252 research outputs found
Optical and dc conductivities of cuprates: Spin-fluctuation scattering in the t-J model
A microscopic theory of the electrical conductivity within
the t-J model is developed. An exact representation for is
obtained using the memory-function technique for the relaxation function in
terms of the Hubbard operators, and the generalized Drude law is derived. The
relaxation rate due to the decay of charge excitations into particle-hole pairs
assisted by antiferromagnetic spin fluctuations is calculated in the
mode-coupling approximation. Using results for the spectral function of spin
excitations calculated previously, the relaxation rate and the optical and dc
conductivities are calculated in a broad region of doping and temperatures. The
reasonable agreement of the theory with experimental data for cuprates proves
the important role of spin-fluctuation scattering in the charge dynamics.Comment: 13 pages,15 figures, v.2, publication referenc
Dynamic spin susceptibility of superconducting cuprates: A microscopic theory of the magnetic resonance mode
A microscopic theory of the dynamic spin susceptibility (DSS) in the
superconducting state within the t-J model is presented. It is based on an
exact representation for the DSS obtained by applying the Mori-type projection
technique for the relaxation function in terms of Hubbard operators. The static
spin susceptibility is evaluated by a sum-rule-conserving generalized
mean-field approximation, while the self-energy is calculated in the
mode-coupling approximation. The spectrum of spin excitations is studied in the
underdoped and optimally doped regions. The DSS reveals a resonance mode (RM)
at the antiferromagnetic wave vector Q = \pi(1,1) at low temperatures due to a
strong suppression of the damping of spin excitations. This is explained by an
involvement of spin excitations in the decay process besides the particle-hole
continuum usually considered in random-phase-type approximations. The spin gap
in the spin-excitation spectrum at Q plays a dominant role in limiting the
decay in comparison with the superconducting gap which results in the
observation of the RM even above in the underdoped region. A good
agreement with inelastic neutron-scattering experiments on the RM in YBCO
compounds is found.Comment: 15 pages, 20 figures, references adde
Superconductivity of strongly correlated electrons on the honeycomb lattice
A microscopic theory of the electronic spectrum and of superconductivity
within the t-J model on the honeycomb lattice is developed. We derive the
equations for the normal and anomalous Green functions in terms of the Hubbard
operators by applying the projection technique. Superconducting pairing of d +
id'-type mediated by the antiferromagnetic exchange is found. The
superconducting Tc as a function of hole doping exhibits a two-peak structure
related to the van Hove singularities of the density of states for the two-band
t-J model. At half-filling and for large enough values of the exchange
coupling, gapless superconductivity may occur. For small doping the coexistence
of antiferromagnetic order and superconductivity is suggested. It is shown that
the s-wave pairing is prohibited, since it violates the constraint of
no-double-occupancy.Comment: 10 pages, 3 figures, to be published in Eur. Phys. J.
Spin excitations and thermodynamics of the t-J model on the honeycomb lattice
We present a spin-rotation-invariant Green-function theory for the dynamic
spin susceptibility in the spin-1/2 antiferromagnetic t-J Heisenberg model on
the honeycomb lattice. Employing a generalized mean-field approximation for
arbitrary temperatures and hole dopings, the electronic spectrum of
excitations, the spin-excitation spectrum and thermodynamic quantities
(two-spin correlation functions, staggered magnetization, magnetic
susceptibility, correlation length) are calculated by solving a coupled system
of self-consistency equations for the correlation functions. The temperature
and doping dependence of the magnetic (uniform static) susceptibility is
ascribed to antiferromagnetic short-range order. Our results on the doping
dependencies of the magnetization and susceptibility are analyzed in comparison
with previous results for the t_J model on the square lattice.Comment: 9 pages, 7 figures, submitted to European Physical Journal B. arXiv
admin note: text overlap with arXiv:1703.0839
Dynamic spin susceptibility in the t-J model
A relaxation-function theory for the dynamic spin susceptibility in the
-- model is presented. By a sum-rule-conserving generalized mean-field
approximation (GMFA), the two-spin correlation functions of arbitrary range,
the staggered magnetization, the uniform static susceptibility, and the
