One-component Order Parameter Research Articles

Higher-Tc Superconducting Phase in Sr2RuO4 Induced by In-Plane Uniaxial Pressure

We revealed that the superconducting transition temperature Tc of the multi-component superconductor Sr2RuO4 is enhanced to 3.3 K under in-plane uniaxial pressure that reduces the tetragonal crystal symmetry. This result suggests that new superconducting phases with a one-component order parameter are induced. We have also clarified the inplane pressure direction dependence of the emergence of this higher-Tc superconducting phase: pressure along the [100] direction is more favorable than pressure along the [110] direction. This result is probably closely related to the direct shortening of the in-plane Ru-O bond length along the pressure direction and the approach of the gamma Fermi surface to the van Hove singularity under the pressure parallel to the [100] direction.

Incommensurate Phase Transition in a Thin Film

The solutions of the exact boundary value problems for the order parameter and susceptibility were obtained numerically for the incommensurate thin films with the one-component order parameter. The spatial variations of the order parameter and susceptibility and their temperature dependences were calculated for some particular sets of phenomenological parameters in the Landau expansion. The non-monotone dependences of the order parameter and the temperature of the phase transition into the incommensurate phase on the film thickness were obtained in contrast to approximate solutions. It was shown that the critical behavior of the mean susceptibility becomes more pronounced for thinner films.

Acoustic anisotropy in uniaxial tungsten bronze ferroelectric single crystals studied by Brillouin light scattering

Acoustic properties were investigated for four tungsten bronze (TB) uniaxial ferroelectric crystals, i.e., (Sr0.61Ba0.39)5Nb10O30 (SBN61), Cu-doped (K0.5Na0.5)1.0(Sr0.75Ba0.25)4.5Nb10O30 (KNSBN:Cu), K5.80Li3.82Nb10.12O30, and K4.74Li3.07Nb10.44O30 of which the spontaneous polarization is directed along the polar c axis. Large acoustic anisotropy between the two elastic constants C11 and C33 have been observed from all samples. C33 exhibits a significant softening on approaching the diffuse phase transition temperature from high-temperature side while C11 does not show any substantial change in the same temperature range. This softening is accompanied by substantial growth of hypersonic damping, appearance and growth of central peak (CP), and slowing down of the relevant dynamics of CP represented by the reducing half width. All these results indicate that the lattice motions along the c axis couple strongly to the one-component order parameter of the polar nanoregions (PNRs) or precursor polar clusters which form and grow below a certain temperature in the paraelectric phase. The inverse dielectric constant measured along the c axis of SBN61 and KNSBN:Cu can be described by two linear regions divided by a crossover temperature at which the change in the magnitude of dipole moments and the strength of the dipole couplings are expected due to the formation of PNRs. C33 of SBN61 becomes continuously softened upon cooling even when the temperature crosses the Burns temperature TB at which PNRs begin to appear. It may suggest that additional relaxation process other than that of PNRs may exist at high temperatures above TB and couple to the longitudinal acoustic waves propagating along the polar axis. Recent observation of a single-particle relaxation at high temperatures and its transformation into a collective relaxation of PNRs by dielectric spectroscopy [Belous et al., J. Appl. Phys. 102, 014111 (2007)] might be related to the anomalous acoustic behavior of SBN61 observed above TB.

Intermediate States at Structural Phase Transition: Model with a One-Component Order Parameter Coupled to Strains

We study a Ginzburg–Landau model of structural phase transition in two dimensions, in which a single order parameter is coupled to the tetragonal and dilational strains. Such elastic coupling terms in the free energy much affect the phase transition behavior particularly near the tricriticality. A characteristic feature is appearance of intermediate states, where the ordered and disordered regions coexist on mesoscopic scales in nearly steady states in a temperature window. The window width increases with increasing the strength of the dilational coupling. It arises from freezing of phase ordering in inhomogeneous strains. No impurity mechanism is involved. We present a simple theory of the intermediate states to produce phase diagrams consistent with simulation results.

Theory of improper decay of binary solid solutions

A phenomenological theory of improper decay of solid solutions has been developed for the first time. This theory differs from the previous versions by consideration of three- and four-particle interactions. One of possible two-dimensional cross sections of the complete phase diagram is presented, which includes all possible versions of improper decay at the one-component order parameter, describing the proper ordering that leads to decay. This cross section makes it possible to understand the dissolution of ordered nanoregions in PbMg 1/3Nb2/3O3 with a decrease in temperature.

