Monday, November 30, 2009

What are deconfined spinons?

They are the spin-1/2 quasi-particle excitations associated with a spin liquid ground state of a quantum antiferromagnet. Perhaps the easiest way to understand them is in contrast to the low-lying excitations in an antiferromagnet with an ordered ground state. Spontaneously broken symmetry is the key concept behind understanding the nature of these excitations. Specifically, for infinite systems the ground state is usually degenerate and is not invariant under the samesymmetries as the system Hamiltonian. This family of ground states is described by an ``order parameter'' which describes the extent of the symmetry breaking. For example, quantum antiferromagnets can be described by a Heisenberg model Hamiltonian which describes a lattice of spins which interact with their nearest (and sometimes next-nearest) neigbours on the lattice. The model Hamiltonian is invariant under rotations of all the spins and under lattice translations. However, both these symmetries can be broken by the ground states.

The figure (a) above shows a cartoon picture of a line of alternating spins in the common antiferromagnetic ground state. This symmetry breaking can be seen by the presence of new Bragg peaks in elastic neutron scattering. In contrast, a spin liquid can be defined as a ground state in which there is no broken spin or translational symmetries.

When there is a continuous symmetry that is broken, the lowest lying excitations are weakly interacting bosons, known as Goldstone modes, and are associated with small
long wave length ``rotations'' of the order parameter. (see Figure (a) above). For an antiferromagnet, these modes have spin-1, and are also referred to as magnons. They can also be viewed as the propagation of a spin flip through the lattice.

The Heisenberg antiferromagnet in one spatial dimension has distinctly different properties from in three dimensions. In one dimension, there is no symmetry breaking or long range magnetic order; the ground state is a spin liquid. The low-lying excitations have spin 1/2, and can be viewed as domains walls or solitons in the background fluctuating magnetic order (see Figure (b) above. Haldane showed these excitations obey fractional statistics (semions which are intermediate between fermions and bosons) in contrast to the spin 1 magnons which are bosons, in three dimensions. Scattering of neutrons creates spin 1 excitations which are composed of pairs of spinons with different momenta. These spinons are deconfined, i.e., they can propagate independently of one another. (See figure above).

It is possible to directly ``see'' the quasi-particles, and measure their energy and lifetimes using inelastic neutron scattering. One scatters a beam of neutrons off a magnetic material and measures the momentum and energy of the scattered neutrons.
If there are well-defined quasi-particles they will have a particular energy for each wavevector q. The neutron scattering cross section is proportional to the dynamical spin structure factor S(E,q) will then show well-defined peaks when E=h ω(q) (see Figure above).

In an actual material the deconfinement of spinons was first seen in the compound KCuF3 which is composed of linear chains of spin-1/2 copper ions. The experimental signature of deconfined spinons was the presence of substantial spectral weight at energies above the magnon dispersion (the lower dashed line in the Figure above) one would see in a semi-classical antiferromagnet.

The Figures above are taken from a review, Mapping atomic motions in materials, by Toby Perring (ISIS) in Materials Today. The spectrometers for inelastic neutron scattering that Toby has built at ISIS have produced much of the beautiful data (such as that shown above) which is keeping theorists such as myself very busy.

Whether or not spinons exist in any real two-dimensional material is controversial. A News and Views piece I wrote for Nature Physics several years ago briefly reviews some of the issues.

Something I am still not clear on is whether a spin liquid ground state is a necessary and/or a sufficient condition for the existence of spinons.

Any ideas?

Sunday, November 29, 2009

THE question

A good (and painful) question to ask when evaluating research, both our own and others, is:
What does the scientific community know now that we did not know when you began the research?
There is a similar probing question to ask yourself before you start a project (or a new sub-project). Suppose everything goes as well as can be hoped (i.e., you are able to complete the calculation, do the measurement, get the new technique to work, or make the compound). Then will you be able to say something new? If not, is it worth even trying?

The before question is a good one for both students and supervisors to contemplate. It is too easy for supervisors (including me) to say do this extra calculation (or make this extra compound and measure all its properties) without considering enough the time cost to the student or postdoc.

