Interacting many-body systems in quantum wells: Evidence for exciton-trion-electron correlations

We report on the nonlinear optical dynamical properties of excitonic complexes in CdTe modulation-doped quantum wells, due to many-body interactions among excitons, trions and electrons. These were studied by time and spectrally resolved pump-probe experiments. The results reveal that the nonlinearities induced by trions differ from those induced by excitons, and in addition they are mutually correlated. We propose that the main source of these subtle differences comes from the Pauli exclusion-principle through phase-space filling and short-range fermion exchange.Comment: 5 pages, 4 figures. accepted for publications in Phys. Rev.

Mapping of quantum well eigenstates with semimagnetic probes

We present results of transmission measurements on CdTe quantum wells with thin semimagnetic CdMnTe probe layers embedded in various positions along the growth axis. The presence of the probes allow us to map the probability density functions by two independent methods: analyzing the exciton energy position and the exciton Zeeman splitting. We apply both approaches to map the first three quantum well eigenstates and we find that both of them yield equally accurate results.Comment: Accepted for publication in Physical Review

Growth and properties of ferromagnetic In(1-x)Mn(x)Sb alloys

We discuss a new narrow-gap ferromagnetic (FM) semiconductor alloy, In(1-x)Mn(x)Sb, and its growth by low-temperature molecular-beam epitaxy. The magnetic properties were investigated by direct magnetization measurements, electrical transport, magnetic circular dichroism, and the magneto-optical Kerr effect. These data clearly indicate that In(1-x)Mn(x)Sb possesses all the attributes of a system with carrier-mediated FM interactions, including well-defined hysteresis loops, a cusp in the temperature dependence of the resistivity, strong negative magnetoresistance, and a large anomalous Hall effect. The Curie temperatures in samples investigated thus far range up to 8.5 K, which are consistent with a mean-field-theory simulation of the carrier-induced ferromagnetism based on the 8-band effective band-orbital method.Comment: Invited talk at 11th International Conference on Narrow Gap Semiconductors, Buffalo, New York, U.S.A., June 16 - 20, 200

External control of the direction of magnetization in ferromagnetic InMnAs/GaSb heterostructures

In this paper, we demonstrate external control over the magnetization direction in ferromagnetic (FM) In_{1-x}Mn_{x}As/GaSb heterostructures. FM ordering with T_C as high as 50 K is confirmed by SQUID magnetization, anomalous Hall effect (AHE), and magneto-optical Kerr effect (MOKE) measurements. Even though tensile strain is known to favor an easy axis normal to the layer plane, at low temperatures we observe that the magnetization direction in several samples is intermediate between the normal and in-plane axes. As the temperature increases, however, the easy axis rotates to the direction normal to the plane. We further demonstrate that the easy magnetization axis can be controlled by incident light through a bolometric effect, which induces a pronounced increase in the amplitude of the AHE. A mean-field-theory model for the carrier-mediated ferromagnetism reproduces the tendency for dramatic reorientations of the magnetization axis, but not the specific sensitivity to small temperature variations.Comment: 11 pages, 3 figures, submitted to NGS-1

PlatoSim: An end-to-end PLATO camera simulator for modelling high-precision space-based photometry

PLAnetary Transits and Oscillations of stars (PLATO) is the ESA M3 space mission dedicated to detect and characterise transiting exoplanets including information from the asteroseismic properties of their stellar hosts. The uninterrupted and high-precision photometry provided by space-borne instruments such as PLATO require long preparatory phases. An exhaustive list of tests are paramount to design a mission that meets the performance requirements, and as such, simulations are an indispensable tool in the mission preparation. To accommodate PLATO's need of versatile simulations prior to mission launch - that at the same time describe accurately the innovative but complex multi-telescope design - we here present the end-to-end PLATO simulator specifically developed for the purpose, namely PlatoSim. We show step-by-step the algorithms embedded into the software architecture of PlatoSim that allow the user to simulate photometric time series of CCD images and light curves in accordance to the expected observations of PLATO. In the context of the PLATO payload, a general formalism of modelling, end-to-end, incoming photons from the sky to the final measurement in digital units is discussed. We show the strong predictive power of PlatoSim through its diverse applicability and contribution to numerous working groups within the PLATO Mission Consortium. This involves the on-going mechanical integration and alignment, performance studies of the payload, the pipeline development and assessments of the scientific goals. PlatoSim is a state-of-the-art simulator that is able to produce the expected photometric observations of PLATO to a high level of accuracy. We demonstrate that PlatoSim is a key software tool for the PLATO mission in the preparatory phases until mission launch and prospectively beyond.Comment: 30 pages, 22 figure

"Quasi two-dimensional" spin distributions in II-VI magnetic semiconductor heterostructures: Clustering and dimensionality

Spin clustering in diluted magnetic semiconductors (DMS) arises from antiferromagnetic exchange between neighboring magnetic cations and is a strong function of reduced dimensionality. Epitaxially-grown single monolayers and abrupt interfaces of DMS are, however, never perfectly two-dimensional (2D) due to the unavoidable inter-monolayer mixing of atoms during growth. Thus the magnetization of DMS heterostructures, which is strongly modified by spin clustering, is intermediate between that of 2D and 3D spin distributions. We present an exact calculation of spin clustering applicable to arbitrary distributions of magnetic spins in the growth direction. The results reveal a surprising insensitivity of the magnetization to the form of the intermixing profile, and identify important limits on the maximum possible magnetization. High-field optical studies of heterostructures containing "quasi-2D" spin distributions are compared with calculation.Comment: 5 pages (RevTeX), 5 embedded EPS figs, published in PRB v61 p1736 (2000

