Additional Coulomb Interaction Research Articles

Proton halo effects in the 8B+64Zn collision around the Coulomb barrier

The 8B+64Zn reaction at 38.5 MeV has been studied at HIE-ISOLDE CERN to investigate proton halo effect on the reaction dynamics. For the first time it was used the only existing post-accelerated 8B beam. The measured elastic scattering angular distribution showed a small suppression of the Coulomb-nuclear interference peak, opposite to what observed for the one-neutron halo nucleus 11Be on the same target where a large suppression was observed instead. Inclusive angular and energy distributions of breakup fragments were also measured showing that, both, elastic and non-elastic breakup contribute. The presence of the additional Coulomb interactions halo-core and halo-target in 8B makes the reaction dynamics in this proton-halo nucleus different than the neutron-halo case.

Probing proton halo effects in the 8B+64Zn collision around the Coulomb barrier

Proton halo effects in the 8B+64Zn reaction at an energy around 1.5 times the Coulomb barrier have been studied at HIE-ISOLDE CERN using, for the first time, the only existing postaccelerated 8B beam. This, together with the use of a high granularity and large solid angle detection system, allowed for a careful mapping of the elastic angular distribution, especially in the Coulomb-nuclear interference region. Contrary to what is observed for the one-neutron halo nucleus 11Be on the same target in a similar energy range, the analysis of the elastic scattering angular distribution shows only a modest suppression of the Coulomb-nuclear interference peak, with no remarkable enhancement of the total reaction cross-section. Inclusive angular and energy distributions of 7Be produced in direct reaction processes have also been measured. The comparison of these data with the results of theoretical calculations for the elastic and non-elastic breakup contributions indicate that both processes are important. Overall, the experimental data suggest a 8B collision dynamics at the barrier very different from the one of neutron halo nuclei, showing only modest effects of coupling to continuum. This behaviour can be interpreted as due to the presence of the additional Coulomb interactions halo-core and halo-target together with the presence of the centrifugal barrier felt by the valence proton of 8B.

Enhanced capacitance characteristic of microporous carbon spheres through surface modification by oxygen-containing groups

The synthesis of porous carbon spheres with good dispersity and oxygen-rich content from phenolic resin remains challenging due to weak interactions between carbon precursors and soft templates, as well as fast cross-linking rate of precursors. Herein, we demonstrate a facile and efficient organic-organic self-assembly hydrothermal method to prepare microporous carbon spheres (MCSs) with perfect dispersity and rich oxygen-containing groups under highly acidic conditions. Both the precursors and templates are partly protonated by high concentration acid and thus provide an additional Coulombic interactions between resol and F108 via the I+X−S+ mechanism. Such enhanced interaction improve the self-assembly ability of resol and F108, lead to more matching the cross-linking rate of resol, which efficient avoid macroscopic phase separation and aggregate. Results indicate that the MCSs are modified with a considerable number of oxygen-containing functional groups (hydroxyl and carbonyl) under acidic conditions, and the oxygen content increases with the concentration of HCl. The optimal MCSs with a high specific surface area of 1449 m2 g−1, large pore volume of 0.77 cm3 g−1 and high oxygen content of 8.5 at%, exhibits an enhanced specific capacitance of 361 F g−1 and excellent long-term stability (93.8% retention after 10 000 cycles) when used as the electrode materials for supercapacitors. Furthermore, the MCSs based symmetrical supercapacitors delivers a maximum energy density of 18.5 W h kg−1 at power density of 750 W kg−1. This feasible hydrothermal method develops a new strategy for the fabrication of functionalized porous carbon spheres.

Magic angle for barrier-controlled double quantum dots

We show that the exchange interaction of a singlet-triplet spin qubit confined in double quantum dots, when being controlled by the barrier method, is insensitive to a charged impurity lying along certain directions away from the center of the double-dot system. These directions differ from the polar axis of the double dots by the magic angle, equaling $\arccos\left(1/\sqrt{3}\right)\approx 54.7^\circ$, a value previously found in atomic physics and nuclear magnetic resonance. This phenomenon can be understood from an expansion of the additional Coulomb interaction created by the impurity, but also relies on the fact that the exchange interaction solely depends on the tunnel coupling in the barrier-control scheme. Our results suggest that for a scaled-up qubit array, when all pairs of double dots rotate their respective polar axes from the same reference line by the magic angle, cross-talks between qubits can be eliminated, allowing clean single-qubit operations. While our model is a rather simplified version of actual experiments, our results suggest that it is possible to minimize unwanted couplings by judiciously designing the layout of the qubits.

