The other blog that has been keeping me away from this one…

September 21, 2011

Well over a year has passed since my last post and I think that requires an explanation. Since September 2010 I have been busy setting up a small theory outpost of the University of Kent’s Functional Materials Group at the Rutherford Appleton Laboratory. The new research group, which is part of SEPnet and of the Hubbard Theory Consortium, has its own blog (much more active than this one) at

The new blog is of a rather more technical nature than this one so I will keep Condensed Matters open to post the occasional wild speculation or unfettered musing as inspiration calls.

Superconductivity in the Hubbard model: an exact solution

June 15, 2010

If you read our article on strong correlations, you may remember that we still do not have an exact solution the Hubbard model in more than one dimension.You might also recall how important it was to learn something definite about superconductivity in this model. Now Raghu, Kivelson and Scalapino have published a paper proposing a way to obtain the superconducting phase diagram of this model in the weak-coupling limit:

S. Raghu, S. A. Kivelson, and D. J. Scalapino,
“Superconductivity in the repulsive Hubbard model: An asymptotically exact weak-coupling solution”,
Phys. Rev. B 81, 224505 (2010)
DOI: 10.1103/PhysRevB.81.224505

Follow the link for a synopsis of the above article in Physics.

Superconductivity in the repulsive Hubbard model: An asymptotically exact weak-coupling solution

Superconductivity in the repulsive Hubbard model: An asymptotically exact weak-coupling solution

John Hubbard’s challenge 45 years on

September 22, 2009

A very brief Highlight feature we wrote some time ago on the complementarity between condensed matter and cold-atom experiments has now appeared on the ISIS website:

* J Quintanilla, C Hooley, ST Carr, JJ Betouras, VL Campo Jr and K Capelle, “John Hubbard’s challenge 45 years on”.

Strong correlations in ultra-cold atom gases and at the RHIC – the string connection

September 17, 2009

You may remember a brief mention of Brookhaven’s Relativistic Heavy Ion Collider (RHIC) in our article on strong correlations of a couple of months ago. This month Physics World carries another article, by Barbara Jacak, that discusses that type of strongly-correlated quantum matter in a lot more detail. The new article explains how string theory can be used to connect the experiments at RHIC to others carried out on another type of strongly-correlated system: ultra-cold atomic gases. Well worth reading:

PS: there was also a much more technical article on strongly interacting matter (as the quark-gluon soup is now known) in Rev. Mods. Phys. a few months ago:

Colloquium: Phase diagram of strongly interacting matter
P. Braun-Munzinger and J. Wambach, Rev. Mod. Phys. 81, 1031 (2009)

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!)

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.