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1.
We determine the bifurcation phase diagrams with isotropic (I), uniaxial (N(U)) and biaxial (N(B)) nematic phases for model bent-core mesogens using Onsager-type theory. The molecules comprise two or three Gay-Berne interacting ellipsoids of uniaxial and biaxial shape and a transverse central dipole. The Landau point is found to turn into an I-N(B) line for the three-center model with a large dipole moment. For the biaxial ellipsoids, a line of Landau points is observed even in the absence of the dipoles.  相似文献   

2.
Initial dynamics of photo-induced ionic (I) to neutral (N) phase transition in tetrathiafulvalene- p -chloranil (TTF-CA) was investigated by femtosecond reflection spectroscopy with various excitation intensities. A charge transfer excitation in the I phase produces N donor (D 0 )-acceptor (A 0 ) strings within 2 ps. For a weak excitation intensity, these initial N states decay with a life time of 300 ps at 4 K, but rather multiply leading to macroscopic I-N conversion within 20 ps at 77 K just below the N-I transition temperature T NI . Near T NI , we also found the coherent motion of the macroscopic N-I domain boundary with a period of 85 ps.  相似文献   

3.
Results of the extended Landau-de Gennes model analysis and experimental studies of the isotropic-nematic (I-N) and isotropic-smectic-A (I-SmA) phase transitions in rod-like liquid crystalline n-alkylcyanobiphenyls are presented. Experiments were carried out as a function of temperature and pressure using the static dielectric permittivity and its 'nonlinear' (strong electric field related) counterpart-the low-frequency nonlinear dielectric effect. Precise estimations of the values of the discontinuity of the isotropic-mesophase transitions (ΔT) for nCB from n?=?3-14 have been obtained. It is suggested that for each nCB a unique, characteristic minimal value of ΔT, associated with the I-N-SmA triple point, exists. For 'shorter' nCBs it can be hidden in the negative pressures domain. The possibility of the extension of the 'melting curve' into the negative pressures region as well as the appearance of the 'melting inversion' at high enough pressures is indicated.  相似文献   

4.
Ultrafast optical switching from an ionic ( I) to a neutral ( N) state in TTF-CA was observed in femtosecond reflection spectroscopy. Charge transfer excitation in the I phase produces N donor ( D0) acceptor ( A0) strings within 2 ps. These initial N states decay with a lifetime of 300 ps at 4 K, but rather multiply leading to macroscopic I-N conversion within 20 ps at 77 K just below the N-I transition temperature T(NI). Near T(NI), we also found the evidence for the coherent motion of the macroscopic N-I domain boundary with a period of 85 ps.  相似文献   

5.
We study a frustrated 3D antiferromagnet of stacked J(1)-J(2) layers. The intermediate 'quantum spin liquid' phase, present in the 2D case, narrows with increasing interlayer coupling and vanishes at a triple point. Beyond this, there is a direct first-order transition from Néel to columnar order. Possible applications to real materials are discussed.  相似文献   

