Showing papers in "Physical Review B in 2004"

Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons

Abstract: We have studied the interaction of polyaromatic hydrocarbons ~PAHs! with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene, and ovalene at submonolayer coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV, and 2.1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of two-dimensional adsorbate islands up to the desorption temperature. Preexponential frequency factors are found to be in the range 10 14 ‐10 21 s 21 as obtained from both FalconerMadix ~isothermal desorption! analysis and Antoine’s fit to vapor pressure data. The resulting binding energy per carbon atom of the PAH is 5265 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is 6165 meV/atom, which is considerably larger than previously reported experimental values.

Misfit-energy-increasing dislocations in vapor-deposited CoFe/NiFe multilayers

Abstract: Recent molecular dynamics simulations of the growth of $[{\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}/\mathrm{Au}]$ multilayers have revealed the formation of misfit-strain-reducing dislocation structures very similar to those observed experimentally. Here we report similar simulations showing the formation of edge dislocations near the interfaces of vapor-deposited (111) [NiFe/CoFe/Cu] multilayers. Unlike misfit dislocations that accommodate lattice mismatch, the dislocation structures observed here increase the mismatch strain energy. Stop-action observations of the dynamically evolving atomic structures indicate that during deposition on the (111) surface of a fcc lattice, adatoms may occupy either fcc sites or hcp sites. This results in the random formation of fcc and hcp domains, with dislocations at the domain boundaries. These dislocations enable atoms to undergo a shift from fcc to hcp sites, or vice versa. These shifts lead to missing atoms, and therefore a later deposited layer can have missing planes compared to a previously deposited layer. This dislocation formation mechanism can create tensile stress in fcc films. The probability that such dislocations are formed was found to quickly diminish under energetic deposition conditions.

Rapid estimation of elastic constants by molecular dynamics simulation under constant stress

Abstract: Molecular simulations, when they are used to understand properties characterizing the mechanical strength of solid materials, such as stress-strain relation or Born stability criterion, by using elastic constants, are sometimes seriously time consuming. In order to resolve this problem, we propose an efficient simulation approach under constant external stress and temperature, modifying Parrinello-Rahman (PR) method using useful sampling techniques developed recently---massive Nos\'e-Hoover chain method and hybrid Monte Carlo method. Test calculations on the Ni crystal employing the embedded atom method have shown that our method greatly improved the efficiency in sampling the elastic properties compared with the conventional PR method.

First-principles prediction of redox potentials in transition-metal compounds with LDA+ U

Abstract: First-principles calculations within the local density approximation (LDA) or generalized gradient approximation (GGA), though very successful, are known to underestimate redox potentials, such as those at which lithium intercalates in transition metal compounds. We argue that this inaccuracy is related to the lack of cancellation of electron self-interaction errors in LDA/GGA and can be improved by using the DFT+U method with a self-consistent evaluation of the U parameter. We show that, using this approach, the experimental lithium intercalation voltages of a number of transition metal compounds, including the olivine Li xMPO4 (M = Mn, Fe Co, Ni), layered LixMO2 (x = Co, Ni) and spinel-like LixM2O4 (M = Mn, Co), can be reproduced accurately.

Surface tension effect on the mechanical properties of nanomaterials measured by atomic force microscopy

Abstract: The effect of reduced size on the elastic properties measured on silver and lead nanowires and on polypyrrole nanotubes with an outer diameter ranging between 30 and 250 nm is presented and discussed. Resonant-contact atomic force microscopy (AFM) is used to measure their apparent elastic modulus. The measured modulus of the nanomaterials with smaller diameters is significantly higher than that of the larger ones. The latter is comparable to the macroscopic modulus of the materials. The increase of the apparent elastic modulus for the smaller diameters is attributed to surface tension effects. The surface tension of the probed material may be experimentally determined from these AFM measurements.

