46,001 research outputs found
Impurity spin textures across conventional and deconfined quantum critical points of two-dimensional antiferromagnets
We describe the spin distribution in the vicinity of a non-magnetic impurity
in a two-dimensional antiferromagnet undergoing a transition from a
magnetically ordered Neel state to a paramagnet with a spin gap. The quantum
critical ground state in a finite system has total spin S=1/2 (if the system
without the impurity had an even number of S=1/2 spins), and recent numerical
studies in a double layer antiferromagnet (K. H.Hoglund et al.,
cond-mat/0611418) have shown that the spin has a universal spatial form
delocalized across the entire sample. We present the field theory describing
the uniform and staggered magnetizations in this spin texture for two classes
of antiferromagnets: (i) the transition from a Neel state to a paramagnet with
local spin singlets, in models with an even number of S=1/2 spins per unit
cell, which are described by a O(3) Landau-Ginzburg-Wilson field theory; and
(ii) the transition from a Neel state to a valence bond solid, in
antiferromagnets with a single S=1/2 spin per unit cell, which are described by
a deconfined field theory of spinons.Comment: 30 pages, 9 figure
Welfare Reform and Labor Participation: Are There Urban and Rural Differences?
Although welfare reform began in 1996 at the national level, Iowa was one of the earliest states to obtain a waiver to initiate the Iowa Family Investment Program (FIP) in 1993. To gain a better understanding of welfare recidivism, we use Iowa administrative quarterly data between October1993 and September 1995, impute the education attainment for the caseheads with missing education attainment using fractional imputation and study the factors that affect the probability of working, the potential wage for the caseheads and the possibility of leaving FIP based on the potential wage. We find higher education (i.e. higher skills) leads to higher labor force participation, especially for single-mothers with children. Metro or urban location is associated with the probability of working and potential wage earnings, but has no effect on FIP participation. The local unemployment rate does not affect labor participation of low-income individuals, but does affect the potential wage and FIP status. Those with lower education, and nonwhites are more affected by the local labor market environment than others. If an individual moves once in a year, he or she will earn more money than in the original job; no gains are achieved through moving more than once. The possibility of leaving FIP is relatively high if there is only one move.Labor and Human Capital,
Morphological characterization of shocked porous material
Morphological measures are introduced to probe the complex procedure of shock
wave reaction on porous material. They characterize the geometry and topology
of the pixelized map of a state variable like the temperature. Relevance of
them to thermodynamical properties of material is revealed and various
experimental conditions are simulated. Numerical results indicate that, the
shock wave reaction results in a complicated sequence of compressions and
rarefactions in porous material. The increasing rate of the total fractional
white area roughly gives the velocity of a compressive-wave-series.
When a velocity is mentioned, the corresponding threshold contour-level of
the state variable, like the temperature, should also be stated. When the
threshold contour-level increases, becomes smaller. The area increases
parabolically with time during the initial period. The curve goes
back to be linear in the following three cases: (i) when the porosity
approaches 1, (ii) when the initial shock becomes stronger, (iii) when the
contour-level approaches the minimum value of the state variable. The area with
high-temperature may continue to increase even after the early
compressive-waves have arrived at the downstream free surface and some
rarefactive-waves have come back into the target body. In the case of energetic
material ... (see the full text)Comment: 3 figures in JPG forma
Nanoscale Impurity Structures on the Surface of -wave Superconductors
We study the effects of nanoscale impurity structures on the local electronic
structure of -wave superconductors. We show that the interplay
between the momentum dependence of the superconducting gap, the geometry of the
nanostructure and its orientation gives rise to a series of interesting quantum
effects. Among these are the emergence of a zero bias conductance peak in the
superconductor's density of states and the suppression of impurity states for
certain nanostructures. The latter effect can be used to screen impurity
resonances in the superconducting state.Comment: 4 pages, 5 figure
Josephson scanning tunneling microscopy
We propose a set of scanning tunneling microscopy experiments in which the
surface of superconductor is scanned by a superconducting tip. Potential
capabilities of such experimental setup are discussed. Most important
anticipated results of such an experiment include the position-resolved
measurement of the superconducting order parameter and the possibility to
determine the nature of the secondary component of the order parameter at the
surface. The theoretical description based on the tunneling Hamiltonian
formalism is presented.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Valence bond solid order near impurities in two-dimensional quantum antiferromagnets
Recent scanning tunnelling microscopy (STM) experiments on underdoped
cuprates have displayed modulations in the local electronic density of states
which are centered on a Cu-O-Cu bond (Kohsaka et. al., cond-mat/0703309). As a
paradigm of the pinning of such bond-centered ordering in strongly correlated
systems, we present the theory of valence bond solid (VBS) correlations near a
single impurity in a square lattice antiferromagnet. The antiferromagnet is
assumed to be in the vicinity of a quantum transition from a magnetically
ordered Neel state to a spin-gap state with long-range VBS order. We identify
two distinct classes of impurities: i) local modulation in the exchange
constants, and ii) a missing or additional spin, for which the impurity
perturbation is represented by an uncompensated Berry phase. The `boundary'
critical theory for these classes is developed: in the second class we find a
`VBS pinwheel' around the impurity, accompanied by a suppression in the VBS
susceptibility. Implications for numerical studies of quantum antiferromagnets
and for STM experiments on the cuprates are noted.Comment: 41 pages, 6 figures; (v2) Minor changes in terminology, added
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