Saturday, October 30, 2010

Grant application post-mortems

This week the Australian Research Council announced which grant applications were successful for funding for next year. So roughly 20 per cent of people were happy and 80 per cent were sad. After the exhiliration or devastation inevitably come the post-mortems, particularly from those who are unsuccessful. We offer each other a multitude of possible reasons for failure or success....

Professor Z was on the committee and he doesn't like my Ph.D advisor.... All they care about is number of publications.... They must have liked the bit I wrote about... Clearly they want people who work at the interface of chemistry and physics... I need more Nature papers.... I should have promised less... It is because I did not have a big name person on the grant... it is because I am working on such a hot topic.... Obviously they aren't going to fund 2 groups working in my area.... I think Dr. X must have been that negative referee... You need to promise lots ....People think my group has too much money....

The problem with this is that this is all speculation. There is usually NO evidence for any of these ideas. Most funding decisions involve a black box. Input is your application which contains tens of pages of information. The output is binary information: yes/no. The output has a large random and subjective element to it.

As scientists we should only draw conclusions based on the evidence at hand.
If you did not succeed, don't take it personally and give up. Keep applying.
If you did succeed, don't let it go to your head.

de Broglie on quantum foundations

In the latest APS News there is a This Month in Physics History column on Louis de Broglie. It contains an interesting quote:
[de Broglie] tried to develop a causal model to replace the probabilistic models of quantum mechanics, which was refined by David Bohm in the 1950s and known as the de Broglie-Bohm theory. While most of his colleagues embraced the notion that the statistical nature of atomic physics was all that could be known, de Broglie believed that “the statistical theories hide a completely determined and ascertainable reality behind variables which elude our experimental techniques.
This was similar to Einstein's view and has been given a very concrete expression in Stephen Adler's work discussed here.

Friday, October 29, 2010

Deconstructing charge transport in organic semiconductors

A key question about charge transport in organic molecular materials is:

What is the relative importance of disorder and dielectric relaxation [small polarons = Marcus-Hush theory] in determining the charge mobility?

There is a nice clear and succinct review article in Chemical Reviews from 2007 by Coropceanu et al.

The view that disorder is dominant has been advocated by Bassler and collaborators, in
terms of a Gaussian density of states. This leads to a  mobility with the temperature dependence

[I have not seen an analytical derivation, this seems to be based on curved fitting to the results of Monte Carlo simulations].
This is in contrast, to an activated form.


Aside: Coropceanu et al. claim "there is no full theoretical justification for such an Arrhenius like expression". I am mystified by this claim. Small polaron theory [and equivalently Marcus-Hush theory, together with the fluctuation-dissipation theorem] give such a form. Indeed, in the review article they later give such expressions.
But, that is not my main point.

It is also pointed out that distinguishing between these two models is difficult
with experimental data from a limited temperature range.
This can be seen clearly in the Figure below taken from a 2003 paper Low-k insulators as the choice of dielectrics in Organic Field-Effect Transistors




Hence, just because one can fit the data to one of the models one should NOT conclude that model is correct. Unfortunately, this is often forgotten...

Presumably measurements down to 1 K may help distinguish the two models, although apparently these devices can malfunction at lower temperature.

I have more to say about this data, and what it may say about the charge transport mechanism,  but will leave that for another day...

Thursday, October 28, 2010

A Reconstructed view of the "Fermi surface".

Mike Norman writes some of the nicest and most helpful review articles about the cuprate superconductors. They are balanced and insightful. This week at the cake meeting we are discussing his latest, Fermi surface re-construction and the origin of high-temperature superconductivity. It gives a succinct summary of the issues raised by the observation of quantum oscillations associated with Fermi surface features iin the underdoped cuprates. It is impressive the way Norman is so objective and does not push any of his own significant contributions as being the "answer".


A few things I learnt

The importance of the negative sign of the Hall constant.

One can only produce electron pockets near x=1/8 with a magnetic stripe potential.

It is not clear where the "spin zeroes" associated with quantum oscillations are.

A few minor comments

Perhaps he gives too much credit to the initial claims from 1992 of observation of quantum oscillations, based on explosive experiments at Los Alamos. He mentioned criticisms by Springford et al. that the data was all noise. Should people get any credit for making unjustified claims based on dodgy data, just because the claimed effect turns out to be real. 

The unresolved issue of closed Fermi surface pockets vs. open arcs is perhaps not emphasized enough. Perhaps Norman leans too much towards pockets.