Phenomenological theory of first-order phase transitions characterized by a one-component order parameter

The Landau theory of phase transitions is generalized with regard to the change Δρinv in the charge distribution probability density that does not break the symmetry of the high-symmetry phase. It is shown that the inclusion of Δρinv makes it possible to describe first-order phase transitions in terms of a fourth-order non-equilibrium potential. The suggested theory leads to the conclusion that the ferroelastic transition in TeO2 is a first-order phase transition.

Solliton-like order parameter distributions in the critical region

Some exact one-component order parameter distributions for the Michelson thermodynamic potential are obtained. The phase transition of second kind in Ginzburg-Landau type model is investigated. The exact partial distribution of the order parameter in the form of Jakobi elliptic function is obtained. The energy of this distribution is lower at some temperature interval than for the best known models.

CePt3Si:An unconventional superconductor without inversion center

Most superconducting materials have an inversion center in their crystal lattices. One of few exceptions is the recently discovered heavy-fermion superconductor CePt3Si [E. Bauer et al, Phys. Rev. Lett. 92, 027003 (2004)]. In this paper, we analyze the implications of the lack of inversion symmetry for the superconducting pairing. We show that the order parameter is an odd function of momentum, and that there always are lines of zeros in the excitation energy gap for one-component order parameters, which seems to agree with the experimental data. The superconducting phase can be non-uniform, even without external magnetic field, due to the presence of unusual gradient terms in the Ginzburg-Landau free energy. Also, we performed ab initio electronic structure calculations for CePt3Si, which showed that the spin-orbit coupling in this material is strong, and the degeneracy of the bands is lifted everywhere except along some high symmetry lines in the Brillouin zone.

Characteristics of irreducible vectors rotating in superstructures with two one-component order parameters

It is shown that incommensurate long-period structures formed by two synchronously rotating irreducible magnetic vectors are unstable and split into two interacting superstrucures. The behavior of such states away from and close to the phase-transition line is considered.

Exact one-component order parameter distributions for the thermodynamic potential of Landau-Ginzburg type with higher gradient and power nonlinearities

A number of exact one-component order parameter distributions for the Michelson thermodynamic potential is obtained. The exact partial distribution with energy, which is lower at some temperature interval than for the best of the known order parameter models in the form of an elliptic Jacobi sine, is obtained for the thermodynamic potential of the Ginzburg-Landau type. By varying its coefficients, the model $\ensuremath{\varphi}(x)=a\mathrm{sn}(bx,k)\ensuremath{\cdot}\mathrm{dn}(bx,k)$ is obtained which is able to reflect the properties of an incommensurate phase in systems with a one-component order parameter in a better way, particularly in the proper uniaxial ${\mathrm{Sn}}_{2}{\mathrm{P}}_{2}{\mathrm{Se}}_{6}$ ferroelectric.

“Latent” paramagnetism and incommensurate structures in three-sublattice magnets

The formation of superstructures due to the interaction of one-component and two-component order parameters in three-sublattice magnets is considered. It is shown that, in the general case, the propagation vector is located in the XOY plane and the irreducible vectors can rotate either within one plane or in two mutually perpendicular planes. Under the conditions of “latent” paramagnetism, no superstructure is formed.

Equilibrium Nonlinear Distributions of One-Component Order Parameter in Systems with Competitive Interaction

The approach which enables to obtain the solutions of the variational equation as the series [Formula: see text] is considered. It is shown that when N=1 it gives well-known cos x solutions, while for N>1 it leads to essentially nonlinear functions. The φ2φ′2-term in the thermodynamical potential provides the proper nonlinear behavior of the order parameter and the possibility of existence of kink-like distributions, particularly. In special case N=2 there exist the exact partial solutions which, however, do not give the absolute minima to the thermodynamical potential.

Field theory of finite-size effects for systems with a one-component order parameter

The field-theoretic renormalization-group approach is used to describe finite-size effects near the critical point of the ϕ 4 model with a one-component order parameter. Problems of previous perturbation approaches for T < T c are discussed and an improved perturbation theory is employed that is applicable both above and below T c. The susceptibility, order parameter, specific heat, and a cumulant ratio are calculated for fixed d < 4 in one-loop order for the case of a cube with periodic boundary conditions. Finite-size scaling functions are evaluated in three dimensions without using the ε = 4 − d expansion. Quantitative agreement with Monte-Carlo (MC) data of the three-dimensional Ising model is found in most cases. Additional MC data of larger systems would be desirable in order to test the detailed predictions of the finite-size field theory more conclusively in the asymptotic region.

