At the end of the day, we want a
thin superconducting layer to produce the skin of a magnetic field exclusion
vessel (MFEV), also known as an UFO. We
are here one step closer.
It has been gratifying to see
steady progress in these necessary areas since I published my piece on the
reverse engineering of an UFO. It is
going very well and we should be able to mock up a device inside of the decade.
This was one of the key technologies:
manufacture of a single superconducting layer of high temperature
superconductor. A single layer cooling
substrate would also be good.
Exploring the Superconducting Transition in Ultra Thin Films
by Staff WritersUpton
NY
Ivan Bozovic.
Like atomic-level bricklayers, researchers from the
The technical break-through, which is described in the April 28,
2011, issue of Nature, is already leading to advances in understanding
high-temperature superconductivity, and could also accelerate the development
of resistance-free electronic devices.
"Understanding exactly what happens when a normally insulating
copper-oxide material transitions from the insulating to the superconducting
state is one of the great mysteries of modern physics," said Brookhaven physicist
Ivan Bozovic, lead author on the study.
One way to explore the transition is to apply an external electric field to
increase or decrease the level of "doping" - that is, the
concentration of mobile electrons in
the material - and see how this affects the ability of the material to carry
current.
But to do this in copper-oxide (cuprate) superconductors, one needs
extremely thin films of perfectly uniform composition - and electric fields
measuring more than 10 billion volts per meter. (For comparison, the electric
field directly under a power transmission line is 10 thousand volts per meter.)
Bozovic's group has employed a technique called molecular beam epitaxy
(MBE) to uniquely create such perfect superconducting thin films one atomic
layer at a time, with precise control of each layer's thickness. Recently,
they've shown that in such MBE-created films even a single cuprate layer can
exhibit undiminished high-temperature superconductivity.*
Now, they've applied the same technique to build ultrathin
superconducting field effect devices that allow them to achieve the charge
separation, and thus electric field strength, for these critical studies.
These devices are similar to the field-effect transistors (FETs) that
are the basis of all modern electronics, in which a semiconducting material
transports electrical current from the "source" electrode on one end
of the device to a "drain" electrode on the other end.
FETs are controlled by a third electrode, called a "gate,"
positioned above the source-drain channel - separated by a thin insulator -
which switches the device on or off when a particular gate voltage is applied
to it.
But because no known insulator could withstand the high fields required
to induce superconductivity in the cuprates, the standard FET scheme doesn't
work for high-temperature superconductor FETs. Instead, the scientists used
electrolytes, liquids that conduct electricity, to separate the charges.
In this setup, when an external voltage is applied, the electrolyte's
positively charged ions travel to the negative electrode and the negatively
charged ions travel to the positive electrode. But when the ions reach the
electrodes, they abruptly stop, as though they've hit a brick wall.
The electrode "walls" carry an equal amount of opposite
charge, and the electric field between these two oppositely charged layers can
exceed the 10 billion volts per meter goal.
The result is a field effect device in which the critical temperature
of a prototype high-temperature superconductor compound
(lanthanum-strontium-copper-oxide) can be tuned by as much as 30 degrees
Kelvin, which is about 80 percent of its maximal value - almost ten times more
than the previous record.
The scientists have now used this enhanced device to study some of the
basic physics of high-temperature superconductivity.
One key finding: As the density of mobile charge carriers is
increased, their cuprate film transitions from insulating to superconducting
behavior when the film sheet resistance reaches 6.45 kilo-ohm. This is exactly
equal to the Planck quantum constant divided by twice the electron charge
squared - h/(2e)2.
Both the Planck constant and electron charge are "atomic"
units - the minimum possible quantum of action and of electric charge,
respectively, established after the advent of quantum mechanics early in the
last century.
"It is striking to see a signature of such clearly
quantum-mechanical behavior in a macroscopic sample (up to millimeter scale)
and at a relatively high temperature," Bozovic said. Most people associate
quantum mechanics with characteristic behavior of atoms and molecules.
This result also carries another surprising message. While it has been
known for many years that electrons are paired in the superconducting state,
the findings imply that they also form pairs (although localized and immobile)
in the insulating state, unlike in any other known material.
That sets the scientists on a more focused search for what gets these
immobilized pairs moving when the transition to superconductivity occurs.
Superconducting FETs might also have direct practical applications.
Semiconductor-based FETs are power-hungry, particularly when packed very
densely to increase their speed. In contrast, superconductors operate with no
resistance or energy loss. Here, the atomically thin layer construction is in
fact advantageous - it enhances the ability to control superconductivity using
an external electric field.
"This is just the beginning," Bozovic said. "We still
have so much to learn about high-temperature superconductors. But as we
continue to explore these mysteries, we are also striving to make ultrafast and
power-saving superconducting electronics a reality."
Coauthors on the paper include: I.
Bozovic, A. Bollinger, J. Yoon, and J. Misewich from Brookhaven Lab and G.
Dubuis and D. Pavuna from Ecole Polytechnique Federale deLausanne
No comments:
Post a Comment