We explore theoretically the feasibility of functionalizing carbon nanostructures for hydrogen storage, focusing on the coating of C60 fullerenes with light alkaline-earth metals. Our first-principles density functional theory studies show that both Ca and Sr can bind strongly to the C60 surface, and highly prefer monolayer coating, thereby explaining existing experimental observations. The strong binding is attributed to an intriguing charge transfer mechanism involving the empty d levels of the metal elements. The charge redistribution, in turn, gives rise to electric fields surrounding the coated fullerenes, which can now function as ideal molecular hydrogen attractors. With a hydrogen uptake of >8.4 wt % on Ca32C60, Ca is superior to all the recently suggested metal coating elements.

We present a method to perform fault-tolerant single-qubit gate operations using Landau-Zener tunneling. In a single Landau-Zener pulse, the qubit transition frequency is varied in time so that it passes through the frequency of the radiation field. We show that a simple three-pulse sequence allows eliminating errors in the gate up to the third order in errors in the qubit energies or the radiation frequency.

Endohedral metallofullerenes constitute an appealing class of nanoscale building blocks for fabrication of a wide range of materials. One open question of fundamental importance is the precise nature of charge redistribution within the carbon cages ͑C n ͒ upon metal encapsulation. Using ab initio density functional theory, we systematically study the electronic structure of metallofullerenes, focusing on the spatial charge redistribution. For large metallofullerenes ͑n Ͼ 32͒, the valence electrons of the metal atoms are all transferred to the fullerene states. Surprisingly, the transferred charge is found to be highly localized inside the cage near the metal cations rather than uniformly distributed on the surfaces of the carbon cage as traditionally believed. This counterintuitive charge localization picture is attributed to the strong metal-cage interactions within the systems. These findings may prove to be instrumental in the design of fullerene-based functional nanomaterials.

We study the classical dynamics of resonantly modulated large-spin systems in a strong magnetic field, where the Zeeman energy exceeds the anisotropy energy. We show that these systems have special symmetry. It leads to characteristic nonlinear effects. They include abrupt switching between magnetization branches with varying modulating field without hysteresis and a specific pattern of switching in the presence of multistability and hysteresis. Along with steady forced vibrations, the transverse spin components can display incoherent vibrations at a combination of the Larmor frequency and a smaller frequency determined by the anisotropy constant. The analysis is based on a microscopic theory that takes into account relaxation mechanisms important for single-molecule magnets and other large-spin systems. We find how the Landau-Lifshitz model should be modified in order to describe the classical spin dynamics. The occurrence of incoherent oscillations depends on the interrelation between the relaxation parameters.

We study nonlinear response of a spin $S>1/2$ with easy-axis anisotropy. The response displays sharp dips or peaks when the modulation frequency is adiabatically swept through multiphoton resonance. The effect is a consequence of a special symmetry of the spin dynamics in a magnetic field for the anisotropy energy $\propto S_z^2$. The occurrence of the dips or peaks is determined by the spin state. Their shape strongly depends on the modulation amplitude. Higher-order anisotropy breaks the symmetry, leading to sharp steps in the response as function of frequency. The results bear on the dynamics of molecular magnets in a static magnetic field.Comment: Submitted to PR