Using a combination of Z-contrast imaging and atomically resolved electron energy-loss spectroscopy on a scanning transmission electron microscope, we show that the chemical bonding of individual impurity atoms can be deduced experimentally. We find that when a Si atom is bonded with four atoms at a double-vacancy site in graphene, Si 3d orbitals contribute significantly to the bonding, resulting in a planar sp(2) d-like hybridization, whereas threefold coordinated Si in graphene adopts the preferred sp(3) hybridization. The conclusions are confirmed by first-principles calculations and demonstrate that chemical bonding of two-dimensional materials can now be explored at the single impurity level.

Using density matrix equations of motion, we predict a femtosecond collective spin tilt triggered by nonlinear, near-ultraviolet (∼3eV), coherent photoexcitation of (Ga,Mn)As ferromagnetic semiconductors with linearly polarized light. This dynamics results from carrier coherences and nonthermal populations excited in the {111} equivalent directions of the Brillouin zone and triggers a subsequent uniform precession. We predict nonthermal magnetization control by tuning the laser frequency and polarization direction. Our mechanism explains recent ultrafast pump-probe experiments.PACS numbers: 78.47. 78.20.Ls, 78.47.Fg, 42.50.Md Long range magnetic order arises from the interactions between itinerant and localized spins in a wide variety of systems, such as EuO, EuS, chrome spinels, pyrochlore, manganese oxides, or (III,Mn)V ferromagnetic semiconductors [1,2]. With ferromagnetic semiconductors one can envision multifunctional devices combining information processing and storage on a single chip with low power consumption. Fast spin manipulation is of great importance for such spin-electronic, spin-photonic, magnetic storage, and quantum computation applications.One of the challenges facing magnetic devices concerns their speed. The magnetic properties of carrier-induced ferromagnets respond strongly to carrier density tuning via light, electrical gates, or current [3]. While magnetic field pulses and spin currents can be used to manipulate spin on the many-picosecond time scale, femtosecond spin manipulation requires the use of laser pulses [4,5]. In ultrafast pump-probe magneto-optical spectroscopy, the pump optical pulse excites e-h coherences and corresponding carrier populations, whose subsequent interactions trigger a magnetization dynamics monitored as function of time via the Faraday or Kerr rotation [6].The physical processes leading to femtosecond magnetization dynamics (femto-magnetism) are under debate. Open questions include the possibility of direct photonspin coupling, the distinction of coherent and incoherent effects, and the exact role of the spin-orbit interaction. Following the pioneering work of Ref. [7], many ultrafast spectroscopy experiments were interpreted in terms of a decrease in the magnetization amplitude due to transient thermal effects [7,8]. Observations of lightinduced changes in the magnetization orientation were also mostly attributed to the temperature elevation, which leads to transient changes in the magnetic easy axes [9,10,11]. Most desirable is nonthermal magnetization control within the femtosecond coherent [12] temporal regime, which promises more flexibility limited only by the optical pulse duration. Experiments in ferrimagnetic garnets were interpreted in terms of an interplay between the inverse Faraday effect [13] and long-lived changes in the magneto-crystalline anisotropy [4]. In (Ga,Mn)As, Ref.[14] reported magnetization precession triggered by changes of magnetic anisotropy on a ∼100ps time scale due to carrier relaxation, while Ref.[15] demonstrated coherent ...

We address the role of correlations between spin and charge degrees of freedom on the dynamical properties of ferromagnetic systems governed by the magnetic exchange interaction between itinerant and localized spins. For this we introduce a general theory that treats quantum fluctuations beyond the random phase approximation based on a correlation expansion of the Green's function equations of motion. We calculate the spin susceptibility, spin-wave excitation spectrum, and magnetization precession damping. We find that correlations strongly affect the magnitude and carrier concentration dependence of the spin stiffness and magnetization Gilbert damping.