Effective interactions in medium heavy nuclei

We present a brief overview of microscopic nuclear structure approaches to nuclei with mass number from 100 to 132. The emphasis is on the shell model and theories for deriving effective interactions starting from the free interactions between nucleons. New results for (105,106,107)Sb are presented.Comment: Proceedings of RNB5, 3-8 April 2000, Divonne, France. 6 pages, Elsevier latex style. To appear in Nucl. Phys.

The bacterial strains characterization problem

The accurate characterization of collections of bacterial strains is a major scientific challenge, since bacteria are indeed responsible of significant plant diseases and thus subjected to official control procedures (e.g., in Europe, Directive 2000/29/EC). The development of diagnostic tests is therefore an important issue in order to routinely identify strains of these species

LO-phonon assisted polariton lasing in a ZnO based microcavity

Polariton relaxation mechanisms are analysed experimentally and theoretically in a ZnO-based polariton laser. A minimum lasing threshold is obtained when the energy difference between the exciton reservoir and the bottom of the lower polariton branch is resonant with the LO phonon energy. Tuning off this resonance increases the threshold, and exciton-exciton scattering processes become involved in the polariton relaxation. These observations are qualitatively reproduced by simulations based on the numerical solution of the semi-classical Boltzmann equations

Patterned silicon substrates: a common platform for room temperature GaN and ZnO polariton lasers

A new platform for fabricating polariton lasers operating at room temperature is introduced: nitride-based distributed Bragg reflectors epitaxially grown on patterned silicon substrates. The patterning allows for an enhanced strain relaxation thereby enabling to stack a large number of crack-free AlN/AlGaN pairs and achieve cavity quality factors of several thousands with a large spatial homogeneity. GaN and ZnO active regions are epitaxially grown thereon and the cavities are completed with top dielectric Bragg reflectors. The two structures display strong-coupling and polariton lasing at room temperature and constitute an intermediate step in the way towards integrated polariton devices

Alkali Doping Leads to Charge-Transfer Salt Formation in a Two-Dimensional Metal–Organic Framework

Efficient charge transfer across metal–organic interfaces is a key physical process in modern organic electronics devices, and characterization of the energy level alignment at the interface is crucial to enable a rational device design. We show that the insertion of alkali atoms can significantly change the structure and electronic properties of a metal–organic interface. Coadsorption of tetracyanoquinodimethane (TCNQ) and potassium on a Ag(111) surface leads to the formation of a two-dimensional charge transfer salt, with properties quite different from those of the two-dimensional Ag adatom TCNQ metal–organic framework formed in the absence of K doping. We establish a highly accurate structural model by combination of quantitative X-ray standing wave measurements, scanning tunnelling microscopy, and density-functional theory (DFT) calculations. Full agreement between the experimental data and the computational prediction of the structure is only achieved by inclusion of a charge-transfer-scaled dispersion correction in the DFT, which correctly accounts for the effects of strong charge transfer on the atomic polarizability of potassium. The commensurate surface layer formed by TCNQ and K is dominated by strong charge transfer and ionic bonding and is accompanied by a structural and electronic decoupling from the underlying metal substrate. The consequence is a significant change in energy level alignment and work function compared to TCNQ on Ag(111). Possible implications of charge-transfer salt formation at metal–organic interfaces for organic thin-film devices are discussed

Orbifold projection in supersymmetric QCD at N_f\leq N_c

Supersymmetric orbifold projection of N=1 SQCD with relatively small number of flavors (not larger than the number of colors) is considered. The purpose is to check whether orbifolding commutes with the infrared limit. On the one hand, one considers the orbifold projection of SQCD and obtains the low-energy description of the resulting theory. On the other hand, one starts with the low-energy effective theory of the original SQCD, and only then perfoms orbifolding. It is shown that at finite N_c the two low-energy theories obtained in these ways are different. However, in the case of stabilized run-away vacuum these two theories are shown to coincide in the large N_c limit. In the case of quantum modified moduli space, topological solitons carrying baryonic charges are present in the orbifolded low-energy theory. These solitons may restore the correspondence between the two theories provided that the soliton mass tends to zero in the large N_c limit.Comment: 10 pages; misprint corrected, reference adde

Quartic scaling of sound attenuation with frequency in vitreous silica

Several theoretical approaches to disordered media predict that acoustic waves should undergo a quartic increase in their attenuation coefficient with increasing frequency in the sub-terahertz region. Such Rayleigh-type scattering would be related to the anomalous low-temperature plateau in the thermal conductivity and to the so-called boson peak, i.e. an excess of vibrational modes above the Debye density of states at around 1 THz. Brillouin scattering of light allows the measurement of sound absorption and velocity dispersion up to about 0.1 THz while inelastic x-ray scattering is limited to frequencies larger than about 1 THz. We take advantage of the advent of ultrafast optical techniques to explore the acoustical properties of amorphous SiO2 layers in the difficult but crucial frequency region within this gap. A quartic scaling law with frequency is clearly revealed between 0.2 and 0.9 THz, which is further shown to be independent of temperature. This strongly damped regime is accompanied by a decrease in the sound velocity already starting from about 0.5 THz, in line with theories. Our study assists to clarify the anomalous acoustical properties in glasses at frequencies entering the boson peak region.Comment: 4 figures, 11 page