393 research outputs found
Coupling to optical phonons in the one-dimensional t-J model: Effects on superconducting fluctuations and phase separation
The one-dimensional (1D) - Holstein model is studied by exact
diagonalization of finite rings using a variational approximation for the
phonon states. Due to renormalization effects induced by the phonons, for
intermediate electron-phonon coupling, the phase separation (PS) boundary, and
with it the region of dominating superconducting fluctuations is shifted
substantially to smaller values of as compared to the pure - model.
Superconducting correlations are weakened through charge density wave
interactions mediated by the phonons. Possible consequences for the high
oxides are discussed.Comment: 4 pages, Latex2
Phonon Dispersion Relations in PrBa2Cu3O6+x (x ~ 0.2)
We report measurements of the phonon dispersion relations in
non-superconducting, oxygen-deficient PrBa2Cu3O6+x (x ~ 0.2) by inelastic
neutron scattering. The data are compared with a model of the lattice dynamics
based on a common interaction potential. Good agreement is achieved for all but
two phonon branches, which are significantly softer than predicted. These modes
are found to arise predominantly from motion of the oxygen ions in the CuO2
planes. Analogous modes in YBa2Cu3O6 are well described by the common
interaction potential model.Comment: 4 pages, 3 figures. Minor changes following referees' comment
Phenomenological BCS theory of the high- cuprates
A BCS model characterized by a phenomenological pair potential with on-site
(), nearest (), and next nearest () neighbour coupling
constants, and an empirical quasiparticle dispersion taken from angle-resolved
photoemission spectra is considered. The model can consistently explain the
experimental data concerning the pair state of the hole doped cuprates. Three
ingredients are required to make the interpretation possible: the existence of
flat bands, a very small effective on-site repulsion, and a slightly dominating
effective nnn attraction of the order of 60-80meV with a ratio .Comment: 13 pages, uuencoded Postscrip
Anomalous behaviors of the charge and spin degrees of freedom in the CuO double chains of PrBaCuO
The density-matrix renormalization-group method is used to study the
electronic states of a two-chain Hubbard model for CuO double chains of
PrBaCuO. We show that the model at quarter filling has the charge
ordered phases with stripe-type and in-line--type patterns in the parameter
space, and in-between, there appears a wide region of vanishing charge gap; the
latter phase is characteristic of either Tomonaga-Luttinger liquid or a
metallic state with a spin gap. We argue that the low-energy electronic state
of the CuO double chains of PrBaCuO should be in the metallic state
with a possibly small spin gap.Comment: REVTEX 4, 10 pages, 9 figures; submitted to PR
Phenomenological Models for the Gap Anisotropy of Bi-2212 as Measured by ARPES
Recently, high resolution angle-resolved photoemission spectroscopy has been
used to determine the detailed momentum dependence of the superconducting gap
in the high temperature superconductor Bi-2212. In this paper, we first
describe tight binding fits to the normal state dispersion and superlattice
modulation effects. We then discuss various theoretical models in light of the
gap measurements. We find that the simplest model which fits the data is the
anisotropic s-wave gap , which within a one-band BCS frame-
work suggests the importance of next near neighbor Cu-Cu interactions. Various
alternative interpretations of the observed gap are also discussed, along with
the implications for microscopic theories of high temperature superconductors.Comment: 14 pages, revtex, 9 uuencoded postscript 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
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
Transport and Magnetic Studies on the Spin State Transition of Pr1-xCaxCoO3 up to High Pressure
Transport and magnetic measurements and structural and NMR studies have been
carried out on (Pr1-yR'y)1-xAxCoO3 {R'=(rare earth elements and Y); A=(Ca, Ba
and Sr)} at ambient pressure or under high pressure. The system exhibits a
phase transition from a nearly metallic to an insulating state with decreasing
temperature T, where the low spin (LS) state of Co3+ is suddenly stabilized.
For y=0, we have constructed a T-x phase diagram at various values of the
external pressure p. It shows that the (T, x) region of the low temperature
phase, which is confined to a very narrow region around x=0.5 at ambient
pressure, expands as p increases, suggesting that the transition is not due to
an order-disorder type one. For the occurrence of the transition, both the Pr
and Ca atoms seem to be necessary. The intimate relationship between the local
structure around the Co ions and the electronic (or spin) state of Co3+ ions is
discussed: For the smaller unit cell volume or the smaller volume of the CoO6
octahedra and for the larger tilting angle of the octahedra, the temperature of
the transition becomes higher. The role of the carriers introduced by the
doping of the A atoms, is also discussed. By analyzing the data of 59Co-NMR
spectra and magnetic susceptibilities of Pr1-xCaxCoO3 the energy separations
among the different spin states of Co3+ and Co4+ are roughly estimated.Comment: 15 pages, 15 figures, 2 tables, submitted to J. Phys. Soc. Jp
Problems With the Vortex-Boson Mapping in 1+1 Dimensions
Using the well known boson mapping, we relate the transverse magnetic
susceptibility of a system of flux vortices in 1+1 dimensions to an
appropriately defined conductivity of a one-dimensional boson system. The tilt
response for a system free of disorder is calculated directly, and it is found
that a subtle order of limits is required to avoid deceptive results.Comment: 4 Pages (REVTeX 3.0). Postscript file for this paper is available on
the World Wide Web at http://cmtw.harvard.edu/~simon/
Critical temperature and superfluid density suppression in disordered high- cuprate superconductors
We argue that the standard Abrikosov-Gorkov (AG) type theory of in
disordered -wave superconductors breaks down in short coherence length
high- cuprates. Numerical calculations within the Bogoliubov-de Gennes
formalism demonstrate that the correct description of such systems must allow
for the spatial variation of the order parameter, which is strongly suppressed
in the vicinity of impurities but mostly unaffected elsewhere. Suppression of
as measured with respect to the attendant decrease in the superfluid
density is found to be significantly weaker than that predicted by the AG
theory, in good agreement with experiment.Comment: REVTeX, 4 pages, 3 ps figures included [The version to appear in PRB
Sept. 1. Conclusions of the paper unchanged; several changes in text and
figures for added clarity, discussion of phase fluctuations added.
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