Sinusoidal incommensurate structure in spiral antiferromagnets

It is shown, that in hexagonal antiferromagnets, the form of incommensurate structure depends on the ratio of dipole-dipole energy to the anisotropic interaction energy at fixed values of exchange constants. In CsCuCl₃-type compounds, in which the spiral magnetic structure realizes along the c-axis and 120° structure - in the c-plane, there appears a new sinusoidal phase in the external field H>H_c, where H_c is a critical field. Thus, in result, two modulated structures form at intermediate temperatures.     

Analysis of incommensurate structure in [N(CH3)4]2ZnCL4 crystal

We analyzed the temperature dependence of the amplitude of modulated structures of [N(CH₃)₄]₂ZnCl₄ crystal in the incommensurate (INC) phase by measuring the EPR spectra for Mn²⁺ doped single crystal. The amplitude of the vibrational modulation linearly increased with decreasing temperature. Applying Frank and Van der Merwe model, we simulated the EPR spectra in the INC phase. The simulated spectra agreed with the observed one very well. We, therefore, revealed that the vibrational modulation in this INC phase is due to the interaction between the harmonic chain of inter-particles and the modulation due to underlying potential which comes from the commensurate structure.     

Growth of commensurate and incommensurate smectics on crystal substrates

The nematic-smectic transition near a crystal substrate with a periodic potential relief is investigated. It is shown that structures which are commensurate or incommensurate with the substrate potential can form, depending on the values of the physical parameters of the system and the temperature. The possibility of realizing and observing such structures is discussed. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 708–712 (10 May 1996)     

The commensurate and incommensurate TGBC^* liquid crystal phases

We propose complements to the Renn model of the liquid crystal TGBC^* phase. We argue that the rotation angle per helislab is spontaneously in the radian range, not too small to limit the energy cost of the twist grain boundaries between the helislabs, not too large to preserve the double twisting efficiency. Taking the elastic interactions between the helislabs into account, we show that the structure may undergo two different lock-ins, provided that the uncontrolled interactions at the sample surfaces are small enough. First, for appropriate values of the elastic constants, an angular lock-in may fix the rotation angle per helislab at values exactly commensurate to π. Three characteristic lengths of the TGBC^* phase, the TGB period and the thicknesses of both the smectic blocks and of the helislabs are then commensurate to one another, with moreover, integer ratios at the most efficient commensurabilities. The TGBC^* phase could thus exist in several versions, incommensurate and commensurate, according to the steps of a restricted Devil's staircase. A second elastic lock-in should then arise in the commensurate TGBC^* phases, to set the arrays of disclination lines between the helislabs in simple rectangular lattices, arranged in a helical manner. Being placed right behind one another, the disclination lines then coincide when observed along the TGB axis. This could explain the typical textures with square or hexagonal grids, observed in oriented samples. The commensurability of the TGBC^* phases could be analyzed with X-ray scattering experiments in the same manner as already reported for the TGBC phase. Received 30 November 1999 and Received in final form 5 May 2000     

Relaxation processes in the incommensurate phase of a crystal with defects

The nature of the relaxation of the incommensurate superstructure of a ferroelectric to the equilibrium state is investigated experimentally. It is shown that near a phase transition the temperature dependence of the relaxation time of the incommensurate phase of the defective crystal is exponential. This law agrees qualitatively with the notion of domain wall motion in an inhomogeneous medium containing “random local phase-transition temperature” type defects. Fiz. Tverd. Tela (St. Petersburg) 41, 513–515 (March 1999)     

Thermal hysteresis and memory effects in TlGaSe2 crystal with incommensurate phase

Temperature dependencies of dielectric permittivity of TlGaSe₂ have been measured under various thermal cycles. Peculiarities of anomalies in temperature dependencies of dielectric permittivity corresponding to structural phase transitions at 108 and 115?K are discussed. The coexistence of two different incommensurate structures in TlGaSe₂ was proposed. The phase transitions at 108 and 115?K are considered as commensurate lock-in transitions. As a result a new model of the structural phase transitions in TlGaSe₂ has been suggested.     

