Symmetry of `molecular' configurations of interacting electrons in a quantum dot in strong magnetic fields

A molecular description for magic-number configurations of interacting electrons in a quantum dot in high magnetic fields developed by one of the authors has been elaborated for four, five and six electron dots. For four electrons, the magic spin-singlet states are found to alternate between two different resonating valence bond (RVB)-like states. For the five-electron spin-polarized case, the molecular description is shown to work for the known phenomenon of magic-number sequences that correspond to both the N-fold symmetric ring configuration and a $(N-1)$-fold symmetric one with a center electron. A six-electron dot is shown here to have an additional feature in which inclusion of quantum mechanical mixing between classical configurations, which are deformed and degenerate, restores the N-fold symmetry and reproduces the ground-state energy accurately.Comment: 4 pages, to be published in Physisca

A Tidal Disruption Flare in Abell 1689 from an Archival X-ray Survey of Galaxy Clusters

Theory suggests that a star making a close passage by a supermassive black hole at the center of a galaxy can under most circumstances be expected to emit a giant flare of radiation as it is disrupted and a portion of the resulting stream of shock-heated stellar debris falls back onto the black hole itself. We examine the first results of an ongoing archival survey of galaxy clusters using Chandra and XMM-selected data, and report a likely tidal disruption flare from SDSS J131122.15-012345.6 in Abell 1689. The flare is observed to vary by a factor of >30 over at least 2 years, to have maximum L_X(0.3-3.0 keV)> 5 x 10^{42} erg s^{-1} and to emit as a blackbody with kT~0.12 keV. From the galaxy population as determined by existing studies of the cluster, we estimate a tidal disruption rate of 1.2 x 10^{-4} galaxy^{-1} year^{-1} if we assume a contribution to the observable rate from galaxies whose range of luminosities corresponds to a central black hole mass (M_bh) between 10^6 and 10^8 M_sun.Comment: 24 pages, including 6 figures and 2 tables Accepted for publication in the Astrophysical Journa

Spin-Blockade in Single and Double Quantum Dots in Magnetic Fields: a Correlation Effect

The total spin of correlated electrons in a quantum dot changes with magnetic field and this effect is generally linked to the change in the total angular momentum from one magic number to another, which can be understood in terms of an `electron molecule' picture for strong fields. Here we propose to exploit this fact to realize a spin blockade, i.e., electrons are prohibited to tunnel at specific values of the magnetic field. The spin-blockade regions have been obtained by calculating both the ground and excited states. In double dots the spin-blockade condition is found to be less stringent than in single dots.Comment: 4pages, to be published in Phys. Rev. B (Rapid Communication

Hall mobility enhancement caused by annealing of Si0.2Ge0.8/Si0.7Ge0.3/Si(001) p-type modulation-doped heterostructures

The effect of post-growth furnace thermal annealing (FTA) on the Hall mobility and sheet carrier density measured at 9–300 K in the Si0.2Ge0.8/Si0.7Ge0.3/Si(001) p-type modulation-doped heterostructures was studied. FTA treatments in the temperature range of 600–900 °C for 30 min were performed on similar heterostructures but with two Si0.2Ge0.8 channel thicknesses. The annealing at 600 °C is seen to have a negligible effect on the Hall mobility as well as on the sheet carrier density. Increases in the annealing temperature resulted in pronounced successive increases of the mobility. For both samples the maximum Hall mobility was observed after FTA at 750 °C. Further increases of the annealing temperature resulted in a decrease in mobility. The sheet carrier density showed the opposite behavior with an increase in annealing temperature. The mechanism causing this behavior is discussed. Structural characterization of as-grown and annealed samples was done by cross-sectional transmission electron microscopy

Deep Spectroscopy of the MV∼−14.8M_V\sim -14.8 Host Galaxy of a Tidal Disruption Flare in A1795

A likely tidal disruption of a star by the intermediate-mass black hole (IMBH) of a dwarf galaxy was recently identified in association with Abell 1795. Without deep spectroscopy for this very faint object, however, the possibility of a more massive background galaxy or even a disk-instability flare from a weak AGN could not be dismissed. We have now obtained 8 hours of Gemini spectroscopy which unambiguously demonstrate that the host galaxy is indeed an extremely low-mass $(M_\ast\sim 3\times 10^8\; {\rm M}_{\odot})$ galaxy in Abell 1795, comparable to the least-massive galaxies determined to host IMBHs via other studies. We find that the spectrum is consistent with the X-ray flare being due to a tidal disruption event rather than an AGN flare. We also set improved limits on the black hole mass $({\rm log}[M_{\bullet}/{\rm M}_{\odot}] \sim 5.3 - 5.7)$ and infer a 15-year X-ray variability of a factor of $> 10^4$. The confirmation of this galaxy-black hole system provides a glimpse into a population of galaxies that is otherwise difficult to study, due to the galaxies' low masses and intrinsic faintness, but which may be important contributors to the tidal disruption rate.Comment: 9 pages, 4 figures. Accepted by MNRA

Mapping Seyfert and LINER Excitation Modes in the Inner kpc of NGC 3393

We have mapped the extended narrow line region (ENLR) of NGC 3393 on scales of $r\lesssim4^{\prime\prime}$ ($\sim1\,$kpc) from the nucleus using emission line images of H$\alpha\,\lambda6563$, [O III]$\lambda5007$, and [S II]$\lambda\lambda6717,6731$ taken with HST as part of the CHandra survey of Extended Emission line Regions in nearby Seyfert galaxies (CHEERS). By mapping these lines onto a spatially resolved Baldwin-Phillips-Terlevich (BPT) diagram, we investigate the impact of feedback from a Compton-thick AGN on its circumnuclear ISM. We find the expected Seyfert-like emission within the ionization bicone ($\lesssim3^{\prime\prime}$; 770 pc). We also find a new, figure 8 shaped LINER cocoon enveloping the bicone and defining a sharp ($\lesssim100\;$pc) transition between higher and lower ionization zones. These data illustrate the morphological dependence of ionization states of the ENLR relative to bicone and host gas geometries.Comment: Accepted by ApJ and published (2016, ApJ, 829, 46). Updated to reflect the accepted versio