Thursday, November 26, 2009

The most important letter in your scientific career?

Hopefully, the title got your attention. This is mostly directed at people applying for postdocs.
The cover letter is key.
I believe most postdoc (and many faculty) applications live or die [i.e., get to the long short list] based on the quality of the cover letter.

You need to specifically answer the following specific questions:
  • Why are you interested in this specific job?
  • Why are you interested in this specific research?
  • Why should they hire specifically you for this specific job?
Most cover letters I receive are generic. People tend to write the same letter for every job they apply for. Furthermore, the research achievements and research interests they list are usually generic.

So to be specific!
Write something like:
"One of the scientific questions I am most interested in is "What is the physical mechanism for XXXX in material YYY? I recently read your nice paper "blah blah" in journal YY and I have been wondering if a similar approach might be relevant to answering my question. I welcome any suggestions from you in this regard....."
Dont write:
"Dear Professor,
I did my Ph.D on topic X and want to continue working on it [even though I have no idea as to whether you have ever worked in this area or have any interest in it]. I published lots of papers [even though most listed on the CV are "in preparation"] and will publish lots more if I come and work with you....]"
The letter is so important you should spend at least a day writing it, even though is should fit on a single page. Should should get a range of faculty to read it and provide feeback.

Wednesday, November 25, 2009

Diverse career options for physicists

Here are the slides of the fascinating talk that Joel Gilmore gave at the Careers session organised by the Australian Institute of Physics (Qld branch) last wednesday.

Deconstructing charge transport in complex materials

Consider a material in which there are two distinct charge carriers (e.g., electrons and protons or electrons and oxygen vacancies). A measurement of the conductivity of a sample will just yield the sum of the conductivities of the two individual charge carriers. Given that the physical conduction mechanism for the two carriers may be distinctly different (e.g., small polaron hopping vs. vacancy diffusion) the temperature, pressure, and composition dependence of the two components may be completely different. Is there a way to extract for each of the carriers the conductivity, density of carriers, and mobility? A few weeks ago I thought this was hopeless, but I was wrong.

So my favourite paper for this week has the weighty title
by Wei Lai and Sossina Haile from Caltech.
The paper is in a journal I have never looked at before, Journal of the American Ceramic Society (n.b. that is Ceramic not Chemical!)

The physics underlying the impedance technique is fascinating. It makes use of the concept of chemical capacitance:
The chemical capacitance has certain similarities to conventional dielectric capacitance. While the latter is a measure of the ability of the system to store electrical energy in the form of polarized electric dipoles, the former is a measure of the ability of the system to store chemical energy in the form of changes in stoichiometry
One measures the frequency dependence of the impedance of a sample of finite thickness. The sample acts like a circuit with a finite RC time constant, but the capacitance is due to the chemical capacitance. One then plots the imaginary part of the impedance vs. the real part (this is known as a Nyquist plot). Qualitatively it should look like one of the plots below.

The left one is what one obtains for a mixed ionic and electronic conductor where the two specifies have distinctly different conductivities. The right one occurs when the two components have comparable conductivities.

Lai and Haile apply the technique to a solid oxide fuel cell material (samaria doped cerium oxide at a range of oxygen partial pressures). They extract a considerable amount of information about the electronic and ionic conduction which is of great interest to Elvis Shoko, Michael Smith and myself. Elvis brought the paper to our attention.

I wonder whether this technique would also be useful for understanding charge transport in hydrated melanin, Gratzel cells, and conjugated polymers with charged functional groups and counterions.

Tuesday, November 24, 2009

Quantum frustration in a nutshell

Understanding lattice models for strongly correlated electron systems is a major challenge. Widely studied (and still poorly understood) models include the Hubbard and Heisenberg models. But some insight can be gained from studying model Hamiltonians on small clusters such as four lattice sites. Although, such a small system is far from the thermodynamic limit, these models can illustrate some of the essential physics associated with the interplay of strong electronic correlations, frustration, and quantum fluctuations. They illustrate the quantum numbers of important low-lying quantum states, the dominant short-range correlations, and how frustration changes the competition between these states.