Wednesday, October 27, 2010

From a biophysics Ph.D to energy consulting

A group of Biophysics students made a very impressive video featuring my former Ph.D student Joel Gilmore discussing his job working on renewable energy policy for Roam Consulting.

Monday, October 25, 2010

Cooking up a new physics course

Last week the New York Times had a fascinating article [in the Dining and Wine section!] about a new physics class at Harvard, Science and Cooking: from Haute Cuisine to Soft Matter Science. The course is for liberal arts majors, and features guest lectures from and problems assigned by (!) famous chefs.

Saturday, October 23, 2010

Physics research rocks on prime time!

A great scene in The Big Bang Theory (Season 3, Episode 4) is where Sheldon and Raj stare at a whiteboard trying to figure out how to detect dark matter. It is done to the tune of The Eye of the Tiger.

Friday, October 22, 2010

Male frog protein whips up a broth

For the biophysics class BIPH3001 students are required to select one recent scientific paper of interest to them and give a presentation to the class. This week one student, Heather Nutt, selected a 2009 Biophysical Journal paper, 
Ranaspumin-2: Structure and Function of a Surfactant Protein from the Foam Nests of a Tropical Frog
Her slides are here.
At first I thought this topic was a little obscure but it turns out to be fascinating. Below I highlight some of the things I found particularly interesting from the paper abstract. 
Ranaspumin-2 (Rsn-2) is a monomeric, 11 kDa surfactant protein identified as one of the major foam nest components of the túngara frog (Engystomops pustulosus), with an amino acid sequence unlike any other protein described so far. We report here on its structure in solution as determined by high-resolution NMR analysis, together with investigations of its conformation and packing at the air-water interface using a combination of infrared and neutron reflectivity techniques. Despite the lack of any significant sequence similarity, Rsn-2 in solution adopts a compact globular fold characteristic of the cystatin family, comprising a single helix over a four-stranded sheet, in a motif not previously associated with surfactant activity. The NMR structure of Rsn-2 shows no obvious amphiphilicity that might be anticipated for a surfactant protein. This suggests that it must undergo a significant conformational change when incorporated into the air-water interface that may involve a hinge-bending, clamshell opening of the separate helix and sheet segments to expose hydrophobic faces to air while maintaining the highly polar surfaces in contact with the underlying water layer. This model is supported by direct observation of the relative orientations of secondary structure elements at the interface by infrared reflection absorption spectroscopy, and by protein packing densities determined from neutron reflectivity profiles.

Thursday, October 21, 2010

Does the computer always give the right answer?


I thank Eric Bittner for sending me this in response to the last post. He says:

this on a road sign in Wales. The part in Welsh reads "we are out off the office at the moment, we'll translate this when we return". (or something like this). What had happened was that some one had sent a phrase to an automatic translation service, got an out of office reply, and not knowing Welsh, figured it was the correctly translated phrase. The point is that you can't blindly trust a computer or translation software to get things right.

Wednesday, October 20, 2010

A different kind of OLEDS!

I found this on amazon.com for an author blurb:

His research is in the area of quantum dynamics as applied to organic polymer semiconductors, object linking and embedding directory services (OLEDS), solar cells, and energy transport in biological systems.
I think it should be Organic Light Emitting Diodes!

A decent theory for characterising some organic solar cells

The current-voltage characteristics of organic heterojunctions (HJs) are often modeled using the generalized Shockley equation derived for inorganic diodes. However, since this description does not rigorously apply to organic semiconductor donor-acceptor (D-A) HJs, the extracted parameters lack a clear physical meaning. Here, we derive the current density-voltage (J-V) characteristic specifically for D-A HJ solar cells and show that it predicts the general dependence of dark current, open-circuit voltage (Voc), and short-circuit current (Jsc) on temperature and light intensity as well as the maximum Voc for a given D-A material pair....


 This is the beginning of the abstract for a recent PRB paper from Stephen Forrest and collaborators, that I want to understand. I certainly agree with the first two sentences!

