Conduction electrons localized by charged magneto-acceptors A2−^{2-} in GaAs/GaAlAs quantum wells

A variational theory is presented of A$^{1-}$ and A$^{2-}$ centers, i.e. of a negative acceptor ion localizing one and two conduction electrons, respectively, in a GaAs/GaAlAs quantum well in the presence of a magnetic field parallel to the growth direction. A combined effect of the well and magnetic field confines conduction electrons to the proximity of the ion, resulting in discrete repulsive energies above the corresponding Landau levels. The theory is motivated by our experimental magneto-transport results which indicate that, in a heterostructure doped in the GaAs well with Be acceptors, one observes a boil-off effect in which the conduction electrons in the crossed-field configuration are pushed by the Hall electric field from the delocalized Landau states to the localized acceptor states and cease to conduct. A detailed analysis of the transport data shows that, at high magnetic fields, there are almost no conducting electrons left in the sample. It is concluded that one negative acceptor ion localizes up to four conduction electrons.Comment: 8 pages, 5 figure

Spin splitting in open quantum dots

We present results from a theoretical and experimental study of spin-splitting in small open lateral quantum dots (i.e. in the regime when the dot is connected to the reservoirs via leads that support one or more propagating modes). We demonstrate that the magnetoconductance shows a pronounced splitting of the conductance peaks (or dips) which persists over a wide range of magnetic fields (from zero field to the edge-state regime) and is virtually independent of magnetic field. A numerical analysis of the conductance and the dot eigenspectrum indicates that this feature is related to a lifting of the spin degeneracy in the corresponding closed dot associated with the interaction between electrons of opposite spin.Comment: 4 pages, 4 figures 1 misdirected figure reference corrected mismatch between spin-up/spin-down notation in figure 3-4 and discussion corrected, clarifications in text adde

Un estudio sobre indicios perceptivos para el resto en el servicio de tenis

Using a kinematic analysis, perceptual cues were studied through observing the ball toss in tennis serves in order to facilitate the receiver's anticipated response. Eight female professionals (m = 67.13, SD = 29.38 WTA ranking) participated in the study during an official competition. The main result of the multivariate analysis, which involved twelve space and time variables, revealed a relationship between ball tossing and the depth of the serve. The quantitative values obtained were: R = .917 y R2= .840 (F (9, 96) = 56.007, p <.000), with β = 1.040 (p < .001) for the times in which the player released the ball. This specific result underscores the relevance of the concept of "perceptual cues" to achieve faster and more accurate responses in tennis and also in other sports in which it is possible to study the opponent

Temperature dependence of the electron spin g factor in GaAs

The temperature dependence of the electron spin $g$ factor in GaAs is investigated experimentally and theoretically. Experimentally, the $g$ factor was measured using time-resolved Faraday rotation due to Larmor precession of electron spins in the temperature range between 4.5 K and 190 K. The experiment shows an almost linear increase of the $g$ value with the temperature. This result is in good agreement with other measurements based on photoluminescence quantum beats and time-resolved Kerr rotation up to room temperature. The experimental data are described theoretically taking into account a diminishing fundamental energy gap in GaAs due to lattice thermal dilatation and nonparabolicity of the conduction band calculated using a five-level kp model. At higher temperatures electrons populate higher Landau levels and the average $g$ factor is obtained from a summation over many levels. A very good description of the experimental data is obtained indicating that the observed increase of the spin $g$ factor with the temperature is predominantly due to band's nonparabolicity.Comment: 6 pages 4 figure

Spin Hall Effect

The intrinsic spin Hall effect in semiconductors has developed to a remarkably lively and rapidly growing branch of research in the field of semiconductor spintronics. In this article we give a pedagogical overview on both theoretical and experimental accomplishments and challenges. Emphasis is put on the the description of the intrinsic mechanisms of spin Hall transport in III-V zinc-blende semiconductors, and on the effects of dissipation.Comment: 22 pages, minor adjustments, version as publishe

Non-locality of Foldy-Wouthuysen and related transformations for the Dirac equation

Non-localities of Foldy-Wouthuysen and related transformations, which are used to separate positive and negative energy states in the Dirac equation, are investigated. Second moments of functional kernels generated by the transformations are calculated, the transformed functions and their variances are computed. It is shown that all the transformed quantities are smeared in the coordinate space by the amount comparable to the Compton wavelength $\lambda_c=\hbar/mc$.Comment: 7 pages, two figure

Zitterbewegung of electrons and holes in III-V semiconductor quantum wells

The notion of zitterbewegung is a long-standing prediction of relativistic quantum mechanics. Here we extend earlier theoretical studies on this phenomenon for the case of III-V zinc-blende semiconductors which exhibit particularly strong spin-orbit coupling. This property makes nanostructures made of these materials very favorable systems for possible experimental observations of zitterbewegung. Our investigations include electrons in n-doped quantum wells under the influence of both Rashba and Dresselhaus spin-orbit interaction, and also the two-dimensional hole gas. Moreover, we give a detailed anaysis of electron zitterbewegung in quantum wires which appear to be particularly suited for experimentally observing this effect.Comment: 10 pages, 3 figures include

The intrinsic features of the specific heat at half-filled Landau levels of two-dimensional electron systems

The specific heat capacity of a two-dimensional electron gas is derived for two types of the density of states, namely, the Dirac delta function spectrum and that based on a Gaussian function. For the first time, a closed form expression of the specific heat for each case is obtained at half-filling. When the chemical potential is temperature-independent, the temperature is calculated at which the specific heat is a maximum. Here the effects of the broadening of the Landau levels are distinguished from those of the different filling factors. In general, the results derived herein hold for any thermodynamic system having similar resonant states.Comment: 11 pages, 1 figure, to appear in J Low Temp Phys (2010

Hydroacoustic forcing function modeling using DNS database

A wall pressure frequency spectrum model (Blake 1971 ) has been evaluated using databases from Direct Numerical Simulations (DNS) of a turbulent boundary layer (Na & Moin 1996). Good agreement is found for moderate to strong adverse pressure gradient flows in the absence of separation. In the separated flow region, the model underpredicts the directly calculated spectra by an order of magnitude. The discrepancy is attributed to the violation of the model assumptions in that part of the flow domain. DNS computed coherence length scales and the normalized wall pressure cross-spectra are compared with experimental data. The DNS results are consistent with experimental observations