Nematic Phase Transition Research Articles

A molecular field approach to pressure-induced phase transitions in liquid crystals: Smectic–nematic transition

Since a rigorous microscopic treatment of a nematic fluid system based on a pairwise interaction potential is immensely complex, we had introduced a simple mean field potential, which was a modification of the Maier–Saupe potential in a previous paper [S. DasGupta et al., “Pressure-induced phase transitions in liquid crystals: A molecular field approach,” Phys. Rev. E 98, 022701 (2018)]. Building upon that, here we have modified that potential to take into account the various aspects of a smectic A–nematic phase transition. In particular, we have studied the dependence of the phase transition on the coupling coefficient between the nematic and smectic order parameters, which in turn depends on the length of alkyl chain, variation of density, entropy, and specific heat. Detailed investigation on the coupling parameter shows the existence of a smectic A–nematic–isotropic triple point as well as a tricritical point where the smectic–nematic phase transition changes its nature from the second to the first order. It is also seen that the application of pressure can result in the appearance of a nematic phase.

Mössbauer spectroscopy study of nematicity in Ba(Fe0.962Cu0.038)2As2 single crystal: enhanced orbital effect

The origin of the nematic order remains unclear due to the strong coupling between orbital, spin and lattice degrees of freedom in iron-based superconductors. Although the driving force of hole-doped BeFe2As2 is still controversial, the nematic fluctuation of electron-doped compounds is generally believed to be spin fluctuation driven. Here, we present a comprehensive study of the nematic phase transition in Ba(Fe0.962Cu0.038)2As2 single crystal by using Mössbauer spectroscopy. The electric field gradient and its in-plane asymmetry on Fe nucleus, which are directly determined by the occupation of individual t 2g orbital, are sensitive to the local nematicity of Fe ions. The nematic phase transition happens at T nem ≈ 73.8 K in the compound while the band splitting between d xz /d yz orbitals begins far above T nem and reaches 18.8 meV at 30 K. The temperature evolution of the hyperfine parameters proves the existence of electron–phonon interaction and non-Fermi-liquid behaviour near T nem. However, the spin–lattice relaxation signal is only evident below T nem. These observations show that the role of orbital degrees of freedom is more active in driving nematicity than in Co- or Ni-doped BaFe2As2 compounds, and can be attributed to enhanced electronic localization caused by Cu doping.

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.

Effect of the surfactant head-group size dependence of the dye-surfactant interactions on the lyotropic uniaxial to biaxial nematic phase transitions

In the present study, the interactions of the anionic azo dye Sunset Yellow with dodecylalkylammonium bromide (DAABr) surfactants in lyotropic mixtures have been investigated for the stabilization of the different lyotropic nematic phases. Sunset Yellow doped mixtures of DAABr surfactants, with different head-group size, were studied by polarizing optical microscopy, laser conoscopy, and small-angle X-ray scattering. It was found that the extent of the interaction of Sunset Yellow with DAABr surfactant head groups on micelle surfaces affects the stabilization of different nematic phases, by causing the change in the micellar structural parameters. The results also indicated that the micelle surface curvature should be different in the three nematic phases.

Quantum coherence and spin nematic to nematic quantum phase transitions in biquadratic spin-1 and spin-2 XY chains with rhombic single-ion anisotropy

We investigate quantum phase transitions and quantum coherence in infinite biquadratic spin-1 and -2 XY chains with rhombic single-ion anisotropy. All considered coherence measures such as the ${l}_{1}$ norm of coherence, the relative entropy of coherence, and the quantum Jensen-Shannon divergence, and the quantum mutual information show consistently that singular behaviors occur for the spin-1 system, which enables to identify quantum phase transitions. For the spin-2 system, the relative entropy of coherence and the quantum mutual information properly detect no singular behavior in the whole system parameter range, while the ${l}_{1}$ norm of coherence and the quantum Jensen-Shannon divergence show a conflicting singular behavior of their first-order derivatives. Examining local magnetic moments and spin quadrupole moments lead to the explicit identification of novel orderings of spin quadrupole moments with zero magnetic moments in the whole parameter space. We find the three uniaxial spin nematic quadrupole phases for the spin-1 system and the two biaxial spin nematic phases for the spin-2 system. For the spin-2 system, the two orthogonal biaxial spin nematic states are connected adiabatically without an explicit phase transition, which can be called quantum crossover. The quantum crossover region is estimated by using the quantum fidelity. Whereas for the spin-1 system, the two discontinuous quantum phase transitions occur between three distinct uniaxial spin nematic phases. We discuss the quantum coherence measures and the quantum mutual information in connection with the quantum phase transitions including the quantum crossover.

