Can a chemist synthesize this molecule?

Research in chemistry is all about making new molecules.

I have written a few previous posts about organometallic compounds and transition metal catalysts. Hence, it was interesting to see the 2010 Nobel Prize in Chemistry awarded for "palladium-catalyzed cross couplings in organic synthesis".

I got the picture above from the scientific background which is worth reading. The basic idea seems to be that one whats to form a carbon-carbon bond between R and R'' and this can be catalysed by first forming the R''-Pd-R intermediate.

What can theorists say about this chemistry? I found an interesting looking paper by Shaik (one of my favourite quantum chemsists) and collaborators which uses DFT to study this problem.

Interlayer magnetoresistance talk

Today I am giving a seminar at Bristol University, on Interlayer magnetoresistance in strongly correlated electron materials. The current version of the slides are here. The main point is that measurements of the dependence of the interlayer resistance on the direction of the magnetic field provides a powerful probe to map out anisotropies in intralayer Fermi surface properties.
The figure above is taken from a nice review article by Mark Kartsovnik, one of the pioneers of the field.

A low stakes game

This weekend's Sunday Times Magazine had a fascinating and sad article O The Wild Charges He Made about Orlando Figes, Professor of History at Birkbeck College, London who "brought himself to the brink of academic ruin by posting anonymous reviews disparaging colleagues and praising his own work."
[Unfortunately, the article is only available online with a subscription].

The article raises issues about mental health in the academy and the dangers of the internet particularly because of potential or perceived anonymity.

The article also quotes Sayre's law (giving too much credit to Henry Kissinger) which can be stated as "The reason academic politics are so bitter is that the stakes are so low." 
I only remember hearing this once before, from Andy Schofield in a discussion of The Masters by C.P. Snow. It is worth reading the Wikipedia page.

So bear this in mind next time you are worked up about something....

Laughing students learn

Previously I posted about Shankar's public lectures in Aspen on relativity and quantum physics. His lecture course Fundamentals of Physics at Yale is available online.
On his website you can also read a collection of his one liners.

Time to clean your desk

I saw this PhDComics in a hard copy of last week's Times Higher Education Supplement.

Anomalous magnetoresistance in the pseudogap state

Previously I wrote a post about a wide range of strongly correlated electron materials that exhibit a very puzzling and unexplained magnetoresistance. In particular, the dependence on the direction of the magnetic field is the opposite to what one expects for the Lorentz force.

I recently became aware of another example, the underdoped cuprate superconductor
Y_1-xPr_xBa₂Cu₃O_7, 
The angular dependence of the intra-layer magnetoresistance is described in this PRL and interlayer magnetoresistance data is available in a 2006 PRB.

The authors suggest that the anomalous angular dependence arises because the magnetoresistance is not dominated by quasi-particles but rather the flux flow from fluctuating superconducting vortices associated with pseudogap state.

There is an alternative explanation of the data due to Dora, Maki, and Virosztek. They consider an underlying d-density wave state [which has a pseudogap]. They claim that their theory describes some of the other materials in my original post and the heavy fermion material CeCoIn5. Hence, I need to understand this theory better. One question I have is how it deals with the violation of Kohler's rule. [I suspect it comes about due to thermal excitations across the pseudogap].

From chemical exotica to rich physics

I had a really nice meeting yesterday in the Clarendon lab with Amalia Coldea. She has been doing some magnetoresistance measurements on an interesting class of organic charge transfer salts, κ-β′′-(BEDT-TTF)₂(PO-CONHC₂H₄SO₃), where PO = 2,2,5,5 Tetramethyl-3-pyrrolin-1-oxyl Free Radical, described in a recent Chemistry of Materials paper.

Why should physicists care about such exotica?
A few things I think are particularly interesting about these new materials are the following.

• The anion is a free radical (i.e., has a localised spin 1/2). These spins interact via an exchange interaction with the itinerant electrons in the BEDT-TTF layers. Thus, the system is something like a Kondo lattice model. In some senses this is the spin 1/2 analogue of the magnetic field induced superconductor lambda-(BETS)2FeCl4 which has spin-5/2 (see this PRB for a discussion of the relevant theory).
• The crystal structure is such that there are alternating layers of BEDT-TTF molecules with two different stacking motifs (kappa and beta''). It is claimed that this leads to the two layers are doped away from half filling. The average filling is one half as for individual kappa and beta'' layers, but because they have different band structures the kappa (beta'') layers are at greater (less) than half filling. [This is a bit like what happens in TTF-TCNQ.] If correct this fulfills a long sought goal of doping organic charge transfer salts!
• The alternation of beta'' and kappa layers also means the interlayer charge transport could be particularly interesting because the interlayer hopping integral could vary significantly with intralayer momentum, as it does in the cuprates. This could lead to unusual angle-dependent magnetoresistance, as described in a theory paper by Yagi and Iye.

What are the prospects of seeing Kondo lattice type physics in these materials? Not good, I fear. The scale of the Heisenberg exchange interaction J_eff between the free radical spins is estimated to be of the order of 1 K. Assuming this is due an RKKY interaction, J_eff ~ J^2 D(E_F) where J is the Kondo exchange interaction and D(E_F) is the metallic density of states, estimated to be about 6 states/eV from the Pauli susceptibility. 

The Kondo temperature ~ E_F exp( -1/JD(E_F)), using the above estimates I obtain JD(E_F) ~ 0.02 and so the Kondo temperature will be many orders of magnitude less than a mK and so not experimentally accessible. However, as for the reasons outline above there is a lot of other interesting strongly correlated electron physics to be explored in these materials!