Triplet-Singlet Spin Relaxation in Quantum Dots with Spin-Orbit Coupling

We estimate the triplet-singlet relaxation rate due to spin-orbit coupling assisted by phonon emission in weakly-confined quantum dots. Our results for two and four electrons show that the different triplet-singlet relaxation trends observed in recent experiments under magnetic fields can be understood within a unified theoretical description, as the result of the competition between spin-orbit coupling and phonon emission efficiency. Moreover, we show that both effects are greatly affected by the strength of the confinement and the external magnetic field, which may give access to very long-lived triplet states as well as to selective population of the triplet Zeeman sublevels.Comment: 5 pages, 3 figures. Closely related to recent experiments in cond-mat/060972

Physical Properties of Galactic Planck Cold Cores revealed by the Hi-GAL survey

Previous studies of the initial conditions of massive star formation have mainly targeted Infrared-Dark Clouds (IRDCs) toward the inner Galaxy. This is due to the fact that IRDCs were first detected in absorption against the bright mid-IR background, requiring a favourable location to be observed. By selection, IRDCs represent only a fraction of the Galactic clouds capable of forming massive stars and star clusters. Due to their low dust temperatures, IRDCs are bright in the far-IR and millimeter and thus, observations at these wavelengths have the potential to provide a complete sample of star-forming massive clouds across the Galaxy. Our aim is to identify the clouds at the initial conditions of massive star formation across the Galaxy and compare their physical properties as a function of their Galactic location. We have examined the physical properties of a homogeneous galactic cold core sample obtained with the Planck satellite across the Galactic Plane. With the use of Herschel Hi-GAL observations, we have characterized the internal structure of them. By using background-subtracted Herschel images, we have derived the H2 column density and dust temperature maps for 48 Planck clumps. Their basic physical parameters have been calculated and analyzed as a function of location within the Galaxy. These properties have also been compared with the empirical relation for massive star formation derived by Kauffmann & Pillai (2010). Most of the Planck clumps contain signs of star formation. About 25% of them are massive enough to form high mass stars. Planck clumps toward the Galactic center region show higher peak column densities and higher average dust temperatures than those of the clumps in the outer Galaxy. Although we only have seven clumps without associated YSOs, the Hi-GAL data show no apparent differences in the properties of Planck cold clumps with and without star formation.Comment: 22 pages, 11 figures, accepted for publication in A&

Effect of the Coulomb interaction on the electron relaxation of weakly-confined quantum dot systems

We study acoustic-phonon-induced relaxation of charge excitations in single and tunnel-coupled quantum dots containing few confined interacting electrons. The Full Configuration Interaction approach is used to account for the electron-electron repulsion. Electron-phonon interaction is accounted for through both deformation potential and piezoelectric field mechanisms. We show that electronic correlations generally reduce intradot and interdot transition rates with respect to corresponding single-electron transitions, but this effect is lessened by external magnetic fields. On the other hand, piezoelectric field scattering is found to become the dominant relaxation mechanism as the number of confined electrons increases. Previous proposals to strongly suppress electron-phonon coupling in properly designed single-electron quantum dots are shown to hold also in multi-electron devices. Our results indicate that few-electron orbital degrees of freedom are more stable than single-electron ones.Comment: 20 pages (preprint format), 7 figures, submitted to Phys. Rev.

The Luminosity Function of Cluster Galaxies. III

We investigated the optical properties of 7 clusters of galaxies observed in three colors over the range of absolute magnitudes -24 < M < -12. Our aim is to estimate the Luminosity Function and the total cluster luminosity of our sample in order to have information about the formation and the evolution of galaxies in clusters. In this paper, we present the main points of our analysis and give the formal parameters obtained by fitting the data using the maximum likelihood algorithm. We find consistency between our results and other works in literature confirming the bimodal nature of the luminosity function of cluster galaxies. More important, we find that the relation L_opt/ L_X versus L_X is color dependent: Low X ray Luminosity clusters have a bluer galaxy population.Comment: 22 pages. Accepted for publication on A&

Phonon-induced electron relaxation in weakly-confined single and coupled quantum dots

We investigate charge relaxation rates due to acoustic phonons in weakly-confined quantum dot systems, including both deformation potential and piezoelectric field interactions. Single-electron excited states lifetimes are calculated for single and coupled quantum dot structures, both in homonuclear and heteronuclear devices. Piezoelectric field scattering is shown to be the dominant relaxation mechanism in many experimentally relevant situations. On the other hand, we show that appropriate structure design allows to minimize separately deformation potential and piezolectric field interactions, and may bring electron lifetimes in the range of microseconds.Comment: 20 pages (preprint format), 7 figures, submitted to Physical Review

Effect of electron-electron interaction on the phonon-mediated spin relaxation in quantum dots

We estimate the spin relaxation rate due to spin-orbit coupling and acoustic phonon scattering in weakly-confined quantum dots with up to five interacting electrons. The Full Configuration Interaction approach is used to account for the inter-electron repulsion, and Rashba and Dresselhaus spin-orbit couplings are exactly diagonalized. We show that electron-electron interaction strongly affects spin-orbit admixture in the sample. Consequently, relaxation rates strongly depend on the number of carriers confined in the dot. We identify the mechanisms which may lead to improved spin stability in few electron (>2) quantum dots as compared to the usual one and two electron devices. Finally, we discuss recent experiments on triplet-singlet transitions in GaAs dots subject to external magnetic fields. Our simulations are in good agreement with the experimental findings, and support the interpretation of the observed spin relaxation as being due to spin-orbit coupling assisted by acoustic phonon emission.Comment: 12 pages, 10 figures. Revised version. Changes in section V (simulation of PRL 98, 126601 experiment

ISO-LWS observations of IRAS16293-2422

We obtained LWS grating spectra toward IRAS 16293-2422 and the surrounding region, which covers the entire extent of the molecular outflow. The LWS spectra show that the region is relatively uncontaminated by PhotoDissociationRegion (PDR)-like emission, showing only a weak diffuse CII emission. The on-source spectrum revealed the presence of the OI(63μm) line and several lines from CO, H2O and OH molecules. In this work we derive the macroscopic quantities associated with the UV-illuminated emitting gas which surrounds IRAS16293-2422 and compare it with previous studies. We show that the molecular lines originate in a hot (~1600 K), dense (~ 3·104cm-3) and extended (~ 8·1016cm) region, that we interprete as the shock of the wind impacting obliquely with the walls of the cavity created by the wind itself. The OI(63μm) line observed by the Kuiper Airborne Observatory (KAO: Ceccarelli et al. 1997a) at ~ 1.2·1017cm west from the central source is hence interpreted as the head of the shock where the wind strikes the ambient gas. Finally we speculate that the OI(63μm) line emission seen on-source originates in the collapsing envelope that surrounds the central object(s

Spectral Curves and Localization in Random Non-Hermitian Tridiagonal Matrices

Eigenvalues and eigenvectors of non-Hermitian tridiagonal periodic random matrices are studied by means of the Hatano-Nelson deformation. The deformed spectrum is annular-shaped, with inner radius measured by the complex Thouless formula. The inner bounding circle and the annular halo are stuctures that correspond to the two-arc and wings observed by Hatano and Nelson in deformed Hermitian models, and are explained in terms of localization of eigenstates via a spectral duality and the Argument principle.Comment: 5 pages, 9 figures, typographical error corrected in reference