3,167 research outputs found
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
Disorder effect in low dimensional superconductors
The quasiparticle density of states (DOS), the energy gap, the superfluid
density , and the localization effect in the s- and d-wave
superconductors with non-magnetic impurity in two dimensions (2D) are studied
numerically. For strong (unitary) scatters, we find that it is the range of the
scattering potential rather than the symmetry of the superconducting pairing
which is more important in explaining the impurity dependences of the specific
heat and the superconducting transition temperature in Zn doped YBCO. The
localization length is longer in the d-wave superconducting state than in the
normal state, even in the vicinity of the Fermi energy.Comment: 2 pages, uuencoded compressed postscript file, IRC-940610
Extinction of quasiparticle interference in underdoped cuprates with coexisting order
Recent scanning tunnelling spectroscopy measurements [Y. Koksaka et al.,
Nature 454, 1072 (2008)] have shown that dispersing quasiparticle interference
peaks in Fourier transformed conductance maps disappear as the bias voltage
exceeds a certain threshold corresponding to the coincidence of the contour of
constant quasiparticle energy with the antiferromagnetic zone boundary. Here we
argue that this is caused by quasistatic short-range coexisting order present
in the d-wave superconducting phase, and that the most likely origin of this
order is disorder-induced incommensurate antiferromagnetism. We show explicitly
how the peaks are extinguished in the related situation with coexisting
long-range antiferromagnetic order, and discuss the connection with the
realistic disordered case. Since it is the localized quasiparticle interference
peaks rather than the underlying antinodal states themselves which are
destroyed at a critical bias, our proposal resolves a conflict between scanning
tunneling spectroscopy and photoemission regarding the nature of these states.Comment: 10 pages, 9 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
Raising the critical temperature by disorder in unconventional superconductors mediated by spin fluctuations
We propose a mechanism whereby disorder can enhance the transition
temperature Tc of an unconventional superconductor with pairing driven by
exchange of spin fluctuations. The theory is based on a self-consistent real
space treatment of pairing in the disordered one-band Hubbard model. It has
been demonstrated before that impurities can enhance pairing by softening the
spin fluctuations locally; here, we consider the competing effect of
pair-breaking by the screened Coulomb potential also present. We show that,
depending on the impurity potential strength and proximity to magnetic order,
this mechanism results in a weakening of the disorder-dependent Tc-suppression
rate expected from Abrikosov-Gor'kov theory, or even in disorder-generated Tc
enhancements.Comment: 6 pages, 4 figures + Supplementary Materia
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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