13,436 research outputs found
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 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 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 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
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.
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 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
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 .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
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