Sunday, November 28, 2010

Frank Fenner (1914-2010): a legacy of public science

Frank Fenner (1914-2010), one of Australia's most distinguished scientists died this week. He was an immunologist who is best known for leading the world-wide eradication of the smallpox virus and for introducing myxomatosis to stop the rabbit plague in Australia. The latter led to an interesting study in evolution and genetics, described in a recent Cambridge University Press book, Myxomatosis he recently co-authored (in his nineties!).

He was also author of a classic text, Medical Virology, first published in 1970, now in its fourth edition.

I partly know of Fenner because my father knew him, through working at the John Curtin School of Medical Research (JCSMR) at the ANU in Canberra. The prolific Fenner also wrote an exhaustive history of the JCSMR. [Three Nobel Prizes have been awarded for work done in the JCSMR].

What has immunology got to do with emergence and physics? I have always been fascinated by the existence of the Reviews of Modern Physics article, Immunology for physicists.

Saturday, November 27, 2010

Abramowitz and Stegun online

This morning I was at home struggling with some Bessel function identities. I really wanted to look at the copy of Abramowitz and Stegun: Handbook of Mathematical Functions  that I have in my office but, I discovered that the complete text  is available online.
I know Wolfram Mathworld can be useful but it does not have the same level of detail as A&S.

Friday, November 26, 2010

Research income is a good measure of ...

While on the subject of money .... Research income is a good measure of .... research income, and not much more. Consider the following:

Obituaries and Nobel Prize citations do not mention how much research income someone received.

Grants are a means to an end, not an end in themselves. Sometimes they are necessary to do good research, either to hire people to do the work, or to purchase or build equipment. But, grants are not a sufficient condition to do good research.

Many significant discoveries, especially experimental ones, come from people doing "tabletop" science with small budgets. The discovery of graphene is a significant example.

A distinguished elderly colleague expressed his disappointment to me that his department newsletter was always trumpeting the grants that people got. He asked, "Why aren't there any articles about what discoveries they make with the money?"

I find it easier to get grants than to do really significant and original research. As I have posted before, the latter is very hard work. I doubt I am alone in this.

Grant writing involves a different skill set from actually doing the research. Most people are more adept at one than the other.

A more interesting metric than research income or total number of papers is the ratio of the two quantities. Citations and the h-index are better. But, in the end there is only one meaningful and worthwhile measure of research productivity: creation of significant new scientific knowledge. Don't forget it and get distracted by endlessly chasing money or comparing your income to others.

Money changes you

Phil Anderson finishes his classic More is Different article from Science in 1972 with the cheeky and amusing conclusion:
Marx said that "Quantitative differences become qualitative ones." But a dialogue in Paris from the 1920’s sums it up even more clearly:
FITZGERALD: The rich are different from us.
HEMMINGWAY: Yes, they have more money.
I often wondered what this was all about. It is worth reading Quote/Counterquote which explains that the dialogue never actually happened.

    Thursday, November 25, 2010

    Faculty position in Melbourne available

    I was asked to publicise a position in Condensed Matter Theory that has been advertised at University of Melbourne. I am always keen to see more Condensed Matter Theory in Australia!
    All the details are here.
    I believe that applications will still be received for a week or two after the advertised closing date (30 November), but after the closing date should also be submitted directly to Professor Les Allen or Professor Lloyd Hollenberg

    Wednesday, November 24, 2010

    Possible origin of anisotropic scattering in cuprates

    Previously I posted about the anisotropic scattering scattering rate in the optimally doped to overdoped cuprates. Both Angle-Dependent Magnetoresistance (ADMR) and Angle-Resolved PhotoEmission Spectroscopy (ARPES) suggest that it  has a d-wave variation around the Fermi surface and that it has a "marginal Fermi liquid" dependence on energy and temperature. ADMR measurements found that the strength of this scattering scales with the transition temperature, and hence increases as one moves towards optimal doping.

    This raises three important questions:

    1. What is the physical origin of this scattering [and the associated self energy]?  Superconducting, D-density wave, antiferromagnetic, or gauge fluctuations?

