Electronic properties of model quantum-dot structures in zero and finite magnetic fields

We have computed electronic structures and total energies of circularly confined two-dimensional quantum dots and their lateral dimers in zero and finite uniform external magnetic fields using different theoretical schemes: the spin-density-functional theory (SDFT), the current-and-spin-density-functional theory (CSDFT), and the variational quantum Monte Carlo (VMC) method. The SDFT and CSDFT calculations employ a recently-developed, symmetry-unrestricted real-space algorithm allowing solutions which break the spin symmetry. Results obtained for a six-electron dot in the weak confinement limit and in zero magnetic field as well as in a moderate confinement and in finite magnetic fields enable us to draw conclusions about the reliability of the more approximative SDFT and CSDFT schemes in comparison with the VMC method. The same is true for results obtained for the two-electron quantum dot dimer as a function of inter-dot distance. The structure and role of the symmetry-breaking solutions appearing in the SDFT and CSDFT calculations for the above systems are discussed. Received 16 October 2001 and Received in final form 17 January 2002     

Positron studies of hydrogen-defect interactions in proton irradiated molybdenum

Molybdenum single crystals are irradiated at 20 K with 6 MeV protons. The radiation damage and lattice defect annealing is studied by positron lifetime spectroscopy in the temperature range from 15 to 720 K. Loss of vacancies due to recombination with mobile interstitials is observed at 40 K (Stage I) in agreement with resistivity measurements. This is the first time Stage I is observed by positrons below 77 K. The implanted hydrogen decorates the vacancies around 100 K, which is consistent with a hydrogen migration energy in molybdenum:E _M ^H = 0.3–0.4 eV. Clustering of spatially correlated vacancies takes place in a wide temperature region below the usual vacancy clustering stage (Stage III). Stage III is observed at rather low temperatures (400–480 K) due to the very high vacancy concentration. Hydrogen bound to vacancies and vacancy clusters is released above 540 K, which puts an upper limit to the hydrogen binding energy:E _B ^H 1.4 eV. The present work emphasizes the advantage of employing a vacancy sensitive technique to study hydrogen in metals, where its intrinsic solubility is low. In such metals (as molybdenum) both the effective solubility and the effective mobility of hydrogen are strongly influenced by the presence of vacancies.     

Probing organic layers on the TiO2(110) surface

In this work, we use first principles simulations to provide features of the dynamic scanning force microscopy imaging of adsorbed organic layers on insulating surfaces. We consider monolayers of formic (HCOOH) and acetic (CH(3)COOH) acid and a mixed layer of acetic and trifluoroacetic acids (CF(3)COOH) on the TiO(2)(110) surface and study their interaction with a silicon dangling bond tip. The results demonstrate that the silicon tip interacts more strongly with the substrate and the COO(-) group than the adsorbed acid headgroups, and, therefore, molecules would appear dark in images. The pattern of contrast and apparent height of molecules is determined by the repulsion between the tip and the molecular headgroups and by significant deformation of the monolayer and individual molecules. The height of the molecule on the surface and the size of the headgroup play a large role in determining access of the tip to the substrate and, hence, the contrast in images. Direct imaging of the molecules themselves could be obtained by providing a functionalized tip with attraction to the molecular headgroups, for example, a positive potential tip.     

Computed positron lifetimes in vacancies and vacancy-iron clusters in gold

Annihilation characteristics are calculated for positrons trapped in clean and impurity decorated vacancy clusters in Au. The positron lifetime depends strongly on the structure of the clusters. In a strongly relaxed vacancy cluster, the lifetime can become smaller than the lifetime in a single vacancy. The substitution of some neighbour atoms of a vacancy cluster by Fe atoms has only a minor effect on the positron lifetimes.     

