Confinement Theory Research Articles

Optical properties of the low-energy Ge-implanted and annealed SiO2 films

Ultraviolet–visible light emissions from nanocrystalline (nc) Ge-embedded SiO2 films fabricated by ion implantation and rapid annealing techniques are studied as a function of different fabricating conditions (implanting dose and annealing temperature). The samples exhibit seven photoluminescence (PL) peaks peaked at 1.68, 1.75, 1.84, 1.93, 2.00, 2.70, and 3.10eV. There are also two excitation bands in the PL excitation (PLE) spectra peaked at 4.90 and 5.19eV. Raman spectra are employed to observe and understand structural variations in SiO2 matrix during the formation of nc-Ge and defects. Within the frame of the quantum confinement (QC) theory and Oswald ripening growth model, the origin and evolution of the seven PL peaks and two PLE bands are identified clearly demonstrated by the proposed energy-level and generated process diagrams. Our results indicate that both the implanting dose and annealing temperature play a dominant role in modulating the optical properties of the nc-Ge-embedded SiO2 films.

A Hybrid Quantum Mechanical Approach: Intimate Details of Electron Transfer between Type-I CdSe/ZnS Quantum Dots and an Anthraquinone Molecule.

We report a hybrid computational approach to calculate electron transfer between a type-I CdSe/ZnS core/shell quantum dot (QD) with a varying shell thickness and the functionalized anthraquinone (AQ) molecule. This novel approach combines the traditional electron/hole confinement theory in the effective mass approximation for the QD and molecular orbital theory for the AQ molecule. In the present study, the QD's electron and hole envelope wave functions are solutions of the effective-mass Schrödinger equation, and the AQ wave function is obtained at the density functional level. Electron-transfer rate calculations are based on Marcus's theory with the coupling strength computed according to an one-electron orbital perturbation model. We show that in a heptane solution, the LUMO of AQ and the 1Se electron orbital of QD are involved in the charge separation (CS) process. The charge recombination (CR) process, on the other hand, occurs from the singly occupied molecular orbital of the AQ radical (which corresponds to the LUMO in AQ) to a trapped hole state of the QD within the band gap. The calculations support previously reported interpretations of the role of the ZnS shell as a hindrance in the CS and CR process.

Electronic and spectroscopic properties of GeC superlattice nanocrystals: A first-principle study using diamondoid structures

Germanium carbide superlattices are used as building blocks to investigate GeC nanocrystal properties using diamondoid structures. Convergence of electronic and vibrational properties of GeC diamondoids to GeC bulk limits is observed using density functional theory. Energy gap, HOMO and LUMO vary according to confinement theory with shape fluctuations. Ge–C force constants reach 2.67mDyne/Å which is close to that of silicon. Ge–Ge vibrations embodied inside GeC superlattice vibrations at 305.6cm−1 are observed with reduced masses 33.77amu and force constant 1.91mDyne/Å which are nearly the values of pure bulk Ge. Bulk longitudinal optical (LO) vibrational mode of GeC at 812cm−1 is reached within 6 GeC diamondoid cages. LO mode is confined to the range 603–812cm−1 in moving from molecular, nano to bulk. Radial breathing mode (RBM) varies from 603cm−1 to bulk 0cm−1 limit. Size variation of UV–Vis and 1H NMR spectra of GeC diamondoids is analyzed. Present work results show that all the investigated spectroscopic tools are sensitive to nanocrystals size. Natural bond orbital (NBO) population analysis shows present compound deviation from ideal sp3 bonding to Ge(4s0.814p1.315p0.02) C(2s1.262p4.643p0.01) near the center of the present largest investigated GeC nanocrystal.

Ab initio structural and vibrational properties of GaAs diamondoids and nanocrystals

Gallium arsenide diamondoids structural and vibrational properties are investigated using density functional theory at the PBE/6-31(d) level and basis including polarization functions. Variation of energy gap as these diamondoids increase in size is seen to follow confinement theory for diamondoids having nearly equiaxed dimensions. Density of energy states transforms from nearly single levels to band structure as we reach larger diamondoids. Bonds of surface hydrogen with As atoms are relatively localized and shorter than that bonded to Ga atoms. Ga-As bonds have a distribution range of values due to surface reconstruction and effect of bonding to hydrogen atoms. Experimental bulk Ga-As bond length (2.45 Å) is within this distribution range. Tetrahedral and dihedral angles approach values of bulk as we go to higher diamondoids. Optical-phonon energy of larger diamondoids stabilizes at 0.037 eV (297 cm-1) compared to experimental 0.035 eV (285.2 cm-1). Ga-As force constant reaches 1.7 mDyne/Å which is comparable to Ga-Ge force constant (1.74 mDyne/Å). Hydrogen related vibrations are nearly constant and serve as a fingerprint of GaAs diamondoids while Ga-As vibrations vary with size of diamondoids.