Revisiting the neutral C-vacancy in diamond: Localization of electrons through DFT+U

The neutral C-vacancy is investigated using density functional theory calculations. We show that local functionals, such as PBE, can predict the correct stability order of the different spin states, and that the success of this prediction is related to the accurate description of the local magnetic configuration. Despite the correct prediction of the stability order, the PBE functional still fails predicting the defect states correctly. Introduction of a fraction of exact exchange, as is done in hybrid functionals such as HSE06, remedies this failure, but at a steep computational cost. Since the defect states are strongly localized, the introduction of additional on-site Coulomb and exchange interactions, through the DFT+U method, is shown to resolve the failure as well, but at a much lower computational cost. In this work, we present optimized U and J parameters for DFT+U calculations, allowing for the accurate prediction of defect states in defective diamond. The transferability of the U and J parameters is tested through the study of the ⟨001⟩ split-interstitial.

ChemInform Abstract: Interactions Between Ionic Liquids and Radicals

AbstractReview: 102 refs.

Shift of the photoemission threshold in III–V and II–VI semiconductors

The reasons why the photoemission threshold energy of semiconductors is lower than the ionization energy of constituent atoms have been investigated. It has been indicated that the previously proposed interpretation of this phenomenon based on the inclusion of an additional intra-atomic Coulomb interaction between the valence electrons is insufficient. It has been shown that the calculation of the electronic energy structure of semiconductors, in particular, of the photoemission threshold, requires taking into account a change in the localization region of the valence electrons when a free atom is embedded into a crystal. A way of taking this change into account in the tight-binding theory has been demonstrated. Corrections to the tight-binding Hamiltonian have been found. The photoemission thresholds of III–V and II–VI semiconductors have been calculated with the inclusion of these corrections. Comparison of the results with the experimental data has been performed.

Crystal Structures of the Novel Cytosolic 5′-Nucleotidase IIIB Explain Its Preference for m7GMP

5′-nucleotidases catalyze the hydrolytic dephosphorylation of nucleoside monophosphates. As catabolic enzymes they contribute significantly to the regulation of cellular nucleotide levels; misregulation of nucleotide metabolism and nucleotidase deficiencies are associated with a number of diseases. The seven human 5′-nucleotidases differ with respect to substrate specificity and cellular localization. Recently, the novel cytosolic 5′-nucleotidase III-like protein, or cN-IIIB, has been characterized in human and Drosophila. cN-IIIB exhibits a strong substrate preference for the modified nucleotide 7-methylguanosine monophosphate but the structural reason for this preference was unknown. Here, we present crystal structures of cN-IIIB from Drosophila melanogaster bound to the reaction products 7-methylguanosine or cytidine. The structural data reveal that the cytosine- and 7-methylguanine moieties of the products are stacked between two aromatic residues in a coplanar but off-centered position. 7-methylguanosine is specifically bound through π-π interactions and distinguished from unmodified guanosine by additional cation-π coulomb interactions between the aromatic side chains and the positively charged 7-methylguanine. Notably, the base is further stabilized by T-shaped edge-to-face stacking of an additional tryptophan packing perpendicularly against the purine ring and forming, together with the other aromates, an aromatic slot. The structural data in combination with site-directed mutagenesis experiments reveal the molecular basis for the broad substrate specificity of cN-IIIB but also explain the substrate preference for 7-methylguanosine monophosphate. Analyzing the substrate specificities of cN-IIIB and the main pyrimidine 5′-nucleotidase cN-IIIA by mutagenesis studies, we show that cN-IIIA dephosphorylates the purine m7GMP as well, hence redefining its substrate spectrum. Docking calculations with cN-IIIA and m7GMP as well as biochemical data reveal that Asn69 does not generally exclude the turnover of purine substrates thus correcting previous suggestions.