Furthermore, understanding these small clusters is a pre-requisite for cluster extensions of dynamical mean-field theory and rotationally invariant slave boson mean-field theory which describes band selective and momentum space selective Mott transitions. See for example my earlier post on that.

I now give a concrete example which illustrates how frustration can change the quantum numbers of the ground and first excited states. This is taken from a 1996 PRL, Plaquette Resonating valence bond ground state of CaV4O9, by Ueda, Kontani, Sigrist, and Lee. They first consider a single

Sunday, November 22, 2009

Tough times for science in California

My wife brought to my attention an article in the New York Times about the consequences of California's budget woes for the University of California system, and especially Berkeley. People interviewed include Bob Birgeneau (famous for inelastic neutron scattering studies of strongly correlated electron materials, now Chancellor at Berkeley) and Richard Mathies (pioneer in femtosecond spectroscopy, now Dean of the College of Chemistry, Berkeley)

Saturday, November 21, 2009

Teaching high school physics

Dr Richard Walding gave some insights into the current state of high school Physics teaching in Queensland at the AIP Careers seminar on wednesday. He was Head of Science for the past 20 years at Moreton Bay College and now tutors Senior Physics student teachers at Griffith Uni. Richard said tha the demand for physics teachers was very strong, not only here in Queensland but everywhere in the word. Teacher recruiting companies have an unmet demand for physics teachers and it would seem they can place you at countries all over the world. Richard said the starting wage for a graduate was about $51,000 rising to $72,000 after 7 years or so. He said there were 7000 Senior Physics students in Queensland in Yr 11 and 12 from 176 schools and taught by about 220 physics teachers. Most of the teachers had BSc degrees but, surprisingly enough, only a handful majored in physics at university.

To a question about whether you'd have to teach other subjects beside Physics and Junior Science if you were employed as a Physics teacher, Richard said that from his knowledge you may get some Junior Maths but would be unlikely to get anything else unless you asked for it and had been trained for it in your PostGrad Education qualification (e.g. Senior Maths). Richard made the point that teaching didn't suit everyone and some student teachers drop out either during their coursework or soon after. He said he had several former scientists and engineers as student teachers who wanted to become Physics teachers and in most cases they were a success; however, some retuned to their former profession within a year or so as it wasn't what they expected.

Richard said that he never planned to be a Physics teacher; he started his career as an Industrial Chemist but became a teacher after 5 years in industry. As he had physics in his degree the Principal of his school made him take Senior Physics even though he did not do it in his GradDipEd degree. He said he grew to enjoy it and has taught it for the past 30 years. He also said that there was plenty of scope for original research while teaching. He undertook an MSc, MPhil and PhD to satisfy his curiosity about aspects of science education and he said that many teachers were doing postgrad studies (Masters and Doctorates) as part of their Professional development and to help with their promotion prospects. Richard said that a Senior Teacher's wage is $75000 and a Head of Department from $84000 to $88000 - although the recent pay deal would see that increase by a further 8% or so over the next 3 years.

He stressed that Physics teaching may be different today that it was when people in the audience were in high school. There was more emphasis on extended experimental investigations and the report writing that entailed, and with more questions and situations involving evaluating and justifying conclusions (than with the older style emphasis on quantitative, algorithmic, closed problem solving questions). He said it would be worth having a look at the 2007 Senior Physics syllabus to get a better idea. This is available online at
If you have any questions, you could email Richard at:

Richard is also co-author of a high school physics textbook,
that Joel Gilmore (Science communicator extraordinaire and my former Ph.D student) was so impressed with he bought it off a student he was tutoring when she finished high school!

Friday, November 20, 2009

Our tendency to scientific fantasy not reality

More great quotes from Bob Laughlin, A Different Universe: Reinventing Physics from the Bottom Down
“The great power of science is its ability, through brutal objectivity, to reveal to us truth we did not anticipate.”
(p. xvi)

``mythologies are immensely powerful things, and sometimes we humans go to enormous lengths to see the world as we think it should be, even when the evidence says we are mistaken.’’
(p. 114)

“ideologies preclude discovery. All of us see the world as we wish it were rather than as it actually is.”
(p. 116).

