Uniaxial Phase Research Articles

Uniaxial Tensile-Induced Phase Transition in Graphynes

Uniaxial Tensile-Induced Phase Transition in Graphynes

Synergistic performance of microwave-activated coal gangue with limestone in low-carbon cement

This study aims to apply microwave-activated coal gangue (MCG) to develop limestone calcined clay cement (LC3). The impact of microwave activation schemes (microwave temperature and holding time) on the pozzolanic activity of coal gangue were explored. After that, LC3 cements were prepared by replacing cement clinker with limestone powder (LS) and MCG at different ratios. The uniaxial compressive strength, phase composition, microstructure, and pore structure of the MCG-based LC3 cements were analyzed. The results indicate that the maximum volcanic ash activity index is achieved when the material is microwave-activated at 800 °C with a holding time of 20 min. The MCG based LC3 cements achieve the highest strength at an MCG content of 20 % with an MCG:LS ratio of 2. Then, the response surface methodology was employed to predict the optimal mixture proportions for achieving the best strength of the material. The life cycle analysis shows that the carbon emission of the MCG-based LC3 material is 30 % lower than that of Ordinary Portland Cement. This study develops a new high-performance material with low carbon emission for construction industry.

Uniaxial - biaxial lyotropic cholesteric phase transition of lyotropic liquid crystals

ABSTRACT A Landau model is proposed to describe the uniaxial – biaxial lyotropic cholesteric phase transition of lyotropic liquid crystals (LLC). The model free energy is based on a mixture of rod-shaped and disc-shaped micelles. The possibility of the existence of two biaxial lyotropic cholesteric phases and two uniaxial lyotropic cholesteric phases of LLC is discussed. The phase transitions between two biaxial lyotropic cholesteric phases and two uniaxial lyotropic cholesteric phases are explored. Qualitative comparisons between theoretical predictions and recent experimental results are discussed.

Phase Equilibria and Critical Behavior in Nematogenic MBBA-Isooctane Monotectic-Type Mixtures.

The transition from the isotropic (I) liquid to the nematic-type (N) uniaxial phase appearing as the consequence of the elongated geometry of elements seems to be a universal phenomenon for many types of suspensions, from solid nano-rods to biological particles based colloids. Rod-like thermotropic nematogenic liquid crystalline (LC) compounds and their mixtures with a molecular solvent (Sol) can be a significant reference for this category, enabling insights into universal features. The report presents studies in 4'-methoxybenzylidene-4-n-butylaniline (MBBA) and isooctane (Sol) mixtures, for which the monotectic-type phase diagram was found. There are two biphasic regions (i) for the low (TP1, isotropic liquid-nematic coexistence), and (ii) high (TP2, liquid-liquid coexistence) concentrations of isooctane. For both domains, biphasic coexistence curves' have been discussed and parameterized. For TP2 it is related to the order parameter and diameter tests. Notable is the anomalous mean-field type behavior near the critical consolute temperature. Regarding the isotropic liquid phase, critical opalescence has been detected above both biphasic regions. For TP2 it starts ca. 20 K above the critical consolute temperature. The nature of pretransitional fluctuations in the isotropic liquid phase was tested via nonlinear dielectric effect (NDE) measurements. It is classic (mean-field) above TP1 and non-classic above the TP2 domain. The long-standing problem regarding the non-critical background effect was solved to reach this result.

Scanning tunneling microscopy study of hidden phases in atomically thin 1T-TaS$$_2$$

Lower thermal stability due to thinning often leads to unprecedented hidden phases in low-dimensional materials. Such hidden phases can coexist or compete with preexisting electronic phases. We investigate hidden phases observed in atomically thin (6–8 layers) 1T-TaS $$_2$$ with scanning tunneling microscopy. First, we can electrically induce a hidden stripe phase at room temperature. Such a uniaxial stripe phase has three equivalent orientations by breaking threefold symmetry of 1T-TaS $$_2$$ . We also reveal that the hidden stripe phase coexists with nearly commensurate charge-density-wave phase. Next, we observe that the emergent stripe phase spontaneously appears without any electric excitation on a tiny flake ( $$160\times 80$$ nm $$^2$$ ). Our findings may provide a plausible explanation for the previously observed phase transition and twofold optical response in thin 1T-TaS $$_2$$ devices at room temperature. Furthermore, the hidden stripe phase would be crucial to understand exotic CDW-related phenomena in 1T-TaS $$_2$$ for potential applications.

