40,798 research outputs found
Quasi-exact solvability of Dirac equation with Lorentz scalar potential
We consider exact/quasi-exact solvability of Dirac equation with a Lorentz
scalar potential based on factorizability of the equation. Exactly solvable and
-based quasi-exactly solvable potentials are discussed separately in
Cartesian coordinates for a pure Lorentz potential depending only on one
spatial dimension, and in spherical coordinates in the presence of a Dirac
monopole.Comment: 10 pages, no figure
Higher Dimensional Geometries from Matrix Brane constructions
Matrix descriptions of even dimensional fuzzy spherical branes in
Matrix Theory and other contexts in Type II superstring theory reveal, in the
large limit, higher dimensional geometries , which have an
interesting spectrum of harmonics and can be up to 20 dimensional,
while the spheres are restricted to be of dimension less than 10. In the case
, the matrix description has two dual field theory formulations. One
involves a field theory living on the non-commutative coset which
is a fuzzy fibre bundle over a fuzzy . In the other, there is a U(n)
gauge theory on a fuzzy with instantons. The two
descriptions can be related by exploiting the usual relation between the fuzzy
two-sphere and U(n) Lie algebra. We discuss the analogous phenomena in the
higher dimensional cases, developing a relation between fuzzy
cosets and unitary Lie algebras.Comment: 28 pages (Harvmac big) ; version 2 : minor typos fixed and ref. adde
D0-branes in an H-field Background and Noncommutative Geometry
It is known that if we compactify D0-branes on a torus with constant B-field,
the resulting theory becomes SYM theory on a noncommutative dual torus. We
discuss the extension to the case of a H-field background. In the case of
constant H-field on a three-torus, we derive the constraints to realize this
compactification by considering the correspondence to string theory. We carry
out this work as a first step to examine the possibility to describe transverse
M5-branes in Matrix theory.Comment: 15 pages, LaTeX; some comments added, typos corrected, to appear in
Nucl. Phys.
The vertical metal insulator semiconductor tunnel transistor: A proposed Fowler-Nordheim tunneling device
We propose a new field-effect transistor, the vertical metal insulator semiconductor tunnel transistor (VMISTT) which operates using gate modulation of the Fowler-Nordheim tunneling current through a metal insulator semiconductor (M-I-S) diode. The VMISTT has significant advantages over the metal-oxide-semiconductor field-effect transistor in device scaling. In order to allow room-temperature operation of the VMISTT, the tunnel oxide has to be optimized for the metal-to-insulator barrier height and the current-voltage characteristics. We have grown TiO2 layers as the tunnel insulator by oxidizing 7 and 10 nm thick Ti metal films vacuum-evaporated on silicon substrates, and characterized the films by current-voltage and capacitance-voltage techniques. The quality of the oxide films showed variations, depending on the oxidation temperatures in the range of 450-550 degrees C. Fowler-Nordheim tunneling was observed at low temperatures at bias voltage of 2 V and above and a barrier height of approximately 0.4 eV was calculated. Leakage currents present were due Schottky-barrier emission at room-temperature, and hopping at liquid nitrogen temperature
Optimal Photon Generation from Spontaneous Raman Processes in Cold Atoms
Spontaneous Raman processes in cold atoms have been widely used in the past
decade for generating single photons. Here, we present a method to optimize
their efficiencies for given atomic coherences and optical depths. We give a
simple and complete recipe that can be used in present-day experiments,
attaining near-optimal single photon emission while preserving the photon
purity.Comment: 6+6 pages, 3 figures, 1 tabl
Financing Direct Democracy: Revisiting the Research on Campaign Spending and Citizen Initiatives
The conventional view in the direct democracy literature is that spending against a measure is more effective than spending in favor of a measure, but the empirical results underlying this conclusion have been questioned by recent research. We argue that the conventional finding is driven by the endogenous nature of campaign spending: initiative proponents spend more when their ballot measure is likely to fail. We address this endogeneity by using an instrumental variables approach to analyze a comprehensive dataset of ballot propositions in California from 1976 to 2004. We find that both support and opposition spending on citizen initiatives have strong, statistically significant, and countervailing effects. We confirm this finding by looking at time series data from early polling on a subset of these measures. Both analyses show that spending in favor of citizen initiatives substantially increases their chances of passage, just as opposition spending decreases this likelihood
A Rigorous Theory of Many-Body Prethermalization for Periodically Driven and Closed Quantum Systems
Prethermalization refers to the transient phenomenon where a system
thermalizes according to a Hamiltonian that is not the generator of its
evolution. We provide here a rigorous framework for quantum spin systems where
prethermalization is exhibited for very long times. First, we consider quantum
spin systems under periodic driving at high frequency . We prove that up
to a quasi-exponential time , the
system barely absorbs energy. Instead, there is an effective local Hamiltonian
that governs the time evolution up to , and hence this
effective Hamiltonian is a conserved quantity up to . Next, we consider
systems without driving, but with a separation of energy scales in the
Hamiltonian. A prime example is the Fermi-Hubbard model where the interaction
is much larger than the hopping . Also here we prove the emergence of an
effective conserved quantity, different from the Hamiltonian, up to a time
that is (almost) exponential in .Comment: 21 pages, file updated to match published versio
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