Mean Field Approximation

Abstract Time-resolved Kerr rotation experiments show that two kinds of spin modes exist in diluted magnetic semiconductor quantum wells:(i) strongly coupled electron-magnetic ion spin excitations and (ii) excitations of magnetic ion spin subsystem decoupled from ...

The phase diagram of the t-t'-J model is investigated with use of a slave-fermion mean-field approach. States with spiral antiferromagnetic order are found to be stable for small t/J and hole concentration δ. For larger t/J and δ, corresponding more closely to the physical parameter values of the oxide superconductor, a flux phase exhibiting a uniform chiral order is found to be more stable than the spiral states. This state breaks parity and time-reversal symmetry.

A Hartree-Bose mean-field approximation for the IBM-3 is presented. A Hartree- Bose transformation from spherical to deformed bosons with charge-dependent parameters is proposed which allows bosonic pair correlations and includes higher angular momentum bosons. The formalism contains previously proposed IBM-2 and IBM-3 intrinsic states as particular limits.

We present calculations of magnetic exchange interactions and critical temperature T_c in Mn:GaAs, Cr:GaAs and Cr:GaN. The local spin density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg hamiltonion, but no significant further approximations are made. Special quasi-random structures in large unit cells are used to accurately model the disorder. T_c is computed

Depletion interactions in correlated fluids are investigated both theoretically and experimentally. A formally exact derivation of a general expression for depletion interactions is presented and then specialized to the case of critical correlations in the depletant by employing a long wavelength approximate analysis. A scaling expression is obtained in the critical region, suggesting a close connection to the critical Casimir effect. As a result we are able to compute the full scaling function of the critical Casimir effect in terms of the known scaling form of the depletant equation of state. These predictions are experimentally tested in a colloidal suspension with a micellar solution as depletion agent. Colloids are seen to aggregate reversibly when the micellar concentration exceeds a temperature dependent value which becomes remarkably small as the temperature approaches the lower consolution point of the micellar suspension. Continuity between the standard depletion picture at low temperature and the Casimir effect in the critical region is demonstrated by identifying several approximate scaling laws which compare favorably with the theoretical analysis. The transition line is seen to lie close to the curve of maximum susceptibility of the depletant. A model, analyzed within mean field approximation, is shown to reproduce the main qualitative features of the phenomenon.

... Phys. 121 4246. [55] Ramos JAP, Granato E, Achim CV, Ying SC, Elder KR and Ala-Nissila T 2008 Phys. Rev. E 78 031109. ... Rev. E 76 056706. [79] Jaatinen A, Achim CV, Elder KR and Ala-Nissila T 2009 Phys. Rev. E 80 031602. [80] Chu MW et al 2004 Nat. Mater. 3 87. ...

In this work we study the collective behaviors arising in a model of a simplified homogeneous society. Each agent is modeled as a binary ``perceptron'', receiving neighbors' opinions as inputs and outputting one of two possible opinions, according to the conformist attitude and to the external pressure of mass media. For a neighborhood size greater than three, the system shows a very complex phase diagram, including a disordered phase and chaotic behavior. We present analytic calculations, mean fields approximation and numeric simulations for different values of the parameters.