37 research outputs found
Efficient simulations with electronic open boundaries
We present a reformulation of the Hairy Probe method for introducing electronic open boundaries that is appropriate for steady state calculations involving non-orthogonal atomic basis sets. As a check on the correctness of the method we investigate a perfect atomic wire of Cu atoms, and a perfect non-orthogonal chain of H atoms. For both atom chains we find that the conductance has a value of exactly one quantum unit, and that this is rather insensitive to the strength of coupling of the probes to the system, provided values of the coupling are of the same order as the mean inter-level spacing of the system without probes. For the Cu atom chain we find in addition that away from the regions with probes attached, the potential in the wire is uniform, while within them it follows a predicted exponential variation with position. We then apply the method to an initial investigation of the suitability of graphene as a contact material for molecular electronics. We perform calculations on a carbon nanoribbon to determine the correct coupling strength of the probes to the graphene, and obtain a conductance of about two quantum units corresponding to two bands crossing the Fermi surface. We then compute the current through a benzene molecule attached to two graphene contacts and find only a very weak current because of the disruption of the π-conjugation by the covalent bond between the benzene and the graphene. In all cases we find that very strong or weak probe couplings suppress the current
Nonlinear modes for the Gross-Pitaevskii equation -- demonstrative computation approach
A method for the study of steady-state nonlinear modes for Gross-Pitaevskii
equation (GPE) is described. It is based on exact statement about coding of the
steady-state solutions of GPE which vanish as by reals. This
allows to fulfill {\it demonstrative computation} of nonlinear modes of GPE
i.e. the computation which allows to guarantee that {\it all} nonlinear modes
within a given range of parameters have been found. The method has been applied
to GPE with quadratic and double-well potential, for both, repulsive and
attractive nonlinearities. The bifurcation diagrams of nonlinear modes in these
cases are represented. The stability of these modes has been discussed.Comment: 21 pages, 6 figure
Enhanced thermoelectric properties in hybrid graphene-boron nitride nanoribbons
The thermoelectric properties of hybrid graphene-boron nitride nanoribbons
(BCNNRs) are investigated using the non-equilibrium Green's function (NEGF)
approach. We find that the thermoelectric figure of merit (ZT) can be
remarkably enhanced by periodically embedding hexagonal BN (h-BN) into graphene
nanoribbons (GNRs). Compared to pristine GNRs, the ZT for armchair-edged BCNNRs
with width index 3p+2 is enhanced up to 10~20 times while the ZT of nanoribbons
with other widths is enhanced just by 1.5~3 times. As for zigzag-edge
nanoribbons, the ZT is enhanced up to 2~3 times. This improvement comes from
the combined increase in the Seebeck coefficient and the reduction in the
thermal conductivity outweighing the decrease in the electrical conductance. In
addition, the effect of component ratio of h-BN on the thermoelectric transport
properties is discussed. These results qualify BCNNRs as a promising candidate
for building outstanding thermoelectric devices.Comment: 21 pages, 7 figure
Nonequilibrium Bose systems and nonground-state Bose-Einstein condensates
The theory of resonant generation of nonground-state Bose-Einstein
condensates is extended to Bose-condensed systems at finite temperature. The
generalization is based on the notion of representative statistical ensembles
for Bose systems with broken global gauge symmetry. Self-consistent equations
are derived describing an arbitrary nonequilibrium nonuniform Bose system. The
notion of finite-temperature topological coherent modes, coexisting with a
cloud of noncondensed atoms, is introduced. It is shown that resonant
generation of these modes is feasible for a gas of trapped Bose atoms at finite
temperature.Comment: Latex file, 16 pages, no figure