Tuesday, October 19, 2010

Lawrie Lyons (1922-2010): a pioneer in organic photonics and electronics

Professor Lawrie Lyons died last thursday in Brisbane. He was the Foundation Professor of Physical Chemistry at University of Queensland from 1963 until his retirement in 1987. Long before it became a "hot" field he made many of the first measurements the optical and electronic properties of organic molecular crystals such as anthracene.  
In 1967 with Felix Gutmann (UNSW) he published, Organic Semiconductors  (Wiley, 1967) 858 pages. This work was of sufficient influence and importance that in 1983 Hendrik Keyzer revised and expanded it, Organic Semiconductors: Part 2 (Wiley, 1983). [I think it is the ultimate compliment if you write a book and someone else publishes a later edition. Other famous examples include Coulson's Valence and Lehringer's Biochemistry].
In 1971 he was elected a Fellow of the Australian Academy of Science. 
Lawrie was the scientific "grandfather" of Andrew Taylor, Head of Rutherford Appleton Laboratory and a driving force in making ISIS the world's leading neutron and muon source. At Sydney University, Laurie supervised the B.Sc. (Honours) thesis of John White (now at ANU), who was subsequently, Taylor's D.Phil supervisor at Oxford.
Following retirement, Lawrie pursued full-time a life-long passion. In 1989  he founded ISCAST, the Institute for the Study of Christianity in an Age of Science andTechnology.


I thank John Mainstone for providing some of the chronology.

Monday, October 18, 2010

There is no perfect Ph.D project

Students tend to pick the topic not the supervisor. This is a mistake. A colleague once said to me "students are very good at picking bad supervisors."

My view is you should find a supervisor who will train you to do good research. Later you can pursue your own very particular interests and tastes, if you have the opportunity. Pursuing them as a novice at the Ph.D level is usually the kiss of career death.

You need to own your project, whatever it is. Don't wait for your supervisor to make it work. If your project seems a struggle, a bit boring at times, .... welcome to research! If it was a breeze and exciting someone else would probably have already done it!
It is your responsibility to find what works and what does not. If you do make progress, solve a problem, or discover something new and unexpected you will find that in itself may be a greater reward than pursuing your favourite topic which you thought would be interesting.

Sunday, October 17, 2010

A cool Father's day present II

My family did it again. As they did last year, they gave me a DVD of a season of The Big Bang Theory. I am looking forward to watching all of Season 3. Most of the episodes I have not seen because we do not own a TV. But a few I have seen on planes. In the first episode Sheldon is dis-illusioned when he finds out that he did not detect magnetic monopoles and thus confirm string theory on his trip to the Arctic.... He was actually detecting noise from the electric can opener... Too bad he had already sent an email to everyone in the university about his results...

Thursday, October 14, 2010

"Seeing" quasi-particles go over the waterfall

Check out this experimental data!
I am very impressed with how over the past two decades the quality and resolution of Angle Resolved PhotoEmission Spectroscopy (ARPES) data has improved so significantly.

Nigel Hussey brought to my attention a beautiful paper  Anisotropic quasiparticle scattering rates in slightly underdoped to optimally doped high-temperature La2−xSrxCuO4 superconductors

This allows one to clearly see real quantum many-body effects with the "naked eye".

The Figure below shows an intensity plot as a function of energy and momentum from
an cuprate superconductor with approximately optimum doping. The intensity is proportional to the one electron spectral function (imaginary part of the Greens function).
If there are well defined quasi-particles this should have a clear maximum which disperses (i.e. defines an energy vs. momentum curve). The blue dashed line is the bare dispersion one estimates from band structure calculations. One can see that near the Fermi energy the actual dispersion curve has a much smaller slope indicative of signficant renormalisation due to many-body effects. The almost vertical slope between EI and EII is known as the "waterfall" (WF).
Notice that the quasi-particle peak gets broader as one moves away from the Fermi energy as one would expect for a "Fermi liquid" (or something similar).
Using a model bare dispersion one can extract both the energy and momentum dependence of the self energy. It turns out that the imaginary part has the form
where phi is the angle around the Fermi surface.
The first term is associated with disorder and the second with inelastic scattering in a marginal Fermi liquid (which scales linearly with frequency).
[In a Fermi liquid the scattering scales quadratically with frequency.].
The momentum (phi) dependence has a "d-wave" form will cold spots near the nodes.


I also found this particularly interesting because angle-dependent magnetoresistance measurements from Nigel Hussey's group on an overdoped cuprate found a similar angular dependence for the inelastic scattering rate and an approximately linear temperature dependence. (A Nature Physics paper describes this result and a PRL shows how the strength of the anisotropic scattering increases as the doping is reduced.)

If two different groups using different techniques on different materials observe the same physics it suggests that there is a significant effect here.

