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June 26, 2015

Invitations to edit this blog sent to all HTC members 26 June 2015. Let me know if you didn’t get yours – thanks!

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

First light at B18

November 19, 2009

Two days ago, beamline B18 at the Diamond Light Source had its first light – exactly as planned. Here is the picture that Silvia Ramos sent me. It is a view down a vacuum tube. The long strip in the centre of the picture is a fluorsecent screen, beautifully lit up by the X-ray photons. The additional brightness surrounding it comes from reflections on the walls of the tube. Hurray!

B18 is an EXAFS beamline: a technique enabling the elucidation of local structure around atoms of a specific atomic species that exploits the ability of X-ray photons to excite electrons so they get out of the atoms and explore the immediate environment.

Are high-energy physicists about to give us yet another probe?

October 17, 2009

In some recent posts I have highlighted the fact that some of the cutting-edge research in high-energy physics (HEP) is providing us with novel form of condensed matter – in particular, the quark-gluon plasma. This type of interaction between (or merging of) condensed matter and high-energy physics is new, since in the past HEP looked at process involving a few particles at a time, i.e. it was not concerned with collective states of matter or with phase diagrams.

On the other hand, there is a different short of interaction between HEP and condensed matter physics that goes a long way back, namely we owe to high-energy physicists some of the most powerful probes available of condensed matter systems. For example, the most advanced X-ray and neutron sources are based on particle accelerators (e.g. the electron and proton synchrotrons employed by Diamond and ISIS, respectively, here on the Harwell campus). Such machines ride on the back of advances in technology that were spurred by research at the frontier of particle physics some decades ago.

Now Andreas Ipp and Christoph Keitel propose that a new probe of matter may be provided by the quark-gluon itself:

Ipp, A., Keitel, C.H. & Evers, J., 2009. Yoctosecond Photon Pulses from Quark-Gluon Plasmas. Physical Review Letters, 103(15), 152301-4.

For a short summary see

Physics – The shortest known photon pulses.

The article argues that ultra-short pulses of light are emitted by the quark-gluon plasma formed in heavy-ion collisions and propose it as a probe of ultra-fast processes, such as those taking place inside the nucleus. The pulses are ~ 1 Yoctosecond = 1E-24 seconds in duration (look it up on Wikipedia). This corresponds to approximately 4 GeV. The question I ask is: what about condensed matter? Could this be useful there, too?

While all condensed matter processes are much less energetic than that, the energy of the probe need not be the energy of the excitation being probed – the energy of the probe just limits the resolution (the energy of the excitation being probed is the energy change of the photons in the pulse as they go through the sample). In that sense these ultra-short, high-energy pulses could be useful to probe excitations at much lower energies (e.g. ~ 10 meV, which is relevant for example for magnetic excitations in superconductors) but with potentially much more resolution than can be currently achieved.

The question is: are our current probes limited by their resolution? After all, what could we possibly learn that would be relevant to understand, say, a superconductor, that happens on the yoctosecond timescale? I’d love to know what people think about this.

One final though: the Yoctosecond pulses are obtained because a ys is the typical lifetime of a quark-gluon plasma (QGP). This raises the following difficulty, from the point of view of regarding the QGP itself as a condensed system and an object of study: if it only lasts for a ys, we are going to need even faster pulses to study its dynamics. In other words, if we regard the QGP as a condensed matter system, it is going to be really hard to invent the equivalents of neutron and X-ray scattering for this system. Perhaps we could use photons created in one QGP to probe another one…?

Nobel Prizes for Physics and Chemistry 2009

October 8, 2009

The Swedish Academy of Sciences has awarded this year’s Nobel prize for Physics to Charles K. Kao for “groundbreaking achievements concerning the transmission of light in fibers for optical communication” and to Willard S. Boyle and George E. Smith for “the invention of an imaging semiconductor circuit—the CCD sensor.” Physics Today offers a brief, and very readable, account of the breakthroughs that deserved this accolade.

In addition, the Nobel prize for chemistry this year has gone to Venkatraman Ramakrishnan, Thomas A. Steitz, and Ada E. Yonath “for studies of the structure and function of the ribosome” in which, as Physics Today points out, X-ray crystallography played a crucial role. Thus their achievement straddles beautifully across the fields of Physics, Chemistry and Biology.

The Nobel Prizes serve a dual purpose: on the one hand, they are an opportunity that the scientific community has to honor and reward those that have made the most important contributions. On the other hand, they represent the chance to send a message to the rest of society. Let us hope that these prizes will remind decision-makers and the public everywhere of, firstly, the crucial role that today’s research plays in creating tomorrow’s technologies and, secondly, that all fields of science are strongly inter-dependent, so it is not profitable to cherry-pick individual areas for support while others are neglected.

Are robots the next condensed matter?

September 23, 2009
Modular robots on the march... Is this the next condensed matter?

Modular robots on the march... Is this the next condensed matter?

Because condensed matter deals with concepts as fundamental as as scale invariance and broken symmetry, it is sometimes hard to predict where the next big condensed matter physics problem will turn up. For example (continuing from my latest post) who would have guessed twenty years ago that atomic physicists and high-energy physicists would today be furnishing some of the most interesting examples of strongly correlated matter? So it is interesting to be wild and speculate where the next big realm of condensed matter physics may lay. (After all, wild speculation is one of the things a blog may be useful for.)

In this spirit, I dare suggesting a look at modular reconfigurable robotics: robots made up of many individual, but interacting, identical elements. There are a number of groups around the world working on it. At the time of writing, there is a fairly detailed overview of the field on Wikipedia.

The Wikipedia article list a number of challenges for the future. It starts with the following (I quote):

Demonstration of a system with >1000 units. Physical demonstration of such a system will inevitably require rethinking key hardware and algorithmic issues, as well as handling noise and error.

A way to phrase this problem is to say that we want a large assembly of robots to behave like a condensed matter system, where a very large collection of individual, interacting elements (e.g. all the individual atoms in a magnetic material) conspire to produce a collective behaviour (e.g. ferromagnetism) in spit of the presence of errors and imperfections (e.g. impurities, missing atoms at indivudal lattice sites, and so on). So in some sense this problem of modular reconfigurable robotics has already been solved by Nature in condensed matter systems. Thus some important problems in modular reconfigurable robotics might be solved by looking to condensed matter for inspiration (e.g. find the conditions to achieve in the robots the equivalent of generalised rigidity).

Indeed researchers in that field are already having to draw on some elementary condensed matter concepts. See, for example, the PARC Modular Robotics website: the section on the Proteo project even has a good old-fashioned discussion of close packing structures.

Conversely, and perhaps even more interestingly (at least from a condensed matter theorist’s point of view) the robots could be used to realize new states of classical condensed matter – just as novel forms of quantum condensed matter are currently being created through chemical synthesis and in ultra-cold atom labs. Interestingly, the individual building blocks in the case of robots can be a lot more complex than in any form of condensed matter we currently know of e.g. the rules governing element-element interactions may be very complicated – for example, the interactions could be time-dependent or depend on the history of previous interactions for each individual particle (i.e. each individual robot module). It will be interesting to see whether such complexity of the individual particles will find a manifestation at the collective (macro) level or rather we will find that the beahviour of the whole always obeys simpler, emergent organisational principles. The latter is the case of, for example, a human crowd – though the interactions between individual components of a human crowd are in fact simple, while in the case of robots we might engineer them to be very complex.


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