3,256 research outputs found
Multicolored quantum dimer models, resonating valence-bond states, color visons, and the triangular-lattice t_2g spin-orbital system
The spin-orbital model for triply degenerate t_2g electrons on a triangular
lattice has been shown to be dominated by dimers: the phase diagram contains
both strongly resonating, compound spin-orbital dimer states and quasi-static,
spin-singlet valence-bond (VB) states. To elucidate the nature of the true
ground state in these different regimes, the model is mapped to a number of
quantum dimer models (QDMs), each of which has three dimer colors. The generic
multicolored QDM, illustrated for the two- and three-color cases, possesses a
topological color structure, "color vison" excitations, and broad regions of
resonating VB phases. The specific models are analyzed to gain further insight
into the likely ground states in the superexchange and direct-exchange limits
of the electronic Hamiltonian, and suggest a strong tendency towards VB order
in all cases.Comment: 16 pages, 12 figure
The Effects of Negative Legacies on the Adjustment of Parentally Bereaved Children and Adolescents
This is a report of a qualitative analysis of a sample of bereaved families in which one parent died and in which children scored in the clinical range on the Child Behavior Check List. The purpose of this analysis was to learn more about the lives of these children. They were considered to be at risk of developing emotional and behavioral problems associated with the death. We discovered that many of these “high risk” children had a continuing bond with the deceased that was primarily negative and troubling for them in contrast to a comparison group of children not at risk from the same study. Five types of legacies, not mutually exclusive, were identified: health related, role related, personal qualities, legacy of blame, and an emotional legacy. Coping behavior on the part of the surviving parent seemed to make a difference in whether or not a legacy was experienced as negative
Dzyaloshinskii-Moriya anisotropy and non-magnetic impurities in the kagome system ZnCu_3(OH)_6Cl_2
Motivated by recent nuclear magnetic resonance experiments on
ZnCu(OH)Cl, we present an exact-diagonalization study of the
combined effects of non-magnetic impurities and Dzyaloshinskii-Moriya (DM)
interactions in the kagome antiferromagnet. The local response to an
applied field and correlation-matrix data reveal that the dimer freezing which
occurs around each impurity for persists at least up to , where and denote respectively the exchange and DM interaction
energies. The phase transition to the () semiclassical, 120
state favored at large takes place at . However, the dimers
next to the impurity sites remain strong up to values , far above
this critical point, and thus do not participate fully in the ordered state. We
discuss the implications of our results for experiments on
ZnCu(OH)Cl.Comment: 11 pages, submitted to PR
Real Output in Mental Health Care During the 1990s
Health accounts document changes over time in the level and composition of health spending. There has been a continued evolution in the ability to track such outlays. Less rapid has been the ability to interpret changes in spending. In this paper we apply quality adjusted price indexes for several major mental disorders to national mental health account estimates to assess changes in real "output". We show that using the new price indexes reveals large gains in real output relative to application of BLS indexes.
Gallium self-interstitial relaxation in Gallium Arsenide: an {ab initio} characterization
Ga interstitials in GaAs () are studied using the local-orbital
{ab-initio} code SIESTA in a supercell of {216+1} atoms. Starting from eight
different initial configurations, we find five metastable structures: the two
tetrahedral sites in addition to the 110-split,
111-split, and 100-split. Studying
the competition between various configuration and charges of , we find
that predominant gallium interstitials in GaAs are charged +1, neutral or at
most -1 depending on doping conditions and prefer to occupy the tetrahedral
configuration where it is surrounded by Ga atoms. Our results are in excellent
agreement with recent experimental results concerning the dominant charge of
, underlining the importance of finite size effects in the calculation
of defects.Comment: v1) 18 pages, 5 figures, submitted to PRB (Latex preprint version)
v2) 9 pages, 5 figures, reviewed version resubmitted to PRB (correction to
equation 1, some changes and reformulations, minor grammatical and typo
corrections, added reference
Binary continuous random networks
Many properties of disordered materials can be understood by looking at
idealized structural models, in which the strain is as small as is possible in
the absence of long-range order. For covalent amorphous semiconductors and
glasses, such an idealized structural model, the continuous-random network, was
introduced 70 years ago by Zachariasen. In this model, each atom is placed in a
crystal-like local environment, with perfect coordination and chemical
ordering, yet longer-range order is nonexistent. Defects, such as missing or
added bonds, or chemical mismatches, however, are not accounted for. In this
paper we explore under which conditions the idealized CRN model without defects
captures the properties of the material, and under which conditions defects are
an inherent part of the idealized model. We find that the density of defects in
tetrahedral networks does not vary smoothly with variations in the interaction
strengths, but jumps from close-to-zero to a finite density. Consequently, in
certain materials, defects do not play a role except for being thermodynamical
excitations, whereas in others they are a fundamental ingredient of the ideal
structure.Comment: Article in honor of Mike Thorpe's 60th birthday (to appear in J.