There are similarities to the cautions of Walter Kauzmann, in his Reminiscences of a Life in Protein Chemistry.

Thursday, November 19, 2009

Careering out of control

Here is a copy of the talk I gave tonight on academic careers.

Some of the feedback included:

* the importance of mental health issues (I will try and do a few future posts on this).

* dogged perseverance is often a key component to success

It would be good to get some discussion going on some of the issues I raise in the talk.

Are your perceptions of stress objectively accurate?

Self-esteem, stress, and depression among graduate students.
Kreger DW. Wright Institute, Berkeley, CA 94704, USA.
Psychological Reports 1995 Feb;76(1):345-6.

In a study of 29 graduate students, self-ratings of stress correlated with low scores on self-esteem but were not related to an objective indicator of actual stress. Both self-rated stress and low self-esteem scores were related to scores on depression, with a weak interaction effect.

Wednesday, November 18, 2009

Advice on a research career

Tomorrow evening I am giving a talk on career advice for people who want to pursue a career in scientific research for a careers night of the local branch of the Australian Institute of Physics. Currently, points I am planning to make include:

There is more to life than a research career.

Be realistic and consider alternative careers.

Learn to write, to get along with other people, ....

plus previous career advice I have posted, especially this advice to Ph.D students.

When was the first BEC observed?

I am getting tired of hearing talks and reading reports which state, "The first Bose-Einstein condensate was observed in 1995." I think a more accurate statement would be "The first BEC in a dilute atomic gas was observed in 1995." Many would argue that superfluid 4He is a BEC. This new phase of matter was first observed around 1930. In 1932 Fritz London proposed that this was a BEC. (BTW, this is the same London as in the Heitler-London wave function, the London penetration depth, and London dispersion forces...).
But it should be noted that the case of a BEC in superfluid He is not as clear cut as in dilute atomic gases. Nevertheless, I dont think these subtleties validate ignoring 80 years of research on superfluid helium. A very useful summary of the history and the associated physical issues is contained in this nice article by Sebastian Balibar.

I think that people who are supervising Ph.D students on BEC's should be familiar with these issues, make sure their students are aware of this history, and present their work in the appropriate context. But then I am just a grumpy old condensed matter physicist....

Tuesday, November 17, 2009

Organic LED's in nature

Just how efficient are biomolecular systems? For a long time it was claimed that fireflies had a quantum efficiency close to 100 per cent. However, it was recently found this is not the case, the efficiency being about 40%. A nice summary of the work is in this News and Views article in Nature Photonics last year.

Besides measuring the quantum efficiency Ando et al. find that there are three components to the light emission and all are pH dependent. One component is of unknown origin. Clearly there is a correlation between the colour of the emission and the protonation state of the chromophore.

Only in a 2006 Nature paper was a structural basis for two different emission states proposed. I am curious as to how much quantum chemistry has been done on these issues. This may help address questions such as:

What determines the quantum efficiency of emission?
How are non-radiative decay channels suppressed?
What role does the protein environment play?

Monday, November 16, 2009

What is your goal?

I just went to a session in the department where 6 new staff members each had 5 minutes to introduce themselves and their research. If you have such an opportunity I would advise trying to just answer the following questions.

What is the scientific question you want to answer in the next few years?

Why is this important? Why are you excited about it?

Where is Brisbane anyway?

Americans are known for scientific prowess but not their knowledge of geography! A colleague recently told me that he now begins all his talks overseas with a map of Australia and shows where Brisbane is. On my last trip to Europe I also did this. However, it took me a while to find the pictures I really wanted, those which I had seen on postcards. I eventually found them and will include one in all my future talks overseas.

They really put the size of Australia in perspective.

