Evidence of Sn adatoms quantum tunneling at the alpha-Sn/Si(111) surface

We present a low-temperature scanning tunneling microscopy study of the alpha-Sn/Si(111) surface that demonstrates the fluctuating behavior of the Sn adatoms. The dynamical fluctuation model, successfully applied in describing the alpha-Sn/Ge(111) surface, is proposed for the related alpha-Sn/Si(111) surface too, although with a much lower transition temperature. In addition, a new phenomenon appears responsible for the unexpected evidence that the average oscillation frequency remains constant at temperatures lower than 15 K, in contradiction to the Arrhenius law. We explain this phenomenon as quantum tunneling of Sn adatoms.     

Multipoint holographic optical velocimetry in microfluidic systems

We show how holographic optical trapping can be used for the multipoint measurement of fluid flow in microscopic geometries. An array of microprobes can be simultaneously trapped and used to map out the fluid flow in a microfluidic device. The optical traps are alternately turned on and off such that the probe particles are displaced by the flow of the surrounding fluid and then retrapped. The particles' displacements are monitored by digital video microscopy and directly converted into velocity field values. This technique enables the measurement of a two-dimensional flow field at points arbitrarily distributed in a three-dimensional volume. The validity of the technique is demonstrated for the case of the flow around a spinning sphere and the flow at the outlet of a microchannel.     

Shallow donor states induced by in-diffused Cu in ZnO: a combined HREELS and hybrid DFT study

A combined experimental and first principles study of Cu defects in bulk ZnO is presented. Cu particles are epitaxially deposited on the polar O-ZnO(0001) surface at room temperature. Upon heating, a broadening of the quasielastic peak in high resolution electron energy loss spectra is observed, corresponding to an electronic doping effect of Cu atoms in bulk ZnO with an ionization energy of 88 meV. Cu impurities in ZnO, although commonly acting as acceptors, are presently observed to induce shallow donor states. We assign these to interstitial Cu species on the basis of a hybrid density functional study.     

Solar modulation of cosmic deuteron fluxes in the PAMELA experiment

The preliminary results from measurements of deuteron fluxes in galactic cosmic rays (GCR) in the vicinity of the Earth in 2006–2009 are presented. The results are obtained by analyzing data from the PAMELA experiment aboard the Resurs DK-1 satellite. High-precision detection instruments provided an opportunity to identify GCR deuterons and measure their spectrum in the energy interval of 90–650MeV/nucleon. Spectra averaged over six-month intervals from the summer of 2006 to the summer of 2009 (the solar activity minimum) are presented. The influence of solar modulation on the observed spectrum is clearly seen in the results.     

Vibrationally assisted transitions and phonon spectra in crystalline solids

The vibrationally assisted electronic (vibronic) transitions of localized centers in crystalline solids provide relevant information regarding the phonon spectra of the host materials. We present the vibronic spectra of some compounds with particular attention to the case of the transition metal ions V²⁺ and Cr³⁺ embedded in simple ionic crystals such as MgO or more complex systems such as YAG. The vibronic spectra are interpreted in light of the radiative selection rules and are compared with phonon data obtained with other techniques such as neutron scattering, infrared and Raman spectroscopies. Conclusions regarding the effectiveness of vibronic spectra in uncovering the phonon spectral distributions are presented.     

The case of RIB at intermediate energies: Isospin effects on nuclear dynamics

In the reaction dynamics of intermediate-energy radioactive beams we can probe highly asymmetric nuclear matter in compressed as well as dilute phases. In this report some predictions are presented, based on analytical results as well as on reaction simulations. We suggest a series of experiments with RIB aimed to shed light on isospin properties of nuclear interactions in the medium. Received: 1 May 2001 / Accepted: 4 December 2001     

Application of high-resolution magic-angle spinning NMR spectroscopy to define the cell uptake of MRI contrast agents

A new method, based on proton high-resolution magic-angle spinning ((1)H HR-MAS) NMR spectroscopy, has been employed to study the cell uptake of magnetic resonance imaging contrast agents (MRI-CAs). The method was tested on human red blood cells (HRBC) and white blood cells (HWBC) by using three gadolinium complexes, widely used in diagnostics, Gd-BOPTA, Gd-DTPA, and Gd-DOTA, and the analogous complexes obtained by replacing Gd(III) with Dy(III), Nd(III), and Tb(III) (i.e., complexes isostructural to the ones of gadolinium but acting as shift agents). The method is based on the evaluation of the magnetic effects, line broadening, or induced lanthanide shift (LIS) caused by these complexes on NMR signals of intra- and extracellular water. Since magnetic effects are directly linked to permeability, this method is direct. In all the tests, these magnetic effects were detected for the extracellular water signal only, providing a direct proof that these complexes are not able to cross the cell membrane. Line broadening effects (i.e., the use of gadolinium complexes) only allow qualitative evaluations. On the contrary, LIS effects can be measured with high precision and they can be related to the concentration of the paramagnetic species in the cellular compartments. This is possible because the HR-MAS technique provides the complete elimination of bulk magnetic susceptibility (BMS) shift and the differentiation of extra- and intracellular water signals. Thus with this method, the rapid quantification of the MRI-CA amount inside and outside the cells is actually feasible.     

Relative position determination of a lunar rover using high-accuracy multi-frequency same-beam VLBI

Multi-frequency same-beam VLBI means that two explorers with a small separation angle are simultaneously observed with the main beam of receiving antennas. In the same-beam VLBI, the differential phase delay between two explorers and two receiving telescopes can be obtained with a small error of several picoseconds. The differential phase delay, as the observable of the same-beam VLBI, gives the separation angular information of the two explorers in the celestial sphere. The two-dimensional relative position on the plane-of-sky can thus be precisely determined with an error of less than 1 m for a distance of 3.8×105 km far away from the earth, by using the differential phase delay obtained with the four Chinese VLBI stations. The relative position of a lunar rover on the lunar surface can be determined with an error of 10 m by using the differential phase delay data and the range data for the lander when the lunar topography near the rover and the lander can be determined with an error of 10 m.