Shell Contributions Research Articles

Core and Shell Contributions to the Phonon Spectra of CdTe/CdS Quantum Dots

The parameters of the shell and interface in semiconductor core/shell nanocrystals (NCs) are determinant for their optical properties and charge transfer but are challenging to be studied. Raman spectroscopy was shown earlier to be a suitable informative probe of the core/shell structure. Here, we report the results of a spectroscopic study of CdTe NCs synthesized by a facile route in water, using thioglycolic acid (TGA) as a stabilizer. Both core-level X-ray photoelectron (XPS) and vibrational (Raman and infrared) spectra show that using thiol during the synthesis results in the formation of a CdS shell around the CdTe core NCs. Even though the spectral positions of the optical absorption and photoluminescence bands of such NCs are determined by the CdTe core, the far-infrared absorption and resonant Raman scattering spectra are dominated by the vibrations related with the shell. The physical mechanism of the observed effect is discussed and opposed to the results reported before for thiol-free CdTe Ns as well as CdSe/CdS and CdSe/ZnS core/shell NC systems, where the core phonons were clearly detected under similar experimental conditions.

Probes of axial and nonaxial hexadecapole deformation effects in nuclei around 230U

The structure properties for even–even nuclei around 230U, located on the hexadecapole-deformation island, are investigated using the potential-energy-surface calculation within the framework of the macroscopic−microscopic model. The impact of different deformation degrees of freedom (including axial and nonaxial quadrupole and hexadecapole deformations) on total energy, shell, and pairing contributions is analyzed, based on the projected energy maps and curves. The single-particle structure is presented and briefly discussed. To a large extent, a much better agreement with experimental data and other theoretical results is obtained if the hexadecapole deformations, especially the axial one, are taken into account. These results could provide useful insights into understanding the effects of different quadrupole and hexadecapole deformations.

Atomic isotropic hyperfine properties for second row elements (Al-Cl).

Isotropic hyperfine properties have been obtained for the second row elements Al-Cl using a systematic composite approach consisting of a sequence of core/valence correlation consistent basis sets, up through aug-cc-pCV7Z, along with configuration interaction and coupled cluster methods. The best nonrelativistic final values for the atomic ground states (in MHz) are -1.80 27Al (2Po 1/2), -24.31 29Si (3P0), 63.70 31P (4So 3/2), 20.77 33S (3P2), and 35.42 35Cl (2Po 3/2). We find a large K shell contribution to the spin density at the nucleus that is almost canceled by the L and M shell contributions. The spin density in atomic units is approximately linear with respect to the atomic number.

Hydration Thermodynamics of Familial Parkinson's Disease-Linked Mutants of α-Synuclein.

The hydration thermodynamics of different mutants of α-synuclein (α-syn) related to familial Parkinson's disease (PD) is explored using a computational approach that combines both molecular dynamics simulations in water and integral equation theory of molecular liquids. This analysis focuses on the change in conformational entropy, hydration free energy (HFE), and partial molar volume of α-syn upon mutation. The results show that A53T, A30P, E46K, and H50Q mutants aggregate more readily and display increased HFE and less negative interaction volume than the wild-type α-syn. In contrast, an opposite trend is observed for the G51D mutant with a lower experimental aggregation rate. The residuewise decomposition analysis of the HFE highlights that the dehydration/hydration of the hydrophilic residue-rich N- and C-termini of α-syn majorly contributes to the change upon mutation. The hydration shell contributions of different residues to the interaction volume are consistent with its increase/decrease upon mutation. This work shows that both HFE and interaction volume determine the aggregation kinetics of α-syn upon mutation and may serve as an appropriate benchmark for the treatment of PD.

