
of interaction. An increased range of interaction has a similar effect to an
increased (less adiabatic) phonon frequency: specifically, a reduction in the
effective mass.Comment: 27 pages, 16 figures, to appear Phys Rev B. Introduction rewritten,
comparison with other authors extended, description of method shortened,
improved treatment of weak coupling theor
Two-dimensional Hubbard-Holstein bipolaron
We present a diagrammatic Monte Carlo study of the properties of the
Hubbard-Holstein bipolaron on a two-dimensional square lattice. With a small
Coulomb repulsion, U, and with increasing electron-phonon interaction, and when
reaching a value about two times smaller than the one corresponding to the
transition of light polaron to heavy polaron, the system suffers a sharp
transition from a state formed by two weakly bound light polarons to a heavy,
strongly bound on-site bipolaron. Aside from this rather conventional bipolaron
a new bipolaron state is found for large U at intermediate and large
electron-phonon coupling, corresponding to two polarons bound on
nearest-neighbor sites. We discuss both the properties of the different
bipolaron states and the transition from one state to another. We present a
phase diagram in parameter space defined by the electron-phonon coupling and U.
Our numerical method does not use any artificial approximation and can be
easily modified to other bipolaron models with longer range electron-phonon
and/or electron-electron interaction.Comment: 14 pages, 12 figure
Electron-phonon relaxation and excited electron distribution in gallium nitride
We develop a theory of energy relaxation in semiconductors and insulators
highly excited by the long-acting external irradiation. We derive the equation
for the non-equilibrium distribution function of excited electrons. The
solution for this function breaks up into the sum of two contributions. The
low-energy contribution is concentrated in a narrow range near the bottom of
the conduction band. It has the typical form of a Fermi distribution with an
effective temperature and chemical potential. The effective temperature and
chemical potential in this low-energy term are determined by the intensity of
carriers' generation, the speed of electron-phonon relaxation, rates of
inter-band recombination and electron capture on the defects. In addition,
there is a substantial high-energy correction. This high-energy 'tail' covers
largely the conduction band. The shape of the high-energy 'tail' strongly
depends on the rate of electron-phonon relaxation but does not depend on the
rates of recombination and trapping. We apply the theory to the calculation of
a non-equilibrium distribution of electrons in irradiated GaN. Probabilities of
optical excitations from the valence to conduction band and electron-phonon
coupling probabilities in GaN were calculated by the density functional
perturbation theory. Our calculation of both parts of distribution function in
gallium nitride shows that when the speed of electron-phonon scattering is
comparable with the rate of recombination and trapping then the contribution of
the non-Fermi 'tail' is comparable with that of the low-energy Fermi-like
component. So the high-energy contribution can affect essentially the charge
transport in the irradiated and highly doped semiconductors.Comment: 15 pages, 6 figure
A strange metal with a small Fermi surface and strong collective excitations
We develop a theory of a hybrid state, where quasi-particles coexist with
strong collective modes, taking as a starting point a model of infinitely many
1D Mott insulators coupled by a weak interchain tunneling. This state exists at
an intermediate temperature range and undergoes an antiferromagnetic phase
transition at temperatures much smaller than the Mott-Hubbard gap. The most
peculiar feature of the hybrid state is that the volume of the Fermi surface is
unrelated to the electron density. We present a self-consistent derivation of
the low energy effective action for our model.Comment: 12 pages, 7 figure
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