191 research outputs found
Weak localization effects in granular metals
The weak localization correction to the conductivity of a granular metal is
calculated using the diagrammatic technique in the reciprocal grain lattice
representation. The properties of this correction are very similar to that one
in disordered metal, with the replacement of the electron mean free path by the grain diameter and the dimensionless conductance by the
tunnelling dimensionless conductance . In particular, we demonstrate
that at zero temperature no conducting phase can exist for dimensions . We also analyze the WL correction to magnetoconductivity in the weak field
limit.Comment: 4 pages, 3 figures; minor corrections adde
Two-gap model for underdoped cuprate superconductors
Various properties of underdoped superconducting cuprates, including the
momentum-dependent pseudogap opening, indicate a behavior which is neither BCS
nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a
two-gap model. This model assumes an anisotropic pairing interaction among two
kinds of fermions with small and large Fermi velocities representing the
quasiparticles near the M and the nodal points of the Fermi surface
respectively. We find that a gap forms near the M points resulting into
incoherent pairing due to strong fluctuations. Instead the pairing near the
nodal points sets in with phase coherence at lower temperature. By tuning the
momentum-dependent interaction, the model allows for a continuous evolution
from a pure BCS pairing (in the overdoped and optimally doped regime) to a
mixed boson-fermion picture (in the strongly underdoped regime).Comment: 5 pages, 1 enclosed figure. For further information see
http://htcs.or
Temperature dependence of the collective mode and its influence on the band splitting in bilayer cuprates
The recently observed bilayer splitting in high-T cuprates is analyzed
within a model where the charge carriers are coupled to a phenomenological
bosonic spectrum which interpolates between the marginal Fermi liquid structure
and collective mode type behavior as a function of temperature. We argue that
the origin of the collective mode is probably associated with dynamic
incommensurate charge density waves. Moreover it is shown that the resulting
temperature dependence of the self-energy is in good agreement with
as extracted from angle-resolved photoemission data.Comment: 6 pages, 4 figures, accepted for PR
Dynamical charge susceptibility in layered cuprates: the influence of screened inter-site Coulomb repulsion
The analytical expression for dynamical charge susceptibility in layered
cuprates has been derived in the frame of singlet-correlated band model beyond
random-phase-approximation (RPA) scheme. Our calculations performed near
optimal doping regime show that there is a peak in real part of the charge
susceptibility at {\bf Q} = (, ) at strong
enough inter-site Coulomb repulsion. Together with the strong maximum in the Im
at 15 meV it confirms the formation of low-energetic
plasmons or charge fluctuations. This provides a jsutification that these
excitations are important and together with a spin flcutuations can contribute
to the Cooper pairing in layered cuprates. Analysing the charge susceptibilitiy
with respect to an instability we obtain a new plasmon branch, , along the Brillouin Zone. In particular, we have found that it goes to
zero near {\bf Q}
Effects of an electronic topological transition for anisotropic low-dimensional superconductors
We study the superconducting properties of a two-dimensional superconductor
in the proximity to an electronic topological transition (ETT). In contrast to
the 3D case, we find that the superconducting gap at T=0, the critical
temperature Tc, and the impurity scattering rate are characterized by a
nonmonotonic behavior, with maxima occurring close to the ETT. We derive
analytical expressions for the value of such maxima both in the s-wave and in
the d-wave case. Such expressions are in good qualitative agreement with the
phenomenological trend recently observed for Tc^max as a function of the
hopping ratio t'/t across several cuprate compounds. We further analyze the
effect of an ETT on the Ginzburg-Landau stiffness eta. Instead of vanishing at
the ETT, as could be expected, thus giving rise to an increase of the
fluctuation effects, in the case of momentum-independent electron-electron
interaction, we find eta different from 0, as a result of an integration over
the whole Fermi surface.Comment: to be published in Phys. Rev.
