Interface water-induced hydrophobic carbon chain unfolding in water

The folding and unfolding of the carbon chain, which is the basic constitutional unit of polymers,are important to the performance of the material. However, it is difficult to regulate conformational transition of the carbon chain, especially in an aqueous environment. In this paper, we propose a strategy to regulate the conformational transition of the carbon chain in water based on the all-atom molecular dynamics simulations. It is shown that the unfolded carbon chain will spontaneously collapse into the folded state, while the folded carbon chain will unfold with an external electric field. The regulation ability of the electric field is attributed to the electric field-induced redistribution of interface water molecules near the carbon chain. The demonstrated method of regulating conformational transition of the carbon chain in water in this study provides an insight into regulating hydrophobic molecules in water, and has great potential in drug molecule design and new polymer material development.     

Superfluid molecular hydrogen

In order to produce a supercooled liquid phase of molecular hydrogen that may possibly change at a sufficiently low temperature to a superfluid state, it is suggested to reduce the temperature of its equilibrium coexistence with the solid phase by means of developing different pressures in these phases through the use of linear mechanical pressure on the solid phase or of external electric field. The thermodynamic functions of hydrogen are calculated in both the stable and metastable regions; its phase diagram and the region of possible transition to a superfluid state are also found. The values of excess pressure on the solid phase and of external electric field intensity are estimated, which are necessary for the stabilization of this state.     

On Anomalous Influence of an External Electric Field on a Second-Order Transition in a Thin Ferroelectric Film

Physics of the Solid State - The specific features of a phase transition in a thin ferroelectric film in an external electric film have been studied in terms of the...     

Formation of phase domain structures in thin films under conditions of a first-order magnetic phase transition

This paper reports on the results of a theoretical investigation into the magnetic and resonance properties of thin films in the range of the transition from a paramagnetic state to a ferromagnetic state in the case where the magnetic transition is a first-order phase transformation. It is demonstrated that, in an external magnetic field directed perpendicular to the film plane, the formation of a specific domain structure consisting of domains of the coexisting paramagnetic and ferromagnetic phases can appear to be energetically favorable. The parameters of the equilibrium system of stripe phase domains and their dependences on the temperature, the magnetic field, and the characteristics of the material are calculated. The specific features of the magnetic resonance spectra under the conditions of formed stripe phase domains are considered. A relationship is derived for the dependence of the resonance field of the system of ferromagnetic domains on the magnetization and temperature. It is shown that the alternating external field can fulfill an orientation function in the formation of stripe phase domains.     

Phase transitions in uniaxial ferromagnetic film covered by superconducting layers

The phase diagram and the single-domain uniform state for a uniaxial ferromagnetic film with the superconducting layers covering one or both sides of a ferromagnet are investigated. The superconductor is supposed to be a second-order one and the interaction between the magnetic sub-system and with the conductivity electrons in a superconductor is purely electromagnetic and the vortices in a superconductor are pinned. The critical thickness of the magnetic film for which the uniform state becomes absolutely stable is calculated when the external magnetic field is supposed to be in-plane of the film. It is shown that the critical thickness of the film from the magnetic material with the quality factor Q>1 monotonically decreases as the magnetic field increases in the range from zero value to the value of the transition field where the collinear phase transforms into the angular (canted) phase. Further the critical thickness increases with the increase of the field. The quasi-single-domain magnetic film states were considered when the film thickness was close to the critical one. It is shown that for a thin isolated magnetic film the domain period exponentially increases with the decrease of the film thickness. Such dependence, however for the film with double-side superconducting cover and close to the transition into the single domain state becomes logarithmic and for the film covered by superconductor only on the one side varies as the power series. The single-domain state existence and the asymptotic behaviour of the domain structure is explained by the features of the asymptotic behaviour of the domain walls within the system. As for isolated magnetic film and for a film with the superconductor cover layers the transition from the collinear phase to the inhomogeneous state is the second-order phase transition and the transition from the uniform angular phase to the inhomogeneous phase is the first-order transition.     

