Showing papers on "Landau theory published in 2016"
TL;DR: In this article, the electronic nematic phase in electron-and hole-doped BaFe 2 As 2 and FeSe was analyzed using a three-point-bending method in a capacitance dilatometer.
122 citations
TL;DR: In this paper, the paramagnetoelectric (PME) coefficient in high-resistive ceramics made of multiferroic Pb(Fe 1/2Nb1/2)O3 (PFN) and PFN-based solid solution 0.91PFN-0.09PbTiO3 was analyzed.
Abstract: We have observed the magnetoelectric (ME) response at room temperature and above in high-resistive ceramics made of multiferroic Pb(Fe1/2Nb1/2)O3 (PFN) and PFN-based solid solution 0.91PFN-0.09PbTiO3 (PFN-PT). The value of the paramagnetoelectric (PME) coefficient shows a pronounced maximum near the ferroelectric-to-paraelectric phase transition temperature, T
C, and then decreases sharply to zero for T > T
C. The maximal PME coefficient in PFN is about 4 × 10−18 s/A. The theoretical description of the PME effect, within the framework of a Landau theory of phase transitions allowing for realistic temperature dependences of spontaneous polarization, dielectric and magnetic susceptibilities, qualitatively reproduces well the temperature dependence of the PME coefficient. In particular, the Landau theory predicts the significant increase of the PME effect at low temperatures and near the temperature of the paraelectric-to-ferroelectric phase transition, since the PME coefficient is equal to the product of the spontaneous polarization, dielectric permittivity, square of magnetic susceptibility, and the coefficient quantifying the strength of the biquadratic ME coupling. An atomistic technique is further developed and used to further demonstrate that the PME effect can also be sensitive to the frequency of the applied magnetic field (when this frequency is of the order of GHz).
53 citations
TL;DR: The main finding is that the double gyroid phase can exist as a thermodynamically stable phase.
Abstract: We study the phase diagram of a fluid with spherically symmetric competing pair interactions that consist of a short-ranged attraction and a longer-ranged repulsion in addition to a hard core. To this end we perform free minimizations of three-dimensional triple periodic structures within the framework of classical density functional theory. We compare our results to those from Landau theory. Our main finding is that the double gyroid phase can exist as a thermodynamically stable phase.
39 citations
TL;DR: In this article, the authors introduce a dynamical field theory to describe the glassy behavior in supercooled liquids, which predicts that the transition is avoided and transformed into a crossover, as observed in experiments and simulations.
Abstract: I introduce a dynamical field theory to describe the glassy behavior in supercooled liquids. The mean-field approximation of the theory predicts a dynamical arrest transition, as in the ideal mode-coupling theory and mean-field discontinuous spin-glass models. Instead, beyond the mean-field approximation, the theory predicts that the transition is avoided and transformed into a crossover, as observed in experiments and simulations. To go beyond mean-field, a standard perturbative loop expansion is performed at first. Approaching the ideal critical point this expansion is divergent at all orders and I show that the leading divergent term at any given order is the same as a dynamical stochastic equation, called stochastic-beta relaxation (SBR) in Europhys. Lett. 106, 56003 (2014). At variance with the original theory, SBR can be studied beyond mean-field directly, without the need to resort to a perturbative expansion. Thus it provides a qualitative and quantitative description of the dynamical crossover. For consistency reasons, it is important to establish the connection between the dynamical field theory and SBR beyond perturbation theory. This can be done with the help of a stronger result: the dynamical field theory is exactly equivalent to a theory with quenched disorder. Qualitatively, the nonperturbative mechanism leading to the crossover is therefore the same as the mechanism of SBR. Quantitatively, SBR is equivalent to making the mean-field approximation once the quenched disorder has been generated.
30 citations
TL;DR: In this paper, the authors proposed a phenomenological interpretation on the isothermal phase transition of a relaxor ferroelectric and a ferroelectroelectric, which is induced by increasing the waiting time at certain temperatures.
30 citations
TL;DR: In this article, the authors apply variational principles from statistical physics and the Landau theory of phase transitions to multicomponent alloys using the multiple-scattering theory of Korringa-Kohn-Rostoker (KKR) and the coherent potential approximation (CPA).
