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In the recently developed correlation factor (CF) model [Precechtelova et al., J. Chem. Phys. 143, 144102 (2015)], the exchange-correlation (XC) hole is approximated. Since various constraints satisfied by the XC-hole are known, approximations to it can be designed which largely avoid empirical adjustments. In the CF approach, the XC-hole is written as a product of an exchange hole times a CF. An important constraint satisfied by the CF model is that it correctly reproduces the exact exchange energy in the high density limit. This is achieved by employing the exact exchange-energy per particle ( ϵ X r ) as an input variable, i.e., the CF model builds on exact exchange. Variations of the initial CF model are proposed which ensure that the exact answer is obtained in the homogeneous limit. Furthermore, we apply a correction to the depth of the XC-hole that is designed to capture strong correlation. EC functionals that build on exact exchange, such as hybrids, often fail for systems that exhibit sizeable electron correlation. Despite this fact and despite the reduction of empiricism to a single parameter within CF, accurate atomization energies are obtained for strongly-correlated transition metal compounds. The CF model significantly improves upon widely used functionals such as Perdew-Burke-Ernzerhof (PBE), PBE hybrid, and Tao-Perdew-Staroverov-Scuseria (TPSS). [ABSTRACT FROM AUTHOR]

We calculate strains, polarizations, and electric fields in InGaN/GaN quantum wells (lattice matched to GaN) under the influence of hydrostatic and uniaxial (along the c-axis) pressure. We calculate the confinement energies for electrons and holes, and we derive simple expressions for the transition energies and their pressure derivatives. We include the changes of the dielectric constant with pressure. The results seem compatible with the experimental data. [ABSTRACT FROM AUTHOR]

We theoretically investigate the electronic transport and Klein tunneling in AA-stacked bilayer graphene (AA-BLG) encapsulated by dielectric materials. Using the four-band continuum model, we evaluate the transmission and reflection probabilities along with the respective conductances. We find that the interlayer mass-term difference induced by the dielectric materials opens a gap in the energy spectrum and couples the upper and lower Dirac cones in AA-BLG. This cone coupling induces an inter-cone transport that is asymmetric with respect to the normal incidence in the presence of the asymmetric mass-term. The energy spectrum of the gapped AA-BLG exhibits electron-hole asymmetry that is reflected in the associated intra- and inter-cone transport channels. We also find that even though Klein tunneling exists in gated and biased AA-BLG, it is precluded by the interlayer mass-term difference and instead Fabry-Pérot resonances appear. [ABSTRACT FROM AUTHOR]

A new formalism was recently proposed to improve random phase approximation (RPA) correlation energies by including approximate exchange effects [B. Mussard et al., J. Chem. Theory Comput. 12, 2191 (2016)].Within this framework, by keeping only the electron-hole contributions to the exchange kernel, two approximations can be obtained: An adiabatic connection analog of the second order screened exchange (AC-SOSEX) and an approximate electron-hole time-dependent Hartree-Fock (eh-TDHF). Here we show how this formalism is suitable for an efficient implementation within the plane-wave basis set. The response functions involved in the AC-SOSEX and eh-TDHF equations can indeed be compactly represented by an auxiliary basis set obtained from the diagonalization of an approximate dielectric matrix. Additionally, the explicit calculation of unoccupied states can be avoided by using density functional perturbation theory techniques and the matrix elements of dynamical response functions can be efficiently computed by applying the Lanczos algorithm. As shown by several applications to reaction energies and weakly bound dimers, the inclusion of the electron-hole kernel significantly improves the accuracy of ground-state correlation energies with respect to RPA and semi-local functionals. [ABSTRACT FROM AUTHOR]

When twisted to angles near 1°, graphene multilayers provide a window on electron correlation physics. Here, we report the discovery of a correlated electron-hole state in double-bilayer graphene twisted to 2.37°. At this angle, the moiré states retain much of their isolated bilayer character, allowing their bilayer projections to be separately controlled by gates. We use this property to generate an energetic overlap between narrow isolated electron and hole bands with good nesting properties. Our measurements reveal the formation of ordered states with reconstructed Fermi surfaces, consistent with a density-wave state. This state can be tuned without introducing chemical dopants, enabling studies of correlated electron-hole states and their interplay with superconductivity. [ABSTRACT FROM AUTHOR]

The authors observe the significant penetration of electrically injected holes through InGaN/GaN quantum wells (QWs) with an indium mole fraction of 40%. This effect and its current density dependence were analysed by studies on micro-pixel light-emitting diodes, which allowed current densities to be varied over a wide range up to 5 kA/cm2. The systematic changes in electroluminescence spectra are discussed in the light of the piezoelectric field in the high-indium-content QWs and its screening by the carriers. Simulations were also carried out to clarify the unusual hole transport mechanism and the underlying physics in these high-indium QWs. [ABSTRACT FROM AUTHOR]

The energetic stability, electronic and structural properties of MoS2 adsorbed on an amorphous a-HfO2 surface (MoS2/HfO2) are examined through ab initio theoretical investigations. Our total energy results indicate that the formation of MoS2/HfO2 is an exothermic process with an adsorption energy of 34 meV/Ų, which means that it is more stable than similar systems like graphene/HfO2 and MoS2/SiO2. There are no chemical bonds at the MoS2-HfO2 interface. Upon formation of MoS2/HfO2, the electronic charge distribution is mostly localized at the interface region with no net charge transfer between the adsorbed MoS2 sheet and -HfO2 surface. However, the MoS2 sheet becomes n-type doped when there are oxygen vacancies in the HfO2 surface. Further investigation of the electronic distribution reveals that there are no electron- and hole-rich regions (electron-hole puddles) on the MoS2 sheet, which makes this system promising for use in high-speed nanoelectronic devices. [ABSTRACT FROM AUTHOR]