Robust Fermi-Surface Morphology of CeRhIn_{5} across the Putative Field-Induced Quantum Critical Point.

We report a comprehensive de Haas-van Alphen (dHvA) study of the heavy-fermion material CeRhIn_{5} in magnetic fields up to 70T. Several dHvA frequencies gradually emerge at high fields as a result of magnetic breakdown. Among them is the thermodynamically important β_{1} branch, which has not been observed so far. Comparison of our angle-dependent dHvA spectra with those of the non-4f compound LaRhIn_{5} and with band-structure calculations evidences that the Ce 4f electrons in CeRhIn_{5} remain localized over the whole field range. This rules out any significant Fermi-surface reconstruction, either at the suggested nematic phase transition at B^{*}≈30 T or at the putative quantum critical point at B_{c}≃50 T. Our results rather demonstrate the robustness of the Fermi surface and the localized nature of the 4f electrons inside and outside of the antiferromagnetic phase.

Signatures of a quantum Griffiths phase close to an electronic nematic quantum phase transition

In the vicinity of a quantum critical point, quenched disorder can lead to a quantum Griffiths phase, accompanied by an exotic power-law scaling with a continuously varying dynamical exponent that diverges in the zero-temperature limit. Here, we investigate a nematic quantum critical point in the iron-based superconductor FeSe0.89S0.11 using applied hydrostatic pressure. We report an unusual crossing of the magnetoresistivity isotherms in the non-superconducting normal state which features a continuously varying dynamical exponent over a large temperature range. We interpret our results in terms of a quantum Griffiths phase caused by nematic islands that result from the local distribution of Se and S atoms. At low temperatures, the Griffiths phase is masked by the emergence of a Fermi liquid phase due to a strong nematoelastic coupling and a Lifshitz transition that changes the topology of the Fermi surface.

Optical and thermal characterization of thermotropic hydrogen bonded liquid crystal mixture

A novel intermolecular hydrogen-bonded liquid crystal mixture (HBLC) has been synthesized from non-mesogenic itaconic acid (IA) and mesogenic 4-undecyloxybenzoic acid (11OBA). The formation of hydrogen bond has been analyzed by using spectroscopic studies. Polarizing optical microscope (POM) confirms the existence of liquid crystal phase corresponding to the transition temperature. LC transition temperature, enthalpy values, stability factors, and thermal width have been calculated using thermal analyzer. Band gap energies are evaluated by using ultraviolet visible and photoluminescence spectroscopy studies. The resultant POM and DSC studies reveal the present mixture enclose nematic and smectic mesogenic phase transitions. Induced thermoluminescence with parachromic variations are observed and represented as novel detection in the present HBLC mixture. This synthesized mixture is more suitable for HD display devices.

Colloidal aggregation in anisotropic liquid crystal solvent.

