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Prof. Leggett was a soft-spoken, kind person who was also a brilliant theoretical physicist. I was fortunate enough to first meet him back when I was a graduate student working in Doug Osheroff's lab. Doug had discovered (along with his thesis advisors Bob Richardson and Dave Lee) the superfluid phases of the rare isotope of helium, 3He in 1972.
[Science digression: 3He atoms are fermions - if you add up the spin angular momentum from the two protons, the neutron, and the two electrons, you end up with a net spin of 1/2. To condense into a superfluid state, by analogy with electrons in superconductors, the 3He atoms need to pair up, and it's the pairs that condense into the superfluid. This pairing ends up being quite complicated; the pair of 3He atoms end up having \(\ell = 1\) orbital angular momentum, and this implies that the nuclear spins of the 3He atoms in the pair have to form a triplet. Prof. Leggett figured out a ton of the insights on this topic - see here for an early paper on this, and here for a definitive review c. 1975.]
Prof. Leggett made many contributions beyond 3He. For example, he and others studied the problem of a tunneling particle coupled to some dissipative environment (like phonons, say), and similarly of a two-state quantum system coupled to a "bath", as in this paper with several thousand citations. These both had close connections to the "measurement" problem in quantum mechanics - how in detail do you go from a highly quantum system (e.g., a particle tunneling out of a bound state, or a particle coherently oscillating back and forth) and end up with more classical-looking outcomes due to coupling to "baths" with large numbers of degrees of freedom? He was interested in these kinds of foundational quantum issues all the way along (see this 1980 paper) and was still writing about them within the last couple of years. Prof. Leggett also wrote important tutorial reviews of superfluidity and of Bose-Einstein condensation in ultracold gases. When I got to meet him on a trip through Stanford, I was introduced to the ideas that he and Clare Yu developed about tunneling two-level systems in solids - looking at the big question of why the properties of TLS in disordered solids are so universal even though the materials can be very different at the microscopic level. He was a great scientist while also being a kind person.