Driven colloidal suspensions in confinement and density functional theory: Microstructure and wall-slip

We theoretically investigate general properties of driven (sheared) colloidal suspensions in confinement, based on methods of classical density functional theory. In the absence of an exact closed (Smoluchowski-) equation for the one-particle density under shear, we formulate a set of general conditions for approximations, and show that a simple closure fulfills them. The exact microscopic stress tensor is identified. Exemplifying the situation near a wall (oriented parallel to the direction of shear), we note that the microscopic shear stress is not necessarily homogeneous. Formulating a second equation additional to the Smoluchowski equation, we achieve a homogeneous shear stress, and thereby compute the local flow velocity near the wall. This finally leads to a slip length of the complex fluid at the wall.

Performance and calculations of recycled aggregate concrete-filled steel tubular (RACFST) short columns under axial compression

This paper describes a series of tests on steel tubular short columns of circular section filled with normal concrete (NA) and recycled aggregate concrete (RAC). Twenty-two specimens, including 11 small diameter of RAC-filled steel tubular (RACFST) columns named CA series and 11 larger diameter of RACFST columns called CB series, were tested to research the influence of variations in confinement index and replacement ratio of recycled coarse aggregate (RCA), from 0% to 100% with 10% increasing. The test results show that both types of RACFST columns and normal concrete-filled steel tubular (CFST) columns failed due to oblique shear pressure damage with a drum-like. Comparisons are made with predicted bearing capacities of RACFST columns using the existing theories, such as Unified Strengthen theory (I), Confinement theory (II) and Superposition theory (III). The equation for axial compression stress-strain curves of the whole process are proposed in this investigation for RACFST short columns, which can be directly used in theoretical and numerical analysis as well as practical engineering design and evaluation of RACFST structures.

Structural and optical properties of size controlled Si nanocrystals in Si3N4 matrix: The nature of photoluminescence peak shift

Superlattices of Si3N4 and Si-rich silicon nitride thin layers with varying thickness were prepared by plasma enhanced chemical vapor deposition. After high temperature annealing, Si nanocrystals were formed in the former Si-rich nitride layers. The control of the Si quantum dots size via the SiNx layer thickness was confirmed by transmission electron microscopy. The size of the nanocrystals was well in agreement with the former thickness of the respective Si-rich silicon nitride layers. In addition X-ray diffraction evidenced that the Si quantum dots are crystalline whereas the Si3N4 matrix remains amorphous even after annealing at 1200 °C. Despite the proven Si nanocrystals formation with controlled sizes, the photoluminescence was 2 orders of magnitude weaker than for Si nanocrystals in SiO2 matrix. Also, a systematic peak shift was not found. The SiNx/Si3N4 superlattices showed photoluminescence peak positions in the range of 540–660 nm (2.3–1.9 eV), thus quite similar to the bulk Si3N4 film having peak position at 577 nm (2.15 eV). These rather weak shifts and scattering around the position observed for stoichiometric Si3N4 are not in agreement with quantum confinement theory. Therefore theoretical calculations coupled with the experimental results of different barrier thicknesses were performed. As a result the commonly observed photoluminescence red shift, which was previously often attributed to quantum-confinement effect for silicon nanocrystals, was well described by the interference effect of Si3N4 surrounding matrix luminescence.

A higher-dimensional model of the nucleon-nucleon central potential

Based on a theory of extra dimensional confinement of quantum particles [E. R. Hedin, Physics Essays, 2012, 25(2): 177], a simple model of a nucleon-nucleon (NN) central potential is derived which quantitatively reproduces the radial profile of other models, without adjusting any free parameters. It is postulated that a higher-dimensional simple harmonic oscillator confining potential localizes particles into three-dimensional (3D) space, but allows for an evanescent penetration of the particles into two higher spatial dimensions. Producing an effect identical with the relativistic quantum phenomenon of zitterbewegung, the higher-dimensional oscillations of amplitude ħ/(mc) can be alternatively viewed as a localized curvature of 3D space back and forth into the higher dimensions. The overall spatial curvature is proportional to the particle’s extra-dimensional ground state wave function in the higher-dimensional harmonic confining potential well. Minimizing the overlapping curvature (proportional to the energy) of two particles in proximity to each other, subject to the constraint that for the two particles to occupy the same spatial location one of them must be excited into the 1st excited state of the harmonic potential well, gives the desired NN potential. Specifying only the nucleon masses, the resulting potential well and repulsive core reproduces the radial profile of several published NN central potential models. In addition, the predicted height of the repulsive core, when used to estimate the maximum neutron star mass, matches well with the best estimates from relativistic theory incorporating standard nuclear matter equations of state. Nucleon spin, Coulomb interactions, and internal nucleon structure are not considered in the theory as presented in this article.