We study the unitary propagation of a two-particle one-dimensional SchrÃdinger equation by means of the Split-Step Fourier method, to study the coherent evolution of a spatially indirect exciton (IX) in semiconductor heterostructures. The mutual Coulomb interaction of the electron-hole pair and the electrostatic potentials generated by external gates and acting on the two particles separately are taken into account exactly in the two-particle dynamics. As relevant examples, step/downhill and barrier/well potential profiles are considered. The space- and time-dependent evolutions during the scattering event as well as the asymptotic time behavior are analyzed. For typical parameters of GaAs-based devices, the transmission or reflection of the pair turns out to be a complex two-particle process, due to comparable and competing Coulomb, electrostatic, and kinetic energy scales. Depending on the intensity and anisotropy of the scattering potentials, the quantum evolution may result in excitation of the IX internal degrees of freedom, dissociation of the pair, or transmission in small periodic IX wavepackets due to dwelling of one particle in the barrier region. We discuss the occurrence of each process in the full parameter space of the scattering potentials and the relevance of our results for current excitronic technologies. [ABSTRACT FROM AUTHOR]

We report new potential energy surfaces for the ground state Ar-NO(X²II) van der Waals system calculated using the unrestricted Hartree-Fock (UHF) method with the addition of the Becke-Roussel correlation functional and exchange-hole dipole moment dispersion correction (XDM). We compare UHFBR-XDM surfaces and those previously reported by Alexander from coupled cluster CCSD(T) calculations [J. Chem. Phys. 111, 7426 (1999)]. The bound states of Ar-NO have been investigated with these new UHFBR-XDM surfaces, including relative energy-level spacing, adiabatic bender states and wave functions, and spectroscopic data. These results have been found to be in good agreement with calculations based on the CCSD(T) PESs. These new PESs are used to investigate the inelastic scattering of NO(X) by Ar. Full close-coupling integral cross sections at collision energies of 442 cm-1, 1774 cm-1 and differential cross sections at collision energy of 530 cm-1 were determined for transitions out of the lowest NO(X) rotational level (j = 1/2, f). These cross sections are in good agreement with those calculated with CCSD(T) and accordingly in good agreement with the most recent initial and final state resolved experimental data. The UHFBR-XDM scheme yields high-quality potential surfaces with computational cost comparable to the Hartree-Fock method and our results may serve as a benchmark for application of this scheme to collisions between larger molecules. [ABSTRACT FROM AUTHOR]

We investigated the translational incidence energy (E¡) and surface temperature (Ts) dependence of CO vibrational excitation upon scattering from a clean Au(111) surface. We report absolute v = 0 → 1 excitation probabilities for E¡ between 0.16 and 0.84 eV and Ts between 473 and 973 K. This is now only the second collision system where such comprehensive measurements are available - the first is NO on Au(111). For CO on Au(111), vibrational excitation occurs via direct inelastic scattering through electron hole pair mediated energy transfer - it is enhanced by incidence translation and the electronically non-adiabatic coupling is about 5 times weaker than in NO scattering from Au(111). Vibrational excitation via the trapping desorption channel dominates at E¡ = 0.16 eV and quickly disappears at higher E¡. [ABSTRACT FROM AUTHOR]

The effects of electric field and shape on the ground state energy of an exciton bound to an ionized donor are studied in a weakly prolate (oblate) GaAs ellipsoidal quantum dot within the effective-mass approximation using a variational method in the framework of perturbation theory. Both the heavy-hole and light-hole excitons are considered. The results show that the energy shift not only depends on the electric field but also on the dot shape and size. The influence of the dot shape on the light hole case is more pronounced than that of the heavy hole case, but the influence of the electric field on the heavy hole case is more pronounced. [ABSTRACT FROM AUTHOR]

Auger decay of the C2H2 double core-hole (DCH) states, including the single-site DCH (C1s-2), two-site DCH (C1s-1C1s-1), and satellite (C1s-2π-1π*+1) states, has been investigated experimentally using synchrotron radiation combined with multi-electron coincidence method, and theoretically with the assumption of the two-step sequential model for Auger decay of the DCH states. The theoretical calculations can reproduce the experimental two-dimensional Auger spectra of the C2H2 single-site DCH and satellite decays, and allow to assign the peaks appearing in the spectra in terms of sequential two-electron vacancy creations in the occupied valence orbitals. In case of the one-dimensional Auger spectrum of the C2H2 two-site DCH decay, the experimental and calculated results agree well, but assignment of peaks is difficult because the first and second Auger components overlap each other. The theoretical calculations on the Auger decay of the N2 single-site DCH state, approximately considering the effect of nuclear motion, suggest that the nuclear motion, together with the highly repulsive potential energy curves of the final states, makes an important effect on the energy distribution of the Auger electrons emitted in the second Auger decay. [ABSTRACT FROM AUTHOR]

