Determination of Cleavage Energy and Efficient Nanostructuring of Layered Materials by Atomic Force Microscopy.

A method is presented to use atomic force microscopy to measure the cleavage energy of van der Waals materials and similar quasi-two-dimensional materials. The cleavage energy of graphite is measured to be 0.36 J/m2, in good agreement with literature data. The same method yields a cleavage energy of 0.6 J/m2 for MoS2 as a representative of the dichalcogenides. In the case of the weak topological insulator Bi14Rh3I9 no cleavage energy is obtained, although cleavage is successful with an adapted approach. The cleavage energies of these materials are evaluated by means of density-functional calculations and literature data. This further validates the presented method and sets an upper limit of about 0.7 J/m2 to the cleavage energy that can be measured by the present setup. In addition, this method can be used as a tool for manipulating exfoliated flakes, prior to or after contacting, which may open a new route for the fabrication of nanostructures.

Reproducibility in density functional theory calculations of solids.

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.

Ferromagnetism induced by orbital order in the charge-transfer insulator Cs2AgF4: an electronic structure study.

Cs2AgF4 was proposed to be an orbitally ordered ferromagnet based on recent neutron scattering data. Here, we report a detailed electronic structure study within the local spin density approximation also including strong Coulomb repulsion U. We investigate the influence of an orthorhombic distortion of the Ag environment and the importance of the on-site Coulomb repulsion. We find good quantitative agreement with both the experimentally observed exchange coupling and structural changes. Thus, our results strongly support that Cs2AgF4 is a strongly correlated charge-transfer insulator where the ferromagnetism is driven by orbital order.