Photo-excited semiconductor superlattices as constrained excitable media: Motion of dipole domains and current self-oscillations

A model for charge transport in undoped, photo-excited semiconductor superlattices, which includes the dependence of the electron-hole recombination on the electric field and on the photo-excitation intensity through the field-dependent recombination coefficient, is proposed and analyzed. Under dc voltage bias and high photo-excitation intensities, there appear self-sustained oscillations of the current due to a repeated homogeneous nucleation of a number of charge dipole waves inside the superlattice. In contrast to the case of a constant recombination coefficient, nucleated dipole waves can split for a field-dependent recombination coefficient in two oppositely moving dipoles. The key for understanding these unusual properties is that these superlattices have a unique static electric-field domain. At the same time, their dynamical behavior is akin to the one of an extended excitable system: an appropriate finite disturbance of the unique stable fixed point may cause a large excursion in phase space before returning to the stable state and trigger pulses and wave trains. The voltage bias constraint causes new waves to be nucleated when old ones reach the contact.Comment: 19 pages, 8 figures, to appear in Phys. Rev.

Chaos in resonant-tunneling superlattices

Spatio-temporal chaos is predicted to occur in n-doped semiconductor superlattices with sequential resonant tunneling as their main charge transport mechanism. Under dc voltage bias, undamped time-dependent oscillations of the current (due to the motion and recycling of electric field domain walls) have been observed in recent experiments. Chaos is the result of forcing this natural oscillation by means of an appropriate external microwave signal.Comment: 3 pages, LaTex, RevTex, 3 uuencoded figures (1.2M) are available upon request from [email protected], to appear in Phys.Rev.

Disordered eating attitudes and behaviors : comparing lesbian, gay, bisexual, transgender and queer women to women who identify as straight

This study used a quantitative, fixed-method research design to explore the trends of eating disordered attitudes and behaviors in women. Specifically, it investigated whether there were differences in prevalence of eating disordered attitudes and behaviors between heterosexual women and women who identify as Lesbian, Gay, Bisexual, Transgender or Queer. Participants were found using snowball sampling. 556 women participated in the research by completing an online survey. The survey instrument they were then asked to complete was EAT-26, used to asses if a person may be at high risk for an eating disorder. The ages of the participants ranged from 18-83 years old. The mean age of participants was 31.63. There were 14 women who self -identified as Transgender. 338 women identified as heterosexual (60.8%), 61 women identified as Lesbian, 6 women identified as Gay, 73 identified as Bisexual, 62 identified as Queer and 16 identified as other. The results of this study show that there were no significant differences in disordered eating between women who identified as heterosexual and women who identified as homosexual. However, differences were found among homosexual women based on a more detailed breakdown of their sexual identity. A higher percent of women who identified as lesbian were at risk for an eating disorder (23%) than those who identified as bisexual (8.2%) queer (6.2%) or gay (0%). This study also looked at eating disorder rates for transgendered women, and found the percent of transgendered women with disordered eating (14.3%) fell within national averages (Gordon, 1990)

American Legion v. American Humanist Association

The separation of church and state is a key element of American democracy, but its interpretation has been challenged as the country grows more diverse. In American Legion v. American Humanist Association, the Supreme Court adopted a new standard to analyze whether a religious symbol on public land maintained by public funding violated the Constitution’s Establishment Clause

FMC Corp. v. Shoshone-Bannock Tribes

In 1998, FMC Corporation agreed to submit to the Shoshone-Bannock Tribes’ permitting processes, including the payment of fees, for clean-up work required as part of consent decree negotiations with the Environmental Protection Agency. Then, in 2002, FMC refused to pay the Tribes under a permitting agreement entered into by both parties, even though the company continued to store hazardous waste on land within the Shoshone-Bannock Fort Hall Reservation in Idaho. FMC challenged the Tribes’ authority to enforce the $1.5 million permitting fees first in tribal court and later challenged the Tribes’ authority to exercise civil regulatory and adjudicatory jurisdiction over the non-Indian corporation in federal court. FMC Corp. v. Shoshone-Bannock Tribes demonstrates the complexities and fraught nature of tribal civil jurisdiction

