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?
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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.

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!

A Ph.D is not enough!

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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.

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?
  

Hiding the truth

In their wonderful (and provocative) PNAS article, The Theory of Everything, Laughlin and Pines introduced the term protectorate to describe the insensitivity of higher level laws (organising principles) to the details of lower level laws. For example, the laws of thermodynamics are the same regardless of whether the microscopic dynamics of the constituent particles is quantum or classical. Universality in the theory of continuous phase transitions is another important example. Near the liquid-vapour critical point the critical exponents are independent of the chemical composition of the system or of the interatomic forces involved.

Conference highlights

Here are a few things I learnt this week:

Dirk Manske showed that it is very difficult to use electron-phonon coupling to simultaneously explain more than one set of experimental results in the cuprates. Those, such as Devereux and Shen (Stanford) who claim they can must use values for the coupling constants which are as much as an order of magnitude different from what electronic structure calculations give (a case of where the details matter). A nice (but perhaps understated) summary of the results is here.

Jurgen Haase showed how NMR experiments on the cuprates exhibit very large line widths which can be explained in terms of a large spatially inhomogeneous distribution of dopings. Here is one reference, but I would like to find others.
He did not discuss it but he is also a proponent (with Slichter and Williams) of the two-component model for the cuprates, supporting Baryzkin and Pines.

more to follow...