Period Color and Amplitude Color relations for MACHO project LMC RR Lyraes

In this paper, we analyze period color and amplitude color relations at minimum, mean and maximum $V$ band light for 6391 RRab stars in the Large Magellanic Cloud obtained by the MACHO project. Specifically, we find that color and amplitude are nearly independent of period at minimum light but that there exists a definite relation between period and color and amplitude and color at maximum light. These two properties are easily explained by the application of the Stefan Boltzmann law and the interaction of the photosphere and hydrogen ionization front at minimum light. When we examine the slope of the period color relation as a function of phase, we find that the slope varies significantly with phase and is small for a wide range of phases around minimum light. This suggests that another factor that needs to be considered when trying to understand RR Lyrae observed properties is their behavior at different phases during a pulsation cycle.Comment: Sumitted for publication to MNRAS Letter

Correction to the geometric phase by structured environments: the onset of non-Markovian effects

We study the geometric phase of a two-level system under the presence of a structured environment, particularly analysing its correction with the ohmicity parameter $s$ and the onset of non-Markovianity. We firstly examine the system coupled to a set of harmonic oscillators and studied the decoherence factor as function of the environment's ohmicity parameter. Secondly, we propose the two-level system coupled to a non-equilibrium environment, and show that these environments display non-Markovian effects for all values of the ohmicity parameter. The geometric phase of the two-level system is therefore computed under the presence of both types of environment. The correction to the unitary geometric phase is analysed in both, Markovian and non-Markovian regimes. Under Markovian environments, the correction induced on the system's phase is mainly ruled by the coupling constant between the system and the environment, while in the non-Markovian regime, memory effects seem to trigger a significant correction to the unitary geometric phase. The result is significant to the quantum information processing based on the geometric phase in quantum open systemsComment: 7 figures. Accepted for publication in Phys. Rev. A. arXiv admin note: text overlap with arXiv:1303.493

Decoherence in composite quantum open systems: the effectiveness of unstable degrees of freedom

The effect induced by an environment on a composite quantum system is studied. The model considers the composite system as comprised by a subsystem A coupled to a subsystem B which is also coupled to an external environment. We study all possible four combinations of subsystems A and B made up with a harmonic oscillator and an upside down oscillator. We analyzed the decoherence suffered by subsystem A due to an effective environment composed by subsystem B and the external reservoir. In all the cases we found that subsystem A decoheres even though it interacts with the environment only through its sole coupling to B. However, the effectiveness of the diffusion depends on the unstable nature of subsystem A and B. Therefore, the role of this degree of freedom in the effective environment is analyzed in detailComment: 20 pages, 4 figures. Version to appear in Int. J. Mod. Phys.

Decoherence of a solid-state qubit by different noise correlation spectra

The interaction between solid-state qubits and their environmental degrees of freedom produces non-unitary effects like decoherence and dissipation. Uncontrolled decoherence is one of the main obstacles that must be overcome in quantum information processing. We study the dynamically decay of coherences in a solid-state qubit by means of the use of a master equation. We analyse the effects induced by thermal Ohmic environments and low-frequency 1/f noise. We focus on the effect of longitudinal and transversal noise on the superconducting qubit's dynamics. Our results can be used to design experimental future setups when manipulating superconducting qubits.Comment: 14 pages, 9 figures. Version to appear in Physics Letters A. arXiv admin note: text overlap with arXiv:0809.4716 by other author

Energy Level Alignment in Organic-Organic Heterojunctions: The TTF-TCNQ Interface

The energy level alignment of the two organic materials forming the TTF-TCNQ interface is analyzed by means of a local orbital DFT calculation, including an appropriate correction for the transport energy gaps associated with both materials. These energy gaps are determined by a combination of some experimental data and the results of our calculations for the difference between the TTF_{HOMO} and the TCNQ_{LUMO} levels. We find that the interface is metallic, as predicted by recent experiments, due to the overlap (and charge transfer) between the Density of States corresponding to these two levels, indicating that the main mechanism controlling the TTF-TCNQ energy level alignment is the charge transfer between the two materials. We find an induced interface dipole of 0.7 eV in good agreement with the experimental evidence. We have also analyzed the electronic properties of the TTF-TCNQ interface as a function of an external bias voltage \Delta, between the TCNQ and TTF crystals, finding a transition between metallic and insulator behavior for \Delta~0.5 eV

Excitons in core-only, core-shell and core-crown CdSe nanoplatelets: interplay between in-plane electron-hole correlation, spatial and dielectric confinement

Using semi-analytical models we calculate the energy, effective Bohr radius and radiative lifetime of neutral excitons confined in CdSe colloidal nanoplatelets (NPLs). The excitonic properties are largely governed by the electron-hole in-plane correlation, which in NPLs is enhanced by the quasi-two-dimensional motion and the dielectric mismatch with the organic environment. In NPLs with lateral size $L \gtrsim 20$ nm the exciton behavior is essentially that in a quantum well, with superradiance leading to exciton lifetimes of 1 ps or less, only limited by the NPL area. However, for $L < 20$ nm excitons enter an intermediate confinement regime, hence departing from the quantum well behavior. In heterostructured NPLs, different response is observed for core/shell and core/crown configurations. In the former, the strong vertical confinement limits separation of electrons and holes even for type-II band alignment. The exciton behavior is then similar to that in core-only NPL, albeit with weakened dielectric effects. In the latter, charge separation is also inefficient if band alignment is quasi-type-II (e.g. in CdSe/CdS), because electron-hole interaction drives both carriers into the core. However, it becomes very efficient for type-II alignment, for which we predict exciton lifetimes reaching $\mu s$.Comment: typographical errors fixed (with respect to v1 and PRB) in eqs. 9,12 and definition of overla

Symmetry induced hole-spin mixing in quantum dot molecules

We investigate theoretically the spin purity of single holes confined in vertically coupled GaAs/AlGaAs quantum dots (QDs) under longitudinal magnetic fields. A unique behavior is observed for triangular QDs, by which the spin is largely pure when the hole is in one of the dots, but it becomes strongly mixed when an electric field is used to drive it into molecular resonance. The spin admixture is due to the valence band spin-orbit interaction, which is greatly enhanced in C3h symmetry environments. The strong yet reversible electrical control of hole spin suggests that molecules with C3-symmetry QDs, like those obtained with [111] growth, can outperform the usual C2-symmetry QDs obtained with [001] growth for the development of scalable qubit architectures.Comment: 5-pages manuscript + supplementary information. Version to be published in PRB Rapid Communication

Macroscopic tunneling, decoherence and noise-induced activation

We study the effects of the environment at zero temperature on tunneling in an open system described by a static double-well potential. We show that the evolution of the system in an initial Schrodinger cat state, can be summarized in terms of three main physical phenomena, namely decoherence, quantum tunneling and noise-induced activation. Using large-scale numerical simulations, we obtain a detailed picture of the main stages of the evolution and of the relevant dynamical processesComment: Contribution to the Proceedings of DICE'0