Nonlinear Optical studies of the Transient Coherence in the Quantum Hall System

We review recent investigations of the femtosecond non-linear optical response of the two-dimensional electron gas (2DEG) in a strong magnetic field. We probe the Quantum Hall (QH) regime for filling factors $\nu \sim 1$. Our focus is on the transient coherence induced via optical excitation and on its time evolution during early femtosecond timescales. We simultaneously study the interband and intraband coherence in this system by using a nonlinear spectroscopic technique, transient three-pulse four wave mixing optical spectroscopy, and a many-body theory. We observe striking differences in the temporal and spectral profile of the nonlinear optical signal between a modulation doped quantum well system (with the 2DEG) and a similar undoped quantum well (without a 2DEG). We attribute these qualitative differences to Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG. We show, in particular, that intraband many-particle coherences assisted by the inter-Landau-level magnetoplasmon excitations of the 2DEG dominate the femtosecond nonlinear optical responce. The most striking effect of these exciton-magnetoplasmon coherences is a large off-resonant four-wave-mixing signal in the case of very low photoexcited carrier densities, not observed in the undoped system, with strong temporal oscillations and unusually symmetric temporal profile.Comment: 22 pages, 9 figures; review article to be published in Solid State Communication

Combining Magnetic and Electric Sails for Interstellar Deceleration

The main benefit of an interstellar mission is to carry out in-situ measurements within a target star system. To allow for extended in-situ measurements, the spacecraft needs to be decelerated. One of the currently most promising technologies for deceleration is the magnetic sail which uses the deflection of interstellar matter via a magnetic field to decelerate the spacecraft. However, while the magnetic sail is very efficient at high velocities, its performance decreases with lower speeds. This leads to deceleration durations of several decades depending on the spacecraft mass. Within the context of Project Dragonfly, initiated by the Initiative of Interstellar Studies (i4is), this paper proposes a novel concept for decelerating a spacecraft on an interstellar mission by combining a magnetic sail with an electric sail. Combining the sails compensates for each technologys shortcomings: A magnetic sail is more effective at higher velocities than the electric sail and vice versa. It is demonstrated that using both sails sequentially outperforms using only the magnetic or electric sail for various mission scenarios and velocity ranges, at a constant total spacecraft mass. For example, for decelerating from 5% c, to interplanetary velocities, a spacecraft with both sails needs about 29 years, whereas the electric sail alone would take 35 years and the magnetic sail about 40 years with a total spacecraft mass of 8250 kg. Furthermore, it is assessed how the combined deceleration system affects the optimal overall mission architecture for different spacecraft masses and cruising speeds. Future work would investigate how operating both systems in parallel instead of sequentially would affect its performance. Moreover, uncertainties in the density of interstellar matter and sail properties need to be explored

Many-body Correlation Effects in the Ultrafast Nonlinear Optical Response of Confined Fermi Seas

The dynamics of electrons and atoms interacting with intense and ultrashort optical pulses presents an important problem in physics that cuts across different materials such as semiconductors and metals. The currently available laser pulses, as short as 5 fs, provide a time resolution shorter than the dephasing and relaxation times in many materials. This allows for a systematic study of many-body effects using nonlinear optical spectroscopy. In this review article, we discuss the role of Coulomb correlations in the ultrafast dynamics of modulation-doped quantum wells and metal nanoparticles. We focus in particular on the manifestations of non-Markovian memory effects induced by strong electron-hole and electron-plasmon correlations.Comment: 107 pages including 15 figures. Review article to appear in Surf. Sci. Report

3G networks in emergency telemedicine - An in-depth evaluation & analysis

The evolution of telecommunications technologies in connection with the robustness and the fidelity these new systems provide, have opened up many new horizons as regards the provision of healthcare and the quality of service from the side of the experts to that of the patients. The purpose of this paper is to evaluate the third generation telecommunications systems that are only recently being deployed in Europe, as well as argue on why a transition from 2G and 2.5G to 3G telecommunications systems could prove to be crucial, especially in relation to emergency telemedicine. The experimental results of the use of these systems are analyzed, the implementation of a tele-consultation unit is presented and their exploitation capabilities are explored

Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion

With a variational three--body calculation we study the role of the interplay between the onsite Coulomb, Hund's rule, and superexchange interactions on the spinwave excitation spectrum of itinerant ferromagnets. We show that correlations between a Fermi sea electron--hole pair and a magnon result in a very pronounced zone boundary softening and strong deviations from the Heisenberg spinwave dispersion. We show that this spin dynamics depends sensitively on the Coulomb and exchange interactions and discuss its possible relevance to experiments in the manganites.Comment: 4 pages, 4 figures, published in Physical Review B as rapid communication

Three--body Correlation Effects on the Spin Dynamics of Double--Exchange Ferromagnets

We present a variational calculation of the spin wave excitation spectrum of double--exchange ferromagnets in different dimensions. Our theory recovers the Random Phase approximation and 1/S expansion results as limiting cases and can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites. In particular, we treat exactly the long range three--body correlations between a Fermi sea electron--hole pair and a magnon excitation and show that they strongly affect the spin dynamics in the parameter range relevant to experiments in the manganites. The manifestations of these correlations are many-fold. We demonstrate that they significantly change the ferromagnetic phase boundary. In addition to a decrease in the magnon stiffness, we obtain an instability of the ferromagnetic state against spin wave excitations close to the Brillouin zone boundary.Within a range of intermediate concentrations, we find a strong softening of the spin wave dispersion as compared to the Heisenberg ferromagnet with the same stiffness, which changes into hardening for other concentrations. We discuss the relevance of these results to experiments in colossal magnetoresistance ferromagnets.Comment: 14 pages, 11 figures, published in Phys. Rev. B (1 figure added, references added

Use of 3G mobile phone links for teleconsultation between a moving ambulance and a hospital base station

The importance of this paper lays in its suggestion: minimise the time for the initial treatment of a patient. As soon as an ambulance receives a patient, a videoconferencing session can be initiated between the moving vehicle and a base station in a hospital where a medical consultant resides. The communication link is implemented using 3G networks. Effectively, the use of such a system can decrease fatality on patients being transferred to A&E, as expert opinion can be obtained straight away. This paper was repeatedly cited in IEEE papers and formed the basis for an EPSRC proposal that was recently submitted