Energy sharing based on microscopic level densities

The transformation of a moderately excited heavy nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape. The resulting Brownian motion in the multi-dimensional deformation space is guided by the shape-dependent level density which has been calculated microscopically for each of nearly ten million shapes (given in the three-quadratic-surfaces parametrization) by using a previously developed combinatorial method that employs the same single-particle levels as those used for the calculation of the pairing and shell contributions to the five-dimensional macroscopic-microscopic potential-energy surface. The stochastic shape evolution is followed until a small critical neck radius is reached, at which point the mass, charge, and shape of the two proto-fragments are extracted. The available excitation energy is divided statistically on the basis of the microscopic level densities associated with the two distorted fragments. Specific fragment structure features may cause the distribution of the energy disvision to deviate significantly from expectations based on a Fermi-gas level density. After their formation at scission, the initially distorted fragments are being accelerated by their mutual Coulomb repulsion as their shapes relax to their equilibrium forms. The associated distortion energy is converted to additional excitation energy in the fully accelerated fragments. These subsequently undergo sequential neutron evaporation which is calculated using again the appropriate microscopic level densities. The resulting dependence of the mean neutron multiplicity on the fragment mass, as well as the dependence of on the initial excitation energy of the fissioning compound nucleus, exhibit features that are similar to the experimentally observed behavior, suggesting that the microscopic energy sharing mechanism plays an important role in low-energy fission.

Electrophoresis of composite soft particles with differentiated core and shell permeabilities to ions and fluid flow

Within the framework of analytical theories for soft surface electrophoresis, soft particles are classically defined by a hard impermeable core of given surface charge density surrounded by a polyelectrolyte shell layer permeable to both electroosmotic flow and ions from background electrolyte. This definition excludes practical core-shell particles, e.g. dendrimers, viruses or multi-layered polymeric particles, defined by a polyelectrolytic core where structural charges are distributed and where counter-ions concentration and electroosmotic flow velocity can be significant. Whereas a number of important approximate expressions has been derived for the electrophoretic mobility of hard and soft particles, none of them is applicable to such generic composite core-shell particles with differentiated ions- and fluid flow-permeabilities of their core and shell components. In this work, we elaborate an original closed-form electrophoretic mobility expression for this generic composite particle type within the Debye-Hückel electrostatic framework and thin double layer approximation. The expression explicitly involves the screening Debye layer thickness and the Brinkman core and shell hydrodynamic length scales, which favors so-far missing analysis of the respective core and shell contributions to overall particle mobility. Limits of this expression successfully reproduce results from Ohshima’s electrophoresis theory solely applicable to soft particles with or without hard core.

Fission dynamics with microscopic level densities

Working within the Langevin framework of nuclear shape dynamics, we study the dependence of the evolution on the degree of excitation. As the excitation energy of the fissioning system is increased, the pairing correlations and the shell effects diminish and the effective potential-energy surface becomes ever more liquid-drop like. This feature can be included in the treatment in a formally well-founded manner by using the local level densities as a basis for the shape evolution. This is particularly easy to understand and implement in the Metropolis treatment where the evolution is simulated by means of a random walk on the five-dimensional lattice of shapes for which the potential energy has been tabulated. Because the individual steps between two neighboring lattice sites are decided on the basis of the ratio of the statistical weights, what is needed is the ratio of the local level densities for those shapes, evaluated at the associated local excitation energies. For this purpose, we adapt a recently developed combinatorial method for calculating level densities which employs the same single-particle levels as those that were used for the calculation of the pairing and shell contributions to the macroscopic-microscopic deformation-energy surface. For each nucleus under consideration, the level density (for a fixed total angular momentum) is calculated microscopically for each of the over five million shapes given in the three-quadratic-surface parametrization. This novel treatment, which introduces no new parameters, is illustrated for the fission fragment mass distributions for selected uranium and plutonium cases.

Precision measurement of theRb87tune-out wavelength in the hyperfine ground stateF=1at 790 nm

We report on a precision measurement of the $D$ line tune-out wavelength of $^{87}$Rubidium in the hyperfine ground state $|F=1, m_F=0,\pm1 \rangle$ manifold at 790 nm, where the scalar ac Stark shifts of the $D_1$ and the $D_2$ lines cancel. This wavelength is sensitive to usually neglected contributions from vector and tensor ac Stark shifts, transitions to higher principle quantum numbers, and core electrons. The ac Stark shift is probed by Kapitza-Dirac scattering of a Rubidium Bose-Einstein condensate in a one-dimensional optical lattice in free space and controlled magnetic environment. The tune-out wavelength of the magnetically insensitive $m_F=0$ state was determined to 790.01858(23) nm with sub pm accuracy. An in situ absolute polarization, and magnetic background field measurement is performed by employing the ac vector Stark shift for the $m_F=\pm 1$ states. Comparing our findings to theory, we get quantitative insight into atomic physics beyond commonly used two-level atom approximations or the neglect of inner shell contributions.

Enhanced magnetic properties in antiferromagnetic-core/ferrimagnetic-shell nanoparticles.

