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Register a new userThermodynamically Stable Blue Phases
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We show theoretically that flexoelectricity stabilizes blue phases in chiral liquid crystals. Induced internal polarization reduces the elastic energy cost of splay and bend deformations surrounding singular lines in the director field. The energy of regions of double twist is unchanged. This in turn reduces the free energy of the blue phase with respect to that of the chiral nematic phase, leading to stability over a wider temperature range. The theory explains the discovery of large temperature range blue phases in highly flexoelectric bimesogenic and bent-core materials, and predicts how this range may be increased further.
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Blue phases are networks of disclination lines, which occur in cholesteric liquid crystals near the transition to the isotropic phase. They have recently been used for the new generation of fast switching liquid crystal displays. Here we study numerically the steady states and switching hydrodynamics of blue phase I (BPI) and blue phase II (BPII) cells subjected to an electric field. When the field is on, there are three regimes: for very weak fields (and strong anchoring at the boundaries) the blue phases are almost unaffected, for intermediate fields the disclinations twist (for BPI) and unzip (for BPII), whereas for very large voltages the network dissolves in the bulk of the cell. Interestingly, we find that a BPII cell can recover its original structure when the field is switched off, whereas a BPI cell is found to be trapped more easily into metastable configurations. The kinetic pathways followed during switching on and off entails dramatic reorganisation of the disclination networks. We also discuss the effect of changing the director field anchoring at the boundary planes and of varying the direction of the applied field.
Semiflexible polymer models are widely used as a paradigm to understand structural phases in biomolecules including folding of proteins. Since stable knots are not so common in real proteins, the existence of stable knots in semiflexible polymers has not been explored much. Here, via extensive replica exchange Monte Carlo simulation we investigate the same for a bead-stick and a bead-spring homopolymer model that covers the whole range from flexible to stiff. We establish the fact that the presence of stable knotted phases in the phase diagram is dependent on the ratio $r_b/r_{rm{min}}$ where $r_b$ is the equilibrium bond length and $r_{rm{min}}$ is the distance for the strongest nonbonded contacts. Our results provide evidence for both models that if the ratio $r_b/r_{rm{min}}$ is outside a small window around unity then depending on the bending stiffness one always encounters stable knotted phases along with the usual frozen and bent-like structures at low temperatures. These findings prompt us to conclude that knots are generic stable phases in semiflexible polymers.
We predict theoretically a carbon-based clathrate in the bipartite sodalite structure, SrB3C3, that is thermodynamically stable at high pressure. This clathrate is predicted to be a dynamically stable superconductor with an estimated Tc of 42 K at ambient pressure. Calculated stress-strain relations for SrB3C3 clathrate demonstrate its intrinsic hard nature with Vickers hardness of 24-31 GPa. Boron substitution aids in the stabilization of SrB3C3 clathrate, and offers valuable insights into design guidelines for various carbon-based materials.
We report on a construction for smectic blue phases, which have quasi-long range smectic translational order as well as long range cubic or hexagonal order. Our proposed structures fill space with a combination of minimal surface patches and cylindrical tubes. We find that for the right range of material parameters, the favorable saddle-splay energy of these structures can stabilize them against uniform layered structures.
The appropriateness of including Hg among the transition metals has been debated for a long time. Although the synthesis of HgF$_{4}$ molecules in gas phase was reported before, the molecules show strong instabilities and dissociate. Therefore, the transition metal propensity of Hg remains an open question. Here, we propose that high pressure provides a controllable method for preparing unusual oxidation states of matter. Using an advanced structure search method based on first-principles electronic structure calculations, we predict that under high pressures, Hg can transfer the electrons in its outmost $d$ shell to F atoms, thereby acting as a transition metal. Oxidation of Hg to the +4 state yielded thermodynamically stable molecular crystals consisting of HgF$_{4}$ planar molecules, a typical geometry for $d^{8}$ metal centers.
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