3,754 research outputs found
Facing human rights attributes of copyright in Europe in the context of the EU Digital Single Market
The principle of equality as a fundamental norm in law and political philosophy, Jurysprudencja 8., Wojciechowski B., Bekrycht T., Cern K.M., (eds.), Wydawnictwo Uniwersytetu Łódzkiego, Łódź 2017The project was financed by National Science Centre Poland
(decision no. DEC-2012/05/B/HS5/01111)
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
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
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
Enhancing magnetic stripe order in iron pnictides by RKKY exchange interactions
Recent experimental studies have revealed several unexpected properties of
Mn-doped BaFe2As2. These include extension of the stripe-like magnetic (pi,0)
phase to high temperatures above a critical Mn concentration only, the presence
of diffusive and weakly temperature dependent magnetic (pi,pi) checkerboard
scattering, and an apparent absent structural distortion from tetragonal to
orthorhombic. Here, we study the effects of magnetic impurities both below and
above the N\'eel transition temperature within a real-space five-band model
appropriate to the iron pnictides. We show how these experimental findings can
be explained by a cooperative behavior of the magnetic impurities and the
conduction electrons mediating the Ruderman-Kittel-Kasuya-Yosida (RKKY)
interactions between them.Comment: 5 pages, 4 figure
Impurity bound states and disorder-induced orbital and magnetic order in the s+- state of Fe-based superconductors
We study the presence of impurity bound states within a five-band Hubbard
model relevant to iron-based superconductors. In agreement with earlier
studies, we find that in the absence of Coulomb correlations there exists a
range of repulsive impurity potentials where in-gap states are generated. In
the presence of weak correlations, these states are generally pushed to the
edges of the gap, whereas for larger correlations the onsite impurity potential
induces a local magnetic region which reintroduces the low-energy bound states
into the gap
Impurity-induced sub-gap bound gap states in alkali doped iron chalcogenide superconductors
Measurements of the local density of states near impurities can be useful for
identifying the superconducting gap structure in alkali doped iron chalcogenide
superconductors K_xFe_{2-y}Se_2. Here, we study the effects of nonmagnetic and
magnetic impurities within a nearest neighbor d-wave and next-nearest neighbor
s-wave superconducting state. For both repulsive and attractive nonmagnetic
impurities, it is shown that sub-gap bound states exist only for d-wave
superconductors with the positions of these bound states depending rather
sensitively on the electron doping level. Further, for such disorder Coulomb
interactions can lead to local impurity-induced magnetism in the case of d-wave
superconductivity. For magnetic impurities, both s-wave and d-wave
superconducting states support sub-gap bound states. The above results can be
explained by a simple analytic model that provides a semi-quantitative
understanding of the variation of the impurity bound states energies as a
function of impurity potential and chemical doping level.Comment: 8 pages, 7 figure
Origin of electronic dimers in the spin-density wave phase of Fe-based superconductors
We investigate the emergent impurity-induced states arising from point-like
scatterers in the spin-density wave phase of iron-based superconductors within
a microscopic five-band model. Independent of the details of the band-structure
and disorder potential, it is shown how stable magnetic (pi,pi) unidirectional
nematogens are formed locally by the impurities. Interestingly, these
nematogens exhibit a dimer structure in the electronic density, are directed
along the antiferromagnetic a-axis, and have typical lengths of order 10
lattice constants in excellent agreement with recent scanning tunnelling
experiments. These electronic dimers provide a natural explanation of the
dopant-induced transport anisotropy found e.g. in the 122 iron pnictides.Comment: 5 pages, 4 figure
Impurity states and cooperative magnetic order in Fe-based superconductors
We study impurity bound states and impurity-induced order in the
superconducting state of LiFeAs within a realistic five-band model based on the
band structure and impurity potentials obtained from density functional theory
(DFT). In agreement with recent experiments, we find that Co impurities are too
weak produce sub-gap bound states, whereas stronger impurities like Cu do. We
also obtain the bound state spectrum for magnetic impurities, such as Mn, and
show how spin-resolved tunnelling may determine the nature of the various
defect sites in iron pnictides, a prerequisite for using impurity bound states
as a probe of the ground state pairing symmetry. Lastly we show how impurities
pin both orbital and magnetic order, providing an explanation for a growing set
of experimental evidence for unusual magnetic phases in doped iron pnictides.Comment: 5 pages, 5 fig
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