The mutual attraction between colloidal particles in an anisotropic fluid, such as the nematic liquid crystal phase, leads to the formation of hierarchical aggregate morphologies distinct from those that tend to form in isotropic fluids. Previously it was difficult to study this aggregation process for a large number of colloids due to the difficulty of achieving a well dispersed initial colloid distribution under good imaging conditions. In this paper, we report the use of a recently developed self-assembling colloidal system to investigate this process. Hollow, micron-scale colloids are formed in situ in the nematic phase and subsequently aggregate to produce fractal structures and colloidal gels, the structures of which are determined by colloid concentration and temperature quench depth through the isotropic to nematic phase transition point. This self-assembling colloidal system provides a unique method to study particle aggregation in liquid crystal over large length scales. We use fluorescence microscopy over a range of length scales to measure aggregate structure as a function of temperature quench depth, observe ageing mechanisms and explore the driving mechanisms in this unique system. Our analyses suggest that aggregate dynamics depend on a combination of Frank elasticity relaxation, spontaneous defect line annihilation and internal aggregate fracturing.

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.

Calorimetric study of critical behaviour near the smectic A to nematic phase transition in a polar-polar binary system

ABSTRACT Studies on critical behaviour associated with different phase transitions are of fundamental importance in condensed matter physics and may suitably be explored in the realm of soft materials. In this work, we report on high-resolution measurements of specific heat capacity (C p) from modulated differential scanning calorimetry (MDSC) to investigate the critical behaviour at the smectic A to nematic (SmA–N) phase transition in a binary system consisting of nonyloxy-cyanobiphenyl (9OCB) and heptylcyanobiphenyl (7CB). All the investigated mixtures exhibit detectable SmA–N heat capacity anomaly with gradually increasing peak height with enhancement in the concentration of 9OCB. Critical exponent α, obtained from the analysis of temperature dependence of the specific heat capacity data at the SmA–N phase transition, has been observed to assume values between those predicted for the 3D-XY and tricritical systems. The outcomes are also compared with those obtained from the high-resolution optical birefringence measurements.

Effect of glycerol on the order of the mesophase transitions of supercooled itraconazole

Itraconazole, an antifungal drug, is a thermotropic liquid crystal that exhibits nematic (N) and smectic A (SmA) phases, when cooled from the melt. By means of high-resolution adiabatic scanning calorimetry (ASC), we have obtained the temperature dependence of the heat capacity as well as the enthalpy (including latent heats) of the nematic to smectic A (N-SmA) and the isotropic to nematic (I-N) phase transitions. The N-SmA transition is weakly first-order, with substantial pretransitional heat capacity increases. The critical exponent α, obtained from power law fits to the heat capacity data, is 0.50 ± 0.05, suggesting that the N-SmA transition must be very close to a tricritical point. Indeed, with this character, the small molecule dopant glycerol (in binary mixtures with ITZ), causes interesting changes to the mesomorphic phase sequence and to the order of the phase transitions. With increasing glycerol content, the temperature width of the nematic phase systematically reduces, until a critical concentration, at which the nematic phase disappears, leading to a direct isotropic-smectic A (I-SmA) transition. The I-SmA transitions of the ITZ-glycerol mixtures show stronger first-order character with substantial latent heats and wide two-phase regions, when compared to the (N-SmA and I-N) transitions of neat itraconazole. The ability of glycerol to drive the ITZ transitions to stronger first-order character indicates a possible coupling of the additive concentration to the smectic order parameter, which leads to the development of highly ordered, stable smectic structures.

Prediction of the density–pressure–temperature behavior in the isotropic–nematic phase transition of MBBA liquid crystal

In this work, it is proposed a perturbation theory in conjunction with the Convex Peg HERSW model and an adjustable intermolecular potential. The perturbation theory developed can quantitatively predict the density–pressure–temperature experimental behavior at the isotropic–nematic phase transition of MBBA at 1 atm.

Biaxiality-driven twist-bend to splay-bend nematic phase transition induced by an electric field.

Although the existence of the twist-bend (NTB) and splay-bend (NSB) nematic phases was predicted long ago, only the former has as yet been observed experimentally, whereas the latter remains elusive. This is especially disappointing because the NSB nematic is promising for applications in electro-optic devices. By applying an electric field to a planar cell filled with the compound CB7CB, we have found an NTB-NSB phase transition using birefringence measurements. This field-induced transition to the biaxial NSB occurred, although the field was applied along the symmetry axis of the macroscopically uniaxial NTB Therefore, this transition is a counterintuitive example of breaking of the macroscopic uniaxial symmetry. We show by theoretical modeling that the transition cannot be explained without considering explicitly the biaxiality of both phases at the microscopic scale. This strongly suggests that molecular biaxiality should be a key factor favoring the stability of the NSB phase.

