Excitons and charged excitons in semiconductor quantum wells

A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width.Comment: 9 pages, 11 figure

Magnetic field dependence of the energy of negatively charged excitons in semiconductor quantum wells

A variational calculation of the spin-singlet and spin-triplet state of a negatively charged exciton (trion) confined to a single quantum well and in the presence of a perpendicular magnetic field is presented. We calculated the probability density and the pair correlation function of the singlet and triplet trion states. The dependence of the energy levels and of the binding energy on the well width and on the magnetic field strength was investigated. We compared our results with the available experimental data on GaAs/AlGaAs quantum wells and find that in the low magnetic field region (B<18 T) the observed transition are those of the singlet and the dark triplet trion (with angular momentum $L_z=-1$), while for high magnetic fields (B>25 T) the dark trion becomes optically inactive and possibly a transition to a bright triplet trion (angular momentum $L_z=0$) state is observed.Comment: 9 pages, 10 figures submitted to Phys. Rev.

Effect of fermionic components on trion-electron scattering

To test the validity of replacing a composite fermion by an elementary fermion, we here calculate the transition rate from a state made of one free electron and one trion to a similar electron-trion pair, through the time evolution of such a pair induced by Coulomb interaction between elementary fermions. To do it in a convenient way, we describe the trion as one electron interacting with one exciton, and we use the tools we have developed in the new composite-exciton many-body theory. The trion-electron scattering contains a direct channel in which ``in'' and ``out'' trions are made with the same fermions, and an exchange channel in which the ``in'' free electron becomes one of the ``out'' trion components. As expected, momenta are conserved in these two channels. The direct scattering is found to read as the bare Coulomb potential between elementary particles multiplied by a form factor which depends on the ``in'' and ``out'' trion relative motion indices $\eta$ and $\eta'$, this factor reducing to $\delta_{\eta\eta'}$ in the zero momentum transfer limit: In this direct channel, the trion at large distance reacts as an elementary particle, its composite nature showing up for large momentum transfer. On the contrary, the fact that the trion is not elementary does affect the exchange channel for all momentum transfers. We thus conclude that a 3-component fermion behaves as an elementary fermion for direct processes in the small momentum transfer limit only

The Late Cretaceous to recent tectonic history of the Pacific Ocean basin

A vast ocean basin has spanned the region between the Americas, Asia and Australasia for well over 100 Myr, represented today by the Pacific Ocean. Its evolution includes a number of plate fragmentation and plate capture events, such as the formation of the Vancouver, Nazca, and Cocos plates from the break-up of the Farallon plate, and the incorporation of the Bellingshausen, Kula, and Aluk (Phoenix) plates, which have been studied individually, but never been synthesised into one coherent model of ocean basin evolution. Previous regional tectonic models of the Pacific typically restrict their scope to either the North or South Pacific, and global kinematic models fail to incorporate some of the complexities in the Pacific plate evolution (e.g. the independent motion of the Bellingshausen and Aluk plates), thereby limiting their usefulness for understanding tectonic events and processes occurring in the Pacific Ocean perimeter. We derive relative plate motions (with 95% uncertainties) for the Pacific-Farallon/Vancouver, Kula-Pacific, Bellingshausen-Pacific, and early Pacific-West Antarctic spreading systems, based on recent data including marine gravity anomalies, well-constrained fracture zone traces and a large compilation of magnetic anomaly identifications. We find our well-constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the North and South Pacific. In conjunction with recently published and well-constrained relative plate motions for other Pacific spreading systems (e.g. Aluk-West Antarctic, Pacific-Cocos, recent Pacific-West Antarctic spreading), we explore variations in the age of the oceanic crust, seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern Pacific. Asymmetries in crustal accretion within the overall Pacific basin (where both flanks of the spreading system are preserved) have typically deviated less than 5% from symmetry, and large variations in crustal accretion along the southern East Pacific Rise (i.e. Pacific-Nazca/Farallon spreading) appear to be unique to this spreading corridor. Through a relative plate motion circuit, we explore the implied convergence history along the North and South Americas, where we find that the inclusion of small tectonic plate fragments such as the Aluk plate are critical for reconciling the history of convergence with onshore geological evidence. © 2015 Elsevier B.V.Australian Research Council, M.S.I. Foundatio

