62 research outputs found
Scanning tunneling microscopy simulations of poly(3-dodecylthiophene) chains adsorbed on highly oriented pyrolytic graphite
We report on a novel scheme to perform efficient simulations of Scanning
Tunneling Microscopy (STM) of molecules weakly bonded to surfaces. Calculations
are based on a tight binding (TB) technique including self-consistency for the
molecule to predict STM imaging and spectroscopy. To palliate the lack of
self-consistency in the tunneling current calculation, we performed first
principles density-functional calculations to extract the geometrical and
electronic properties of the system. In this way, we can include, in the TB
scheme, the effects of structural relaxation upon adsorption on the electronic
structure of the molecule. This approach is applied to the study of
regioregular poly(3-dodecylthiophene) (P3DDT) polymer chains adsorbed on highly
oriented pyrolytic graphite (HOPG). Results of spectroscopic calculations are
discussed and compared with recently obtained experimental datComment: 15 pages plus 5 figures in a tar fil
Correlations Between Charge Ordering and Local Magnetic Fields in Overdoped YBaCuO
Zero-field muon spin relaxation (ZF-SR) measurements were undertaken on
under- and overdoped samples of superconducting YBaCuO to
determine the origin of the weak static magnetism recently reported in this
system. The temperature dependence of the muon spin relaxation rate in
overdoped crystals displays an unusual behavior in the superconducting state. A
comparison to the results of NQR and lattice structure experiments on highly
doped samples provides compelling evidence for strong coupling of charge, spin
and structural inhomogeneities.Comment: 4 pages, 4 figures, new data, new figures and modified tex
Planar 17O NMR study of Pr_yY_{1-y}Ba_2Cu_3O_{6+x}
We report the planar ^{17}O NMR shift in Pr substituted YBa_{2}Cu_{3}O_{6+x},
which at x=1 exhibits a characteristic pseudogap temperature dependence,
confirming that Pr reduces the concentration of mobile holes in the CuO_{2}
planes. Our estimate of the rate of this counterdoping effect, obtained by
comparison with the shift in pure samples with reduced oxygen content, is found
insufficient to explain the observed reduction of T_c. From the temperature
dependent magnetic broadening of the ^{17}O NMR we conclude that the Pr moment
and the local magnetic defect induced in the CuO_2 planes produce a long range
spin polarization in the planes, which is likely associated with the extra
reduction of T_c. We find a qualitatively different behaviour in the oxygen
depleted Pr_yY_{1-y}Ba_2Cu_3O_{6.6}, i.e. the suppression of T is nearly
the same, but the magnetic broadening of the ^{17}O NMR appears weaker. This
difference may signal a weaker coupling of the Pr to the planes in the
underdoped compound, which might be linked with the larger Pr to CuO_2 plane
distance, and correspondingly weaker hybridization.Comment: 8 pages, 9 figures, accepted in Phys Rev
Angle-resolved photoemission study of untwinned PrBaCuO: undoped CuO plane and doped CuO chain
We have performed an angle-resolved photoemission study on untwinned
PrBaCuO, which has low resistivity but does not show
superconductivity. We have observed a dispersive feature with a band maximum
around (/2,/2), indicating that this band is derived from the undoped
CuO plane. We have observed another dispersive band exhibiting
one-dimensional character, which we attribute to signals from the doped CuO
chain. The overall band dispersion of the one-dimensional band agrees with the
prediction of model calculation with parameters relevant to cuprates
except that the intensity near the Fermi level is considerably suppressed in
the experiment.Comment: 6 pages, 10 figure
Friedel oscillations in a two-band Hubbard model for CuO chains
Friedel oscillations induced by open boundary conditions in a two-band
Hubbard model for CuO chains are numerically studied. We find that for
physically realistic parameters and close to quarter filling, these
oscillations have a 2k_F modulation according with experimental results on
YBa_2Cu_3O_{7-delta}. In addition, we predict that, for the same parameters, as
hole doping is reduced from quarter filling to half filling, Friedel
oscillations would acquire a 4k_F modulation, typical of a strongly correlated
electrons regime. The 4k_F modulation dominates also in the electron doped
region. The range of parameters varied is very broad, and hence the results
reported could apply to other cuprates and other strongly correlated compounds
with quasi-one dimensional structures. On a more theoretical side, we stress
the fact that the copper and oxygen subsystems should be described by two
different Luttinger liquid exponents.Comment: 7 pages, 7 eps figure
Spin dynamics and antiferromagnetic order in PrBa2Cu4O8 studied by Cu nuclear respnance
Results of the nuclear resonance experiments for the planar Cu sites in
PrBa2Cu4O8 are presented. The NMR spectrum at 1.5 K in zero magnetic field
revealed an internal field of 6.1 T, providing evidence for an
antiferromagnetic order of the planar Cu spins. This confirms that the CuO2
planes are insulating, therefore, the metallic conduction in this material is
entirely due to the one-dimensional zigzag Cu2O2 chains. The results of the
spin-lattice relaxation rates measured by zero field NQR above 245 K in the
paramagnetic state are explained by the theory for a Heisenberg model on a
square lattice.Comment: 4 pages, 2 figure
Angle-resolved photoemission study of insulating and metallic Cu-O chains in PrBaCuO and PrBaCuO
We compare the angle-resolved photoemission spectra of the hole-doped Cu-O
chains in PrBaCuO (Pr123) and in PrBaCuO (Pr124).
While, in Pr123, a dispersive feature from the chain takes a band maximum at
(momentum along the chain) and loses its spectral weight
around the Fermi level, it reaches the Fermi level at in
Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4-filled,
they show contrasting behaviors: While the chains in Pr123 have an instability
to charge ordering, those in Pr124 avoid it and show an interesting spectral
feature of a metallic coupled-chain system.Comment: 4 pages, 5 figures, to be published in PR
Superconducting Gap and Strong In-Plane Anisotropy in Untwinned YBa2Cu3O7-d
With significantly improved sample quality and instrumental resolution, we
clearly identify in the (pi,0) ARPES spectra from YBa2Cu3O6.993, in the
superconducting state, the long-sought `peak-dip-hump' structure. This advance
allows us to investigate the large a-b anisotropy of the in-plane electronic
structure including, in particular, a 50% difference in the magnitude of the
superconducting gap that scales with the energy position of the hump feature.
This anisotropy, likely induced by the presence of the CuO chains, raises
serious questions about attempts to quantitatively explain the YBa2Cu3O7-d data
from various experiments using models based on a perfectly square lattice.Comment: Phys. Rev. Lett., in press. Revtex, 4 pages, 4 postscript figures
embedded in the tex
suppression in co-doped striped cuprates
We propose a model that explains the reduction of due to the pinning of
stripes by planar impurity co-doping in cuprates. A geometrical argument about
the planar fraction of carriers affected by stripe pinning leads to a a linear
suppression as a function of impurity concentration . The critical
value for the vanishing of superconductivity is shown to scale like
in the under-doped regime and becomes universal in the optimally- and
over-doped regimes. Our theory agrees very well with the experimental data in
single- and bi-layer cuprates co-doped with Zn, Li, Co, etc...Comment: 4 pages, 4 figure
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