6.
The phase behaviour of binary mixtures of hard rod-like particles has been studied using Parsons—Lee theory (Parsons, J. D., 1979, Phys. Rev. A, 19, 1225); Lee, S. D., 1987, J. Chem. Phys., 87, 4972). The stability of the isotropic-nematic (I-N) transition with respect to isotropic—isotropic (I-I), and nematic—nematic (N-N) demixing is investigated. The individual components in the mixtures are modelled as hard cylinders of diameters Di and lengths Li (i = 1,2). The aspect ratios ki = Li/Di of the components are kept fixed (with values of k 1 = 15 and k 2 = 150), and the phase behaviour of the mixtures is studied for varying diameter ratios d = D 1/D 2. When the diameter ratio is relatively large, e.g., for values of d = 50, component 1 may be considered a large colloidal particle, while the second component plays the role of a weakly interacting solvent. This mixture exhibits only an I-N phase transition which is driven by the excluded volume interaction between the large particles (no I-I or N-N demixing is seen). A decrease in the diameter ratio enhances the contribution of the smaller component to the free energy (especially in terms of the unlike excluded volume term), and I-I as well as N-N demixing transitions are observed. The character of the N-N transition is rather unusual, a single region bounded by a lower critical point (in the pressure—composition plane) is seen for a diameter ratio of d = 3.2, while two demixed nematic regions bounded by lower and upper critical points are observed for d = 3.13. A further decrease in the diameter ratio (e.g., to d = 3) leads to systems with a phase behaviour in which the two demixed N-N regions meet, giving rise to a large demixed region with very strong fractionation in composition, and no N-N critical points. The I-I demixing transition is always accompanied by a lower critical point and occurs for systems with intermediate size (diameter) ratios. A diameter ratio of d = 4.5 corresponds to systems with significant like and unlike excluded volume interactions, and in this case the I-N transition takes place over the whole composition range with weak fractionation and one azeotropic point. Surprisingly, the coexisting nematic phase is of lower packing fraction than the isotropic phase for some of the compositions, i.e., an inversion of packing fraction takes place. In addition to this, the longer rods can be less ordered that the shorter rods for certain values of the composition.  相似文献   

7.
Differential scanning calorimetry (DSC) and X-ray measurements give evidence on a complete miscibility within the solid solution system [(CH3)4N]2CuBrxCl4−x. In addition to the known phase sequences of pure components, some new phases appear for intermidiate concentrations. Out of five triple points observed in the phase diagram, one seems to be a Lifshitz point between a paraelectric, an incommensurate and a ferroelectric phase.  相似文献   

8.
The melting curve of silicon has been determined up to 15 GPa using a miniaturized Kawai-type apparatus with second-stage cubic anvils made of X-ray transparent sintered diamond. Our results are in good agreement with the melting curve determined by electrical resistivity measurements [V.V. Brazhkin, A.G. Lyapin, S.V. Popova, R.N. Voloshin, Nonequilibrium phase transitions and amorphization in Si, Si/GaAs, Ge, and Ge/GaSb at the decompression of high-pressure phases, Phys. Rev. B 51 (1995) 7549] up to the phase I (diamond structure)—phase II (β-tin structure)—liquid triple point. The triple point of phase XI (orthorhombic, Imma)—phase V (simple hexagonal)—liquid has been constrained to be at 14.4(4) GPa and 1010(5) K. These results demonstrate that the combination of X-ray transparent anvils and monochromatic diffraction with area detectors offers a reliable technique to detect melting at high pressures in the multianvil press.  相似文献   

9.
We calculate the dynamic phase transition (DPT) temperatures and present the dynamic phase diagrams in the kinetic spin-5/2 Blume–Capel model under the presence of a time-dependent oscillating external magnetic field. First, we employ the Glauber transition rates to construct the mean-field dynamic equation. Then, we study the time variation of the average magnetization to find the phases in the system. We also investigate the behaviour of the dynamic magnetization to characterize the nature (continuous and discontinuous) of transition and to obtain the DPT points. We present the dynamic phase diagrams in two different planes. The phase diagrams include the ferromagnetic-5/2 (f5/2), the ferromagnetic-1/2 (f1/2) and paramagnetic (p) fundamental phases. In addition to these fundamental phases, we find 10 mixed phases, depending on the interaction parameters. The phase diagrams display many special points, such as a dynamic tricritical point, a double critical end point, a triple point and a quadruple point.  相似文献   

10.
Adsorption isotherms of nitric oxide on the cleavage face of fourteen lamellar halides (NiCl2, CoCl2, FeCl2, NiBr2, CoBr2, FeBr2, MgBr2, CdCl2, CoI2, CdBr2, MnI2, CdI2, CaI2 and Pbl2) have been determined. Adsorbents with a uniform surface have been prepared by sublimation in a rapid current of dry nitrogen. The isotherms show vertical steps corresponding to two-demensional phase transitions. For all the studied systems, except NOCaI2, a two-dimensional triple point exists. Below the temperature of this point, a single transition occurs from a dilute adsorbed phase to a dense one, which we call β. Above this temperature, two transitions are observed: one is from the dilute phase to another dense phase called α, and a second one from a to β. From the analysis of our experimental results, we infer that: (i) the β phase is a monolayer having a structure similar to that of the (001) plane of the bulk crystalline adsorbate (the dimers of nitric oxide standing almost perpendicular to the adsorbent surface) and (ii) in the vicinity of the two-dimensional triple point the a phase is a monolayer essentially composed of dimers lying flat on the surface. We are able to explain the variations of the two-dimensional triple point temperature and of the free energy of the β phase in terms of the nature and the crystal parameter of the adsorbent.  相似文献   

