344 research outputs found
The Significance of Federal Taxes as Automatic Stabilizers
Using the TAXSIM model for the period 1962-95, we consider the federal tax system's impact as an automatic stabilizer. Despite the many changes in the tax system, there has been relatively little change in its role as an automatic stabilizer. We estimate that individual federal taxes offset perhaps as much as 8 percent of initial shocks to GDP. We also suggest that the progressive income tax may help to stabilize output via its effect on the supply of labor, an additional effect that may even be of similar magnitude to the more traditional path of stabilization through aggregate demand.
A geometrical approach to the dynamics of spinor condensates I: Hydrodynamics
In this work, we derive the equations of motion governing the hydrodynamics
of spin-F spinor condensates. We pursue a description based on standard
physical variables (total density and superfluid velocity), alongside 2F
`spin-nodes': unit vectors that describe the spin F state, and also exhibit the
point-group symmetry of a spinor condensate's mean-field ground state. The
hydrodynamic equations of motion consist of a mass continuity equation, 2F
Landau-Lifshitz equations for the spin-nodes, and a modified Euler equation. In
particular, we provide a generalization of the Mermin-Ho relation to spin one,
and find an analytic solution for the skyrmion texture in the incompressible
regime of a spin-half condensate. These results exhibit a beautiful geometrical
structure that underlies the dynamics of spinor condensates.Comment: 12 pages. First paper in two-part serie
Generation of Tunable Narrow Bandwidth Nanosecond Pulses in the Deep Ultraviolet for Efficient Optical Pumping and High Resolution Spectroscopy
Nanosecond optical pulses with high power and spectral brightness in the deep ultraviolet (UV) region have been produced by sum frequency mixing of nearly transform-limited-bandwidth IR light originating from a home-built injection-seeded ring cavityoptical parametric oscillator(OPO) and the fourth harmonic beam of an injection-seeded Nd:YAG laser used simultaneously to pump the OPO with the second harmonic. We demonstrate UV output, tunable from 204 to 207 nm, which exhibits pulse energies up to 5 mJ with a bandwidth better than [Math Processing Error]. We describe how the approach shown in this paper can be extended to wavelengths shorter than 185 nm. The injection-seeded OPO provides high conversion efficiency ([Math Processing Error] overall energy conversion) and superior beam quality required for highly efficient downstream mixing where sum frequencies are generated in the UV. The frequency stability of the system is excellent, making it highly suitable for optical pumping. We demonstrate high resolution spectroscopy as well as optical pumping using laser-induced fluorescence and stimulated emission pumping, respectively, in supersonic pulsed molecular beams of nitric oxide
Generation of Tunable Narrow Bandwidth Nanosecond Pulses in the Deep Ultraviolet for Efficient Optical Pumping and High Resolution Spectroscopy
Nanosecond optical pulses with high power and spectral brightness in the deep ultraviolet (UV) region have been produced by sum frequency mixing of nearly transform-limited-bandwidth IR light originating from a home-built injection-seeded ring cavityoptical parametric oscillator(OPO) and the fourth harmonic beam of an injection-seeded Nd:YAG laser used simultaneously to pump the OPO with the second harmonic. We demonstrate UV output, tunable from 204 to 207 nm, which exhibits pulse energies up to 5 mJ with a bandwidth better than [Math Processing Error]. We describe how the approach shown in this paper can be extended to wavelengths shorter than 185 nm. The injection-seeded OPO provides high conversion efficiency ([Math Processing Error] overall energy conversion) and superior beam quality required for highly efficient downstream mixing where sum frequencies are generated in the UV. The frequency stability of the system is excellent, making it highly suitable for optical pumping. We demonstrate high resolution spectroscopy as well as optical pumping using laser-induced fluorescence and stimulated emission pumping, respectively, in supersonic pulsed molecular beams of nitric oxide
Self-consistent spin-wave theory for a frustrated Heisenberg model with biquadratic exchange in the columnar phase and its application to iron pnictides
Recent neutron scattering studies revealed the three dimensional character of
the magnetism in the iron pnictides and a strong anisotropy between the
exchange perpendicular and parallel to the spin stripes. We extend studies of
the J1-J2-Jc Heisenberg model with S = 1 using self-consistent spin-wave
theory. A discussion of two scenarios for the instability of the columnar phase
is provided. The relevance of a biquadratic exchange term between in-plane
nearest neighbors is discussed. We introduce mean-field decouplings for
biquadratic terms using the Dyson-Maleev and the Schwinger boson
representation. Remarkably their respective mean-field theories do not lead to
the same results, even at zero temperature. They are gauged in the N'eel phase
in comparison to exact diagonalization and series expansion. The J1-J2-Jc model
is analyzed under the influence of the biquadratic exchange Jbq and a detailed
description of the staggered magnetization and of the magnetic excitations is
given. The biquadratic exchange increases the renormalization of the in-plane
exchange constants which enhances the anisotropy between the exchange parallel
and perpendicular to the spin stripes. Applying the model to iron pnictides, it
is possible to reproduce the spin-wave dispersion for CaFe2As2 in the direction
perpendicular to the spin stripes and perpendicular to the planes.
