From quantum chemistry to many-body theory of frustrated antiferromagnets

There are a wide range of organic charge transfer salts that can be described by a Hubbard model at half filling on the anisotropic triangular lattice. [See this review for a full discussion]. This model can also be viewed as a square lattice with hopping t' along one diagonal.
An important parameter is the ratio t'/t. The values t'=0, t'=t, and t=0, correspond to the square lattice, isotropic triangular lattice, and decoupled chains, respectively.

The actual value of t'/t is critical because the ground state of the Heisenberg model for the Mott insulating state [Neel, spiral order, valence bond crystal, spin liquid] is quite sensitive to the value of J'/J=(t'/t)^2. For example, as J'/J increases from 0.6 to 0.9 the order can change from Neel to valence bond crystal or spin liquid to spiral order.

So, what value do specific materials have? Do they vary with the counter anion?

Previously the parameters t and t' have been estimated from Huckel theory. An earlier post discusses recent progress at calculating these parameters for BEDT-TTF materials from computational methods based on Density Functional Theory (DFT).

My UQ colleagues Edan Scriven and Ben Powell recently reported new results for dmit materials in this preprint. J'/J varies from 0.4 to 1.4 with the counterion. Furthermore, if the calculated values are combined with the results of many-body theories of the corresponding Heisenberg model one obtains a consistent picture between theory and experiment.

The results should be compared with the figure below (taken from a recent review article by Kanoda and Kato) attempts to present a unified picture of the relationship between the ground state and the value of t'/t for a range of materials. However, the values used are based on Huckel calculations and the DFT calculations give significantly different values.

Outstanding challenges include

• finding a simple physical explanation for the origin of the parameter variation with counterion.
• calculating the pressure dependence of t'/t
• calculating Hubbard U values consistent with experiment [it seems screening is important]
• the previous two need to be combined to describe the pressure-temperature phase diagram, including the transition from a Mott insulator to a superconducting state

Universal magnetic excitations in the cuprates

I heard a nice talk by Bernhard Keimer which included a discussion of recent RIXS [Resonant Inelastic X-ray Scattering] results on the cuprates. The relevant Nature Physics article  states

As a major result, we demonstrate the existence of spin-wave-like dispersive magnetic excitations (paramagnons) deep inside the electron–hole spin-flip continuum (up to ~300 meV), for all the investigated doping levels, with spectral weights comparable to those of magnons in the undoped parent compounds.

The work is nicely put in context in a News and Views article by Matthias Vojta.

Extracting Fermi surface information from magnetoresistance

Today I am giving a talk Interlayer magnetoresistance as a probe of Fermi surface anisotropies at the GG conference on Strong Correlations and Spins.

Interlayer charge transport in the pseudogap state

Previously I have posted about the unusual anisotropies present in the temperature dependence of the resistivity in the pseudogap phase of the cuprates. In particular, the intralayer resistivity exhibits a metallic temperature dependence whereas the the interlayer resistivity exhibits a semi-conductor like temperature dependence. Much had been made of this previously with exotic explanations in terms of spin-charge separation. However, I posted about recent work showing how the data has a natural explanation in terms of the presence of a pseudogap with nodes in the same direction as that at which the interlayer hopping vanishes.
[This interlayer hopping anisotropy is quite important in understanding Angle-Dependent-MagnetoResistance (ADMR)].

I recently became aware of earlier phenomenological work by Tao Xiang and collaborators which also gives this explanation. In particular, this PRB gives a nice "universal" phenomenological form for the temperature dependence of the interlayer conductivity. This form is compared to data from a wide range of cuprates.

Seeking a unified theory for unconventional superconductors

I am currently in Sydney at the Gordon Godfrey workshop on Spins and Strong Correlations.
Yesterday Rajiv Singh began his talk on orbital effects in the iron pnictide superconductors with some general remarks about the relationship between magnetism and superconductivity. The two were once considered inimical (one of Bernd Matthias' rules). But we now see classes of superconductors (heavy fermions, organics, cuprates, and pnictides) where they appear to be intimately connected.

Rajiv then took a philosophical position: there should be a common physics for all of these non-electron-phonon superconductors.

This certainly reflects a physicists desire for universality and simplicity. This is in distinct contrast to a chemists focus on particularity.

I am not convinced that it is or should be the case that there is some common underlying physics. Here are a few disordered thoughts.

• For pnictides it seems that orbital (multiple band) effects matter. In contrast, in cuprates and organic it seems a single band is sufficient.
• If there is a unified theory I think the strongest candidate is a weak-coupling spin fluctuation RPA picture (with renormalised interactions and Fermi liquid quasi-particles). To hold to this one will have to show that "exotica" (e.g., non-Fermi liquid effects) are just some higher order perturbative effects.
• Similar sentiments of a unified picture of cuprates and pnictides is presented by Basov and Chubukov.
• To me the two biggest problems for a "common physics" scenario are the pseudogap state in the cuprates and the spin liquid states in organics. They represent a "discontinuity" from the other materials and from any weak-coupling picture.

I welcome comments. Is there a common physics for non-electron-phonon coupled superconductors?

Should you use Turnitin?

Turnitin is commercial software that detects student plagiarism by comparing submitted assignments to everything on the web and a vast database of other assignments.

A few years ago I would have thought this was might be relevant to people teaching large courses of undergraduates in the humanities. However, I was wrong. Unfortunately, experience has shown that plagiarism does occur, even in physics courses, and at the graduate level. There are cases of students submitting Ph.D proposals and literature reviews that involve cutting and pasting text from papers and the internet. Although certainly not confined to them this can be more of a problem with students from non-Western countries. There do seem to be some "cultural" differences as to what is acceptable practice and what is not. This does not excuse it, but does mean that sometimes first-time offenders need to be gently cautioned and educated.

A range of offences can occur, ranging from sloppy referencing to blatantly copying large swathes of text and presenting them as ones own.

So if you don't use Turnitin (or something equivalent) try it. You won't know if there really is a problem until you check.

If you do detect plagiarism make sure you report it to the relevant academic authority. Do not just give the student a private warning. It is important that someone is keeping track.  Otherwise repeat offenders may not really understand the severity of their offence and get appropriately disciplined.

Another issue, which is harder to detect, is that of ghost writing.

Superconductivity video goes viral

Today Ben Powell gave a great colloquium at UQ on 100 years of superconductivity. During it he showed this great video. A shorter version went viral on the internet receiving more than 3 million views within less than a week of being posted!
It is great to see superconductivity is so popular.