antiferromagnetic correlation length are calculated in a wide region of hole
doping and temperaturs. A good agreement with available exact diagonalization
(ED) data is found. The correlation length is in reasonable agreement with
neutron-scattering experiments on La_{2-\delta}Sr_\delta)CuO_4. Going beyond
the GMFA, the self-energy is calculated in the mode-coupling approximation. The
spin dynamics at arbitrary frequencies and wave vectors is studied for various
temperatures and hole doping. At low doping a spin-wave-type behavior is found
as in the Heisenberg model, while at higher doping a strong damping caused by
hole hopping occurs, and a relaxation-type spin dynamics is observed in
agreement with the ED results. The local spin susceptibility and its (\omega/T)
scaling behavior are calculated in a reasonable agreement with experimental and
ED data.Comment: 13 pages, 14 figure
Doping - dependent superconducting gap anisotropy in the two-dimensional 10-3-8 pnictide Ca(PtAs)[(FePt)As]
The characteristic features of
Ca(PtAs)[(FePt)As] ("10-3-8")
superconductor are relatively high anisotropy and a clear separation of
superconductivity and structural/magnetic transitions, which allows studying
the superconducting gap without complications due to the coexisting order
parameters. The London penetration depth, measured in underdoped single
crystals of 10-3-8 ( 0.028, 0.041, 0.042, and 0.097), shows behavior
remarkably similar to other Fe-based superconductors, exhibiting robust
power-law, . The exponent decreases from 2.36
( 0.097, close to optimal doping) to 1.7 ( 0.028, a heavily
underdoped composition), suggesting that the superconducting gap becomes more
anisotropic at the dome edge. A similar trend is found in low-anisotropy
superconductors based on BaFeAs ("122"), implying that it is an
intrinsic property of superconductivity in iron pnictides, unrelated to the
coexistence of magnetic order and superconductivity or the anisotropy of the
normal state. Overall this doping dependence is consistent with
pairing competing with intra-band repulsion
Thermal drag revisited: Boltzmann versus Kubo
The effect of mutual drag between phonons and spin excitations on the thermal
conductivity of a quantum spin system is discussed. We derive general
expression for the drag component of the thermal current using both Boltzmann
equation approach and Kubo linear-response formalism to leading order in the
spin-phonon coupling. We demonstrate that aside from higher-order corrections
which appear in the Kubo formalism both approaches yield identical results for
the drag thermal conductivity. We discuss the range of applicability of our
result and provide a generalization of our consideration to the cases of
fermionic excitations and to anomalous forms of boson-phonon coupling. Several
asymptotic regimes of our findings relevant to realistic situations are
highlighted.Comment: 14 pages, 3 figures, published version, extended discussio
Perspectiveness of correction of accompanying diseases in patients with autism by course of interior use of naturally low mineralized water
The article reviews the pathogenesis and the course of autism. Authors define autism as a psychosomatic disorder. A distinctive feature of mineral waters, as natural therapeutic factors of weak intensity, is their pronounced biological activity, the absence of side effects and the possibility of long-term use in combination with medication therapy. The paper presents data substantiating the possibility of using mineral water with an increased content of organic substances for the correction of the somatic component of autism (problems of the gastrointestinal tract, carbohydrate metabolism disorders, neurosis)
Josephson surface plasmons in spatially confined cuprate superconductors
In this work, we generalize the theory of localized surface plasmons to the
case of high-Tc cuprate superconductors, spatially confined in the form of
small spherical particles. At variance from ordinary metals, cuprate
superconductors are characterized by a low-energy bulk excitation known as the
Josephson plasma wave (JPW), arising from interlayer tunneling of the
condensate along the c-axis. The effect of the JPW is revealed in a
characteristic spectrum of surface excitations, which we call Josephson surface
plasmons. Our results, which apply to any material with a strongly anisotropic
electromagnetic response, are worked out in detail for the case of multilayered
superconductors supporting both low-frequency (acoustic) and transverse-optical
JPW. Spatial confinement of the Josephson plasma waves may represent a new
degree of freedom to engineer their frequencies and to explore the link between
interlayer tunnelling and high-Tc superconductivity
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