Discrepancies remain in the direction parallel to the spin stripes which can be
resolved by passing from S = 1 to S = 2. In addition, results for the dynamical
structure factor within the self-consistent spin-wave theory are provided.Comment: 18 pages, 12 figures. Updated version, several references adde
On the Path Integral Representation for Spin Systems
We propose a classical constrained Hamiltonian theory for the spin. After the
Dirac treatment we show that due to the existence of second class constraints
the Dirac brackets of the proposed theory represent the commutation relations
for the spin. We show that the corresponding partition function, obtained via
the Fadeev-Senjanovic procedure, coincides with the one obtained using coherent
states. We also evaluate this partition function for the case of a single spin
in a magnetic field.Comment: To be published in J.Phys. A: Math. and Gen. Latex file, 12 page
Instability of charge ordered states in doped antiferromagnets
We analyze the induced interactions between localized holes in weakly-doped
Heisenberg antiferromagnets due to the modification of the quantum zero point
spin wave energy; i.e. the analogue of the Casimir effect. We show that this
interaction is uniformly attractive and falls off as r^{-2 d+1} in d
dimensions. For ``stripes'', i.e parallel (d-1)-dimensional hypersurfaces of
localized holes, the interaction energy per unit hyperarea is attractive and
falls, generically, like r^{-d}. We argue that, in the absence of a long-range
Coulomb repulsion between holes, this interaction leads to an instability of
any charge-ordered state in the dilute doping limit.Comment: Revtex, 5 pages two-column format, 3 ps figures (epsf). Two
references added and some textual change
The Spin Density Matrix II: Application to a system of two quantum dots
This work is a sequel to our work "The Spin Density Matrix I: General Theory
and Exact Master Equations" (eprint arXiv:0708.0644 [cond-mat]). Here we
compare pure- and pseudo-spin dynamics using as an example a system of two
quantum dots, a pair of localized conduction-band electrons in an n-doped GaAs
semiconductor. Pure-spin dynamics is obtained by tracing out the orbital
degrees of freedom, whereas pseudo-spin dynamics retains (as is conventional)
an implicit coordinate dependence. We show that magnetic field inhomogeneity
and spin-orbit interaction result in a non-unitary evolution in pure-spin
dynamics, whereas these interactions contribute to the effective pseudo-spin
Hamiltonian via terms that are asymmetric in spin permutations, in particular,
the Dzyaloshinskii-Moriya (DM) spin-orbit interaction. We numerically
investigate the non-unitary effects in the dynamics of the triplet states
population, purity, and Lamb energy shift, as a function of interdot distance
and magnetic field difference. The spin-orbit interaction is found to produce
effects of roughly four orders of magnitude smaller than those due to magnetic
field difference in the pure-spin model. We estimate the spin-orbit interaction
magnitude in the DM-interaction term. Our estimate gives a smaller value than
that recently obtained by Kavokin [Phys. Rev. B 64, 075305 (2001)], who did not
include double occupancy effects. We show that a necessary and sufficient
condition for obtaining a universal set of quantum logic gates, involving only
two spins, in both pure- and pseudo-spin models is that the magnetic field
inhomogeneity and the Heisenberg interaction are both non-vanishing. We also
briefly analyze pure-spin dynamics in the electron on liquid helium system
recently proposed by Lyon [Phys. Rev. A 74, 052338 (2006)].Comment: 16 pages including 12 figures. Sequel to "The Spin Density Matrix I:
General Theory and Exact Master Equations", arXiv:0708.064
Efficient noninteractive certification of RSA moduli and beyond
In many applications, it is important to verify that an RSA public key (N; e) speci es a
permutation over the entire space ZN, in order to prevent attacks due to adversarially-generated
public keys. We design and implement a simple and e cient noninteractive zero-knowledge
protocol (in the random oracle model) for this task. Applications concerned about adversarial
key generation can just append our proof to the RSA public key without any other modi cations
to existing code or cryptographic libraries. Users need only perform a one-time veri cation of
the proof to ensure that raising to the power e is a permutation of the integers modulo N. For
typical parameter settings, the proof consists of nine integers modulo N; generating the proof
and verifying it both require about nine modular exponentiations.
We extend our results beyond RSA keys and also provide e cient noninteractive zero-
knowledge proofs for other properties of N, which can be used to certify that N is suitable
for the Paillier cryptosystem, is a product of two primes, or is a Blum integer. As compared to
the recent work of Auerbach and Poettering (PKC 2018), who provide two-message protocols for
similar languages, our protocols are more e cient and do not require interaction, which enables
a broader class of applications.https://eprint.iacr.org/2018/057First author draf
Kondo Insulator: p-wave Bose Condensate of Excitons
In the Anderson lattice model for a mixed-valent system, the
hybridization can possess a -wave symmetry. The strongly-correlated
insulating phase in the mean-field approximation is shown to be a -wave Bose
condensate of excitons with a spontaneous lattice deformation. We study the
equilibrium and linear response properties across the insulator-metal
transition. Our theory supports the empirical correlation between the lattice
deformation and the magnetic susceptibility and predicts measurable ultrasonic
and high-frequency phonon behavior in mixed-valent semiconductors.Comment: 5 pages, 3 encapsulated PostScript figure
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