Summary of the magnetic confinement theory and modelling activity presented at the 24th IAEA Fusion Energy Conference

This paper presents a summary of the papers presented at the 24th IAEA Fusion Energy Conference (San Diego, CA, October 2012) on magnetic confinement theory and modelling.

Soft confinement in a 3-d spin system

We consider a 1+3 dimensional spin system. The spin-wave (magnon) field is described by the O(3) non-linear sigma model with a symmetry-breaking potential. This interacts with a slow spin SU(2) doublet Schrodinger fermion. The interaction is described by a generalized nonperturbative Yukawa coupling, and the self-consistency condition is solved with the aid of a non-relativistic Gribov equation. When the Yukawa coupling is sufficiently strong, the solution exhibits supercriticality and soft confinement, in a way that is quite analogous to Gribov's light-quark confinement theory. The solution corresponds to a new type of spin polaron, whose condensation may lead to exotic superconductivity.

Size dependence of the optical gap of “small” silicon quantum dots: Ab initio and empirical correlation schemes

We present a comparative study of the energy-gap dependence on diameter d of “small” (d<20Å) hydrogen-terminated Si quantum dots, using density functional theory (DFT) with the hybrid functional of Becke, Lee, Parr and Yang (B3LYP). These accurate real space ab initio calculations [see [1], Garoufalis et al., Phys. Rev. Lett. 87 (2001) 276402] are used to compare the size dependence of the band gap according to quantum confinement theory in relation to the empirical bond-order-length-strength (BOLS) correlation mechanism, usually applied to larger nanocrystals. Our results for the gap variation, in the range of diameters considered here, are in very good agreement with quantum confinement theory and they reproduce by extrapolation the experimental band gap of bulk silicon with high accuracy (error smaller than 1%). On the contrary, extrapolation of fitted band gaps by BOLS scheme, grossly overestimates the band gap of bulk (by almost 80%); whereas, forcing the agreement of bulk band gap results in largely underestimated dot band gaps, in the range of diameters considered here.

Influence of the embedding matrix on optical properties of Ge nanocrystals-based nanocomposite

Germanium nanocrystals were prepared by a nanocluster source and characterized by photoluminescence and spectroscopic photometry methods. The optical measurements were carried out in order to estimate the effective bandgap of the Ge nanocrystals. Both Mie theory and the quantum confinement theory were applied to interpret the extracted absorption data. We found that the quantum confinement theory enables to explain the nanocrystal size and the host matrix dependence of the nanocrystal bandgap. On the other hand, the photoluminescence measurements have not allowed to confirm the bandgap evaluated from absorption data. This is explained as due to the dominant effect of the recombination at the nanocrystals surface.

EXPERIMENTAL STUDY AND THEORETICAL ANALYSIS ON AXIAL COMPRESS CAPACITY OF ERCYCLED AGGREGATE CONCRETE-FILLED CIRCLE STEEL TUBE SHORT COLUMN

In order to study the axial compress capacity of recycled aggregate concrete-filled steel tube short-columns, 22 specimens were designed to test, and the recycled coarse aggregate replacement percentage and confinement index were regarded as major variable parameters. By the experiment, the whole mechanical process and failure mode of all specimens were observed, and stress-strain curves of the specimens were obtained. The influence of the aggregate replacement rate and confinement index on the bearing behaviour of the specimens was analyzed. The test results indicate that the failure mode of the columns is similar to that of an ordinary concrete-filled steel tube, and recycled coarse aggregate replacement rate has little effect on a recycled aggregate concrete-filled steel tube, but confinement index has a significant effect on the bearing behaviour and deformation performance. The bearing capacity of the specimens was calculated and analyzed according to the theories of Unified Strengthen, Confinement and Superposition Calculation. Compared with the experimental values, the results show that calculated values are low on the basis of the first and the third calculation theories, but the calculated values are slightly high based on the second calculation theory. Finally, stress-strain curves were fitted and analyzed according to experimental data. Stress-strain whole curve equations of specimens were put forward. The research conclusions could be a reference for the scientific study and engineering application of recycled aggregate concrete-filled steel tubes.

The Effect of the MeV Si-Ion Irradiation on the Photoluminescence of Silicon Nanocrystals

Silicon nanocrystals embedded in silicon nitride films were irradiated with Si-ions at 8 MeV in order to modify their optical response. The samples were characterized by means of Rutherford Backscattering Spectrometry, Elastic Recoil Detection Analysis, High-Resolution Transmission Electronic Microscopy and Photoluminescence analysis. It was found a blue-shift in the photoluminescence emission from the as-grown films after they were irradiated with high energetic silicon ions. According to the quantum confinement theory, this fact is related to a decrease in size of the silicon nanocrystals, which means that a higher silicon fluence irradiation is related with a diminishing in silicon nanocrystal size.

