When crystal vibrations' inner clock drives superconductivity

November 9, 2016

Superconductivity is like an Eldorado for electrons, as they flow without resistance through a conductor. However, it only occurs below a very low critical temperature. Physicists now believe they can enhance superconductivity - the idea is to externally drive its underlying physical phenomena by changing how ions vibrating in the crystal lattice of the conductor material, called phonons, interact with electron flowing in the material. Andreas Komnik from the University of Heidelberg and Michael Thorwart from the University of Hamburg, Germany, adapted the simplest theory of superconductivity to reflect the consequences of externally driving the occurrence of phonons. Their main result, published in EPJ B, is a simple formula explaining how it is theoretically possible to raise the critical temperature using phonon driving.

The authors studied the all-important autocorrelation function of in the BCS theory, named after John Bardeen, Leon Cooper and John Robert Schrieffer. Superconductivity is induced by phonons cooperating to bring the together in the so-called Cooper pairs—against their spontaneous tendancy to repel each other. In this work, the authors focused on the scenario of an X-ray light field rattling the phonons. They realised that a kind of driving, which controls the frequency of vibration of the lattice sites, can profoundly change the pairing of the electrons.

However, electrons and phonons do not follow the same internal clock. This means that change at what is perceived as a normal pace for phonons comes across as extremely slow to electrons. To eliminate this discrepancy, the author devised a solution based on bringing a clever time-averaging procedure into the BCS equations. This theoretical approach, the authors found, reveals the controlled elevation of the critical temperature. They thus managed to integrate the external phonon drive into the standard BCS theory. The advantge is that the critical temperature can be computed from this simple formula and can, in theory, be considerably elevated using the driving procedure. Suprisingly, the was even higher up when further refinment to the equations accounted for phonon-phonon interactions.

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More information: Andreas Komnik et al, BCS theory of driven superconductivity, The European Physical Journal B (2016). DOI: 10.1140/epjb/e2016-70528-1

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Sonhouse
not rated yet Nov 09, 2016
Sounds like this is so far just theory. When will they do a real experiment showing this effect and how far up the temperature scale they can go.
Macksb
not rated yet Nov 22, 2016
This result supports my "coupled periodic oscillators" theory of superconductivity.

The "crystal vibrations" here are periodic oscillations--phonons.

The "inner clock" of these vibrations is "extremely slow" compared to that of electrons. But "driving the frequency of the vibrations" can "profoundly change the pairing of the electrons." The critical temperature of superconductivity onset can be increased with such phonon "driving."

Moreover, such critical temperature can be increased even higher by "accounting for phonon-phonon interactions." These phonon interactions are also a form of coupled periodic oscillations.

So the highest "super" temp results when (a) phonons couple their oscillations and (b) the periodic oscillation frequency of the phonons more closely matches the periodic oscillation frequency of the electrons.

The product is Cooper pairs. Cooper pairs are two electrons in which the periodic oscillations of the electrons are coupled in two ways.
Macksb
not rated yet Nov 22, 2016
Continuing my post above: Electrons have two periodic oscillations. Orbit and spin. And these two periodic oscillations are exactly coupled: half a spin for every orbit. Fermions.

Cooper pairs are joined perfectly in two ways. Spin up for A and spin down for B. Clockwise orbit for A and counterclockwise orbit for B. In both cases the coupling is exactly anti-synchronous. Exactly 180 degrees apart. So paired, the system is a boson, not a fermion.

At every pairing step in this progress towards superconductivity, the pairing is a pairing of periodic oscillations. In a two oscillator system, pairing can only take one of two forms: exactly anti-synchronous, or exactly synchronous.

Think of Huygens' clocks, where the two pendulums coupled 350 years ago. That was anti-synchronous coupling. Or think of our moon, where two different types of periodic oscillations, orbit and spin, are coupled synchronously. Mercury has a 3:2 period coupling of spin to orbit.

Macksb
not rated yet Nov 25, 2016
See also "Light-driven atomic rotations excite magnetic waves," Physorg October 24. It provides a broad and well illustrated explanation of how light (a term that includes the X ray light field used to reset the "inner clock" in the above article) can influence matter.

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