Empty States Research Articles

Cerium-Doped Zirconium Dioxide, a Visible-Light-Sensitive Photoactive Material of Third Generation

The dispersion of small amounts of Ce(4+) ions in the bulk of ZrO2 leads to a photoactive material sensitive to visible light. This is shown by monitoring with EPR the formation and the reactivity of photogenerated (λ > 420 nm) charge carriers. The effect, as confirmed by DFT calculations, is due to the presence in the solid of empty 4f Ce states at the mid gap, which act as intermediate levels in a double excitation mechanism. This solid can be considered an example of a third-generation photoactive material.

Mapping Proximity within Proteins using Fluorescence Spectroscopy: Tyr as Well as Trp can be used for Distance-Dependent Fluorescence Quenching Studies

We have been developing a site-directed fluorescence labeling (SDFL) approach called TrIQ to assess distances within and between proteins, in a way complementary to traditional FRET methods. TrIQ, or Tryptophan-Induced Quenching, exploits the fact that some fluorophores are quenched by nearby Trp residues in a distance dependent fashion. Here we report two new advances to TrIQ, gleaned from recent studies of T4 lysozyme (T4L).First, we find Tyr can also be used in TrIQ studies. Interestingly, Tyr has some key differences from Trp. Tyr has a smaller sphere of for the fluorophore bimane, and only shows significant quenching for Cα-Cα distance of less than ∼10A, compared to ∼15A for Trp. Also, unlike Trp, Tyr cannot quench the fluorophore BODIPY.Second, we find TrIQ can reliably assess the magnitude and energetics of protein movements, especially when a combination of Tyr and Trp are employed. We used TrIQ to measure a key movement in T4L by placing a bimane and Trp (or Tyr) on opposite ends of a “hinge” in T4L, sites predicted to be ∼14.5A apart (Cα-Cα distance) in the substrate bound state, and ∼10.5A in the empty state. The only substantial TrIQ was observed for the Trp sample in the empty state, and this quenching was abolished in a mutant (T26E) that covalently binds substrate. Tyr did not dramatically quench bimane in either the empty or bound conformation. Together, these results are consistent with the different distance constraints for quenching described above. Arrhenius analysis of the effect of hinge-bending movement on fluorophore lifetime suggests an activation energy on the order of 2-4 kcal/mol, and this value is not significantly affected by the “active site mutation” T26E.

Activation of Inhibitory G Protein Catalyzed by GPCR: Molecular Dynamics Simulations of the Activated Cannabinoid CB2 Receptor/Gαi1β1γ2 Protein Complex

The GPCR signaling cascade via the G protein pathway begins with an agonist binding to an inactive GPCR, causing conformational changes that activate the GPCR. Previously, we used molecular dynamics simulations to study the activation of the CB2 receptor (a Class A GPCR), by the endogenous ligand, 2-arachidonoylglycerol (2-AG) via the lipid bilayer (Hurst et al., 2010). In the next step of our study of the G-protein signaling cascade, we studied G-protein activation by an activated GPCR. In this step, GDP is released via the separation of the Gαi1 ras-like and helical domains of (Van Eps et al., 2011). To this end, we used our 2-AG activated CB2 model to produce an initial 2-AG/CB2/Gαi1β1γ2 complex based on the crystal structure of β2 adrenoreceptor in complex with Gαsβ1γ2 (Rasmussen et al., 2011). The complex was immersed in a fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer and NPT NAMD2 (Phillips et al., 2005) molecular dynamics (MD) simulations were initiated for four different trajectories of this system. Results from our longest MD simulation (3μs) suggest that the C-terminal α5 helix of Gαi prefers a different orientation in the CB2 activated receptor relative to the orientation seen in the empty state (GDP-GTP less) β2 adrenoreceptor/Gαsβ1γ2 complex. Initial hydrophobic interactions between P139 on CB2 intracellular loop 2 (IC-2 loop) and a hydrophobic pocket on Gαi consisting of residues V34 (N terminus); F336, T340, I343 and I344 (C terminus); L194 (β1-sheet); and F196 (β2-sheet) stabilize the receptor / Gαi1 protein interface during the first 0.5μs of the simulation. Later, between 1.4 - 1.6 μs, electrostatic interactions between R229 (CB2 IC-3 loop) and Q304/E308 (Gαi1 α4 helix) facilitate hydration of GDP. [Support: RO1 DA003934 and KO5 DA021358 (PHR)].

Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3

The organic-inorganic hybrid perovskite CH3NH3PbI3 is a novel light harvester, which can greatly improve the solar-conversion efficiency of dye-sensitized solar cells. In this article, a first-principle theoretical study is performed using local, semi-local and non-local exchange-correlation approximations to find a suitable method for this material. Our results, using the non-local optB86b + vdWDF functional, excellently agree with the experimental data. Thus, consideration of weak van der Waals interactions is demonstrated to be important for the accurate description of the properties of this type of organic-inorganic hybrid materials. Further analysis of the electronic properties reveals that I 5p electrons can be photo-excited to Pb 6p empty states. The main interaction between the organic cations and the inorganic framework is through the ionic bonding between CH3 and I ions. Furthermore, I atoms in the Pb-I framework are found to be chemically inequivalent because of their different chemical environments.

Role of HOPG density of empty electronic states above vacuum on electron emission spectra induced by ions and UV photons

We present results of secondary electron emission (SEE) induced by UV photons and low energy He ions interacting with a pristine and ion damaged surface of highly-oriented pyrolytic graphite (HOPG). A critical examination of the changes in the SEE spectra produced by the radiation damage allows us to identify the origin of the different features of the SEE spectra. In this way, we conclude that the low energy peak at ~3eV is strongly determined by the structure of the (empty) HOPG density of states above vacuum level. In contrast the high-energy structure at ~14eV could not be explained by such an argument but rather by exciton autoionization.

Some Anomalies of Farsighted Strategic Behavior

We investigate the loss in optimality due to the presence of selfish players in sequential games, a relevant subclass of extensive form games with perfect information recently introduced in Paes Leme et al. (Proceedings of innovations in theoretical computer science (ITCS), ACM, New York, pp. 60---67, 2012). In such a setting, the notion of subgame perfect equilibrium is preferred to that of Nash equilibrium since it better captures the farsighted rationality of the players who can anticipate future strategic opportunities. We prove that the sequential price of anarchy, that is the worst-case ratio between the social performance at a subgame perfect equilibrium and that of the best possible solution, is exactly 3 in cut and consensus games. Moreover, we improve the known Ω(n) lower bound for unrelated scheduling to $2^{\varOmega(\sqrt{n})}$ and refine the corresponding upper bound to 2 n , where n is the number of players. Finally, we determine essentially tight bounds for fair cost sharing games by proving that the sequential price of anarchy is between n+1?H n and n. A surprising lower bound of (n+1)/2 is also determined for the singleton case. Our results are quite interesting and counterintuitive, as they show that a farsighted behavior may lead to a worse performance with respect to a myopic one: in fact, either Nash equilibria and simple Nash rounds, consisting of a single (myopic) move per player starting from the empty state, achieve a price of anarchy which results to be lower or equivalent to the sequential price of anarchy in almost all the considered cases.

Random-phase approximation correlation energies from Lanczos chains and an optimal basis set: theory and applications to the benzene dimer.

A new ab initio approach is introduced to compute the correlation energy within the adiabatic connection fluctuation dissipation theorem in the random phase approximation. First, an optimally small basis set to represent the response functions is obtained by diagonalizing an approximate dielectric matrix containing the kinetic energy contribution only. Then, the Lanczos algorithm is used to compute the full dynamical dielectric matrix and the correlation energy. The convergence issues with respect to the number of empty states or the dimension of the basis set are avoided and the dynamical effects are easily kept into account. To demonstrate the accuracy and efficiency of this approach the binding curves for three different configurations of the benzene dimer are computed: T-shaped, sandwich, and slipped parallel.