Electronic and Magnetic Anisotropies in FeSe Family of Iron-Based Superconductors

Most parent compounds of iron-based superconductors (FeSCs) exhibit a tetragonal-to-orthorhombic lattice distortion below Ts associated with an electronic nematic phase that breaks the four-fold (C4) rotational symmetry of the underlying lattice, and then forms collinear antiferromagnetic (AF) below TN (TN ≤ Ts). Optimal superconductivity emerges upon suppression of the nematic and AF phases. FeSe, which also exhibits a nematic phase transition below Ts but becomes superconducting in the nematic phase without AF order, provides a unique platform to study the interplay amongst the nematic phase, AF order, and superconductivity. In this review, we focus on the experiments done on uniaxial pressure detwinned single crystals of FeSe and other FeSCs and highlight the importance of understanding the electronic and magnetic anisotropy in elucidating the nature of unconventional superconductivity.

Effect of the anionic azo dye Sunset Yellow in lyotropic mixtures with uniaxial and biaxial nematic phases

The effects of the anionic azo dye Sunset Yellow on the stabilization of lyotropic uniaxial and biaxial nematic phases and on the uniaxial to biaxial phase transitions were studied. The dye was added in different concentrations to the host ternary mixture of cationic surfactant dodecyltrimethylammonium bromide, 1-dodecanol, and water. Furthermore, an investigation examined the role of the dye in the lyotropic host mixture in comparison to some inorganic salts containing anions of the Hofmeister series. The following inorganic salts were added to the same host mixture: NaBr, NaNO3, NaClO3, NaI, NaSCN, and NaClO4. All the samples were investigated by polarizing optical microscopy, laser conoscopy, and small-angle X-ray scattering. The results indicated that (a) the anionic dye Sunset Yellow has a chaotropic character between that of the I− and SCN− ions, being similar to the last one, (b) it gives a larger biaxial nematic phase temperature range in the partial phase diagram in comparison with the SCN− ion and other inorganic salts, and (c) it shifts the temperatures of both uniaxial to the biaxial nematic phase transitions. The small-angle X-ray scattering results also indicated that, at least in the investigated concentration range, the dye is mainly located on the micelles' surfaces, just like a conventional ion, and that on increasing concentration the dye molecules induces higher aggregation number of the micelles.

Spontaneous Alignment of Self‐Assembled Cationic and Amphiphilic β‐Sheet Peptides

AbstractAligned peptide assemblies are of interest for both fundamental understanding of biological systems as well as development of novel biomaterials. Herein, Pro‐Lys‐(Phe‐Lys)5‐Pro (PFK), a cationic and amphiphilic β‐sheet synthetic peptide that self‐assembles into fibrils, capable of undergoing spontaneous isotropic to nematic phase transition, that intrinsically aligns in solution, is presented. Upon dissolution PFK forms isotropically dispersed fibrils due to delicate balance between attractive H bonds and hydrophobic interactions to electrostatic repulsive forces. The transition from isotropic to nematic phase alignment occurring over a period of several days is monitored in situ by both small‐angle X‐ray scattering and cryogenic transmission electron microscopy. The degree of alignment and the time required to achieve a complete isotropic–nematic phase transition of PFK fibrils in aqueous solution is found to be concentration dependent as predicted by Onsager's excluded volume theory. Remarkably, PFK nematic phases remain aligned over the course of several months without the application of any external stimuli. Aligned PFK gels can be formed by mild shearing through a salty medium. This system provides a novel route for producing nanoscale aligned materials for potential biomedical applications.

