Light-Shaping of Valley States.

The established paradigm to create valley states, excitations at local band extrema ("valleys"), is through selective occupation of specific valleys via circularly polarized laser pulses. Here we show a second way exists to create valley states, not by valley population imbalance but by "light-shaping" in momentum space, i.e. controlling the shape of the distribution of excited charge at each valley. While noncontrasting in valley charge, such valley states are instead characterized by a valley current, identically zero at one valley and finite and large at the other. We demonstrate that these (i) are robust to quantum decoherence, (ii) allow lossless toggling of the valley state with successive femtosecond laser pulses, and (iii) permit valley contrasting excitation both with and without a gap. Our findings open a route to robust ultrafast and switchable valleytronics in a wide scope of 2d materials, bringing closer the promise of valley-based electronics.

Laser-Induced Intersite Spin Transfer.

Laser pulses induce spin-selective charge flow that we show to generate dramatic changes in the magnetic structure of materials, including a switching of magnetic order from antiferromagnetic (AFM) to transient ferromagnetic (FM) in multisub-lattice systems. The microscopic mechanism underpinning this ultrafast switching of magnetic order is dominated by spin-selective charge transfer from one magnetic sublattice to another. Because this spin modulation is purely optical in nature (i.e., not mediated indirectly via the spin-orbit interaction) this is one of the fastest means of manipulating spin by light. We further demonstrate this mechanism to be universally applicable to AFM, FM, and ferri-magnets in both multilayer and bulk geometry and provide three rules that encapsulate early-time magnetization dynamics of multisub-lattice systems.

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.