Deep level states and negative photoconductivity in n-ZnO/p-Si hetero-junction diodes

Negative photoconductivity (NPC) was observed in n-ZnO/p-Si heterojunction diode grown by ultra-high vacuum sputtering method under nitrogen ambient. Under the illumination of ultra-violet light, positive photoconductivity was observed at low bias voltages, whereas NPC was observed at high bias voltages. The defect states in the ZnO layers grown on Si were analyzed by photoluminescence and deep level transient spectroscopy measurements. Two deep levels were measured at Ec-0.51 eV and Ec-0.54 eV, which might be originated from oxygen vacancy and nitrogen atom related defects, respectively. Based on the simulation of band diagram, the defect states were located below Fermi level at zero bias voltage. However, as increasing the bias voltages, NPC was observed due to the increase of empty defect states. This analysis allowed us to consider the possibility that the NPC phenomenon in n-ZnO/p-Si heterojunction diode is originated dominantly from the defect states as a carrier recombination center in ZnO layer.

On angular momentum and rest mass of the photon

AbstractA reconsideration is made on the basic concepts of the individual photon, including its angular momentum (spin) and a possibly existing very small rest mass. In terms of conventional classical theory, as well as of its quantum mechanical counterpart, the results from a so far established Standard Model of an empty vacuum state are not found to be reconcilable with an experimentally relevant photon model. The main properties of such a model would on the other hand become compatible with the results of a recently established revised quantum electrodynamic theory based on a non-zero electric field divergence in the vacuum and a corresponding symmetry breaking of the electromagnetic field.

Elimination of the mott interband s-d enhancement of the electrical resistance of nickel and platinum owing to the excitation of electrons by femtosecond laser pulses

The dependence of electrical, σ, and thermal, κ, conductivities of metals on the electron temperature T e at high (∼1 eV) T e values has been calculated. The two-temperature states for which the temperature T e of heated electrons exceeds the temperature T i of ions in the crystal lattice result from the excitation of electrons by femtosecond laser pulses. It is well known that the existence of empty d levels with a high density of states near the Fermi surface (as, e.g., in nickel, platinum, and iron) leads to a pronounced enhancement of the electrical resistance (Mott, 1936). This is due to an increase in the statistical factor related to the electron transitions to the empty states induced by collisions with phonons. It is found that the excitation of the electron subsystem significantly reduces the electron-phonon scattering to unoccupied d states since the chemical potential μ(T e ) rises above the upper edge of the d band. The decrease in the scattering probability leads to the anomalous behavior of the conductivity σel-ph, which increases with the temperature T e . Such a behavior turns out to be inverse with respect to the usual situation in condensed matter.

Soft x-ray appearance potential spectroscopy study of MgO (100) and α-Al2O3 (100) single crystals

Soft x-ray appearance potential spectroscopy (SXAPS) measurements was used to measure on MgO (100) and α-Al2O3 (100) single crystals. Mg 1s, Al 1s, and O 1s SXAPS self-deconvoluted (SD) spectra were obtained. The features of the Mg 1s and O1s SD spectra are in fair agreement with those of the near-edge x-ray absorption fine structure spectra for an MgO thin film (3 ML) on Ag (100). This suggests that the SXAPS spectra reflect electronic states of the relaxed MgO (100) surface. The features of the Al 1s and O 1s SD spectra are in qualitative agreement with those of the electron energy-loss spectroscopy. The SXAPS SD spectra are discussed in terms of antibonding states and partial density of empty states obtained by theoretical calculations for MgO and α-Al2O3, respectively. The present result suggests that the “approximate dipole selection rule” is applicable to the SXAPS spectra of MgO and α-Al2O3, as well as 3d transition metal oxides.

