Raman signatures of classical and quantum phases in coupled dots: A theoretical prediction

We study electron molecules in realistic vertically coupled quantum dots in a strong magnetic field. Computing the energy spectrum, pair correlation functions, and dynamical form factor as a function of inter-dot coupling via diagonalization of the many-body Hamiltonian, we identify structural transitions between different phases, some of which do not have a classical counterpart. The calculated Raman cross section shows how such phases can be experimentally singled out.Comment: 9 pages, 2 postscript figures, 1 colour postscript figure, Latex 2e, Europhysics Letters style and epsfig macros. Submitted to Europhysics Letter

A new Manifestation of Atomic Parity Violation in Cesium: a Chiral Optical Gain induced by linearly polarized 6S-7S Excitation

We have detected, by using stimulated emission, an Atomic Parity Violation (APV) in the form of a chiral optical gain of a cesium vapor on the 7S - 6P$_{3/2}$ transition,consecutive to linearly polarized 6S-7S excitation. We demonstrate the validity of this detection method of APV, by presenting a 9% accurate measurement of expected sign and magnitude. We underline several advantages of this entirely new approach in which the cylindrical symmetry of the set-up can be fully exploited. Future measurements at the percent level will provide an important cross-check of an existing more precise result obtained by a different method.Comment: 4 pages, 2 figure