Offer a novel method for size appraise of NiO nanoparticles by PL analysis: Synthesis by sonochemical method

In this work, we will discuss the optical properties of NiO nanoparticles that we have investigated recently by photoluminescence (PL) spectroscopy. In particular, we will show the blue-shifts of PL, originating from the electron–hole recombination of the self-trapped exciton (STE), observed in smaller-sized NiO nanoparticles. To explain the size effect in relating to the STE PL shift, a question has been raised on whether it is appropriate to apply him quantum confinement (QC) theory usually used for the Mott-Winner type excitons in semiconductors to wide band-gap material, such as silica. Variations in several parameters and their effects on the structural (crystal size and morphology) properties of nanoparticles were investigated. Characterizations were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal stability (TGA and DTA), solid state UV and solid state florescent (PL).

A feasible scheme to search for fractional charge particles in low energy accelerator experiments

In the Standard Model of particle physics, quarks and anti-quarks have fractional charges equal to ±1/3 or ±2/3 of the electron's charge. There has been a large number of experiments searching for fractional charge particles using a variety of methods, including dE/dx ionization energy loss measurements in e+e− collisions, but no evidence has been found to confirm existence of free fractional charge particles, which leads to the quark confinement theory. In this paper, a proposal to search for fractional charge particles is presented, which is based on the conservation law of four-momentum and the Lorentz force law. To show the power of the new strategy, a fully BESIII detector simulated signal sample mixed with a fully simulated background sample is analyzed and lower signal cross section limits to discover 5σ signal as a function of quark mass are estimated for the background. The analysis result shows that in comparison with ionization energy loss measurements, the advantage of the strategy is that it can identify fractional charge particles without using any theoretical or empirical formulae to calculate any expected observation values.

Effective confinement theory from Abelian variables in [formula omitted] gauge theory

In this Letter we use the Julia–Toulouse approach for condensation of defects in order to obtain an effective confinement theory for external chromoelectric probe charges in SU(3) gauge theory in the regime with condensed chromomagnetic monopoles. We use the Cho decomposition of the non-Abelian connection in order to reveal the Abelian sector of the non-Abelian gauge theory and the associated topological defects (monopoles) without resorting to any gauge fixing procedure. Using only the Abelian sector of the theory, we construct a hydrodynamic effective theory for the regime with condensed defects in such a way that it is compatible with the Elitzurʼs theorem. The resulting effective theory describes the interaction between external chromoelectric probe charges displaying a short-range Yukawa interaction plus a linear confining term that governs the long distance physics.

Quantum theory of the electromagnetic response of metal nanofilms

We develop a quantum theory of electron confinement in metal nanofilms. The theory is used to compute the nonlinear response of the film to a static or low-frequency external electric field and to investigate the role of boundary conditions imposed on the metal surface. We find that the sign and magnitude of the nonlinear polarizability depends dramatically on the type of boundary condition used.

Confinement-induced resonances in anharmonic waveguides

We develop the theory of anharmonic confinement-induced resonances (ACIR). These are caused by anharmonic excitation of the transverse motion of the center of mass (COM) of two bound atoms in a waveguide. As the transverse confinement becomes anisotropic, we find that the COM resonant solutions split for a quasi-1D system, in agreement with recent experiments. This is not found in harmonic confinement theories. A new resonance appears for repulsive couplings ($a_{3D}>0$) for a quasi-2D system, which is also not seen with harmonic confinement. After inclusion of anharmonic energy corrections within perturbation theory, we find that these ACIR resonances agree extremely well with anomalous 1D and 2D confinement induced resonance positions observed in recent experiments. Multiple even and odd order transverse ACIR resonances are identified in experimental data, including up to N=4 transverse COM quantum numbers.

On Wilson’s theory of confinement

According to recent results, the Gell-Mann-Low function β(g) of four-dimensional φ4 theory is nonalternating and has a linear asymptotics at infinity. According to the Bogoliubov and Shirkov classification, it means the possibility of constructing a continuous theory with finite interaction at large distances. This conclusion is in visible contradiction to the lattice results indicating triviality of φ4 theory. This contradiction is resolved by a special character of renormalizability in φ4 theory: to obtain the continuous renormalized theory, there is no need to eliminate a lattice from the bare theory. In fact, such kind of renormalizability is not accidental and can be understood in the framework of Wilson’s many-parameter renormalization group. Application of these ideas to QCD shows that Wilson’s theory of confinement is not purely illustrative, but has a direct relation to a real situation. As a result, the problem of analytical proof of confinement and a mass gap can be considered solved, at least on the physical level of rigor.