    2. Is this scattering relevant to the superconductivity? i.e., do the same interactions produce the superconductivity and/or do these interactions make the metallic state unstable to superconductivity?

    3. Is this relevant to formation of the pseudogap in the underdoped region? e.g., as the self energy increases in magnitude with decreasing doping does the pseudogap just result from new poles in the spectral function?

    I focus here on possible answers to 1. as there are already some attempts to answer this question in the literature. Back in 1998, Ioffe and Millis published a PRB paper focusing on the phenomenology of such a scattering rate but Section IV of their paper considered how superconducting fluctuations could produce an anisotropic scattering rate. They suggested that in the overdoped region the rate should scale with T^2, but it should be kept in mind this depends on what assumptions one makes about the temperature dependence of the correlation length.

    Walter Metzner and colleagues have been investigating D-density wave fluctuations near a quantum critical point associated with a Pomeranchuk instability [A PRL with Dell'Anna summarises the main results, including a scattering rate which scales with temperature]. Their starting point is an effective Hamiltonian which has a d-wave form factor built into it. But this is motivated by an earlier PRL which found that the forward scattering they deduced for the Hubbard model from renormalisation group flows.

    Maslov and Chubukov have also published papers on the subject, such as this PRB.

    A key question is what experiments might be a smoking gun to distinguish the different origin of the scattering. I wonder whether the observed weak dependence of the scattering on magnetic field [at least up to 50 Tesla] may help.

    Monday, November 22, 2010

    A beast of an issue

    If you don't think mental health problems will strike anyone you know it is worth reading this column by Kathleen Noonan which appeared in our local newspaper a few weeks back.

    Sunday, November 21, 2010

    What is wrong with these colloquia?

    Are you preparing a talk? There was a provocative article What's Wrong with Those Talks by David Mermin, published by Physics Today back in 1992. It is worth digesting, even if you do not agree with it. He does practice what he preaches. I once remember him reading a referee report from PRL once in a talk on quasi-crystals. (He claimed the referee was Linus Pauling). One of his main points is we need to be very modest about what we hope we can achieve in a talk, particularly a colloquium. People will rarely complain if the talk is too basic and they understand most of it. The primary purpose is to help people understand why you thought the project was so interesting that you embarked on it.

    A string theorist learns basic solid state physics

    The Big Bang Theory seems to be getting better all the time, both in terms of humour and scientific content. This weekend we watched The Einstein Approximation (Season 3, Episode 14). Sheldon is obsessed with understanding how the electrons in graphene are "massless". His brain is stuck and so he seeks out a mind numbing job that he hopes will remove this mental block [inspired by Einstein working in the Swiss patent office, hence the title]. Eventually, he realises the problem was that he was thinking of the electrons as particles rather than as waves diffracted off the hexagonal lattice formed by the carbon nuclei. The Big Blog Theory has a good discussion about graphene.
    I could not find a clip on YouTube of the relevant scenes where the physics is discussed. Let me know if you know of one.
    A synopsis is here.
    Finally, I wonder if this episode helped sway the Nobel Committee?

    Saturday, November 20, 2010

    The Fermi surface of overdoped cuprates

    When I was in Bristol a few months ago Nigel Hussey gave me a proof copy of a nice paper that has just appeared in the New Journal of Physics,
    A detailed de Haas–van Alphen effect study of the overdoped cuprate Tl2Ba2CuO6+δ
    by P M C Rourke, A F Bangura, T M Benseman, M Matusiak, J R Cooper, A Carrington and N E Hussey