Grain-size effects on the ferroelectric behavior of dense nanocrystalline BaTiO 3 ceramics

Abstract: A progressive reduction of tetragonal distortion, heat of transition, Curie temperature, and relative dielectric constant has been observed on dense ${\mathrm{BaTiO}}_{3}$ ceramics with grain size decreasing from 1200 to $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The correlations between grain size, extent of tetragonal distortion, and ferroelectric properties strongly support the existence of an intrinsic size effect. From the experimental trends the critical size for disappearance of ferroelectricity has been evaluated to be $10--30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The strong depression of the relative permittivity observed for the nanocrystalline ceramics can be ascribed to the combination of the intrinsic size effect and of the size-dependent ``dilution'' effect of a grain boundary ``dead'' layer.

Numerical atomic basis orbitals from H to Kr

Abstract: We present a systematic study for numerical atomic basis orbitals ranging from H to Kr, which could be used in large scale $\mathrm{O}(N)$ electronic structure calculations based on density-functional theories (DFT). The comprehensive investigation of convergence properties with respect to our primitive basis orbitals provides a practical guideline in an optimum choice of basis sets for each element, which well balances the computational efficiency and accuracy. Moreover, starting from the primitive basis orbitals, a simple and practical method for variationally optimizing basis orbitals is presented based on the force theorem, which enables us to maximize both the computational efficiency and accuracy. The optimized orbitals well reproduce convergent results calculated by a larger number of primitive orbitals. As illustrations of the orbital optimization, we demonstrate two examples: the geometry optimization coupled with the orbital optimization of a ${\mathrm{C}}_{60}$ molecule and the preorbital optimization for a specific group such as proteins. They clearly show that the optimized orbitals significantly reduce the computational efforts, while keeping a high degree of accuracy, thus indicating that the optimized orbitals are quite suitable for large scale DFT calculations.

Excitonic fine structure and recombination dynamics in single-crystalline ZnO

Abstract: The optical properties of a high quality bulk $\mathrm{ZnO}$, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at $3.3605\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ having a linewidth of $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and dominating the PL spectrum at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This bound exciton transition was visible only below $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, whereas the A-free exciton transition at $3.3771\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ persisted up to room temperature. A-free exciton binding energy of $60\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ is obtained from the position of the excited states of the free excitons. Additional intrinsic and extrinsic fine structures such as polariton, two-electron satellites, donor-acceptor pair transitions, and longitudinal optical-phonon replicas have also been observed and investigated in detail. Time-resolved PL measurements at room temperature reveal a biexponential decay behavior with typical decay constants of $\ensuremath{\sim}170$ and $\ensuremath{\sim}864\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for the as-grown sample. Thermal treatment is observed to increase the carrier lifetimes when performed in a forming gas environment.

Equations of state of six metals above 94 GPa

Abstract: Compression versus pressure at ambient temperature has been measured for tantalum, gold, and platinum to $94\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and for aluminum, copper, and tungsten to $153\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, in a diamond anvil cell. Standard synchrotron x-ray diffraction accuracy in the volume determination could be achieved to the maximum pressure. The current data set is used to recalibrate the static pressure scale based on the ruby luminescence, confirming recent suggestions of an underestimation of pressure. Using an updated pressure calibration, the consistency between ultrasonic, dynamic, and static measurements of the equations of state is improved for these six equations of state. This consistency allows us to test the predictive power of density functional theory, with different approximations, for equation-of-state calculations. For example, the generalized gradient approximation leads to very accurate results, except for gold, the heaviest element.

Evidence for a structurally-driven insulator-to-metal transition in VO 2 : A view from the ultrafast timescale

Abstract: We apply ultrafast spectroscopy to establish a time-domain hierarchy between structural and electronic effects in a strongly correlated electron system. We discuss the case of the model system ${\mathrm{VO}}_{2}$, a prototypical nonmagnetic compound that exhibits cell doubling, charge localization, and a metal-insulator transition below 340 K. We initiate the formation of the metallic phase by prompt hole photo-doping into the valence band of the low-$T$ insulator. The insulator-to-metal transition is, however, delayed with respect to hole injection, exhibiting a bottleneck time scale, associated with the phonon connecting the two crystallographic phases. This structural bottleneck is observed despite faster depletion of the $d$ bands and is indicative of important bandlike character for this controversial insulator.

