556 research outputs found
Boundary Friction on Molecular Lubricants: Rolling Mode?
A theoretical model is proposed for low temperature friction between two
smooth rigid solid surfaces separated by lubricant molecules, admitting their
deformations and rotations. Appearance of different modes of energy dissipation
(by ''rocking'' or ''rolling'' of lubricants) at slow relative displacement of
the surfaces is shown to be accompanied by the stick-and-slip features and
reveals a non-monotonic (mean) friction force {\it vs} external loadComment: revtex4, 4 pages, 5 figure
On the theory of high- superconductivity of doped cuprates
A theoretical analysis is presented on possible effects of disorder by
dopants in high-temperature superconducting cuprate perovskites, to define
their basic spectra of spin and electronic excitations, and the subsequent
observable properties, especially doping dependence of superconducting order
parameter. The central point in the proposed physical picture is formation of
specific impurity subband within the insulating bandgap of initial undoped
material, serving as a source for the system metallization and further
transition into superconducting state with anisotropic order parameter.Comment: 20 pages, 11 figure
Transport Processes in Metal-Insulator Granular Layers
Tunnel transport processes are considered in a square lattice of metallic
nanogranules embedded into insulating host to model tunnel conduction in real
metal/insulator granular layers. Based on a simple model with three possible
charging states (, or 0) of a granule and three kinetic processes
(creation or recombination of a pair, and charge transfer) between
neighbor granules, the mean-field kinetic theory is developed. It describes the
interplay between charging energy and temperature and between the applied
electric field and the Coulomb fields by the non-compensated charge density.
The resulting charge and current distributions are found to be essentially
different in the free area (FA), between the metallic contacts, or in the
contact areas (CA), beneath those contacts. Thus, the steady state dc transport
is only compatible with zero charge density and ohmic resistivity in FA, but
charge accumulation and non-ohmic behavior are \emph{necessary} for conduction
over CA. The approximate analytic solutions are obtained for characteristic
regimes (low or high charge density) of such conduction. The comparison is done
with the measurement data on tunnel transport in related experimental systems.Comment: 10 pages, 11 figures, 1 reference corrected, acknowlegments adde
Specifics of impurity effects in ferropnictide superconductors
Effects of impurities and disorder on quasiparticle spectrum in
superconducting iron pnictides are considered. Possibility for occurrence of
localized energy levels due to impurities within the superconducting gap and
the related modification of band structure and of superconducting order
parameter are discussed. The evolution of superconducting state with impurity
doping is traced.Comment: 9 pages, 8 figure
Quantum effects for ballistic transport in spintronic devices
Recent fabrication of atomic precision nanodevices for spintronics greatly
boosted their performance and also revealed new interesting features, as
oscillating magnetoresistance with number of atomic layers in a multilayered
structure. This motivates the need to go beyond the usual theoretical approach
of semi-classical continuous layers. Here the simple tight-binding dynamics is
used to describe quantum conduction in a multicomponent system with
spin-polarized electrodes separated by an ultrathin and atomically coherent
non-magnetic spacer (either metallic or insulating). A possibility is indicated
for obtaining a huge resonant enhancement of magnetoresistance in such device
by a special choice of gate voltage on the spacer element.Comment: 9 pages, 9 figure
Low-temperature orientational order and possible domain structures in C(_{60}) fullerite
Based on a simple model for ordering of hexagons on square planar lattice, an
attempt has been made to consider possible structure of C(_{60}) fullerite in
its low temperature phase. It is shown that hexagons, imitating fullerens
oriented along (C_{3}) axes of \emph{sc} lattice, can be ordered into an ideal
structure with four non-equivalent molecules in unit cell. Then the energy
degeneracy for each hexagon rotations by (\pi /3) around its (C_{3}) axis
leaves the translational and orientational order in this structure, but leads
to a random distribution of (\pi /3) rotations and hence to {}``averaged{}''
unit cell with two molecules. However the most relevant structural defects are
not these intrinsic \char`\"{}misorientations\char`\"{} but certain walls
between the domains with different sequencies of the above-mentioned two