Abstract: We apply variational principles from statistical physics and the Landau theory of phase transitions to multicomponent alloys using the multiple-scattering theory of Korringa-Kohn-Rostoker (KKR) and the coherent potential approximation (CPA). This theory is a multicomponent generalization of the S(2) theory of binary alloys developed by G. M. Stocks, J. B. Staunton, D. D. Johnson and others. It is highly relevant to the chemical phase stability of high-entropy alloys as it predicts the kind and size of finite-temperature chemical fluctuations. In doing so it includes effects of rearranging charge and other electronics due to changing site occupancies. When chemical fluctuations grow without bound an absolute instability occurs and a second-order order-disorder phase transition may be inferred. The S(2) theory is predicated on the fluctuation-dissipation theorem; thus we derive the linear response of the CPA medium to perturbations in site-dependent chemical potentials in great detail. The theory lends itself to a natural interpretation in terms of competing effects: entropy driving disorder and favorable pair interactions driving atomic ordering. Moreover, to further clarify interpretation we present results for representative ternary alloys CuAgAu, NiPdPt, RhPdAg, and CoNiCu within a frozen charge (or band-only) approximation. These results include the so-called Onsager mean field correction thatmore » extends the temperature range for which the theory is valid.« less
25 citations
TL;DR: In this article, the traditional derivation of the elastic anomalies associated with ferroelectric phase transitions in the framework of the Landau theory is combined with the piezoelectric constitutive relations instead of being explicitly carried out with a definite expression of the FE part of the free energy.
Abstract: The traditional derivation of the elastic anomalies associated with ferroelectric (FE) phase transitions in the framework of the Landau theory is combined with the piezoelectric constitutive relations instead of being explicitly carried out with a definite expression of the FE part of the free energy. In this manner it is shown that the softening within the FE phase is of electrostrictive and hence piezoelectric origin. Such a piezoelectric softening may be canceled by the better known piezoelectric stiffening, when the piezoelectric charges formed during the vibration are accompanied by the depolarization field, as for example in Brillouin scattering experiments. As experimental validation, we present new measurements on Zr-rich PZT, where the FE phase transforms into antiferroelectric on cooling or doping with La, and a comparison of existing measurements made on FE PZT with low frequency and Brillouin scattering experiments.
19 citations
TL;DR: In this article, the magnetocaloric properties of polycrystalline (La 0.7Y0.3)0.7Sr 0.3MnO3 compound have been studied.
14 citations
TL;DR: The set of elastic constants of the charge density wave (CDW) rare earth tritelluride TbTe3 has been measured at 15MHz in the temperature range 300-360 K as discussed by the authors.
13 citations
TL;DR: Detailed measurements of the angular anisotropy of the nonlinear magnetization are presented that reveal a cos^{4}θ Ising anisotropic both at and above the ordering transition, implying a strongly Ising character of the itinerant hidden order parameter.
Abstract: The heavy fermion compound ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ continues to attract great interest due to the unidentified hidden order it develops below 17.5 K. The unique Ising character of the spin fluctuations and low-temperature quasiparticles is well established. We present detailed measurements of the angular anisotropy of the nonlinear magnetization that reveal a ${\mathrm{cos}}^{4}\ensuremath{\theta}$ Ising anisotropy both at and above the ordering transition. With Landau theory, we show this implies a strongly Ising character of the itinerant hidden order parameter.
12 citations
TL;DR: In this paper, it was shown that in the so-called quenched approximation the fermionic spectrum can be computed exactly to all orders in the coupling constant, and extensive detailed analysis that the nonperturbative low-energy dynamics are indeed that of a non-Fermi liquid.
Abstract: Landau theory of Fermi liquids is the foundational concept behind our understanding of conventional metals. Its basic postulate is that the low-energy physics can be described by weakly interacting quasiparticles --- a dressed but effectively free version of electrons formally represented by poles of the single-particle Green's function. However, this concept notoriously fails for strongly interacting systems such as high-${T}_{c}$ cuprate superconductors, and our understanding of properties of these ``non-Fermi'' quantum liquids is far from complete. One well-known candidate theory for non-Fermi liquids is Hertz-Millis theory: a finite-density fermion system coupled to a massless critical boson. In (2+1) dimensions, this theory is strongly interacting at low energies, but as of yet there is no controlled insight in this regime; conventional perturbation theory fails. In this article, the authors show that in the so-called quenched approximation the fermionic spectrum can be computed exactly to all orders in the coupling constant. They show extensive detailed analysis that, in this quenched limit, the nonperturbative low-energy dynamics are indeed that of a non-Fermi liquid. Instead of coherent quasiparticles one finds the emergence of critical fermionic excitations described by branch cuts in the single particle Green's function.
TL;DR: The "bunch" of dimers in CuTi2S4 is a new type of self-organization of atoms in frustrated spinel-like structures and it is shown that Ti-atoms also form other types of metal nanoclusters: trimers and tetrahedra.