Size-dependent surface luminescence in ZnO nanowires

Abstract: Nanometer sized whiskers (nanowires) offer a vehicle for the study of size-dependent phenomena. While quantum-size effects are commonly expected and easily predicted, size reduction also causes more atoms to be closer to the surface. Here we show that intensity relations of below-band-gap and band-edge luminescence in ZnO nanowires depend on the wire radius. Assuming a surface layer wherein the surface-recombination probability is 1 (surface-recombination approximation), we explain this size effect in terms of bulk-related to surface-related material-volume ratio that varies almost linearly with the radius. This relation supports a surface-recombination origin for the deep-level luminescence we observe. The weight of this surface-luminescence increases as the wire radius decreases at the expense of the band-edge emission. Using this model, we obtain a radius of 30 nm, below which in our wires surface-recombination prevails. More generally, our results suggest that in quantum-size nanowires, surface-recombination may entirely quench band-to-band recombination, presenting an efficient sink for charge carriers that unless deactivated may be detrimental for electronic devices.

Nonintrinsic origin of the colossal dielectric constants in Ca Cu 3 Ti 4 O 12

Abstract: The dielectric properties of $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$, a material showing colossal values of the dielectric constant, were investigated over a broad temperature and frequency range extending up to $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$. A detailed equivalent-circuit analysis of the results and two crucial experiments, employing different types of contacts and varying the sample thickness were performed. The results provide clear evidence that the apparently high values of the dielectric constant in $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ are nonintrinsic and due to electrode polarization effects. The intrinsic properties of $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ are characterized by charge transport via hopping of localized charge carriers and a relatively high dielectric constant of the order of 100.

Lieb-Schultz-Mattis in higher dimensions

Abstract: A generalization of the Lieb-Schultz-Mattis theorem to higher-dimensional spin systems is shown. The physical motivation for the result is that such spin systems typically either have long-range order, in which case there are gapless modes, or have only short-range correlations, in which case there are topological excitations. The result uses a set of loop operators, analogous to those used in gauge theories, defined in terms of the spin operators of the theory. We also obtain various cluster bounds on expectation values for gapped systems. These bounds are used, under the assumption of a gap, to rule out the first case of long-range order, after which we show the existence of a topological excitation. Compared to the ground state, the topologically excited state has, up to a small error, the same expectation values for all operators acting within any local region, but it has a different momentum.

Thermopower enhancement in lead telluride nanostructures

Abstract: We demonstrate an enhancement in the thermopower of PbTe nanostructures with grain sizes on the order of $30--50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, relative to bulk. The enhancement is similar in magnitude to that reported in the literature for $\mathrm{Pb}\mathrm{Te}∕\mathrm{Pb}{\mathrm{Se}}_{x}{\mathrm{Te}}_{1\ensuremath{-}x}$ quantum dot superlattices. We provide proof, based on measurements of the transverse Nernst effect, that the enhancement has its origin in electron energy filtering induced by an alteration of the scattering mechanism.

Frequency-dependent local interactions and low-energy effective models from electronic structure calculations

Abstract: We propose a systematic procedure for constructing effective models of strongly correlated materials. The parameters, in particular the on-site screened Coulomb interaction $U$, are calculated from first principles, using the random-phase approximation. We derive an expression for the frequency-dependent $U(\ensuremath{\omega})$ and show, for the case of nickel, that its high-frequency part has significant influence on the spectral functions. We propose a scheme for taking into account the energy dependence of $U(\ensuremath{\omega})$, so that a model with an energy-independent local interaction can still be used for low-energy properties.