Bi-magnetic core/shell nanoparticles are gaining increasing interest due to their foreseen applications. Inverse antiferromagnetic(AFM)/ferrimagnetic(FiM) core/shell nanoparticles are particularly appealing since they may overcome some of the limitations of conventional FiM/AFM systems. However, virtually no simulations exist on this type of morphology. Here we present systematic Metropolis Monte Carlo simulations of the exchange bias properties of such nanoparticles. The coercivity, HC, and loop shift, Hex, present a non-monotonic dependence with the core diameter and the shell thickness, in excellent agreement with the available experimental data. Additionally, we demonstrate novel unconventional behavior in FiM/AFM particles. Namely, while HC and Hex decrease upon increasing FiM thickness for small AFM cores (as expected), they show the opposite trend for large cores. This presents a counterintuitive FiM size dependence for large AFM cores that is attributed to the competition between core and shell contributions, which expands over a wider range of core diameters leading to non-vanishing Hex even for very large cores. Moreover, the results also hint different possible ways to enhance the experimental performance of inverse core/shell nanoparticles for diverse applications.

Does the 4f-shell contribute to bonding in tetravalent lanthanide halides?

Lanthanide tetrahalide molecules LnX4 (Ln = Ce, Pr, Tb; X = F, Cl, Br, I) have been investigated by density functional theory at the levels of the relativistic Zero Order Regular Approximation and the relativistic energy-consistent pseudopotentials, using frozen small- and medium-cores. The calculated bond lengths and vibrational frequencies are close to the experimental data. Our calculations indicate 4f shell contributions to bonding in LnX4, in particular for the early lanthanides, which show significant overlap between the Ln 4f-shell and the halogen np-shells. The 4f shells contribute to Ln-X bonding in LnX4 about one third more than in LnX3.

Dissecting ion-specific dielectric spectra of sodium-halide solutions into solvation water and ionic contributions.

Using extensive equilibrium molecular dynamics simulations we determine the dielectric spectra of aqueous solutions of NaF, NaCl, NaBr, and NaI. The ion-specific and concentration-dependent shifts of the static dielectric constants and the dielectric relaxation times match experimental results very well, which serves as a validation of the classical and non-polarizable ionic force fields used. The purely ionic contribution to the dielectric response is negligible, but determines the conductivity of the salt solutions. The ion-water cross correlation contribution is negative and reduces the total dielectric response by about 5%-10% for 1 M solutions. The dominating water dielectric response is decomposed into different water solvation shells and ion-pair configurations, by this the spectral blue shift and the dielectric decrement of salt solutions with increasing salt concentration is demonstrated to be primarily caused by first-solvation shell water. With rising salt concentration the simulated spectra show more pronounced deviations from a single-Debye form and can be well described by a Cole-Cole fit, in quantitative agreement with experiments. Our spectral decomposition into ionic and different water solvation shell contributions does not render the individual contributions more Debye-like, this suggests the non-Debye-like character of the dielectric spectra of salt solutions not to be due to the superposition of different elementary relaxation processes with different relaxation times. Rather, the non-Debye-like character is likely to be an inherent spectral signature of solvation water around ions.

Dielectric response of II–VI semiconductor core–shell ensembles: Study of the lossless optical condition

We theoretically investigate optical properties of II–VI core–shell distribution mixtures made of two type-I sized-nanoshells as a plausible negative dielectric function material. The nonlocal optical response of the semiconductor QD is described by using a resonant excitonic dielectric function, while the shell response is modeled with Demangeot formula. Achieving the zero-loss at an optical frequency ω, i.e., ϵeff=ϵeff′+iϵeff″ with ϵeff′<0 and ϵeff″=0, is of fundamental importance in nanophotonics. Resonant states in semiconductors provide a source for negative dielectric function provided that the dipole strength and the oscillator density are adequate to offset the background. Furthermore, the semiconductor offers the prospect of pumping, either optically or electrically, to achieve a gain mechanism that can offset the loss. We analyse optimal conditions that must be satisfied to achieve semiconductor-based negative index materials. By comparing with II–VI semiconductor quantum dots (QDs) previously reported in the literature, the inclusion of phonon and shell contributions in the ϵeff along with the finite barrier Effective Mass Approximation (EMA) approach, we found similar qualitative behaviours for the ϵeff. The lossless optical condition along with ϵeff′<0 is discussed in terms of sizes, volume fractions and embedding medium of the mixtures׳ distributions. Furthermore, we estimated optical power to maintain nanocrystals density in excited states and this value is less than that previously obtained in II–VI semiconductor QDs.