    Here are a few things I found particularly interesting and significant about the paper.
    1. It is beautiful data!
    2. Estimating the actual doping level and "band filling" in cuprates is a notoriously difficult problem. But, measuring the dHvA oscillation frequency gives a very accurate measure of the Fermi surface area (via Onsager's relation). Luttingers theorem then gives the doping level. [see equation 15 in the paper].
    3. The intralayer Fermi surface and anisotropy of the interlayer hopping determined are consistent with independent determinations from Angle-Dependent MagnetoResistance (ADMR) performed by Hussey's group earlier. This increases confidence in the validity of both methods. Aside: It should be stressed that both are bulk low-energy probes, whereas ARPES and STM are surface probes. 
    4. The effective mass m*(therm) determined from the temperature dependence of the oscillations agrees with that determined from the specific heat and from ARPES. This is approximately 3 times the band mass, showing the presence of strong correlations, even in overdoped systems. The effective mass does not vary significantly with doping [see my earlier post on this].
    5. The presence of the oscillations put upper bounds on the amount of inhomogeneity in these samples, ruling out some claims concerning inhomogeneous doping.
    6. The Figure above shows how well the observed Fermi surface agrees with that calculated from the Local Density Approximation (LDA) of Density Functional Theory (DFT). I think this means that the momentum dependence of the real part of the self energy on the Fermi surface must be small.
    7. The "spin zeroes" allow one to determine the "effective mass" m*(susc) associated with the Zeeman splitting. [This could also be viewed as a effective g-factor g*]. 
    A few small comments:
    a. The equation below relates the specific heat coefficient gamma to the effective mass determined from dHvA
    This is often derived for a parabolic band. It is not widely that this is a very general relationship which holds for any quasi-two-dimensional band structure. Furthermore, the formula for the effective mass in terms of the derivative of the area of the Fermi surface gives this formula too. Hence, taking that derivative numerically, as done by Rourke et al., is unnecessary. This is shown and discussed in detail in a paper I wrote with Jaime a decade ago, Cyclotron effective masses in layered metals.
    b.  The ratio of the two effective masses, m*(susc) and m*(therm) equals the Sommerfeld-Wilson ratio, something I pointed out  in a preprint [but was not able to publish] long ago. 

    Friday, November 19, 2010

    Should debatable data generate theoretical hyper-activity?

    At the physics colloquium today recent experimental data was highlighted that has been interpreted as evidence for dark matter. I thought this looked familiar and recalled I wrote an earlier blog post Trust but verify, urging caution. If one does a search on the arxiv with the words "dark matter AND positron AND FERMI" one finds more than one hundred papers, many proposing exotic theoretical scenarios.
    It will be interesting to see in a decade whether all this theoretical hyper-activity was justified.

    Thursday, November 18, 2010

    Is chemical accuracy possible?

    Seth Olsen and I had a nice discussion this week about Monkhorst's paper, Chemical physics without the Born-Oppenheimer approximation: The molecular coupled-cluster method, who emphasizes that corrections to the Born-Oppenheimer approximation can be as large as ten per cent. This raises important questions about the dreams and dogmas of computational quantum chemists. The goal is to calculate energies (especially heats of reaction, binding energies, and activation energies) to "chemical accuracy" which is of the order of kBT, about 1 kcal/mol or 0.03 eV. This is much better than most methods can do. 

    The claim of computational chemists is that it just a matter of more computational power. In principle, if one uses a large enough basis set (for the atomic orbitals) and a sophisticated enough treatment of the electronic correlations, then one will converge on the correct answer. However, this is all done assuming the Born-Oppenheimer approximation and "clamped" nuclei. 
    But what about corrections to Born-Oppenheimer? Are there any specific cases of molecules for which any deviations between experiment and computations might be due to non-BOA corrections?
    Furthermore, there is a tricky issue of "parametrising" and "benchmarking" functionals, pseudo-potentials, and results against experiment. How can one separate out effects due to correlations and those due to corrections to BOA?

    How to do better on physics exams

    Since I am marking many first year exams here are a few tips. They are all basic but it is amazing and discouraging how few students do the following:

    Clearly state any assumptions you make. Don't just write down equations. e.g, explicitly state, "Because of the Newtons second law ..." or "we neglect transfer of heat to the environment."

    Keep track of units at each stage of the calculation. Don't just add them in at the end. All physical quantities DO have units. If you make a mistake it will often show up in getting the wrong units.

    Clearly state the answer you obtain. Maybe draw a box around it. e.g, "The change in entropy of the gas is 9.3 J/K".

