484 research outputs found
Slow antiferromagnetic dynamics in the low temperature tetragonal phase of La_{2-x}Sr_xCuO_4 as revealed by ESR of Gd spin probes
Measuring the ESR of Gd spin probes we have studied the magnetic properties
of the copper oxide planes in the low temperature tetragonal (LTT) phase of Eu
doped La_{2-x}Sr_xCuO_4. The data give evidence that at particular levels of Sr
and Eu doping the frequency of the antiferromagnetic fluctuations in the LTT
phase dramatically decreases at low temperatures by almost three orders of
magnitude. However, no static magnetic order has been found for T>8K in
contrast to the observation by neutron scattering of stripe ordering of spins
below 50K in a Nd doped La_{2-x}Sr_xCuO_4 single crystal. To our opinion static
order in the Nd doped compound is induced due to the interaction between the Cu
spins with the rare earth magnetic moments. Therefore, a really characteristic
property of the magnetism in the LTT structural phase may be not static
magnetic order at elevated temperatures but rather extremely slow
antiferromagnetic dynamics.Comment: 12 pages RevTex, 2 EPS figures, to appear in Phys.Rev.B, Feb.,9
Revisiting and modeling the magnetism of hole-doped CuO_2 spin chains in Sr{14-x}Ca_xCu_{24}O_{41}
Magnetization measurements of Sr{14-x}Ca_xCu_{24}O_{41} with 0 <= x <=12 in
magnetic fields up to 16 T show that the low-temperature magnetic response of
the CuO_2 spin chains changes strongly upon Ca doping. For x=0 quantum
statistical simulations yield that the temperature and field dependence of the
magnetization can be well described by an effective Heisenberg model in which
the ground state configuration is composed of spin dimers, trimers, and
monomers. For x>0 a constant contribution to the low-temperature magnetic
susceptibility is observed which cannot be explained in terms of simple chain
models. Alternative scenarios are outlined.Comment: 2 pages, submitted to the proceedings of the ICM, Kyoto, Japan,
August 200
Ising magnets with mobile defects
Motivated by recent experiments on cuprates with low-dimensional magnetic
interactions, a new class of two-dimensional Ising models with short-range
interactions and mobile defects is introduced and studied. The non-magnetic
defects form lines, which, as temperature increases, first meander and then
become unstable. Using Monte Carlo simulations and analytical low- and
high-temperature considerations, the instability of the defect stripes is
monitored for various microscopic and thermodynamic quantities in detail for a
minimal model, assuming some of the couplings to be indefinitely strong. The
robustness of the findings against weakening the interactions is discussed as
well
Preferential antiferromagnetic coupling of vacancies in graphene on SiO_2: Electron spin resonance and scanning tunneling spectroscopy
Monolayer graphene grown by chemical vapor deposition and transferred to
SiO_2 is used to introduce vacancies by Ar^+ ion bombardment at a kinetic
energy of 50 eV. The density of defects visible in scanning tunneling
microscopy (STM) is considerably lower than the ion fluence implying that most
of the defects are single vacancies. The vacancies are characterized by
scanning tunneling spectroscopy (STS) on graphene and HOPG exhibiting a peak
close to the Fermi level. The peak persists after air exposure up to 180 min,
albeit getting broader. After air exposure for less than 60 min, electron spin
resonance (ESR) at 9.6 GHz is performed. For an ion flux of 10/nm^2, we find a
signal corresponding to a g-factor of 2.001-2.003 and a spin density of 1-2
spins/nm^2. The ESR signal consists of a mixture of a Gaussian and a Lorentzian
of equal weight exhibiting a width down to 0.17 mT, which, however, depends on
details of the sample preparation. The g-factor anisotropy is about 0.02%.
Temperature dependent measurements reveal antiferromagnetic correlations with a
Curie-Weiss temperature of -10 K. Albeit the electrical conductivity of
graphene is significantly reduced by ion bombardment, the spin resonance
induced change in conductivity is below 10^{-5}.Comment: 10 pages, 5 figures, discussion on STM images in the literature of
defects in graphene adde
Photoemission induced gating of topological insulator
The recently discovered topological insulators exhibit topologically
protected metallic surface states which are interesting from the fundamental
point of view and could be useful for various applications if an appropriate
electronic gating can be realized. Our photoemission study of Cu intercalated
Bi2Se3 shows that the surface states occupancy in this material can be tuned by
changing the photon energy and understood as a photoemission induced gating
effect. Our finding provides an effective tool to investigate the new physics
coming from the topological surface states and suggests the intercalation as a
recipe for synthesis of the material suitable for electronic applications.Comment: + resistivity data and some discussio
Resistivity and Hall effect of LiFeAs: Evidence for electron-electron scattering
LiFeAs is unique among the broad family of FeAs-based superconductors,
because it is superconducting with a rather large K under
ambient conditions although it is a stoichiometric compound. We studied the
electrical transport on a high-quality single crystal. The resistivity shows
quadratic temperature dependence at low temperature giving evidence for strong
electron-electron scattering and a tendency towards saturation around room
temperature. The Hall constant is negative and changes with temperature, what
most probably arises from a van Hove singularity close to the Fermi energy in
one of the hole-like bands. Using band structure calculations based on angular
resolved photoemission spectra we are able to reproduce all the basic features
of both the resistivity as well as the Hall effect data.Comment: 6 pages, 3 figures included; V2 has been considerably revised and
contain a more detailed analysis of the Hall effect dat
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