News

1. New theory of electron liquid crystals

Four decades after Pierre de Gennes transformed the theory of liquid crystals using ideas from the study of low-temperature superconductors, liquid crystal science may be about to return the favour with their high-temperature cousins.

High-temperature superconductors operate at hotter temperatures than the 30 K limit of the Bardeen, Cooper and Schrieffer ‘BCS’ theory of superconductivity, which was applied universally before their discovery in the 1980s. Since then, understanding them theoretically has been a prime goal in condensed matter physics. One observation was that the electron wavefunctions can break the orientational symmetry of the crystal, and can also form stripes which break its translational symmetry – analogous to nematics and smectics, respectively. This happens particularly in the ‘pseudogap’ state, which is electronically similar to the superconducting state but does not superconduct. It occurs at a higher temperature and is thought to compete with the superconducting phase.

A group of researchers based in the United States, Holland and Japan studied fluctuations in these orderings in the pseudogap state using electron microscopy. They found numerous topological defects in the smectic ordering and they also found a correlation between the orderings; smectic defects occurred in areas where the nematic ordering was close to the mean value. Inspired by the very same paper where de Gennes drew his analogy with low-temperature superconductivity, the researchers developed a Ginzburg–Landau theory of the coupling between the orderings which explained their results. They conclude with the hope that this might be the first step towards understanding ‘the interplay between the different broken electronic symmetries and the superconductivity’.

Nor is this the only liquid crystals phenomenon found recently in condensed matter. In July a team from Karlsruhe in Germany published for the first time a conclusive observation of an analogue of the liquid crystalline blue phase in a chiral magnetic material. And these results follow simulations from the University of Slovenia earlier in the year showing that a lattice of skyrmions, a topological entity found in certain condensed matter systems, could be seen in a thin film of chiral nematic liquid crystal. Clearly there is still much to be gained from the exchange of ideas between these fields.

2. Oil, crowds and a 30-year homework assignment

Ideas from liquid crystal research have been heading further afield than other parts of physics.

Back in the 1970s, Peter Palffy-Muhoray of Kent State University decided that he would like to model the molecules of a liquid crystalline material as ellipsoids, rather than the cylinders or spherocylinders generally used. The dominant interaction in a thermotropic liquid crystal being the excluded volume repulsion between molecules, this involved calculating their distance of closest approach for a given angle between their major axes. However, he was shocked by how difficult this was – in fact, even in two dimensions, for ellipses there was no analytical solution known to mathematics.

At a major liquid crystals conference in 1983, he offered a good bottle of whisky to whoever could solve the problem, but despite its appearance as a ‘high-school geometry homework assignment’ the whisky went undrunk. Finally, with the help of Xiaoyu Zheng from the Kent State maths department, the two-dimensional problem was solved analytically in 2006. Using this result, in 2008 they and Wilder Iglesias (also at Kent State) devised a simple algorithm for the three-dimensional case.

It turns out, though, that liquid crystal theorists are not the only people interested in touching ellipsoids. Since 2009, the team have had numerous inquiries about their method. One person wanted to use the result to model pedestrian traffic. Another was from a senior geologist at the Chesapeake Energy Corporation. They drill horizontal oil wells, which they wish to avoid crashing into each other and the probability distribution of the well's position is ellipsoidal.

3. Liquid crystal adjustable glasses arrive in America

The first of the EmPower range of adjustable focus glasses by company PixelOptics went on sale in parts of America this June. At the touch of a button, the glasses can switch between near and far focus modes, using a film of liquid crystals embedded in the lens. Throughout the rest of the year the product will be rolled out across the rest of the United States, with the rest of the world following soon after.

In the last published design from the company's academic partners at the University of Arizona, applying a voltage across patterned indium-tin-oxide electrodes turns a 5 μm nematic layer into a Fresnel lens. Two of these layers are required, with planar surface alignments orthogonal to each other, to focus unpolarised light. The glasses have an inbuilt battery which lasts two to three days after several hours of charging, and are retailing at around U$1100.

4. E-paper loses the ‘E’

A liquid-crystal-based e-paper has been developed which requires no electrical connections for reading or writing. The paper, called ‘i2R’ and made by researchers at the Industrial Technology Research Institute in Taiwan, consists of a layer of a chiral nematic on a plastic film. Like old fax-machine paper, it darkens in response to heat, and can be written to using a thermal printer. It can be erased by applying a voltage.

The paper can be rewritten over 200 times and has been costed at US$2 for an A4 sheet. They plan to market their work in the next two years (although the company in the previous story said the same thing in 2006) and name shop labels and tickets on public transport as early targets.

Nicholas Kasch

School of Physics and Astronomy

University of Manchester

UK