10,387 research outputs found
Quantum Spin Hall Effect in Graphene
We study the effects of spin orbit interactions on the low energy electronic
structure of a single plane of graphene. We find that in an experimentally
accessible low temperature regime the symmetry allowed spin orbit potential
converts graphene from an ideal two dimensional semimetallic state to a quantum
spin Hall insulator. This novel electronic state of matter is gapped in the
bulk and supports the quantized transport of spin and charge in gapless edge
states that propagate at the sample boundaries. The edge states are non chiral,
but they are insensitive to disorder because their directionality is correlated
with spin. The spin and charge conductances in these edge states are calculated
and the effects of temperature, chemical potential, Rashba coupling, disorder
and symmetry breaking fields are discussed.Comment: 4 pages, published versio
The supervisory implications of financial globalization: three views
At the 1989 Lake Bluff conference on globalization, three authorities presented their personal—and conflicting—views on international financial regulation in general, and the 1988 BIS-sponsored Basle agreement, in particular.International finance ; Bank capital
Magnetic field-assisted manipulation and entanglement of Si spin qubits
Architectures of donor-electron based qubits in silicon near an oxide
interface are considered theoretically. We find that the precondition for
reliable logic and read-out operations, namely the individual identification of
each donor-bound electron near the interface, may be accomplished by
fine-tuning electric and magnetic fields, both applied perpendicularly to the
interface. We argue that such magnetic fields may also be valuable in
controlling two-qubit entanglement via donor electron pairs near the interface.Comment: 4 pages, 4 figures. 1 ref and 1 footnote adde
Roe v. Wade and American Fertility
We consider the effect of abortion legalization on births in the United States. A simple theoretical model demonstrates that the impact of abortion legalization on the birth rate is ambiguous, because both pregnancy and abortion decisions could be affected. We use variation in the timing of legalization across states in the early 1970's to estimate the effect of abortion on birth rates. Our findings indicate that states legalizing abortion experienced a 5% decline in births relative to other states. The decline among teens, women over 35, and nonwhite women was even greater: 13%, 8%, and 12% respectively. Out-of-wedlock births declined by twice as much as births in wedlock. If legalization in some states affected birth rates in neighboring states (through travel to obtain an abortion), comparing births between states will underestimate the actual reduction. Using more distant comparison states increases the estimated impact of abortion legalization on birth rates to about 8%. Applying this estimate to the current level of births, a complete recriminalization of abortion would result in 320,000 additional births per year.
Storm-time changes of geomagnetic field at MAGSAT altitudes (325-550 Km) and their comparison with changes at ground locations
The values of H, X, Y, Z at MAGSAT altitudes were first expressed as residuals delta H, delta X, delta Y, delta Z after subtracting the model HMD, XMD, YMD, ZMC. The storm-time variations of H showed that delta H (Dusk) was larger (negative) than delta H (Dawn) and occurred earlier, indicating a sort of hysteresis effect. Effects at MAGSAT altitudes were roughly the same (10% accuracy) as at ground, indicating that these effects were mostly of magnetospheric origin. The delta Y component also showed large storm-time changes. The latitudinal distribution of storm-time delta H showed north-south asymmetries varying in nature as the storm progressed. It seems that the central plane of the storm-time magnetospheric ring current undergoes latitudinal meanderings during the course of the storm
Does Practice-Based Teacher Preparation Increase Student Achievement? Early Evidence from the Boston Teacher Residency
The Boston Teacher Residency is an innovative practice-based preparation program in which candidates work alongside a mentor teacher for a year before becoming a teacher of record in Boston Public Schools. We find that BTR graduates are more racially diverse than other BPS novices, more likely to teach math and science, and more likely to remain teaching in the district through year five. Initially, BTR graduates for whom value-added performance data are available are no more effective at raising student test scores than other novice teachers in English language arts and less effective in math. The effectiveness of BTR graduates in math improves rapidly over time, however, such that by their fourth and fifth years they out-perform veteran teachers. Simulations of the program’s overall impact through retention and effectiveness suggest that it is likely to improve student achievement in the district only modestly over the long run.
Hydrogenic Spin Quantum Computing in Silicon: A Digital Approach
We suggest an architecture for quantum computing with spin-pair encoded
qubits in silicon. Electron-nuclear spin-pairs are controlled by a dc magnetic
field and electrode-switched on and off hyperfine interaction. This digital
processing is insensitive to tuning errors and easy to model. Electron
shuttling between donors enables multi-qubit logic. These hydrogenic spin
qubits are transferable to nuclear spin-pairs, which have long coherence times,
and electron spin-pairs, which are ideally suited for measurement and
initialization. The architecture is scalable to highly parallel operation.Comment: 4 pages, 5 figures; refereed and published version with improved
introductio
Versatile liquid helium scintillation counter of large volume design
Design and performance of large liquid helium scintillation counter for meson experiment
Many-body spin related phenomena in ultra-low-disorder quantum wires
Zero length quantum wires (or point contacts) exhibit unexplained conductance
structure close to 0.7 X 2e^2/h in the absence of an applied magnetic field. We
have studied the density- and temperature-dependent conductance of
ultra-low-disorder GaAs/AlGaAs quantum wires with nominal lengths l=0 and 2 mu
m, fabricated from structures free of the disorder associated with modulation
doping. In a direct comparison we observe structure near 0.7 X 2e^2/h for l=0
whereas the l=2 mu m wires show structure evolving with increasing electron
density to 0.5 X 2e^2/h in zero magnetic field, the value expected for an ideal
spin-split sub-band. Our results suggest the dominant mechanism through which
electrons interact can be strongly affected by the length of the 1D region.Comment: 5 Pages, 4 figure
Elementary analysis of the special relativistic combination of velocities, Wigner rotation, and Thomas precession
The purpose of this paper is to provide an elementary introduction to the
qualitative and quantitative results of velocity combination in special
relativity, including the Wigner rotation and Thomas precession. We utilize
only the most familiar tools of special relativity, in arguments presented at
three differing levels: (1) utterly elementary, which will suit a first course
in relativity; (2) intermediate, to suit a second course; and (3) advanced, to
suit higher level students. We then give a summary of useful results, and
suggest further reading in this often obscure field.Comment: V1: 25 pages, 6 figures; V2: 22 pages, 5 figures. The revised version
is shortened and the arguments streamlined. Minor changes in notation and
figures. This version matches the published versio
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