Saturday, November 14, 2009

Frustrated quantum spin models in a nutshell

Subir Sachdev has a Physics article which provides a background to recent work using tensor networks (inspired by quantum information theory) to find the ground state of quantum spin lattice models. I really like the following succinct summary of the problem:

The simplest of these problems involve only the spin operators Si of electrons residing on the sites, i, of a regular lattice. Each electron can have its spin oriented either up or down, leading to a Hilbert space of 2N states, on a lattice of N sites. On this space acts the Heisenberg Hamiltonian

H=i<j JijSiSj, (1)

where the Jij are a set of short-range exchange interactions, the strongest of which have Jij>0, i.e., are antiferromagnetic. We would like to map the ground-state phase diagram of H as a function of the Jij for a variety of lattices in the limit of N. Note that we are not interested in obtaining the exact wave function of the ground state: this is a hopeless task in dimensions greater than one. Rather, we would be satisfied in a qualitative characterization of each phase in the space of the Jij. Among the possible phases are

(i) a Néel phase, in which the spins have a definite orientation just as in the classical antiferromagnet, with the spin expectation values all parallel or antiparallel to each other;

(ii) a spiral antiferromagnet, which is magnetically ordered like the Néel phase, but the spins are not collinear;

(iii) a valence bond solid (VBS), with the spins paired into S=0 valence bonds, which then crystallize into a preferred arrangement that breaks the lattice symmetry; and

(iv) a spin liquid, with no broken symmetries, neutral S=1/2 elementary excitations, and varieties of a subtle “topological” order.

Specific examples occur for Heisenberg models on the (i) square lattice, (ii), triangular lattice, (iii) kagome lattice (probably).

An example of a model which contains all three is here.

We are still searching for an example of (iv).

Friday, November 13, 2009

Twisting charges apart

This follows up with the earlier post about a paper, Conical Intersections, Charge Localization and Photoisomerization Pathway Selection in a Minimal Model of a Degenerate Monomethine Dye by Seth Olsen and I, which been accepted for publication in Journal of Chemical Physics.

An important issue is after a organic molecule absorbs a photon what conformational change will occur. Below are several options involving bond twists.

A second issue is how the charge distribution in the molecule changes upon twisting.

This kind of physics is at the heart of how your eye works. When retinal absorbs a photon it undergoes a conformational change which produces a charge separation which eventually leads to an electrical signal in your brain. It is also at the heart of designing better organic solar cells.

We considered a model Hamiltonian for a large class of dyes. The figure below shows contour plots for the first excited state potential energy surface for several parameter values. The lower part of the figure shows how the charge distribution in the molecule changes for different conformations. It is amazing how such a simple model Hamiltonian can capture such rich physics.

Thursday, November 12, 2009

Time management tip

I have not read the book but found the title very helpful and challenging!

Wednesday, November 11, 2009

Computational modeling of complex chemical systems: the state of the art

If you were going to an isolated island to do computational chemistry and you could take this year’s computers and 10-year-old algorithms or this year’s algorithms and 10-year-old computers, which would choose to take?

This is a question that Donald Truhlar asks in a JACS Editorial for a Select issue of 23 papers on Molecular Modeling of Complex Chemical Systems.

How would you answer the question?
You can look in the article to see how most computational chemists would answer the question.

The article is a very nice read to a physicist because it provides a very helpful and concise summary of historical landmarks in the computational modeling of large chemical systems.

However, I disagree and am concerned with one of the opening statements in the article:
Almost all modern theoretical chemistry is computational chemistry, because most of the progress that can be made with pencil and paper without a computer has been already made. Computations on complex systems are, in my opinion, the current frontier of theoretical chemistry.
I fear this does reflect the view of most in the theoretical chemistry community. However, (as a physicist) I think a much greater emphasis needs to be placed on gaining insights, finding organisation principles, and developing analytical models that complement simulations. But, that is what I am trying to do (and excited about!).

Tuesday, November 10, 2009

Many worlds or many words?

