Virtual Electron Excitations Research Articles

Theoretical proposals to measure resonator-induced modifications of the electronic ground state in doped quantum wells

Recent interest in the physics of non-perturbative light-matter coupling led to the development of solid-state cavity quantum electrodynamics setups in which the interaction energies are comparable with the bare ones. In such a regime the ground state of the coupled system becomes interaction-dependent and is predicted to contain a population of virtual excitations which, notwithstanding having been object of many investigations, remain still unobserved. In this paper we investigate how virtual electronic excitations in quantum wells modify the ground-state charge distribution, and propose two methods to measure such a cavity-induced perturbation. The first approach we consider is based on spectroscopic mapping of the electronic population at a specific location in the quantum well using localised defect states. The second approach exploits instead the photonic equivalent of a Kelvin probe to measure the average change distribution across the quantum well. We find both effects observable with present-day or near-future technology. Our results thus provide a route toward a demonstration of cavity-induced modulation of ground-state electronic properties.

Magnetism of metallacrown single-molecule magnets: From a simplest model to realistic systems

Electronic and magnetic properties of molecular nanomagnets are determined by competing energy scales due to the crystal field splitting, the exchange interactions between transition metal atoms, and relativistic effects. We present a comprehensive theory embracing all these phenomena based on first-principles calculations. In order to achieve this goal, we start from the ${\mathrm{FeNi}}_{4}$ cluster as a paradigm. The system can be accurately described on the ab initio level yielding all expected electronic states in a range of multiplicities from 1 to 9, with a ferromagnetic ground state. By adding the spin-orbit coupling between them we obtain the zero-field splitting. This allows to introduce a spin Hamiltonian of a giant spin model, which operates on a smaller energy scale. We compare the computed parameters of this Hamiltonian with the experimental and theoretical magnetic anisotropy energies of the monolayer Ni/Cu(001). In line with them, we find that the anisotropy almost entirely originates from the second-order spin-orbit coupling, the spin-spin coupling constitutes only a small fraction. Finally, we include the ligand atoms in our consideration. This component has a decisive role for the stabilization of molecules in experimental synthesis and characterization, and also substantially complicates the theory by bringing the superexchange mechanisms into play. Since they are higher-order effects involving two hopping matrix elements, not every theory can describe them. Our generalization of the corresponding perturbation theory substantiates the use of complete active space methods for the description of superexchange. At the same time, our numerical results for the $\left\{{\mathrm{CuFe}}_{4}\right\}$ system demonstrate that the Goodenough-Kanamori rules, which are often used to determine the sign of these exchange interactions, cannot deliver quantitative predictions due to the interplay of other mechanisms, e. g., involving multicenter Coulomb integrals. We conclude by comparing ab initio values of the exchange interaction constants for the $\left\{{\mathrm{CuCu}}_{4}\right\}$ and $\left\{{\mathrm{CuFe}}_{4}\right\}$ metallacrown magnetic molecules with experimental values determined by fitting of the magnetic susceptibility curves ${\ensuremath{\chi}}_{M}T(T)$, and attribute the remaining discrepancy between them to the role of virtual electron excitations into and out of the active space (dynamical correlations).

Interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys

The interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys in which monolyers (submonolayers) of transition metals are embedded into a semiconductor matrix is studied theoretically. A mechanism of an indirect exchange between ferromagnetic δ layers is proposed; it is based on the confinement of carriers in two-dimensional spin-polarized states inside the energy gap of the semiconductor. These appear due to strong potential and exchange carrier scattering by the δ layers. The interlayer exchange coupling is shown to occur through a nondegenerate semiconductor interlayer because of virtual electron excitations through an energy barrier separating these partly filled two-dimensional spin-polarized states and the edge of the bulk semiconductor band. The interlayer coupling intensity decreases exponentially with increasing distance between neighboring δ layers, and the type of this coupling can change from ferromagnetic into antiferromagnetic or vice versa as the interlayer thickness or the degree of filling the two-dimensional states increases.

Ferromagnetic Properties in Diluted Magnetic Semiconductors (Al,Mn)N grown by PEMBE

We present the structural, magnetic, and electrical properties in the (Al,Mn)N films with various Mn concentrations grown by plasma-enhanced molecular beam epitaxy. X-ray diffraction analyses reveal that the (Al,Mn)N films have the wurtzite structure without secondary phases. All (Al,Mn)N films showed the ferromagnetic ordering. Particularly, (<TEX>$Al_{1-x}Mn_{x}$</TEX>)N film with x = 0.028 exhibited the highest magnetic moment per Mn atom at room temperature. Since all the films exhibit the insulating characteristics, the origin of ferromagnetism in (Al,Mn)N might be attributed to either indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples or a percolation of bound magnetic polarons arisen from exchange interaction of localized carriers with magnetic impurities in a low carrier density regime.

Observation of room-temperature ferromagnetism in (Al,Mn)N thin films

We present the structural, electrical and magnetic properties in the (Al,Mn)N thin films with various Mn concentrations grown by plasma-enhanced molecular beam epitaxy. The X-ray diffraction patterns reveal that the (Al,Mn)N films have the wurtzite structure without secondary phases. The (Al 1− x Mn x )N films with x = 1.7 % and 2.6% were obviously found to exhibit the ferromagnetic ordering with Curie temperature exceeding room temperature while the film with x = 3.0 % was observed to show the ferromagnetic ordering at 5 K, indicating that a critical Mn concentration exists. Since all the films exhibit insulating characteristics, the origin of ferromagnetism in (Al,Mn)N might be attributed to indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples.

