Field-Dependent Tilt and Birefringence of Electroclinic Liquid Crystals: Theory and Experiment

An unresolved issue in the theory of liquid crystals is the molecular basis of the electroclinic effect in the smectic-A phase. Recent x-ray scattering experiments suggest that, in a class of siloxane-containing liquid crystals, an electric field changes a state of disordered molecular tilt in random directions into a state of ordered tilt in one direction. To investigate this issue, we measure the optical tilt and birefringence of these liquid crystals as functions of field and temperature, and we develop a theory for the distribution of molecular orientations under a field. Comparison of theory and experiment confirms that these materials have a disordered distribution of molecular tilt directions that is aligned by an electric field, giving a large electroclinic effect. It also shows that the net dipole moment of a correlated volume of molecules, a key parameter in the theory, scales as a power law near the smectic-A--smectic-C transition.Comment: 18 pages, including 9 postscript figures, uses REVTeX 3.0 and epsf.st

SiO2 glass density to lower-mantle pressures

The convection or settling of matter in the deep Earth’s interior is mostly constrained by density variations between the different reservoirs. Knowledge of the density contrast between solid and molten silicates is thus of prime importance to understand and model the dynamic behavior of the past and present Earth. SiO2 is the main constituent of Earth’s mantle and is the reference model system for the behavior of silicate melts at high pressure. Here, we apply our recently developed x-ray absorption technique to the density of SiO2 glass up to 110 GPa, doubling the pressure range for such measurements. Our density data validate recent molecular dynamics simulations and are in good agreement with previous experimental studies conducted at lower pressure. Silica glass rapidly densifies up to 40 GPa, but the density trend then flattens to become asymptotic to the density of SiO2 minerals above 60 GPa. The density data present two discontinuities at ∼17 and ∼60  GPa that can be related to a silicon coordination increase from 4 to a mixed 5/6 coordination and from 5/6 to sixfold, respectively. SiO2 glass becomes denser than MgSiO3 glass at ∼40  GPa, and its density becomes identical to that of MgSiO3 glass above 80 GPa. Our results on SiO2 glass may suggest that a variation of SiO2 content in a basaltic or pyrolitic melt with pressure has at most a minor effect on the final melt density, and iron partitioning between the melts and residual solids is the predominant factor that controls melt buoyancy in the lowermost mantle

Superconducting phase diagram and FFLO signature in λ\lambda-(BETS)2_2GaCl4_4 from rf penetration depth measurements

We report the phase diagram of $\lambda$-(BETS)$_2$GaCl$_4$ from rf penetration depth measurements with a tunnel diode oscillator in a pulsed magnetic field. We examined four samples with 1100 field sweeps in a range of angles with the magnetic field parallel and perpendicular to the conducting planes. In the parallel direction, $H_{c2}$ appears to include a tricritical point at 1.6 K and 10 T with a phase line that increases to 11 T as the temperature is decreased to} 500 mK. The second phase line forms a clearly defined high field low temperature region satisfying several of the conditions of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We show remarkably good fits of $H_{c2}$ to WHH in the reentrant $\alpha>1$, $\lambda_{so}=0$ regime. We also note a sharp angle dependence of the phase diagram about the field parallel orientation that characterizes Pauli paramagnetic limiting and further supports the possibility of FFLO behavior. Unrelated to the FFLO study, at fields and temperatures below $H_{c2}$ and $T_c$, we find rich structure in the penetration depth data that we attribute to impurities at the surface altering the superconducting properties while maintaining the same crystallographic axes as $H_{c2}$.Comment: Fina

Characterization of carbon nanotube–thermotropic nematic liquid crystal composites

Dispersions of carbon nanotubes (CNTs) in liquid crystals (LCs) have attracted attention due to their unique properties and possible applications in photonics and electronics. However, these are hard to stabilize, and the loading level in the equilibrium state in LC hosts is small. A practical way to monitor the quality and CNT incorporation in such equilibrium dispersions is required. Here, we compare different methods for characterising equilibrium CNT–LC composite materials