Abstract: The theory of structural phase transition in CuTi2S4 is proposed. The symmetry of order parameters, thermodynamics and the mechanism of the atomic structure formation of the rhombohedral Cu–Ti–thiospinel have been studied. The critical order parameter inducing the phase transition has been found. Within the Landau theory of phase transitions, it is shown that the phase state may change from the high-symmetry cubic disordered Fdm phase to the low-symmetry ordered rhombohedral Rm phase as a result of phase transition of the first order close to the second order. It is shown that the rhombohedral structure of CuTi2S4 is formed as a result of the displacements of all types of atoms and the ordering of Cu-atoms (1 : 1 order type in tetrahedral spinel sites), Ti-atoms (1 : 1 : 6 order type in octahedral spinel sites), and S-atoms (1 : 1 : 3 : 3 order type). The Cu- and Ti-atoms form metal nanoclusters which are named a “bunch” of dimers. The “bunch” of dimers in CuTi2S4 is a new type of self-organization of atoms in frustrated spinel-like structures. It is shown that Ti-atoms also form other types of metal nanoclusters: trimers and tetrahedra.
TL;DR: In this article, the authors demonstrate electrical switching among three stable out-of-plane polarizations in bismuth ferrite thin films grown on gadolinium scandate substrates by the use of an asymmetric external electric field at the step edge of a bottom electrode.
Abstract: By realizing a nonvolatile third intermediate ferroelectric state through anisotropic misfit strain, we demonstrate electrical switching among three stable out-of-plane polarizations in bismuth ferrite thin films grown on ${(110)}_{\mathrm{pc}}$-oriented gadolinium scandate substrates (where pc stands for pseudocubic) by the use of an asymmetric external electric field at the step edge of a bottom electrode. We employ phenomenological Landau theory, in conjunction with electrical poling experiments using piezoresponse force microscopy, to understand the role of anisotropic misfit strain and an in-plane electric field in stabilization of multiple ferroelectric states and their competition. Our finding provides a useful insight into multistep ferroelectric switching in rhombohedral ferroelectrics.
TL;DR: The rotational instability of the electric polarization during phase transformations between ferroelectric phases is of great practical interest, since it may be accompanied by extremely large values of the piezoelectric coefficient, and a divergence of the coupled shear compliance contributes to such enhancements.
Abstract: The rotational instability of the electric polarization $\mathbf{P}$ during phase transformations between ferroelectric phases is of great practical interest, since it may be accompanied by extremely large values of the piezoelectric coefficient, and a divergence of the coupled shear compliance contributes to such enhancements. In the literature, this has been explicitly calculated in the framework of the Landau theory and discussed with specific numerical simulations involving tetragonal, orthorhombic, and rhombohedral ferroelectric phases. When monoclinic phases are involved, such an approach is practically impossible, and an approximated treatment has been proposed, based on the observation that in those cases there are shear strains almost linearly coupled to the transverse component of $\mathbf{P}$, implying a divergence of the Curie-Weiss type in the associated compliances. Here the argument is extended to the general case of transitions whose major effect is a rotation of the polarization, and the limits of its validity are discussed. As experimental verification, the elastic response of ${\mathrm{BaTiO}}_{3}$ is measured and analyzed, together with those of other ferroelectric perovskites available in the literature, such as KNN.
TL;DR: In this article, the authors investigated the validity and usefulness of the theoretical modeling in La 0.6 Pr 0.1 Ba 0.3 MnO 3 perovskite based on the Landau mean-field analysis of the magnetic-entropy change (−Δ S M ).
TL;DR: A generalized Landau-Brazovskii theory for the solidification of chiral molecules on a spherical surface is presented and the predictions of the theory are compared with recent numerical simulations.
Abstract: We present a generalized Landau-Brazovskii theory for the solidification of chiral molecules on a spherical surface. With increasing sphere radius one encounters first intervals where robust achiral density modulations appear with icosahedral symmetry via first-order transitions. Next, one en- counters intervals where fragile but stable icosahedral structures still can be constructed but only by superposition of multiple irreducible representations. Chiral icoshedral structures appear via continuous or very weakly first-order transitions. Outside these parameter intervals, icosahedral symmetry is broken along a three-fold axis or a five-fold axis. The predictions of the theory are compared with recent numerical simulations.
TL;DR: In this article, the authors used the Landau theory of phase transitions to obtain the global phase diagram concerning the uniaxial nematic, biaxia-nematic, unia-xial smectic-A and biaXia-smectic -A phases.
Abstract: We use the Landau theory of phase transitions to obtain the global phase diagram concerning the uniaxial nematic, biaxial nematic, uniaxial smectic-A and biaxial smectic-A phases. The transition between the biaxial nematic and biaxial smectic is continuous as well as the transition between the nematic phases and the transition between the smectic phases. The transition from uniaxial nematic and uniaxial smectic is continuous with a tricritical point. The tricritical point may be absent and the entire transition becomes continuous. The four phases meet at a tetracritical point.
