Singlet-triplet splitting, correlation and entanglement of two electrons in quantum dot molecules

Starting with an accurate pseudopotential description of the single-particle states, and following by configuration-interaction treatment of correlated electrons in vertically coupled, self-assembled InAs/GaAs quantum dot-molecules, we show how simpler, popularly-practiced approximations, depict the basic physical characteristics including the singlet-triplet splitting, degree of entanglement (DOE) and correlation. The mean-field-like single-configuration approaches such as Hartree-Fock and local spin density, lacking correlation, incorrectly identify the ground state symmetry and give inaccurate values for the singlet-triplet splitting and the DOE. The Hubbard model gives qualitatively correct results for the ground state symmetry and singlet-triplet splitting, but produces significant errors in the DOE because it ignores the fact that the strain is asymmetric even if the dots within a molecule are identical. Finally, the Heisenberg model gives qualitatively correct ground state symmetry and singlet-triplet splitting only for rather large inter-dot separations, but it greatly overestimates the DOE as a consequence of ignoring the electron double occupancy effect.Comment: 13 pages, 9 figures. To appear in Phys. Rev.

Charge order induced by electron-lattice interaction in NaV2O5

We present Density Matrix Renormalization Group calculations of the ground-state properties of quarter-filled ladders including static electron-lattice coupling. Isolated ladders and two coupled ladders are considered, with model parameters obtained from band-structure calculations for $\alpha^\prime$-NaV$_2$O$_5$. The relevant Holstein coupling to the lattice causes static out-of-plane lattice distortions, which appear concurrently with a charge-ordered state and which exhibit the same zigzag pattern observed in experiments. The inclusion of electron-lattice coupling drastically reduces the critical nearest-neighbor Coulomb repulsion $V_c$ needed to obtain the charge-ordered state. No spin gap is present in the ordered phase. The charge ordering is driven by the Coulomb repulsion and the electron-lattice interaction. With electron-lattice interaction, coupling two ladders has virtually no effect on $V_c$ or on the characteristics of the charge-ordered phase. At V=0.46\eV, a value consistent with previous estimates, the lattice distortion, charge gap, charge order parameter, and the effective spin coupling are in good agreement with experimental data for NaV$_2$O_5$.Comment: 7 pages, 9 figure

Scalar Field Oscillations Contributing to Dark Energy

We use action-angle variables to describe the basic physics of coherent scalar field oscillations in the expanding universe. These analytical mechanics methods have some advantages, like the identification of adiabatic invariants. As an application, we show some instances of potentials leading to equations of state with $p<-\rho/3$, thus contributing to the dark energy that causes the observed acceleration of the universe.Comment: 17 pages, 6 figures, Latex file. Sec.II reduced, discussion on sound speed added in Sec.IV, new references added. Accepted for publication in Physical Review

Optical and ROSAT X-ray observations of the dwarf nova OY Carinae in superoutburst and quiescence

We present ROSAT X-ray and optical light curves of the 1994 February superoutburst of the eclipsing SU UMa dwarf nova OY Carinae. There is no eclipse of the flux in the ROSAT HRI light curve. Contemporaneous `wide B' band optical light curves show extensive superhump activity and dips at superhump maximum. Eclipse mapping of these optical light curves reveals a disc with a considerable physical flare, even three days into the superoutburst decline. We include a later (1994 July) ROSAT PSPC observation of OY Car that allows us to put constraints on the quiescent X-ray spectrum. We find that while there is little to choose between OY Car and its fellow high inclination systems with regard to the temperature of the emitting gas and the emission measure, we have difficulties reconciling the column density found from our X-ray observation with the column found in HST UV observations by Horne et al. (1994). The obvious option is to invoke time variability.Comment: 16 pages, 14 figures, accepted for publication in MNRA

Teleportation in a noisy environment: a quantum trajectories approach

We study the fidelity of quantum teleportation for the situation in which quantum logic gates are used to provide the long distance entanglement required in the protocol, and where the effect of a noisy environment is modeled by means of a generalized amplitude damping channel. Our results demonstrate the effectiveness of the quantum trajectories approach, which allows the simulation of open systems with a large number of qubits (up to 24). This shows that the method is suitable for modeling quantum information protocols in realistic environments.Comment: 9 pages, 2 figure

The Fe XXII I(11.92 A)/I(11.77 A) Density Diagnostic Applied to the Chandra High Energy Transmission Grating Spectrum of EX Hydrae

Using the Livermore X-ray Spectral Synthesizer, which calculates spectral models of highly charged ions based primarily on HULLAC atomic data, we investigate the temperature, density, and photoexcitation dependence of the I(11.92 A)/I(11.77 A) line ratio of Fe XXII. We find that this line ratio has a critical density n_c \approx 5x10^13 cm^-3, is approximately 0.3 at low densities and 1.5 at high densities, and is very insensitive to temperature and photoexcitation, so is a useful density diagnostic for sources like magnetic cataclysmic variables in which the plasma densities are high and the efficacy of the He-like ion density diagnostic is compromised by the presence of a bright ultraviolet continuum. Applying this diagnostic to the Chandra High Energy Transmission Grating spectrum of the intermediate polar EX Hya, we find that the electron density of its T_e \approx 12 MK plasma is n_e = 1.0^{+2.0}_{-0.5} x 10^14 cm^-3, orders of magnitude greater than that typically observed in the Sun or other late-type stars.Comment: 11 pages including 3 encapsulated postscript figures; LaTeX format, uses aastex.cls; accepted on 2003 April 3 for publication in The Astrophysical Journa

Unusual Radiographic Presentation of Pneumocystis Pneumonia in a Patient with AIDS.

Pneumocystis jirovecii pneumonia (PCP) typically presents as an interstitial and alveolar process with ground glass opacities on chest computed tomography (CT). The absence of ground glass opacities on chest CT is thought to have a high negative predictive value for PCP in individuals with AIDS. Here, we report a case of PCP in a man with AIDS who presented to our hospital with subacute shortness of breath and a nonproductive cough. While his chest CT revealed diffuse nodular rather than ground glass opacities, bronchoscopy with bronchoalveolar lavage and transbronchial biopsies confirmed the diagnosis of PCP and did not identify additional pathogens. PCP was not the expected diagnosis based on chest CT, but it otherwise fit well with the patient's clinical and laboratory presentation. In the era of combination antiretroviral therapy, routine prophylaxis for PCP, and increased use of computed tomography, it may be that PCP will increasingly present with nonclassical chest radiographic patterns. Clinicians should be aware of this presentation when selecting diagnostic and management strategies