    Try and be neat and set out your work clearly. If you make a mess just cross out the whole section and rewrite it. I see many students exam papers where I really have no idea what the student is doing. It is just a random collection of scribbles, equations, and numbers....

    Think about the answers you get and whether they make physical sense. e.g. if you calculate the efficiency of the turbine is 150 per cent or that the car is travelling at 1600 metres per second you have probably made a mistake!

    Wednesday, November 17, 2010

    Recommended summer school

    Telluride Science Research Center runs some great programs each northern summer, mostly with a chemistry orientation. One I would recommend, especially to physics postdocs and graduate students who want to see how quantum and stat. mech.  is relevant to chemistry is the Telluride School of Theoretical Chemistry which will run July 10-16 next year.

    Non-Markovian quantum dynamics in photosynthesis

    Understanding how photosynthetic systems convert photons into separated charge is of fundamental scientific interest and relevant to the desire to develop efficient photovoltaic cells. Systematic studies could also provide a laboratory to test theories of quantum dynamics in complex environments, for reasons I will try and justify below. When I was at U. Washington earlier this year Bill Parson brought to my attention two very nice papers from the group of Neal Woodbury at Arizona State.

    The Figures below are taken from the latter paper. I think the basic processes involved here are 

    P + H_A + photon ->  P* + H_A -> P+ + H_A-

    where a photon is absorbed by the P, producing the excited state P* which then decays non-radiatively by transfer of an electron to a neighbouring molecule H_A. The graphs below show the electron population on P as a function of time, for a range of different temperatures and with different mutants of the protein.  

    The different mutants correspond to substituting amino acids which are located near the special pair. These change the relative free energy of the P+ state.
    The simplest possible theory which might describe these experiments is Hush-Marcus theory. However, it predicts

    * the decay should be exponential with a single decay constant
    * the rate should decrease with decreasing temperature
    * the activation energy for the rate should be smallest when the energy difference between the two charge states epsilon equals the environmental re-organisation energy.

    The above two papers contained several important results which are inconsistent with the simplest version of Hush-Marcus theory:

    • The population does not exhibit simple exponential decay, but rather there are several different times scales, suggesting that the dynamics is non-Markovian, and may be correlated with the dynamics of the protein environment.
    • The temperature dependence: the rate increases with decreasing temperature.
    • A quantitative description of the data could be given in terms of a modification of Hush-Marcus to include the slow dynamics of the protein environment. This allows extraction of epsilon for the different mutants.
    Later I will present my perspective on this....

    Tuesday, November 16, 2010

    Finding the lost twin

    This beautiful picture is on the cover of A Chemist's guide to Valence Bond Theory by Shaik and Hiberty. It summarises the main idea in a paper, The Twin-Excited State as a Probe for the Transition State in Concerted Unimolecular Reactions: The Semibullvalene Rearrangement.
    It illustrates how the use of diabatic states (K1 and K2) based on chemical intuition can lead to adiabatic potential energy surfaces with complex structure. Furthermore, it illustrates the notion of an excited state (K1 - K2) which is a "twin state" to the ground state, K1+K2. The relevant vibrational frequency is higher in the excited state than in the ground state.
    An earlier post discussed the analogous picture for benzene.

