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

Abstract We present time-integrated four-wave mixing measurements on monolayer MoSe 2 in magnetic fields up to 25 T. The experimental data together with time-dependent density function theory calculations provide interesting insights into the biexciton formation and dynamics. In presence of coherence at negative positive time delays is dominated by intervalley biexcitons. demonstrate that can serve as a control enhance help search for more exotic states matter, including creation multiple...

This paper reports on successful application of electron energy loss spectroscopy (EELS) for probing low-energy electronic excitations at all values momentum transfer with atomic-scale spatial resolution. In this regard EELS appears to be complementary optical which probes only direct limited The authors suggest that can used study structural, electronic, magnetic and vibrational properties systems having defects impurities as well.

Using density matrix equations of motion, we predict a femtosecond collective spin tilt triggered by nonlinear, near-ultraviolet ($\ensuremath{\sim}3\text{ }\text{ }\mathrm{eV}$), coherent photoexcitation (Ga,Mn)As ferromagnetic semiconductors with linearly polarized light. This dynamics results from carrier coherences and nonthermal populations excited in the ${111}$ equivalent directions Brillouin zone triggers subsequent uniform precession. We magnetization control tuning laser frequency...

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

Abstract We present a theoretical framework for calculating probe-position-dependent electron energy-loss near-edge structure the scanning transmission microscope by combining density functional theory with dynamical scattering theory. show how simpler approaches to fail include fundamental physics needed understand evolution of as function probe position and investigate dependence on size. It is within this that should be presented, in order ensure variations are truly due local electronic...

With a variational three-body calculation we study the role of interplay between on-site Coulomb, Hund's rule, and superexchange interactions on spin-wave excitation spectrum itinerant ferromagnets. We show that correlations Fermi sea electron-hole pair magnon result in very pronounced zone boundary softening strong deviations from Heisenberg dispersion. this spin dynamics depends sensitively Coulomb exchange discuss its possible relevance to recent experiments manganites.

Using density matrix equations of motion and a tight-binding band calculation, we predict all-optical switching between four metastable magnetic states (III,Mn)As ferromagnets. This is initiated non-thermally within 100fs, during nonlinear coherent photoexcitation. For single optical pulse, magnetization reversal completed after $\sim$100 ps controlled by the femtosecond Our predicted comes from nonlinearities triggered tilt that sensitive to un--adiabatic light--induced spin interactions.

We present a variational calculation of the spin wave excitation spectrum double-exchange ferromagnets in different dimensions. Our theory recovers random phase approximation and $1∕S$ expansion results as limiting cases can be used to study intermediate exchange coupling electron concentration regime relevant manganites. In particular, we treat exactly long range three-body correlations between Fermi sea electron-hole pair magnon show that they strongly affect dynamics. The manifestations...

We describe a mechanism for insulator-to-metal transition triggered by spin canting following femtosecond laser excitation of insulating antiferromagnetic (AFM) states colossal magnetoresistive (CMR) manganites. show that photoexcitation composite fermion quasiparticles dressed fluctuations results in the population broad metallic conduction band due to AFM background spins via strong electron-spin local correlation. By inducing canting, can increase quasiparticle energy dispersion and...

Transition-metal oxides often exhibit complex magnetic behavior due to the strong interplay between atomic-structure, electronic and degrees of freedom. Cobaltates, especially, because cobalt's ability adopt various valence spin state configurations. The case oxygen-deficient perovskite Sr3YCo4O10+x (SYCO) has attracted considerable attention persisting uncertainties about its structure origin observed room temperature ferromagnetism. Here we report a combined investigation SYCO using...

Using a many-body theory, we discuss some fundamental issues of femtomagnetism in magnetic electronic systems. We address the question how spin may couple to transient optical coherence during time scales shorter than photoexcitation duration and characteristic times interaction with lattice. also role competition between exchange spin–orbit interactions nonthermal temporal evolution regime. density matrix equations motion, predict femtosecond collective tilt leading magnetization modulation...

Abstract Substitutional doping of graphene by impurity atoms such as boron and nitrogen, followed atom‐by‐atom manipulation via scanning transmission electron microscopy, can allow for accurate tailoring its electronic structure, plasmonic response, even the creation single atom devices. Beyond identification individual dopant means “Z contrast” imaging, spectroscopic characterization is needed to understand modifications induced in structure response. Here, atomic scale imaging extreme...

The detection and identification of impurities other point defects in materials is a challenging task. Signatures for are typically obtained using spectroscopies without spatial resolution. Here we demonstrate the power valence electron-energy-loss spectroscopy (VEELS) an aberration-corrected scanning transmission-electron microscope (STEM) to provide energy-resolved atomically resolved maps electronic excitations individual which, combined with theoretical simulations, yield unique...

We study spin relaxation and dynamics of collective excitations in correlated double-exchange ferromagnets. For this, we introduce an expansion the Green's functions equations motion that treats nonperturbatively all correlations between a given number charge becomes exact within subspace states. Our method beyond Fermi's golden rule while recovering previous variational results for spin-wave dispersion. find momentum dependence dephasing rate changes qualitatively due to on-site Coulomb...

Abstract Light–wave quantum electronics utilizes the oscillating carrier wave to control electronic properties with intense laser pulses. Without direct light–spin interactions, however, magnetic can only be indirectly affected by light electric field, mostly at later times. A grand challenge is how establish a universal principle for of charge and spin fluctuations, which allow faster-than-THz clock rates. Using kinetic equations density matrix describing non–equilibrium states Hubbard...

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – 8, 2013.

The introduction of aberration-correction in scanning transmission electron microscopy (STEM) has allowed the realization Feynman's long sought dream [1], atom-by-atom structural and elemental identification materials [2], by simply looking "at thing".Moreover, higher current probes achieved with aberration correction also identification, spectroscopic means, individual impurities [3], as well bonding characteristic atoms two-dimensional (2D) [4][5][6].In this talk, we will present our...

fold and 4-fold impurities reproduce the bright features at lower energies, only coordination produces correct contrast between 22-44 eV.The dependence of VEELS on local bonding offers new way studying properties 2dimensional materials.With development latest generation monochomators, even finer details are potentially available for closer study using this approach a range 2-dimensional materials with many applications to vital field renewable energy.

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Using density matrix equations of motion and a tightbinding bandstructure calculation, we demonstrate nonthermal ultrafast spin switching in (Ga,Mn)As triggered by coherent photoexcitation controlled pump fluence external magnetic field.