Phys: Cond Matt.
The Medical Treatment of Depression, 1991-1996: Productive Inefficiency, Expected Outcome Variations, and Price Indexes
We examine the price of treating episodes of acute phase major depression over the 1991-1996 time period. We combine data from a large retrospective medical claims data base (MarketScanTM, from the MedStat Group) with clinical literature and expert clinical opinion elicited from a two-state Delphi procedure. This enables us to construct a variety of treatment price indexes that include variations over time in the proportion of off-frontier' production, as well as the corresponding variations in expected treatment outcomes. We also incorporate the fact that the no treatment option ( waiting list') frequently results in spontaneous remission of depressive symptoms. We find that in general the incremental cost of successfully treating an episode of acute phase major depression has generally fallen over the 1991-96 time period. Based on hedonic regression equations that account for the effects of changing patient mix, we find price reductions that range from about -1.66% to -2.13% per year. An implication of this is that, since expenditures on depression are thought to be increasing since at least 1991, the source of the spending increases is volume (quantity) increases, and not price increases.
Susceptibility of the Endangered Karner Blue Butterfly (Lepidoptera: Lycaenidae) to \u3ci\u3eBacillus Thuringiensis\u3c/i\u3e Var. \u3ci\u3eKurstaki\u3c/i\u3e Used for Gypsy Moth Suppression in Michigan
We investigated the phenological and physiological susceptibility of the endangered Karner blue butterfly (Lycaeides melissa samuelis) to Bacillus thuringiensis var. kurstaki (Bt), a product widely used for gypsy moth (Lymantria dispar) suppression in Michigan and other infested states. We monitored phenology of the bivoltine Karner blue in two regions of Michigan from 1993 to 1995 to determine if larval stages overlapped temporally with the period of Bt application for gypsy moth suppression. Karner blue larvae of the spring generation were found during the period that Bt was applied in nearby areas in 1993 only. However, spring-generation adults or newly laid eggs were observed up to 11 days before applications in 1994 and 1995. Since Karner blue eggs develop within one week, summer-generation larvae were most likely present during or shortly after 1994 and 1995 Bt application periods. These larvae would have been at risk, assuming Bt persistence of 4 to 6 days.
Physiological susceptibility of Karner blue larvae to Bt was determined in a laboratory bioassay. Larvae were reared on wild lupine (Lupinus perennis) foliage that was untreated, or sprayed with Bt formulations at rates of 30-37 or 90 BIU/ha. A similar bioassay with second instar gypsy moth larvae on similarly treated white oak (Quercus alba) foliage was conducted concurrently. Karner blue survival was 100%, 27% and 14% on control, low and high Bt treatments, respectively. Early and late Karner blue instars were equally susceptible to Bt. Survival of gypsy moth was 80%, 33% and 5% on control, low and high Bt treatments, respectively, and did not differ significantly from Karner blue survival. We conclude that Karner blue is both phenologically and physiologically susceptible to Bt used for gypsy moth suppression, although the larval generation at risk and extent of phenological overlap may vary from year to year
Block orthogonal polynomials: I. Definition and properties
Constrained orthogonal polynomials have been recently introduced in the study
of the Hohenberg-Kohn functional to provide basis functions satisfying particle
number conservation for an expansion of the particle density. More generally,
we define block orthogonal (BO) polynomials which are orthogonal, with respect
to a first Euclidean scalar product, to a given -dimensional subspace of polynomials associated with the constraints. In addition, they are
mutually orthogonal with respect to a second Euclidean scalar product. We
recast the determination of these polynomials into a general problem of finding
particular orthogonal bases in an Euclidean vector space endowed with distinct
scalar products. An explicit two step Gram-Schmidt orthogonalization (G-SO)
procedure to determine these bases is given. By definition, the standard block
orthogonal (SBO) polynomials are associated with a choice of equal
to the subspace of polynomials of degree less than . We investigate their
properties, emphasizing similarities to and differences from the standard
orthogonal polynomials. Applications to classical orthogonal polynomials will
be given in forthcoming papers.Comment: This is a reduced version of the initial manuscript, the number of
pages being reduced from 34 to 2
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