Fractionalization of minimal excitations in integer quantum Hall edge channels

A theoretical study of the single electron coherence properties of Lorentzian and rectangular pulses is presented. By combining bosonization and the Floquet scattering approach, the effect of interactions on a periodic source of voltage pulses is computed exactly. When such excitations are injected into one of the channels of a system of two copropagating quantum Hall edge channels, they fractionalize into pulses whose charge and shape reflects the properties of interactions. We show that the dependence of fractionalization induced electron/hole pair production in the pulses amplitude contains clear signatures of the fractionalization of the individual excitations. We propose an experimental setup combining a source of Lorentzian pulses and an Hanbury Brown and Twiss interferometer to measure interaction induced electron/hole pair production and more generally to reconstruct single electron coherence of these excitations before and after their fractionalization.Comment: 18 pages, 10 figures, 1 tabl

Real time decoherence of Landau and Levitov quasi-particles in quantum Hall edge channels

Quantum Hall edge channels at integer filling factor provide a unique test-bench to understand decoherence and relaxation of single electronic excitations in a ballistic quantum conductor. In this Letter, we obtain a full visualization of the decoherence scenario of energy (Landau) and time (Levitov) resolved single electron excitations at filling factor $\nu=2$. We show that the Landau excitation exhibits a fast relaxation followed by spin-charge separation whereas the Levitov excitation only experiences spin-charge separation. We finally suggest to use Hong-Ou-Mandel type experiments to probe specific signatures of these different scenarios.Comment: 14 pages, 8 figure

Leptoquarks decaying to a top quark and a charged lepton at hadron colliders

We study the sensitivity of the Tevatron and the 7 TeV LHC to a leptoquark S coupling to a top quark and a charged lepton L (= e, mu, or tau). For the Tevatron, we focus on the case m_S < m_t, where the leptoquark pair production cross section is large, and the decay is three-body: S --> W b L^{\pm}. We argue that existing Tevatron observations could exclude m_S \lsim 160 GeV. For m_S > m_t, we show that the LHC experiments with low integrated luminosity could be sensitive to such leptoquarks decaying to tl^{\pm} with l= mu or tau.Comment: 13 pages, 6 figures, minor changes (typos

Integer and fractional charge Lorentzian voltage pulses analyzed in the frame of Photon-assisted Shot Noise

The periodic injection $n$ of electrons in a quantum conductor using periodic voltage pulses applied on a contact is studied in the energy and time-domain using shot noise computation in order to make comparison with experiments. We particularly consider the case of periodic Lorentzian voltage pulses. When carrying integer charge, they are known to provide electronic states with a minimal number of excitations, while other type of pulses are all accompanied by an extra neutral cloud of electron and hole excitations. This paper focuses on the low frequency shot noise which arises when the pulse excitations are partitioned by a single scatterer in the framework of the Photo Assisted Shot Noise (PASN) theory. As a unique tool to count the number of excitations carried per pulse, shot noise reveals that pulses of arbitrary shape and arbitrary charge show a marked minimum when the charge is integer. Shot noise spectroscopy is also considered to perform energy-domain characterization of the charge pulses. In particular it reveals the striking asymmetrical spectrum of Lorentzian pulses. Finally, time-domain information is obtained from Hong Ou Mandel like noise correlations when two trains of pulses generated on opposite contacts collide on the scatterer. As a function of the time delay between pulse trains, the noise is shown to measure the electron wavepacket autocorrelation function for integer Lorentzian thanks to electron antibunching. In order to make contact with recent experiments all the calculations are made at zero and finite temperature

Magnetization Measurements on Single Crystals of Superconducting Ba0.6K0.4BiO3

Extensive measurements of the magnetization of superconducting single crystal samples of Ba0.6K0.4BiO3} have been made using SQUID and cantilever force magnetometry at temperatures ranging between 1.3 and 350 K and in magnetic fields from near zero to 27 T. Hysteresis curves of magnetization versus field allow a determination of the thermodynamic critical field, the reversibility field, and the upper critical field as a function of temperature. The lower critical field is measured seperately and the Ginzburg-Landau parameter is found to be temperature dependent. All critical fields have higher T = 0 limits than have been previously noted and none of the temperature dependence of the critical fields follow the expected power laws leading to possible alternate interpretation of the thermodynamic nature of the superconducting transition.Comment: 33 pages, 11 figures, accepted for publication in Philosophical Magazine B on 7 August 1999. This paper supplies the experimental details for the argument presented in our PRL 82 (1999) p. 4532-4535 (also at cond-mat/9904288

Radio to Gamma-Ray Emission from Shell-type Supernova Remnants: Predictions from Non-linear Shock Acceleration Models

Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency and spectral considerations, producing GeV/TeV intensity ratios that are quite different from test particle predictions. The Sedov scaling solution for SNR expansions is used to estimate important shock parameters for input into the Monte Carlo simulation. We calculate ion and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding complete photon spectra from radio frequencies to gamma-ray energies. The cessation of acceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cutoffs in the TeV energy range; these are consistent with Whipple's TeV upper limits on unidentified EGRET sources. Supernova remnants in lower density environments generate higher energy cosmic rays that produce predominantly inverse Compton emission at super-TeV energies; such sources will generally be gamma-ray dim at GeV energies.Comment: 62 pages, AASTeX format, including 1 table and 11 figures, accepted for publication in The Astrophysical Journal (Vol 513, March 1, 1999

Fostering SME's co-development of innovative projects in biotech clusters: Extending the sets of enablers for the knowledge creation process

We explore the linkage of specific sets of enablers for the knowledge-creation process (KCP) mobilized in innovative projects co-developed by biotech SMEs and the role of industrial clusters in easing the access to enablers. Two French and one Brazilian high-tech SMEs belonging to two biotechnological clusters were investigated. Deductive and inductive approaches were applied in a complementary manner. Findings revealed a larger set of enablers at the company's level than predicted in the literature. We identified two sets of KCP enablers at the cluster's level, classified as open access and restricted access, which are used for different purposes and simultaneously. These dynamics enabled the studied SMEs to improve their co-developed innovative projects in a more effective way. Data also revealed that the enablers that benefit SMEs are more likely provided by the cluster organisations

Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9}

Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions, we study the effect of iron disproportionation on the magnetic ordering, and describe the first-order magnetic transition occurring in the perovskite Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of the exchange integrals for the antiferromagntic and ferromagnetic interactions, in agreement with previous works on kindered compounds. Spin-wave theory yields a magnon spectrum with a gapless antiferromagnetic mode together with two gapped ferromagnetic ones.Comment: 8 pages of RevTex, 5 figures (available upon request), submitted to J. Mag. Mag. Ma