Impact of nematicity on the relationship between antiferromagnetic fluctuations and superconductivity in FeSe0.91S0.09 under pressure

The sulfur substituted FeSe system, FeSe$_{1-x}$S$_{x}$, provides a versatile platform for studying the relationship between nematicity, antiferromagnetism, and superconductivity. Here, by nuclear magnetic resonance (NMR) and resistivity measurements up to 4.73 GPa on FeSe$_{0.91}$S$_{0.09}$, we established the pressure($p$)-temperature($T$) phase diagram in which the nematic state is suppressed with pressure showing a nematic quantum phase transition (QPT) around $p$ = 0.5 GPa, two SC regions, separated by the QPT, appear and antiferromagnetic (AFM) phase emerges above $\sim$3.3 GPa. From the NMR results up to 2.1 GPa, AFM fluctuations are revealed to be characterized by the stripe-type wavevector which remains the same for the two SC regions. Furthermore, the electronic state is found to change in character from non-Fermi liquid to Fermi liquid around the nematic QPT and persists up to $\sim$ 2.1 GPa. In addition, although the AFM fluctuations correlate with $T_{\rm c}$ in both SC states, demonstrating the importance of the AFM fluctuations for the appearance of SC in the system, we found that, when nematic order is absent, $T_{\rm c}$ is strongly correlated with the AFM fluctuations, whereas $T_{\rm c}$ weakly depends on the AFM fluctuations when nematic order is present. Our findings on FeSe$_{0.91}$S$_{0.09}$ were shown to be applied to the whole FeSe$_{1-x}$S$_{x}$ system and also provide a new insight into the relationship between AFM fluctuations and SC in Fe-based superconductors.

Momentum-resolved measurement of electronic nematic susceptibility in the FeSe0.9S0.1 superconductor

Unveiling the driving force for a phase transition is normally difficult when multiple degrees of freedom are strongly coupled. One example is the nematic phase transition in iron-based superconductors. Its mechanism remains controversial due to a complex intertwining among different degrees of freedom. In this paper, we report a method for measuring the nematic susceptibly of FeSe$_{0.9}$S$_{0.1}$ using angle-resolved photoemission spectroscopy (ARPES) and an $in$-$situ$ strain-tuning device. The nematic susceptibility is characterized as an energy shift of band induced by a tunable uniaxial strain. We found that the temperature-dependence of the nematic susceptibility is strongly momentum dependent. As the temperature approaches the nematic transition temperature from the high temperature side, the nematic susceptibility remains weak at the Brillouin zone center while showing divergent behavior at the Brillouin zone corner. Our results highlight the complexity of the nematic order parameter in the momentum space, which provides crucial clues to the driving mechanism of the nematic phase transition. Our experimental method which can directly probe the electronic susceptibly in the momentum space provides a new way to study the complex phase transitions in various materials.

‘Quasi-Tricritical’ and Glassy Dielectric Properties of a Nematic Liquid Crystalline Material

Results of dielectric studies in the nematic and isotropic liquid phases of pentylcyanobiphenyl (5CB), a model rod-like liquid crystalline material, are shown. They are based on the discussion of the evolution of dielectric constant ( ε ), its changes under the strong electric field (nonlinear dielectric effect, NDE), and finally, the primary relaxation time. It is shown that changes in ε T and NDE are entirely dominated by the impact of pretransitional fluctuations (pre-nematic and pre-isotropic, respectively) which are associated with the weakly discontinuous character of the isotropic–nematic phase transition. This influence also extends for the low-frequency, ionic species dominated, region. Notable, that the derivative-based and distortions sensitive analysis revealed the tricritical nature of the I-N transition. Although the glass transition in 5CB occurs in the deeply supercooled state at T g ≈ − 68 ° C , the dynamics (changes of the primary relaxation time) follow a previtreous pattern both in the isotropic and in the nematic phase. Finally, the discussion of the ’molecular’ vs. ‘quasi-critical’ characterizations of the isotropic and nematic phases is presented. It shows the evident prevalence of the ‘quasi-critical-picture’, which offers the consistent temperature parameterization in the total tested temperature range.