16 research outputs found
Multiple scattering of elastic waves by pinned dislocation segments in a continuum
The coherent propagation of elastic waves in a solid filled with a random
distribution of pinned dislocation segments is studied to all orders in
perturbation theory. It is shown that, within the independent scattering
approximation, the perturbation series that generates the mass operator is a
geometric series that can thus be formally summed. A divergent quantity is
shown to be renormalizable to zero at low frequencies. At higher frequencies
said quantity can be expressed in terms of a cut-off with dimensions of length,
related to the dislocation length, and physical quantities can be computed in
terms of two parameters, to be determined by experiment. The approach used in
this problem is compared and contrasted with the scattering of de Broglie waves
by delta-function potentials as described by the Schr\"odinger equation
Anisotropic interaction of two-level systems with acoustic waves in disordered crystals
We apply the model introduced in Phys. Rev. B 75, 064202 (2007),
cond-mat/0610469, to calculate the anisotropy effect in the interaction of two
level systems with phonons in disordered crystals. We particularize our
calculations to cubic crystals and compare them with the available experimental
data to extract the parameters of the model. With these parameters we calculate
the interaction of the dynamical defects in the disordered crystal with phonons
(or sound waves) propagating along other crystalographic directions, providing
in this way a method to investigate if the anisotropy comes from the two-level
systems being preferably oriented in a certain direction or solely from the
lattice anisotropy with the two-level systems being isotropically oriented.Comment: 10 page
Tuning the electrical transport properties of double-walled carbon nanotubes by semiconductor and semi-metal filling
Manipulating the electrical properties of carbon nanotubes through semi-metal or semiconductor filling is of paramount importance in the realization of nano-electronic devices based on one dimensional composite materials. From low temperature electrical conductivity measurements of a network, of empty and filled double-walled carbon nanotubes (DWNT’s), we report a transition in electrical transport features from hopping to weakly activated conduction by HgTe filling and also semi-metallic conduction in selenium (Se) filled DWNT’s. Magneto-resistance (MR) studies of the filled DWNT’s show suppression of the hopping conduction and a signature of 3D weak localization for Se@DWNT’s at low temperatures and high magnetic fields. These results are discussed on the basis of strength of interaction between the filler material and the inner-walls of the host DWNT’s, which enhances the electronic density of states (DOS) in the material as well as the change in the property of the filler material due to constrained encapsulation
“Ah, what is moving there along the river…” To the 95th anniversary of the VI Congress of the Russian Association of Physicists
Background and Objectives: In August 1928, the VI Congress of the Russian Association of Physicists was held in the USSR with broad international participation. The congress was distinguished by: a qualified composition of participants (it was attended by 6 future Nobel Prize winners and up to 30 future members of the USSR Academy of Sciences): broad state support; an unusual mobile format; after holding plenary sessions at Moscow University. The congress, called Volga Congress in the foreign press, continued its work on the steamer “Alexey Rykov” and in the Volga university cities (Nizhny Novgorod, Kazan, Saratov). The materials of the congress are now considered as a certain cross-section of the achievements of world physics in the late 1920s. Materials and Methods: The purpose of the article is to supplement the published materials on the history of the congress with an analysis of information reflecting the events at the congress and around it and contained in the domestic central and regional periodicals of August 1928, as well as in the memoirs of participants in the events of those days. Conclusion: The attracted materials testify to the understanding of the significance and originality of the Physics Congress-1928, which was the last congress in the history of the Russian Association of Physicists, by the authorities, the scientific and teaching community, and students
Anisotropic weakly localized transport in nitrogen-doped ultrananocrystalline diamond films
We establish the dominant effect of anisotropic weak localization (WL) in
three dimensions associated with a propagative Fermi surface, on the
conductivity correction in heavily nitrogen doped ultrananocrystalline diamond
(UNCD) films based on magneto-resistance studies at low temperatures. Also, low
temperature electrical conductivity can show weakly localized transport in 3D
combined with the effect of electron-electron interactions in these materials,
which is remarkably different from the conductivity in 2DWL or strong
localization regime. The corresponding dephasing time of electronic
wavefunctions in these systems described as ~ T^-p with p < 1, follows a
relatively weak temperature dependence compared to the generally expected
nature for bulk dirty metals having . The temperature dependence of
Hall (electron) mobility together with an enhanced electron density has been
used to interpret the unusual magneto-transport features and show delocalized
electronic transport in these n-type UNCD films, which can be described as
low-dimensional superlattice structures.Comment: 27 pages, 6 figures, To be published in Physical Review
Origin of conductivity cross over in entangled multi-walled carbon nanotube network filled by iron
A realistic transport model showing the interplay of the hopping transport
between the outer shells of iron filled entangled multi-walled carbon nanotubes
(MWNT) and the diffusive transport through the inner part of the tubes, as a
function of the filling percentage, is developed. This model is based on
low-temperature electrical resistivity and magneto-resistance (MR)
measurements. The conductivity at low temperatures showed a crossover from
Efros-Shklovski (E-S) variable range hopping (VRH) to Mott VRH in 3 dimensions
(3D) between the neighboring tubes as the iron weight percentage is increased
from 11% to 19% in the MWNTs. The MR in the hopping regime is strongly
dependent on temperature as well as magnetic field and shows both positive and
negative signs, which are discussed in terms of wave function shrinkage and
quantum interference effects, respectively. A further increase of the iron
percentage from 19% to 31% gives a conductivity crossover from Mott VRH to 3D
weak localization (WL). This change is ascribed to the formation of long iron
nanowires at the core of the nanotubes, which yields a long dephasing length
(e.g. 30 nm) at the lowest measured temperature. Although the overall transport
in this network is described by a 3D WL model, the weak temperature dependence
of inelastic scattering length expressed as L_phi ~T^-0.3 suggests the
possibility for the presence of one-dimensional channels in the network due to
the formation of long Fe nanowires inside the tubes, which might introduce an
alignment in the random structure.Comment: 29 pages,10 figures, 2 tables, submitted to Phys. Rev.