885 research outputs found
Preparation of decoherence-free, subradiant states in a cavity
The cause of decoherence in a quantum system can be traced back to the
interaction with the environment. As it has been pointed out first by Dicke, in
a system of N two-level atoms where each of the atoms is individually dipole
coupled to the environment, there are collective, subradiant states, that have
no dipole coupling to photon modes, and therefore they are expected to decay
slower. This property also implies that these type of states, which form an N-1
dimensional subspace of the atomic subsytem, also decohere slower. We propose a
scheme which will create such states. First the two-level atoms are placed in a
strongly detuned cavity and one of the atoms, called the control atom is
excited. The time evolution of the coupled atom-cavity system leads to an
appropriately entangled state of the atoms. By applying subsequent laser pulses
at a well defined time instant, it is possible to drive the atomic state into
the subradiant, i. e., decoherence free subspace. Up to a certain average
number of the photons, the result is independent of the state of the cavity.
The analysis of the conditions shows that this scheme is feasible with present
day techniques achieved in atom cavity interaction experiments.Comment: 5 page
Quantum Correlated Interstitials and the Hall Resistivity of the Magnetically Induced Wigner Crystal
We study a trial wavefunction for an interstitial in a Wigner crystal. We
find that the electron correlations, ignored in a conventional Hartree-Fock
treatment, dramatically lower the interstitial energy, especially at fillings
close to an incompressible liquid state. The correlation between the
interstitial electron and the lattice electrons at is introduced by
constructing a trial wave- function which bears a Jastrow factor of a Laughlin
state at . For fillings close to but just below , we find
that a perfect Wigner crystal becomes unstable against formation of such
interstitials. It is argued that conduction due to correlated interstitials in
the presence of weak disorder leads to the {\it classical} Hall resistivity, as
seen experimentally.Comment: 10 pages, RevTe
Impurity and spin effects on the magneto-spectroscopy of a THz-modulated nanostructure
We present a grid-free DFT model appropriate to explore the time evolution of
electronic states in a semiconductor nanostructure. The model can be used to
investigate both the linear and the nonlinear response of the system to an
external short-time perturbation in the THz regime. We use the model to study
the effects of impurities on the magneto-spectroscopy of a two-dimensional
electron gas in a nanostructure excited by an intense THz radiation. We do
observe a reduction in the binding energy of the impurity with increasing
excitation strength, and at a finite magnetic field we find a slow onset of
collective spin-oscillations that can be made to vanish with a stronger
excitation.Comment: LaTeX,10 pages with 11 embedded postscript figure
Generalized second law of thermodynamics in f(T) gravity
We investigate the validity of the generalized second law (GSL) of
gravitational thermodynamics in the framework of f(T) modified teleparallel
gravity. We consider a spatially flat FRW universe containing only the
pressureless matter. The boundary of the universe is assumed to be enclosed by
the Hubble horizon. For two viable f(T) models containing
and , we first
calculate the effective equation of state and deceleration parameters. Then, we
investigate the null and strong energy conditions and conclude that a sudden
future singularity appears in both models. Furthermore, using a cosmographic
analysis we check the viability of two models. Finally, we examine the validity
of the GSL and find that for both models it is satisfied from the early times
to the present epoch. But in the future, the GSL is violated for the special
ranges of the torsion scalar T.Comment: 16 pages, 10 figures, accepted by JCAP 201
A spectrum deconvolution method based on grey relational analysis
The extensive usage of X ray spectroscopies in studying complex material systems is not only intended to reveal underlying mechanisms that govern physical phenomena, but also used in applied studies focused on an insight driven performance improvement of a wide range of devices. However, the traditional analysis methods for X ray spectroscopic data are rather time consuming and sensitive to errors in data pre processing e.g., normalization or background subtraction . In this study, a method based on grey relational analysis, a multi variable statistical method, is proposed to analyze and extract information from X ray spectroscopic data. As a showcase, the valence bands of microcrystalline silicon suboxides probed by hard X ray photoelectron spectroscopy HAXPES were investigated. The results obtained by the proposed method agree well with conventionally derived composition information e.g., curve fit of Si 2p core level of the silicon suboxides . Furthermore, the uncertainty of chemical compositions derived by the proposed method is smaller than that of traditional analysis methods e.g., the least square fit , when artificial linear functions are introduced to simulate the errors in data pre processing. This suggests that the proposed method is capable of providing more reliable and accurate results, especially for data containing significant noise contributions or that is subject to inconsistent data pre processing. Since the proposed method is less experience driven and error prone, it offers a novel approach for automate data analysis, which is of great interest for various applications, such as studying combinatorial material librarie
Electron-Electron Interactions and the Hall-Insulator
Using the Kubo formula, we show explicitly that a non-interacting electron
system can not behave like a Hall-insulator, {\it ie.,} a DC resistivity matrix
and finite in the zero temperature
limit, as has been observed recently in experiment. For a strongly interacting
electron system in a magnetic field, we illustrate, by constructing a specific
form of correlations between mobile and localized electrons, that the Hall
resistivity can approximately equal to its classical value. A Hall-insulator is
realized in this model when the density of mobile electrons becomes vanishingly
small. It is shown that in non-interacting electron systems, the
zero-temperature frequency-dependent conductacnce generally does not give the
DC conductance.Comment: 11 pages, RevTeX3.
Spin Susceptibility and Superexchange Interaction in the Antiferromagnet CuO
Evidence for the quasi one-dimensional (1D) antiferromagnetism of CuO is
presented in a framework of Heisenberg model. We have obtained an experimental
absolute value of the paramagnetic spin susceptibility of CuO by subtracting
the orbital susceptibility separately from the total susceptibility through the
Cu NMR shift measurement, and compared directly with the theoretical
predictions. The result is best described by a 1D antiferromagnetic
Heisenberg (AFH) model, supporting the speculation invoked by earlier authors.
We also present a semi-quantitative reason why CuO, seemingly of 3D structure,
is unexpectedly a quasi 1D antiferromagnet.Comment: 7 pages including 4 tables and 9 figure
Global Distribution of Rubella Virus Genotypes
Phylogenetic analysis of a collection of 103 E1 gene sequences from rubella viruses isolated from 17 countries from 1961 to 2000 confirmed the existence of at least two genotypes. Rubella genotype I (RGI) isolates, predominant in Europe, Japan, and the Western Hemisphere, segregated into discrete subgenotypes; intercontinental subgenotypes present in the 1960s and 1970s were replaced by geographically restricted subgenotypes after ~1980. Recently, active subgenotypes include one in the United States and Latin America, one in China, and a third that apparently originated in Asia and spread to Europe and North America, starting in 1997, indicating the recent emergence of an intercontinental subgenotype. A virus that potentially arose as a recombinant between two RGI subgenotypes was discovered. Rubella genotype II (RGII) showed greater genetic diversity than did RGI and may actually consist of multiple genotypes. RGII viruses were limited to Asia and Europe; RGI viruses were also present in most of the countries where RGII viruses were isolated
Band structure related wave function symmetry of amphoteric Si dopants in GaAs
Autocompensated Si-doped GaAs is studied with cross-sectional scanning
tunneling spectroscopy (X-STS). The local electronic contrasts of
substitutional Si(Ga) donors and Si(As) acceptors under the (110) cleavage
plane are imaged with high resolution. Si(Ga) donor atoms exhibit radially
symmetric contrasts. Si(As) acceptors have anisotropic features. The
anisotropic acceptor contrasts are traced back to a tunnel process at the
valence band edge. They reflect the probability density distribution of the
localized acceptor hole state.Comment: 10 pages, 3 figure
- …