Three dual-wavelength InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) with increasing indium content are grown by metal-organic chemical vapor deposition, which contain six periods of low-In-content MQWs and two periods of high-In-content MQWs. For the low-In-content MQWs of three studied samples, their internal quantum efficiency (IQE) shows a rising trend as the emission wavelength increases from 406 nm to 430 nm due to the suppression of carriers escape from the wells to the barriers. However, for the high-In-content MQWs, the sample IQE falls rapidly with a further increase of emission wavelength from 496 nm to 575 nm. Theoretical calculation reveals that the electron-hole wave function overlap in the high-In-content MQWs is reduced because of an increase in the internal polarization field as indium content is increased. In addition, time-resolved photoluminescence decay curves show that the carriers generated in the low-In-content MQWs can be effectively transferred to the high-In-content part through the reabsorption process. However, the transfer time gradually becomes longer as emission wavelength increases, which means a reduction of carrier transfer rate between the different indium content MQWs. Furthermore, nonradiative recombination is enhanced in the high-In-content MQWs with longer emission wavelength due to the decline of crystal quality. Therefore, the fast drop of IQE for high-In-content MQWs can be attributed to the increase of the internal polarization field, the decrease of carrier transfer efficiency, and the enhanced nonradiative recombination. This research has a certain guiding value for an understanding of the recombination mechanism in the InGaN/GaN MQWs and for achieving high quality multiple-wavelength LEDs with better performance. [ABSTRACT FROM AUTHOR]

We present fully atomistic calculations of the electron and hole mobilities in Ge nanowires with diameter up to 10 nm. We show that the phonon-limited mobility is strongly dependent on the diameter and on the orientation of the nanowire, and is also very responsive to unaxial strains. The similarities and differences with the case of Si nanowires are highlighted. In strained Ge nanowires, the mobility can reach >3000 cm2/V/s for electrons and 12000 cm2/V/s for holes. Ge nanowires are therefore promising nanostructures for ultimate electronic devices. [ABSTRACT FROM AUTHOR]

By using electric field induced optical second harmonic generation measurement, charge accumulation at the double-layer interface of ITO/α-NPD/Alq3/Al diodes was verified under two electroluminescence (EL) operational modes, which were activated in the low and high frequency regions, respectively, with application of large ac square voltage. Results supported our proposed idea [A. Sadakata et al., J. Appl. Phys. 110, 103707 (2011)] that accumulated holes suppress hole injection in the low frequency region and lead to the decrease of the EL intensity activated by the recombination of holes and electrons injected from opposite electrodes. On the one hand, the accumulated holes assist electron injection in the high frequency region and result in the increase of EL intensity activated by the recombination of the interfacial accumulated holes and injected electrons from Al electrode. [ABSTRACT FROM AUTHOR]

We have studied survival and rotational excitation probabilities of H2(vi = 1, Ji = 1) and D2(vi = 1, Ji = 2) upon scattering from Cu(111) using six-dimensional (6D) adiabatic (quantum and quasi-classical) and non-adiabatic (quasi-classical) dynamics. Non-adiabatic dynamics, based on a friction model, has been used to analyze the role of electron-hole pair excitations. Comparison between adiabatic and non-adiabatic calculations reveals a smaller influence of non-adiabatic effects on the energy dependence of the vibrational deexcitation mechanism than previously suggested by low-dimensional dynamics calculations. Specifically, we show that 6D adiabatic dynamics can account for the increase of vibrational deexcitation as a function of the incidence energy, as well as for the isotope effect observed experimentally in the energy dependence for H2(D2)/Cu(100). Furthermore, a detailed analysis, based on classical trajectories, reveals that in trajectories leading to vibrational deexcitation, the minimum classical turning point is close to the top site, reflecting the multidimensionally of this mechanism. On this site, the reaction path curvature favors vibrational inelastic scattering. Finally, we show that the probability for a molecule to get close to the top site is higher for H2 than for D2, which explains the isotope effect found experimentally. [ABSTRACT FROM AUTHOR]

Sensing with sub-wavelength hole arrays is being actively researched as a means to improve detection sensitivity and reduce the size of the developed sensor. One of the approaches to sensing with hole arrays is to use a shift of the main transmittance peak generated by analyte exposure. In this report, the effect of the shape of the holes on the peak shift is investigated with a view to improve further the main transmittance peak shift. Rectangular holes are studied by simulation and experiments with a palladium metallic matrix. Palladium permittivity is varied by exposure to hydrogen and generates main transmittance peak shifts toward longer wavelengths. The simulation results of the propagation constant and electric field distribution revealed that the peak shift is controlled by the short side length of the rectangular holes. The short side of the rectangular holes normalized by the peak wavelength should be below 1/10 for the rectangular holes to achieve their maximum effect. [ABSTRACT FROM AUTHOR]

The electron-hole explicitly correlated Hartree-Fock method (eh-XCHF) is presented as a general strategy for investigation of electron-hole correlation and computation of electron-hole recombination probability. The eh-XCHF method is a variational method which uses explicitly correlated wavefunction that depends on the electron-hole inter-particle distances. It is shown that the explicitly correlated ansatz provides a systematic route to variationally minimize the total energy. The parabolic quantum dot is used as the benchmark system and the eh-XCHF method is used for computation of the ground state energy and electron-hole recombination probability. The results are compared to Hartree-Fock and explicitly correlated full configuration interaction (R12-FCI) calculations. The results indicate that an accurate description of the electron-hole wavefunction at short electron-hole inter-particle distances is crucial for qualitative description of the electron-hole recombination probability. The eh-XCHF method successfully addresses this issue and comparison of eh-XCHF calculations with R12-FCI shows good agreement. The quality of the mean field approximation for electron-hole system is also investigated by comparing HF and R12-FCI energies for electron-electron and electron-hole systems. It was found that performance of the mean field approximation is worse for the electron-hole system as compared to the corresponding electron-electron system. [ABSTRACT FROM AUTHOR]

