2,609 research outputs found
Multimode Phonon Cooling via Three Wave Parametric Interactions with Optical Fields
We discuss the possible cooling of different phonon modes via three wave
mixing interactions of vibrational and optical modes. Since phonon modes
exhibit a variety of dispersion relations or frequency spectra with diverse
spatial structures, depending on the shape and size of the sample, we formulate
our theory in terms of relevant spatial mode functions for the interacting
fields in any given geometry. We discuss the possibility of Dicke like
collective effects in phonon cooling and present explicit results for
simultaneous cooling of two phonon modes via the anti-Stokes up conversions. We
show that the bimodal cooling should be observable experimentally
Polaron Crossover and Bipolaronic Metal-Insulator Transition in the Holstein model at half-filling
The evolution of the properties of a finite density electronic system as the
electron-phonon coupling is increased are investigated in the
Holstein model using the Dynamical Mean-Field Theory (DMFT).
We compare the spinless fermion case, in which only isolated polarons can be
formed, with the spinful model in which the polarons can bind and form
bipolarons. In the latter case, the bipolaronic binding occurs through a
metal-insulator transition. In the adiabatic regime in which the phonon energy
is small with respect to the electron hopping we compare numerically exact DMFT
results with an analytical scheme inspired by the Born-Oppenheimer procedure.
Within the latter approach,a truncation of the phononic Hilbert space leads to
a mapping of the original model onto an Anderson spin-fermion model. In the
anti-adiabatic regime (where the phonon energy exceeds the electronic scales)
the standard treatment based on Lang-Firsov canonical transformation allows to
map the original model on to an attractive Hubbard model in the spinful case.
The separate analysis of the two regimes supports the numerical evidence that
polaron formation is not necessarily associated to a metal-insulator
transition, which is instead due to pairing between the carriers. At the
polaron crossover the Born-Oppenheimer approximation is shown to break down due
to the entanglement of the electron-phonon state.Comment: 19 pages, 15 figure
Bimodal grain-size scaling of thermal transport in polycrystalline graphene from large-scale molecular dynamics simulations
Grain boundaries in graphene are inherent in wafer-scale samples prepared by
chemical vapor deposition. They can strongly influence the mechanical
properties and electronic and heat transport in graphene. In this work, we
employ extensive molecular dynamics simulations to study thermal transport in
large suspended polycrystalline graphene samples. Samples of different
controlled grain sizes are prepared by a recently developed efficient
multiscale approach based on the phase field crystal model. In contrast to
previous works, our results show that the scaling of the thermal conductivity
with the grain size implies bimodal behaviour with two effective Kapitza
lengths. The scaling is dominated by the out-of-plane (flexural) phonons with a
Kapitza length that is an order of magnitude larger than that of the in-plane
phonons. We also show that in order to get quantitative agreement with the most
recent experiments, quantum corrections need to be applied to both the Kapitza
conductance of grain boundaries and the thermal conductivity of pristine
graphene and the corresponding Kapitza lengths must be renormalized
accordingly.Comment: Accepted to Nano Lett.; Numerical samples and computer codes
availabl
Bimodal grain-size scaling of thermal transport in polycrystalline graphene from large-scale molecular dynamics simulations
Grain boundaries in graphene are inherent in wafer-scale samples prepared by
chemical vapor deposition. They can strongly influence the mechanical
properties and electronic and heat transport in graphene. In this work, we
employ extensive molecular dynamics simulations to study thermal transport in
large suspended polycrystalline graphene samples. Samples of different
controlled grain sizes are prepared by a recently developed efficient
multiscale approach based on the phase field crystal model. In contrast to
previous works, our results show that the scaling of the thermal conductivity
with the grain size implies bimodal behaviour with two effective Kapitza
lengths. The scaling is dominated by the out-of-plane (flexural) phonons with a
Kapitza length that is an order of magnitude larger than that of the in-plane
phonons. We also show that in order to get quantitative agreement with the most
recent experiments, quantum corrections need to be applied to both the Kapitza
conductance of grain boundaries and the thermal conductivity of pristine
graphene and the corresponding Kapitza lengths must be renormalized
accordingly.Comment: Accepted to Nano Lett.; Numerical samples and computer codes
availabl
Surface Polaron Formation in the Holstein model
The effect of a solid-vacuum interface on the properties of a strongly
coupled electron-phonon system is analyzed using dynamical mean-field theory to
solve the Holstein model in a semi-infinite cubic lattice. Polaron formation is
found to occur more easily (i.e., for a weaker electron-phonon coupling) on the
surface than in the bulk. On the other hand, the metal-insulator transition
associated to the binding of polarons takes place at a unique critical strength
in the bulk and at the surface.Comment: 5 pages, 3 figure
A Variational Approach to Nonlocal Exciton-Phonon Coupling
In this paper we apply variational energy band theory to a form of the
Holstein Hamiltonian in which the influence of lattice vibrations (optical
phonons) on both local site energies (local coupling) and transfers of
electronic excitations between neighboring sites (nonlocal coupling) is taken
into account. A flexible spanning set of orthonormal eigenfunctions of the
joint exciton-phonon crystal momentum is used to arrive at a variational
estimate (bound) of the ground state energy for every value of the joint
crystal momentum, yielding a variational estimate of the lowest polaron energy
band across the entire Brillouin zone, as well as the complete set of polaron
Bloch functions associated with this band. The variation is implemented
numerically, avoiding restrictive assumptions that have limited the scope of
previous assaults on the same and similar problems. Polaron energy bands and
the structure of the associated Bloch states are studied at general points in
the three-dimensional parameter space of the model Hamiltonian (electronic
tunneling, local coupling, nonlocal coupling), though our principal emphasis
lay in under-studied area of nonlocal coupling and its interplay with
electronic tunneling; a phase diagram summarizing the latter is presented. The
common notion of a "self-trapping transition" is addressed and generalized.Comment: 33 pages, 11 figure
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