Formation and structure of a soliton in an antiferroelectric liquid crystal in an electric field

The soliton structure in an antiferroelectric liquid crystal in an electric field is calculated in the discrete phenomenological Landau model for phase transitions. The anticlinic structure of the antiferroelectric crystal gives rise to nontrivial features of the soliton structure and the stability of the soliton. The soliton is topologically stable in a metastable state if the electric field is much higher than the field of antiferroelectric-helix unwinding. A structural transition with a step-like change in the orientation of molecules occurs as the soliton field varies. A peculiar soliton (synclinic pair) can be formed if the barrier between the anti- and ferroelectric structures is large. The calculation shows the possibility of a large electroclinic effect in the soliton, i.e., a variation in the molecular tilt angle in an electric field in the vicinity of the transition of the structure to the synclinic state.     

A self-consistent approach for the SmA–SmC* phase transition

By using an analogy with the Maier–Saupe theory of the nematic phase, a mean field theory of the chiral smectic-C (SmC*) phase is constructed. As an order parameter of the SmC* phase, the tilt angle θ is selected, and the feedback effect of θ is introduced into the hindered rotational potential about the molecular long axis. By solving the self-consistent equation for the order parameter θ, the second-order phase transition appears. Also, to describe the SmA–SmC* phase transition under an electric field, a coupling term between a molecular dipole moment and an external electric field is introduced to the potential function. The electroclinic effect in the SmA phase and the hysteresis effect in the SmC* phase, which were described phenomenologically in the past, are found directly from the microscopic theory.     

ZnSe在外电场下的基态性质和激发特性研究

以LANL2DZ为基组, 采用Hartree-Fock(HF)方法研究了不同外电场(-0.025–0.040 a.u.)对ZnSe分子的基态几何结构、电荷分布、能量、电偶极矩、最高占据轨道(HOMO)能级、最低空轨道(LUMO)能级、能隙、红外光谱特性的影响; 继而采用含时的TD-HF方法研究了ZnSe分子在外电场下前9 个激发态的吸收谱、激发能、振子强度等激发特性. 研究结果表明: 当电场从-0.025 a.u.变化到0.04 a.u. 时, 键长先减小后增加; 分子偶极矩先由正减小到0, 然后又反向增加; 体系总能量一直减小; 谐振频率先增加后减小, 红外光谱强度先减小后又增加. ZnSe分子的LUMO能级一直增加, HOMO能级先增加后又减小, 变化趋势较小, 而能隙一直增大. 外电场对ZnSe分子的激发特性影响较大, 当电场从-0.025 a.u.变化到0.04 a.u.时, 激发能增加, 相应的激发波长减小; 对应的振子强度也受到很大影响, 原来振子强度最强的激发态变得很弱, 而原来振子强度很弱的激发态变得最强. 因此, 可以通过改变电场来控制ZnSe的激发特性.     

强电场作用下反铁电锆钛酸铅的介电行为研究

采用电滞回线方法和偏置直流电场中叠加小交变电场方法研究了锆钛酸铅反铁电陶瓷材料在强电场作用下的介电行为.测量结果显示，锆钛酸铅反铁电材料的介电常数随外加电场强度呈非线性变化，在反铁电 铁电转变的电场区间形成介电峰.表征极化强度随电场强度变化率的微分介电常数εd峰值出现在反铁电 铁电转换电场强度处，最高达到41000.随着偏置电场增加反铁电向铁电体转变过程中，小信号介电常数εc减小；在电场降低铁电回复成反铁电过程中，小信号介电常数εc增大，小信号介电常数εc峰先于微分介电常数εd峰出现.根据电场作用下反铁电 关键词： 锆钛酸铅反铁电陶瓷 介电行为 强电场条件     

Abrupt change of ionic conductivity induced by an electric field

A theory of abrupt disordering of one of ionic crystal sublattices in an external electric field is developed. It is shown that it is possible to transfer a crystal into a high-conductivity state without heating by means of a field-induced phase transition.     