Ferromagnetism in magnetically doped III-V semiconductors.

The origin of ferromagnetism in semimagnetic III-V materials is discussed. The indirect exchange interaction caused by virtual electron excitations from magnetic impurity acceptor levels to the valence band can explain ferromagnetism in GaAs(Mn) in both degenerate and nondegenerate samples. Formation of ferromagnetic clusters and the percolation picture of phase transition describes well all available experimental data and allows us to predict the Mn-composition dependence of transition temperature in wurtzite (Ga,In,Al)N epitaxial layers.

Theory of Anomalous X-Ray Scattering in Orbital-Ordered Manganites

We study theoretically the anomalous X-ray scattering as a new probe to observe the orbital orderings and excitations in perovskite manganites. The scattering matrix is given by the virtual electronic excitations from Mn $1s$ level to unoccupied Mn $4p$ level. We find that orbital dependence of the Coulomb interaction between Mn $3d$ and Mn $4p$ electrons is essential to bring about the anisotropy of the scattering factor near the K edge. The calculated results in $MnO_6$ clusters explain the forbidden reflections observed in $La_{0.5}Sr_{1.5}MnO_4$ and $LaMnO_3$. A possibility of the observation of the orbital waves by the X-ray scattering is discussed.

Influence of d orbitals on the nonlinear optical response of transparent transition-metal oxides.

The bond-orbital theory of linear and nonlinear electronic response in optically transparent materials, developed earlier for pretransition-metal halides and chalcogenides, is expanded to embrace the transition-metal (TM) oxides. The extension requires an explicit recognition of the influence of cationic empty d orbitals on electronic polarizability. Two competing mechanisms, involving, respectively, virtual electronic excitations to the d orbitals and to the conduction-band ``sp orbitals,'' are shown to be essentially additive for linear polarizability ${\mathrm{\ensuremath{\chi}}}^{(1)}$ and lowest-order nonlinear polarizability ${\mathrm{\ensuremath{\chi}}}^{(2)}$, but not for ${\mathrm{\ensuremath{\chi}}}^{(3)}$. The d-orbital contributions to linear and nonlinear response are found to be negligible for bond lengths d\ensuremath{\gtrsim}2.3 \AA{}, but to increase rapidly as a function of decreasing bond length within each TM series to become dominant when d\ensuremath{\lesssim}2.0 \AA{}. Numerical evaluations of nonlinear refractive index ${\mathit{n}}_{2}$ are presented for each series of TM oxides.

Substrate induced specificities in atom-adsorbate van der Waals scattering

We investigate the substrate induced specificities of the van der Waals potentials acting between gas phase atoms and chemisorbed CO molecules, and the effect of these specificities on the enhancement of the magnitude of the total scattering cross section in He → CO/metal collisions. The role of these effects in the relative enhancement of the cross section can be quantitatively estimated for prototype systems of weak and strong CO chemisorption, viz. CO/Cu and CO/Ni, respectively, for which other electronic and structural properties are known. Since the strengths of the atom-adsorbate van der Waals potentials depend on the spectra of virtual electronic excitations in each electronic subsystem involved, the specificity of the cross section arises from the particularities of the screening properties of the substrate surface, on the one hand, and on the strength of the adsorbate-substrate chemisorptive bond, on the other. Using the response functions of Cu and Ni surfaces and the valence electronic properties of the 2π ∗ derived resonances of CO chemisorbed on Cu and Ni substrates, we are able to calculate and discuss qualitatively the scattering potentials and the corresponding cross sections for He → CO/Cu and He → CO/Ni collision events. The results of our calculations demonstrate that the strong chemisorption system CO/Ni represents also a stronger scattering centre for thermal He beam atoms than the weak chemisorption system CO/Cu, and that for equal kinetic parameters of the collision the total cross sections may in these sysems differ by up to 10%.

Substrate induced specificities in atom-adsorbate van der Waals scattering

We investigate the substrate induced specificities of the van der Waals potentials acting between gas phase atoms and chemisorbed CO molecules, and the effect of these specificities on the enhancement of the magnitude of the total scattering cross section in He → CO/metal collisions. The role of these effects in the relative enhancement of the cross section can be quantitatively estimated for prototype systems of weak and strong CO chemisorption, viz. CO/Cu and CO/Ni, respectively, for which other electronic and structural properties are known. Since the strengths of the atom-adsorbate van der Waals potentials depend on the spectra of virtual electronic excitations in each electronic subsystem involved, the specificity of the cross section arises from the particularities of the screening properties of the substrate surface, on the one hand, and on the strength of the adsorbate-substrate chemisorptive bond, on the other. Using the response functions of Cu and Ni surfaces and the valence electronic properties of the 2π∗ derived resonances of CO chemisorbed on Cu and Ni substrates, we are able to calculate and discuss qualitatively the scattering potentials and the corresponding cross sections for He → CO/Cu and He → CO/Ni collision events. The results of our calculations demonstrate that the strong chemisorption system CO/Ni represents also a stronger scattering centre for thermal He beam atoms than the weak chemisorption system CO/Cu, and that for equal kinetic parameters of the collision the total cross sections may in these sysems differ by up to 10%.

A piezoelectric hypothesis for interface superconductivity in CuCl

A new model for “excitonic” superconductivity is proposed to explain the anomalous diamagnetism seen in CuCl. The superconductivity is presumed to occur in thin layers of copper deposited in grain boundaries of the CuCl. Pairing of the cooper electrons is ascribed to virtual electronic excitations between the metal and adjacent, two-dimensional potential minima in the CuCl resulting from the piezoelectric polarization due to the elastic strain of grain boundry dislocations.