390 research outputs found
Toward a Unified Magnetic Phase Diagram of the Cuprate Superconductors
We propose a unified magnetic phase diagram of cuprate superconductors. A new
feature of this phase diagram is a broad intermediate doping region of
quantum-critical, , behavior, characterized by temperature independent
and linear , where the spin waves are not completely
absorbed by the electron-hole continuum. The spin gap in the moderately doped
materials is related to the suppression of the low-energy spectral weight in
the quantum disordered, , regime. The crossover to the regime, where
T_1T/T_{\rm 2G}^2 \simeq \mbox{const}, occurs only in the fully doped
materials.Comment: 14 pages, REVTeX v2.1, PostScript file for 3 figures attached,
UIUC-P-93-06-04
Stability of homogeneous magnetic phases in a generalized t-J model
We study the stability of homogeneous magnetic phases in a generalized t-J
model including a same-sublattice hopping t' and nearest-neighbor repulsion V
by means of the slave fermion-Schwinger boson representation of spin operators.
At mean-field order we find, in agreement with other authors, that the
inclusion of further-neighbor hopping and Coulomb repulsion makes the
compressibility positive, thereby stabilizing at this level the spiral and Neel
orders against phase separation. However, the consideration of Gaussian
fluctuation of order parameters around these mean-field solutions produces
unstable modes in the dynamical matrix for all relevant parameter values,
leaving only reduced stability regions for the Neel phase. We have computed the
one-loop corrections to the energy in these regions, and have also briefly
considered the effects of the correlated hopping term that is obtained in the
reduction from the Hubbard to the t-J model.Comment: 5 pages, 5 figures, Revte
Spin dynamics in stripe-ordered La5/3Sr1/3NiO4
Polarized and unpolarized neutron inelastic scattering has been used to
measure the spin excitations in the spin-charge-ordered stripe phase of
La5/3Sr1/3NiO4. At high energies, sharp magnetic modes are observed
characteristic of a static stripe lattice. The energy spectrum is described
well by a linear spin wave model with intra- and inter-stripe exchange
interactions between neighbouring Ni spins given by J = 15 +/- 1.5 meV and J' =
7.5 +/- 1.5 meV respectively. A pronounced broadening of the magnetic
fluctuations in a band between 10 meV and 25 meV is suggestive of coupling to
collective motions of the stripe domain walls.Comment: ReVTeX 4, 4 pages inc. 4 Fig
Direct Observation of a One Dimensional Static Spin Modulation in Insulating La1.95Sr0.05CuO4
We report the results of an extensive elastic neutron scattering study of the
incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4 which is an
insulating spin glass at low temperatures. The present neutron scattering
experiments on the same x=0.05 crystal employ a narrower instrumental
Q-resolution and thereby have revealed that the crystal has only two
orthorhombic twins at low temperatures with relative populations of 2:1. We
find that, in a single twin, only two satellites are observed at (1, +/-0.064,
L)(ortho) and (0, 1+/-0.064, L)(ortho), that is, the modulation vector is only
along the orthorhombic b*-axis. This demonstrates unambiguously that
La1.95Sr0.05CuO4 has a one-dimensional static diagonal spin modulation at low
temperatures, consistent with certain stripe models. We have also reexamined
the x=0.04 crystal that previously was reported to show a single commensurate
peak. By mounting the sample in the (H, K, 0) zone, we have discovered that the
x=0.04 sample in fact has the same IC structure as the sample. The
incommensurability parameter d for x=0.04 and 0.05, where d is the distance
from (1/2, 1/2) in tetragonal reciprocal lattice units, follows the linear
relation d=x. These results demonstrate that the insulator to superconductor
transition in the under doped regime (0.05 </= x </= 0.06) in La2-xSrxCuO4 is
coincident with a transition from diagonal to collinear static stripes at low
temperatures thereby evincing the intimate coupling between the one dimensional
spin density modulation and the superconductivity.Comment: 9 pages 8 figure
Exploratory Biomarker Analysis In The Ph 3 Echelonâ1 Study: Worse Outcome With Abvd In Patients With Elevated Baseline Levels Of Scd30 And Tarc
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149507/1/hon99_2630.pd
Upper critical field for underdoped high-T_c superconductors. Pseudogap and stripe--phase
We investigate the upper critical field in a stripe--phase and in the