FUNTIMES – Future Navigation and Timing Evolved Signals

International audienceThe European Galileo system moves clear steps forward towards the completion of its space and ground segment infrastructures, after starting providing early services in 2016 and with the plan to achieve the full operational capability (FOC) in 2020. Also the user segment is rapidly expanding, with the increasing introduction of mass market chipsets fully supporting Galileo in a constantly growing number of smartphones. In this context a strong need for R&D activities in the field of navigation signal engineering has been identified by various Programme's stakeholders. Considering the long process required for introducing new signals and features in a system that is already deployed and finds itself in the exploitation phase, early R&D activities become essential to investigate potential evolutions and new concepts to improve the Galileo signals and services in the short, medium and long term. The Future Navigation and Timing Evolved Signals (FUNTIMES) project is a European GNSS mission evolution study funded by the European Commission within the Horizon 2020 Framework for Research and Development. It aims at identifying, studying and recommending mission evolution directions and at preliminary supporting the definition, design and implementation of the future generation of Galileo signals. The project is led by Airbus Defence and Space as prime contractor, supported by Ecole Nationale de l‘Aviation Civile (ENAC) and Istituto Superiore Mario Boella (ISMB) as subcontractors and was run under the supervision of the European Commission and its Joint Research Centre. The research activities were conducted according to the following high level evolution directions: - Improve the Galileo OS reliability by providing an enhanced authentication service based on both navigation message authentication and spreading code authentication, in such a way that the two solutions can take advantage of their combination. - Improve the sensitivity and/or reduce the complexity of the acquisition of the Galileo OS signals, e.g. by studying the potential introduction of a new signal component for this purpose. - Make use of new concepts and techniques for the delivery of the data messages, to improve the time-to-data performance and robustness. - Consider options for providing an effective high data rate component suitable for satellite navigation purposes, e.g. in view of a possible evolution of the signals providing the Galileo Commercial Service. The project started by defining the key elements characterizing GNSS signals, describing the current signal plans of the major global and regional satellite systems and carrying out a literature survey on the various proposals for the evolution and optimization of navigation signals. A key role in the project was then played by a specific task on the definition of signal user requirements, which, besides providing by themselves an added-value to the project outcomes, were taken into account to select and consolidate the R&D topics defined at the beginning of the study. For what concerns the core navigation signal R&D activity, various solutions belonging to the following areas were considered: new and evolved modulations and multiplexing techniques, new concepts and techniques for the data message, solutions providing services with higher reliability, solutions for improved navigation performance. In the followings, some highlights about the main project tasks are provided. *Adding New Signal Components to Galileo E1 OS* Due to backward compatibility constraints, the Galileo legacy signals defined in the current SIS-ICD do not offer much space for further modifications. The possibility to add new signal components to the Galileo E1 signal was investigated with the goals of providing a fast and reliable authentication service and better acquisition performance while keeping the complexity of the acquisition process low. Various options were investigated, considering new components centered at E1 or ones presenting a carrier offset. The options were studied in terms of ranging performance, compatibility with other signals in E1/L1, multiplexing efficiency and backward compatibility. The outcome of this task was then combined with the other solutions investigated during the project and briefly introduced in the followings. *Signal User Requirements Survey* This task aimed at identifying and understanding the current and future needs of various GNSS user groups in order to derive requirements and evolution directions for the Galileo signals. The work logic followed was based on a 3-step approach: - Definition of the user communities - Analysis of available documentation and state-of-the-art for each user communities to extract high level and, if possible, low level requirements - Consultation of representative of the various user communities by means of questionnaire on signal user requirements. The considered user communities are representative of 7 classes of users: - Traditional Safety-of-Life Applications (Navigation of Civil Aviation aircrafts, Train Control) - Automotive Location-Based Charging (LBC) and Vehicle Motion Sensing (VMS) - Mobile Location-Based Services (LBS) - Surveying - Timing and Synchronization - Search and Rescue - Remotely-Piloted Aircraft Systems (RPAS). As mentioned above, the consortium prepared a questionnaire which was distributed to companies and organizations representative of various GNSS user communities. After collecting the answers, personal interviews were conducted to deepen the outcomes of the survey and collect more details about their expectations. From the received answers, the following points were considered particularly relevant for the identification/consolidation of signal evolution directions: - The need for integrity and authentication is present also in non-safety of life applications (e.g. precise positioning) - Very wide-spread need for fast authenticated PVT (fast data and pseudo-range authentication) - Interest in fast Time-To-First-Fix (TTFF) Data, or in other words, fast provision of the Clock error corrections and satellites Ephemeris Data (CED). - Need for precise clock and orbit data, freely accessible through the navigation message transmitted through conventional signals (at L1/E1 or L5/E5) - Importance of stand-alone operation mode despite the increasing number of connected users (network connection still judged not reliable enough). - Need for multipath/NLOS resistant signals - Need for RFI resistant signals - Interest for an alert/emergency service. *Reed-Solomon Codes for the Improvement of the I/NAV Message* Despite the growing number of connected user devices, the reception of the clock and ephemeris data (CED) is still a major factor impacting the TTFF. The current approach for the dissemination of these data can be defined as "data carouseling": the data are repeatedly sent to the users with a certain repetition rate. For example the repetition rate of the CED contained in the Galileo E1 OS message is equal to 1 every 30 s. A different approach is offered by Maximum Distance Separable (MDS) codes like Reed-Solomon codes, whose erasure correction capability allows to retrieve the entire information contained in k data blocks from any combination of k received blocks of the codeword. During the project, the performance of Reed-Solomon codes when applied to the Galileo I/NAV message as proposed in [1] were studied, in terms of Time-to-Data, with extensive simulations in the AWGN and mobile channel. The results were then compared with the legacy implementation and with the performance of the GPS L1C signal and showed a very significant improvement, with a reduction of the Galileo E1 OS TTFF by up to 50% in difficult urban environments. Also received processing scheme and complexity aspects were taken into account in the work. *Spreading Code Authentication Techniques* The increasing awareness concerning the vulnerability of GNSS signals to potential spoofing attacks suggested to dedicate an important part of the project R&D activities to investigating new concepts and ideas to improve the reliability of the provided PNT service. This need was also confirmed by the conducted user requirements survey. The investigation of possible authentication techniques has been carried out on the basis of both quantitative results and qualitative analyses, considering a set of criteria useful to weight the overall performance of different options in realistic scenarios. The methodology used to trade-off different options took into account four main criteria: - the authentication performance, aiming to assess the techniques mainly in terms of Time Between Authentications (TBA) and Time To Alarm (TTA) metrics; - the spoofing robustness, that measures the level of resilience to different specific spoofing attacks; - the implementation readiness, that assesses the level of complexity required both at the system and receiver levels and the backward compatibility; - the legacy signal valorization, with the objective to assess the level of reuse and valorization of today’s signal and messages structures, e.g. considering the current Galileo plans to provide navigation message authentication for his Open Service. When considering authentication solutions, it is important not to focus only on the benefits of future participant users, i.e., those able to exploit the features of the authenticated signals, but also to take into account the possible impact on the existing satellites, ground segment, and other receivers (i.e. non-participant users). Therefore the activities included the assessment of the impact of authentication schemes on user receivers. In detail, the analysis covered the possible degradation of the performance of non-participant users, in terms of C/N0 degradation and impact on acquisition and tracking, and the evaluation of the performance of participant users in relation with the authentication technique parameters. In addition, a novel high-level concept for spreading code authentication, based on the idea of reusing the E1-B OS NMA data, was investigated. The proposed concept, already anticipated in [2], foresees the use of two types of SCA bursts, inserted in the open Pseudo-Random Noise (PRN) code sequence at different rates: - “Slow rate” SCA bursts, which are intended for a robust a-posteriori verification with moderate latency (i.e., TBA of about 10 seconds); - “Fast rate” SCA bursts, potentially suitable to improve the authentication performance (e.g. TBA of about 2 seconds) under a wide set of spoofing attacks. The proposed solution can potentially exploit the information received from all the in-view satellites by means of a two-steps authentication procedure. *CSK Modulation and Channel Codes for a High Data Rate Component* The Code Shift Keying (CSK) modulation is an orthogonal M-ary modulation (M orthogonal symbols are used in order to transmit U =log_2?(M) bits) which was specially designed to increase the bandwidth efficiency of a DS-SS signal, i.e. the bit rate to signal bandwidth ratio, without affecting the PRN code structure. The usage of CSK for the improvement of GNSS data delivery was already investigated in the past (e.g. in [3]). Within the FUNTIMES project the main scope of this task was to prove the expected benefits of this technique by applying it to a number of signal design options, considering various data rates, power distributions between data and pilot components and demodulation strategies at the receiver. The first advantage of CSK is the possibility to increase the bit rate of a DS-SS signal without increasing the PRN code number of bits and without increasing the signal chip rate (and thus signal bandwidth). The increased data rate could be used to increase the number of services provided by the signal and/or to improve the services already available, e.g. by sending correction data. The second benefit is enhanced flexibility of the signal bit rate as the CSK modulation allows to change the number of symbols of the modulation alphabet from one codeword to another one. This allows the GNSS signal to provide more robustness to fundamental data and less robustness to less relevant or optional data since the bit rate is directly relate to the demodulation sensitivity. The third major benefit of a CSK modulation is the possibility of implementing a non-coherent demodulation process that does not require the estimation of the incoming signal carrier phase. Therefore, when in degraded environments and/or for high dynamic users, the PLL cannot be in lock for a certain time, the GNSS receiver could still be able to demodulate the data signal. The results obtained in terms of signal availability and reduced Time-to-First-Fix are very promising and bring a significant improvement when compared with the data delivery performance of today's navigation signals. For what concerns the study of channel codes that could be best suited for high data rate transmission and, especially, in combination with a CSK scheme, the investigation focused on LDPC codes with a bit interleaved coded modulation (BICM/BCIM-ID). As Galileo transmits a navigation signal intended to deliver value-added data in a significant amount (high accuracy service through the E6-B signal), it was decided to study a potential application of the studied CSK schemes to a similar use case. From the results obtained, depending on the C/N0 value considered, an increase of the information bit rate from the current 500 bps up to 5000 bps can be feasible, while still reaching a WER equal to 10-3 for a signal component C/N0 equal to 37 dB-Hz. The project allowed to study new elements in the field of GNSS signal engineering and to consolidate solutions that were already investigated in the recent literature, paving the way to the evolution of the Galileo signal plan but also offering elements and ideas that can be adopted by any other GNSS. The variety of solutions proposed presents different levels of maturity. In some cases the solutions are ready to be implemented in the currently deployed systems, while in other cases they would require a corresponding evolution of the space and ground segments. Where deemed necessary, specific recommendations for future R&D work in the areas studied in the project were provided

