Topological invariants of time-reversal-invariant band structures

The topological invariants of a time-reversal-invariant band structure in two dimensions are multiple copies of the $\mathbb{Z}_2$ invariant found by Kane and Mele. Such invariants protect the topological insulator and give rise to a spin Hall effect carried by edge states. Each pair of bands related by time reversal is described by a single $\mathbb{Z}_2$ invariant, up to one less than half the dimension of the Bloch Hamiltonians. In three dimensions, there are four such invariants per band. The $\mathbb{Z}_2$ invariants of a crystal determine the transitions between ordinary and topological insulators as its bands are occupied by electrons. We derive these invariants using maps from the Brillouin zone to the space of Bloch Hamiltonians and clarify the connections between $\mathbb{Z}_2$ invariants, the integer invariants that underlie the integer quantum Hall effect, and previous invariants of ${\cal T}$-invariant Fermi systems.Comment: 4 page

Assessing the Welfare Impact of the 2001 Tax Reform on Dual-earner Families

The welfare impact of the 2001 income tax reform is assessed across dual-earner families with different characteristics. A household labor supply model is estimated to account for variable behavioral responses by family type. It was found that while higher education families received a larger share of the welfare gain generated from lower marginal tax rates, it was the lower education families that provided the bulk of the additional labor supply motivated by the tax reform. Differing welfare gains across families with different numbers of children were also found, highlighting the importance of allowing responses to vary across family characteristics when assessing the welfare impact of a policy change. Working Paper 07-3

Assessing the welfare impact of the 2001 tax reform on dual-earner families

We assess the 2001 income tax reform to determine its welfare impact across families with different characteristics. A household labor supply model is estimated to account for variable behavioral responses by family type. We find that while higher-education families received a larger share of the welfare gain generated from lower marginal tax rates, it was the lower-education families that provided the bulk of the additional labor supply motivated by the tax reform. We also find differing welfare gains across families with different numbers of children, highlighting the importance of allowing responses to vary across family characteristics when assessing the welfare impact of a policy change.

Configuration interaction in the helium continuum

Configuration interaction in helium continuum and autoionization level

The impact of the 1996 Summer Olympic Games on employment and wages in Georgia

Using the standard differences-in-differences (DD) technique and a modified DD technique in the slopes, this paper determines that hosting the 1996 Summer Olympic Games boosted employment by 17% in the counties of Georgia affiliated with and close to Olympic activity, relative to employment increases in other counties in Georgia (the rate of growth increased by 0.002 percentage points per quarter). Estimation of a random-growth model confirms a positive impact of the Olympics on employment. In addition, the employment impact is shown not to be merely a "metropolitan statistical area (MSA) effect"; employment in the northern Olympic venue areas was found to increase 11% more post- versus pre-Olympics than it did in other, similar southern MSAs. The evidence of an Olympic impact on wages is weak

Development of a 1000V, 200A, low-loss, fast-switching, gate-assisted turn-off thyristor

The results of a program to develop a fast high power thyristor that can operate in switching circuits at frequencies of 10 to 20 kHz with very low power loss are given. Feasibility was demonstrated for a thyristor that blocks 1000V forward and reverse, conducts 200A, turns on in little more than 2 more microseconds with only 2A of gate drive, turns off in 3 microseconds with 2A of gate assist current and has an energy dissipation of only 12 mJ per pulse for a 20 microsecond half sine wave 200A pulse. Data were generated that clearly showed the tradeoffs that can be made between the turn off time and forward drop. The understanding of this relationship is necessary in the selection of deliverable thyristors with turn off times up to 7 microseconds to give improved efficiency in a series resonant dc to dc inverter application

Global phase diagram of the spin-1 antiferromagnet with uniaxial anisotropy on the kagome lattice

The phase diagram of the XXZ spin-1 quantum magnet on the kagome lattice is studied for all cases where the $J_z$ coupling is antiferromagnetic. In the zero magnetic field case, the six previously introduced phases, found using various methods, are: the nondegenerate gapped photon phase which breaks no space symmetry or spin symmetry; the six-fold degenerate phase with plaquette order, which breaks both time reversal symmetry and translational symmetry; the "superfluid" (ferromagnetic) phase with an in-plane global U(1) symmetry broken, when $J_{xy} 0$; the nematic phase when $D < 0$ and large; and a phase with resonating dimers on each hexagon. We obtain all of these phases and partial information about their quantum phase transitions in a single framework by studying condensation of defects in the six-fold plaquette phases. The transition between nematic phase and the six-fold degenerate plaquette phase is potentially an unconventional second-order critical point. In the case of a nonzero magnetic field along $\hat{z}$, another ordered phase with translation symmetry broken is opened up in the nematic phase. Due to the breaking of time-reversal symmetry by the field, a supersolid phase emerges between the six-fold plaquette order and the superfluid phase. This phase diagram might be accessible in nickel compounds, BF$_4$ salts, or optical lattices of atoms with three degenerate states on every site.Comment: 17 pages, 16 figure