631 research outputs found
Time-dependent backgrounds of 2D string theory: Non-perturbative effects
We study the non-perturbative corrections (NPC) to the partition function of
a compactified 2D string theory in a time-dependent background generated by a
tachyon source. The sine-Liouville deformation of the theory is a particular
case of such a background. We calculate the leading as well as the subleading
NPC using the dual description of the string theory as matrix quantum
mechanics. As in the minimal string theories, the NPC are classified by the
double points of a complex curve. We calculate them by two different methods:
by solving Toda equation and by evaluating the quasiclassical fermion wave
functions. We show that the result can be expressed in terms of correlation
functions of the bosonic field associated with the tachyon source and identify
the leading and the subleading corrections as the contributions from the
one-point (disk) and two-point (annulus) correlation functions.Comment: 37 pages, 2 figure
Breakdown of the Wiedemann-Franz law in strongly-coupled electron-phonon system, application to the cuprates
With the superconducting cuprates in mind, a set of unitary transformations
was used to decouple electrons and phonons in the strong-coupling limit. While
phonons remain almost unrenormalised, electrons are transformed into itinerent
singlet and triplet bipolarons and thermally excited polarons. The
triplet/singlet exchange energy and the binding energy of the bipolarons are
thought to account for the spin and charge pseudogaps in the cuprates,
respectively. We calculated the Hall Lorenz number of the system to show that
the Wiedemann-Franz law breaks down due to the interference of the polaron and
bipolaron contributions to heat flow. The model provides a quantitative fit to
magnetotransport data in the cuprates. Furthermore we are able to extract the
phonon component of the thermal conductivity with the use of experimental data
and the model. Our results further validate the use of a charged Bose gas model
to describe normal and superconducting properties of unconventional
superconductors.Comment: 9 pages, 6 figures. Submitted to Physical Review
2D String Theory as Normal Matrix Model
We show that the bosonic string theory at finite temperature has two
matrix-model realizations related by a kind of duality transformation. The
first realization is the standard one given by the compactified matrix quantum
mechanics in the inverted oscillator potential. The second realization, which
we derive here, is given by the normal matrix model. Both matrix models exhibit
the Toda integrable structure and are associated with two dual cycles (a
compact and a non-compact one) of a complex curve with the topology of a sphere
with two punctures. The equivalence of the two matrix models holds for an
arbitrary tachyon perturbation and in all orders in the string coupling
constant.Comment: lanlmac, 21 page
Comment on `Dynamical properties of small polarons'
We show that the conclusion on the breakdown of the standard small polaron
theory made recently by E.V. deMello and J. Ranninger (Phys. Rev. B 55, 14872
(1997)) is a result of an incorrect interpretation of the electronic and
vibronic energy levels of the two-site Holstein model. The small polaron
theory, when properly applied, agrees well with the numerical results of these
authors. Also we show that their attempt to connect the properties of the
calculated correlation functions with the features of the intersite electron
hopping is unsuccessful.Comment: To appear in Phys. Rev.
Screening effects in the electron-optical phonon interaction
We show that recently reported unusual hardening of optical phonons
renormalized by the electron-phonon interaction is due to the neglect of
screening effects. When the electron-ion interaction is properly screened
optical phonons soften in three dimension. It is important that for
short-wavelength optical phonons screening is static while for long-wavelength
optical phonons screening is dynamic. In two-dimensional and one-dimensional
cases due to crossing of the nonperturbed optical mode with gapless plasmons
the spectrum of renormalized optical phonon-plasmon mode shows split momentum
dependence.Comment: 7 page
Neutron charge radius and the Dirac equation
We consider the Dirac equation for a finite-size neutron in an external
electric field. We explicitly incorporate Dirac-Pauli form factors into the
Dirac equation. After a non-relativistic reduction, the Darwin-Foldy term is
cancelled by a contribution from the Dirac form factor, so that the only
coefficient of the external field charge density is , i. e. the
root mean square radius associated with the electric Sachs form factor . Our
result is similar to a recent result of Isgur, and reconciles two apparently
conflicting viewpoints about the use of the Dirac equation for the description
of nucleons.Comment: 7 pages, no figures, to appear in Physical Review
Integrable flows in c=1 string theory
In these notes we review the method to construct integrable deformations of
the compactified c=1 bosonic string theory by primary fields (momentum or
winding modes), developed recently in collaboration with S. Alexandrov and V.
Kazakov. The method is based on the formulation of the string theory as a
matrix model. The flows generated by either momentum or winding modes (but not
both) are integrable and satisfy the Toda lattice hierarchy.Comment: sect.1 extended and typos correcte
Jahn-Teller distortions and phase separation in doped manganites
A "minimal model" of the Kondo-lattice type is used to describe a competition
between the localization and metallicity in doped manganites and related
magnetic oxides with Jahn-Teller ions. It is shown that the number of itinerant
charge carriers can be significantly lower than that implied by the doping
level x. A strong tendency to the phase separation is demonstrated for a wide
range of intermediate doping concentrations vanishing at low and high doping.
The phase diagram of the model in the x-T plane is constructed. At low
temperatures, the system is in a state with a long-range magnetic order:
antiferromagnetic (AF), ferromagnetic (FM), or AF-FM phase separated (PS)
state. At high temperatures, there can exist two types of the paramagnetic (PM)
state with zero and nonzero density of the itinerant electrons. In the
intermediate temperature range, the phase diagram includes different kinds of
the PS states: AF-FM, FM-PM, and PM with different content of itinerant
electrons. The applied magnetic field changes the phase diagram favoring the FM
ordering. It is shown that the variation of temperature or magnetic field can
induce the metal-insulator transition in a certain range of doping levels.Comment: 14 pages, 7 figures, submitted to Phys. Rev. B.; v.2 contains the
changes introduced according to comments of the PRB Referees; in v. 3, some
misprints are correcte
Time-dependent backgrounds of 2D string theory
We study possible backgrounds of 2D string theory using its equivalence with
a system of fermions in upside-down harmonic potential. Each background
corresponds to a certain profile of the Fermi sea, which can be considered as a
deformation of the hyperbolic profile characterizing the linear dilaton
background. Such a perturbation is generated by a set of commuting flows, which
form a Toda Lattice integrable structure. The flows are associated with all
possible left and right moving tachyon states, which in the compactified theory
have discrete spectrum. The simplest nontrivial background describes the
Sine-Liouville string theory. Our methods can be also applied to the study of
2D droplets of electrons in a strong magnetic field.Comment: 28 pages, 2 figures, lanlma
Synthesis and research of polyfunctional silylureas used in electric deposition of tin-indium alloy
Polyfunctional silylureas were synthesized by the interaction of 3-aminopropyltriethoxysilane with isocyanates of various structures in an inert aromatic solvent. Commercially available diisocyanates such as isophorone diisocyanate, hexamethylene diisocyanate, 2,4-toluene diisocyanate were used as isocyanates. In this case, freshly distilled toluene was used as a solvent. The structures of the obtained compounds were confirmed by the data of IR and NMR1H spectroscopy. Using the synthesized compounds, formulations of compositions for electrodeposition of a tin-indium alloy on a copper wire were developed. The possibility of using silylureas of various structures as effective surfactants used in the electrodeposition of the tin-indium alloy is shown. The operational characteristics of the obtained wire were investigated, including the wire diameter, coating thickness, tensile strength, electrical resistance, and direct current electrical resistivity
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