Theory Of Electric Polarization Research Articles

A quantized microwave quadrupole insulator with topologically protected corner states.

The theory of electric polarization in crystals defines the dipole moment of an insulator in terms of a Berry phase (geometric phase) associated with its electronic ground state. This concept not only solves the long-standing puzzle of how to calculate dipole moments in crystals, but also explains topological band structures in insulators and superconductors, including the quantum anomalous Hall insulator and the quantum spin Hall insulator, as well as quantized adiabatic pumping processes. A recent theoretical study has extended the Berry phase framework to also account for higher electric multipole moments, revealing the existence of higher-order topological phases that have not previously been observed. Here we demonstrate experimentally a member of this predicted class of materials-a quantized quadrupole topological insulator-produced using a gigahertz-frequency reconfigurable microwave circuit. We confirm the non-trivial topological phase using spectroscopic measurements and by identifying corner states that result from the bulk topology. In addition, we test the critical prediction that these corner states are protected by the topology of the bulk, and are not due to surface artefacts, by deforming the edges of the crystal lattice from the topological to the trivial regime. Our results provide conclusive evidence of a unique form of robustness against disorder and deformation, which is characteristic of higher-order topological insulators.

Theory of the piezo-spintronic effect

The theoretical prediction that some materials might develop pure spin currents in response to strain is presented. Such piezo-spintronic effect is studied and shown to be allowed by symmetry in several systems. This mechanism opens up a way to obtain and measure pure spin currents. From a close analogy with the theory of electric polarization and the piezoelectric effect, we show that such piezo-spintronics response can be represented in a geometrical form in terms of spin Berry phases. Additionally, we illustrate the ideas by the use of two toy models that displays a non-trivial piezo-spintronic response and discuss on possible experimental realizations on antiferromagnetic insulators.

Effective attractive potential between identical dielectric molecules via Lifshitz theory

A general theory of Van der Waals forces developed by Lifshitz based on quantum electrodynamics theory is applied in the range R 0 (where the Casimir effects may be neglected) to construct Van der Waals attractive potential between identical dielectric molecules in rarefied media in order that the effective attractive potential between the like-molecules (including the repeat units) is offered. A closed form solution for the integral formulation of the attractive potential between like-particles is first obtained based on certain assumptions made in this work. On the basis of the theory of electric polarization, the derived expression in terms of bulk properties is then compared with the well-known London formula, the former differs from the latter only by the factor 4/π or 4/π(e ∞ + 2/3) 2 .The validity of the effective potential can be verified by testing cases composed of several types of dielectric materials. The computed results are presented in this paper, and comparisons with the results computed by London dispersion formula, as well as the recommended values in virtue of the experimental and theoretical techniques, are also presented. The effective potential of polyethylene is also computed to demonstrate the effectiveness of the developed model, and it is found that the computed well depth fall within a reasonable range of accuracy.

Recent developments in the theory of electric polarization in solids

Polarization, the dipole moment per unit volume of a material, is one of the basic quantities in physics which is essential to the theory of dielectrics, effective charges in lattice dynamics, piezoelectricity, ferroelectricity and other phenomena. For extended matter such as crystals, however, a theory of polarization formulated directly in terms of the quantum mechanical wavefunction of the electrons has only recently been derived. Quantum mechanics is the problem: because the electrons do not belong to an individual unit cell of a crystal and their density is continuous, there is no way to uniquely “cut” the density and derive the dipole moment of a cell. The new approach consists in relating the change in polarization to the integrated bulk current flowing through the unit cell and quantum mechanics provides the solution: unique expressions for the integrated current in an insulator can be found from a geometric Berry's phase involving integrals over the phases of the electronic wavefunctions. This method has been used to calculate many properties such as effective charges and ferroelectric moments, and also to discover new phenomena such as divergences of effective charges at magnetic phase transitions.

Polarizability and molecular radius of bromoform and hydrocarbon liquids

Abstract Aminabhavi, T.M., Aralaguppi, M.I., Harogoppad, S.B. and Balundgi, R.H., 1992. Polarizability and molecular radius of bromoform and hydrocarbon liquids. Fluid Phase Equilibria, 72: 211-225. Densities and refractive indices of bromoform, n -heptane, n -octane, 2,2,4-trimethylpen- tane, n -nonane, n -decane, n -tetradecane, n -hexadecane and 1,2,3,4-tetrahydronaphthalene have been measured over the temperature interval of 298.15-323.15 K. Polarizabilities and molecular radii were calculated on the basis of Bottcher's theory of electric polarization and compared with the molecular radii evaluated from the molar volumes of the liquids and from free volume considerations. Bottcher's theory is found to be satisfactory in estimating the molecular radii and polarizability of long-chain molecules. Lorenz-Lorentz and Eykman relations have been used to calculate the thermal expansion coefficients and the polarizabil- ities of liquids.

Polarizability and molecular radius of dimethyl-sulfoxide and dimethylformamide from refractive index data

Densities and refractive indicies of dimethylsulfoxide and dimethylformamide were measured at various temperatures and wave-lengths. Polarizabilities and molecular radii were calculated on the basis of Bottcher's theory of electric polarization and compared with the molecular radii evaluated from the molar volumes of the liquids and from geometrical considerations.

Theory of Electric Polarization,

Theory of Electric Polarization,

A theory of electric polarization in liquids

Abstract The main approximations used in the theory of dielectric liquids previously developed by the author (1976) are discussed. They are shown to be widely justifiable in the framework of the Yvon-Kirkwood model, treating the polarisability as a molecular constant, independent of the thermodynamical state of the liquid.

Theory of Electric Polarization

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A theory of electric polarization, electro-optical Kerr effect and electric saturation in liquids and solutions

I recently published some papers (Piekara 1937 a , 1938) concerning the theory of dielectric polarization in polar liquids and their solutions in non-polar solvents. I have assumed the existence of two kinds of intermolecular coupling forces. The coupling forces of the first kind are due to the directional field produced by the surrounding molecules (ordered as in a crystal lattice) of potential energy — W 1 cos θ 1 ; θ 1 is the angular displacement of the dipole axis from the momentary axis of equilibrium, which can take all possible directions in space. This idea has been derived from R. H. Fowler (1935) and P. Debye (1935 a ), who developed with the help of this conception a theory of dielectric polarization. In order to obtain a satisfactory agreement with experiments on liquids, it proved, however, necessary to assume still a second kind of coupling forces. ‘Simultaneously it has been shown, that these latter play a more important role than the first ones. The coupling forces of the second kind are due not to aggregates of well-ordered molecules, but to the next molecule, which happens to be in the neighbourhood of the molecule just considered. As a matter of fact there are several such molecules in the vicinity, the action of which should be taken into account.