Matter & Energy
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Metal crust, superconductor filling
NEWS BRIEFS
had conclusively proven that this superconductivity is any different from the
run-of-the-mill variety discovered in
mercury a century ago.
To probe the material, Yoichi Ando
of Osaka University and colleagues in
Japan injected current into it using a
gold wire. This excited electrons at the
surface, creating ripples of energy. Conventional superconductors have a dead
spot in their surfaces that prevents low-energy, slow-wobbling ripples from
forming. But a close look at this material revealed a sea of waves moving both
quickly and slowly.
Ando says that this pattern of
ripples is “unambiguous
evidence” of a type of
superconductivity never
seen before: topological
superconductivity, in
which electrons become
waves molded into a complex shape that resembles the outside
of a doughnut. These waves, says Ando,
seem to be behaving like exotic two-dimensional particles at the surface
of the material — specifically, Majorana
fermions.
“This is the best evidence so far for
Majorana fermions in a solid material,” says Taylor Hughes, a theoretical
physicist at the University of Illinois
at Urbana-Champaign. Still, the new
experiment didn’t actually reveal the flat
fermions themselves — only their supposed influence. No one has yet figured
out how to directly detect them in solids. So it’s probably going to take many
sources of indirect evidence to make
the case that Majorana fermions actually exist in this material, says Hughes.
One way to test the idea would be to
use magnetic fields to create swirling
vortices on the material’s surface. These
vortices should be able to trap a Majorana fermion.
Ando says that this pattern of
ripples is “unambiguous
which electrons become
waves molded into a com-
By Devin Powell
A new kind of superconductor can’t
make up its mind about how to conduct
electricity. Current passes through its
interior without any resistance, as in
a typical superconductor. But its skin
behaves like a metal, conducting electricity but with some resistance.
This split personality, described in the
Nov. 18 Physical Review Letters, could
be the handiwork of something strange
hiding on the surface — a two-dimensional
entity behaving like a Majorana fermion.
First proposed more than 70 years ago,
a Majorana fermion is a theoretical type of particle that
is its own antiparticle.
Electrons and quarks
and other particles of
matter all have oppositely charged antimatter
partners.
Some theorists who suspect that neutrinos are their own antiparticles would
be excited to find evidence that anything
can act like a Majorana fermion, even the
surface of the superconductor in the new
study. Others hope that such particles
could be useful for storing information
in new kinds of computers.
“This is great,” says Robert Cava, a
chemist at Princeton University who
wasn’t involved with the study. “Hope-
fully it will get people excited about
this material.”
Cava and colleagues were the first to
create the material, made of copper, bis-
muth and selenium layered like lasagna.
They showed that it’s a superconductor
at temperatures within a few degrees
of absolute zero. But until now, no one
Diamonds are forever
(entangled)
Tiffany’s take note: Physicists
have succeeded in joining two
tiny diamonds together in the
bizarre quantum state known as
entanglement. Usually demon-
strated in ultracold, microscopic
systems involving atoms or par-
ticles of light, entanglement links
two objects in a sort of quantum
embrace such that measuring
the state of one instantly reveals
the state of the other, even if they
are separated. Researchers in
England, Canada and Singapore
have now linked the vibrational
states of two millimeter-sized
diamonds at room temperature in
the lab. The work, reported in the
Dec. 2 Science, could point the way
to using macroscopic objects in
quantum information processing.
—Alexandra Witze
This material may contain exotic par-
ticles that could explain why the stuff
behaves like a metal at the surface and
like a superconductor on the inside.
How water chills out
A new study reveals why – 48°
Celsius is the lowest temperature
at which pure water can remain
a liquid. Ice formation in such
supercooled water happens too
quickly to watch in the lab. But
using a computer simulation, sci-
entists at the University of Utah
in Salt Lake City show that water
molecules suddenly come together
in groups of four at about –41°.
This change to an unstable icelike
liquid drives the creation of ice
crystals, a process that happens
quickly at ;rst and then slows
down as the temperature drops.
The slowdown suggests that ice
in clouds forms more slowly than
previously thought, the research-
ers report in the Nov. 24 Nature.
— Devin Powell