Phase transitions and modulated structure of the incommensurate phase in Mg[H2O]6SiF6 crystal

The temperature and angular dependences of the EPR spectra of Mg[H₂O]₆SiF₆:Mn²⁺ crystal were investigated in order to clarify the successive phase transitions and existence of the incommensurate phase. Five successive phase transitions were found to occur, and phase II was found to be incommensurately modulated. The modulated structure is caused mainly by the vibrational displacement of the Mg[H₂O]²⁺ ₆ ion along the c-axis. The soliton density of this phase is almost independent of temperature and remains equal to unity.     

Topology and ferroelectricity in group-V monolayers

The group-V monolayers(MLs) have been studied intensively after the experimental fabrication of two-dimensional(2D) graphene and black phosphorus. The observation of novel quantum phenomena, such as quantum spin Hall effect and ferroelectricity in group-V elemental layers, has attracted tremendous attention because of the novel physics and promising applications for nanoelectronics in the 2D limit. In this review, we comprehensively review recent research progress in engineering of topology and ferroelectricity, and several effective methods to control the quantum phase transition are discussed. We then introduce the coupling between topological orders and ferroelectric orders. The research directions and outlooks are discussed at the end of the perspective. It is expected that the comprehensive overview of topology and ferroelectricity in 2D group-V materials can provide guidelines for researchers in the area and inspire further explorations of interplay between multiple quantum phenomena in low-dimensional systems.     

Optical properties of group-V atom-vacancy pairs in silicon

Abstract

Two optical absorption bands are identified as arising from the negative charge state of each of the P-, As-, and Sb-vacancy pairs in silicon. The more prominent band for each is at 6150, 6000, and 5500 cm^?1, respectively. A second broader partially overlapping band at ～8500 cm^?1 is also present for each. Dichroism produced in the bands by uniaxial stress reveals a static Jahn-Teller distortion of opposite sign to that for the neutral state previously studied by EPR. The absorption bands can be identified as electron transitions from filled to empty orbitals of a simple one-electron molecular orbital model for the defect.     

An exactly solvable model of a multidimensional incommensurate structure

The paper considers the class of Schrödinger multidimensional discrete operators with quasi-periodic unbounded potential for which essentially complete spectral analysis may be carried out. In the case of sufficiently high incommensurability of almost-periods, the spectrum of such operators is found to be pure point and simple, the eigenfunctions exponentially localized and the low frequency conductivity exponentially small. In the one-dimensional case, for any incommensurability, the spectrum does not contain the absolutely continuous component, while for small incommensurability the spectrum is singular continuous.     

On the structure of the microscopic dielectric tensor in incommensurate crystals

A microscopic model for calculating the optical response of incommensurately modulated phases in insulating crystals is presented. The dominant contribution to the dielectric permittivity tensor is shown to originate from the lowest-index reciprocal lattice vectors, thus proving the validity of the mesoscopic approach developed in several earlier studies. The expression for the mesoscopic Fourier component of the dielectric tensor is obtained. These results may be useful in relation to the controversial problem of the optical activity observed in the incommensurate phases of some A₂BX₄ family crystals.     

Existence of an incommensurate phase in a Li2B4O7 crystal

The x-ray diffraction spectra of Li₂B₄O₇ single crystals are investigated in the temperature range 80–300 K, and the lattice parameter c is determined in the same temperature range in the presence of a periodically varying temperature field. An incommensurate phase is not observed anywhere in the temperature range investigated, regardless of whether the crystals are subjected to a periodic temperature field. Fiz. Tverd. Tela (St. Petersburg) 39, 1461–1463 (August 1997)     

Dynamics of the domain structure in ferroelectrics with an incommensurate phase

The dynamics of the domain structure of ferroelectrics with an incommensurate phase (like sodium nitrite) is considered. The polarization properties of these ferroelectrics are described using the pseudospin formalism. The equation for the polarization is numerically solved together with the equation for acoustic waves. The obtained results are analyzed for various values of the parameters of the crystal and different initial conditions. It is shown that, after a sufficiently large time, the domain structure of the crystal becomes localized.     

Melting properties of group-V elements and InSb

Summary In this paper, we study the structural properties of liquid group-V elements (and a compound InSb) and we analyse the differences with their room temperature crystalline phases. Neutron diffraction experiments are performed using short-wavelength neutrons (0.7 ?). We show different melting behaviours of a light element (As) and a heavy element (Sb) under study. Whereas As keeps its coordination three upon melting, Sb is six-coordinated in the melt. Similarly, InSb has a coordination number increasing from 4 to about 6 upon melting. Large entropies of melting are related to important structural variations. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.