Pesquisa avançada de Supercondutores

1. Descrevendo supercondutores
Os físicos criam antiferromagnetic que podem ajudar a desenvolver, monitorar
materiais-chave
Physics Professor Markus Greiner (in back) and his team — Anton Mazurenko (from left), senior grad student,
Daniel Greif, postdoc fellow, Geoffrey Ji, graduate student, and Christie Chiu (not shown) — have created an
antiferromagnet, an exotic form of matter that can give important insights into how room temperature
superconductors might be created
From the moment when physicists discovered superconductors —
materials that conduct electricity without resistance at extremely low
temperatures — they wondered whether they might be able to develop materials
that exhibit the same properties at warmer temperatures.
The key to doing so, a group of Harvard scientists say, may lie in another exotic
material known as an antiferromagnet.
Led by physics professor Markus Greiner, a team of physicists has taken a
crucial step toward understanding those materials by creating a quantum
antiferromagnet from an ultracold gas of hundreds of lithium atoms. The work
is described in a May 25 paper published in the journal Nature.

2. “We have created a model system for real materials … and now, for the first
time, we can study this model system in a regime where classical computers get
to their limit,” Greiner said. “Now, we can poke and prod our antiferromagnet.
It’s a beautifully tunable system, and we can even freeze time to take a snapshot
of where the atoms are. That’s something you won’t be able to do with an actual
solid.”
But what, exactly, is an antiferromagnet?
Traditional magnets, the kind that you can stick to your refrigerator, work
because the electron spins in the material are aligned, allowing them to work in
unison. In an antiferromagnet, however, those spins are arranged in a
checkerboard pattern. One spin may be pointed north, while the next is pointing
south, and so on.
Understanding antiferromagnets is important, Greiner and physics professor
Eugene Demler said, because experimental work has suggested that, in the most
promising high-temperature superconductors — a class of copper-containing
compounds known as cuprates — the unusual state may be a precursor to high-
temperature superconductivity.
Currently, Demler said, the best cuprates display superconductivity at about
minus 160 degrees Fahrenheit, which is cold by everyday standards, but far
higher than for any other type of superconductor. That temperature is also
warm enough to allow practical applications of cuprate superconductors in
telecommunications, transportation, and in the generation and transmission of
electric power.
“This antiferromagnet stage is a crucial stepping-stone for understanding
superconductors,” said Demler, who led the team providing theoretical support
for the experiments. “Understanding the physics of these doped
antiferromagnets may be the key to high-temperature superconductivity.”
To build one, Greiner and his team trapped a cloud of lithium atoms in a
vacuum and then used a technique they dubbed “entropy redistribution” to cool
them to just 10 billionths of a degree above absolute zero, which allowed them
to observe the unusual physics of antiferromagnets.
“We have full control over every atom in our experiment,” said Daniel Greif, the
postdoctoral fellow working in Greiner’s lab. “We use this control to implement

3. a new cooling scheme, which allows us to reach the lowest temperatures so far
in such systems.”
“The problem in trying to come up with better superconductors is that if you take a
material and change one parameter … lots of things are changing,” Demler said. “With
this simulation, we have full control of parameters. So we can actually understand what
helps and what suppresses superconductivity.” Rose Lincoln/Harvard Staff
Photographer
That degree of control has enabled Greiner and his team to photograph the
system with enough detail to identify and extract information about individual
atoms. The team can also change the atomic density of the antiferromagnet to
search for a superconducting state.
The system isn’t just a model, but is a special-purpose quantum computer that
can simulate the complex physics of antiferromagnets and how their
transformation into superconductors can work.
Though scientists can simulate the quantum properties of simple atoms, and
even relatively simple materials, more exotic compounds like cuprates are
simply too complex to be modeled accurately by conventional computers, and
many in the field believe that quantum computers may be the answer.

4. “Many people expect that the first field where quantum computers will make a
major impact is in quantum simulation,” Demler said. “If scientists want to test
the airflow and other flight characteristics of an airplane, they would build a
wind tunnel to test that. This is, essentially, a quantum wind tunnel for real
materials.
“So what we have done in the past is to come up with what we think are simple
models. The truth is we still cannot solve those models,” Demler said. “The end
result is that our predictions disagree with experimental results, but we don’t
know if our model was incorrect or if we didn’t compute it correctly. With this
system, we know exactly which model describes it. And now … if we make a
prediction, they can tell us if it is accurate.”
Though the system may one day play a role in designing a new generation of
superconductors, Demler said its ultimate importance may lie in helping
researchers build a foundation of knowledge for materials science.
“The problem in trying to come up with better superconductors is that if you
take a material and change one parameter … lots of things are changing,”
Demler said. “With this simulation, we have full control of parameters. So we
can actually understand what helps and what suppresses superconductivity, and
then we can become wiser in terms of choosing elements” to investigate.
This research was supported by the Air Force Office of Scientific Research, Army
Research Office, the Gordon and Betty Moore Foundation EPiQS Initiative, the
Harvard Quantum Optics Center, the National Science Foundation and the
Swiss National Science Foundation.