Magnetic Percolation and the Phase Diagram of the Disordered RKKY model

We consider ferromagnetism in spatially randomly located magnetic moments, as in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect exchange RKKY interaction. We obtain via Monte Carlo the magnetic phase diagram as a function of the impurity moment density $n_{i}$ and the relative carrier concentration $n_{c}/n_{i}$. As evidenced by the diverging correlation length and magnetic susceptibility, the boundary between ferromagnetic (FM) and non-ferromagnetic (NF) phases constitutes a line of zero temperature critical points which can be viewed as a magnetic percolation transition. In the dilute limit, we find that bulk ferromagnetism vanishes for $n_{c}/n_{i}>.1$. We also incorporate the local antiferromagnetic direct superexchange interaction between nearest neighbor impurities, and examine the impact of a damping factor in the RKKY range function.Comment: 5 pages, 3 figures; figure formatting modified, typos fixe

Critical behavior of diluted magnetic semiconductors: the apparent violation and the eventual restoration of the Harris criterion for all regimes of disorder

Using large-scale Monte Carlo calculations, we consider strongly disordered Heisenberg models on a cubic lattice with missing sites (as in diluted magnetic semiconductors such as Ga_{1-x}Mn_{x}As). For disorder ranging from weak to strong levels of dilution, we identify Curie temperatures and calculate the critical exponents nu, gamma, eta, and beta finding, per the Harris criterion, good agreement with critical indices for the pure Heisenberg model where there is no disorder component. Moreover, we find that thermodynamic quantities (e.g. the second moment of the magnetization per spin) self average at the ferromagnetic transition temperature with relative fluctuations tending to zero with increasing system size. We directly calculate effective critical exponents for T > T_{c}, yielding values which may differ significantly from the critical indices for the pure system, especially in the presence of strong disorder. Ultimately, the difference is only apparent, and eventually disappears when T is very close to T_{c}.Comment: 11 pages, 9 figure

Video Microscopy of Selective Laser Sintering

This paper presents the design and implementation of a video microscopy system that enables real time observation and archival of selective laser sintering of polymer and metal materials. The design objectives and selection of system components are discussed in the first section of this paper. Experimental results from preliminary experiments conducted on polycarbonate, wax and nylon powders are also presented.Mechanical Engineerin

Boundary Conditions in Stepwise Sine-Gordon Equation and Multi-Soliton Solutions

We study the stepwise sine-Gordon equation, in which the system parameter is different for positive and negative values of the scalar field. By applying appropriate boundary conditions, we derive relations between the soliton velocities before and after collisions. We investigate the possibility of formation of heavy soliton pairs from light ones and vise versa. The concept of soliton gun is introduced for the first time; a light pair is produced moving with high velocity, after the annihilation of a bound, heavy pair. We also apply boundary conditions to static, periodic and quasi-periodic solutions.Comment: 14 pages, 8 figure

Plasmons in coupled bilayer structures

We calculate the collective charge density excitation dispersion and spectral weight in bilayer semiconductor structures {\it including effects of interlayer tunneling}. The out-of-phase plasmon mode (the ``acoustic'' plasmon) develops a long wavelength gap in the presence of tunneling with the gap being proportional to the square root (linear power) of the tunneling amplitude in the weak (strong) tunneling limit. The in-phase plasmon mode is qualitatively unaffected by tunneling. The predicted plasmon gap should be a useful tool for studying many-body effects.Comment: 10 pages, 6 figures. to appear in Phys. Rev. Let

Magnetic Hydro-Dynamic Propulsion of Blood

The problem with current Ventricular Assist Devices is that the pump design imposes unnatural behavior to the flow of the blood such as stagnation and impingement regions due to the many moving parts, thus promoting the formation of blood clots. To solve this problem, a magneto hydromagnetic drive or MHD can replace the pump in the flow loop. MHD takes advantage of Lorenz’s force, which states that if a magnetic field is perpendicular to an electric field, a particle in the conducting fluid will experience a force orthogonal to both the magnetic and electric field thus to propelling the blood. The result is a unrestricted flow propelled by a electromagnetic force. The first step is to design and build a simple, low cost, and effective electromagnetic flow sensor that is easy to operate and integrates well with a bench-top flow loop. This loop is made up of half inch vinyl tubing, a voltage pump, and a commercial electromagnetic flow sensor. The next step is to use the knowledge from the flow sensor to develop a magnetohydrodynamic drive. The flow sensor design and operation are based on the magnetic flow sensing principle which is built upon Faraday’s Law. This law states that the voltage induced across any conductor as it moves orthogonally through a magnetic field is proportional to the velocity of that conductor. When applying this to a flow sensor, the resulting principle means that any conducting fluid flowing through a magnetic field will induce a voltage which can be read by two electrodes placed orthogonal to the magnetic field. Essentially, the job of the flow sensor is to read these voltage changes that are proportional to and arise from the changes in flow speed. After some post processing of the voltage readings the flow rate can be determined. To reduce the error in the system, the polarity of the magnetic field is changed at a certain rate. This changing field will produce two outputs for the readings of the induced voltages which can be subtracted to cancel out any external readings. Those external readings when taken out of the system will result in a proper flow rate value. After completing the flow sensor, the MHD design will be finalized using a 3D model finite element analysis done in COMSOL Multiphysics

Radiation from a class of string theoretic black holes

The emission of a scalar with low energy $\omega$, from a $D (4\le D\le 8 )$ dimensional black hole with n charges is studied in both string and semiclassical calculations. In the lowest order in $\omega$, the weak coupling string and semiclassical calculations agree provided that the Bekenstein--Hawking formula is valid and the effective central charge $c_{eff}=6$ for any D. When the next order in $\omega$ is considered however, there is no agreement between the two schemes unless D=5, n=3 or D=4, n=4.Comment: 20 pages, Late

Two endosomal NHX-type Na+/H+ antiporters are involved in auxin-mediated development in Arabidopsis thaliana

In Arabidopsis thaliana, the endosomal-localized Na+/H+ antiporters NHX5 and NHX6 regulate ion and pH homeostasis and are important for plant growth and development. However, the mechanism by which these endosomal NHXs function in plant development is not well understood. Auxin modulates plant growth and development through the formation of concentration gradients in plant tissue to control cell division and expansion. Here, we identified a role for NHX5 and NHX6 in the establishment and maintenance of auxin gradients in embryo and root tissues. We observed developmental impairment and abnormal cell division in embryo and root tissues in the double knockout nhx5 nhx6, consistent with these tissues showing high expression of NHX5 and NHX6. Through confocal microscopy imaging with the DR5:: GFP auxin reporter, we identify defects in the perception, accumulation and redistribution of auxin in nhx5 nhx6 cells. Furthermore, we find that the steady-state levels of the PIN-FORMED (PIN) auxin efflux carriers PIN1 and PIN2 are reduced in nhx5 nhx6 root cells. Our results demonstrate that NHX5 and NHX6 function in auxin-mediated plant development by maintaining PIN abundance at the plasma membrane, and provide new insight into the regulation of plant development by endosomal NHX antiporters