(non-ideal) sublattices. Numeric estimates have been made for the anisotropic
inter-molecular potential showing that the anisotropy is noticeably smaller for
molecules in walls than in domains
Remarks on the tight-binding model of graphene
We address a simple but fundamental issue arising in the study of graphene,
as well as of other systems that have a crystalline structure with more than
one atom per unit cell. For these systems, the choice of the tight-binding
basis is not unique. For monolayer graphene two bases are widely used in the
literature. While the expectation values of operators describing physical
quantities should be independent of basis, the form of the operators may depend
on the basis, especially in the presence of disorder or of an applied magnetic
field. Using the inappropriate form of certain operators may lead to erroneous
physical predictions. We discuss the two bases used to describe monolayer
graphene, as well as the form of the most commonly used operators in the two
bases. We repeat our analysis for the case of bilayer graphene.Comment: 15 pages, 4 figure
Fourier Transform Scanning Tunneling Spectroscopy: the possibility to obtain constant energy maps and the band dispersion using a local measurement
We present here an overview of the Fourier Transform Scanning Tunneling
spectroscopy technique (FT-STS). This technique allows one to probe the
electronic properties of a two-dimensional system by analyzing the standing
waves formed in the vicinity of defects. We review both the experimental and
theoretical aspects of this approach, basing our analysis on some of our
previous results, as well as on other results described in the literature. We
explain how the topology of the constant energy maps can be deduced from the FT
of dI/dV map images which exhibit standing waves patterns. We show that not
only the position of the features observed in the FT maps, but also their shape
can be explained using different theoretical models of different levels of
approximation. Thus, starting with the classical and well known expression of
the Lindhard susceptibility which describes the screening of electron in a free
electron gas, we show that from the momentum dependence of the susceptibility
we can deduce the topology of the constant energy maps in a joint density of
states approximation (JDOS). We describe how some of the specific features
predicted by the JDOS are (or are not) observed experimentally in the FT maps.
The role of the phase factors which are neglected in the rough JDOS
approximation is described using the stationary phase conditions. We present
also the technique of the T-matrix approximation, which takes into account
accurately these phase factors. This technique has been successfully applied to
normal metals, as well as to systems with more complicated constant energy
contours. We present results recently obtained on graphene systems which
demonstrate the power of this technique, and the usefulness of local
measurements for determining the band structure, the map of the Fermi energy
and the constant-energy maps.Comment: 33 pages, 15 figures; invited review article, to appear in Journal of
Physics D: Applied Physic
Magnetic and transport properties of diluted granular multilayers
The magnetic and transport properties of Co80Fe20t /Al2O34 nm multilayers with low nominal thicknesses t=0.7 and 0.9 nm of Co80Fe20 granular layers are studied. Magnetic studies find a superparamagnetic state above the blocking temperature Tb of field-cooled/zero-field-cooled splitting that grows with t and decreases with H. The low-voltage Ohmic tunnel transport passes to
non-Ohmic IV3/2 law for applied fields above 500 V/cm. At fixed V, the temperature dependence of conductance reveals an anomalous dip around 220 K, which can be attributed to the effect of surface contamination by supercooled water. Current-in-plane tunnel magnetoresistance MR ratio tends, at lower t, to higher maximum values 8% at room temperature but to lower field sensitivity. This may indicate growing discorrelation effect e.g., between shrinking areas of correlated moments in this regime and corroborates the deficit of granule magnetization estimated from the Inoue–Maekawa MR fit, compared to that from direct magnetization measurements. MR displays a mean-field-like critical behavior when t approaches the point of superparamagnetic/ superferromagnetic transition tc1.3 nm at room temperature from below, different from the formerly reported percolationlike behavior at approaching it from above.With growing temperature, MR reveals, beyond the common decrease, an anomalous plateau from Tb30–50 K up to some
higher value T150–200 K, not seen at higher t
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