A new interpretation of the optical interference figures observed in a lyotropic biaxial nematic phase for a ternary mixture of DTMABr, 1-DDeOH and H2O

The lyotropic ternary mixture of dodecyltrimethylammonium bromide (DTMABr), 1-dodecanol (DDeOH) and water (H2O) was studied by optical conoscopy and Abbe refractometer techniques. The present mixture showed a large biaxial phase domain (NB) located between two different uniaxial ones in the partial phase diagram, one optically positive (ND) and the other negative (NC). The NB phase exhibits two optic signs as a function of the temperature , each of which was equal to the neighboring uniaxial phases. The temperature dependence of the refractive indices was measured along the NC-NB-ND phase transitions. In addition, we are reporting for the first time a new interpretation of the optical interference figures observed along the NB phase.

Optical signals of uniaxial and biaxial cholesteric lyotropic phases

Lyotropic cholesteric liquid crystalline phases were prepared by doping the ternary mixture of potassium laurate (KL)/decanol (DeOH)/ deuterium oxide (D2O) with brucine sulphate, in two different concentrations. Using the variation of the average color in a heating cycle, with and without the insertion of compensator a gypsum plate (1 λ), in the light path, the optical signal of the cholesteric phases ChD-(ChC+) was confirmed as being negative(positive). When the biaxial cholesteric phase (ChB) is between two uniaxial phases (ChD-ChB-ChD) or (ChD-ChB-ChC), the sign of the ChB phase depends on the uniaxial phase in its vicinity. In the case (ChD--ChB--ChD-), the phase ChB has a negative optical signal (ChB-) across the domain. However, if the ChB phase is between uniaxial cholesteric phases of opposite optical signals, it cannot have just one signal, but must have a change at some point in the biaxial domain showing (ChD--ChB--ChB+-ChC+) phase sequence.

Topological phase transitions in two-dimensional bent-core liquid crystal models.

Two-dimensional liquid crystal (LC) models of interacting V-shaped bent-core molecules with two rigid rodlike identical segments connected at a fixed angle (θ=112^{∘}) are investigated. The model assigns equal biquadratic tensor coupling among constituents of the interacting neighboring molecules on a square lattice, allowing for reorientations in three dimensions (d=2,n=3). We find evidence of two temperature-driven topological transitions mediated by point disclinations associated with the three ordering directors, condensing the LC medium successively into uniaxial and biaxial phases. With Monte Carlo simulations, temperature dependencies of the system energy, specific heat, orientational order parameters, topological order parameters, and densities of unbound topological defects of the different ordering directors are computed. The high-temperature transition results in topological ordering of disclinations of the primary director, imparting uniaxial symmetry to the phase. The low-temperature transition precipitates simultaneous topological ordering of defects of the remaining directors, resulting in biaxial symmetry. The correlation functions, quantifying spatial variations of the orientational correlations of the molecular axes show exponential decays in the high-temperature (disordered) phase, and power-law decays in the low-temperature (biaxial) phase. Differing temperature dependencies of the topological parameters point to a significant degree of cross coupling among the uniaxial and biaxial tensors of interacting molecules. This simplified Hamiltonian leaves θ as the only free model parameter, and the system traces a θ-dependent trajectory in a plane of the phenomenological parameter space.

Spin-momentum locked interface modes based on transverse resonance and Zak phase in finite thickness dielectric slabs

The field of topological photonics has made great strides in the past decade with many new designs based on bandgap and band inversion structures that provide robust, unidirectional, and reflection-free propagation of energy. The topological invariant or Chern number of a metamaterial guarantees the existence of topologically protected edge modes. However, its mathematical application to real systems is not always straightforward and can be greatly simplified by reducing the dimensional complexity of the problem by the calculation of a Zak phase that determines the topological phase in just one dimension. This work explores two methods of creating interface modes with finite height dielectric slabs: (1) transverse resonance through variable edge truncation of a photonic crystal (PhC) or cell sliding and (2) a uni-axial topological phase by means of scaling the internal features of a unit cell. The proposed metamaterial devices use the same C4v symmetric unit cell structure on both sides of the interface and are finite in all three dimensions, allowing for easy fabrication, excitation, and implementation in real-world applications. The all-dielectric design also enables an easy transition to and from conventional PhC waveguides and lends itself well to operation in frequencies spanning across the microwave and optical spectrum without concerns of additional metallic losses in the THz region.

Lattice Monte Carlo study of orientational order in a confined system of biaxial particles: Effect of an external electric field.