Low-energy fission within the Lublin–Strasbourg drop and Yukawa folded model

The evolution of fragment mass distribution in low-energy fission is studied as a function of the composition of the fissioning system over a wide range, from Hg to Fm isotopes. The relative importance of symmetric and asymmetric fission modes is investigated within a static picture based on elaborate, four-dimensional potential energy spaces using the modified Funny–Hills deformation coordinates. The potential energy is computed with the macroscopic-microscopic method. The Lublin–Strasbourg drop model is used to calculate the macroscopic part of the potential energy, while the Yukawa folded single-particle potential with an improved Strutinsky method and the Bardeen–Cooper–Schrieffer theory are employed to evaluate the shell and pairing contributions to the macroscopic energy. From the analysis of the potential energy surfaces, equilibrium deformations and saddle-point shapes are determined. A coexistence of symmetric and asymmetric fission valleys is observed. The relative depth of these valleys depends on the fissioning nucleus. The valleys' competition is used to explain the evolution of the fission-fragment mass distribution, from asymmetric fission in Hg to symmetric fission in Fm. Good qualitative agreement is found with the experiment.

The Periodic Pyramid

The chemical elements present in the modern periodic table are arranged in terms of atomic numbers and chemical periodicity. Periodicity arises from quantum mechanical limitations on how many electrons can occupy various shells and subshells of an atom. The shell model of the atom predicts that a maximum of 2, 8, 18, and 32 electrons can occupy the shells identified by the principle quantum numbers n = 1, 2, 3, and 4, respectively. The numbers 2, 8, 18, and 32 are shown in this work to be related to the triangular numbers from mathematical number theory. The relationship to the triangular numbers, in turn, suggests an alternate method for arranging elements in terms of periodicity. The resulting three-dimensional “periodic pyramid” is highly symmetric in shape. Just as is true in the modern periodic table, each layer of the periodic pyramid can be separated into shell and subshell contributions. Examining the pyramid’s structure is arguably a pedagogically useful activity for college-level introductory or physical chemistry students, as it provides an opportunity to further ponder the shell model of the atom and the origins of periodicity. The connections to number theory are used to show that the outermost subshell of a given shell contains (2n – 1) orbitals.

Theoretical design of the biradical character in 1,3-diphosphacyclobutanediyl and homologous structures

The electronic nature of 1,3-diphosphacyclobutane-2,4-diyl is explored with wavefunction based and density functional methods. According to MCSCF calculations the singlet state of the title compound is a biradicaloid with closed shell character, the number of unpaired electrons, assigned upon the analysis of the natural orbitals, is close to one. The participation of closed shell contributions in the overall wavefunction arises from a strong mixing of canonical structures, which emphasizes (a) the phosphorane type of bonding as well as (b) π-delocalization within the ring system. The bonding situation changes when σ-attracting substituents, e.g. amino groups, are attached to the phosphorus atoms. They inhibit possible cyclic π-delocalization and enhance the biradical character within the ring system.

Roles of Nucleus Accumbens Core and Shell in Incentive-Cue Responding and Behavioral Inhibition

The nucleus accumbens (NAc) is involved in many reward-related behaviors. The NAc has two major components, the core and the shell. These two areas have different inputs and outputs, suggesting that they contribute differentially to goal-directed behaviors. Using a discriminative stimulus (DS) task in rats and inactivating the NAc by blocking excitatory inputs with glutamate antagonists, we dissociated core and shell contributions to task performance. NAc core but not shell inactivation decreased responding to a reward-predictive cue. In contrast, inactivation of either subregion induced a general behavioral disinhibition. This reveals that the NAc actively suppresses actions inappropriate to the DS task. Importantly, selective inactivation of the shell but not core significantly increased responding to the nonrewarded cue. To determine whether the different contributions of the NAc core and shell depend on the information encoded in their constituent neurons, we performed electrophysiological recording in rats performing the DS task. Although there was no firing pattern unique to either core or shell, the reward-predictive cue elicited more frequent and larger magnitude responses in the NAc core than in the shell. Conversely, more NAc shell neurons selectively responded to the nonrewarded stimulus. These quantitative differences might account for the different behavioral patterns that require either core or shell. Neurons with similar firing patterns could also have different effects on behavior due to their distinct projection targets.