Relaxation of the domain structure in doped triglycine sulfate crystals in an internal field

This paper reports on the results of an investigation into the relaxation of the domain structure of triglycine sulfate ferroelectric crystals from a nonequilibrium state induced by rapid cooling of the sample through the phase transition point, in response to an external electric field, and under thermal annealing. The role played by the internal field in the processes of relaxation of the domain structure to the equilibrium state in defect-containing triglycine sulfate crystals is elucidated.     

Evolution of the optical properties of Pb<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>Sc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (PBSN-6) solid solution single crystals caused by a static electric field

The electric-field-induced variation of the optical properties (small-angle light scattering, birefringence) of PBSN-6 solid solutions was studied. It was found that in the absence of an electric field, the cubic nonpolar matrix contains, at temperatures below the dielectric permittivity maximum, spontaneously polarized regions of the ferroelectric phase not less than 10⁴ Å in size. It was shown that a weak electric field (～0.4 kV/cm) is capable of inducing a kinetic phase transition to the ferroelectric state, with the temperature of this transformation depending on the sample heating rate. The destruction of the induced state was accompanied by a sharp peak in the temperature dependence of the small-angle light scattering intensity (indicating the percolation nature of the transition) and was independent of the sample heating rate. The boundaries of stability of the induced state in various modes of application of an external electric field were determined, and the E-T phase diagram was constructed.     

Effect of a weak external electric field on the kinetics of the ordering of ferroelectrics upon first-order phase transitions

The kinetics of the formation and growth of 180° domains in a weak quasi-stationary external electric field has been considered in the framework of the phenomenological Ginzburg–Landau model using the example of sodium nitrite (NaNO₂) crystals that undergo a first-order ferroelectric phase transition of the order–disorder type. The influence of the rate and temperature of quenching, as well as the strength of an external electric field, on the subsequent evolution of the system toward the thermodynamic equilibrium state has been analyzed. It has been shown that, by varying a weak external electric field applied to the ferroelectric crystal after quenching, it is possible to obtain both single-domain and multi-domain ordered structures. It has been established that the formation of nonequilibrium (“virtual”) multi-domain structures of the asymmetric type is possible for particular strengths of the electric field applied to the ferroelectric after quenching. A similar effect can be achieved by varying the depth of quenching of the sample. It has been found that, if the size of the order parameter inhomogeneities formed at the stage of quenching does not exceed a critical value, they can be reoriented partially or completely into domains of opposite sign. For this purpose, the relaxation after quenching should be performed in an external electric field of the appropriate sign.     

Suppression of the domain structure in uniaxial ferromagnetic films with a superconducting coating

Specific models of domain walls are used to investigate conditions for the single-domain state and quasi-single-domain states in structures with magnetic materials having a quality factor higher than one. It is shown that the critical thickness of the magnetic film in a tangentially magnetized system decreases monotonically as the magnetizing field increases from zero to the transition from the collinear to the homogeneous angular phase and then increases monotonically with increasing external field. In a thin isolated magnetic film, the size of the domains increases exponentially with decreasing thickness. This dependence is logarithmic near the transition to the single-domain state for a film coated on two sides and obeys a power law for a film coated on one side. The establishment of a single-domain state and characteristic features in the asymptotic behavior of the domain structure in magnetic films with and without coatings can be attributed to differences in the asymptotic behavior of the field of a single domain wall. Fiz. Tverd. Tela (St. Petersburg) 40, 1068–1074 (June 1998)     

Investigation of the Disordered Antiferromagnetic Ising Model with the Effects of Random Magnetic Fields

Computer simulation of phase transitions is made by the Monte Carlo method using a three-dimensional disordered antiferromagnetic Ising model in the external magnetic field. It is found that in the case where the spin concentration in a system is lower than a threshold one, the effects of random magnetic fields destroy the second-order phase transition and lead to the first-order phase transition into a new phase state of the system characterized by a ground spin-glassy state and metastable energy states at finite temperatures. The dependence of the threshold concentration on the external magnetic field is revealed.     