In this work we have used lattice Monte Carlo to determine the orientational order of a system of biaxial particles confined between two walls inducing perfect order and subjected to an electric field perpendicular to the walls. The particles are set to interact with their nearest neighbors through a biaxial version of the Lebwohl-Lasher potential. A particular set of values for the molecular reduced polarizabilities defining the potential used was considered; the Metropolis sampling algorithm was used in the Monte Carlo simulations. The relevant order parameters were determined in the middle plane of the sample and for some cases across the whole thickness of the sample. We have determined the temperature-electric field phase diagram for this system and found, as expected, five different system configurations corresponding to three different mesophases. At low temperatures and low fields the system finds itself in an undistorted biaxial phase. On increasing the field at low temperatures, a Freedericksz transition takes place and the secondary directors reorient towards the field while the primary director stays undistorted and parallel to the walls. On increasing the field further, a second Freedericksz transition occurs and the primary director orients also towards the field direction. The orientational order measured at the field strengths tested is not affected by the field. On increasing the temperature, a transition to a uniaxial phase occurs and within the range of this phase a field increase leads also to a Freedericksz transition where the main director reorients towards the field. At higher temperature a transition to a disordered phase is found. We have performed finite size scaling analysis for the Freedericksz critical fields and found that they scale with the distance L between the walls as L^{-1} as expected from continuum theory. From these fields we have also determined the temperature dependence of two elastic constant ratios. Critical exponents and critical temperatures for the order parameter and the correlation length for the biaxial-uniaxial phase transition and the uniaxial to disordered phase transition were also determined by finite size scaling and are discussed.

Real-space renormalization-group treatment of the Maier-Saupe-Zwanzig model for biaxial nematic structures.

The Maier-Saupe-Zwanzig model for the nematic phase transitions in liquid crystals is investigated in a diamond hierarchical lattice. The model takes into account a parameter to describe the biaxiality of the microscopic units. Also, a suitably chosen external field is added to the Hamiltonian to allow the determination of critical parameters associated with the nematic phase transitions. Using the transfer-matrix technique, the free energy and its derivatives are obtained in terms of recursion relations between successive generations of the hierarchical lattice. In addition, a real-space renormalization-group approach is developed to obtain the critical parameters of the same model system. Results of both methods are in excellent agreement. There are indications of two continuous phase transitions. One of them corresponds to a uniaxial-isotropic transition, in the class of universality of the three-state Potts model on the diamond hierarchical lattice. The transition between the biaxial and the uniaxial phases is in the universality class of the Ising model on the same lattice.

Uniaxial strain-induced phase transition in the 2D topological semimetal IrTe2

Strain is ubiquitous in solid-state materials, but despite its fundamental importance and technological relevance, leveraging externally applied strain to gain control over material properties is still in its infancy. In particular, strain control over the diverse phase transitions and topological states in two-dimensional transition metal dichalcogenides remains an open challenge. Here, we exploit uniaxial strain to stabilize the long-debated structural ground state of the 2D topological semimetal IrTe2, which is hidden in unstrained samples. Combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy data reveal the strain-stabilized phase has a 6 × 1 periodicity and undergoes a Lifshitz transition, granting unprecedented spectroscopic access to previously inaccessible type-II topological Dirac states that dominate the modified inter-layer hopping. Supported by density functional theory calculations, we show that strain induces an Ir to Te charge transfer resulting in strongly weakened inter-layer Te bonds and a reshaped energetic landscape favoring the 6×1 phase. Our results highlight the potential to exploit strain-engineered properties in layered materials, particularly in the context of tuning inter-layer behavior.

Conoscopic image of a biaxial negative in a reentrant discotic – biaxial nematic phase transition

ABSTRACT Conoscopic images have confirmed the occurrence of biaxial positive ( and biaxial negative () nematic phases between the two uniaxial nematic (discotic – and calamitic – ) phases. Biaxial positive () nematic phase was also optically characterised between two uniaxial phases. These nematic phases are found in different phase diagram lines of potassium laurate (), decanol () and deuterium oxide () lyotropic mixtures. In limited concentration range between the two phase diagram lines the following phase sequence ( ) is determined here through conoscopic images. An analysis of the meaning of this result leads us to conclude that the biaxial nematic phase range between uniaxial nematic phases of phase diagrams with nematic lyotropic materials is separated in two regions: one has an optically positive signal and the other, having an optical negative signal.

Temperature dependence of 2V angle in biaxial negative and positive nematic lyotropic phases

ABSTRACTRefractive index measurements and conoscopic images has confirmed the occurrence of biaxial negative () and biaxial positive () nematic phases between the two uniaxial nematic (calamitic and discotic ) phases. These nematic phases are found in a phase diagram line of potassium laurate (KL), decanol (, potassium sulfate and water. Biaxial positive ( nematic phase is also optically characterized between two uniaxial phases in a KL, DeOH, and mixture. Temperature dependence of the angles ( and ) between two optic axes of ( and ) phases is obtained from refractive index data. The transition point ( characterized by an angle peak corresponds to the change in the biaxial configuration of the optical axes and optical signal of the biaxial nematic phase. The micellar configuration of biaxial nematic phases is discussed as a consequence of our experimental data.

Phase behavior of hard C_{2h}-symmetric particle systems.