Interactions between ionic liquids and radicals

Ionic liquids were investigated with both stable radicals on the basis of 2,2,6,6-tetramethylpiperidine-1-yloxyl (TEMPO) and photogenerated lophyl radicals. The ionic liquids are composed either of bis(trifluoromethylsulfonyl)imide (NTf2) as anion and various cations or they contain an imidazolium ion in combination with various anions. The cations include imidazolium, pyrrolidinium, piperidinium, polymethine or ammonium ions. Furthermore, BF4−, PF6−, triflate, camphorsulfonate, lactate, tosylate or tris(pentafluoroethyl)trifluorophosphate (FAP) are the counter ions in the imidazolium salts. The structural variation of the ionic liquids results in differences in glass formation, semicrystallinity, or crystallinity, as well as in viscosity differences. Furthermore, a vinyl substituent at the imidazolium ion and a methacryloyloxyethyl substituent at the ammonium ion result in polymerizable ionic liquids that were converted via a radical mechanism in amorphous polymerized ionic liquids with a glass transition temperature, which is significantly higher compared to the ionic liquids. An additional substituent at TEMPO causes additional hydrogen bond formation or additional Coulomb interactions with the individual ions of the ionic liquids compared to TEMPO. This influences the mobility of these radicals in the ionic liquid expressed by differences in the average rotational correlation time (τrot). The mobility of the radicals in the ionic liquids as function of the temperature describes ionic liquids either as continuum in analogy to molecular solvents using the Stokes–Einstein model, that is the case for 1-butyl-3-methylimidazolium NTf2, or as medium where free volume effects are important for the mobility of a solute in the ionic liquid using the model of Spernol, Gierer, and Wirtz. The 1-butyl-3-methylimidazolium BF4− fits well into the latter. Furthermore, the isotropic hyperfine coupling constant (Aiso(14N)) of the stable radicals gives information about micropolarity of the ionic liquids only if the mobility of the radical is high enough in the ionic liquid. In addition to the rotational mobility of the stable radicals, the photogenerated lophyl radicals give information about translational diffusion of radicals and solvent cage effects in the ionic liquids. The application of the Eyring equation results mostly in the expected negative values of the activation entropy for the transition state that is typical for bimolecular reactions. Only few examples show a less negative or positive activation entropy for the bimolecular reaction, which may be attributed to radical recombination within the solvent cage to a high extent. The results obtained during investigation of radicals in ionic liquids are important to understand the radical processes in ionic liquids that may occur for example in dye sensitized solar cells, photo or thermally induced reactions or radical polymerizations in ionic liquids.

Magnetic anisotropy of FeO and CoO: the influence of gradient corrections on exchange and correlation

We study the formation of local magnetic moments and the magnetic anisotropy of the antiferromagnetic transition-metal compounds FeO and CoO in the framework of the density functional theory. Exchange and correlation are described within two (semi-)local approximations, the local spin-density approximation (LDA) and the generalized gradient approximation (GGA), in order to investigate the role of gradient corrections. An additional on-site Coulomb interaction U is considered in an LDA+U/GGA+U approach and spin–orbit interaction is taken into account. We find that the orbital magnetization is strongly quenched in the GGA+U approach compared to LDA+U results and experimental findings. The quenching is explained in terms of an overestimated exchange enhancement for the partially filled 3d shells due to the gradient corrections inherent in GGA+U.

Quantum theory of longitudinal momentum transfer in above-threshold ionization

In the ionization process, longitudinal momentum along the direction of propagation is transferred to the photoelectrons due to the action of the magnetic component of Lorentz force. In a recent experiment by Smeenk et al. [Phys. Rev. Lett. 106, 193002 (2011)], such a transfer is observed in the ionization of argon and neon atoms by circularly polarized light at 800 and 1400 nm in the intensity range of ${10}^{14}--{10}^{15}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. They accounted for the results by a purely classical model. We present a fully quantum-mechanical calculation of the transfer of longitudinal momentum to the photoelectrons. The results are in agreement with the observations of Smeenk et al. at high intensities, but clear evidence for additional Coulomb interactions emerges at low intensities.