硼球烯B_(40)在外电场下的基态性质和光谱特性

以6-31G*为基组,采用密度泛函PBE0方法研究了不同外电场(0—0.060 a.u.)对硼球烯B_(40)的基态几何结构、电荷分布、能量、电偶极矩、能隙、红外及拉曼光谱特性的影响;继而采用含时的TD-PBE0方法研究了硼球烯B_(40)在外电场下的电子光谱.研究结果表明:外电场的加入导致分子对称性降低,当电场从0 a.u.变化到0.060 a.u.时,偶极矩逐渐增加,体系总能量和能隙一直减小;外电场的加入将改变红外和拉曼光谱特征,如谐振频率的移动以及红外和拉曼峰的增强或减弱;外电场对硼球烯B_(40)的电子光谱影响较大,当电场从0 a.u.变化到0.060 a.u.时,电子光谱发生红移,同时对振子强度有很大影响,原来振子强度最强的激发态变弱或成为禁阻跃迁,而原来振子强度很弱或禁阻的激发态变得最强.可以通过改变外电场来改变B_(40)的基态性质,以及控制B_(40)的光谱特性.     

Angular dependence of the photothermally modulated magnetic resonance in Gd thin films

In this paper we use electron spin resonance and photothermally modulated magnetic resonance techniques to investigate gadolinium thin films as a function of the orientation of the film surface with respect to the external magnetic field and of the temperature, around the magnetic phase transition temperature. We observe that, in the ferromagnetic phase, the resonance line is shifted up to higher external magnetic fields when the angle between the film surface and the field increases, revealing the magnetic anisotropy of the sample. At the same time, when the temperature is augmented to values higher than the phase transition temperature, the external field of the resonance collapses back to the expected value in the paramagnetic phase for all orientations. We also demonstrated that, even for the perpendicular orientation (magnetic field perpendicular to the sample surface), the photothermally modulated magnetic resonance signal is maximized near the magnetic phase transition temperature. Furthermore, in the ferromagnetic phase the photothermally modulated magnetic resonance intensity is very sensitive to the orientation, showing a significant enhancement in the perpendicular direction.     

Electrical and structural properties of VO2 in an electric field

We examined the electrical and local structural properties of a VO₂ film at different electric fields using electrical resistance and x-ray absorption fine structure (XAFS) measurements at the V K edge in the temperature range of 30–100 °C. The T_c value of the metal-to-insulator transition (MIT) during both heating and cooling decreases with electric field. When the electric field exceeds a certain value, the MIT becomes sharper due to Joule heating. The MIT, the structural phase transition (SPT), and the pre-edge peak transition of the VO₂ do not congruently occur at a uniform temperature. A metallic VO₂ is observed in only the rutile (or M2) symmetry. An electric field induces a substantial amount of conduction electrons in insulating VO₂. Simultaneously measured resistance and XAFS reveal that Joule heating caused by an external electric field significantly affects the MIT and SPT of VO₂.     

Control of the transmission spectrum of a photonic crystal with lattice defects

The transmission spectrum of a one-dimensional photonic crystal with structural defects is investigated. An anisotropic layer of a nematic liquid crystal is considered as a defect. The possibility of effective external control of the transmission spectrum of the photonic crystal by changing either the orientational state of the defect liquid crystal layer (for example, using an electric field) or the angle of incidence of light is shown. It is established that the spectral properties of the photonic crystal change radically with a change in the optical characteristics of the defect layer in the vicinity of the temperature phase transition of the nematic liquid crystal to the isotropic state.     

Molecular field theory for nematic liquid crystal film with finite layers

The nematic liquid crystal film composed of n molecular layers is studied based upon a spatially anisotropic pair potential, which reproduces approximately the elastic free energy density. On condition that the system has perfect nematic order, as in the Lebwohl—Lasher model, the director in the film is isotropic. The effect of the temperature is investigated by means of molecular field theory. Some new results are obtained. Firstly, symmetry breaking takes place when taking account of the temperature, and the state with the director along the normal of the film has the lowest free energy. Secondly, the N—I phase transition temperature increases as an effect of finite sizes instead of decreasing as in the Lebwohl—Lasher model. Thirdly, the nematic order is induced in the layers near the surface in the isotropic phase.