The authors theoretically investigate quantum confinement and transition energies in quantum wells (QWs) asymmetrically positioned in wrinkled nanomembranes. Calculations reveal that the wrinkle profile induces both blue- and redshifts, depending on the lateral position of the QW probed. Relevant radiative transitions include the ground state of the electron (hole) and excited states of the hole (electron). Energy shifts as well as stretchability of the structure are studied as a function of wrinkle amplitude and period. Large tunable bandwidths of up to 70 nm are predicted for highly asymmetric, wrinkled QWs. [ABSTRACT FROM AUTHOR]

Due to their effective short channel length of only a few hundred nanometers, vertical organic triodes (VOTs) have a high potential to overcome problems of low current densities and switching speed in current organic field effect transistors (OFETs). Furthermore, VOTs are easy to build because no sub-structuring of the base contact is necessary. Nevertheless, these devices are poorly investigated. In literature, two different working mechanisms are suggested: hot carrier transport through the metallic base or transport of charge carriers through a permeable base electrode. As a strong asymmetry is expected for function principle based on hot carriers, we are able to distinguish between both mechanisms by examining the bidirectional transmission properties of the VOT consisting of electron transporting materials. We show that high transmission values (>95%) are possible for both directions, suggesting a base contact with openings forming a grid electrode. [ABSTRACT FROM AUTHOR]

A simple optical model based on the transfer matrix method was used to simulate photon absorption in oxide semiconductor systems with varying insulator thickness in the thin film transistor (TFT) structure. For comparison with actual experimental results, hole current was measured in transparent metal/semiconductor/insulator/metal capacitor stacks under light illumination, and the threshold voltage shift under negative bias illumination stress conditions was also measured in the TFT structure. In each structure, experimental data showed variance as the insulator thickness changed, and these results agreed well with the simulations. The results showed that light interference in multi-layered devices has a crucial influence on the reliability of them under illumination and that they should be considered when designing systems that work under these conditions. The accuracy of the simulations suggests they can be implemented to minimize instability issues in oxide TFTs for display. [ABSTRACT FROM AUTHOR]

The time dynamics of self-pumped reflection gratings in a commonly used photorefractive polymer PDCST:PVK:ECZ-BBP:C60 with no additional electron sources or traps is investigated. While holes are normally the mobile charges and responsible for grating formation, our experimental observations, analyzed using multi-exponential fitting curves, show evidence of electrons in addition to holes as charge carriers, particularly above an applied field of 40 V/μm. The dependence of effective carrier mobilities on the applied electric field, deduced from experimental results, show stronger field dependence of electron mobility at high electric fields. At an applied field of 70 V/μm, electron and hole mobilities become approximately equal, and the contribution of electrons on grating formation becomes significant. [ABSTRACT FROM AUTHOR]

Strong modifications of semiconductors can be provoked by high-density electronic excitation. We report on surface structuring of monocrystalline wurtzite O-face (0001) ZnO excited by UV femtosecond laser pulses (248 nm) below the ablation threshold. At fluences above 11 mJ/cm2, nanoholes of D=10 nm diameter appear quasi-periodically separated by a distance ∼30 nm (=3 D). Dual-pulse (pump-pump) experiments permit estimation of the electronic excitation lifetime responsible for this nanostructuring, which is in agreement with the electron-hole plasma lifetime 220 ps. The nanostructuring results in a smaller monocrystalline domain of ∼0.1 μm size and increases the crystalline interplane c-distance by 0.11%. The excitonic luminescence of the irradiated sample is found to increase by about 10 times. The nanostructuring remains stable in a limited range of laser fluences: above 40 mJ/cm2 the surface melts, which accelerates the photoinduced bonds breaking leading to surface erosion. We tentatively ascribe the related mechanism to the nucleation-growth of cluster vacancies at crystal dislocations accelerated by the non-thermal (electronic) melting of the surface layer. At fluences lower than 11 mJ/cm2, larger volcano-like features of 60-nm diameter were observed. The characteristic crater shape and irregular surface repartition permit their assignment to thermal explosion of impurities due to multiple exciton condensation. [ABSTRACT FROM AUTHOR]

We have studied the effect of the yttria content on the paramagnetic centers in electron-irradiated yttria-stabilized zirconia (ZrO2: Y3+) or YSZ. Single crystals with 9.5 mol % or 18 mol % Y2O3 were irradiated with electrons of 1.0, 1.5, 2.0, and 2.5 MeV. The paramagnetic center production was studied by X-band electron paramagnetic resonance (EPR) spectroscopy. The same paramagnetic centers were identified for both chemical compositions, namely two electron centers, i.e., (i) F+-type centers (involving singly ionized oxygen vacancies), and (ii) so-called T centers (Zr3+ in a trigonal symmetry site), as well as hole-centers. A strong effect is observed on the production of hole-centers that is strongly enhanced when doubling the yttria content. However, no striking effect is found on the electron centers (except the enhancement of an extra line associated with the F+-type centers). It is concluded that hole-centers are produced by inelastic interactions, whereas F+-type centers are produced by elastic collisions with no effect of the yttria content on the defect production rate. In the latter case, the threshold displacement energy (Ed) of oxygen is estimated from the electron-energy dependence of the F+-type center production rate, with no significant effect of the yttria content on Ed. An Ed value larger than 120 eV is found. This is supported by classical molecular dynamics (MD) simulations with a Buckingham-type potential that show Ed values for Y and O are likely to be in excess of 200 eV. Due to the difficulty in displacing O or Y atoms, the radiation-induced defects may alternatively be a result of Zr atom displacements for Ed = 80 ± 1 eV with subsequent defect rearrangement. [ABSTRACT FROM AUTHOR]