The Rodriguez Triple Junction (Indian Ocean): Structure and evolution for the past one million years

The Rodriguez Triple Junction (RTJ) corresponds to the junction of the 3 Indian Ocean spreading ridges. A detailed survey of an area of 90 km by 85 km, centered at 25 degree 30 degree S and 70 degree E, allows detailed mapping (at a scale of 1/100,000) of the bathymetry (Seabeam) and the magnetic anomalies. The Southeast Indian Ridge, close to the triple junction, is a typical intermediate spreading rate ridge (2.99 cm/a half rate) trending N140 degree . The Central Indian Ridge rift valley prolongs the Southeast Indian Ridge rift valley with a slight change of orientation (12 degree ). The half spreading rate and trend of this ridge are 2.73 cm/a and N152 degree respectively. A model is proposed to explain the evolution of the unstable RRF Rodriguez Triple Junction

Etude geophysique des dorsales de l'ocean Indien dans la region du point triple de Rodriguez

The Indian Ocean is dominated by as system of 3 mid-ocean ridges converging at the Rodriguez Triple Junction located near 25 degree 30'S and 70 degree E. Numerous studies carried out on these 3 ridges provided the basic information on spreading rates and spreading directions. In 1984 the R/V Jean Charcot conducted a detailed survey over the Rodriguez Triple Junction and over an axial ridge segment located on each of the 3 associated ridges. The ridge axes are cut by distinct rift valleys; the depth of these rift valleys is a function of spreading rates. The rift valleys are not in isostatic equilibrium, but show clear density and mass deficits (0.4 to 0.8 g.cm super(-3) for spreading rate of 1 cm/a and 0.1 to 0.3 g.cm super(-3) for spreading rates of 3 cm/a). The Rodriguez Triple Junction is unstable. The axis of the Central Indian Ridge is offset from the axis of the Southeast Indian Ridge at a velocity of 0.14 cm/a

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Structure and tectonic history of the southern Kerguelen Plateau (Indian Ocean) deduced from seismic reflection data

Early single-channel and recent multichannel seismic reflection data together reveal major tectonic events in the history of the Southern Kerguelen Plateau (SKP). The SKP was created by Early Cretaceous volcanism which was mainly subaerial. We estimate the age of the adjacent ocean basins to be similar to that of the plateau. Following termination of the main volcanism, the SKP began to subside and to accumulate sediment. A lower sedimentary megasequence was deposited at this time, primarily in depressions on the plateau. At about 88 Ma, tectonism affected the eastern Raggatt Basin; this tectonism may be related to faulting which created the steep eastern margin of the SKP. At about 72 Ma, predominant tectonic activity, characterized by widespread extension and uplift forming the tilted block morphology, occurred over large areas of the SKP. The extension centered along several NW-SE trending rift systems, in places creating well-preserved axial rifts. The largest structure of this kind, the Central SKP Uplift, lies along the center of the SKP; its relief increases from the southernmost part of the plateau toward the NW. The northernmost part of this uplift, the Banzare Bank, was elevated during rifting above sea level and was eroded. A second extensional structure, the SW Uplift, lies in the SW corner of the plateau, and other, smaller structures may also be present. The extension appears to have culminated in the initiation of the Southeast Indian Ridge at 43 Ma, but, at least in the Raggatt Basin, the two events were not continuous. Tectonic subsidence that was associated with the extension corresponds to deposition of a second megasequence, estimated to be about 1000 m thick on Banzare Bank. The subparallel trend of the rift systems on the SKP to the Southeast Indian Ridge and their timing suggest that the rearrangement of spreading in the South Indian Ocean at 43 Ma was not solely the result of the collision of India with Asia; it started earlier in association with other plate motions in the area