Research of Collaborative Filtering Recommendation Algorithm based on Network Structure

This paper combines the classic collaborative filtering algorithm with personalized recommendation algorithm based on network structure. For the data sparsity and malicious behavior problems of traditional collaborative filtering algorithm, the paper introduces a new kind of social network-based collaborative filtering algorithm. In order to improve the accuracy of the personalized recommendation technology, we first define empty state in the state space of multi-dimensional semi-Markov processes and obtain extended multi-dimensional semi-Markov processes which are combined with social network analysis theory, and then we get social network information flow model. The model describes the flow of information between the members of the social network. So, we propose collaborative filtering algorithm based on social network information flow model. The algorithm uses social network information and combines user trust with user interest and find nearest neighbors of the target user and then forms a project recommended to improve the accuracy of recommended. Compared with the traditional collaborative filtering algorithm, the algorithm can effectively alleviate the sparsity and malicious behavior problem, and significantly improve the quality of the recommendation system recommended.

Evolution of Defect-Associated Subband Energy States in Nanocrystalline TiO2 Films on Si and Ge Substrates

We identified the electronic states in the conduction- and valence-band edges associated with intrinsic defects in nanocrystalline TiO2 layers on Si and Ge substrates. This was accomplished through spectroscopic study with soft X-ray photoemission spectroscopy and visible-ultraviolet spectroscopic ellipsometry. The interpretation of the spectra based on molecular orbital (MO) theory well explains the origin of empty and occupied states of band edge in TiO2 with the correct assignment of MO states. The evolution of these band-edge states under thermal nanocrystalline growth and interfacial chemical mixing as a function of type of substrate was investigated, taking into consideration the asymmetric local bonding distortion and fast atomic diffusion at the grain boundary. The engineering solution to utilize TiO2 as a gate dielectric on Ge substrate is demonstrated by implanting SiON/Si interfacial layer. This study suggests that the control of electronically active defect density and energy level in nanoscale TiO2 thin films is strongly affected by thermal grain expansion and interfacial chemistry, depending on the semiconductor substrates used.

Strong anisotropic influence of local-field effects on the dielectric response ofα-MoO3

Dielectric properties of {\alpha}-MoO3 are investigated by a combination of valence electron-energy loss spectroscopy and ab initio calculation at the random phase approximation level with the inclusion of local-field effects (LFE). A meticulous comparison between experimental and calculated spectra is performed in order to interpret calculated dielectric properties. The dielectric function of MoO3 has been obtained along the three axes and the importance of LFE has been shown. In particular, taking into account LFE is shown to be essential to describe properly the intensity and position of the Mo-N2,3 edges as well as the low energy part of the spectrum. A detailed study of the energy-loss function in connection with the dielectric response function also shows that the strong anisotropy of the energy-loss function of {\alpha}-MoO3 is driven by an anisotropic influence of LFE. These LFE significantly dampen a large peak in {\epsilon}2, but only along the [010] direction. Thanks to a detailed analysis at specific k-points of the orbitals involved in this transition, the origin of this peak has not only been evidenced but a connection between the inhomogeneity of the electron density and the anisotropic influence of local-field effects has also been established. More specifically, this anisotropy is governed by a strongly inhomogeneous spatial distribution of the empty states. This depletion of the empty states is localized around the terminal oxygens and accentuates the electron inhomogeneity.

Unified description for hopping transport in organic semiconductors including both energetic disorder and polaronic contributions

We developed an analytical model to describe hopping transport in organic semiconductors including both energetic disorder and polaronic contributions due to geometric relaxation. The model is based on a Marcus jump rate in terms of the small-polaron concept with a Gaussian energetic disorder, and it is premised upon a generalized effective medium approach yet avoids shortcomings involved in the effective transport energy or percolation concepts. It is superior to our previous treatment [Phys. Rev. B 76, 045210 (2007)] since it is applicable at arbitrary polaron activation energy ${E}_{a}$ with respect to the energy disorder parameter $\ensuremath{\sigma}$. It can be adapted to describe both charge-carrier mobility and triplet exciton diffusion. The model is compared with results from Monte Carlo simulations. We show (i) that the activation energy of the thermally activated hopping transport can be decoupled into disorder and polaron contributions whose relative weight depend nonlinearly on the $\ensuremath{\sigma}/{E}_{a}$ ratio, and (ii) that the choice of the density of occupied and empty states considered in configurational averaging has a profound effect on the results of calculations of the Marcus hopping transport. The $\ensuremath{\sigma}/{E}_{a}$ ratio governs also the carrier-concentration dependence of the charge-carrier mobility in the large-carrier-concentration transport regime as realized in organic field-effect transistors. The carrier-concentration dependence becomes considerably weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality. This model bridges a gap between disorder and polaron hopping concepts.

