Gravitational Instabilities In A Protoplanetary Disk Including The Effects Of Magnetic-Fields

We investigate the gravitational instability of a thin, Keplerian protoplanetary disk including the effects of a largely azimuthal magnetic field. The model follows that of our previous work (Noh, Vishniac, & Cochran 1991) except for the inclusion of a magnetic field. The disk is assumed to consist of neutral and ionized gas and neutral dust which are coupled by gravity and friction. The growth rates and eigenfunctions are calculated numerically using nonaxisymmetric linear perturbation methods. The results show that the growth rate has a maximum at some intermediate azimuthal number m, but for each value of m it is reduced relative to the unmagnetized case. The effects of the magnetic field appear more strongly on small scales. As the strength of the equilibrium magnetic field increases the growth rates decrease, and the maximum instability occurs at a lower value of m due to the increasing magnetic pressure. The response of each component to the magnetic field is discussed using the behavior of the eigenfunctions in the radial direction. With the inclusion of the magnetic field, the effects of the ionization fraction and friction on the growth rates also appear to be important for high m modes. Increasing the ionization fraction or the friction suppresses instability, but only slightly changes the maximally unstable azimuthal scales. The enhanced growth rates due to a dust component for which thermal pressure is negligible are somewhat reduced by the inclusion of a magnetic field. The effects of different boundary conditions (reflecting and transmitting) on the growth rates are also shown.NASA NAGW 2418Astronom

Defect Formation and Kinetics of Atomic Terrace Merging

Pairs of atomic scale terraces on a single crystal metal surface can be made to merge controllably under suitable conditions to yield steps of double height and width. We study the effect of various physical parameters on the formation of defects in a kinetic model of step doubling. We treat this manifestly non- equilibrium problem by mapping the model onto a 1-D random sequential adsorption problem and solving this analytically. We also do simulations to check the validity of our treatment. We find that our treatment effectively captures the dynamic evolution and the final state of the surface morphology. We show that the number and nature of the defects formed is controlled by a single dimensionless parameter $q$. For $q$ close to one we show that the fraction of defects rises linearly with $\epsilon \equiv 1-q$ as $0.284 \times \epsilon$. We also show that one can arrive at the final state faster and with fewer defects by changing the parameter with time.Comment: 17 pages, 8 figures. To be submitted to Phys. Rev.

Coulomb Drag near the metal-insulator transition in two-dimensions

We studied the drag resistivity between dilute two-dimensional hole systems, near the apparent metal-insulator transition. We find the deviations from the $T^{2}$ dependence of the drag to be independent of layer spacing and correlated with the metalliclike behavior in the single layer resistivity, suggesting they both arise from the same origin. In addition, layer spacing dependence measurements suggest that while the screening properties of the system remain relatively independent of temperature, they weaken significantly as the carrier density is reduced. Finally, we demonstrate that the drag itself significantly enhances the metallic $T$ dependence in the single layer resistivity.Comment: 6 pages, 5 figures; revisions to text, to appear in Phys. Rev.

Orbital-selective Mass Enhancements in Multi-band Ca2−x_{2-x}Srx_{x}RuO4_{4} Systems Analyzed by the Extended Drude Model

We investigated optical spectra of quasi-two-dimensional multi-band Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ systems. The extended Drude model analysis on the ab-plane optical conductivity spectra indicates that the effective mass should be enhanced near $x=0.5$. Based on the sum rule argument, we showed that the orbital-selective Mott-gap opening for the $d_{yz/zx}$ bands, the widely investigated picture, could not be the origin of the mass enhancement. We exploited the multi-band effects in the extended Drude model analysis, and demonstrated that the intriguing heavy mass state near $x=0.5$ should come from the renormalization of the $d_{xy}$ band.Comment: 4 figure

An Electronic Mach-Zehnder Quantum Eraser

We propose an electronic quantum eraser in which the electrons are injected into a mesoscopic conductor at the quantum Hall regime. The conductor is composed of a two-path interferometer which is an electronic analogue of the optical Mach-Zehnder interferometer, and a quantum point contact detector capacitively coupled to the interferometer. While the interference of the output current at the interferometer is shown to be suppressed by the which-path information, we show that the which-path information is erased by the zero-frequency cross correlation measurement between the interferometer and the detector output leads. We also investigate a modified setup in which the detector is replaced by a two-path interferometer.We show that the path distinguishability and the visibility of the joint detection can be controlled in a continuous manner, and satisfy a complementarity relation for the entangled electrons.Comment: 5 pages, 2 figure

Predictability of reset switching voltages in unipolar resistance switching

In unipolar resistance switching of NiO capacitors, Joule heating in the conducting channels should cause a strong nonlinearity in the low resistance state current-voltage (I-V) curves. Due to the percolating nature of the conducting channels, the reset current IR, can be scaled to the nonlinear coefficient Bo of the I-V curves. This scaling relationship can be used to predict reset voltages, independent of NiO capacitor size; it can also be applied to TiO2 and FeOy capacitors. Using this relation, we developed an error correction scheme to provide a clear window for separating reset and set voltages in memory operations

Observation of inhomogeneous domain nucleation in epitaxial Pb(Zr,Ti)O3 capacitors

We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O3 capacitors under a modified piezoresponse force microscope. We obtained domain evolution images during polarization switching process and observed that domain nucleation occurs at particular sites. This inhomogeneous nucleation process should play an important role in an early stage of switching and under a high electric field. We found that the number of nuclei is linearly proportional to log(switching time), suggesting a broad distribution of activation energies for nucleation. The nucleation sites for a positive bias differ from those for a negative bias, indicating that most nucleation sites are located at ferroelectric/electrode interfaces