Wednesday, October 13, 2010

100 most influential living British scientists

Last week The Times (London) newspaper published in their Eureka magazine an ordered list of the 100 most important people in British science and engineering. Dame Athene Donald  (Soft condensed matter, Cambridge) was one of four panelists who made the selection. Condensed matter physicists who I could see on the list included Andre Geim, John Pendry (meta materials), Richard Friend (OLEDs and plastic electronics), and Steve Bramwell (spin ice).
[I have to confess I bought the newspaper for other reasons... to get the latest news for my son about the Liverpool FC sale...]


Perhaps people who could be argued might be on the list include Peter Littlewood (Head of the Cavendish lab, Cambridge), Andrew Taylor (Director of ISIS),  and Gabe Aeppli (Director, London Centre for Nanotechnology).

Monday, October 11, 2010

Bardeen International Airport?

I was intrigued and impressed that the Belgrade airport is called Belgrade Nikola Tesla Airport. This the only case I have encountered of an airport named after a scientist. They mostly seem to be named after politicians (e.g. JFK and Ronald Reagan) and rock stars (John Lennon in Liverpool). Here are a few proposals:

LAX - Linus Pauling
Urbana - John Bardeen
Newark - Phil Anderson
Adelaide - William Bragg

Any other ideas?

It is interesting reading the wikipedia page about Tesla, particularly the observation that he probably suffered from obessive-compulsive disorder. He is another example of where the dividing line between genius and mental illness is a fine one.

Sunday, October 10, 2010

An iconic disaster


While in Belgrade, my gracious host Darko Tanaskovic took me to afternoon tea at a really nice restaurant, The Balkan Express, which had spectacular views of the Danube. The restaurant uses the iconic picture below on many of its publicity material. 


I was curious if the train wreck actually was the Balkan Express. It turns to actually have been a French domestic train at Montparnasse station in Paris in 1895.

Saturday, October 9, 2010

Correlations clean up crystals

Yesterday I had some nice discussions at the Institute of Physics (Belgrade) with Darko Tanaskovic. One thing fascinating thing I learnt was about how near the Mott transition strong correlations can screen the effect of site disorder in the metallic phase. This is described in a PRL which contains the Figure below.

It shows the scattering rate at zero temperature [normalised to the non-interacting value] as a function of U/Uc where Uc is the value of U at which the Mott transition occurs. The different curves corresponding to increasing disorder strength [bottom to top]. Note that even a long way from the Mott transition [e.g. when U/Uc = 0.5 and so the effective mass may only be enhanced by a factor of 2] the scattering rate can be reduced an order of magnitude by the correlations.

Some of this can be capture semi-quantitatively for weak disorder by a slave boson treatment which gives the following analytical expression for the scattering rate
When I first started working on organic charge transfer salts about 15 years ago I remember people asking Jim Brooks why the crystals were so pure that one could observe beautiful quantum oscillations? After all they are just grown using $100s worth of electrochemistry equipment as opposed to the $M molecular beam epitaxy (MBE) machines used to make semiconductor heterostructures. Jim said that somehow "self assembly" and "self purification" occurs in the crystal growth.

The scattering rate in the actual materials can be less than 0.1 meV, which seems small given that it is easy to imagine variations in the site potential of the order of 10-100's meV [see e.g, the discussion here].

However, Darko's work may give another explanation for the apparent purity of these materials. Most of these organic metals are quite close to a Mott insulating phase and so the correlations actually screen the disorder that is present.

Friday, October 8, 2010

Talk in Belgrade

Tomorrow I am giving a colloquium, "Destruction of quasi-particles near the Mott transition," at the Institute of Physics Belgrade. Here is the current version of the slides for the talk. The key reference is this PRL.

Thursday, October 7, 2010

A key property of the cuprate superconductors

First, some trivia. Neville Mott wrote his seminal paper on the metal-insulator transition while at Bristol University. I am writing this in the H.H. Wills building where he worked and down the corridor from the Mott lecture theatre!

This week Nigel Hussey taught me something important about the cuprate superconductors: the electronic density of states (or effective mass) does not vary significantly with doping. This is inconsistent with the Brinkmann-Rice picture which predicts that the effective mass is inversely proportional to the doping.
This figure is taken from a paper by Wade et al.
The effective mass that one deduces from the optical conductivity also shows a weak doping dependence as summarised in the figure below, taken from this  PRB paper.

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.

Wednesday, October 6, 2010

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.

Monday, October 4, 2010

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.

Saturday, October 2, 2010

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
Y1-xPrxBa2Cu3O7, 
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].

Friday, October 1, 2010

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!