In 1998 Max Tegmark wrote a paper with the great title, The Interpretation of Quantum Mechanics: Many worlds or many words. He suggested that the validity of different interpretations of quantum theory cannot be decided empirically, but are:
“purely a matter of taste, roughly equivalent to whether or not one believes mathematical language or human language to be more fundamental.”
But it is interesting that he is now writing articles about multiverses...

Monday, November 9, 2009

Schrodinger was right on the money!

In 1983 Austria introduced this bank note featuring Erwin Schroedinger. Note the Psi! But where is the cat?

It is interesting they also introduced a 50 Schilling note with Sigmund Freud and a 5000 Schilling note for Mozart. Is this a relative measure of their contributions to culture and society?

The decay path taken

I am very happy that a paper, Conical Intersections, Charge Localization and Photoisomerization Pathway Selection in a Minimal Model of a Degenerate Monomethine Dye by Seth Olsen and I, has been accepted for publication in Journal of Chemical Physics.

A key question concerning optically active molecules is what is dynamics of the excited states?
Specifically, what are the predominant non-radiative decay mechanisms. The schematic below shows several options for the energy of the potential energy surfaces versus some configurational co-ordinate. On the left both S1 and S2 excited states decay to a conical intersection with the ground state. In contrast, on the right they have distinctly different decay paths.

But how does one go beyond such schematics. It turns out that for a broad class of dyes one can justify from high level quantum chemistry calculations a description in terms of just three valence bond states (see below).

The description in terms of the three diabatic states allow us to consider a somewhat "generic" or minimal model which exhibits a number of significant features, including:
  • Conical intersections between the S1 and S0 surfaces only occur for large twist angles.
  • In contrast, S2/S1 intersections can occur near the Franck-Condon region.
  • When the molecule has left-right symmetry, all intersections are associated with con- or dis-rotations and never with single bond twists.
  • For asymmetric molecules (i.e. where the bridge couples more strongly to one end) then the S2 and S1 surfaces bias torsion about different bonds.
  • Charge localization and torsion pathway biasing are correlated.
I may write more about some specific results later.

BTW, if you look at the paper look at the great job Seth did at writing Informative section headings!

Sunday, November 8, 2009

MO vs. VB for ketone dyes

I am thinking more about photophysical properties of ketocyanine dyes. I have puzzled through the basics of the molecular orbitals. Some material on this site is useful, including a visualisation of the molecular orbitals of formaldehyde. A key point is that the HOMO is a non-bonding orbital centred on the O atom and lying perpendicual to the C=O bond.

Thinking in the alternative resonating valence bond picture there will be three alternative Lewis structures
C -R

C -R

C -R+

The extent to which the lower two structures contribute will increase the C-O bond length and reduce the C-O stretch frequency.
It should be possible to describe the low lying excited states in terms of a complete active space with 4 electrons in 3 orbitals (a pi* orbital on the C=O bridge, and a pi orbital on the left and right fragments).

Saturday, November 7, 2009

Inventing your mother-in-law or More is Different II

P. W. Anderson, “Emergence, Reductionism and the Seamless Web: When and Why Is Science Right,” Current Science 78:6 (2000), 1. [Based on the Pagels lecture, Aspen, 1999].

Anderson suggests that emergence is the mechanism for consilience (the unifying of disparate pieces of knowledge) and reduction is the evidence for it. Theories may be under-determined, i.e., there may be many possible theories that can explain what is actually known. Hence, a successful theory may not actually correspond to what is happening. If there are only a few constraints (hypotheses, observations) that a theory must satisfy it has sometimes been the case that more than one theory can satisfy the constraints. However, as the number of constraints increases, acceptance of a theory is more likely and it becomes hard to conceive of alternative theories that could satisfy these constraints. Reduction can greatly increase the number of conditions that a theory must satisfy. For example, any alternative to quantum theory must be able to explain all known principles of atomic physics and of chemistry. Anderson concludes ``it is as impossible to `socially construct’ science as it is to invent A. Abrikosov or your mother-in-law.’’

Anderson mentions work of Kirkpatrick concerning the problem of satisfying many constraints. An example is this Science paper.

