858 research outputs found
Magnetic and superfluid phases of confined fermions in two-dimensional optical lattices
We examine antiferromagnetic and d-wave superfluid phases of cold fermionic
atoms with repulsive interactions in a two-dimensional optical lattice combined
with a harmonic trapping potential. For experimentally realistic parameters,
the trapping potential leads to the coexistence of magnetic and superfluid
ordered phases with the normal phase. We study the intriguing shell structures
arising from the competition between the magnetic and superfluid order as a
function of the filling fraction. In certain cases antiferromagnetism induce
superfluidity by charge redistributions. We furthermore demonstrate how these
shell structures can be detected as distinct anti-bunching dips and pairing
peaks in the density-density correlation function probed in expansion
experiments.Comment: 4 pages, 3 figure
Spin dynamics in the stripe phase of the cuprates
Within a model that supports stripe spin and charge order coexisting with a
d-wave superconducting phase, we study the self-consistently
obtained electronic structure and the associated transverse dynamical spin
susceptibility. In the coexisting phase of superconducting and static stripe
order, the resulting particle-hole continuum can strongly damp parts of the
low-energy spin wave branches. This provides insight into recent inelastic
neutron scattering data revealing the dispersion of the low-energy collective
magnetic modes of lanthanum based cuprate superconductors.Comment: 4 pages, 3 figure
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
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
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
Spin-Orbit Coupling and Magnetic Anisotropy in Iron-Based Superconductors
We determine theoretically the effect of spin-orbit coupling on the magnetic
excitation spectrum of itinerant multi-orbital systems, with specific
application to iron-based superconductors. Our microscopic model includes a
realistic ten-band kinetic Hamiltonian, atomic spin-orbit coupling, and
multi-orbital Hubbard interactions. Our results highlight the remarkable
variability of the resulting magnetic anisotropy despite constant spin-orbit
coupling. At the same time, the magnetic anisotropy exhibits robust universal
behavior upon changes in the bandstructure corresponding to different materials
of iron-based superconductors. A natural explanation of the observed
universality emerges when considering optimal nesting as a resonance
phenomenon. Our theory is also of relevance to other itinerant system with
spin-orbit coupling and nesting tendencies in the bandstructure.Comment: 15 pages, 9 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
Suppression of superfluid stiffness near Lifshitz-point instability to finite momentum superconductivity
We derive the effective Ginzburg-Landau theory for finite momentum (FFLO/PDW)
superconductivity without spin population imbalance from a model with local
attraction and repulsive pair-hopping. We find that the GL free energy must
include up to sixth order derivatives of the order parameter, providing a
unified description of the interdependency of zero and finite momentum
superconductivity. For weak pair-hopping the phase diagram contains a line of
Lifshitz points where vanishing superfluid stiffness induces a continuous
change to a long wavelength Fulde-Ferrell (FF) state. For larger pair-hopping
there is a bicritical region where the pair-momentum changes discontinuously.
Here the FF type state is near degenerate with the Larkin-Ovchinnikov (LO) or
Pair-Density-wave (PDW) type state. At the intersection of these two regimes
there is a "Super-Lifshitz" point with extra soft fluctuations. The instability
to finite momentum superconductivity occurs for arbitrarily weak pair-hopping
for sufficiently large attraction suggesting that even a small repulsive
pair-hopping may be significant in a microscopic model of strongly correlated
superconductivity. Several generic features of the model may have bearing on
the cuprate superconductors, including the suppression of superfluid stiffness
in proximity to a Lifshitz point as well as the existence of subleading FFLO
order (or vice versa) in the bicritical regime
Disorder-induced freezing of dynamical spin fluctuations in underdoped cuprates
We study the dynamical spin susceptibility of a correlated d-wave
superconductor (dSC) in the presence of disorder, using an unrestricted
Hartree-Fock approach. This model provides a concrete realization of the notion
that disorder slows down spin fluctuations, which eventually "freeze out". The
evolution of disorder-induced spectral weight transfer agrees qualitatively
with experimental observations on underdoped cuprate superconductors. For
sufficiently large disorder concentrations, static spin density wave (SDW)
order is created when droplets of magnetism nucleated by impurities overlap. We
also study the disordered stripe state coexisting with a dSC and compare its
magnetic fluctuation spectrum to that of the disorder-generated SDW phase.Comment: 5 pages, 4 figure
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