Origin of the different photoactivity of N -doped anatase and rutile TiO 2

Abstract: We have investigated the origin of the experimentally observed change in photoactivity of anatase and rutile ${\mathrm{TiO}}_{2}$ induced by substitutional $\mathrm{N}$-doping using state-of-the-art density functional theory calculations. Our results show that in both polymorphs $\mathrm{N}\phantom{\rule{0.3em}{0ex}}2p$ localized states just above the top of the $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ valence are present. In anatase these states cause a redshift of the absorption band edge towards the visible region. In rutile, instead, this effect is offset by the concomitant $\mathrm{N}$-induced contraction of the $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ band, resulting in an overall increase of the optical transition energy. Experimental trends are well described by these results.

Atomic disorder effects on half-metallicity of the full-Heusler alloys Co 2 ( Cr 1 − x Fe x ) Al : A first-principles study

Abstract: We investigate the atomic disorder effects on the half-metallicity of the full-Heusler alloy ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ from first principles by using the Korringa-Kohn-Rostoker method with the coherent potential approximation. Our results show that disorder between Cr and Al does not significantly reduce the spin polarization of the parent alloy ${\mathrm{Co}}_{2}\mathrm{CrAl},$ while disorder between Co and Cr makes a considerable reduction of the spin polarization. It is observed that the spin polarization of ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ decreases with increasing Fe concentration x in both the ordered ${L2}_{1}$ and the disordered $B2$ structures, and that the effects of the disorder on the spin polarization is significant at low Fe concentrations. The results suggest that a highly spin-polarized ferromagnet with high Curie temperature will be obtained if a ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ with the ordered ${L2}_{1}$ structure can be fabricated at low Fe concentrations.

Quantum criticality beyond the Landau-Ginzburg-Wilson paradigm

Abstract: We present the critical theory of a number of zero-temperature phase transitions of quantum antiferromagnets and interacting boson systems in two dimensions. The most important example is the transition of the $S=1∕2$ square lattice antiferromagnet between the N\'eel state (which breaks spin rotation invariance) and the paramagnetic valence bond solid (which preserves spin rotation invariance but breaks lattice symmetries). We show that these two states are separated by a second-order quantum phase transition. This conflicts with Landau-Ginzburg-Wilson theory, which predicts that such states with distinct broken symmetries are generically separated either by a first-order transition, or by a phase with co-existing orders. The critical theory of the second-order transition is not expressed in terms of the order parameters characterizing either state, but involves fractionalized degrees of freedom and an emergent, topological, global conservation law. A closely related theory describes the superfluid-insulator transition of bosons at half filling on a square lattice, in which the insulator has a bond density wave order. Similar considerations are shown to apply to transitions of antiferromagnets between the valence bond solid and the ${Z}_{2}$ spin liquid: the critical theory has deconfined excitations interacting with an emergent $\mathrm{U}(1)$ gauge force. We comment on the broader implications of our results for the study of quantum criticality in correlated electron systems.

Trends in the structure and bonding of noble metal clusters

Abstract: We present a systematic study of the electronic properties and the geometric structure of noble metal clusters ${X}_{n}^{\ensuremath{ u}}$ ($X=\mathrm{Cu}$, Ag, Au; $\ensuremath{ u}=\ensuremath{-}1,0,+1$; $n\ensuremath{\leqslant}13$ and $n=20$), obtained from first-principles generalized gradient approximation density functional calculations based on norm-conserving pseudopotentials and numerical atomic basis sets. We obtain planar structures for the ground state of anionic $(\ensuremath{ u}=\ensuremath{-}1)$, neutral $(\ensuremath{ u}=0)$, and cationic $(\ensuremath{ u}=1)$ species of gold clusters with up to 12, 11, and 7 atoms, respectively. In contrast, the maximum size of planar clusters with $\ensuremath{ u}=\ensuremath{-}1,0,+1$ are $n=(5,6,5)$ for silver and (5,6,4) for copper. For ${X}_{20}$ we find a ${T}_{d}$ symmetry for gold and a compact ${C}_{s}$ structure for silver and copper. Our results for the cluster geometries agree partially with previous first-principles calculations, and they are in good agreement with recent experimental results for anionic and cationic gold clusters. The tendency to planarity of gold clusters, which is much larger than in copper and silver, is strongly favored by relativistic effects, which decrease the $s\text{\ensuremath{-}}d$ promotion energy and lead to hybridization of the half-filled $6s$ orbital with the fully occupied $5{d}_{{z}^{2}}$ orbital. That picture is substantiated by analyzing our calculated density matrix for planar and three-dimensional clusters of gold and copper. The trends for the cohesive energy, ionization potentials, electron affinities, and highest accupied and lowest unoccupied molecular orbital gap, as the cluster size increases, are studied in detail for each noble metal and rationalized in terms of two- and three-dimensional electronic shell models. The most probable fragmentation channels for ${X}_{n}^{\ensuremath{ u}}$ clusters are in very good agreement with available experiments.