    Sunday, November 14, 2010

    Quantum decoherence in the brain on prime time TV

    It is great how interesting physics still gets a mention in The Big Bang Theory. Last night my family watched The Maternal Congruence (Season 3, Episode 11). Leonard's mother [Beverley] comes for a visit and on the ride from the airport it turns out she and Sheldon have been in correspondence, unbeknown to Leonard. Here is the scene and dialogue I was delighted about:
    Beverley: Yes, dear. Mommy’s proud. I’ve been meaning to thank you for your notes on my paper disproving quantum brain dynamic theory.
    Sheldon: My pleasure. For a non-physicist, you have a remarkable grasp of how electric dipoles in the brain’s water molecules could not possibly form a Bose condensate.
    Leonard: Wait, wait, wait. When did you send my mom notes on a paper?
    On the Big Blog Theory, David Saltzberg [the UCLA Physics Professor who is a consultant to the show] discusses the "quantum dynamic brain theory" at length and mentions Roger Penrose and says:
    Quantum Brain Dynamicists entertain the idea that the same kind of condensate might exist in a living human brain, at normal body temperature. Does that sound pretty unlikely?  It did to me.  So I poked around a bit.  The amount of published material in refereed scientific journals turns out to be small.  Most of what I found about it was published on webpages and small publishers which is a red flag..... There are a few papers  on these ideas published by Springer, a serious publisher of scientific work.   Usually ideas about how the world works  separate nicely into mainstream (even if speculative) versus crackpot.  Here we find the distinction is not so clear.
    Saltzberg's scepticism is justified. The case again Penrose is actually much stronger than suggested by Saltzberg. Here I mention two papers I recently co-authored which discredit the "Quantum dynamic brain theory." 

    Weak, strong, and coherent regimes of Fröhlich condensation and their applications to terahertz medicine and quantum consciousness published in Proceedings of the National Academy of Sciences (USA)

    Saturday, November 13, 2010

    A grand challenge: calculate the charge mobility of a real organic material

    Previously I have written several posts about charge transport in organic materials for plastic electronic and photovoltaic devices. This week I looked over two recent articles in Accounts of Chemical Research that discuss progress at the very ambitious task of using computer simulations to calculate/predict properties of real disordered molecular materials beginning with DFT calculations of specific molecules. I was pleased to see that Marcus-Hush electron transfer theory plays a central role in both papers,

    Electronic Properties of Disordered Organic Semiconductors via QM/MM Simulations (from the group of Troy Van Voorhis at MIT). 

    Modeling Charge Transport in Organic Photovoltaic Materials (from Jenny Nelson's group at Imperial College London).

    I have several questions and concerns about the latter paper.

    The authors do a rather sophisticated simulation of a time of flight experiment where they put a charge accumulation on one side of the sample, apply an electric field, and measure
    the average charge velocity, and extract the mobility. Intermolecular hopping rates are given by Marcus-Hush theory with parameters extracted from DFT calculations.

    1. Is this the most efficient and reliable way to calculate mobility?
    Generally in solid state physics one uses the fluctuation-dissipation relation. In this case Einstein's relation the mobility can be obtained from the diffusion constant. The diffusion constant is just the intermolecular hopping rate times the square of the intermolecular distance.

    2. The mobility computed depends on the thickness of the sample.

    3. How was the calculation benchmarked? Does this method give reliable
    results for simpler systems, e.g., a naphthalene crystal?

    4. The abstract makes some very strong claims,
    "these computational methods simulate experimental mobilities within an order of magnitude at high electric fields. We ... reproduce the relative values of electron and hole mobility in a conjugated small molecule... We can reproduce the trends in mobility wiht molecular weight ... we quantitatively reproduce...On the basis of these results, we conclude that all of the necessary building blocks are in place for the predictive simulation of charge transport in macromolecular electronic materials and that such methods can be used as a tool toward the future rational design of functional organic electronic materials."

    But when I look at the graph above it looks to me that the method often disagrees with experiment by more than an order of magnitude and fails to capture any electric field dependence. I could not find any discussion of temperature dependence.

    Friday, November 12, 2010

    Beyond Born-Oppenheimer

    This morning I started to read a fascinating paper by Henrik Monkhorst, 

    It begins discussing a controversy I did not know about which 

    Later there is a fascinating discussion of the coupled cluster (CC) method. Note the superlatives in the last sentence.

    Thursday, November 11, 2010

    A sign of true love

    My son and I just watched a great episode of The Big Bang Theory. It is called the Gorilla Experiment and in it Penny decides that she wants to learn some physics so she can talk to Leonard about his experiment testing the Aharonov-Bohm effect with electric fields. She begs Sheldon to teach her leading to this amusing scene.

    Wednesday, November 10, 2010

    Adiabatic, non-adiabatic, or diabatic?

    The Diabatic Picture of Electron Transfer, Reaction Barriers, and Molecular DynamicsThis is the review article that we will be discussing at the cake meeting tomorrow. It is clear and helpful.

