Strong correlations

June 26, 2009

In this month’s issue of Physics World there is and article I wrote with Chris Hooley on strong correlations:

J. Quintanilla & C. Hooley,
The strong-correlations puzzle,
Physics World 22, 32-37 (June 2009)
[ online excerpt; complete print version available here ].

What we basically tried to do is explain what is meant by that term, putting it in a historical context and explaining the role that experiments with ultra-cold atoms, as opposed to “traditional” materials, can play in disentangling the mysteries. We placed a lot of emphasis on the model that John Hubbard (pictured) invented here, in the Harwell campus, in the early sixties, and on neutron scattering, which was responsible for the genesis of that model and is still producing many of the main puzzles today. Polite comments and enlightening  criticism are certainly most welcome.

(By the way, the same issue of Physics World contains Doug Natelson’s article on single-molecule electronics – as he already informed his blog readers – much more promtly than I have, it has to be said!)

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We need crystal growers!

April 23, 2009

In a letter published in this week’s issue of Nature, Christian Kloc relates some of his experience over the years as a crystal grower. He states

Of the laboratories where I have worked and grown crystals over the past 30 years, not one is growing crystals today.

Barring the possibility that the author of the letter actually jinx’s the labs where he works, 😉 the sentence is a pretty harsh indictment, but one that does not surprise me. Indeed, crystal growing (that is, the fabrication in the laboratory of large, near-perfect crystals of the materials that are of interest to current condensed matter research) is a delicate, unpredictable, and relatively costly enterprise.

I emphasize the word ‘relatively’ because growing crystals is not nearly as costly as other, much larger endeavors, such as building and operating large-scale facilities (I should know that, since I work at one). However those are carried out by governments, not universities. Crystal-growing is at the upper end of what, say, a Physics department of a medium-sized university could afford. Moreover I understand (though note this is second-hand knowledge – I am a theorist, after all) that it is a bit of an art form, where breakthroughs are often serendipitous and often a program leads to nothing after much hard work. So in some sense I guess one could use crystal growing as a measure the level of commitment of a country to genuinely novel condensed matter physics research.

If Christian Kloc‘s experience is anything to go by, it looks like many future breakthroughs will come not from Europe or America, but from Asia. Indeed, this is already happening, as the discovery of the pnictide superconductors attests. Ultimately the problem lies, I think, with the pressure university-based academics are put under to obtain concrete results on a fairly short time scale and within rather tight budgets. Perhaps the solution to the problem in the Western countries is for the large facilities to start growing their crystals themselves…


Does a Fermi liquid on a half-filled Landau level have a Pomeranchuk instability?

April 7, 2009

Today, Orion Ciftja and I have made available on the arXiv a preprint reporting our findings on the possibility of Pomeranchuk instabilities in certain quantum Hall devices. What does that mean? It has to do with electrons confined to move in two dimensions and placed under a strong magnetic field. We look at whether under certain conditions the electrons might undergo phase transition in which the distribution of momenta of the electrons changes due to electron-electron interactions, so that they start to move faster in some directions than others. Which direction they choose is random, but once they have decided, they all conform to that choice. So in that sense it is an example of a broken symmetry. This is a problem we have been working on since Orion and I met at SCES’07. Here is the reference:

Does a Fermi liquid on a half-filled Landau level have Pomeranchuk instabilities?
Jorge Quintanilla and Orion Ciftja,
arXiv:0904.0658 [cond-mat.mes-hall].

For the specialists among the readership, here is the abstract:

We present a theory of spontaneous Fermi surface deformations for half-filled Landau levels (filling factors of the form ν = 2n+1/2). We assume the half-filled level to be in a compressible, Fermi liquid state with a circular Fermi surface. The Landau level projection is incorporated via a modified effective electron-electron interaction and the resulting band structure is described within the Hartree-Fock approximation. We regulate the infrared divergences in the theory and probe the intrinsic tendency of the Fermi surface to deform through Pomeranchuk instabilities. We find that the corresponding susceptibility never diverges, though the system is asymptotically unstable in the n → ∞ limit.