105 research outputs found
Modulation of the high mobility two-dimensional electrons in Si/SiGe using atomic-layer-deposited gate dielectric
Metal-oxide-semiconductor field-effect transistors (MOSFET's) using
atomic-layer-deposited (ALD) AlO as the gate dielectric are fabricated
on the Si/SiGe heterostructures. The low-temperature carrier
density of a two-dimensional electron system (2DES) in the strained Si quantum
well can be controllably tuned from 2.5cm to
4.5cm, virtually without any gate leakage current.
Magnetotransport data show the homogeneous depletion of 2DES under gate biases.
The characteristic of vertical modulation using ALD dielectric is shown to be
better than that using Schottky barrier or the SiO dielectric formed by
plasma-enhanced chemical-vapor-deposition(PECVD).Comment: 3 pages Revtex4, 4 figure
Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars
Dynamical instabilities in protoneutron stars may produce gravitational waves
whose observation could shed light on the physics of core-collapse supernovae.
When born with sufficient differential rotation, these stars are susceptible to
a shear instability (the "low-T/|W| instability"), but such rotation can also
amplify magnetic fields to strengths where they have a considerable impact on
the dynamics of the stellar matter. Using a new magnetohydrodynamics module for
the Spectral Einstein Code, we have simulated a differentially-rotating neutron
star in full 3D to study the effects of magnetic fields on this instability.
Though strong toroidal fields were predicted to suppress the low-T/|W|
instability, we find that they do so only in a small range of field strengths.
Below 4e13 G, poloidal seed fields do not wind up fast enough to have an effect
before the instability saturates, while above 5e14 G, magnetic instabilities
can actually amplify a global quadrupole mode (this threshold may be even lower
in reality, as small-scale magnetic instabilities remain difficult to resolve
numerically). Thus, the prospects for observing gravitational waves from such
systems are not in fact diminished over most of the magnetic parameter space.
Additionally, we report that the detailed development of the low-T/|W|
instability, including its growth rate, depends strongly on the particular
numerical methods used. The high-order methods we employ suggest that growth
might be considerably slower than found in some previous simulations.Comment: REVTeX 4.1, 21 pages, 18 figures, submitting to Physical Review
Binary Neutron Stars with Arbitrary Spins in Numerical Relativity
We present a code to construct initial data for binary neutron star systems
in which the stars are rotating. Our code, based on a formalism developed by
Tichy, allows for arbitrary rotation axes of the neutron stars and is able to
achieve rotation rates near rotational breakup. We compute the neutron star
angular momentum through quasi-local angular momentum integrals. When
constructing irrotational binary neutron stars, we find a very small residual
dimensionless spin of . Evolutions of rotating neutron
star binaries show that the magnitude of the stars' angular momentum is
conserved, and that the spin- and orbit-precession of the stars is well
described by post-Newtonian approximation. We demonstrate that orbital
eccentricity of the binary neutron stars can be controlled to . The
neutron stars show quasi-normal mode oscillations at an amplitude which
increases with the rotation rate of the stars.Comment: 20 pages, 22 figure
Gravitational waveforms for neutron star binaries from binary black hole simulations
Gravitational waves from binary neutron star (BNS) and black-hole/neutron star (BHNS) inspirals are primary sources for detection by the Advanced Laser Interferometer Gravitational-Wave Observatory. The tidal forces acting on the neutron stars induce changes in the phase evolution of
the gravitational waveform, and these changes can be used to constrain the nuclear equation of state. Current methods of generating BNS and BHNS waveforms rely on either computationally challenging full 3D hydrodynamical simulations or approximate analytic solutions. We introduce a new method for computing inspiral waveforms for BNS/BHNS systems by adding the post-Newtonian (PN) tidal effects to full numerical simulations of binary black holes (BBHs), effectively replacing the non-tidal terms in the PN expansion with BBH results. Comparing a waveform generated with this method against a full hydrodynamical simulation of a BNS inspiral yields a phase difference of < 1 radian over ~ 15 orbits. The numerical phase accuracy required of BNS simulations to measure the accuracy of the method we present here is estimated as a function of the tidal deformability parameter â‹‹
Black Hole-Neutron Star Mergers with a Hot Nuclear Equation of State: Outflow and Neutrino-cooled Disk for a Low-mass, High-spin Case
Neutrino emission significantly affects the evolution of the accretion tori formed in black hole-neutron star mergers. It removes energy from the disk, alters its composition, and provides a potential power source for a gamma-ray burst. To study these effects, simulations in general relativity with a hot microphysical equation of state (EOS) and neutrino feedback are needed. We present the first such simulation, using a neutrino leakage scheme for cooling to capture the most essential effects and considering a moderate mass (1.4 M_☉ neutron star, 5.6 M_☉ black hole), high-spin (black hole J/M^2 = 0.9) system with the K_0 = 220 MeV Lattimer-Swesty EOS. We find that about 0.08 M_☉ of nuclear matter is ejected from the system, while another 0.3 M_☉ forms a hot, compact accretion disk. The primary effects of the escaping neutrinos are (1) to make the disk much denser and more compact, (2) to cause the average electron fraction Ye of the disk to rise to about 0.2 and then gradually decrease again, and (3) to gradually cool the disk. The disk is initially hot (T ~ 6 MeV) and luminous in neutrinos (L_ν ~ 10^54 erg s^–1), but the neutrino luminosity decreases by an order of magnitude over 50 ms of post-merger evolution
Studying and Treating Schizophrenia Using Virtual Reality: A New Paradigm
Understanding schizophrenia requires consideration of patients’
interactions in the social world. Misinterpretation of other peoples’
behavior is a key feature of persecutory ideation. The occurrence and intensity
of hallucinations is affected by the social context. Negative symptoms such as
anhedonia, asociality, and blunted affect reflect difficulties in social
interactions. Withdrawal and avoidance of other people is frequent in
schizophrenia, leading to isolation and rumination. The use of virtual reality
(VR)—interactive immersive computer environments—allows one
of the key variables in understanding psychosis, social environments, to be
controlled, providing exciting applications to research and treatment. Seven
applications of virtual social environments to schizophrenia are set out:
symptom assessment, identification of symptom markers, establishment of
predictive factors, tests of putative causal factors, investigation of the
differential prediction of symptoms, determination of toxic elements in the
environment, and development of treatment. The initial VR studies of persecutory
ideation, which illustrate the ascription of personalities and mental states to
virtual people, are highlighted. VR, suitably applied, holds great promise in
furthering the understanding and treatment of psychosis
Electrospray/VUV single-photon ionization mass spectrometry for the analysis of organic compounds
Simulations of inspiraling and merging double neutron stars using the Spectral Einstein Code
We present results on the inspiral, merger, and postmerger evolution of a neutron star-neutron star (NSNS) system. Our results are obtained using the hybrid pseudospectral-finite volume Spectral Einstein Code (SpEC). To test our numerical methods, we evolve an equal-mass system for ≈22 orbits before merger. This waveform is the longest waveform obtained from fully general-relativistic simulations for NSNSs to date. Such long (and accurate) numerical waveforms are required to further improve semianalytical models used in gravitational wave data analysis, for example, the effective one body models. We discuss in detail the improvements to SpEC’s ability to simulate NSNS mergers, in particular mesh refined grids to better resolve the merger and postmerger phases. We provide a set of consistency checks and compare our results to NSNS merger simulations with the independent bam code. We find agreement between them, which increases confidence in results obtained with either code. This work paves the way for future studies using long waveforms and more complex microphysical descriptions of neutron star matter in SpEC
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