scholarly journals Failure of mean-field approximation in weakly coupled dilaton gravity

2018 ◽  
Vol 191 ◽  
pp. 07004
Author(s):  
Maxim Fitkevich
Keyword(s):  
Black Hole ◽  
Mean Field ◽  
Direct Calculation ◽  
Scalar Fields ◽  
Field Approximation ◽  
Mean Field Approximation ◽  
Massless Scalar ◽  
Dilaton Gravity ◽  
Black Hole Evaporation ◽  
Weakly Coupled

We investigate black hole evaporation in a weakly coupled model of two-dimensional dilaton gravity paying a particular attention to the validity of the semiclassical mean-field approximation. Our model is obtained by adding a reflecting boundary to the celebrated RST model describing N gravitating massless scalar fields to one-loop level. The boundary cuts off the region of strong coupling. Although our model is explicitly weakly coupled, we find that the mean field approximation inevitably fails at the end of black hole evaporation. We propose an alternative semiclassical method aiming at direct calculation of S-matrix elements and illustrate it in a simple shell model.

scholarly journals Several Physical Properties of Two Interacting Complex Scalar Fields at Finite Density

2011 ◽  
Vol 21 (1) ◽  
pp. 1
Author(s):  
Tran Huu Phat ◽  
Phan Thi Duyen
Keyword(s):  
Mean Field ◽  
Scalar Fields ◽  
Meissner Effect ◽  
Field Approximation ◽  
Mean Field Approximation ◽  
Goldstone Theorem ◽  
Complex Scalar ◽  
Finite Density ◽  
Chemical Potentials ◽  
The Mean

The two interacting complex scalar fields at finite density is considered in the mean field approximation. It is shown that although the symmetry is spontaneously broken for the chemical potentials bigger than the meson masses in vacuum, but the Goldstone theorem is not preserved in broken phase. Then two mesons are condensed and their condensates turn out to be two-gap superconductor which is signaled by the appearance of the Meissner effect as well as the Abrikosov and non-Abrikosov vortices. Finally, there exhibits domain wall which is the plane, where two condensates flowing in opposite directions collide and generate two types of vortices with cores in the wall.

ENTANGLEMENT DYNAMICS OF FLUCTUATIONS IN TWO-MODE BOSE–EINSTEIN CONDENSATES

2010 ◽  
Vol 24 (25) ◽  
pp. 2571-2580
Author(s):  
P. L. SHU ◽  
L. C. WANG ◽  
X. X. YI
Keyword(s):  
Time Evolution ◽  
Mean Field ◽  
Field Approximation ◽  
Mean Field Approximation ◽  
Entanglement Dynamics ◽  
Weakly Coupled ◽  
Tunneling Transition ◽  
The Mean ◽  
Bose Einstein ◽  
Double Well Potential

The entanglement dynamics of fluctuations in two weakly coupled Bose–Einstein condensates (BECs) is studied in this paper. By calculating the time evolution of entanglement between two fluctuations of condensates in a double-well potential, we show that the nonlinear tunneling transition can be reflected in the entanglement dynamics of fluctuations in BECs. This complements the study on the entanglement dynamics of BECs based on the mean-field approximation.

A MEAN FIELD APPROACH TO THE COLORED HUBBARD MODEL

2001 ◽  
Vol 16 (11) ◽  
pp. 2003-2008
Author(s):  
TOBIAS BAIER ◽  
EIKE BICK
Keyword(s):  
Mean Field ◽  
Scalar Fields ◽  
Field Approximation ◽  
Square Lattice ◽  
Mean Field Approximation ◽  
Antiferromagnetic Order ◽  
Expectation Values ◽  
Field Approach ◽  
Antiferromagnetic Ordering ◽  
Measured Phase Diagram

By subdividing a 2d square lattice into plaquettes containing 4 lattice sites each, the Hubbard Hamiltonian is reformulated in terms of different "colored" fermion species. This makes it possible to describe e.g. antiferromagnetic order or d-wave superconductivity in terms of expectation values of composite scalar fields. A suitable mean field approximation indeed reproduces qualitatively the measured phase diagram and shows a competition between phases with antiferromagnetic ordering and dx2-y2-superconductivity at low temperatures.