    Adiabatic states are the eigenstates of the electronic Schrodinger equation in the Born-Oppenheimer approximation. There are several problems with them
    -they can vary significantly in character with changes in nuclear geometry [in particular they can be singular near conical intersections]
    -they are not a good starting point for describing non-adiabatic processes such as transitions between potential energy surface
    -they are hard to connect to chemical intuitive concepts such as valence bond and ionic states

    Diabatic states  overcome some of these problems. They do not vary significantly with changes in nuclear geometry.
    However, determining them is non-trivial. The article describes these issues.

    Tuesday, November 9, 2010

    There is still a Kondo problem

    It was nice having Peter Wolfle visit UQ the past few days and give a colloquium style talk on the Kondo effect.
    A nice accessible article which discusses the basics of the Kondo effect and how it occurs in quantum dots, carbon nanotubes, and "quantum corrals" is this 2001 Physics World article by Leo Kouwenhoven and Leonid Glazman.
    In his talk Peter gave a nice discussion of the problem of the Kondo lattice (a lattice of localised spins interacting with a band of itinerant electrons). There is still an open question (originally posed by Doniach) of what happens in between the two limits of weak coupling (one expects magnetic ordering of the spins due to the RKKY interaction mediated by the itinerant electrons) and strong coupling (where the individual spins are Kondo screened by the itinerant electrons). Is there a quantum critical point? Does a non-Fermi liquid occur near it?
    All of this is discussed in a nice review article Peter co-authored, Fermi liquid instabilities at magnetic phase transitions. It was very useful to Michael Smith and I when we wrote a PRL about possible Weidemann-Franz violations at a quantum critical point.
    If you want a discussion of the RKKY-Kondo competition from the point of view of quantum entanglement, see this PRA paper  Sam Cho and I wrote.

    Spin ice on prime time TV

    In condensed matter one can see many analogues of phenomena in quantum field theory and elementary particle physics. There have been many fruitful illustrations of cross-fertilisation over the years: symmetry breaking, renormalisation, fractional quantum numbers, vortices, confinement by gauge fields, .....
    This raises controversial questions about which is more fundamental? Afterall, if the Higgs boson is just the analogue of gauge symmetry breaking in superconductors, why spend all that money on the LHC...

    Last week I watched a great episode of The Big Bang Theory, "The Vengeance Formulation" [Season 3, Episode 9]. It features some cool condensed matter physics. Sheldon, who is enamoured with string theory is interviewed on National Public Radio (NPR) for the show Science Friday.

    Leonard: Are you really going to be on NPR?
    Sheldon: Yes, they’re interviewing me by phone from my office, regarding the recent so-called discovery of magnetic monopoles in spin-ices. It’s pledge week and they’re trying to goose the ratings with a little controversy.

    [Complete dialogue is here].

    The Big Blog Theory gives a good summary of the science issues, suggesting Sheldon is having a go at two papers that appeared in Science at the same time as the episode was filmed. Michel Gringas wrote a Perspective piece on the two Science papers.

    Saturday, November 6, 2010

    Do literature reviews matter?

    In 1983, when I was a budding young student heading off to Princeton to do a physics Ph.D. I had the privilege of spending some time with an elderly John Edsall, an eminent Harvard biochemist, who was friend and collaborator of my father. I asked him if he had any advice for me. I expected him to say something profound and give me a long list of suggestions. He thought for a while and said, "Well I am not sure but I guess it is important to know the literature on what you are working on."
    I am not sure to what extent I took on board this advice over the next decade.
    But, now I think this was very good advice. The reason is knowing the literature can save you a lot of time.  If you are trying to do something someone else has already done then
    -perhaps there is no point in trying it yourself
    -you may be able to use what they have done to do something even better.

    It is amazing what a discerning Google Scholar search can pick up. On the other hand, you need to be careful you don't spend all your time downloading and reading papers. Also, don't assume your supervisor knows the literature.

    Friday, November 5, 2010

    What is a significant change of opinion?