Maxwell Equations in Complex Form of Majorana - Oppenheimer, Solutions with Cylindric Symmetry in Riemann S_{3} and Lobachevsky H_{3} Spaces
Complex formalism of Riemann - Silberstein - Majorana - Oppenheimer in
Maxwell electrodynamics is extended to the case of arbitrary pseudo-Riemannian
space - time in accordance with the tetrad recipe of Tetrode - Weyl - Fock -
Ivanenko. In this approach, the Maxwell equations are solved exactly on the
background of static cosmological Einstein model, parameterized by special
cylindrical coordinates and realized as a Riemann space of constant positive
curvature. A discrete frequency spectrum for electromagnetic modes depending on
the curvature radius of space and three parameters is found, and corresponding
basis electromagnetic solutions have been constructed explicitly. In the case
of elliptical model a part of the constructed solutions should be rejected by
continuity considerations. Similar treatment is given for Maxwell equations in
hyperbolic Lobachevsky model, the complete basis of electromagnetic solutions
in corresponding cylindrical coordinates has been constructed as well, no
quantization of frequencies of electromagnetic modes arises.Comment: 39 page
Synthesis of 8-phenyl substituted 3-benzazecines with allene moiety, their thermal rearrangement and evaluation as acetylcholinesterase inhibitors
Various 4′-R-substituted phenyl azacyclic allenes were synthesized in good yields, and their thermal transformations were studied. For the first time, the obtained rearrangement products—new N-bridged cyclopenta[a]indenes, and the corresponding parent allenes were evaluated as potential inhibitors of acetyl- and butyrylcholinesterase. Among the tested compounds, the allene derivative 2g proved to competitively inhibit human AChE with inhibition constant value (Ki) in the low micromolar range. Graphic abstract: [Figure not available: see fulltext.
Effects of proximity to an electronic topological transition on normal state transport properties of the high-Tc superconductors
Within the time dependent Ginzburg-Landau theory, the effects of the
superconducting fluctuations on the transport properties above the critical
temperature are characterized by a non-zero imaginary part of the relaxation
rate gamma of the order parameter. Here, we evaluate Im gamma for an
anisotropic dispersion relation typical of the high-Tc cuprate superconductors
(HTS), characterized by a proximity to an electronic topological transition
(ETT). We find that Im gamma abruptly changes sign at the ETT as a function of
doping, in agreement with the universal behavior of the HTS. We also find that
an increase of the in-plane anisotropy, as is given by a non-zero value of the
next-nearest to nearest hopping ratio r=t'/t, increases the value of | Im gamma
| close to the ETT, as well as its singular behavior at low temperature,
therefore enhancing the effect of superconducting fluctuations. Such a result
is in qualitative agreement with the available data for the excess Hall
conductivity for several cuprates and cuprate superlattices.Comment: to appear in Phys. Rev.
DETECTION AND DIFFERENTIATION OF NON-TUBERCULOUS MYCOBACTERIA AND M. TUBERCULOSIS COMPLEX BY REAL TIME PCR
Goal of the study: to define the design of primers and probes specific to DNA of non-tuberculous mycobacteria and evaluate their diagnostic value in case of simultaneous detection of non-tuberculous mycobacteria and M. tuberculosis complex by real time PCR.Materials and methods. Primer 3, Primer BLAST, Ugene Uni Pro were used to design primers and probes. Preliminary assessment of specificity and sensitivity of detection of non-tuberculous mycobacteria DNA was performed on cultures belonging to 18 types of non-tuberculous mycobacteria, 16 strains of M. tuberculosis complex and 14 types of microorganisms being none Mycobacterum. Analytic sensitivity was tested on 284 cultures of non-tuberculous mycobacteria and diagnostic sensitivity was tested on 124 sputum samples. The kit ofM-Sorb-Tub-Avtomat (ZAO Sintol) was used for DNA isolation. Cultures were subcultured on the liquid medium of Middlebrook 7H9 in Bactec MGIT 960. Cultures were identified with the use of standard microbiological techniques. Analysis of DNA isolated from cultures was performed by the reagent kit of GenoTypeCM/AS (Hain Lifescience, Germany).Results. 100% specificity and sensitivity of PCR was demonstrated in mycobacterial cultures and 100% specificity and 69-70% sensitivity was demonstrated in diagnostic material analysis
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