Some issues addressed
What determines the structure in perovskites of the form ABX3, where one has a molecule at the A site?
The structural distortions of the perovskites have been determined by the relative sizes of the atoms comprising it, which are used to define the tolerance factor. In qualitative terms, if one had a small atom at the A site (centre of the cube representing the perovskite), the unit cell volume would decrease. This would lead to shorter bonds between the B and X atoms. As this would increase the repulsion between nearest neighbor B and X atoms, one would have distortions which would lead to B-X-B angles that deviate from 180o. But how would these ideas change when one had a molecule at the A site? This has been addressed in Phys. Rev. B 95, 214118 (2017).
Doping a ferroelectric – could it still be insulating and more importantly would it still be useful?
In several recent papers (Fronteirs in Chemistry 9 (2021); ACS Energy Letters 3 1176 (2018); Phys. Rev. B 96, 024107 (2017); Phys. Rev. B 87 214110 (2013)) various aspects of how one can retain the ferroelectric property even on doping at the Ti site in BaTiO3 has been investigated. This is achieved by doping small concentrations of transition metal atoms or by co-doping pairs of atoms that don’t introduce any carriers. The advantage of this route is to tune the band gap of the ferroelectric material to a regime in which it is useful for capturing a part of the solar spectrum. The electric field associated with the ferroelectric helps in separating the photo-generated electron-hole carriers as well as helps in catalysis.
Understanding flat band formation in twisted bilayers of transition metal dichalcogenides
The twisted bilayers of two-dimensional materials have recently become the playground of strong correlation physics, with the possibilities of charge and spin density wave instabilities, while the untwisted limit represent regular semiconductors. This seems surprising as the two layers are coupled by weak van der Waal’s interactions, and so the effect of one layer on the other is expected to be weak. In order to gauge the effect of the perturbation on the electronic structure, in Phys. Rev B 102, 205415 (2020) we have projected the wavefunction from the twisted bilayer at each k-point to that for the untwisted limit. For a small twist angle in the range typically examined in experiments, we find that while the low energy electronic structure is largely retained, the top of the valence band is split off from the rest of the bands.
For arbitrary angles of rotations in twisted bilayers, do we expect to find any symmetry in the structures?
Twisted bilayers are generated by rotating one layer with respect to the other. One would not expect any symmetries to be present for arbitrary angles of rotation. Considering MoSe2, we show that an unusual symmetry exists that allows us to relate the electronic structure for a twist angle of theta to that found at 60-theta. Specifically considering the spin-splitting at K, one finds that if one has a spin splitting at theta, one finds that it vanishes at 60 – theta. This has appeared in Phys. Rev. B 101 045432 (2020).
More information @ : Priya Mahadevan [Google Scholar]