Using Monte Carlo numerical simulation, this work sketches the phase diagram of systems of certain hard C_{2h}-symmetric particles, formed by gluing two aligned and displaced hard spherocylinders with a cylindrical-length-to-diameter ratio realistically, if viewed not only from the lyotropic colloidal liquid-crystal side but also from the thermotropic low-molecular-mass liquid-crystal side, equal to 5, as a function of the displacement. Several distinctive phases are observed, such as a nonperiodic smectic-B-like phase, a nonperiodic smectic-H-like phase, a smectic-C phase, and a short-layer-spacing uniaxial smectic-A phase but no biaxial nematic phase.

Mean-field model of boomerang nematic liquid crystals with diminished coupling of molecular uniaxial and biaxial susceptibilities.

The mean-field theory approach has been applied to the boomerang type particles from P.I.C. Teixeira, A. Masters, and B. Mulder [Mol. Cryst. Liq. Cryst. 323, 167 (1998)MCLCE91058-725X10.1080/10587259808048440] but with diminished strength of the interaction coefficient responsible for the coupling between molecular uniaxial and biaxial susceptibilities. For the rodlike particles, when the apex boomerang angle is larger than 107.35^{∘}, the stable uniaxial rodlike phase occurs. For smaller angles, beyond the point where the transition is of the second order (the Landau point) and for diminished parameter of molecular biaxial-uniaxial coupling, a biaxial phase is observed with the transition undergoing directly from the isotropic phase. According to the order parameters the character of this transition is of the first order. Such behavior is in accordance with the Sonnet-Durand-Virga model of the biaxial phases. The change in the type of the phase transition order is also illustrated by the changes in the equations of state and the changes in second and third derivatives of the free energy. The possibilities to tailor interaction coefficients of real molecules to obtain such a phase transition scenario are discussed.

Optical signal and optical axes in uniaxial and biaxial nematic phases

ABSTRACTIn this work we will study the behavior of the optical signal and optical axes along the continuous transition of a lyotropic system from a nematic discotic () phase to a nematic calamitic () phase; we will study in detail the case in which there is a biaxial nematic () phase at the interstice between these two uniaxial phases. It will be shown that the optical signal and optical axes change simultaneously in order to agree with the optical signals of the uniaxial phases at their borders: the optical signal is positive at the frontiers with the phase and it turns to be negative at the frontiers with the phase. Concomitantly, the optical axes rotates from the direction to the direction. We will illustrate these findings with experimental data that confirm our results.

Exact equations of state for nematics

We propose a novel approach to the solution of nematic Liquid Crystal models based on the derivation of a system of nonlinear wave equations for order parameters such that the occurrence of uniaxial and biaxial phase transitions can be interpreted as the propagation of a two-dimensional shock wave in the space of thermodynamic parameters. We obtain the exact equations of state for an integrable model of biaxial nematic liquid crystals and show that the classical transition from isotropic to uniaxial phase in absence of external fields is the result of a van der Waals type phase transition, where the jump in the order parameters is a classical shock generated from a gradient catastrophe at a non-zero isotropic field. The study of the equations of state provides the first analytical description of the rich structure of nematics phase diagrams in presence of external fields.

Considerations in the determination of orientational order parameters from X-ray scattering experiments

ABSTRACTAn assessment of the data processing and analysis methods used to obtain the second- and fourth-rank orientational order parameters of liquid crystals from X-ray scattering experiments has been carried out, using experimental data from four extensively studied alkyl-cyanobiphenyls and calculated data generated from two general types of theoretical orientational distribution function. The application of a background subtraction and two different baseline correction methods to the scattering profiles is assessed, along with three different methods to analyse the processed data. The choice of baseline correction method is shown to have a significant effect: an offset to zero overestimates the order parameters from the experimental and calculated data sets, particularly for lower order parameters arising from broad distributions, whereas an offset to a value estimated from regions of low scattering intensity provides experimental values close to those reported from other experimental techniques. By contrast, the three different analysis methods are shown generally to result in relatively small absolute differences between the order parameters. We outline a straightforward general approach to experimental X-ray scattering data processing and analysis for uniaxial phases that results in order parameters that match well with those reported using other experimental techniques.

Uniaxial and biaxial structures in the elastic Maier-Saupe model.

We perform statistical mechanics calculations to analyze the global phase diagram of a fully connected version of a Maier-Saupe-Zwanzig lattice model with the inclusion of couplings to an elastic strain field. We point out the presence of uniaxial and biaxial nematic structures, depending on temperature T and on the applied stress σ. Under uniaxial extensive tension, applied stress favors uniaxial orientation, and we obtain a first-order boundary along which there is a coexistence of two uniaxial paranematic phases, and which ends at a simple critical point. Under uniaxial compressive tension, stress favors biaxial orientation; for small values of the coupling parameters, the first-order boundary ends at a tricritical point, beyond which there is a continuous transition between a paranematic and a biaxially ordered structure. For some representative choices of the model parameters, we obtain a number of analytic results, including the location of critical and tricritical points and the line of stability of the biaxial phase.