9,789 research outputs found
Welfare, the Earned Income Tax Credit, and the Labor Supply of Single Mothers
During 1984-96, welfare and tax policy changed dramatically. The Earned Income Tax Credit was expanded, welfare benefits were cut, welfare time limits were added and cases were terminated, Medicaid for the working poor was expanded, training programs were redirected, and subsidized or free child care was expanded. Many of the program changes were intended to encourage low income women to work. During this same time period there were unprecedented increases in the employment and hours of single mothers, particularly those with young children. In this paper, we first document these large changes in policies and employment. We then examine if the policy changes are the reason for the large increases in single mothers' labor supply. We find evidence that a large share of the increase in work by single mothers can be attributed to the EITC, with smaller shares for welfare benefit reductions, welfare waivers, changes in training programs, and child care expansions. We also find that most of these policies increased hours worked. Our results indicate that financial incentives through the tax and welfare systems have substantial effects on single mothers' labor supply decisions.
FAME, a microprocessor based front-end analysis and modeling environment
Higher order software (HOS) is a methodology for the specification and verification of large scale, complex, real time systems. The HOS methodology was implemented as FAME (front end analysis and modeling environment), a microprocessor based system for interactively developing, analyzing, and displaying system models in a low cost user-friendly environment. The nature of the model is such that when completed it can be the basis for projection to a variety of forms such as structured design diagrams, Petri-nets, data flow diagrams, and PSL/PSA source code. The user's interface with the analyzer is easily recognized by any current user of a structured modeling approach; therefore extensive training is unnecessary. Furthermore, when all the system capabilities are used one can check on proper usage of data types, functions, and control structures thereby adding a new dimension to the design process that will lead to better and more easily verified software designs
Switchable Hardening of a Ferromagnet at Fixed Temperature
The intended use of a magnetic material, from information storage to power
conversion, depends crucially on its domain structure, traditionally crafted
during materials synthesis. By contrast, we show that an external magnetic
field applied transverse to the preferred magnetization of a model disordered
uniaxial ferromagnet is an isothermal regulator of domain pinning. At elevated
temperatures, near the transition into the paramagnet, modest transverse fields
increase the pinning, stabilize the domain structure, and harden the magnet,
until a point where the field induces quantum tunneling of the domain walls and
softens the magnet. At low temperatures, tunneling completely dominates the
domain dynamics and provides an interpretation of the quantum phase transition
in highly disordered magnets as a localization/delocalization transition for
domain walls. While the energy scales of the rare earth ferromagnet studied
here restrict the effects to cryogenic temperatures, the principles discovered
are general and should be applicable to existing classes of highly anisotropic
ferromagnets with ordering at room temperature or above.Comment: 10 pages, 4 figure
Normal Coordinates and Primitive Elements in the Hopf Algebra of Renormalization
We introduce normal coordinates on the infinite dimensional group
introduced by Connes and Kreimer in their analysis of the Hopf algebra of
rooted trees. We study the primitive elements of the algebra and show that they
are generated by a simple application of the inverse Poincar\'e lemma, given a
closed left invariant 1-form on . For the special case of the ladder
primitives, we find a second description that relates them to the Hopf algebra
of functionals on power series with the usual product. Either approach shows
that the ladder primitives are given by the Schur polynomials. The relevance of
the lower central series of the dual Lie algebra in the process of
renormalization is also discussed, leading to a natural concept of
-primitiveness, which is shown to be equivalent to the one already in the
literature.Comment: Latex, 24 pages. Submitted to Commun. Math. Phy
Probing many-body localization in a disordered quantum magnet
Quantum states cohere and interfere. Quantum systems composed of many atoms
arranged imperfectly rarely display these properties. Here we demonstrate an
exception in a disordered quantum magnet that divides itself into nearly
isolated subsystems. We probe these coherent clusters of spins by driving the
system beyond its linear response regime at a single frequency and measuring
the resulting "hole" in the overall linear spectral response. The Fano shape of
the hole encodes the incoherent lifetime as well as coherent mixing of the
localized excitations. For the disordered Ising magnet,
, the quality factor for spectral holes
can be as high as 100,000. We tune the dynamics of the quantum degrees of
freedom by sweeping the Fano mixing parameter through zero via the
amplitude of the ac pump as well as a static external transverse field. The
zero-crossing of is associated with a dissipationless response at the drive
frequency, implying that the off-diagonal matrix element for the two-level
system also undergoes a zero-crossing. The identification of localized
two-level systems in a dense and disordered dipolar-coupled spin system
represents a solid state implementation of many-body localization, pushing the
search forward for qubits emerging from strongly-interacting, disordered,
many-body systems.Comment: 22 pages, 6 figure
Barkhausen noise in the Random Field Ising Magnet NdFeB
With sintered needles aligned and a magnetic field applied transverse to its
easy axis, the rare-earth ferromagnet NdFeB becomes a
room-temperature realization of the Random Field Ising Model. The transverse
field tunes the pinning potential of the magnetic domains in a continuous
fashion. We study the magnetic domain reversal and avalanche dynamics between
liquid helium and room temperatures at a series of transverse fields using a
Barkhausen noise technique. The avalanche size and energy distributions follow
power-law behavior with a cutoff dependent on the pinning strength dialed in by
the transverse field, consistent with theoretical predictions for Barkhausen
avalanches in disordered materials. A scaling analysis reveals two regimes of
behavior: one at low temperature and high transverse field, where the dynamics
are governed by the randomness, and the second at high temperature and low
transverse field where thermal fluctuations dominate the dynamics.Comment: 16 pages, 7 figures. Under review at Phys. Rev.
- …