Chaotic motion of space charge wavefronts in semiconductors under time-independent voltage bias

A standard drift-diffusion model of space charge wave propagation in semiconductors has been studied numerically and analytically under dc voltage bias. For sufficiently long samples, appropriate contact resistivity and applied voltage - such that the sample is biased in a regime of negative differential resistance - we find chaos in the propagation of nonlinear fronts (charge monopoles of alternating sign) of electric field. The chaos is always low-dimensional, but has a complex spatial structure; this behavior can be interpreted using a finite dimensional asymptotic model in which the front (charge monopole) positions and the electrical current are the only dynamical variables.Comment: 12 pages, 8 figure

Synchronization in populations of globally coupled oscillators with inertial effects

A model for synchronization of globally coupled phase oscillators including ``inertial'' effects is analyzed. In such a model, both oscillator frequencies and phases evolve in time. Stationary solutions include incoherent (unsynchronized) and synchronized states of the oscillator population. Assuming a Lorentzian distribution of oscillator natural frequencies, $g(\Omega)$, both larger inertia or larger frequency spread stabilize the incoherent solution, thereby making harder to synchronize the population. In the limiting case $g(\Omega)=\delta(\Omega)$, the critical coupling becomes independent of inertia. A richer phenomenology is found for bimodal distributions. For instance, inertial effects may destabilize incoherence, giving rise to bifurcating synchronized standing wave states. Inertia tends to harden the bifurcation from incoherence to synchronized states: at zero inertia, this bifurcation is supercritical (soft), but it tends to become subcritical (hard) as inertia increases. Nonlinear stability is investigated in the limit of high natural frequencies.Comment: Revtex, 36 pages, submit to Phys. Rev.

Magnetoswitching of current oscillations in diluted magnetic semiconductor nanostructures

Strongly nonlinear transport through Diluted Magnetic Semiconductor multiquantum wells occurs due to the interplay between confinement, Coulomb and exchange interaction. Nonlinear effects include the appearance of spin polarized stationary states and self-sustained current oscillations as possible stable states of the nanostructure, depending on its configuration and control parameters such as voltage bias and level splitting due to an external magnetic field. Oscillatory regions grow in size with well number and level splitting. A systematic analysis of the charge and spin response to voltage and magnetic field switching of II-VI Diluted Magnetic Semiconductor multiquantum wells is carried out. The description of stationary and time-periodic spin polarized states, the transitions between them and the responses to voltage or magnetic field switching have great importance due to the potential implementation of spintronic devices based on these nanostructures.Comment: 14 pages, 4 figures, Revtex, to appear in PR

Dynamics of Electric Field Domains and Oscillations of the Photocurrent in a Simple Superlattice Model

A discrete model is introduced to account for the time-periodic oscillations of the photocurrent in a superlattice observed by Kwok et al, in an undoped 40 period AlAs/GaAs superlattice. Basic ingredients are an effective negative differential resistance due to the sequential resonant tunneling of the photoexcited carriers through the potential barriers, and a rate equation for the holes that incorporates photogeneration and recombination. The photoexciting laser acts as a damping factor ending the oscillations when its power is large enough. The model explains: (i) the known oscillatory static I-V characteristic curve through the formation of a domain wall connecting high and low electric field domains, and (ii) the photocurrent and photoluminescence time-dependent oscillations after the domain wall is formed. In our model, they arise from the combined motion of the wall and the shift of the values of the electric field at the domains. Up to a certain value of the photoexcitation, the non-uniform field profile with two domains turns out to be metastable: after the photocurrent oscillations have ceased, the field profile slowly relaxes toward the uniform stationary solution (which is reached on a much longer time scale). Multiple stability of stationary states and hysteresis are also found. An interpretation of the oscillations in the photoluminescence spectrum is also given.Comment: 34 pages, REVTeX 3.0, 10 figures upon request, MA/UC3M/07/9