Technique reveals quantum phase transition; could lead to superconducting transistors

Apr 27, 2011
Exploring the superconducting transition in ultra thin films
Ivan Bozovic

(PhysOrg.com) -- Like atomic-level bricklayers, researchers from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory are using a precise atom-by-atom layering technique to fabricate an ultrathin transistor-like field effect device to study the conditions that turn insulating materials into high-temperature superconductors. 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 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 , 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 .

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 , 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 . But as we continue to explore these mysteries, we are also striving to make ultrafast and power-saving superconducting electronics a reality."

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that_guy
1 / 5 (1) Apr 27, 2011
so does this mean that supercooled superconducting supercomputers will be coming to a university near you in the next decade?
axemaster
not rated yet Apr 27, 2011
Logic devices will never be able to operate without any heat losses - this is a simple fact of thermodynamics. There is a minimum amount of heat that will always be generated as a direct result of logic operations, which increase entropy during each calculation.
El_Nose
not rated yet Apr 27, 2011
excuse my ignorace but is superconducting currently studied as a quantum mechanical effect ?? or some derivation of an action that comes from a more classical approach? such as from a chemistry standpoint -- how are we tackling super conductivity -- from what angle are the main reseacher coming from. physics, quantum physics, chemistry, ???
ppnlppnl
5 / 5 (1) Apr 27, 2011
"Logic devices will never be able to operate without any heat losses - this is a simple fact of thermodynamics. There is a minimum amount of heat that will always be generated as a direct result of logic operations, which increase entropy during each calculation."

Actually there is no minimum energy if the calculations are thermodynamically reversible. You can in principle make any calculation reversible if you keep track of a lot of information about how it was done. There are reversible versions of the logic gates that do just this. You only generate heat when you dump a bit to the environment or read off an answer.

Quantum computers depend on this reversibility. If a quantum computer dumps a bit to the environment then its internal state becomes entangled with the external world. This causes decoherence that ruins your calculation.

Macksb
3 / 5 (4) Apr 28, 2011
This result is exactly consistent with a theory that I have posted on Phys Org many times. Here, the superconducting transition occurs at a point where the resistance is equal to 6.45 kilo ohm, which is exactly equal to the Planck constant divided by twice the electron charge squared. My theory is that superconductivity results when the normal oscillations of electrons synchronize. Synchrony occurs when the oscillations of electrons are fully interconnected in a mobile environment. The transition point described in this article is the precise point at which that condition (full interconnection) is met in a film. At that point, the electrons will couple in pairs, exactly antisynchronously--because they must. Presto: Cooper pairs. See Art Winfree's theory of coupled oscillators, well developed in biology and math. Steve Strogatz (Cornell, applied math) is the leading proponent of Art's theory. Physicists have ignored it.
Bob_Kob
not rated yet Apr 28, 2011
if you make a superconducting computer, does it not use any power?
Macksb
3 / 5 (4) Apr 28, 2011
Continuing with my post above, friction normally is sufficient to prevent the oscillations of electrons from interacting broadly throughout a system of electrons. When I say "friction" here, think electrical resistance. Normally, some portion of the energy of those interacting oscillations is dissipated in the form of heat. That changes when the interaction of the oscillations is powerful enough to overcome electrical resistance. At that point, the interacting oscillations are felt broadly through the system of electrons. That is the point at which the Winfree theory of coupled oscillators applies. The oscillations must organize themselves broadly, by Winfree's law of coupled oscillators. The easiest way to do that is antisynchronously.

At that point, superconductivity breaks out. Cooper pairs.

When that happens, friction disappears. That also reveals the essence of friction...friction is simply the disorganized manner in which the oscillations of electrons interact.
Macksb
3 / 5 (4) Apr 28, 2011
Continuing with my two posts above, my theory provides a unified explanation for superconductivity and superfluidity. Occam's Razor. At the transition point for superfluidity, the energy of interacting oscillations in the fluid is sufficient to overcome the "resistance" in the fluid, so the interactions of those oscillations are felt broadly through the system. (Before that point is reached, the effects of oscillations are local.)

At that point, the Winfree law of coupled oscillators applies, so these oscillations must organize themselves in an exact Winfree pattern. Synchrony emerges.

At that point, superfluidity breaks out.

When that happens, viscosity (friction) disappears. That also reveals the essence of viscosity...viscosity is simply the disorganized manner in which the oscillations in a fluid interact, pre Winfree effect.

In the case of superfluidity, the relevant oscillations are one type in the case of He 3, and another type in the case of He 4.
Skeptic_Heretic
not rated yet Apr 28, 2011
Actually there is no minimum energy if the calculations are thermodynamically reversible. You can in principle make any calculation reversible if you keep track of a lot of information about how it was done.
To make a calculation thermodynamically reversable, you would require the ability to remove the information created by the operation from existence, which is impossible. Flipping a switch on, then flipping it off isn't reversing the operation, it is performing an operation twice.
wiyosaya
not rated yet Apr 28, 2011
Interesting theory, Macksb. Why not publish?

Sounds like there is coherency here. When a ruby laser is too hot, it will no longer lase, but cool it, it will then lase. Is there a similar mechanism here where the ambient energy makes transition times too long or unable to occur at all - thus - cooling materials removes that ambient energy so that multiple electrons can act in a coherent manner? Different "host" materials with different inter-atomic energetic properties, such as HTSCs, might allow these interactions because of their inherent energy state.

Interesting about h / 2e. Certainly seems to confirm that there is a pair of electrons involved.
MIBO
not rated yet Apr 28, 2011
"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."

can somebody explain how you control the thickness of a single atomic layer?.
Macksb
2.3 / 5 (3) Apr 28, 2011
Thanks, Wiyosaya. To date, I have "published" my theory in bits and pieces on the web under the Macksb pen name. Phys Org is my most active site, but I have also published on other sites, usually in brief posts.

Let me add that this Brookhaven math also calls to mind the math of the fractional quantum Hall effect, which I have long thought to be Winfree patterns at work. In fqHe, the relevant formula is e squared divided by h. The stupendous precision of that effect screams "Winfree patterns."

Since you are interested in lasers, you might want to look at my several comments on a Phys Org article entitled "Light Touch Transforms Material Into a Superconductor," dated January 14, 2011. Also see my several comments on "Physicists Show That Superfluid Light Is Possible," an October 2010 Phys Org article.

Simply put, I believe that Art Winfree's theory of coupled oscillators explains all phases of matter and their transitions, from the simple to the exotic. Thanks again.
TheQuietMan
not rated yet May 01, 2011
Steady state a superconducting computer would be extremely low energy. However, the switching transition of a gate is never zero, it takes some finite amount of time. While it is switching it is neither zero ohms or nonconducting, in other words it is a resistor. This is where the energy use comes in.

It is why the current very energy efficient transistors in their billions make a CPU chip so hot. If they didn't switch the chip would be cool, but it switches thousand or millions of gates billions of times per second. It adds up.