PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 628 March 13, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

LEFT HANDED MATERIALS (LHM), materials with a negative index of refraction,
can in principle focus light without the need for curved surfaces. The first
observation of such a "meta-material" (consisting of alternating layers of
metal rods and "C" shaped rings lodged on a honeycomb array of printed
circuit boards) came three years ago (see Update 476,
www.aip.org/enews/physnews/2000/split/pnu476-1.htm ).  Then some
theorists said it couldn't be done.  Now scientists at several labs say it
can be done.  At last week's meeting of the American Physical Society (APS)
in Austin, Texas, two labs reported devising LHMs of their own and
demonstrating a negative-index behavior when microwaves were sent into a
wedge-shaped LHM "prism."  A group from MIT (represented at the meeting by
Andrew Houck) said that microwaves entered an LHM sample and, sure enough,
the light waves were refracted according to Snell's law, the classic
equation for prescribing what happens when light goes from one medium into
another, but with a negative sign.  The MIT experiment also provides
evidence that light from a point source can be focused with a flat
rectangular slab of LHM material (see also Houck et al.,  upcoming article
in Physical Review Letters).  Patanjali Parimi (Northeastern Univ.) also
reported at the meeting that his team of scientists had observed
negative-index propagation on microwaves through a LHM sample (for
background and some simple movies, see sagar.physics.neu.edu/ ).
Two theorists present at the meeting, Clifford Krowne (Naval Research Lab)
and Alexandre Pokrovski (Univ. Utah), affirmed that the experimental results
had indeed established the existence of working left handed meta-materials
but that an earlier criterion thought necessary for LHM behavior, namely
that the material's permittivity (a measure of the material's response to an
applied electric field) and its permeability (a measure of the material's
response to an applied magnetic field) both had to be negative, was not
strictly required.  Potential applications in the cell-phone industry alone
are many: LHM devices would be handy for filtering, steering, and focusing
microwaves.  Furthermore, one would expect novel optical effects if negative
index-of-refraction materials could be extended into the visible light
range.

THE GIANT PLANAR HALL EFFECT is the name for a new type of
magnetoresistance (MR) seen in an experiment with ferromagnetic
semiconductors performed by a Caltech-UC Santa Barbara team of physicists.
MR effects are important in the huge magnetic read-head industry (where a
tiny magnetic artifact, such as a magnetic bit written in a storage medium,
is transformed into a large electrical artifact signal, such as an abrupt
change in resistance) and are also central to the development of
spintronics, the new form of electronics in which electron spin and not just
electron charge is instrumental in conducting high-speed transactions. In
the usual Hall effect, current flowing along a planar conductor is slightly
swept to the side when a magnetic field, oriented perpendicular to the
current and to the plane, is turned on. In the Caltech-UCSB experiment, the
applied magnetic field lies in the conducting plane, and the result is to
lower resistivity along several specific directions, encouraging a
corresponding pattern of current flow. This type of anisotropic MR has
previously been seen in magnetic metals, but the effect was weak. In the
present experiment, carried out with a magnetic semiconductor (GaMnAs), the
effect is 10^4 times stronger.  For this reason Michael Roukes
(roukes{at}caltech.edu, 626-395-2916) believes that once the temperature at
which the materials can no longer retain a magnetic orientation (the "Curie
temperature") can be raised to more practical levels (this experiment was
carried out at below 45 K), the giant planar Hall effect could hasten the
onset of better magnetic resonance microscopy and magnetic random access
memory (MRAM) components, advanced magnetic sensors and memory components,
and, perhaps ultimately elements for solid-state quantum computers. (Tang et
al., Physical Review Letters, 14 March 2003, contact also David Awschalom,
awsch{at}physics.ucsb.edu).

DNA FUEL FOR FREE-RUNNING NANOMACHINES. More than just a blueprint for
life, DNA is proving to be one of the most versatile materials in
nanotechnology. A DNA molecule is made from 4 building blocks--the chemical
bases A, C, G, and T.  Nanotechnologists take advantage of the fact that
they can obtain DNA strands with any sequence of bases to design strands
that bind together to make novel structures. G always binds to C, and A is
similarly complementary to T.  Researchers at Bell Labs/Lucent Technologies
and the University of Oxford (contact Bernie Yurke, Yurke{at}lucent.com and
Andrew  Turberfield, a.turberfield{at}physics.ox.ac.uk) have previously
constructed short strands of synthetic DNA that bind together to make a
simple molecular machine---a pair of molecular tweezers that can be opened
and closed by adding additional DNA strands (Yurke et al., Nature, 10 August
2000; see http://www.nature.com/nsu/000810/000810-10.html). Now, they have
made a fuel, consisting of DNA loops, that can act as a source of energy for
DNA-based molecular motors.  The loops react very slowly unless a specially
designed DNA strand is present to catalyze the reaction by forcing loops
open. They propose that this principle could be used to make a molecular
motor (not yet built). The motor would act as a catalyst,pulling open two
complementary DNA loops.  The opened loops would bind to each other,
exerting a force in the process which could for example cause the motor to
rotate or move down a track.  The motor would slowly deplete the DNA
fuel--and run freely until the fuel was exhausted. Possible applications of
artificial molecular motors include nanoscale conveyor belts that carry
molecular cargo in a nanoscale asssembly line. (Turberfield et al., Physical
Review Letters, upcoming article).

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