11.
12.
Phase diagrams and magnetic properties of the mixed spin-1 and spin-3/2 Ising film with different single-ion anisotropies are investigated, by the use of Monte Carlo simulation based on heat bath algorithms. The effects of the crystal-fields and the surface coupling on the phase diagrams are investigated in detail and the obtained phase diagrams are presented. Depending on the Hamiltonian parameters, the system exhibits both second-and first-order phase transitions besides tricritical point, triple point, and isolated critical end point.  相似文献   

13.
New beam-foil lifetime measurements of excited levels have been made for N I-N IV using lines in the vacuum ultraviolet spectral range. These values are compared with other experimental results and theoretical data. ‘Best’ values of absolute transition probabilities are proposed.  相似文献   

14.
Employing a mean-field approach, we study the stationary states of the kinetic spin-5/2 Blume–Emery–Griffiths (BEG) model under the presence of a time-varying (sinusoidal) magnetic field by using the Glauber-type stochastic dynamics. We employ the Glauber transition rates to construct the set of dynamic mean-field equations. We investigate the time variation in average order parameters to find the phases in the system, and the thermal behavior of the dynamic order parameters to characterize the nature (continuous or discontinuous) of the dynamic phase transtions and to the dynamic phase transition temperature. The dynamic phase diagrams are presented in three different planes. The phase diagrams contain the ferromagnetic-5/2, the ferromagnetic-3/2, the ferromagnetic-1/2, the ferroquadrupolar, and disordered fundamental phases. They also include the nine coexisting or mixed phases composed of binary and ternary combinations of fundamental phases that strongly depend on the interaction parameters. The phase diagrams display the critical end point, double critical end point, triple point, quadruple point, and one, two, or three special points and the dynamic tricritical point that depends on the interaction parameters.  相似文献   

15.
《中国物理 B》2021,30(6):67505-067505
Recent experiments [Guo et al., Phys. Rev. Lett. 124 206602(2020)] on thermodynamic properties of the frustrated layered quantum magnet SrCu_2(BO_3)_2-the Shastry–Sutherland material-have provided strong evidence for a lowtemperature phase transition between plaquette-singlet and antiferromagnetic order as a function of pressure. Further motivated by the recently discovered unusual first-order quantum phase transition with an apparent emergent O(4) symmetry of the antiferromagnetic and plaquette-singlet order parameters in a two-dimensional "checkerboard J-Q" quantum spin model[Zhao et al., Nat. Phys. 15 678(2019)], we here study the same model in the presence of weak inter-layer couplings. Our focus is on the evolution of the emergent symmetry as the system crosses over from two to three dimensions and the phase transition extends from strictly zero temperature in two dimensions up to finite temperature as expected in SrCu_2(BO_3)_2.Using quantum Monte Carlo simulations, we map out the phase boundaries of the plaquette-singlet and antiferromagnetic phases, with particular focus on the triple point where these two ordered phases meet the paramagnetic phase for given strength of the inter-layer coupling. All transitions are first-order in the neighborhood of the triple point. We show that the emergent O(4) symmetry of the coexistence state breaks down clearly when the interlayer coupling becomes sufficiently large, but for a weak coupling, of the magnitude expected experimentally, the enlarged symmetry can still be observed at the triple point up to significant length scales. Thus, it is likely that the plaquette-singlet to antiferromagnetic transition in SrCu_2(BO_3)_2 exhibits remnants of emergent O(4) symmetry, which should be observable due to additional weakly gapped Goldstone modes.  相似文献   

