How to Control Heat Flow with a Magnet?

In a quirky quantum-mechanical connection, a mag breadic bailiwick rat control the flow of conflagrate cross shipway a interlace of cable. The kindle is carried by quantum waves of electrons, and waves traveling slightly the hand-build carrefour to bolster or limit the flow depending on the strengths of the magnetic field threading the loop, a pair of physicists predicts in the 15 April PRL. antecedent experiments confirm the prediction. Curiously, still as electrons shuttle fondness across the little device, no electric current flows through it.

For decades, physicists substantiate known that a magnetic field disregard meet the flow of electricity through a cable that splits and rejoins to condition a visit resembling a traffic circle in the middle of a road. Known as the Aharonov-Bohm effect, the phenomenon arises because each electron is described by a quantum wave that splits, so that half(prenominal) of it flows around one font of the ring and half flows around the other side. When the two waves recombine on the far side of the ring they can overlap peak-to-peak to maximize the current, or peak-to-valley to smirch it.

How the waves line up depends on the strength of the magnetic field threading the loop. The field shifts the peaks in the waves going around the two halves of the ring by different amounts. Bizarrely, the field can shift the waves even if it is confined to the hole in the ring and does not go by into the metal through which the electrons flow. Thanks to quantum mechanics, the electrons feel the effect of a field that they never travel through.

But in some ways electrons also behave like particles, so they can consider energy and, hence, heat, as they rattle from the hotter end to the colder end of a wire. And a magnetic field can affect the heat flow through a wire with a loop in much the equivalent way the field can affect the flow of electricity, calculate Zhigang Jiang and Venkat Chandrasekar of Northwestern University in Evanston, Illinois. The researchers study a gizmo known as an Andreev interferometer--essentially a wire a few microns long with a loop of superconducting wire attached to it like a side street that branches from a highway and circles around to merge back into it at the same point. The electrons in the super theatre director have wave-like characteristics that are easier to detect than those in median(a) metal.

According to the calculations, the waves of electrons zipping all the way around the superconducting detour cancel or reinforce themselves at the point where they meet the main wire. much(prenominal) interference limits the number of quantum states through which electrons in the ordinary conductor can cross the intersection.



That limits the flow of heat, much as a closed lane limits traffic flow on a highway. So as the magnetic field increases, the flow of heat from the hotter to the colder end of the wire climbs and falls repeatedly, just as the electrical current rises and falls in the Aharonov-Bohm effect. However, in this case, no electrical current flows, Chandrasekhar says. Basically, electrons go from one end to the other, give up their energy, and then go back, he says, so theres no net flow of electric charge. In preliminary experiments, the researchers have seen the predicted heat flow oscillations [1].

At first glance, the superconducting ring and magnetic field step to the fore to be irrelevant, so the result is surprising, says Dan Prober of Yale University in New Haven, Connecticut. Prober notes that to pick out the subtle effect, Chandrasekhar had to bring to addher several experimental techniques: He does elegant, hard, and sometimes crazy hard experiments.

--Adrian Cho

Adrian Cho is a freelance writer in Grosse Pointe Woods, Michigan

References:

[1] Z. Jiang and V. Chandrasekhar, Quantitative measurements of the thermal resistance of Andreev interferometers,

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