Phase separation in colloidal suspensions induced by a solvent phase transition

A colloidal suspension of macroparticles in a solvent is considered near a solvent first-order phase transition. The solvent phase transition is described by a Ginzburg-Landau model with a one-component order parameter which is coupled to the macroparticles coordinates. Wetting of the macroparticle surface by one of the two coexisting phases can induce phase separation of the colloidal particles. This phase separation is first explained by simple thermodynamic arguments and then confirmed by computer simulation of the Ginzburg-Landau model coupled to the macroparticles. Furthermore a topological diagnosis of the interface between the stable and metastable phase is given near phase separation and possible experimental consequences of the phase separation are discussed.

Field theory of finite-size effects in Ising-like systems

We propose a new perturbation approach to finitesize effects within the ϕ4 field theory for a one-component order parameter with periodic boundary conditions. Our approach is applicable both above and belowT c . Renormalization-group calculations of finite-size scaling functions in three dimensions are compared with new Monte Carlo data for theL×L×L Ising model withL=8, 16, 32. The field-theoretic predictions are in good overall agreement with the Monte Carlo data.

Critical exponents in the single-parameter highest catastrophe theory

Abstract On the basis of elementary catastrophe theory a direct calculation of the critical exponents of systems described by a one-component order parameter is performed. It is shown that in the approximation, which is a version of a self-consistent field method, critical exponents can differ from classical values corresponding to the Landau theory. The scheme proposed allows one to get results satisfying the universal Rushbrooke–Griffiths–Fisher–Widom relations connecting the critical exponents which appear to be dependent on natural parameters of the corresponding Landau potential and allows one to obtain strictly determined discrete values. The Ginzburg–Levanyuk criterion, determining the application range of the small fluctuating approximation, in the approach developed here is discussed. In order to account for critical fluctuations, the scale-invariant effective potential, which is analogous to a catastrophe function, is introduced.

Fluctuation-induced first-order phase transitions near mean-field tricritical points in solids

The authors consider displacive phase transitions with a one-component order parameter near what would be a tricritical point (TCP) in the mean-field approximation. As the system in question is a solid, a continuous phase transition is generally impossible. Along a fairly large portion of the phase transition line, a fluctuation-induced first-order phase transition occurs within the region of applicability of the perturbation theory. Close to the mean-field TCP the first-order phase transition proves to be strong, i.e. the discontinuity of the order parameter is of the same order of magnitude as the order parameter at T=0. Below Tc the temperature dependence of the order parameter is given by an equation containing a non-analytical term related to the critical fluctuations, and the other thermodynamic anomalies are governed by this (renormalized) temperature dependence of the order parameter. The theory shows that first-order transitions which are accompanied by strong precursor anomalies in the symmetrical phase, as is the case for ammonium chloride and quartz, would very likely be second order in the mean-field approximation.

Theory of a superconductor with a tricritical point

The Ginzburg-Landau theory of a superconductor with a tricritical point on its phase transition line in the (T, P) plane is formulated. The tricritical point divides the transition line into two parts. One part is the line of first-order transitions, the other that of second-order transitions. The magnetic properties of such a superconductor are different from both type I and type II superconductors. In a part of the phase diagram, the transition into the mixed state is a first-order transition. The Meissner phase can always coexist with the normal phase. The assumption of the existence of a tricritical point makes it possible to interpret the superconducting phase diagram of UPt3 in terms of a one-component order parameter.

Nonmonotonic behavior of a contact angle on approaching critical end points.

We study the behavior of the contact angle of a middle-phase pendant drop at an oil-water interface in an amphiphilic system. A Landau theory with a one-component order parameter is employed. We find that as a weak amphiphilic system departs from its balanced state, the contact angle decreases monotonically to zero at a wetting transition as a critical end point is approached. In a stronger system, the angle initially increases with this departure before ultimately falling to zero. For a very strong system, the angle can increase to 180\ifmmode^\circ\else\textdegree\fi{} before falling to zero. In such a case, three wetting transitions would be encountered.

Paraconductivity in an unconventional layered superconductor

We study the paraconductivity in an impure, unconventional layered superconductor with an underlying square lattice. The overall magnitude of the paraconductivity is found to be reduced by ordinary impurity scattering. For the case of a one-component order parameter, the paraconductivity has the same form as that of ans-wave, apart from the above mentioned reduction factor, and is independent of the coherence length in the plane. For the case of a two component order parameter, the result consists of two parts, the first being simply twice the one component result, the second nonnegative one arising only when the eigenvalues or the eigenvectors of the gradient term in the free energy cannot be chosen to be independent of the angle in the plane.