presence of a phenomenological pseudogap. Our results indicate that the
formation of stripes affects the Landau orbits and results in an enhancement of
. On the other hand, phenomenologically introduced pseudogap leads to a
reduction of the upper critical field. This effect is of particular importance
when the magnitude of the gap is of the order of the superconducting transition
temperature. We have found that a suppression of the upper critical field takes
place also for the gap that originates from the charge--density waves.Comment: 7 pages, 5 figure
Glassy nature of stripe ordering in La(1.6-x)Nd(0.4)Sr(x)CuO(4)
We present the results of neutron-scattering studies on various aspects of
crystalline and magnetic structure in single crystals of
La(1.6-x)Nd(0.4)Sr(x)CuO(4) with x=0.12 and 0.15. In particular, we have
reexamined the degree of stripe order in an x=0.12 sample. Measurements of the
width for an elastic magnetic peak show that it saturates at a finite value
below 30 K, corresponding to a spin-spin correlation length of 200 A. A model
calculation indicates that the differing widths of magnetic and (previously
reported) charge-order peaks, together with the lack of commensurability, can
be consistently explained by disorder in the stripe spacing. Above 30 K, the
width of the nominally elastic signal begins to increase. Interpreting the
signal as critical scattering from slowly fluctuating spins, the temperature
dependence of the width is consistent with renormalized classical behavior of a
2-dimensional anisotropic Heisenberg antiferromagnet. Inelastic scattering
measurements show that incommensurate spin excitations survive at and above 50
K, where the elastic signal is neglible. We also report several results related
to the LTO-to-LTT transition.Comment: 13 pp, 2-col. REVTeX, 11 figures embedded with psfig; expanded
discussion of T-dep. of magnetic peak width; version to appear in Phys. Rev.
B (01Jun99
Stability of metallic stripes in the extended one-band Hubbard model
Based on an unrestricted Gutzwiller approximation (GA) we investigate the
stripe orientation and periodicity in an extended one-band Hubbard model. A
negative ratio between next-nearest and nearest neighbor hopping t'/t, as
appropriate for cuprates, favors partially filled (metallic) stripes for both
vertical and diagonal configurations. At around optimal doping diagonal
stripes, site centered (SC) and bond centered (BC) vertical stripes become
degenerate suggesting strong lateral and orientational fluctuations. We find
that within the GA the resulting phase diagram is in agreement with experiment
whereas it is not in the Hartree-Fock approximation due to a strong
overestimation of the stripe filling. Results are in agreement with previous
calculations within the three-band Hubbard model but with the role of SC and BC
stripes interchanged.Comment: 10 pages, 8 figure
Berry phases and pairing symmetry in Holstein-Hubbard polaron systems
We study the tunneling dynamics of dopant-induced hole polarons which are
self-localized by electron-phonon coupling in a two-dimensional antiferro-
magnet. Our treatment is based on a path integral formulation of the adia-
batic approximation, combined with many-body tight-binding, instanton, con-
strained lattice dynamics, and many-body exact diagonalization techniques. Our
results are mainly based on the Holstein- and, for comparison, on the
Holstein-Hubbard model. We also study effects of 2nd neighbor hopping and
long-range electron-electron Coulomb repulsion. The polaron tunneling dynamics
is mapped onto an effective low-energy Hamiltonian which takes the form of a
fermion tight-binding model with occupancy dependent, predominant- ly 2nd and
3rd neighbor tunneling matrix elements, excluded double occupan- cy, and an
effective intersite charge interactions. Antiferromagnetic spin correlations in
the original many-electron Hamiltonian are reflected by an attractive
contribution to the 1st neighbor charge interaction and by Berry phase factors
which determine the signs of effective polaron tunneling ma- trix elements. In
the two-polaron case, these phase factors lead to polaron pair wave functions
of either -wave symmetry or p-wave symme- try with zero and
nonzero total pair momentum, respectively. Implications for the doping
dependent isotope effect, pseudo-gap and Tc of a superconduc- ting polaron pair
condensate are discussed/compared to observed in cuprates.Comment: 23 pages, revtex, 13 ps figure
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