82 research outputs found
Triplet excitations in graphene-based systems
In this article we investigate the excitations in a single graphene layer and
in a single-walled carbon nanotube, i.e. the spectrum of magnetic excitations
is calculated. In the absence of interactions in these systems there is a
unique gap in the electron-hole continuum. We show that in the presence of
Coulomb correlations new states, magnons, appear in this forbidden region.
Coulomb interaction is examined in the context of Pariser-Parr-Pople (PPP)
model which takes into account long range nature of interaction. The energy of
new bound states depends on the strength of Coulomb forces. The calculations
are performed for arbitrary electron-hole () momentum what
allows to find the magnons dispersion law , effective
mass and velocity . Finally, we determine the critical values of
system parameters when this type of excitations can exist.Comment: 7 pages, 7 figure
Laser-like vibrational instability in rectifying molecular conductors
We study the damping of molecular vibrations due to electron-hole pair
excitations in donor-acceptor(D-A) type molecular rectifiers. At finite voltage
additional non-equilibrium electron-hole pair excitations involving both
electrodes become possible, and contribute to the stimulated emission and
absorption of phonons. We point out a generic mechanism for D-A molecules,
where the stimulated emission can dominate beyond a certain voltage due to
inverted position of the D and A quantum resonances. This leads to
current-driven amplification (negative damping) of the phonons similar to
laser-action. We investigate the effect in realistic molecular rectifier
structures using first principles calculations.Comment: 4 pages, 4 figure
Semi-classical generalized Langevin equation for equilibrium and nonequilibrium molecular dynamics simulation
Molecular dynamics (MD) simulation based on Langevin equation has been widely
used in the study of structural, thermal properties of matters in difference
phases. Normally, the atomic dynamics are described by classical equations of
motion and the effect of the environment is taken into account through the
fluctuating and frictional forces. Generally, the nuclear quantum effects and
their coupling to other degrees of freedom are difficult to include in an
efficient way. This could be a serious limitation on its application to the
study of dynamical properties of materials made from light elements, in the
presence of external driving electrical or thermal fields. One example of such
system is single molecular dynamics on metal surface, an important system that
has received intense study in surface science. In this review, we summarize
recent effort in extending the Langevin MD to include nuclear quantum effect
and their coupling to flowing electrical current. We discuss its applications
in the study of adsorbate dynamics on metal surface, current-induced dynamics
in molecular junctions, and quantum thermal transport between different
reservoirs.Comment: 23 pages, 16 figur
Interaction-induced negative differential resistance in asymmetric molecular junctions
Combining insights from quantum chemistry calculations with master equations,
we discuss a mechanism for negative differential resistance (NDR) in molecular
junctions, operated in the regime of weak tunnel coupling. The NDR originates
from an interplay of orbital spatial asymmetry and strong electron-electron
interaction, which causes the molecule to become trapped in a non-conducting
state above a voltage threshold. We show how the desired asymmetry can be
selectively introduced in individual orbitals in e.g., OPE-type molecules by
functionalization with a suitable side group, which is in linear conjugation to
one end of the molecule and cross-conjugated to the other end.Comment: 8 page
Current-induced forces and hot-spots in biased nano-junctions
We investigate theoretically the interplay of current-induced forces (CIF),
Joule heating, and heat transport inside a current-carrying nano-conductor. We
find that the CIF, due to the electron-phonon coherence, can control the
spatial heat dissipation in the conductor. This yields a significant asymmetric
concentration of excess heating (hot-spot) even for a symmetric conductor. When
coupled to the electrode phonons, CIF drive different phonon heat flux into the
two electrodes. First-principles calculations on realistic biased
nano-junctions illustrate the importance of the effect.Comment: Phys. Rev. Lett. accepted versio
Magnetic field-induced soft mode in spin-gapped high-Tc superconductors
We present an explanation of the dynamical in-gap spin mode in LSCO induced
by an applied magnetic field H as recently observed by J. Chang et al. Our
model consists of a phenomenological spin-only Hamiltonian, and the softening
of the spin mode is caused by vortex pinning of dynamical stripe fluctuations
which we model by a local ordering of the exchange interactions. The spin gap
vanishes experimentally around H=7T which in our scenario corresponds to the
field required for overlapping vortex regions.Comment: 4 pages, 3 fig
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