A CV will be published soon.
By Volker Steger
Itʼs quite a twist when atoms form pairs to allow for superfl uidity, but Tony
Leggett wanted to show exactly how they do it – with his hands!
“But Professor, how can you hold up your drawing to the camera when you
need both of your hands for your presentation of paired atoms?”
“Just tape the drawing onto me!”
Es ist ja eine ziemlich verwickelte Angelegenheit, wenn Atome Paare
bilden, um Superfl uidität zu ermöglichen. Aber Tony Leggett will
exakt darstellen, wie sie das machen
– mit seinen Händen!
„Aber Herr Professor, wie wollen Sie Ihre
Zeichnung in die Kamera halten, wenn Sie
beide Hände für die Darstellung der Paarung
der Atome benötigen?“
„Kleben Sie die Zeichnung einfach an mir fest!“
Pairing-up in the Cold
by Adam SMith
Anthony Leggett’s sketch concerns the behaviour of an isotope of helium at very low temperatures. Close to absolute zero, helium-3 (3He) displays the property of superfluidity, being able to creep up the walls of the vessel that holds it, as if it had no viscosity whatsoever. The drawing depicts both the experimental observation and the theoretical explanation that allowed Leggett to account for this phenomenon.
At the top we see the spinning nuclei of the 3He atoms in the presence of a magnetic field. The nuclei possess a resonance frequency proportional to the magnetic field, and this can be detected by experiment. The lines in black and red, labelled N, A and B, show what happens to that resonance frequency as you lower the temperature of 3He to just a very few thousandth of a degree. Passing towards colder temperatures from left to right, the nuclei spin like tops in a magnetic field down to about three millidegrees (region N), but below that something very odd seems to happen. The resonance frequency starts rising (region A), goes on rising for a bit and then abruptly falls again back to the original value (region B) at still colder temperatures.
Leggett’s work showed that these three regions correspond to completely different phases of 3He, and that the behaviour in region A is explained by a rearrangement of the nuclear spins. In this region, the spins rotate around each other in pairs in the way Leggett has tried to indicate in the left hand diagram (with red and green arrows), but not, importantly, in the way indicated by the right hand diagram. These two states are of almost equivalent energy, so both states should be equally probable, but Leggett showed that the nuclei are forced into just one of the states in much the same way as electrons are forced into so-called Cooper pairs in superconductive states. Cooper pairs of nuclei are formed exactly at the point where the red curve starts, leading to superfluidity, and these pairs grow stronger as the temperature decreases, until the nuclear spins suddenly collapse to the final ground state of 3He.
“It seemed to me so weird,” recalls Leggett, “that I seriously contemplated the possibility that under these very extreme conditions, quantum mechanics was breaking down. But of course that in the end turned out not to be the case.”