Combination of electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy to determine indium concentration in InGaN thin film structures

We demonstrate a method to determine the indium concentration, x, of In x Ga1-x N thin films by combining plasmon excitation studies in electron energy-loss spectroscopy (EELS) with a novel way of quantification of the intensity of x-ray lines in energy-dispersive x-ray spectroscopy (EDXS). The plasmon peak in EELS of InGaN is relatively broad. We fitted a Lorentz function to the main plasmon peak to suppress noise and the influence from the neighboring Ga 3d transition in the spectrum, which improves the precision in the evaluation of the plasmon peak position. As the indium concentration of InGaN is difficult to control during high temperature growth due to partial In desorption, the nominal indium concentrations provided by the growers were not considered reliable. The indium concentration obtained from EDXS quantification using Oxford Instrument ISIS 300 x-ray standard quantification software often did not agree with the nominal indium concentration, and quantification using K and L lines was inconsistent. We therefore developed a self-consistent iterative procedure to determine the In content from thickness-dependent k-factors, as described in recent work submitted to Journal of Microscopy. When the plasmon peak position is plotted versus the indium concentration from EDXS we obtain a linear relationship over the whole compositional range, and the standard error from linear least-squares fitting shows that the indium concentration can be determined from the plasmon peak position to within Δx = ± 0.037 standard deviation