We use capacitance techniques to directly measure the Fermi level at the crystalline/amorphous interface in n-type silicon heterojunction solar cells. The hole density calculated from the Fermi level position and the inferred band-bending picture show strong inversion of (n)crystalline silicon at the interface at equilibrium. Bias dependent experiments show that the Fermi level is not pinned at the interface. Instead, it moves farther from and closer to the crystalline silicon valence band under a reverse and forward bias, respectively. Under a forward bias or illumination, the Fermi level at the interface moves closer to the crystalline silicon valence band thus increases the excess hole density and band bending at the interface. This band bending further removes majority electrons away from the interface leading to lower interface recombination and higher open-circuit voltage. [ABSTRACT FROM AUTHOR]

The transport behaviors of achiral single walled carbon nanotubes (SWNTs) with two di-vacancies have been investigated using a tight-binding model. The obtained results indicate that the number and position of the resonant conductance are related to the longitudinal inter-vacancy distance, and the conductance spectra are very different for different di-vacancy configurations. It is also found that the di-vacancy's lateral distribution has a drastic effect on the SWNT's transport behaviors. A 2π/n rotation invariance exists in the (10,10) armchair SWNT, and instead the 3·2π/n one for the (18,0) zigzag SWNT. Furthermore, our obtained results can give a good estimate of the transport behaviors of the metallic SWNTs with a number of di-vacancies. [ABSTRACT FROM AUTHOR]

Focusing a problem with Rashba spin-orbit interaction (SOI-R), we propose an approach derived from standard multiband Hamiltonians that incorporate several hole band mixing effects. The influence of the SOI-R is shown on both the band structure and the zero-temperature conductance of a quasi-one-dimensional hole system patterned in a two-dimensional hole gas by a repulsive bias. The SOI-R leads the hole spectrum to split, as expected. However, we found appealing additional sub-band extremes only for heavy holes' unfolded sub-bands. The heavy-hole energy splitting exhibits a strong dependency on the wave vector component kz along the wire as well as on the effective Rashba parameter that clearly departs from the two-dimensional (2D) case. Nevertheless, for some low interval of kz, the standard 2D feature is preserved and the splitting naturally follows a cubic proportionality. The two-probe Landauer ladder-steps quantization conductance shifts down to lower Fermi energies for nonzero SOI-R. The existence of heavy hole subband multiple minima lead to additional propagating modes and, thus, anomalous peaks for the ballistic conductance arise at variance with the opposite for light holes. The robustness of the anomalous features in the ballistic conductance can be tuned by manipulating the effective Rashba parameter and the hole-nanowire width, leading to destroy them as well. [ABSTRACT FROM AUTHOR]

Two lowest-lying excited singlets with Bu symmetry of all-trans-oligoenes, the well-known ionic 11Bu+ state as well as the 'hidden' ionic-covalent-mixed 11Bu- state, are calculated within both the Pariser-Parr-Pople (PPP) model at full configuration interaction (FCI) level and ab initio methods. The vertical excitation energies as well as wavefunctions from PPP-FCI calculations are found to be in good agreement with those from high-level multi-reference methods, such as multi-reference complete active space self-consistent field (CASSCF) with second order perturbative corrections (CASPT2), multi-reference Mo\ller-Plesset perturbation theory (MRMP), and complete active space valence bond theory (CASVB). The oscillator strengths from PPP calculation are in good agreement with spectroscopy experiments. The relatively small oscillator strength of 11Bu- is due to the approximate electron-hole symmetry of this state. In addition, the bond lengths in both states are found to show remarkable relativity with the bond orders calculated with ground state geometries, which suggests a possible strategy for initial guess in geometry optimization of excited states. [ABSTRACT FROM AUTHOR]

The energy relaxation of carriers in reduced graphene oxide thin films is studied using optical pump-probe spectroscopy with two probes of different colors. We measure the time difference between peaks of the carrier density at each probing energy by measuring a time-resolved differential transmission and find that the carrier density at the lower probing energy peaks later than that at the higher probing energy. Also, we find that the peak time for the lower probing energy shifts from about 92 to 37 fs after the higher probing energy peak as the carrier density is increased from 1.5×1012 to 3×1013/cm2, while no noticeable shift is observed in that for the higher probing energy. Assuming the carriers rapidly thermalize after excitation, this indicates that the optical phonon emission time decreases from about 50 to about 20 fs and the energy relaxation rate increases from 4 to 10 meV/fs. The observed density dependence is inconsistent with the phonon bottleneck effect. [ABSTRACT FROM AUTHOR]

The tuning effects of substitutional B and N dopings on the electron transport properties of a C60 dimer bridge [(C60)2] are investigated by nonequilibrium Green's functions in combination with density functional theory. It is found that, unlike C60, the equilibrium conductance of (C60)2 is very small. However, it can be controlled by electron (N) doping or hole (B) doping, which shifts the molecular energy levels so that the LUMOs or HOMOs align well with the Fermi level and results in LUMO-mediated transport or HOMO-mediated transport. The conductance increases accordingly with a certain number of N or B atoms doped in the C60s. Interestingly, when one C60 is doped with N atoms and the other is doped with B atoms, new transport behaviors arise and rectification is achieved due to the special alignment of the energy levels of the individual C60s with the Fermi level. It suggests that cluster or molecule assembling can often realize specific functions which are not available by single molecules and should be taken into consideration in the design of molecular devices. [ABSTRACT FROM AUTHOR]

A simple method for computing accurate density-dependent dispersion coefficients is presented. The dispersion coefficients are modeled by a generalized gradient-type approximation to Becke and Johnson's exchange hole dipole moment formalism. Our most cost-effective variant, based on a disjoint description of atoms in a molecule, gives mean absolute errors in the C6 coefficients for 90 complexes below 10%. The inclusion of the missing long-range van der Waals interactions in density functionals using the derived coefficients in a pair wise correction leads to highly accurate typical noncovalent interaction energies. [ABSTRACT FROM AUTHOR]

