From quantum spin liquids to superconductors

Today I am giving a CMT seminar in the Physics Department at the Indian Institute of Science in Bangalore, "Spin frustration in organic Mott insulators: from quantum spin liquids to superconductors." Slides are here.

The talk material is covered in great detail in a review article, written with Ben Powell.

Postdoc available in condensed matter theory at UQ

I have just advertised for a new postdoc to work with me on the theory of strongly correlated electron materials. You can see more details via the official advertisement. The flavour of some of the research can be seen from my blog posts under the "thermoelectric" label on this blog.

Talk on quantum proton transfer in enzymes

On wednesday morning I am giving an informal talk, Quantum proton transfer in enzymes, in the Dept. of Inorganic and Physical Chemistry at IISc Bangalore.
It is largely based on this paper.

Dramatic isotope effects near the Mott insulator

When I gave a seminar last week in the Inorganic and Physical Chemistry department at IISc the slide that generated the most interest and discussion was the one below.

The figure is taken from this PRB.
It shows how as one increases the number of deuterium atoms (replacing H) on the BEDT-TTF molecules (shown below) one gradually moves from the superconducting (S.C.) phase into the antiferromagnetic Mott insulator (A.F.I) phase.
This transition is analogous to what happens if one decreases the pressure or replaces the Br with Cl, i.e. it is generally associated with a change in the intermolecular spacings. Theory suggests this corresponds to increasing U/W [ratio of the Hubbard U to the bandwidth W]. It may also be due to a decrease in electronic frustration.

Why is this surprising?

First, this must be a quantum nuclear effect (e.g. due to zero point motion) because the chemical forces [potential energy surfaces] for H and D are identical. This is the Born-Oppenheimer approximation.

Second, the relevant molecular orbitals are centred [i.e. have the dominant electronic density] on the S and C atoms and not the H atoms.

Third, one can expect some sort of geometric isotope effect where there are small changes in bond lengths and crystal lattice parameters with H to D substitution. However, generally these effects are very small, particularly for systems in which hydrogen bonding plays a small role. Here the H bonding is with the anion. Thus it is hard to see how these small geometric changes might produce large enough changes in the band structure parameters t and t' in the relevant Hubbard model.

Further insight might be gained by doing DFT-based calculations of the parameters t and t' for the geometries of the H and D crystals determined by x-ray diffraction.

Geometric solid state chemistry

A while ago I posted about the incredibly rich crystal structure of boron which has a unit cell of hundreds of atoms.
Today at IISER-TVM Jemmis gave me a nice overview of his work [see this paper] showing how this and the associated defect structures emerge naturally from several building blocks including B_12 icosahedra which turn out to be particularly stable.

One call also make fruitful analogies with Huckel type rules associated with aromatic molecules such as benzene.

Hydrogen bonding in Kerala

On thursday I am visiting the new Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM) [Trivandrum, Kerala], an hours flight from Bangalore.
I will give a talk to a mixed chemistry-physics audience,
"A quantum physicist looks at hydrogen bonding."
Here are the slides of the current version.
It is largely based on this paper.

I recently posted earlier about some nice work my host Jemmis did on blue-shifted hydrogen bonds.

From chemistry to RVB and spin liquids

Here are the slides for my talk this afternoon in the Department of Inorganic and Physical Chemistry at the Indian Institute of Science in Bangalore.
Superconducting organic charge transfer salts: from chemistry to quantum many-body physics

I feel the talk does not quite flow and needs work but I have run out of time so will see how it goes. One can't be a perfectionist, particularly the first time one gives a specific talk. You also have to be careful when making visits you does not spend most of your time preparing your talk, rather than actually talking science with your hosts!