TL;DR: In this article, a generalized Landau-Brazovskii free energy for the solidification of chiral molecules on a spherical surface in the context of the assembly of viral shells is presented.
Abstract: We present a generalized Landau-Brazovskii free energy for the solidification of chiral molecules on a spherical surface in the context of the assembly of viral shells. We encounter two types of solidification transitions. The first type is a conventional first-order phase transition from a uniform to an icosahedral state, described by a single icosahedral spherical harmonic of even l . The chiral pseudo-scalar term in the free energy does not affect the transition. The second type is anomalous: icosahedral spherical harmonics with odd l are unstable. Stability is recovered when admixture with the neighboring l + 1 icosahedral spherical harmonic is included. This is in apparent conflict with the principle of Landau theory that symmetry-breaking transitions are characterized by a single irreducible representation of the symmetry group of the uniform phase. The chiral term selects one of two isomeric mixed-l icosahedral states. A direct transition is possible only over a limited range of parameters. Outside this range, a non-icosahedral state with the symmetry of an isotropy subgroup of the icosahedral group interposes between the uniform and icosahedral states.This paper is dedicated to the memory of our friends and colleagues William Klug and Vladimir Lorman .
TL;DR: Here it is shown precisely how one can employ graphics-processing units (GPUs) for this type of calculations, and obtain physics answers of interest in a reasonable time-frame - with speedup of over 100× compared to best available CPU implementations of the theory on a 2563 grid.
TL;DR: Two Landau points are found in the phase diagram at which continuous transition from biaxial nematic state to isotropic phase occurs, and these are found to be of the first and second order.
Abstract: The phase behavior of a monodisperse melt of polymer molecules consisting of two rod-like segments joined at an angle α has been inspected within the Landau theory of phase transitions. The interactions between monomer units were assumed to be of the Maier-Saupe form. The Landau–de Gennes expansion of the free energy of the melt has been obtained up to the sixth order in powers of the nematic order parameter, the coefficients of this expansion have been calculated from the microscopic model of polymer molecule. The phase diagram contains the regions of stability of isotropic, prolate uniaxial, oblate uniaxial, and biaxial nematic phases. The isotropic-uniaxial nematic and uniaxial-biaxial nematic transitions are of the first and second order, respectively. We found two Landau points in the phase diagram at which continuous transition from biaxial nematic state to isotropic phase occurs.
TL;DR: In this article, the magnetic and magnetocaloric properties of the La0.65Eu0.05Sr0.3Mn0.95Cr 0.05O3 manganite compound were investigated using the Modified Arrott plot method.
TL;DR: In this article, the authors investigated the dependence of dielectric susceptibility on the frequency, temperature, film thickness, surface effects, and various parameters of a ferroelectric thin film under an AC applied field.
Abstract: In this paper, frequency, temperature, film thickness, surface effects, and various parameters dependence of dielectric susceptibility is investigated theoretically for ferroelectric thin films by the modified Landau theory under an AC applied field. The dielectric susceptibility versus AC applied field shows butterfly-shaped behavior, and depends strongly on the frequency and amplitude of the field and temperature. Our study shows that the existence of the surface transition layer can depress the dielectric susceptibility of a ferroelectric thin film. These results are well consistent with the phenomena reported in experiments.
TL;DR: In this article, a phenomenological theory for metamagnetism observed in heavy Fermion compounds by extension of the Landau theory for phase transitions is developed for the crossover behavior in high magnetic fields, indicated for the extended Landau-type free energy density.
Abstract: A phenomenological theory is developed for the metamagnetism observed in heavy Fermion compounds by extension of the Landau theory for phase transitions. From simple analysis, the crossover behavior in high magnetic fields is indicated for the extended Landau-type free energy density. According to the finite temperature behavior of the extended Landau free energy density, theoretical magnetic phase diagrams that resemble those observed in CeRu\(_{2}\)Si\(_{2}\) and Ce(Ru\(_{0.92}\)Rh\(_{0.08}\))\(_{2}\)Si\(_{2}\) are obtained by using two sets of parameters, respectively.
TL;DR: In this article, the formation of a unique hyper-kagome atomic order in the structure of the ordered spinel phase Na3Ir3O8 has been analyzed in terms of the Landau theory of phase transitions.