Search for Direct CP Violation in Non-Leptonic Decays of Charged Ξ\Xi and Λ\Lambda Hyperons

A search for direct CP violation in the non-leptonic decays of hyperons has been performed. In comparing the product of the decay parameters, $\alpha_{\Xi}\alpha_{\Lambda}$, in terms of an asymmetry parameter, $A_{\Xi\Lambda}$, between hyperons and anti-hyperons in the charged $\Xi \to \Lambda \pi$ and $\Lambda \to p \pi$ decay sequence, we found no evidence of direct CP violations. The parameter $A_{\Xi\Lambda}$ was measured to be $0.012 \pm 0.014$.Comment: Submitted for publication; RevTex, 13 pages, 4 figure

Neutral weak currents in pion electroproduction on the nucleon

Parity violating asymmetry in inclusive scattering of longitudinally polarized electrons by unpolarized protons with $\pi^0$ or $\pi^+$ meson production, is calculated as a function of the momentum transfer squared $Q^2$ and the total energy $W$ of the $\pi N$-system. This asymmetry, which is induced by the interference of the one-photon exchange amplitude with the parity-odd part of the $Z^0$-exchange amplitude, is calculated for the $\gamma^*(Z^*)+p\to N+\pi$ processes ($\gamma^*$ is a virtual photon and $Z^*$ a virtual Z-boson) considering the $\Delta$-contribution in the $s-$channel, the standard Born contributions and vector meson ($\rho$ and $\omega$) exchanges in the $t-$channel. Taking into account the known isotopic properties of the hadron electromagnetic and neutral currents, we show that the P-odd term is the sum of two contributions. The main term is model independent and it can be calculated exactly in terms of fundamental constants. It is found to be linear in $Q^2$. The second term is a relatively small correction which is determined by the isoscalar component of the electromagnetic current. Near threshold and in the $\Delta$-region, this isoscalar part is much smaller (in absolute value) than the isovector one: its contribution to the asymmetry depend on the polarization state (longitudinal or transverse) of the virtual photon.Comment: 30 pages 9 figure

Measurement of Decay Parameters for Ξ−→Λπ−\Xi^{-} \to \Lambda \pi^{-} Decay

Based on 1.35 million polarized $\Xi^{-}$ events, we measure the parameter $\phi_{\Xi}$ to be $-1.61^{\circ} \pm 2.66^{\circ} \pm 0.37^{\circ}$ for $\Xi^{-} \to \Lambda \pi^{-}$ decay. New results for the parameters $\beta_{\Xi}$ and $\gamma_{\Xi}$ are also presented. Assuming that the CP-violating phase-shift difference is negligible, we deduce the strong phase-shift difference between the P-wave and S-wave amplitudes of the $\Lambda\pi$ final state to be $3.17^{\circ} \pm 5.28^{\circ} \pm 0.73^{\circ}$. This strong phase-shift difference reduces the theoretical uncertainty in estimating the level of CP violation in $\Xi$-hyperon decay.Comment: To be published in Physical Review Letter

Second-harmonic generation from coupled plasmon modes in a single dimer of gold nanospheres

We show that a dimer made of two gold nanospheres exhibits a remarkable efficiency for second-harmonic generation under femtosecond optical excitation. The detectable nonlinear emission for the given particle size and excitation wavelength arises when the two nanoparticles are as close as possible to contact, as in situ controlled and measured using the tip of an atomic force microscope. The excitation wavelength dependence of the second-harmonic signal supports a coupled plasmon resonance origin with radiation from the dimer gap. This nanometer-size light source might be used for high-resolution near-field optical microscopy.Comment: 6 pages, 5 figure

Evidence of relationship between strain and In-incorporation:Growth of N-polar In-rich InAlN buffer layer by OMCVD

International audienc

Low-Energy Parity-Violation and New Physics

The new physics sensitivity of a variety of low-energy parity-violating (PV) observables is analyzed. A comparison is made between atomic PV for a single isotope, atomic PV using isotope ratios, and PV electron-hadron and electron-electron scattering. The complementarity among these observables, as well as with high-energy processes, is emphasized. Theoretical uncertainties entering the interpretation of low-energy measurements are discussed.Comment: 37 pages, 2 figures. Omissions to Tables I and V corrected along with some minor typographical errors. To appear in Phys. Rev.