Fermi surfaces and quantum oscillations in the underdoped high-Tcsuperconductors YBa2Cu3O6.5and YBa2Cu4O8

We study underdoped high-${T}_{c}$ superconductors YBa${}_{2}$Cu${}_{3}$O${}_{6.5}$ and YBa${}_{2}$Cu${}_{4}$O${}_{8}$ using first-principles pseudopotential methods with additional Coulomb interactions at the Cu atoms and obtain Fermi-surface pocket areas in close agreement with measured Shubnikov-de Haas and de Haas-van Alphen oscillations. With antiferromagnetic order in CuO${}_{2}$ planes, stable in the calculations, small hole pockets are formed near the so-called Fermi-arc positions in the Brillouin zone, which reproduce the low-frequency oscillations. A large electron pocket, necessary for the negative Hall coefficient, is also formed in YBa${}_{2}$Cu${}_{3}$O${}_{6.5}$, giving rise to the high-frequency oscillations as well. Effective masses and specific heats are also calculated. Our results highlight the important role of magnetic order in the electronic structure of underdoped high-${T}_{c}$ superconductors.

Electronic structure theory of the hidden-order materialURu2Si2

We report a comprehensive electronic structure investigation of the paramagnetic (PM), the large moment antiferromagnetic (LMAF), and the hidden order (HO) phases of ${\text{URu}}_{2}{\text{Si}}_{2}$. We have performed relativistic full-potential calculations on the basis of the density-functional theory, employing different exchange-correlation functionals to treat electron correlations within the open $5f$ shell of uranium. Specifically, we investigate---through a comparison between calculated and low-temperature experimental properties---whether the $5f$ electrons are localized or delocalized in ${\text{URu}}_{2}{\text{Si}}_{2}$. The local spin-density approximation (LSDA) and generalized gradient approximation (GGA) are adopted to explore itinerant $5f$ behavior, the GGA plus additional strong Coulomb interaction ($\text{GGA}+U$ approach) is used to approximate moderately localized $5f$ states, and the $5f$-core approximation is applied to probe potential properties of completely localized uranium $5f$ states. We also performed local-density approximation plus dynamical mean-field theory calculations (DMFT) to investigate the temperature evolution of the quasiparticle states at 100 K and above, unveiling a progressive opening of a quasiparticle gap at the chemical potential when temperature is reduced. A detailed comparison of calculated properties with known experimental data demonstrates that the LSDA and GGA approaches, in which the uranium $5f$ electrons are treated as itinerant, provide an excellent explanation of the available low-temperature experimental data of the PM and LMAF phases. We show furthermore that due to a material-specific Fermi-surface instability a large, but partial, Fermi-surface gapping of up to 750 K occurs upon antiferromagnetic symmetry breaking. The occurrence of the HO phase is explained through dynamical symmetry breaking induced by a mode of long-lived antiferromagnetic spin fluctuations. This dynamical symmetry breaking model explains why the Fermi-surface gapping in the HO phase is similar but smaller than that in the LMAF phase and it also explains why the HO and LMAF phases have the same Fermi surfaces yet different order parameters. A suitable order parameter for the HO is proposed to be the Fermi-surface gap, and the dynamic spin-spin correlation function is further suggested as a secondary order parameter.

Electronic structure of GaN and Ga investigated by soft x-ray spectroscopy and first-principles methods