Weak magnetism and non-Fermi liquids near heavy-fermion critical points

Abstract: This paper is concerned with the weak-moment magnetism in heavy-fermion materials and its relation to the non-Fermi liquid physics observed near the transition to the Fermi liquid. We explore the hypothesis that the primary fluctuations responsible for the non-Fermi liquid physics are those associated with the destruction of the large Fermi surface of the Fermi liquid. Magnetism is suggested to be a low-energy instability of the resulting small Fermi surface state. A concrete realization of this picture is provided by a fractionalized Fermi liquid state which has a small Fermi surface of conduction electrons, but also has other exotic excitations with interactions described by a gauge theory in its deconfined phase. Of particular interest is a three-dimensional fractionalized Fermi liquid with a spinon Fermi surface and a U(1) gauge structure. A direct second-order transition from this state to the conventional Fermi liquid is possible and involves a jump in the electron Fermi surface volume. The critical point displays non-Fermi liquid behavior. A magnetic phase may develop from a spin density wave instability of the spinon Fermi surface. This exotic magnetic metal may have a weak ordered moment although the local moments do not participate in the Fermi surface. Experimental signatures of this phase and implications for heavy-fermion systems are discussed.

Water adsorption on metal surfaces: A general picture from density functional theory studies

Abstract: We present a density functional theory study of water adsorption on metal surfaces. Prototype water structures including monomers, clusters, one-dimensional chains, and overlayers have been investigated in detail on a model system-a Pt(111) surface. The structure, energetics, and vibrational spectra are all obtained and compared with available experimental data. This study is further extended to other metal surfaces including Ru(0001), Rh(111), Pd(111), and Au(111), where adsorption of monomers and bilayers has been investigated. From these studies, a general picture has emerged regarding the water-surface interaction, the interwater hydrogen bonding, and the wetting order of the metal surfaces. The water-surface interaction is dominated by the lone pair-d band coupling through the surface states. It is rather localized in the contacting layer. A simultaneous enhancement of hydrogen bonding is generally observed in many adsorbed structures. Some special issues such as the partial dissociation of water on Ru(0001) and in the RT39 bilayer phase, the H-up and H-down conversion, and the quantum-mechanical motions of H atoms are also discussed.

Plasmon emission in photoexcited gold nanoparticles

Abstract: Light emission at the particle plasmon frequency is observed in optically excited spherical gold nanoparticles. We find a photoluminescence efficiency of ${10}^{\ensuremath{-}6}$, which is essentially independent of particle size and four orders of magnitude higher than the efficiencies determined from metal films. Our experimental findings are explained with a process in which excited $d$-band holes recombine nonradiatively with $\mathit{sp}$ electrons, emitting particle plasmons. These plasmons subsequently radiate, giving rise to the photoluminescence observed in the experiment. We determine the quantum efficiencies involved in this process.