The Determination of the Magnetoelectric Coupling Coefficient in Ferroelectric–Ferromagnetic Composite Based on PZT–Ferrite

2013 ◽  
Vol 58 (4) ◽  
pp. 1401-1403 ◽  
Author(s):  
J.A. Bartkowska ◽  
R. Zachariasz ◽  
D. Bochenek ◽  
J. Ilczuk
Keyword(s):  
Dielectric Permittivity ◽  
Coupling Coefficient ◽  
Mean Field ◽  
Field Approximation ◽  
Theoretical Description ◽  
Magnetoelectric Coupling ◽  
Mean Field Approximation ◽  
Dielectric Measurements ◽  
Temperature Dependences ◽  
Multiferroic Composite

Abstract In the present work, the magnetoelectric coupling coefficient, from the temperature dependences of the dielectric permittivity for the multiferroic composite was determined. The research material was ferroelectric-ferromagnetic composite on the based PZT and ferrite. We investigated the temperature dependences of the dielectric permittivity (") for the different frequency of measurement’s field. From the dielectric measurements we determined the temperature of phase transition from ferroelectric to paraelectric phase. For the theoretical description of the temperature dependence of the dielectric constant, the Hamiltonian of Alcantara, Gehring and Janssen was used. To investigate the dielectric properties of the multiferroic composite this Hamiltonian was expressed under the mean-field approximation. Based on dielectric measurements and theoretical considerations, the values of the magnetoelectric coupling coefficient were specified.

scholarly journals Hexagonal-Shaped Spin Crossover Nanoparticles Studied by Ising-Like Model Solved by Local Mean Field Approximation

2021 ◽  
Vol 7 (5) ◽  
pp. 69
Author(s):  
Catherine Cazelles ◽  
Jorge Linares ◽  
Mamadou Ndiaye ◽  
Pierre-Richard Dahoo ◽  
Kamel Boukheddaden
Keyword(s):  
Spin Crossover ◽  
Hexagonal Lattice ◽  
Mean Field ◽  
Field Approximation ◽  
Mean Field Approximation ◽  
Ligand Field ◽  
Order And Disorder ◽  
The Matrix ◽  
Local Mean ◽  
Parameter Values

The properties of spin crossover (SCO) nanoparticles were studied for five 2D hexagonal lattice structures of increasing sizes embedded in a matrix, thus affecting the thermal properties of the SCO region. These effects were modeled using the Ising-like model in the framework of local mean field approximation (LMFA). The systematic combined effect of the different types of couplings, consisting of (i) bulk short- and long-range interactions and (ii) edge and corner interactions at the surface mediated by the matrix environment, were investigated by using parameter values typical of SCO complexes. Gradual two and three hysteretic transition curves from the LS to HS states were obtained. The results were interpreted in terms of the competition between the structure-dependent order and disorder temperatures (TO.D.) of internal coupling origin and the ligand field-dependent equilibrium temperatures (Teq) of external origin.

Geometrical and Transport Properties of Disordered Fibre Networks: Analytical Results

1997 ◽  
Vol 11 (20) ◽  
pp. 867-875 ◽  
Author(s):  
A. A. Rodríaguez ◽  
E. Medina
Keyword(s):  
Probability Distribution ◽  
Transport Properties ◽  
Geometrical Structure ◽  
Mean Field ◽  
Field Approximation ◽  
Mean Field Approximation ◽  
Fracture Networks ◽  
Simplified Models ◽  
Fibre Density ◽  
Line Network

We study novel geometrical and transport properties of a 2D model of disordered fibre networks. To assess the geometrical structure we determine, analytically, the probability distribution for the number of fibre intersections and resulting segment sizes in the network as a function of fibre density and length. We also determine, numerically, the probability distribution of pore perimeters and areas. We find a non-monotonous behavior of the perimeter distribution whose main features can be explained by solving for two simplified models of the line network. Finally we formulate a mean field approximation to conduction, above the percolation threshold, using the derived results. Relevance of the results to fracture networks will be discussed.

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