Number 596, July 2, 2002
by Phil Schewe, James Riordon, and Ben Stein
Liquid Light
Spanish physicists have shown how the photons in a beam of laser light
might be able to condense into "light droplets" with certain
liquidlike properties.
Laser light, passing through a nonlinear optical medium, can undergo
self focusing: the very presence of the intense light, with its strong
electric and magnetic fields, can modify the material's index of refraction,
causing the material to act like a lens.
At some point the streams of laser light making up the beam would have
converged sufficiently to form a condensed state in analogy with the
Van der Waals forces which create liquid drops from a gas cloud. These
"droplets" would not be at rest but would continue to move
at the speed of light.
Humberto Michinel (hmichinel@uvigo.es) and his colleagues at the Universidade
de Vigo, the Universidade de Santiago, and the Chalmers Tekniska Hogskola
(Goteborg, Sweden) argue that the light condensates can be considered
as droplets because his study shows that they have these properties
in common with liquids: they have a surface tension (elastic resistance
to being deflected) and can sustain vortices like those in superfluids.
The light droplets, not yet demonstrated in the lab, would be useful
as robust information bits in future optical computers. (Michinel
et al., Physical Review E, June 2002.)
Spinonics
In electronics the movement of electrons in a circuit can be exploited
to store data, perform calculations, and to excite the playback or broadcast
of music. In spintronics one exploits, in addition to the electron's
charge, the electron's spin.
What if one could have just the spin and not the charge? " Spinonics"
is the term coined by Ganapathy Baskaran of the Institute of Mathematical
Sciences in Madras, India, to describe the manipulation of special chargeless
parcels of spin known as "spinons" (also called "triplet
excitons" when the value of the spin equals 1).
In general collective excitations are to condensed matter physics what
elementary particles are to high energy physics. Spin excitations have
been seen in condensed matter physics before: spin waves are disturbances
which can propagate a spin orientation from one atom to another through
a lattice.
But what Baskaran (baskaran@imsc.ernet.in, 0091-44-254-1856) is proposing
is an actual current of spin moving from place to place. Triplet excitons,
as a packet of spin, can be produced in semiconductors or insulators,
but don't go very far and require special techniques and lasers for
their generation.
Baskaran and his colleague S. A. Jafari predict a spin current could,
however, be created easily and propagate over long distances in graphite
and carbon nanotubes, which are both semi-metals: basically semiconductors
but ones in which the energy gap (the energy difference between electrons
retained by the carbon atoms and electrons free to roam about) is essentially
zero.
The advantages of a spin-only form of transport would include the chance
to explore new quantum effects and a reduction in undesirable scattering
from defects, impurities, and phonons. (Baskaran
and Jafari, Physical Review Letters, 1 July 2002.)
Universal Veins
Despite the stunning diversity of leaf shapes from one plant variety
to another, a universal formula may guide the vein patterns in all leaves.
Researchers at the Laboratorie de Physique Statistique (S. Bohn, 33-1-4432-3447,
bohn@lps.ens.fr) and the Museum National d'Historie Naturalle, both
in Paris, recently studied the networks of veins in leaves from several
plant types.
An analysis of leaf vein networks revealed simple relationships between
the angles that veins form when they intersect and the thickness of
the veins at the intersections. At a three-way junction of similarly
sized veins, the angles between the veins are all roughly 120 degrees
(see image). At
locations where a small vein joins a large vein, however, the angles
between the small vein and the larger one approach 90 degrees.
Imagine, for example, the angles formed by pulling on a light thread
tied to a taut rope. The similarity between a network of threads and
ropes exerting forces on each other and the patterns in leaf veins suggested
to the researchers that there is an underlying mechanism in leaf formation
that is reminiscent of simple mechanics problems.
The model breaks with theories of leaf networks related to soap froths
(which require all intersection angles to equal 120 degrees), crack
propagation (with 90 degree intersections where new cracks meet old
cracks), and Turing diffusion (which leads to patterns that resemble
spreading tree branches, rather than the net-like patterns found in
leaf veins). Some botanists had considered leaf veins as potential tools
for categorizing plants.
The universal similarity of vein structure from one plant to another
suggests that the patterns provide insight to leaf mechanics, but are
of little help in distinguishing or cataloging plants. (S.
Bohn et al., Phys. Rev. E, June 2002.)
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