Abstract: The formation of a unique hyper-kagome atomic order in the structure of the ordered spinel phase Na3Ir3O8 has been analyzed in terms of the Landau theory of phase transitions. We have identified a critical six-component order parameter generating a phase transformation of the spinel parent phase to ordered P4132 (P4332) spinel-like enantiomorphs and examined the structural mechanism behind the development of hyper-kagome order and the detailed crystal chemistry of Na3Ir3O8.
01 Jan 2016
TL;DR: A phenomenological Landau theory is constructed for the two-dimensional helical Fermi liquid found on the surface of a three-dimensional time-reversal invariant topological insulator with a single projected Landau parameter per angular momentum channel that captures the spin-momentum locking or nontrivial Berry phase of the Fermani surface.
Abstract: We construct a phenomenological Landau theory for the two-dimensional helical Fermi liquid found on the surface of a three-dimensional time-reversal invariant topological insulator. In the presence of rotation symmetry, interactions between quasiparticles are described by ten independent Landau parameters per angular momentum channel, by contrast with the two (symmetric and antisymmetric) Landau parameters for a conventional spin-degenerate Fermi liquid. We project quasiparticle states onto the Fermi surface and obtain an effectively spinless, projected Landau theory with a single projected Landau parameter per angular momentum channel that captures the spin-momentum locking or nontrivial Berry phase of the Fermi surface. As a result of this nontrivial Berry phase, projection to the Fermi surface can increase or lower the angular momentum of the quasiparticle interactions. We derive equilibrium properties, criteria for Fermi surface instabilities, and collective mode dispersions in terms of the projected Landau parameters. We briefly discuss experimental means of measuring projected Landau parameters.
TL;DR: In this paper, the same Landau theory can be associated to supercooled liquid described by Mode-Coupling Theory invoking a physical principle of time-scale invariance.
Abstract: Stochastic-Beta-Relaxation provides a characterisation of the glass crossover in discontinuous Spin-Glasses and supercoooled liquid. Notably it can be derived through a rigorous computation from a dynamical Landau theory. In this paper, I will discuss the precise meaning of this connection in a language that does not require familiarity with statistical field theory. I will discuss finite-size corrections in mean-field Spin-Glass models and loop corrections in finite-dimensional models that are both described by the dynamical Landau theory considered. Then I will argue that the same Landau theory can be associated to supercooled liquid described by Mode-Coupling Theory invoking a physical principle of time-scale invariance.
TL;DR: In this article, a group-theoretical, thermodynamic, and structural study of the formation of P4132(P4332) spinel modification has been performed.
Abstract: A group-theoretical, thermodynamic, and structural study of the formation of P4132(P4332) spinel modification has been performed. In particular, the occurrence of unique hyper-kagome atomic order is analyzed. The critical order parameter inducing a phase transition is established. It is shown that the calculated structure of the low-symmetry P4132(P4332) phase is formed as a result of displacements of atoms of all types and due to the cation and anion ordering. The problem of the occurrence of unique hyper-kagome atomic order in the structures of P4132(P4332) spinel modifications is considered theoretically. It is proven within the Landau theory of phase transitions that the P4132(P4332) phase can be formed from the high-symmetry Fd3m phase with an ideal spinel structure only as a result of first-order phase transition. Therefore, the formation of hyper-kagome sublattice in the P4132(P4332) phase is accompanied by a significant transformation of the spinel structure.
TL;DR: In this paper, a new ground state of the textbook compound strontium titanate was obtained by inducing a specific core-shell structure of the particles using a combination of high energy synchrotron and neutron diffraction, demonstrating a lowering of the ferroelastic ground state towards a new antiferrodistortive phase.
Abstract: A new ground state of textbook compound strontium titanate $(\mathrm{SrTi}{\mathrm{O}}_{3})$ is obtained by inducing a specific core-shell structure of the particles. Using a combination of high energy synchrotron and neutron diffraction, we demonstrate a lowering of the ferroelastic ground state towards a new antiferrodistortive phase, accompanied with strong shifts of the critical temperature. This new phase is discussed within the Landau theory and compared with the situation in thin films and during pressure experiments. The crucial competition between particle shape anisotropy, surface tension, and shear strain is analyzed. Inducing a specific core-shell structure is therefore an easy way to tailor structural properties and to stabilize new phases that cannot exist in bulk material, just like film deposition on a substrate.
TL;DR: The effect of uniaxial stress on dielectric properties of NaNO 2 ferroelectric has been investigated in this article, where the real part of the dielectrics susceptibility was measured at the frequency of 1 kHz without and on applying compressive uniaXial stress along different crystallographic directions using a compress meter, and the observed phenomena were interpreted on the base of the phenomenological Landau theory of phase transitions in Na NO 2 by taking into account the uniaaxial compressive stress effect.