The scattering effects in the infrared (IR) spectra of single, isolated bread yeast cells (Saccharomyces cerevisiae) on a ZnSe substrate and in metal microchannels have been probed by Fourier transform infrared imaging microspectroscopy. Absolute extinction [(3.4±0.6)×10-7 cm2 at 3178 cm-1], scattering, and absorption cross sections for a single yeast cell and a vibrational absorption spectrum have been determined by comparing it to the scattering properties of single, isolated, latex microspheres (polystyrene, 5.0 μm in diameter) on ZnSe, which are well modeled by the Mie scattering theory. Single yeast cells were then placed into the holes of the IR plasmonic mesh, i.e., metal films with arrays of subwavelength holes, yielding 'scatter-free' IR absorption spectra, which have undistorted vibrational lineshapes and a rising generic IR absorption baseline. Absolute extinction, scattering, and absorption spectral profiles were determined for a single, ellipsoidal yeast cell to characterize the interplay of these effects. [ABSTRACT FROM AUTHOR]

Using an invariant expansion, we build an Hamiltonian describing the influence of the crystal-field, the electron-hole exchange interaction, and any symmetry-breaking perturbations on the fine structure of excitons bound to systems of reduced symmetry: D2d, C3v, and C2v. Several perturbations are considered, including, but not limited to, an electric field, a magnetic field, a strain field, and their combinations. For each symmetry system considered, symmetrized excitonic wave functions, build from heavy- and light-holes states, are used to expand the Hamiltonian in the form of matrices, whose eigenvalues directly provide the energy of the excitonic states and whose eigenstates can be used to determine oscillator strengths of optical transitions. Using this model, we satisfactorily reproduce the excitonic emission observed from nitrogen dyads in GaAs and tellurium dyads in strained ZnSe. We also present Hamiltonians for independent heavy- and light-hole subsystems. Comparing the two models, we demonstrate that the splitting observed in strained quantum dots of C2v symmetry does not necessarily imply a significant anisotropic exchange interaction. This splitting can be produced by a weak coupling between heavy- and light-hole bands. [ABSTRACT FROM AUTHOR]

New substructures of H4 and H5 hole traps have been revealed using Laplace deep-level transient spectroscopy. Our measurements show that the hole traps H4 and H5 can have at least three components for each. Moreover, the activation energies are deduced and the microscopic nature of these substructures is discussed. [ABSTRACT FROM AUTHOR]

Hot carriers in a doped graphene under dc electric field is described taking into account the intraband energy relaxation due to acoustic phonon scattering and the interband generation-recombination transitions caused by thermal radiation. The consideration is performed for the case when the intercarrier scattering effectively establishes the quasiequilibrium electron-hole distributions, with effective temperature and concentrations of carriers. The concentration and energy balance equations are solved taking into account an interplay between weak energy relaxation and generation-recombination processes. The nonlinear conductivity is calculated for the momentum relaxation caused by the elastic scattering. The current-voltage characteristics, and the transition between bipolar and monopolar regimes of conductivity are obtained and analyzed, for different temperatures and gate voltages. [ABSTRACT FROM AUTHOR]

The average power loss of two-dimensional hole gas (2DHG) due to acoustic and optical phonons is calculated in a Si/Si0.8Ge0.2 heterostructure over a wide temperature range. The power loss of 2DHG due to acoustic phonons via deformation potential coupling and Pekar mechanism is calculated taking account of temperature dependent screening. The hole-acoustic phonon coupling is found to dominate hole power loss for Tc<70 K. The experimental power loss data for Tc<2 K is accounted for by Pekar mechanism and the data for 2 K70 K. Hot phonon effect is found to reduce the power loss of 2DHG by a factor of about 1.5. The power loss calculations are carried out for different carrier concentrations and compared with those in GaAlAs/GaAs heterostructures. [ABSTRACT FROM AUTHOR]

It has been believed that the defect structure of “undoped” BaTi1-ηO3-δ be governed by background acceptor impurities, most likely AlTi′, in its near stoichiometry regime (δ≈0). Its electrical conductivity versus oxygen activity should, thus, be expected to be similar to that of Al-doped BaTiO3 not only in equilibrium state at elevated temperatures, but also in quenched state at low temperatures. In quenched state, however, the “undoped” BaTiO3 from various sources totally betray this expectation: A landmark of the quenched-state defect structure, semiconductor-to-insulator transition falls at an oxygen activity >10 orders lower than that of the Al-doped and it is rather similar to that of variable-valent Mn-doped BaTiO3. The transition is found to systematically depend on the nonmolecularity (η), suggesting the defect structure being governed by the multiply-charged cation vacancies. The hole-trapping energies of VBa″ and VTi⁗ are estimated and compared with those for the fixed-valent AlTi′ and variable-valent MnTi″. [ABSTRACT FROM AUTHOR]

The electric field effect on the carrier capture cross section of deep traps has been studied. The experimental results on the H4F and H5 hole traps in p-type InP show an enhancement of the capture cross section with the increase in the applied electric field. This enhancement depends on the nature of the deep traps and its peak temperature. Increasing the electric field from 4.1×106 to 2.4×107 V/m leads to an increase in the H4F capture cross section by a factor of 3 to 20. While in the case of H5 it increases by a factor of 2 to 5 by increasing the applied electric field from 8.0×106 to 2.4×107 V/m. A theoretical model has been suggested to explain the electric field effect on the capture cross section. This model deals with the cascade and multiphonon processes semiclassically. Applying this model to the above deep traps, we have found that H4F is negatively charged complex and H5 is positively charged complex. [ABSTRACT FROM AUTHOR]

