Showing papers on "Landau theory published in 1978"
TL;DR: Very general symmetry considerations uniquely favor a bcc crystal structure near the melting line as discussed by the authors, which agrees with observations that almost all metals on the left-hand side of the periodic table are bcc at high temperature, and that even where other structures are more stable, the first phase nucleated on rapid cooling can be bcc.
Abstract: Very general symmetry considerations uniquely favor a bcc crystal structure near the melting line. This agrees with observations that almost all metals on the left-hand side of the periodic table are bcc at high temperature, and that, even where other structures are more stable, the first phase nucleated on rapid cooling can be bcc. Furthermore, icosahedral local symmetries are favored in amorphous solids. Fluctuation effects cause the transition from isotropic liquid to crystal to be first order in any dimension.
440 citations
75 citations
TL;DR: In this paper, the minima of the Ginzburg-landau functional for dense neutron matter was shown to be in the same region of parameter space as the BCS functional, where the free energy is minimized by any unitary order parameter.
Abstract: We first find the minima of the general Ginzburg-Landau functional for $^{3}P_{2}$ pairing. The BCS functional lies in a region of the Ginzburg-Landau parameter space where the free energy is minimized by any unitary order parameter. We next estimate the strong-coupling corrections to the BCS functional for dense neutron matter. Using current theoretical values for the transition temperature and Landau parameters, we find that the strong-coupling functional remains in the same region of parameter space as the BCS functional.
51 citations
TL;DR: In this article, an attempt was made to construct a Ginsburg-Landau theory of socalled magnetic superconductors, where two order parameters, the magnetization field and the gap function, are introduced in such a way as to inhibit each others growth.
Abstract: An attempt is made to construct a Ginsburg-Landau theory of socalled magnetic superconductors. Two order parameters, the magnetization field and the gap function, are introduced in such a way as to inhibit each others growth. It is found that the non-local character of the superconducting order parameter must be taken into account in any evaluation of effects of the critical magnetic fluctuations. Some predictions are made within the limits of Ornstein-Zoernicke-like fluctuation theory and some comparison is made with available data.
32 citations
TL;DR: In this paper, a phase diagram with three phases: normal, incommensurate, and commensurate meeting at a triple point was constructed, and the first-order phase transition line ends abruptly at the triple point.
Abstract: We have studied a Landau theory for charge-density waves keeping terms up to sixth order in the free-energy expansion. Even with amplitude modulation, the lock-in transition remains continuous. We construct a phase diagram with three phases: normal, incommensurate, and commensurate meeting at a triple point. With no sixth-order term in the free energy there is a degeneracy of a tricritical point with the triple point, and the first-order phase transition line ends abruptly at the triple point. This is explained in terms of the singular behavior of the derivatives of the entropy in the incommensurate phase near the triple point. With the inclusion of a sixth-order term, the degeneracy of the tricritical point with the triple point is removed and the abrupt ending of the first-order line no longer occurs.
25 citations
TL;DR: In this article, the logarithmic correction factors to classical Landau theory due to order parameter fluctuations close to the Curie point are used to calculate various static and dynamic properties for small t ≡ |T − Tc|/Tc.
Abstract: The logarithmic correction factors to classical Landau theory due to order parameter fluctuations close to the Curie point are used to calculate various static and dynamic properties for small t ≡ |T — Tc|/Tc. The specific heat at constant polarization has below Tc a cusp ∞ |log t|-2/3. The leading term in the spin-lattice relaxation rate 1/Tt, varies as log t just as in classical theory and in agreement with experiment in colemanite. The damping of low-frequency longitudinal sound waves propagating perpendicular to the easy axis varies as t-1|logt|2/3 below Tc (relaxation damping) and as t-1|log t|-1/3 above Tc (fluctuation damping); the ratio of these two terms is material independent, analogously to recent experiments at the superfluid phase transition in helium II. Propagation of sound not perpendicular to the polar axis suppresses the damping due to the relaxation of the long-range order parameter and leaves only the fluctuation damping. The energy of 180° domain walls vanishes as t3/2|log t|1/2 wher...
18 citations
TL;DR: In this article, the ERS spectra of triplet excitons in the charge transfer crystal anthracene-d10−s-tetracyanobenzene (TCNB) were used to investigate changes in the molecular orientation.