Artificial magnetic metamaterial design by using spiral resonators

Abstract: A metallic planar particle, that will be called spiral resonator (SR), is introduced as a useful artificial atom for artificial magnetic media design and fabrication. A simple theoretical model which provides the most relevant properties and parameters of the SR is presented. The model is validated by both electromagnetic simulation and experiments. The applications of SR's include artificial negative magnetic permeability media (NMPM) and left-handed-media (LHM) design. The main advantages of SR's for such purpose are small electrical size at resonance, absence of magnetoelectric coupling (thus avoiding bianisotropic effects in the continuous medium made of these particles), and easy fabrication. Experimental confirmation of NMPM and LHM behavior using SR's is also reported.

Pyrochlore photons: The U ( 1 ) spin liquid in a S = 1 2 three-dimensional frustrated magnet

Abstract: We study the $S=1/2$ Heisenberg antiferromagnet on the pyrochlore lattice in the limit of strong easy-axis exchange anisotropy. We find, using only standard techniques of degenerate perturbation theory, that the model has a $U(1)$ gauge symmetry generated by certain local rotations about the z axis in spin space. Upon addition of an extra local interaction in this and a related model with spins on a three-dimensional network of corner-sharing octahedra, we can write down the exact ground-state wave function with no further approximations. Using the properties of the soluble point we show that these models enter the $U(1)$ spin liquid phase, a fractionalized spin liquid with an emergent $U(1)$ gauge structure. This phase supports gapped ${S}^{z}=1/2$ spinons carrying the $U(1)$ ``electric'' gauge charge, a gapped topological point defect or ``magnetic'' monopole, and a gapless ``photon,'' which in spin language is a gapless, linearly dispersing ${S}^{z}=0$ collective mode. There are power-law spin correlations with a nontrivial angular dependence, as well as $U(1)$ topological order. This state is stable to all zero-temperature perturbations and exists over a finite extent of the phase diagram. Using a convenient lattice version of electric-magnetic duality, we develop the effective description of the $U(1)$ spin liquid and the adjacent soluble point in terms of Gaussian quantum electrodynamics and calculate a few of the universal properties. The resulting picture is confirmed by our numerical analysis of the soluble point wave function. Finally, we briefly discuss the prospects for understanding this physics in a wider range of models and for making contact with experiments.

Structure and transformation of the metastable boron- and oxygen-related defect center in crystalline silicon

Abstract: We analyze the core structure of the carrier-lifetime-reducing boron- and oxygen-related metastable defect center in crystalline silicon by measuring the correlation of the defect concentration with the boron and the oxygen contents on a large number of different silicon materials. The experimental results indicate that the defect is composed of one substitutional boron and two interstitial oxygen atoms. Formation and annihilation of the metastable boron-oxygen complex are found to be thermally activated processes, characterized by two strongly differing activation energies. Measurements of the defect generation rate as a function of light intensity show that the defect generation rate increases proportionally with light intensity below 1 ${\mathrm{m}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ and saturates at higher intensities. All experimental results can be consistently explained using a defect reaction model based on fast-diffusing oxygen dimers $({\mathrm{O}}_{2\mathrm{i}}),$ which are captured by substitutional boron $({\mathrm{B}}_{\mathrm{s}})$ to form a metastable ${\mathrm{B}}_{\mathrm{s}}\ensuremath{-}{\mathrm{O}}_{2\mathrm{i}}$ complex. Based on this model, new strategies for an effective reduction of the light degradation of solar cells made on oxygen-rich silicon materials are derived. The model also explains why no lifetime degradation is observed in aluminum-, gallium-, and indium-doped oxygen-rich silicon.

Phonon effects in molecular transistors: Quantal and classical treatment

Abstract: We present a comprehensive theoretical treatment of the effect of electron-phonon interactions on molecular transistors, including both quantal and classical limits. We study both equilibrated and out of equilibrium phonons. We present detailed results for conductance, noise and phonon distribution in two regimes. One involves temperatures large as compared to the rate of electronic transitions on and off the dot; in this limit our approach yields classical rate equations, which are solved numerically for a wide range of parameters. The other regime is that of low temperatures and weak electron-phonon coupling where a perturbative approximation in the Keldysh formulation can be applied. The interplay between the phonon-induced renormalization of the density of states on the quantum dot and the phonon-induced renormalization of the dot-lead coupling is found to be important. Whether or not the phonons are able to equilibrate in a time rapid compared to the transit time of an electron through the dot is found to affect the conductance. Observable signatures of phonon equilibration are presented. We also discuss the nature of the low-T to high-T crossover.

