186 research outputs found
Comment on 'Hysteresis, Switching, and Negative Differential Resistance in Molecular Junctions: a Polaron Model', by M. Galperin, M.A. Ratner, and A. Nitzan, Nano Lett. 5, 125 (2005)
It is shown that the ``hysteresis'' in a polaron model of electron transport
through the molecule found by M.Galperin et al. [Nano Lett. 5, 125 (2005)] is
an artefact of their ``mean-field'' approximation. The reason is trivial: after
illegitimate replacement where \hat{n} is the
electron number operator, n_{0} the average molecular level occupation,
Galperin et al. obtained non-physical dependence of a renormalized molecular
energy level on the non-integer mean occupation number n_{0} (i.e. the electron
self-interaction) and the resulting non-linearity of current. The exact theory
of correlated polaronic transport through molecular quantum dots (MQDs) that we
proposed earlier [Phys. Rev. B67, 235312 (2003)] proved that there is no
hysteresis or switching in current-voltage characteristics of non-degenerate,
d=1, or double degenerate, d=2, molecular bridges, contrary to the mean-field
result. Switching could only appear in multiply degenerate MQDs with d>2 due to
electron correlations. Most of the molecular quantum dots are in the regime of
weak coupling to the electrodes addressed in our formalism.Comment: 3 pages, no figures; (v3) estimates added showing that most of the
molecules are very resistive, so the actual molecular quantum dots are in the
regime we study, unlike very transparent `molecules' studied by Galperin et
al and other authors. In the latter case the molecules are rather
`transparent' and, obviously, no current hysteresis can exis
The de Haas-van Alphen effect in canonical and grand canonical multiband Fermi liquid
A qualitatively different character of dHvA oscillations has been found in a
multiband (quasi)two dimensional Fermi liquid with a fixed fermion density
(canonical ensemble) compared with an open system where the chemical
potential is kept fixed (grand canonical ensemble). A new fundamental
period appears when is fixed, a damping of the Landau levels is
relatively small and a background density of states is negligible. is
determined by the total density rather than by the partial densities of
carriers in different bands: for spin-split Landau
levels and in the case of spin degenerate levels where
is the flux quantum.Comment: 9 p
Angular dependence of novel magnetic quantum oscillations in a quasi-two-dimensional multiband Fermi liquid with impurities
The semiclassical Lifshitz-Kosevich-type description is given for the angular
dependence of quantum oscillations with combination frequencies in a multiband
quasi-two-dimensional Fermi liquid with a constant number of electrons. The
analytical expressions are found for the Dingle, thermal, spin, and amplitude
(Yamaji) reduction factors of the novel combination harmonics, where the latter
two strongly oscillate with the direction of the field. At the "magic" angles
those factors reduce to the purely two-dimensional expressions given earlier.
The combination harmonics are suppressed in the presence of the non-quantized
("background") states, and they decay exponentially faster with temperature
and/or disorder compared to the standard harmonics, providing an additional
tool for electronic structure determination. The theory is applied to
SrRuO.Comment: 5 pages, 2 figures, minor typos correcte
Three-body scattering problem and two-electron tunneling in molecular wires
We solve the Lippmann-Schwinger equation describing elastic scattering of
preformed pairs (e.g. bipolarons) off a short-range scattering center and find
the two-particle transmission through a thin potential barrier. While the pair
transmission is smaller than the single-electron transmission in the
strong-coupling limit, it is remarkably larger in the weak coupling limit. We
also calculate current-voltage characteristics of a molecule - barrier -
molecule junction. They show unusual temperature and voltage behavior which are
experimentally verifiable at low temperatures.Comment: 5 pages, 2 figure
Phase coexistence and resistivity near the ferromagnetic transition of manganites
Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and
their magnetic pair-breaking in the ferromagnetic phase [the so-called
current-carrier density collapse (CCDC)] has accounted for the first-order
ferromagnetic phase transition, colossal magnetoresistance (CMR), isotope
effect, and pseudogap in doped manganites. Here we propose an explanation of
the phase coexistence and describe the magnetization and resistivity of
manganites near the ferromagnetic transition in the framework of CCDC. The
present quantitative description of resistivity is obtained without any fitting
parameters by using the experimental resistivities far away from the transition
and the experimental magnetization, and essentially model independent.Comment: 10 pages, 3 figure
General Green's function formalism for transport calculations with spd-Hamiltonians and giant magnetoresistance in Co and Ni based magnetic multilayers
A novel, general Green's function technique for elastic spin-dependent
transport calculations is presented, which (i) scales linearly with system size
and (ii) allows straightforward application to general tight-binding
Hamiltonians (spd in the present work). The method is applied to studies of
conductance and giant magnetoresistance (GMR) of magnetic multilayers in CPP
(current perpendicular to planes) geometry in the limit of large coherence
length. The magnetic materials considered are Co and Ni, with various
non-magnetic materials from the 3d, 4d, and 5d transition metal series.
Realistic tight-binding models for them have been constructed with the use of
density functional calculations. We have identified three qualitatively
different cases which depend on whether or not the bands (densities of states)
of a non-magnetic metal (i) form an almost perfect match with one of spin
sub-bands of the magnetic metal (as in Cu/Co spin valves); (ii) have almost
pure sp character at the Fermi level (e.g. Ag); (iii) have almost pure d
character at the Fermi energy (e.g. Pd, Pt). The key parameters which give rise
to a large GMR ratio turn out to be (i) a strong spin polarization of the
magnetic metal, (ii) a large energy offset between the conduction band of the
non-magnetic metal and one of spin sub-bands of the magnetic metal, and (iii)
strong interband scattering in one of spin sub-bands of a magnetic metal. The
present results show that GMR oscillates with variation of the thickness of
either non-magnetic or magnetic layers, as observed experimentally.Comment: 22 pages, 9 figure
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