P-like Electrons Research Articles

A generalized plane-wave formulation of [formula omitted] formalism and continuum-elasticity approach to elastic and electronic properties of semiconductor nanostructures

We present a generalized and flexible plane-wave based implementation of the multiband k·p formalism to study the electronic properties of semiconductor nanostructures. All ingredients of the modeling process, namely the Hamiltonian, the nanostructure’s geometry and the required material parameters, are defined in human-readable input files that can be easily generated and modified. The generalized k·p model can contain an arbitrary number of directly treated bands as well as strain, piezoelectric, and external potentials. All calculations can be performed for arbitrary crystal structures. The nanostructure is described in terms of a real-space composition map that may contain an arbitrary number of base compounds and alloys. We demonstrate the applicability and flexibility of our implementation for the example of (111)-oriented, site-controlled InGaAs quantum dots, where a rotated eight-band k·p Hamiltonian is employed. As a second example, a 14-band k·p model that captures the bulk inversion asymmetry of the zinc-blende lattice is applied for the case of a pyramidal (001)-oriented InAs/GaAs quantum dot. Here we show that the explicit treatment of 14 bands removes the well known shortcoming of eight-band k·p models for (001)-oriented zinc-blende quantum dots which leads to artificially degenerate p-like electron states.

Energy band structure of diamond, cubic silicon carbide, silicon, and germanium

We have developed a method for determining the band structure of crystals which combines the best features of a first-principles approach with the best features of an empirical approach. The first step in our method is a first-principles OPW band calculation based on the free-electron exchange approximation. In the second step, we depart from a purely first-principles approach, and introduce a small empirical correction which brings key features of the calculated energy level scheme into exact (or close) agreement with the most reliably established features of the experimental level scheme. In addition to obtaining reliable energy band models valid over a wide energy range, we obtain the electronic wave functions for a large number of levels, the core and valence electronic charge distributions, and many other “first-principles” quantities. Following a summary of the shortcomings of purely first-principles and purely empirical band calculations, we sketch the essential features of our intermediate treatment. Recent studies of the band structure of diamond, cubic silicon carbide, silicon, and germanium-carried out both by our method and other methods-are then discussed and compared. It is shown how improved band models for these crystals can be generated with the aid of some crucial information about the band structure derived from experiment. Finally, we observe that although the use of a universal exchange potential is an important simplification, and one that has greatly accelerated recent progress in energy band calculations, this approximation is to some extent an oversimplification. It appears likely that some of the empirical corrections that we now have to introduce in order to bring theory and experiment closer together could be reduced substantially if we could define one exchange potential for s-like electrons, another for p-like electrons, and still another for d-like electrons. This idea of l-dependent exchange potentials represents a middle ground between the Hartree-Fock extreme of different exchange potentials for different orbitals, and the Slater extreme of the same exchange potential for all orbitals.

Radiative Recombination of Triexcitons in CdSe Colloidal Quantum Dots

Recent experimental observations of multiexciton recombination in colloidal CdSe quantum dots have revealed a high-energy emission band that has been attributed to the radiative recombination of a p-like electron with a p-like hole in a triexciton complex (three holes + three electrons). The reason for the occupation of p-like hole states in the triexciton, however, has remained elusive. Using atomistic pseudopotential calculations, we show here that p-like hole states are populated even at low temperature because of the relatively small Coulomb repulsion between s-like and p-like hole states, which leads to a non-Aufbau occupation sequence of the hole levels. We also show that the observed temperature dependence of the p-p emission band originates from the dark-bright splitting of the triexciton ground state. Our results provide a consistent explanation of the physical origin of the triexciton emission lines in CdSe quantum dots.

During IR electron transfers in solution

We investigate the real-time probing of nonequilibrium electron transfers in aqueous solutions of disulfide molecules. Our attention is focused on the elementary steps of an IR electron transfer from a charged donor (chloride ion) to a biomolecular acceptor (cystamine dihydrochloride: RSSR, 2HCl, R = (CH 2) 2 NH 2). Following a femtosecond UV excitation of the ionic donor, an ultrafast electron attachment on a SS bond takes place with a time constant of 160 ± 20 fs at 294K. The determination of a UV signature centered around 3.0 eV and characterized by a half band width of 0.6 eV substantiates the subpicosecond formation of a nascent sulfur-centered radical anion (RS∴SR −) with a 2σ/σ ∗ bond. The ultrafast attachment of an IR prehydrated electron on a SS bond competes with the nonadiabatic radiationless transition of IR p-like electrons. These results raise the fundamental importance of solvent caging effects during the efficient coupling between a p-like IR prehydrated electron and a SS bond.

Temperature Dependence of Magnetic Compton Profiles of Ni

The temperature dependence of the magnetic Compton profile of Ni [100] was measured for the first time. It was found that there is a significant difference in shape between the MCPs at room temperature and at 640 K. The result shows that the MCP of Ni above the TC (628 K) has much smaller ratio of the magnetic moment of the s, p-like electrons to that of the 3d magnetic electrons than the MCP at room temperature. This indicates that the spin moment of the s, p-like electrons disappears more rapidly than that of the localized moment due to thermal excitations when temperature approaches to the Curie temperature.

Spin density in ferromagnetic nickel: a magnetic Compton scattering study

The magnetic Compton profiles (MCPs) measured in the [100], [110], [111] and [112] directions in single-crystal nickel with an incident photon beam of energy 224 keV are presented and discussed. The momentum resolution achieved, of 0.43 atomic units, improves on previous studies by almost a factor of two, and facilitates the interpretation of the MCPs in terms of the underlying spin-dependent momentum densities. Calculations have been performed using the linear muffin-tin orbital method, within both the local spin-density approximation (LSDA) and the generalized gradient approximation (GGA). Comparison with experiment reveals the limitations of the LSDA at low momentum, where the GGA is better able to reproduce the contribution of the s- and p-like electrons. All of the calculations overestimate the moment associated with the d-like electrons, for momenta corresponding to the first Brillouin zone. We also confirm the existence of the so-called Umklapp shoulders, which derive from the Fermi surface topology.