Casimir effect of the electromagnetic field inD-dimensional spherically symmetric cavities

Eigenmodes of the electromagnetic field with perfectly conducting or infinitely permeable conditions on the boundary of a $D$-dimensional spherically symmetric cavity is derived explicitly. It is shown that there are ($D\ensuremath{-}2$) polarizations for TE modes and one polarization for TM modes, giving rise to a total of ($D\ensuremath{-}1$) polarizations. In case of a $D$-dimensional ball, the eigenfrequencies of the electromagnetic field with perfectly conducting boundary condition coincides with the eigenfrequencies of gauge 1-forms with relative boundary condition; whereas the eigenfrequencies of the electromagnetic field with infinitely permeable boundary condition coincides with the eigenfrequencies of gauge 1-forms with absolute boundary condition. Casimir energies of single and concentric spherical shells in $D$-dimensions are computed. The Casimir energy of concentric spherical shells can be written as a sum of the single spherical shell contributions and an interacting term, and the latter is free of divergence. The interacting term always gives rise to an attractive force between the two spherical shells. Its leading term is the Casimir force acting between two parallel plates of the same area, as expected by proximity force approximation.

Magnetic properties resulting from core-shell interactions in nanosizedNi0.25Co0.25Zn0.5Fe2O4

Nanosized particles of ${\text{Ni}}_{0.25}{\text{Co}}_{0.25}{\text{Zn}}_{0.5}{\text{Fe}}_{2}{\text{O}}_{4}$ have been synthesized by chemical coprecipitation method. Exchange-bias phenomenon arising from the core-shell interaction has been investigated using a combination of in-field, low-temperature M\ossbauer spectroscopy and dc magnetization. To understand the clear mechanism of interaction and exchange bias, isothermal remanence magnetization-dc demagnetization and $M\text{\ensuremath{-}}H$ loops at different cooling field have been taken. The observed variation in coercivity ${H}_{C}$ and exchange-bias field ${H}_{E}$ confirms that only core is affected by the cooling field. The slope of the Henkel plot is 1.82, indicating the noninteracting nature of the particles. In-field M\ossbauer spectroscopy clearly establishes the core and shell contributions and also confirms that 70% of spins are in the shell. The barrier energy has been estimated to be $17\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\text{ }\text{ergs}$ which accounts for the fact that the shell is not affected by application of as large a field as 5 T.

QCD corrections to charged triple vector boson production with leptonic decay

We compute the O({alpha}{sub s}) QCD corrections to charged triple vector boson production at a hadron collider, i.e., the processes pp{yields}ZZW{sup {+-}}+X and pp{yields}W{sup {+-}}W{sup {+-}}W{sup {+-}}+X. Intermediate Higgs boson exchange effects, spin correlations from leptonic vector boson decays, and off shell contributions are all taken into account. Results are implemented in a fully flexible Monte Carlo program that allows for an easy customization of kinematical cuts and variation of the factorization and renormalization scales. We analyze the dependence of the differential cross sections under scale variations and present distributions where the QCD corrections strongly modify the leading-order results.

Structural anomalies of fluids: Origins in second and higher coordination shells

Compressing or cooling a fluid typically enhances its static interparticle correlations. However, there are notable exceptions. Isothermal compression can reduce the translational order of fluids that exhibit anomalous waterlike trends in their thermodynamic and transport properties, while isochoric cooling (or strengthening of attractive interactions) can have a similar effect on fluids of particles with short-range attractions. Recent simulation studies by Yan [Phys. Rev. E 76, 051201 (2007)] on the former type of system and Krekelberg [J. Chem. Phys. 127, 044502 (2007)] on the latter provide examples where such structural anomalies can be related to specific changes in second and more distant coordination shells of the radial distribution function. Here, we confirm the generality of this microscopic picture through analysis, via molecular simulation and integral equation theory, of coordination shell contributions to the two-body excess entropy for several related model fluids which incorporate different levels of molecular resolution. The results suggest that integral equation theory can be an effective and computationally inexpensive tool for assessing, based on the pair potential alone, whether new model systems are good candidates for exhibiting structural (and hence thermodynamic and transport) anomalies.