Researchers build SQUID device that demonstrates the Josephson effect

Dec 20, 2012 by Bob Yirka report
Josephson heat interferometer. Credit: arXiv:1205.3353 [cond-mat.mes-hall]

(—Italian nano-science researchers Francesco Giazotto and María José Martínez-Pérez have built a superconducting quantum interference device (SQUID) that confirms a theory that describes the Josephson effect, whereby the application of a magnetic field applied to such a device can cause changes in the amount of heat that flows through it. They describe their device and how it works in a paper they've published in the journal Nature.

Half a century ago, physicist Brian Josephson predicted that a device now known as a could be built. Such a device would consist of two connected together with a small amount of between them. Josephson said because of the unique properties of such a device, electrons should be able move from one of the superconductors to the other by "tunneling" through the . Subsequent research proved this to be correct, and devices that incorporate it have been created to measure very small magnetic fields.

But theory also suggested that if a were applied to the device, the amount of heat that moved between the two superconductors could be impacted. Until now, this had not been demonstrated with a real device. To create one, the researchers fashioned two Y shaped pieces of a superconductor then connected them together at their tops, with a small amount of insulating material between them. In this setup the conjoined materials formed a loop.

To test the heat transfer properties, the researchers heated one side of the loop and cooled the other, then measured the amount of heat transfer between the two as variable strength magnetic fields were applied. They found that varying the strength of the magnetic field did indeed cause more or less heat to move from one side of the SQUID to the other. They even found that in some cases, heat could be caused to move from the cold side to the hot.

The researchers explain that varying the magnetic field can cause changes in heat flow because of the quantum phase difference between the two superconductors. Changes in the strength of the magnetic field can cause peaks in the wavefunctions of the two superconductors to line up, thereby increasing heat flow, or force the reverse, resulting in decreased heat flow.

These new findings, the researchers say might help in developing better magnetometers, or perhaps even lead the way to computing devices based on thermal transistors.

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More information: The Josephson heat interferometer, Nature, 492, 401–405 (20 December 2012) doi:10.1038/nature11702 (Arxiv PDF)

The Josephson effect is perhaps the prototypical manifestation of macroscopic phase coherence, and forms the basis of a widely used electronic interferometer—the superconducting quantum interference device (SQUID). In 1965, Maki and Griffin predicted that the thermal current through a temperature-biased Josephson tunnel junction coupling two superconductors should be a stationary periodic function of the quantum phase difference between the superconductors: a temperature-biased SQUID should therefore allow heat currents to interfere, resulting in a thermal version of the electric Josephson interferometer. This phase-dependent mechanism of thermal transport has been the subject of much discussion but, surprisingly, has yet to be realized experimentally. Here we investigate heat exchange between two normal metal electrodes kept at different temperatures and tunnel-coupled to each other through a thermal 'modulator' in the form of a direct-current SQUID. We find that heat transport in the system is phase dependent, in agreement with the original prediction. Our Josephson heat interferometer yields magnetic-flux-dependent temperature oscillations of up to 21 millikelvin in amplitude, and provides a flux-to-temperature transfer coefficient exceeding 60 millikelvin per flux quantum at 235 millikelvin. In addition to confirming the existence of a phase-dependent thermal current unique to Josephson junctions, our results point the way towards the phase-coherent manipulation of heat in solid-state nanocircuits.

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not rated yet Dec 25, 2012
The function of Josephson circuit is based on Cooper pair formation. Inside of atom lattice the movable electrons cannot spread quite freely, but they're are forced to squeeze through holes between atoms and they do accelerate and decelerate during this regularly. The accelerating charge generates the EM waves and electron loses energy with it, which is the main reason of Ohmic loses. But within superconductors the electrons can employ a neat trick, when they condense into so-called Cooper pairs at the distance, which is integral multiple of lattice constant (distance between atoms). After then, when one electron is entering some hole between atoms, then another electron is just leaving another hole and it pulls the former electron. In this way, the pair of electrons can overcome the obstacles in easier way as a team. Now you can actually understand the BCS theory of superconductivity.
not rated yet Dec 25, 2012
Brian Josephson was a first guy, who realized, that the Cooper pairs can overcome not only the natural obstacles inside of superconductors (potential barriers) - but they could overcome even artificial barrier, which is formed when we separate two pieces of superconductor with thin layer of insulator. When the thickness of insulator layer corresponds the distance of electrons within Cooper pairs, then the electrons can tunnel through it nearly as easily, as through potential barriers inside of atom lattice. A Josephson junction is formed.

The significant property of Josephson junction is, it's very sensitive to external magnetic fields (after all, as the whole superconductivity effect itself). The magnetic field separates the electrons within Cooper pairs, thus making the conditions for their easy traveling through Josephson junction progressively more difficult. It enables to detect very subtle magnetic fields with resistance changes of Josephson junction.
not rated yet Dec 25, 2012
When the Josephson junction is exposed to temperature gradient, then the hotter electrons tend to travel from hot side of junction into cooler one. But their motion is balanced with the flux of cool electrons in opposite direction, so it's not so easy to detect it. But every motion of electrons generates the magnetic field, which the Josephson current is very sensitive on. Therefore the thermocurrent can be monitored with pair of Josephson junction in parallel arrangement (SQUID) as pictured here. When electrons are moving through one branch of loop, they induce the magnetic field, which will make this branch more resistant. So that the electrons will use the another branch more preferably and the thermocurrent in the first branch decreases. This will increase the load in the 2nd branch and the whole process will repeat in fast oscillations, the frequency of which will depend on the thermal flux.

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