X-ray Absorption Studies of Ruthenium Silicide

We have investigated the X-ray absorption fine structures (XAFS) of several ruthenium-silicon intermetallic compounds at the Ru L2,3, Ru K and Si K-edges. From the analysis of the Ru and Si K-edge XANES (X-Ray Absorption Near Edge Structures), particularly the relative intensity of the absorption feature just above the threshold, we found that ruthenium gains p-like electrons upon dilution in silicon. Likewise, compound formation of silicon with ruthenium results in the increase of p-like electrons at the silicon site. The Ru L2,3-edge also exhibits variation in the whiteline intensity for ruthenium silicide of different composition. This is used to infer d charge redistribution at the Ru site upon the formation of Ru-Si intermetallic compounds. The implications of the results are discussed.

Experimental widths of excited electron states in metals

From angle-resolved inverse photoemission spectra we have derived estimated for the lifetime-width Γ e of s, p-like electron states in metals. Their compilation together with results obtained from photoemission spectra by other authors indicates a “universal” relationship Γ e ( E) = (0.13 ± 0.01)( E − E f ) for electron energies E ⩽ 50 eV above the Fermi energy E F.

X-ray emission study of amorphous SiO x:H films (0 ⩽ x ⩽ 2.2)

X-ray Si K-, Si L- and O K-emission bands of a-SiO x :H alloy films are presented and compared with available UPS and XPS results on a common energy scale. With the variation of x the Si K- and Si L-emission bands undergo significant changes regarding the shape of the spectra and the position of the main features, in contrast to UPS/XPS spectra which preserve the basic shape throughout the concentration range x. We unambiguously identified all features observable in XES and UPS/XPS spectra with respect to Si 3 s, Si 3 p, O 2 s and O 2 p derived states, and we added new details to a previous UPS/XPS interpretation. Since for the system SiO x our emission bands are more sensitive to sample composition than UPS/XPS spectra we observed a non-trivial behaviour of Si p-like electrons evolving from covalent SiSi to rather ionic SiO bonds.

Si K-emission bands of hydrogenated SiOx alloys

X-ray Si K-emission bands of amorphous SiOx: H alloy films (0 ⩽ x ⩽ 1.7) are presented. The samples were prepared by rf glow discharge decomposition of SiH4-CO2-H2 gas mixtures in a capacitive reactor system. For x < 0.5 the Si K-emission bands are very similar in shape and peak position to that of a-Si: H, for 0.5 < x < 1.7, however, a dramatic reshaping of the Si K-emission bands occurs. For x = 1.7 the spectra resemble that of crystalline SiO2 (β-crystobalite). The resulting Si K-emission bands are compared with UPS/XPS spectra of a-SiOx in which, contrary to XES spectra, no impressive changes of spectral features are observed. Our measurements demonstrate that the Si K-emission bands of the system a-SiOx: H are more sensitive to sample composition than UPS/XPS spectra. The results delineate the nontrivial behaviour of Si p-like electrons evolving from covalent Si-Si to rather ionic Si-O bonds.

Evidence for alkali-metal - carbon hybridization in KC8

Abstract The partial p-like electron densities of states at potassium sites in the graphite intercalation compound KC8 has been investigated by means of soft x-ray emission spectroscopy. It is demonstrated that hybridization between the potassium and graphitic valence states occurs over the 0–10 eV binding energy region. The p-like charge at the intercalated potassium sites is equivalent to 0.6 electrons per potassium atom, i.e. double the value for elemental potassium.

Superconductivity in thin transition-metal films: Its suppression by reduced dimensionality and localization

The effects of reduced dimensionality on the superconductivity of thin films - Nb(100) and V(100) - and Ag/V/Ag(100) sandwiches are investigated using our all-electron self-consistent full potential linearized augmented plane wave (FLAPW) method. Results of calculations of the electronic structures are used along with the rigid-ion approximation and McMillan strong coupling theory to calculate the layer-by-layer contribution to the electron-phonon coupling. Despite a high N( E F) which remains at the surface layer, the electronic contribution to the electron-phonon coupling is found to be strongly suppressed by dimensional constraints. Localization of the surface states and the decrease of p-like electrons at the surface are shown to be behind this suppression. This implies that superconductivity is diminished in reduced dimensional transition metal thin films.

Electrons and phonons in amorphous pyramidally bonded solids

Electronic and phonon state densities of amorphous pyramidally bonded solids are calculated with As as a prototype. Using a Bethe-lattice, the Greaves-Davis random network model and the Cluster-Bethe-lattice method, the results show: (1) the phonon states are isomorphic to, and obtained from, the p-like electron states, (2) the photoemission and infrared measurements are easily interpreted, and (3) the electronic and phonon state densities are extremely sensitive to the local topology.

Band Structure of Metallic Copper and Nickel by a Self-Consistent Procedure

Band structure of metallic copper and nickel are calculated by a self-consistent procedure. A self-consistent potential is constructed by a modified Hartree-Fock-Slater approximation. The general properties of the bands are in good agreement with those obtained previously. The band structure of Ni in a ferromagnetic state is evaluated. The exchange splitting energy for d-like electrons is estimated as about 0.07 Ry, while for s- and p-like electrons as 0.015 Ry. The theory suggests that there exists a neck on the Fermi surfaces of both up- and down-spin electron bands.