    I am puzzled by something. Suppose I ask five people if they prefer tea or coffee? Three say coffee, two say tea. Four years later I ask them again. One of the five people has changed their mind, now two prefer coffee and three tea. Suppose the sample size is much larger, but still only one in five people change their preference. I would not say there was a "massive swing" in preference or that drinks "preference did an almost complete turnabout".

    Furthermore, suppose also that those five people had all previously said they did not wish to register as having a definite opinion on their preference. Then it should hardly be surprising if one in five changed their opinion.

    Hence, I am puzzled by an article in the Wall Street Journal by Gerald F. Seib, Unaligned votes tilt rightward en masse. It states:

    A massive swing by independent voters propelled the Republican Party to a series of key victories.... 
    In House races nationally, Republicans won the votes of independents—voters who said they aren't affiliated with any party—by a 55% to 40% margin, a compilation of exit polls from across the country showed.
    In other words, independents' preference did an almost complete turnabout over the last four years: They favored Democrats by 18 points then, Republicans by 15 points Tuesday.

    Thursday, November 4, 2010

    Trapped by solid state physics

    There is an interesting looking perspective piece in Journal of Physical Chemistry Letters
    Intrinsic Charge Trapping in Organic and Polymeric Semiconductors: A Physical Chemistry Perspective by L. G. Kaake, P. F. Barbara and X.-Y. Zhu

    It begins
    We aim to understand the origins of intrinsic charge carrier traps in organic and polymeric semiconductor materials from a physical chemistry perspective. In crystalline organic semiconductors, we point out some of the inadequacies in the description of intrinsic charge traps using language and concepts developed for inorganic semiconductors. In π-conjugated polymeric semiconductors, we suggest the presence of a two-tier electronic energy landscape, a bimodal majority landscape due to two dominant structural motifs and a minority electronic energy landscape from intrinsic charged defects.

    Tuesday, November 2, 2010

    Helping undergraduate students learn about research

    For the third year biophysics course BIPH3001 I helped run this semester, part of the assessment required that students select a recent research paper and prepare a presentation fo r the class, explaining the basic results and commenting on the strengths and weaknesses of the paper.

    After the experience I realised two important points about this exercise:

    1. Students should explicitly show how material they learnt in the rest of the course helped them understand the paper.

    2. Something very valuable can/should/may happen. Undergraduates learn that just because a research paper claims to have shown something does not mean that it is true. The sooner they learn a healthy scepticism the better.

    Monday, November 1, 2010

    Dielectric relaxation in organic electronic devices

    A recent post mentioned the paper Low-k insulators as the choice of dielectrics in Organic Field-Effect Transistors [by Veres et al.], in the context of the sticky point of how to determine the relative importance of disorder and polaronic effects in molecular conducting materials used in semiconductor type devices. 

    A major point of the paper is something different: how the mobility measured by time of flight (TOF) measurements is distinctly different [it has a larger magnitude and smaller activation energy] from in FETs, and that the mobility in FETs can decrease significantly with increasing the dielectric constant of the material used in the gate insulator material.

    The paper discusses extensively how this may be associated with different kinds of disorder at the interface [which was followed by a theoretical paper in J. Chem. Phys. by Richards, Bird, and Sirringhaus]. It is not clear to me that invoking disorder is necessary to explain the gate dielectric dependence sure this is necessary. The reorganisation energy (polaron binding energy) associated with charge transfer between neighbouring molecules varies significantly with the dielectric constant of the surrounding medium. [Reminder: the activation energy for the mobility is 1/4 of this reorganisation energy].

    For molecules close to the interface between the gate insulator and organic semiconductor, this reorganisation energy will decrease with a decrease in the dielectric constant of the insulator. Furthermore, if the organic semiconductor has a dielectric constant less than the gate dielectric, the reorganisation energy associated with a bulk measurement such as TOF will be less than a FET measurement which measures charge transport close to the interface.

    Indeed the observed dependence of the FET mobility on the gate dielectric constant is observed explained within the framework of small polaron theory in a 2005 Nature Materials paper, Tunable Frohlich polarons in organic single-crystal transistors [see Figure above].