58,273 research outputs found
Local magnetization nucleated by non-magnetic impurities in Fe-based superconductors
We study impurity-induced magnetic order within a five-band Hubbard model
relevant to the normal paramagnetic phase of iron-based superconductors. The
existence of the local magnetic order is explained in terms of an
impurity-enhancement of states near the Fermi level, and we map out the
resulting phase diagram of the existence of magnetization as a function of
impurity strength and Coulomb correlations. In particular, the presence of
impurity-induced magnetism in only a certain range of potential scattering
strengths can be understood from the specific behavior of the impurity resonant
state.Comment: 8 pages, 3 figure
Competing magnetic double-Q phases and superconductivity-induced re-entrance of C2 magnetic stripe order in iron pnictides
We perform a microscopic theoretical study of the generic properties of
competing magnetic phases in iron pnictides. As a function of electron filling
and temperature, the magnetic stripe (single-Q) order forms a dome, but
competing non-collinear and non-uniform double-Q phases exist at the foot of
the dome in agreement with recent experiments. We compute and compare the
electronic properties of the different magnetic phases, investigate the role of
competing superconductivity, and show how disorder may stabilize double-Q
order. Superconductivity is shown to compete more strongly with double-Q
magnetic phases, which can lead to re-entrance of the C2 (single-Q) order in
agreement with recent thermal expansion measurements on K-doped Ba-122
crystals.Comment: 5 pages, 5 figures, Supplementary Materia
Incidence combinatorics of resolutions
We introduce notions of combinatorial blowups, building sets, and nested sets
for arbitrary meet-semilattices. This gives a common abstract framework for the
incidence combinatorics occurring in the context of De Concini-Procesi models
of subspace arrangements and resolutions of singularities in toric varieties.
Our main theorem states that a sequence of combinatorial blowups, prescribed by
a building set in linear extension compatible order, gives the face poset of
the corresponding simplicial complex of nested sets. As applications we trace
the incidence combinatorics through every step of the De Concini-Procesi model
construction, and we introduce the notions of building sets and nested sets to
the context of toric varieties.
There are several other instances, such as models of stratified manifolds and
certain graded algebras associated with finite lattices, where our
combinatorial framework has been put to work; we present an outline in the end
of this paper.Comment: 20 pages; this is a revised version of our preprint dated Nov 2000
and May 2003; to appear in Selecta Mathematica (N.S.
Enhancing Superconductivity by Disorder
We study two mechanisms for enhancing the superconducting transition
temperature Tc by nonmagnetic disorder in both conventional (sign-preserving
gaps) and unconventional (sign-changing gaps) superconductors (SC). In the
first scenario, relevant to multi-band systems in the dilute impurity limit of
both conventional and unconventional SC, we demonstrate how favorable density
of states enhancements driven by resonant states in off-Fermi-level bands, lead
to significant enhancements of Tc in the condensate formed by the
near-Fermi-level bands. The second scenario focuses on the dense impurity limit
where random disorder-generated local density of states modulations cause a
boosted Tc for conventional SC with short coherence lengths. We analyze the
basic physics of both mechanisms within simplified models, and discuss the
relevance to existing materials.Comment: 6 pages, 4 figure
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