Weyl points, first predicted in 1929, observed for the first time.
(July 16, 2015) Part
of a 1929 prediction by physicist Hermann Weyl — of a kind of massless particle
that features a singular point in its energy spectrum called the “Weyl point” —
has finally been confirmed by direct observation for the first time, says an
international team of physicists led by researchers at MIT. The finding could
lead to new kinds of high-power single-mode lasers and other optical devices,
the team says.
For decades, physicists thought that the subatomic particles
called neutrinos were, in fact, the massless particles that Weyl had predicted
— a possibility that was ultimately eliminated by the 1998 discovery that
neutrinos do have a small mass. While thousands of scientific papers have been
written about the theoretical particles, until this year there had seemed
little hope of actually confirming their existence.
“Every single paper written about Weyl points was
theoretical, until now,” says Marin Soljačić, a professor of physics at MIT and
the senior author of a paper published this week in the journal Science
confirming the detection. (Another team of researchers at Princeton University
and elsewhere independently made a different detection of Weyl particles; their
paper appears in the same issue of Science).
Ling Lu, a research scientist at MIT and lead author of that
team’s paper, says the elusive points can be thought of as equivalent to
theoretical entities known as magnetic monopoles. These do not exist in the
real world: They would be the equivalent of cutting a bar magnet in half and
ending up with separate north and south magnets, whereas what really happens is
you end up with two shorter magnets, each with two poles. But physicists often
carry out their calculations in terms of momentum space (also called reciprocal
space) rather than ordinary three-dimensional space, Lu explains, and in that
framework magnetic monopoles can exist — and their properties match those of
Weyl points.
The achievement was made possible by a novel use of a
material called a photonic crystal. In this case, Lu was able to calculate
precise measurements for the construction of a photonic crystal predicted to
produce the manifestation of Weyl points — with dimensions and precise angles
between arrays of holes drilled through the material, a configuration known as
a gyroid structure. This prediction was then proved correct by a variety of
sophisticated measurements that exactly matched the characteristics expected
for such points.