15 citations
01 Jan 1978
TL;DR: A review of group-theory related aspects of the Landau-Thermodynamic Theory of continuous phase transitions in crystals is given in this article, which discusses developments of the theory during several time intervals.
Abstract: A review is given of group-theory related aspects of the Landau-Thermodynamic Theory of continuous phase transitions in crystals. The review discusses developments of the theory during several time intervals. First interval: the 25 years (1937–62) after the theory was presented; Second interval (1962-77) covers the recent 15 years; Third interval (1977- ) presents some possible directions for work in the future. Thus the first 40 years of this theory were characterized inter alia by elaborations and applications of the group theory aspects of the problem which Landau examined: Given a system with group G; What groups G → G′,... may be achieved by continuous transitions i.e. G → G′,...? What order parameters are consistent with transitions G → G′, or G → G′, ...?
13 citations
12 citations
TL;DR: In this article, the authors present measurements of the temperature variation of the electro-optic effect in the low temperature phase of NH4Cl and show how they are related to the order parameter of the order-disorder transition.
Abstract: We present here measurements of the temperature variation of the electro-optic effect in the low temperature phase of NH4Cl and we show how they are related to the order parameter of the order-disorder transition. These measurements are made difficult by the presence of parasitic strain birefringence, and by the existence of domains which produce a change of sign of the electro-optic effect. The results are compared with previous measurements of the order parameter by the piezoelectric effect and second harmonic generation, and their interpretation, following the Landau theory or power law with critical exponents, is discussed in relation to the existence of a multicritical point under high pressure. An exponential tail of the order parameter, extending over several degrees in the high temperature phase is systematically found in all these experiments. The origin of this residual order is probably related to the stresses produces by crystalline defects.
11 citations
TL;DR: In this paper, it was shown that the threshold behavior of an organic dye laser may be interpreted as an example of a second or first order phase transition, which depends on the wavelength dependent internal reabsorption of the laser light.
TL;DR: In this article, a spin-independent coupling is proposed to explain the spin-flip transition in chromium in terms of coupling to the spontaneous strain, which is much larger than the magnetostriction.
Abstract: A theory is presented which explains the spin-flip transition in chromium in terms of coupling to the spontaneous strain. The spin density wave is treated in a Landau formulation and both spin-independent electron-strain coupling and magnetostriction couple the strain to the spin density wave. This coupling explains the observed spontaneous strain and the spin-flip transition. The spin-independent coupling is much larger than the magnetostriction.
TL;DR: In this article, the analysis of the electroclinic effect in a chiral liquid crystal made by Garoff and Meyer was shown to be incorrect in the very vicinity of the T = c.
Abstract: We show that the analysis of the electroclinic effect in a chiral smectic liquid crystal made by Garoff and Meyer is not correct in the very vicinity of ${T}_{c}$. We propose an analysis based on the Landau theory and conclude that the electroclinic coefficient and electric susceptibility do not diverge, but can take large values.
TL;DR: A generalised Landau theory for non-equilibrium phase transitions in chemical reactions is developed in this article, which is based on the construction of a generalised free energy which describes just the fluctuations in excess of equilibrium.
Abstract: A generalised Landau theory for non-equilibrium phase transitions in chemical reactions is developed. This theory is based on the construction of a generalised free energy which describes just the fluctuations in excess of equilibrium. This is accomplished most readily via the Poisson representation. This theory yields results for the spatial correlation function in agreement with those of the stochastic master equation. The range of validity of the Landau theory and the Gaussian approximation to the master equation are found to be equivalent and both are shown to yield satisfactory results for nonequilibrium chemical phase transitions.
TL;DR: In this paper, the velocity of transverse zero sound is adjusted to render mutually consistent a set of equations relating the velocities of longitudinal and zero sound to each other and to the Landau parameters, Fln at 0-33 bar.
Abstract: The velocity of transverse zero sound is adjusted to render mutually consistent a set of equations relating the velocities of longitudinal and transverse zero sound to each other and to the Landau parameters, Fln, at 0-33 bar. The result is incompatible with F2s<0 unless F3s approximately=0, but this conclusion is extremely sensitive to possible small inexactitudes in the Landau theory equations.