PlatoSim: an end-to-end PLATO camera simulator for modelling high-precision space-based photometry

Context. PLAnetary Transits and Oscillations of stars (PLATO) is the ESA M3 space mission dedicated to detect and characterise transiting exoplanets including information from the asteroseismic properties of their stellar hosts. The uninterrupted and high-precision photometry provided by space-borne instruments such as PLATO require long preparatory phases. An exhaustive list of tests are paramount to design a mission that meets the performance requirements and, as such, simulations are an indispensable tool in the mission preparation. Aims. To accommodate PLATO’s need of versatile simulations prior to mission launch that at the same time describe innovative yet complex multi-telescope design accurately, in this work we present the end-to-end PLATO simulator specifically developed for that purpose, namely PlatoSim. We show, step-by-step, the algorithms embedded into the software architecture of PlatoSim that allow the user to simulate photometric time series of charge-coupled device (CCD) images and light curves in accordance to the expected observations of PLATO. Methods. In the context of the PLATO payload, a general formalism of modelling, end-to-end, incoming photons from the sky to the final measurement in digital units is discussed. According to the light path through the instrument, we present an overview of the stellar field and sky background, the short- and long-term barycentric pixel displacement of the stellar sources, the cameras and their optics, the modelling of the CCDs and their electronics, and all main random and systematic noise sources. Results. We show the strong predictive power of PlatoSim through its diverse applicability and contribution to numerous working groups within the PLATO mission consortium. This involves the ongoing mechanical integration and alignment, performance studies of the payload, the pipeline development, and assessments of the scientific goals. Conclusions. PlatoSim is a state-of-the-art simulator that is able to produce the expected photometric observations of PLATO to a high level of accuracy. We demonstrate that PlatoSim is a key software tool for the PLATO mission in the preparatory phases until mission launch and prospectively beyond

Badania układu: belka – zmienna masa skupiona modelującego suwnicę

This paper presents the free vibration problem of beam with additionally attached mass element. The considered system can be treated as simplified to twodimensional model of the bridge crane. In this model the displacement of the hoist along the girder is analyzed and the longitudinal motion of the bridge crane does not occur. Based on the Hamilton’s principle, the boundary problem for this system has been formulated. On the basis of numerical simulations the influence of the position of the mass element (representing the hoist and mass of the load) on the free vibration frequencies of the system has been presented. The accepted mathematical model has been verified by the experimental studies.W niniejszej pracy rozważano problem drgań swobodnych belki z dodatkowo zamocowanym elementem masowym. Układ ten może modelować konstrukcję suwnicy pomostowej, przy założeniu pewnych uproszczeń. Sprowadzono model obiektu rzeczywistego do zagadnienia dwuwymiarowego, w którym rozważa się możliwość przemieszczania wciągnika wzdłuż dźwigara, a ruch wzdłużny suwnicy nie występuje. Sformułowano zagadnienie brzegowe odnośnie do drgań układu na podstawie zasady Hamiltona. Przedstawiono symulacje numeryczne określające wpływ położenia elementu masowego (który reprezentuje układ wciągnika wraz z ładunkiem) na częstości drgań własnych układu. Przeprowadzono badania eksperymentalne odnośnie do drgań rozważanego układu, pozwalające na weryfikację przyjętego modelu matematycznego

Graded Quantum Well Structures Made of Diluted Magnetic Semiconductors

In this paper we review results of studies of two types of spatially graded quantum well structures containing various layers of diluted magnetic semiconductors Cd$\text{}_{1-x}$Mn$\text{}_{x}$Te or Cd$\text{}_{1-x-y}$Mn$\text{}_{x}$Mg$\text{}_{y}$Te. The design of the structures has been recently proposed by us and suitable samples have been grown by a modified molecular beam epitaxy method. In the structures of the first type a digital profiling of the composition of the constituent material in the growth direction allowed to produce quantum wells with a specifically required shape of the confining potential (including parabolic, half-parabolic, triangular, and trapezoidal). Such samples were used for (i) determination of the conduction and valence band offsets in MnTe/CdTe and MgTe/CdTe systems, (ii) for the demonstration of an enhanced exciton binding in a parabolic confining potential as well as for (iii) demonstration of the possibility of "spin-splitting engineering" in diluted magnetic semiconductors quantum structures. In the second type of the structures, a precise in-plane profiling of either quantum well width or the barrier width or n-type doping intensity was realized. These structures were subsequently used for studies of the evolution of optical spectra with an increase in the concentration of confined two-dimensional gas of conduction electrons

Nonlinear optical dynamics of excitons and trions

We investigate the nonlinear optical dynamics of excitons and trions in CdTe modulation-doped quantum wells by time and spectrally resolved pump-probe experiments. We find that the nonlinearities induced by excitons are different from those due to the presence of the trions. From a comparison between the exciton and trion behaviours, we show that, at low temperature, trions are formed from excitons in about 10 picoseconds and, as the temperature is increased, a thermal equilibrium is reached between exciton and trion populations within 5 picoseconds