16.
From experiments with ice or metal crystals, in the vicinity of their crystal/liquid/vapor triple points, it is known that melting of crystals starts on their surfaces and is anisotropic. It is shown here by direct observations under an optical microscope that this anisotropic surface melting phenomenon occurs also in lyotropic systems. In the case of C12EO2/water mixture, it takes place in the vicinity of the peritectic Pn3m/L3/L1 triple point. Above the peritectic triple point, where the Pn3m and L1 phases coexist in the bulk, the surface of a Pn3m-in-L1 crystal is composed of (111)-type facets surrounded by rough surfaces. The angular junction suggests that rough surfaces are wet by a L3-like layer while facets stay “dry”. This is analogous to the pre-melting at rough surfaces in solid crystals. Upon cooling below the peritectic triple point, where L3 and L1 phases coexist in the bulk, a thick layer of the L3 phase grows from the pre-melted, rough Pn3m/L1 interface. Simultaneously, facets stay dry and their radius decreases. In this tri-phasic configuration, stable in a narrow temperature range, the L3/L1 and L3/Pn3m interfaces have shapes of constant mean curvature surfaces having common borders: edges of facets.  相似文献   

17.
Understanding the dynamic process of black hole thermodynamic phase transitions at a triple point is a huge challenge. In this paper, we conduct the first investigation of dynamic phase behavior at a black hole triple point. By numerically solving the Smoluchowski equation near the triple point for a six-dimensional charged Gauss-Bonnet anti-de Sitter black hole, we report that initial small, intermediate, or large black holes can transit to the other two coexistent phases at the triple point, indicating that thermodynamic phase transitions can indeed occur dynamically. More significantly, we observe characteristic weak and strong oscillatory behavior in this dynamic process, which can be understood from an investigation of the rate of first passage from one phase to another. Our results further an understanding of the dynamic process of black hole thermodynamic phase transitions.  相似文献   

18.
Using the Collins model of Liquids the melting of 2D monatomic solids is studied and the melting equation is derived. For the hard disk system our results agree with the computer simulation experiments quite well.For the square well system by means of the LHW theory the triple point and the melting line near the triple point is obtained to establish the phase boundary.  相似文献   

19.
The phase diagram of flexible molecules formed by freely-jointed tangent spheres is studied using the first-order thermodynamic perturbation theory of Wertheim for both fluid and solid phases. A mean-field term is added to the free energy of the fluid and solid phase in order to account for attractive dispersion forces. The approach is used to determine the global (solid-liquid-vapour) phase diagrams and triple points of chain molecules of increasing chain length. It is found that the triple point temperature is not affected strongly by the length of the chain, whereas the gas-liquid critical temperature increases dramatically. The asymptotic limits of the phase diagram for infinitely long chains are discussed. The reduced critical temperature of infinitely long chains as given by the mean-field theory is 2/3, and the reduced triple point temperature is 0.048 56, so that an asymptotic value of T t/T c = 0.07284 for the ratio of the triple to critical point temperatures is obtained. This indicates that fully-flexible tangent chains present an enormous liquid range. The proposed theory, while being extremely simple, provides a useful insight into the phase behaviour of chain molecules, showing the existence of finite asymptotic limits for the triple and critical point temperatures. However, since n-alkanes present an asymptotic limit of about T t/T c, = 0.40, the agreement With experiment is not quantitative. This suggests that fully flexible models may not be appropriate to model the solid phases of real chain molecules.  相似文献   

20.
《Physics letters. A》1988,131(1):38-40
A reentrant trigonal (α)-Hg phase and a new hcp (δ)-Hg phase have been observed in energy dispersive X-ray diffraction measurements on mercury at room temperature and pressures up to 39 GPa. Studies at higher temperatures give first indications for a maximum in the α-β phase line at about 8(1) GPa and 380(10) K and an α-β-δ triple point at about 27(5) GPa and 180(20) K.  相似文献   

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