Showing papers on "Landau theory published in 1971"

Charge conservation and chemical potentials in time-dependent Ginzburg--Landau theory

Abstract: A phenomenological, charge-conserving, time-dependent Ginzburg-Landau theory is found to lead to a charge density and a difference between the quasiparticle electrochemical potential and the pair electrochemical potential when there is a divergence of the supercurrent. Experimental consequences are discussed.

Soft-modes in displacive transitions

Abstract: Displacive phase transitions which do not change the size of the unit cell may be classified under two types depending on their order parameter : in the elastic transitions, this is the amplitude of an elastic strain ; it is the amplitude of a relative displacement of the atoms in an optic transition In this last case, we prove that there always exists at least a whole plane of optical phonons, which are Raman inactive in the high temperature phase, and the frequency of which goes to zero at the transition temperature These soft modes become Raman active in the low temperature phase Should these phonons be Raman active in the high temperature phase, they would induce an elastic transition as shown by Miller and Axe : an elastic constant will pass through zero for a still finite frequency of the optical phonon In the case of an elastic transition, if the Landau theory allows it to be second order, we show by group theory that there always exists at least one sound velocity which passes through zero at the critical temperature We also prove that the optical or elastic soft mode never carries an electric field with it Nevertheless the dielectric constant becomes infinite at the transition temperature, and at low temperature is polar, either if the optical soft mode is infrared active or if the elastic mode induces a piezoelectric strain The damping effect associated with such transitions is not taken into account in this paper

Laser threshold and nonlinear Landau fluctuation theory of phase transitions

Abstract: The threshold region for a single mode laser is treated by Landau's theory of order parameter fluctuations. Taking into account a kinetic energy in the Landau functional, approximate equations of motion for the laser field are derived which include coupling of phase and amplitude. From these it is easy to get the correlation functions and linewidths for intensity and field as well as the transient behaviour of intensity throughout the nonlinear transition region. In addition, a dynamical decoupling scheme of noise theory is applied to calculate the same quantities. Although these methods are very simple the results agree fairly well with those of the much more involved Fokker-Planck theory.

Symmetry aspect of the phase transitions in boracites

Abstract: The phase transitions in boracites are analysed by using the group-theoretical formulation of the Landau theory of phase transitions. It is shown that the orthorhombic, monoclinic and trigonal phase transitions could be induced by the same irreducible representation of the space groupTd5 with the star determined by the wave vectork=1/2(b1+b2). The corresponding free energy function is constructed and the symmetry of normal modes is discussed.

The application of Landau's theory of continuous phase transitions to magnetic phase transitions

Abstract: The conventional group theoretical results of Landau's theory of continuous (i.e. second order) phase transitions are extended to cover the case of a transition between two magnetically ordered phases; this involves the use of the irreducible corepresentations of the magnetic space groups of the two phases between which the transition occurs. An example is given to illustrate the application of this theory. It is shown that while, for some systems, the correct conclusions can be obtained by neglecting the anti-unitary symmetry operations that contain theta , the operation of time reversal, this is not always the case.

Many-body enhancement of the spin susceptibility of metals

Abstract: I t is well known (i) that the electron-electron interactions lead to an enhancement of the paramagnetie susceptibility of a metal over the Pauli free-electron susceptibility. This metallic property is not affected by the electron-phonon interaction and, therefore, offers a better comparison between theory and experiment than the heat capacity. Very recently D v P R ~ and GELDART (2) have reviewed the evidence. A discrepancy seems to exist between the theoretical values determined (3.5) within the framework of the Landau theory of Fermi liquids and the experimental evidence. In this letter we report values of the enhancement factor determined in the framework of the Landau theory on the basis of the t reatment of electron correlations developed by SisGwi, SJbLX~DER, TOSI and LAND (6). This differs from the previous approaches in that short-range Coulomb correlations are taken approximately into account. The results are compatible with the experimental evidence and in good agreement with the alternative theoretical approaches of HAMANN and OVEI~gA~SER (7) and DUPREE and GELDART (2), which also include approximately the Coulomb correlations. The Landau interaction between two quasi-particles in a homogeneous and isotropic electron liquid, defined as the second functional derivative of the ground-state energy with respect to the quasi-particle occupation numbers, depends on the spins of the

Thermomagnetic Effect above the Superconducting Transition

Abstract: The thermomagnetic effect due to the :fluctuation of the superconducting order parameter in magnetic fields slightly above the upper critical :field Hc 2(T) is calculated for a dirty superconductor. Use is made of the time dependent Ginzburg Landau equations. It is found that the Ettingshausen coefficient (or the Nernst coefficient) diverges like (H-Hc 2)-112 and (H-Hc2)-1 for the 3-dimensional and the 2-dimensional sample respectively as the magnetic field approaches Hc2·

Kohn-Sham Self-Consistent Scheme Applied to the Calculation of Atomic Systems and Metallic Sodium

Abstract: Several years ago Kohn and his collaborators1–5 proposed a new approach to study the properties of an interacting inhomogeneous electron system. It is similar in spirit to the Landau theory of Fermi liquids. Both theories try to study certain physical phenomena of an interacting Fermi system in terms of a few parameters. In a way, the two theories supplement each other. The Landau theory is after the single particle excitations, whereas the Kohn theory emphasizes the calculation of static properties and the total energy of the whole system. The basic quantity involved in the latter theory is the total electron density n(r). A review of this theory is given by Sham6 at this conference, and we shall refer the readers to his report, and to the references1–5 for the detailed description of this new approach.