Determination of the Interface States in Amorphous/Crystalline Silicon Using Surface Photovoltage Spectroscopy

The interface state level at amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions is identified with surface photovoltage spectroscopy (SPS). A positive slope alteration of SPS in 1.2 eV indicates there is a primarily empty state at Ev(c-Si)+0.75 eV. Using hydrogen plasma treatment (HPT) on c-Si surface, we observed a systematic variation in the SPS signal upon HPT time, which was associated to the change of interface state density. Our results show that the SPS can be used to optimize the heterojunction with intrinsic thin-layer solar cell preparation process.

Edge state in epitaxial nanographene on 3C-SiC(100)/Si(100) substrate

Epitaxial nanographene grown on SiC substrate is of great interest for electronic and optoelectronic applications. The shape and the size of nanographene dictates its electrical, optical, magnetic, and chemical properties including possible edge states and quantum confinement. Here, we report the epitaxial growth of nanographene on 3C-SiC(100) on silicon substrates. Raman spectroscopy determines the nanographene size to be around 20 nm, making it an ideal high edge density sample. Near edge x-ray absorption fine structure of nanographene reveals the appearance of an additional state located at the Fermi level, interpreted as an empty state corresponding to graphene edges.

Waiting Time Distributions for the Transport through a Quantum-Dot Tunnel Coupled to One Normal and One Superconducting Lead

We have studied the waiting time distributions (WTDs) for subgap transport through a single-level quantum-dot tunnel coupled to one normal and one superconducting lead. The WTDs reveal the internal dynamics of the system, in particular, the coherent transfer of Cooper pairs between the dot and the superconductor. The WTDs exhibit oscillations that can be directly associated to the coherent oscillation between the empty and doubly occupied dot. The oscillation frequency is equal to the energy splitting between the Andreev bound states. These effects are more pronounced when the empty state and double-occupied state are in resonance.

Infrared photoconductivity of Er-doped Si nanoclusters embedded in a slot waveguide

Infrared photoconductive and photovoltaic effects are observed in Er-doped Si nanoclusters incorporated in a silicon p-i-n slot-waveguide device. These effects are ascribed to deep gap states of Si nanoclusters. The room temperature open circuit voltage of the devices is 290 mV under transmission of guided light at 1.5 μm. A power dependence, with the exponent close to 0.5 and 1 for forward and reverse bias, respectively, has been observed for the photocurrent versus light intensity characteristic. The former is attributed to bimolecular recombination (empty deep gap states) and the latter to linear recombination with the states being populated with electrons.

Ligands Raise the Constraint That Limits Constitutive Activation in G Protein-coupled Opioid Receptors

Using a cell-free bioluminescence resonance energy transfer strategy we compared the levels of spontaneous and ligand-induced receptor-G protein coupling in δ (DOP) and μ (MOP) opioid receptors. In this assay GDP can suppress spontaneous coupling, thus allowing its quantification. The level of constitutive activity was 4-5 times greater at the DOP than at the MOP receptor. A series of opioid analogues with a common peptidomimetic scaffold displayed remarkable inversions of efficacy in the two receptors. Agonists that enhanced coupling above the low intrinsic level of the MOP receptor were inverse agonists in reducing the greater level of constitutive coupling of the DOP receptor. Yet the intrinsic activities of such ligands are identical when scaled over the GDP base line of both receptors. This pattern is in conflict with the predictions of the ternary complex model and the "two state" extensions. According to this theory, the order of spontaneous and ligand-induced coupling cannot be reversed if a shift of the equilibrium between active and inactive forms raises constitutive activation in one receptor type. We propose that constitutive activation results from a lessened intrinsic barrier that restrains spontaneous coupling. Any ligand, regardless of its efficacy, must enhance this constraint to stabilize the ligand-bound complexed form.