The microscopic states and performance of organic solar cell are investigated theoretically to explore the effect of the carrier mobility. With Ohmic contacts between the semiconductor and the metal electrodes there are two origins of carriers in the semiconductor: the photocarriers generated by photon absorption and the dark carriers diffused from the electrodes. The power efficiency of the solar cell is limited by the recombination of a carrier with either the photocarrier or a dark carrier. Near the short-circuit condition the photocarrier recombination in the semiconductor bulk decreases as the mobility increases. Near the open-circuit condition the dark carrier recombination increases with the mobility. These two opposite effects balance with one another, resulting in an optimal mobility about 10-2 cm2/V s which gives the highest power conversion efficiency. The balance of the electron and hole mobilities are not necessary to maintain the optimal efficiency also because of the balance of the photocarrier and dark carrier recombination. The efficiency remains about the same as one carrier mobility is fixed at 10-2 cm2/V s while the other one varies from 10-1 to 10-3 cm2/V s. For solar cell with a Schottky barrier between the semiconductor and the metal electrode there is no dark carrier recombination. The efficiency therefore always increases with the mobility. [ABSTRACT FROM AUTHOR]

We fabricate a series of square-lattice subwavelength circular, rectangular, and trapezoidal air-hole arrays drilled in opaque gold thin films and measured the transmission spectra of light passing through these metallic nanostructures in the near-infrared range. The measured results show strong dependence of extraordinary optical transmission on the lattice constant, hole size, and hole shape. The wavelengths of the transmission peaks and dips are mainly determined by the lattice constant. However, they are also influenced by the parameters of the hole size and hole shape. The experimental data agree well with numerical calculation results by means of a plane-wave transfer-matrix method when the detailed geometry of the air holes is fully considered. The results indicate that the extraordinary light transmission through subwavelength metallic nanostructures of air-hole arrays are governed by excitation of surface plasmon polaritons on the metal surface and their scattering by periodic air-hole arrays. [ABSTRACT FROM AUTHOR]

Terahertz (THz) lasers on optically pumped multiple-graphene-layer (MGL) structures as their active region are proposed and evaluated. The developed device model accounts for the interband and intraband transitions in the degenerate electron-hole plasma generated by optical radiation in the MGL structure and the losses in the slot or dielectric waveguide. The THz laser gain and the conditions of THz lasing are found. It is shown that the lasers under consideration can operate at frequencies >=1 THz at room temperatures. [ABSTRACT FROM AUTHOR]

Reduction in electron lifetime induced by excess electrons is a key effect in degradation of solar cells based on boron-doped and oxygen-containing silicon. Although boron related, degradation is controlled by the concentration of holes and not by the boron concentration. This recent finding is the basis for a new degradation model in which the degradation starts from a latent complex BiO2 of an interstitial boron atom and an oxygen dimer. Electron-induced reconstruction of this defect results in a production of recombination centers. This model provides a detailed explanation of the basic features of the degradation, and of subsequent recovery by annealing in the dark. [ABSTRACT FROM AUTHOR]

The differences in drain current and drain voltage (Id-Vd) characteristics of top and bottom contact organic thin film transistors (OTFTs) are analyzed by an OTFT devices simulator, which makes it possible to derive Id-Vd characteristics, potential distribution, and hole concentration distribution by solving Poisson’s equation and current continuity equation. It is found that the intrinsic characteristics of top contact devices are superior to those of the bottom contact ones, which is usually believed to be due to poor contact characteristics and poor semiconductor quality of bottom contact OTFTs. The mechanism behind the intrinsic characteristics differences is the deficiency of carriers at the source-channel interface, resulting to a very high potential drop, which the bottom contact devices suffer more. Remarkable improvements in drain current are expected by only inserting high carrier concentration region around the source/drain contact area, which totally eliminates the potential drop. [ABSTRACT FROM AUTHOR]

The electronic structure as well as the optical response of kesterite and stannite structures of Cu2ZnSnS4 and Cu2ZnSnSe4 are analyzed by a relativistic full-potential linearized augmented plane wave method. The energy dispersion of the conduction-band edge reveals larger effective electron mass of the two Cu2ZnSnS4 compounds (mc1≈0.18m0) compared with Cu2ZnSnSe4 (mc1≈0.07m0). Whereas the effective electron mass tensor is fairly isotropic, the effective hole masses show strong anisotropy. The fundamental band-gap energy is estimated to be Eg≈1.5 eV for Cu2ZnSnS4 and Eg≈1.0 eV for Cu2ZnSnSe4. The larger band gap results in a smaller high-frequency dielectric constant: [variant_greek_epsilon]∞≈6.7 for Cu2ZnSnS4 whereas [variant_greek_epsilon]∞≈8.6 for Cu2ZnSnSe4. The characteristic anisotropy of the dielectric function [variant_greek_epsilon](ω) in the stannite compounds allows for a complementary identification of the crystalline structure type. Overall, however, all four compounds show similar atomic-resolved density-of-states, dielectric function, and optical absorption coefficient α(ω). [ABSTRACT FROM AUTHOR]

