A clear trend is emerging in theoretical reports of dilute magnetic semiconductors: blind enumeration.

do i = all_periodic_table
do j = all_semiconductors
dope $i in $j
run dft_code
if (magnetic_moment > 0)
echo eureka! time to write a paper
else if (magnetic_moment = 0)
echo maybe it's time to co-dope.
enddo
enddo

Standard practice for the brute force approach appears to be: (i) skim previous literature, (ii) forget about intrinsic defects, (iii) ignore any methods post-LDA/GGA, (iv) don't think too hard about the results. This week’s examples are ZnS, and just one more on top of a bottomless series of nitrogen in oxide investigations, this time on TiO2.

Unsurprisingly, aliovalent substitutions where the difference in formal oxidation states is +/- 1 will produce an unpaired electron, e.g. N on an O site or Li on a Zn site. In some cases this can be compensated by intrinsic defects, e.g. in ZrO2, Y(III) doping on the Zr(IV) site is compensated by the formation of oxygen vacancies to be overall charge neutral and closed shell: 2Y_Zr + V_O (YSZ). In a material like MgO, you can form localized oxygen hole states, which act as catalytic centres, but the room temperature stabilization of long range ferromagnetism is highly unlikely. It is possible that (magnetic) aether is staging a comeback, or the experiment on d0 ferromagnetism has gone as bananas as ferroelectrics.

(Update August 2010) An interesting review on the caveats of the theory of dilute magnetic semiconductors appeared in the new APS magazine Physics by Zunger et al. - not entirely objective, but some good points are made.

After an intensive week of talks, I made it back to London alive. Nuggets of wisdom during the last few days included “Theorists needs to provide error bars in their work; where are the error bars?” in relation to structure prediction (CCP5, London) and “The defect is localised... it’s not rocket science” (Defects in Oxides, Dublin).

Structure prediction is a new area to me, and the first day of CCP5 gave a nice introduction, with a fresh stack of papers on energy landscapes and global optimization on my desk waiting to be read. Defects in oxides, I am more familiar with, and there was a nice array of talks covering everything from catalysis to surface chemistry. Messages of the day: (i) an explanation for d0 magnetism in oxides is still elusive (with d0 being representative of a nominally closed shell system with no transition metal ions present); (ii) the band gap is a ground state property, so it is as valid to fit hybrid (exact-exchange) DFT functionals to this as anything other descriptor; (iii) GW theory remains lost in the realm of having no total energy or forces; (iv) TiO2 is still receiving more than its fair share of attention, both theoretically and experimentally.

At a well organized conference, informal conversations tend to surpass the presented material, and this was again the case at Trinity College. The conference dinner, at Fire, definitely finished things on a good note: 'Wood Fired Goats Cheese', 'Panfried Lemon Sole' and 'Traditional Apple Charlotte'.