The electronic structure and chemical bonding of wurtzite-GaN investigated by $\text{N}\text{ }1s$ soft x-ray absorption spectroscopy and $\text{N}\text{ }K$, $\text{Ga}\text{ }{M}_{1}$, and $\text{Ga}\text{ }{M}_{2,3}$ emission spectroscopy is compared to that of pure Ga. The measurements are interpreted by calculated spectra using first-principles density-functional theory (DFT) including dipole transition matrix elements and additional on-site Coulomb interaction $(\text{WC-GGA}+U)$. The $\text{Ga}\text{ }4p\text{-N}\text{ }2p$ and $\text{Ga}\text{ }4s\text{-N}\text{ }2p$ hybridization and chemical bond regions are identified at the top of the valence band between $\ensuremath{-}1.0$ and $\ensuremath{-}2.0$ and further down between $\ensuremath{-}5.5$ and $\ensuremath{-}6.5\text{ }\text{eV}$, respectively. In addition, $\text{N}\text{ }2s\text{-N}\text{ }2p\text{-Ga}\text{ }4s$ and $\text{N}\text{ }2s\text{-N}\text{ }2p\text{-Ga}\text{ }3d$ hybridization regions occur at the bottom of the valence band between $\ensuremath{-}13$ and $\ensuremath{-}15\text{ }\text{eV}$, and between $\ensuremath{-}17.0$ and $\ensuremath{-}18.0\text{ }\text{eV}$, respectively. A bandlike satellite feature is also found around $\ensuremath{-}10\text{ }\text{eV}$ in the $\text{Ga}\text{ }{M}_{1}$ and $\text{Ga}\text{ }{M}_{2,3}$ emission from GaN, but is absent in pure Ga and the calculated ground-state spectra. The difference between the identified spectroscopic features of GaN and Ga are discussed in relation to the various hybridization regions calculated within band-structure methods.

Contributions of the Histidine Side Chain and the N-Terminal α-Amino Group to the Binding Thermodynamics of Oligopeptides to Nucleic Acids as a Function of pH

Interactions of histidine with nucleic acid phosphates and histidine pK(a) shifts make important contributions to many protein-nucleic acid binding processes. To characterize these phenomena in simplified systems, we quantified binding of a histidine-containing model peptide HWKK ((+)NH(3)-His-Trp-Lys-Lys-NH(2)) and its lysine analogue KWKK ((+)NH(3)-Lys-Trp-Lys-Lys-NH(2)) to a single-stranded RNA model, polyuridylate (polyU), by changes in tryptophan fluorescence as a function of salt concentration and pH. For both HWKK and KWKK, equilibrium binding constants, K(obs), and magnitudes of log-log salt derivatives, SK(obs) identical with (partial differential logK(obs)/partial differential log[Na(+)]), decreased with increasing pH in the manner expected for a titration curve model in which deprotonation of the histidine and alpha-amino groups weakens binding and reduces its salt-dependence. Fully protonated HWKK and KWKK exhibit the same K(obs) and SK(obs) within uncertainty, and these SK(obs) values are consistent with limiting-law polyelectrolyte theory for +4 cationic oligopeptides binding to single-stranded nucleic acids. The pH-dependence of HWKK binding to polyU provides no evidence for pK(a) shifts nor any requirement for histidine protonation, in stark contrast to the thermodynamics of coupled protonation often seen for these cationic residues in the context of native protein structure where histidine protonation satisfies specific interactions (e.g., salt-bridge formation) within highly complementary binding interfaces. The absence of pK(a) shifts in our studies indicates that additional Coulombic interactions across the nonspecific-binding interface between RNA and protonated histidine or the alpha-amino group are not sufficient to promote proton uptake for these oligopeptides. We present our findings in the context of hydration models for specific vs nonspecific nucleic acid binding.

Magnetic phases in the correlated Kondo-lattice model

We study magnetic ordering of an extended Kondo-lattice model including an additional on-site Coulomb interaction between the itinerant states. The model is solved in the dynamical mean-field theory using the numerical renormalization group approach of Wilson and coworkers [Phys. Rev. B 21, 1003 (1980); Rev. Mod. Phys. 47, 773 (1975); R. Bulla, et al., arXiv:cond-mat/0701105 (unpublished)] as impurity solver. For a bipartite lattice, we find at half-filling the expected antiferromagnetic phase. Upon doping, this phase is gradually suppressed and hints toward phase separation are observed. For large doping, the model exhibits ferromagnetism, the appearance of which can, at first sight, be explained by the Ruderman-Kittel-Kasuya-Yosida interaction. However, for large values of the Kondo coupling $J$, significant differences from a simple Ruderman-Kittel-Kasuya-Yosida picture can be found. We, furthermore, observe signs of quantum critical points for antiferromagnetic Kondo coupling between the local spins and band states.