We study the quantum-well magnetoexcitons by the variational method in the strong coupling regime of excitons with microcavity photons. In such strong coupling regime, we find that the coupling between the exciton internal state and center of mass motion can be neglected. Through the calculations, we find that the magnetic field can reduce the exciton exchange interaction to 30% as Lw=30 nm with Lw being the quantum well width. This is in contrast to the magnetic-field enhancement in the exciton Rabi splitting. In this work, we obtain that a magnetic field can enhance the exciton Rabi splitting up to 1.6 times in the case of Lw=30 nm. They both originate from the local enhancement in the exciton internal state due to the magnetic field. In addition, we analyze the behavior of the magnetic response of the exciton energy, Rabi splitting, and exchange interaction in manipulating the polariton parametric scattering, and widening the microcavity applications. [ABSTRACT FROM AUTHOR]

The electronic structure as well as the optical response of kesterite and stannite structures of Cu2ZnSnS4 and Cu2ZnSnSe4 are analyzed by a relativistic full-potential linearized augmented plane wave method. The energy dispersion of the conduction-band edge reveals larger effective electron mass of the two Cu2ZnSnS4 compounds (mc1≈0.18m0) compared with Cu2ZnSnSe4 (mc1≈0.07m0). Whereas the effective electron mass tensor is fairly isotropic, the effective hole masses show strong anisotropy. The fundamental band-gap energy is estimated to be Eg≈1.5 eV for Cu2ZnSnS4 and Eg≈1.0 eV for Cu2ZnSnSe4. The larger band gap results in a smaller high-frequency dielectric constant: [variant_greek_epsilon]∞≈6.7 for Cu2ZnSnS4 whereas [variant_greek_epsilon]∞≈8.6 for Cu2ZnSnSe4. The characteristic anisotropy of the dielectric function [variant_greek_epsilon](ω) in the stannite compounds allows for a complementary identification of the crystalline structure type. Overall, however, all four compounds show similar atomic-resolved density-of-states, dielectric function, and optical absorption coefficient α(ω). [ABSTRACT FROM AUTHOR]

We have computed the electronic and hole spectra of a three-dimensional superlattice consisting of layers of GaAs rods of finite height arranged in a hexagonal lattice and embedded in an AldGa1-dAs matrix, alternating with spacer layers of homogeneous AlAs. The spectra are calculated in the envelope function approximation, with both light-hole and heavy-hole subbands and hole spin degeneracy taken into account. The application of thick spacers allows to investigate the band structure of isolated layers of cylindrical rods. We estimate the ultimate efficiency of solar energy conversion in a solar cell based on an array of cylindrical quantum dots versus the dot height, and determine the optimal value of this parameter. [ABSTRACT FROM AUTHOR]

The formation and conversion processes of defect centers in low water peak single mode optical (LWPSM) fiber irradiated with gamma rays were investigated at room temperature using electron spin resonance. Germanium electron center (GEC) and self-trapped hole center (STH) occur when the fibers are irradiated with 1 and 5 kGy cumulative doses, respectively. With the increase in irradiation doses, the GEC defect centers disappear, and new defect centers such as E′ centers (Si and Ge) and nonbridge oxygen hole centers (NBOHCs) generate. The generation of GEC and STH is attributed to the electron transfer, which is completely balanced. This is the main reason that radiation-induced attenuation (RIA) of the LWPSM fiber is only 10 dB/km at communication window. The new defect centers come from the conversion of GEC and STH to E′ centers and NBOHC, and the conversion processes cause bond cleavage, which is the root cause that the RIA of the LWPSM fiber significantly increases up to 180 dB/km at working window. Furthermore, the concentration of new defect centers is saturated easily even by increasing cumulative doses. [ABSTRACT FROM AUTHOR]

For the realization and the improvement of GaN-based optoelectronic devices (light emitting diodes and laser diodes) emitting from the ultraviolet to the red wavelength range GaInN quantum well structures with high internal quantum efficiency are of great importance. To determine parameters which affect the internal quantum efficiency, we have analyzed the emission intensity of GaInN quantum well structures with varied electron and hole wave function overlap by temperature and excitation power dependent and by time-resolved photoluminescence. The quantum confined Stark effect reduces the temperature dependent photoluminescence emission intensity for thick polar quantum wells at low temperature. But near room temperature, these thick polar GaInN quantum wells feature less relative intensity loss than thinner polar quantum wells. This behavior can partially be assigned to increased screening effects and higher quantum well barriers for thicker quantum wells. Additionally, excitation power dependent photoluminescence points to a transition from a radiative recombination based on excitons at 10 K to a bimolecular recombination at room temperature for thick c-plane quantum wells. This transition may also affect the intensity decrease by a changed carrier diffusivity. [ABSTRACT FROM AUTHOR]

Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift mobility, surface band bending, and surface capture coefficient for electrons. Samples grown (unintentionally n-type) by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of (3–6)×1016 cm-3 for one growth run, roughly 5×1014–1×1015 cm-3 for the second, and drift mobility in the range of 500–700 cm2/(V s) for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of ≈3–5 μm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) the thinner (≈100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1–1 μA. Under the same conditions, thicker (151–320 nm) nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 μA. With the light blocked, the dark current was restored in a few minutes for the thinner samples and an hour or more for the thicker ones. The samples with lower carrier concentration showed similar trends. Excitation in the 360–550 nm range produced substantially weaker photocurrent with comparable decay rates. Nanowire photoconductivity arises from a reduction in the depletion layer via photogenerated holes drifting to the surface and compensating ionized surface acceptors. Simulations yielded (dark) surface band bending in the vicinity of 0.2–0.3 V and capture coefficient in the range of 10-23–10-19 cm2. Atomic layer deposition (ALD) was used to conformally deposit ≈10 nm of Al2O3 on several devices. Photoconductivity, persistent photoconductivity, and subgap photoconductivity of the coated nanowires were increased in all cases. TaN ALD coatings showed a reduced effect compared to the Al2O3 coated samples. [ABSTRACT FROM AUTHOR]