The role of MFC/NFC swelling in the rheological behavior and dewatering of high consistency furnishes

The influence of swelling on the rheological and dewatering properties of high consistency nanocellulose based furnishes is considered. Different consistencies of suspensions (1–4 %) and furnishes (5–15 %) were prepared made of two distinctly different grades of nanocellulose containing, micro fibrillated (MFC) and nanofibrillated (NFC) cellulose, and systematic comparison between the rheological and dewatering parameters was conducted. The characterization of the rheological and dewatering properties was performed with a stress controlled rheometer combined with an immobilization cell in parallel plate geometry, as well as with an independent gravimetric dewatering device. The surface charge of nanofibrillated cellulose was found to influence the rheological and dewatering properties of the evaluated suspensions and furnishes due to its impact on swelling and effectively bound water. Due to the complex behavior of the novel materials, the immobilization times were difficult to determine from the changes in the damping factor, as often used for coating colors. Instead, we propose a modified method for determination of immobilization times based on a rheological analysis adopting the rate of change in viscoelastic loss factor over time, d(tan δ = G′′/G′)/dt, describing the critical point(s) in the ratio of the viscous to elastic stress response moduli. With this approach we show that it is possible to characterize immobilization of these materials incorporating the concept of the combined physical interactions of the components and the non-removable bound water, without requiring a direct measure of the nanocellulose surface swelling. Based on the results, we hypothesize that fibrillar swelling impacts the dewatering of MFC and NFC suspensions, and furnishes containing them, by an interfiber pore connectivity blocking/sealing mechanism, which effectively defines the immobilization of the material matrix at the end point of free water extraction caused by the physical blocking imposed by the remaining bound water.     

Singlet–triplet oscillations and far-infrared spectrum of four-minima quantum-dot molecule

We study ground states and far-infrared spectra (FIR) of two electrons in four-minima quantum-dot molecule in magnetic field by exact diagonalization. Ground states consist of altering singlet and triplet states, whose frequency, as a function of magnetic field, increases with increasing dot–dot separation. When the Zeeman energy is included, only the two first singlet states remain as ground states. In the FIR spectra, we observe discontinuities due to crossing ground states. Non-circular symmetry induces anticrossings, and also an additional mode above ω₊ in the spin-triplet spectrum. In particular, we conclude that electron–electron interactions cause only minor changes to the FIR spectra and deviations from the Kohn modes result from the low-symmetry confinement potential.     

Identification of \mathrm{\ensuremath J^{\pi} = 19/2^+} and \mathrm{\ensuremath 23/2^+} isomeric states in 127Sb

The nucleus $\ensuremath {\rm ^{127}Sb}$ , which is on the neutron-rich periphery of the $\ensuremath \beta$ -stability region, has been populated in complex nuclear reactions involving deep-inelastic and fusion-fission processes with $\ensuremath {\rm {}^{136}Xe}$ beams incident on thick targets. The previously known isomer at 2325 keV in $\ensuremath {\rm {}^{127}Sb}$ has been assigned spin and parity $\ensuremath 23/2^+$ , based on the measured $\ensuremath \gamma$ - $\ensuremath \gamma$ angular correlations and total internal conversion coefficients. The half-life has been determined to be 234(12) ns, somewhat longer than the value reported previously. The 2194 keV state has been assigned $\ensuremath J^{\pi} = 19/2^+$ and identified as an isomer with $\ensuremath T_{1/2} = 14(1) {\rm ns}$ , decaying by two $\ensuremath E2$ branches. The observed level energies and transition strengths are compared with the predictions of a shell model calculation. Two $\ensuremath 15/2^+$ states have been identified close in energy, and their properties are discussed in terms of mixing between vibrational and three-quasiparticle configurations.     

Compact and Weakly Compact Composition Operators on BMOA

Any analytic map φ of the unit disc ${\mathbb{D}}$ into itself induces a composition operator C _φ on BMOA, mapping ${f \mapsto f \circ \varphi}$ , where BMOA is the Banach space of analytic functions ${f\colon \mathbb{D} \to \mathbb{C}}$ whose boundary values have bounded mean oscillation on the unit circle. We show that C _φ is weakly compact on BMOA precisely when it is compact on BMOA, thus solving a question initially posed by Tjani and by Bourdon, Cima and Matheson in the special case of VMOA. As a crucial step of our argument we simplify the compactness criterion due to Smith for C _φ on BMOA and show that his condition on the Nevanlinna counting function alone characterizes compactness. Additional equivalent compactness criteria are established. Furthermore, we prove the unexpected result that compactness of C _φ on VMOA implies compactness even from the Bloch space into VMOA.