Instability of Rotationally Tuned Dipolar Bose-Einstein Condensates

The possibility of effectively inverting the sign of the dipole-dipole interaction, by fast rotation of the dipole polarization, is examined within a harmonically trapped dipolar Bose-Einstein condensate. Our analysis is based on the stationary states in the Thomas-Fermi limit, in the corotating frame, as well as direct numerical simulations in the Thomas-Fermi regime, explicitly accounting for the rotating polarization. The condensate is found to be inherently unstable due to the dynamical instability of collective modes. This ultimately prevents the realization of robust and long-lived rotationally tuned states. Our findings have major implications for experimentally accessing this regime.Comment: 9 pages with 5 figure

Analysis of broadband microwave conductivity and permittivity measurements of semiconducting materials

We perform broadband phase sensitive measurements of the reflection coefficient from 45 MHz up to 20 GHz employing a vector network analyzer with a 2.4 mm coaxial sensor which is terminated by the sample under test. While the material parameters (conductivity and permittivity) can be easily extracted from the obtained impedance data if the sample is metallic, no direct solution is possible if the material under investigation is an insulator. Focusing on doped semiconductors with largely varying conductivity, here we present a closed calibration and evaluation procedure for frequencies up to 5 GHz, based on the rigorous solution for the electromagnetic field distribution inside the sample combined with the variational principle; basically no limiting assumptions are necessary. A simple static model based on the electric current distribution proves to yield the same frequency dependence of the complex conductivity up to 1 GHz. After a critical discussion we apply the developed method to the hopping transport in Si:P at temperature down to 1 K.Comment: 9 pages, 10 figures, accepted for publication in the Journal of Applied Physic

The Ca II infrared triplet's performance as an activity indicator compared to Ca II H and K

Aims. A large number of Calcium Infrared Triplet (IRT) spectra are expected from the GAIA- and CARMENES missions. Conversion of these spectra into known activity indicators will allow analysis of their temporal evolution to a better degree. We set out to find such a conversion formula and to determine its robustness. Methods. We have compared 2274 Ca II IRT spectra of active main-sequence F to K stars taken by the TIGRE telescope with those of inactive stars of the same spectral type. After normalizing and applying rotational broadening, we subtracted the comparison spectra to find the chromospheric excess flux caused by activity. We obtained the total excess flux, and compared it to established activity indices derived from the Ca II H & K lines, the spectra of which were obtained simultaneously to the infrared spectra. Results. The excess flux in the Ca II IRT is found to correlate well with $R_\mathrm{HK}'$ and $R_\mathrm{HK}^{+}$, as well as $S_\mathrm{MWO}$, if the $B-V$-dependency is taken into account. We find an empirical conversion formula to calculate the corresponding value of one activity indicator from the measurement of another, by comparing groups of datapoints of stars with similar B-V.Comment: 16 pages, 15 figures. Accepted for publication in Astronomy & Astrophysic

Surveys of Galaxy Clusters with the Sunyaev Zel'dovich Effect

We have created mock Sunyaev-Zel'dovich effect (SZE) surveys of galaxy clusters using high resolution N-body simulations. To the pure surveys we add `noise' contributions appropriate to instrument and primary CMB anisotropies. Applying various cluster finding strategies to these mock surveys we generate catalogues which can be compared to the known positions and masses of the clusters in the simulations. We thus show that the completeness and efficiency that can be achieved depend strongly on the frequency coverage, noise and beam characteristics of the instruments, as well as on the candidate threshold. We study the effects of matched filtering techniques on completeness, and bias. We suggest a gentler filtering method than matched filtering in single frequency analyses. We summarize the complications that arise when analyzing the SZE signal at a single frequency, and assess the limitations of such an analysis. Our results suggest that some sophistication is required when searching for `clusters' within an SZE map.Comment: 8 pages, 7 figure

Hilbert-Schmidt Separability Probabilities and Noninformativity of Priors

The Horodecki family employed the Jaynes maximum-entropy principle, fitting the mean (b_{1}) of the Bell-CHSH observable (B). This model was extended by Rajagopal by incorporating the dispersion (\sigma_{1}^2) of the observable, and by Canosa and Rossignoli, by generalizing the observable (B_{\alpha}). We further extend the Horodecki one-parameter model in both these manners, obtaining a three-parameter (b_{1},\sigma_{1}^2,\alpha) two-qubit model, for which we find a highly interesting/intricate continuum (-\infty < \alpha < \infty) of Hilbert-Schmidt (HS) separability probabilities -- in which, the golden ratio is featured. Our model can be contrasted with the three-parameter (b_{q}, \sigma_{q}^2,q) one of Abe and Rajagopal, which employs a q(Tsallis)-parameter rather than $\alpha$, and has simply q-invariant HS separability probabilities of 1/2. Our results emerge in a study initially focused on embedding certain information metrics over the two-level quantum systems into a q-framework. We find evidence that Srednicki's recently-stated biasedness criterion for noninformative priors yields rankings of priors fully consistent with an information-theoretic test of Clarke, previously applied to quantum systems by Slater.Comment: 26 pages, 12 figure

Engineering Time-Reversal Invariant Topological Insulators With Ultra-Cold Atoms

Topological insulators are a broad class of unconventional materials that are insulating in the interior but conduct along the edges. This edge transport is topologically protected and dissipationless. Until recently, all existing topological insulators, known as quantum Hall states, violated time-reversal symmetry. However, the discovery of the quantum spin Hall effect demonstrated the existence of novel topological states not rooted in time-reversal violations. Here, we lay out an experiment to realize time-reversal topological insulators in ultra-cold atomic gases subjected to synthetic gauge fields in the near-field of an atom-chip. In particular, we introduce a feasible scheme to engineer sharp boundaries where the "edge states" are localized. Besides, this multi-band system has a large parameter space exhibiting a variety of quantum phase transitions between topological and normal insulating phases. Due to their unprecedented controllability, cold-atom systems are ideally suited to realize topological states of matter and drive the development of topological quantum computing.Comment: 11 pages, 6 figure

Anisotropic and long-range vortex interactions in two-dimensional dipolar Bose gases

We perform a theoretical study into how dipole-dipole interactions modify the properties of superfluid vortices within the context of a two-dimensional atomic Bose gas of co-oriented dipoles. The reduced density at a vortex acts like a giant anti-dipole, changing the density profile and generating an effective dipolar potential centred at the vortex core whose most slowly decaying terms go as $1/\rho^2$ and $\ln(\rho)/\rho^3$. These effects modify the vortex-vortex interaction which, in particular, becomes anisotropic for dipoles polarized in the plane. Striking modifications to vortex-vortex dynamics are demonstrated, i.e. anisotropic co-rotation dynamics and the suppression of vortex annihilation.Comment: PRL accepted, 6 pages, 5 figure