Impact of TiN/HfO2 Integration on Carrier Mobility

AbstractWe have investigated the impact of a metal gate (TiN) and high-k dielectric (HfO2) on the carrier mobility. We have shown that strong remote Coulomb scattering (RCS) due to charges in the HfO2 layer (either grown by ALD or MOCVD) mostly degrades the mobility at low/medium field. High amount of charges (>1013cm-2) is needed to explain the 30% degradation observed in devices with a thin interface layer. These additional coulombic interactions are effective for bottom oxide up to 2nm. We have developed a RCS model to fully explain the experimental data. The influence of the metal gate is also evidenced. The latter has a significative impact on the Si/SiO2 interface roughness, and may induce some additional coulombic interactions.

Quantum creep and variable-range hopping of one-dimensional interacting electrons

The variable range hopping results for noninteracting electrons of Mott and Shklovskii are generalized to one-dimensional disordered charge density waves and Luttinger liquids using an instanton approach. Following a recent paper by Nattermann, Giamarchi, and Le Doussal [Nattermann, Giamarchi, and Le Doussal [Phys. Rev. Lett. 91, 56603 (2003)] we calculate the quantum creep of charges at zero temperature and the linear conductivity at finite temperatures for these systems. The hopping conductivity for the short range interacting electrons acquires the same form as for noninteracting particles if the one-particle density of states is replaced by the compressibility. In the present paper we extend the calculation to dissipative systems and give a discussion of the physics after the particles materialize behind the tunneling barrier. It turns out that dissipation is crucial for tunneling to happen. Contrary to pure systems the new metastable state does not propagate through the system but is restricted to a region of the size of the tunneling region. This corresponds to the hopping of an integer number of charges over a finite distance. A global current results only if tunneling events fill the whole sample. We argue that rare events of extra low tunneling probability are not relevant for realistic systems of finite length. Finally we show that an additional Coulomb interaction only leads to small logarithmic corrections.

Solid–liquid coexistence in ionic systems and the properties of the interface

The crystal–melt interfaces of pure KCl and LiCl crystals with (Li,K)Cl melts of various compositions have been investigated by direct simulation of coexisting crystalline and molten regions. This system forms no solid compounds involving both lithium and potassium and exhibits a eutectic close to the equimolar composition. Simulations at various melt compositions allow the liquidus to be determined and compared with experiment. In order to test the precision achievable by a direct simulation of coexisting liquid and solid phases for the thermodynamic properties, we make preliminary calculations on the melting of a model of NaCl, which has recently been studied by Anwar et al. [2003, J. chem. Phys., 118, 728] using thermodynamic integration. Including anionic polarization, effects in the interionic interaction model lower the melting temperature, despite the fact that there is no discernible effect on the liquid or solid structure in these simple monovalent melts. The method of direct simulation of coexistence also allows for the study of the interfacial structure and dynamics. These can be compared with previous studies [Davidchack, R. L., and Laird, B. B., 1998, J. chem. Phys., 108, 9452; Sibug-Aga, R., and Laird, B. B., 2002, J. chem. Phys., 116, 3410] of the solid–liquid interfaces in binary mixtures of hard spheres of sufficiently different size ratio to prevent solid compound formation. Despite the additional Coulombic interactions the ionic interfaces resemble the hard spheres in many regards, but there are differences. For example, when traversing the interface the perpendicular diffusion rises more rapidly than the parallel diffusion, most likely due to charge ordering.

Anomalous g-factors and diamagnetic shifts of biexcitons in ZnS quantum wells

We present a magneto-optical study of biexcitons confined within ZnS quantum wells at fields up to 57 T . We have measured the luminescence energy splitting of these excitonic systems and find that the results contrast with the conventional picture of biexcitons in a magnetic field in which the luminescence splitting is entirely determined by the splitting ( g-factor) of the exciton. Our results suggest that biexcitons are much more complex than previously thought and that the additional confinement and Coulomb interaction within the quantum wells must be taken into account.