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Understanding high-temperature superconductors is one of the biggest
challenges in physics today, according to Eric Hudson, MIT assistant
professor of physics and senior author of the paper.
Most superconductors only superconduct at temperatures near absolute
zero, but about 20 years ago, it was discovered that some ceramics can
superconduct at higher temperatures (but usually still below 100
Kelvin, or -173 Celsius).
Such high-temperature superconductors are now beginning to be used for
many applications, including cell-phone base stations and a demo
magnetic-levitation train. But their potential applications could be
much broader.
�If you could make superconductors work at room temperature, then the
applications are endless,� said Hudson.
Superconductors are superior to ordinary metal conductors such as
copper because current doesn't lose energy as wasteful heat as it
flows through them, thus allowing larger current densities. Once a
current is set in motion in a closed loop of superconducting material,
it will flow forever.
In the Nature Physics study, the MIT researchers looked at a state of
matter that superconductors inhabit just above the temperature at
which they start to superconduct.
When a material is in a superconducting state, all electrons are at
the same energy level. The range of surrounding, unavailable electron
energy levels is called the superconducting gap. It is a critical
component of superconduction, because it prevents electrons from
scattering, thus eliminating resistance and allowing the unimpeded
flow of current.
Just above the transition temperature when a material starts to
superconduct, it exists in a state called the pseudogap. This state of
matter is not at all well understood, said Hudson.
The researchers decided to investigate the nature of the pseudogap
state by studying the properties of electron states that were believed
to be defined by the characteristics of superconductors: the states
surrounding impurities in the material.
It had already been shown that natural impurities in a superconducting
material, such as a missing or replaced atom, allow electrons to reach
energy levels that are normally within the superconducting gap, so
they can scatter. This can be observed using scanning tunneling
microscopy (STM).
The new MIT study shows that scattering by impurities occurs when a
material is in the pseudogap state as well as the superconducting
state. That finding challenges the theory that the pseudogap is only a
precursor state to the superconductive state, and offers evidence that
the two states may coexist.
This method of comparing the pseudogap and superconducting state using
STM could help physicists understand why certain materials are able to
superconduct at such relatively high temperatures, said Hudson.
�Trying to understand what the pseudogap state is is a major
outstanding question,� he said.
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