4,248 research outputs found
Similar glassy features in the NMR response of pure and disordered La1.88Sr0.12CuO4
High Tc superconductivity in La2-xSrxCuO4 coexists with (striped and glassy)
magnetic order. Here, we report NMR measurements of the 139La spin-lattice
relaxation, which displays a stretched-exponential time dependence, in both
pure and disordered x=0.12 single crystals. An analysis in terms of a
distribution of relaxation rates T1^-1 indicates that i) the spin-freezing
temperature is spatially inhomogeneous with an onset at Tg(onset)=20 K for the
pristine samples, and ii) the width of the T1^-1 distribution in the vicinity
of Tg(onset) is insensitive to an ~1% level of atomic disorder in CuO2 planes.
This suggests that the stretched-exponential 139La relaxation, considered as a
manifestation of the systems glassiness, may not arise from quenched disorder.Comment: 7 pages, to be published in Phys. Rev.
Stability of the spiral phase in the 2D extended t-J model
We analyze the t-t'-t''-J model at low doping by chiral perturbation theory
and show that the (1,0) spiral state is stabilized by the presence of t',t''
above critical values around 0.2J, assuming t/J=3.1. We find that the (magnon
mediated) hole-hole interactions have an important effect on the region of
charge stability in the space of parameters t',t'', generally increasing
stability, while the stability in the magnetic sector is guaranteed by the
presence of spin quantum fluctuations (order from disorder effect). These
conclusions are based on perturbative analysis performed up to two loops, with
very good convergence.Comment: 7 pages, 6 figure
Cluster Spin Glass Distribution Functions in LaSrCuO
Signatures of the cluster spin glass have been found in a variety of
experiments, with an effective onset temperature that is frequency
dependent. We reanalyze the experimental results and find that they are
characterized by a distribution of activation energies, with a nonzero glass
transition temperature . While the distribution of activation
energies is the same, the distribution of weights depends on the process.
Remarkably, the weights are essentially doping independent.Comment: 5 pages, 5 ps figure
Exact models for trimerization and tetramerization in spin chains
We present exact models for an antiferromagnetic S=1 spin chain describing
trimerization as well as for an antiferromagnetic S=3/2 spin chain describing
tetramerization. These models can be seen as generalizations of the
Majumdar-Ghosh model. For both models, we provide a local Hamiltonian and its
exact three- or four-fold degenerate ground state wavefunctions, respectively.
We numerically confirm the validity of both models using exact diagonalization
and discuss the low lying excitations.Comment: 7 pages, 8 figure
Cu NMR evidence for enhanced antiferromagnetic correlations around Zn impurities in YBa2Cu3O6.7
Doping the high-Tc superconductor YBa2Cu3O6.7 with 1.5 % of non-magnetic Zn
impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu
NMR spectra, as well as an increase of low-energy magnetic fluctuations
detected in 63Cu spin-lattice relaxation measurements. A model-independent
analysis demonstrates that these effects are due to the development of
staggered magnetic moments on many Cu sites around each Zn and that the
Zn-induced moment in the bulk susceptibility might be explained by this
staggered magnetization. Several implications of these enhanced
antiferromagnetic correlations are discussed.Comment: 4 pages including 2 figure
Enhancement of Pairing Correlation and Spin Gap through Suppression of Single-Particle Dispersion in One-Dimensional Models
We investigate the effects of suppression of single-particle dispersion near
the Fermi level on the spin gap and the singlet-pairing correlation by using
the exact diagonalization method for finite-size systems. We consider strongly
correlated one-dimensional models, which have flat band dispersions near wave
number k=\pi/2, if the interactions are switched off. Our results for strongly
correlated models show that the spin gap region expands as the single-particle
dispersion becomes flatter. The region where the singlet-pairing correlation is
the most dominant also expands in models with flatter band dispersions. Based
on our numerical results, we propose a pairing mechanism induced by the
flat-band dispersion.Comment: 5 pages, including 5 eps figures, to appear in J.Phys.Soc.Jpn Vol.69
No.
Absence of large nanoscale electronic inhomogeneities in the Ba(Fe1-xCox)2As2 pnictide
75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal
for x=6% for various field H values and orientations. The sharpness of the
superconducting and magnetic transitions demonstrates a homogeneity of the Co
doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of
the NMR spectra is found very anisotropic and very narrow for H//ab which
allows to rule out the interpretation of Ref.[6] in terms of strong Co induced
electronic inhomogeneities. We propose that a distribution of hyperfine
couplings and chemical shifts due to the Co effect on its nearest As explains
the observed linewidths and relaxations. All these measurements show that Co
substitution induces a very homogeneous electronic doping in BaFe2As2, from
nano to micrometer lengthscales, on the contrary to the K doping.Comment: 6 pages, 4 figure
Van der Waals forces in density functional theory: perturbational long-range electron interaction corrections
Long-range exchange and correlation effects, responsible for the failure of
currently used approximate density functionals in describing van der Waals
forces, are taken into account explicitly after a separation of the
electron-electron interaction in the Hamiltonian into short- and long-range
components. We propose a "range-separated hybrid" functional based on a local
density approximation for the short-range exchange-correlation energy, combined
with a long-range exact exchange energy. Long-range correlation effects are
added by a second-order perturbational treatment. The resulting scheme is
general and is particularly well-adapted to describe van der Waals complexes,
like rare gas dimers.Comment: 8 pages, 1 figure, submitted to Phys. Rev.
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