TL;DR: In this paper, the second-order phase transition temperatures and tricritical points are determined for several spins in a meanfield Hamiltonian with bilinear and biquadratic pair interactions.
Abstract: Landau’s theory of second-order phase transitions for systems with two order parameters is discussed and applied to a meanfield Hamiltonian for an Ising model with bilinear and biquadratic pair interactions Second-order phase transition temperatures and tricritical points are determined for several spins
TL;DR: In this article, the elastic instability of the cubic-to-tetragonal phase transition in Nb 3 Sn was investigated by means of a simple thermodynamic model with deformation potential coupling.
Abstract: We study the elastic instability of the cubic-to-tetragonal phase transition in Nb 3 Sn. On the basis of Landau theory of phase transitions the softening of the elastic moduli is investigated by means of a simple thermodynamic model with deformation potential coupling. As a generalization of the Labbe-Friedel model our model takes interband transitions into account. The driving mechanism for the phase transition is a band Jahn-Teller effect. Elimination of the electronic coordinates leads to temperature-dependent renormalized elastic moduli. For the low-temperature tetragonal phase we find a splitting of the shear moduli c 11 − c 12 and c 33 − c 13 . In addition, a pronounced difference between the adiabatic and isothermal shear modulus c 33 − c 13 is predicted.
TL;DR: In this paper, the spontaneous polarization of a KFCT single crystal was measured during different temperature cycles in the range from -65°C to 0°C, and the anomalous ferroelectric heat capacity can be explained by the Landau theory with the nonlinearity coefficients ζ = (5.2 ± 0.8).
Abstract: The heat capacity and the spontaneous polarization of a KFCT single crystal were measured during different temperature cycles in the range from -65°C to 0°C. It was found that the low-temperature phase transformation from the tetragonal to the twinned monoclinic modification is not irreversible because at room temperature parts of the twinned crystal successively transform back into the more stable tetragonal phase. The anomalous ferroelectric heat capacity can be explained by the Landau theory with the non-linearity coefficients ζ = (5.2 ± 0.8).1012 J m5 C-4 and ζ = (1.04 + 0.10). 1016 J m9 C-6 within a temperature range of only 12 K. An extended molecular field model based on these coefficients well describes the critical ferroelectric heat capacity in the whole investigated temperature range. Furthermore the spontaneous polarization of an untwinned monoclinic KFCT single crystal was measured, and it verified the molecular field model based on heat capacity data. The saturation polarization of intrinsic...
TL;DR: In this article, the temperature dependence of the tensor component d14 of the nonlinear quadratic susceptibility, measured at atmospheric pressure across the order-disorder phase transition in NH4Cl, was fitted to different expressions derived from the classical Landau theory for discontinuous phase transitions.
TL;DR: In this article, it is shown that the peculiarities of elastic properties in the nonpolar phase of the Rb(H,D)3(SeO3)2 are not described by Landau theory and the elastic anomalies are caused by the presence of the incommensurate structure predicted in these crystals theoretically.
Abstract: Investigations of improper ferroelectric phase transition in the crystals Rb(H,D)3(SeO3)2 are reviewed. It is shown that the peculiarities of elastic properties in the nonpolar phase of the crystals are not described by Landau theory. The elastic anomalies are proposed to be caused by the presence of the incommensurate structure predicted in these crystals theoretically.
TL;DR: In this article, the effect of spin polarization on the propagation of zero sound in dense neutron matter was investigated within the framework of the Landau theory of normal Fermi liquids, and a calculation performed using the quasi-particle interaction derived from the Reid soft core nucleon-nucleon potential was performed.
TL;DR: In this paper, the influence of defects on second-order structural phase transition properties of ferroelectrics is described within a static Landau Theory, and it is shown that point defects may give a significant contribution to the specific heat and cause the temperature dependence of the anomalies, which is close to that measured in the experiments.
Abstract: The influence of defects on some anomalies in the characteristics of crystals with second-order structural phase transitions, including proper ferroelectrics, is described within a static Landau Theory. It is shown that in some cases point defects may give a significant contribution to the specific heat and cause the temperature dependence of the anomalies, which is close to that measured in the experiments.