Friday, November 6, 2009

Pauling and Bardeen on postage stamps

It is wonderful that last year the US Postal Service issued new stamps featuring four prominent scientists. Pauling and Bardeen were indeed masters in unravelling emergent phenomenon using quantum many-body theory.

Update (2016). I just discovered that in 2005 there was one for Josiah Willard Gibbs  and Feynman.

Thursday, November 5, 2009

Modelling a class of organic dyes

Ketocyanine dyes are of considerable interest. An example is shown above. What are the essential ingredients that determine their photophysical properties? The figure below is from a nice paper which compares essential differences between cyanines (CY), ketocyanines (KCY), and squarenes (SQ).

The difference between the upper and lower panels (A and B) relates [I think] to whether one has an even (A) or odd (B) number of p-electron centres on each of the molecular units on the left and right side of the central C=O bridge. Apparently, this is following a "composite molecule" approach in a book by Fabian and Hartmann.

A complementary approach to describing optical properties of these materials is within a resonating valence bond approach. There will be three dominant resonant structures, similar to those advocated by Pauling for urea. [I will try and get a picture]. Such an approach will naturally lead to two low-lying singlet excited states.

This type of resonance is of great biochemical significance since it leads to the planarity of amide groups in polypeptides, something figured out by Pauling and Corey in 1952, an emphasized because it is required for the alpha helix of proteins.

Wednesday, November 4, 2009

A definitive signature of incoherent transport

Following up on previous posts about the absence of energy bands and charge transport in dendrimers. For incoherent transport (equivalent to hopping or small polaron transport) the mobility and conductivity has an activated form where the activation energy is one quarter of the reorganisation energy associated with a charge moving off or on the relevant unit. But how can one distinguish such activated behaviour from what occurs in a regular semiconductor?

Also measure the temperature dependence of the thermopower. The associated energy scale will be much less than the activation energy of the mobility. This is seen nicely in the figure below taken from a review by Salamon and Jaime about colossal mangetoresistance materials.

Failing to see the superconducting transition

Stare at the figure below. Can you see any difference between the top and the bottom panels?
[You can left click on the figure to make it larger.]

Yes. There is essentially no difference. This is not boring. It is fascinating.
The figure is taken from a beautiful Science paper by Seamus Davis group. It represents images from the fourier transform of STM images of the surface of an underdoped cuprate superconductor. From bottom to top the temperature increases from 0.1Tc to 1.5Tc where Tc is the superconducting transition temperature.

Basically this shows that at low energies the pseudogap phase has the identical excitation spectrum as a phase-disordered version of the d-wave superconducting state.

Tuesday, November 3, 2009

Water really matters

Previously, I wrote a post about meeting Roland Wester and the prospect of doing high resolution spectroscopy experiments on trapped ions of organic chromophores. Looking at a review Roland wrote about the technique led me to this beautiful JACS paper which desribes experiments on tryptophan [a flourescent amino acid]. The spectra below show how the protonated Trp has a broad absorption line due to non-radiative decay. Adding one and two water molecules, which hydrogen bond, removes this decay channel resulting in sharp vibrational features in the spectra.

The figures below show how the potential energy surface varies with twisting about the central carbon atom of the molecule.

Similar experiments on methine dyes such as the green flourescent protein chromophore would be wonderful!

Monday, November 2, 2009

A Ph.D is not enough!

[Left click to enlarge and make legible.]

A Ph.D is not enough! A guide to survival in science by Peter Feibelman.
15 years ago I discovered this book, read it, and wrote the enthusiastic review above. I still think it has much wise, practical and helpful advice. I try to get everyone I work with to read it.

Sunday, November 1, 2009

Spin fluctuations in iron pnictide superconductors

Ilya Eremin kindly sent a link to a copy of the nice talk he gave on spin fluctuations in the iron pnictide superconductors. Reading the slides will be more helpful than my rough notes I posted earlier. Important issues addressed include:
  • What are similarities and differences between the cuprates and pnictides?
  • Is the key difference, the pnictides are less strongly correlated?
  • If so, how does one describe/explain the magnetic order in the pnictides?