Propagation of optical excitations by dipolar interactions in metal nanoparticle chains

Abstract: Dispersion relations for dipolar modes propagating along a chain of metal nanoparticles are calculated by solving the full Maxwell equations, including radiation damping. The nanoparticles are treated as point dipoles, which means the results are valid only for $a∕d\ensuremath{\leqslant}\frac{1}{3}$, where $a$ is the particle radius and $d$ the spacing. The discrete modes for a finite chain are first calculated, then these are mapped onto the dispersion relations appropriate for the infinite chain. Computed results are given for a chain of $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.2em}{0ex}}\mathrm{diam}$ Ag spheres spaced by $75\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. We find large deviations from previous quasistatic results: Transverse modes interact strongly with the light line. Longitudinal modes develop a bandwidth more than twice as large, resulting in a group velocity that is more than doubled. All modes for which ${k}_{\text{mode}}\ensuremath{\leqslant}\ensuremath{\omega}∕c$ show strongly enhanced decay due to radiation damping.

Pressure dependence of the lattice dynamics of ZnO: An ab initio approach

Abstract: We have performed first-principles calculations of the electronic structure of ZnO, and applied them to the determination of structural and lattice-dynamical properties and their dependence on pressure. The dynamical matrices have been obtained for the wurtzite, zinc-blende, and rocksalt modifications with several lattice parameters optimized for pressures up to 12 GPa. These matrices are employed to calculate the one-phonon densities of states ~DOS! and the two-phonon DOS associated with either sums or differences of phonons. These results provide the essential tools to analyze the effect of isotope-induced mass disorder and anharmonicity on phonon linewidths, which we discuss here and compare with experimental data from Raman spectroscopy, including first- and second-order spectra. Agreement of calculated properties with experimental results improves considerably when the renormalization due to anharmonicity is subtracted from the experimental data.

First-principles calculation of the structure and magnetic phases of hematite

Abstract: Rhombohedral $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ has been studied by using density-functional theory (DFT) and the generalized gradient approximation (GGA). For the chosen supercell all possible magnetic configurations have been taken into account. We find an antiferromagnetic ground state at the experimental volume. This state is 388 meV/(Fe atom) below the ferromagnetic solution. For the magnetic moments of the iron atoms we obtain $3.4{\ensuremath{\mu}}_{\mathrm{B}},$ which is about $1.5{\ensuremath{\mu}}_{\mathrm{B}}$ below the experimentally observed value. The insulating nature of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ is reproduced, with a band gap of 0.32 eV, compared to an experimental value of about 2.0 eV. Analysis of the density of states confirms the strong hybridization between Fe $3d$ and O $2p$ states in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}.$ When we consider lower volumes, we observe a transition to a metallic, ferromagnetic low-spin phase, together with a structural transition at a pressure of 14 GPa, which is not seen in experiment. In order to take into account the strong on-site Coulomb interaction U present in ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ we also performed $\mathrm{D}\mathrm{F}\mathrm{T}+U$ calculations. We find that with increasing U the size of the band gap and the magnetic moments increase, while other quantities such as equilibrium volume and Fe-Fe distances do not show a monotonic behavior. The transition observed in the GGA calculations is shifted to higher pressures and eventually vanishes for high values of U. Best overall agreement, also with respect to experimental photoemission and inverse photoemission spectra of hematite, is achieved for $U=4\mathrm{eV}.$ The strength of the on-site interactions is sufficient to change the character of the gap from $d\ensuremath{-}d$ to $\mathrm{O}\ensuremath{-}p\ensuremath{-}\mathrm{Fe}\ensuremath{-}d.$