45 research outputs found
Singlet Stripe Phases in the planar t-J Model
The energies of singlet stripe phases in which a plane is broken up into spin
liquid ladders by lines of holes, is examined. If the holes were static then
patterns containing spin liquids with a finite spin gap are favored. The case
of dynamic holes is treated by assembling t-J ladders oriented perpendicular to
the stripes. For a wide region around the hole-hole
correlations in a single ladder are found to be predominantly charge density
wave type but an attraction between hole pairs on adjacent ladders leads to a
stripe phase. A quantum mechanical melting of the hole lines at smaller
values leads to a Bose condensate of hole pairs, i.e. a superconducting phase.Comment: 5 pages, uuencoded compressed PostScript file including 5 figures,
ETH-TH/942
Dynamics of lattice pinned charge stripes
We study the transversal dynamics of a charged stripe (quantum string) and
show that zero temperature quantum fluctuations are able to depin it from the
lattice. If the hopping amplitude t is much smaller than the string tension J,
the string is pinned by the underlying lattice. At t>>J, the string is depinned
and allowed to move freely, if we neglect the effect of impurities. By mapping
the system onto a 1D array of Josephson junctions, we show that the quantum
depinning occurs at t/J = 2 / pi^2. Besides, we exploit the relation of the
stripe Hamiltonian to the sine-Gordon theory and calculate the infrared
excitation spectrum of the quantum string for arbitrary t/J values.Comment: 4 pages, 2 figure
Noninvasive Neuromonitoring of Hypothermic Circulatory Arrest in Aortic Surgery
Background and Aims: Circulatory arrest carries a high risk of neurological damage, but modern monitoring methods lack reliability, and is susceptible to the generalized effects of both anesthesia and hypothermia. The objective of this prospective, explorative study was to research promising, reliable, and noninvasive methods of neuromonitoring, capable of predicting neurological outcome after hypothermic circulatory arrest. Materials and Methods: Thirty patients undergoing hypothermic circulatory arrest during surgery of the thoracic aorta were recruited in a single center and over the course of 4 years. Neuromonitoring was performed with a four-channel electroencephalogram montage and a near-infrared spectroscopy monitor. All data were tested off-line against primary neurological outcome, which was poor if the patient suffered a significant neurological complication (stroke, operative death). Results: A poor primary neurological outcome seen in 10 (33%) patients. A majority (63%) of the cases were emergency surgery, and thus, no neurological baseline evaluation was possible. The frontal hemispheric asymmetry of electroencephalogram, as measured by the brain symmetry index, predicted primary neurological outcome with a sensitivity of 79 (interquartile range; 62%-88%) and specificity of 71 (interquartile range; 61%-84%) during the first 6 h after end of circulatory arrest. Conclusion: The hemispheric asymmetry of frontal electroencephalogram is inherently resistant to generalized dampening effects and is predictive of primary neurological outcome. The brain symmetry index provides an easy-to-use, noninvasive neuromonitoring method for surgery of the thoracic aorta and postoperative intensive care.Peer reviewe
Quantizing Charged Magnetic Domain Walls: Strings on a Lattice
The discovery by Tranquada et al. of an ordered phase of charged domain walls
in the high-Tc cuprates leads us to consider the possible existence of a
quantum domain-wall liquid. We propose minimal models for the quantization, by
meandering fluctuations, of isolated charged domain walls. These correspond to
lattice string models. The simplest model of this kind, a directed lattice
string, can be mapped onto a quantum spin chain or on a classical
two-dimensional solid-on-solid surface model. The model exhibits a rich phase
diagram, containing several rough phases with low-lying excitations as well as
ordered phases which are gapped.Comment: 4 two-column pages, including the 3 Postscript figure
Coexistence of, and Competition between, Superconductivity and Charge-Stripe Order in La(1.6-x)Nd(0.4)Sr(x)CuO(4)
Previously we have presented evidence for stripe order of holes and spins in
La(1.6-x)Nd(0.4)Sr(x)CuO(4) with x=0.12. Here we show, via neutron diffraction
measurements of magnetic scattering, that similar order occurs in crystals with
x=0.15 and 0.20. Zero-field-cooled magnetization measurements show that all 3
compositions are also superconducting, with the superconducting transition
temperature increasing as the low-temperature staggered magnetization
decreases.Comment: REVTEX, 5 pages, 2 col., 4 epsf figures included with psfig.st
Lattice anisotropy as microscopic origin of static stripes in cuprates
Structural distortions in cuprate materials offer a microscopic origin for
anisotropies in electron transport in the basal plane. Using a real-space
Hartree-Fock approach, we consider the ground states of the anisotropic Hubbard
(t_x \ne t_y) and t-J (t_x \ne t_y, J_x \ne J_y) models. Symmetrical but
inhomogeneous (``polaronic'') charge structures in the isotropic models are
altered even by rather small anisotropies to one-dimensional, stripe-like
features. We find two distinct types of stripe, namely uniformly filled,
antiphase domain walls and non-uniform, half-filled, in-phase ones. We
characterize their properties, energies and dependence on the model parameters,
including filling and anisotropy in t (and J). We discuss the connections among
these results, other theoretical studies and experimental observation.Comment: 18 pages, 16 figures, 8 table
Charged domain walls as quantum strings living on a lattice
A generic lattice cut-off model is introduced describing the quantum
meandering of a single cuprate stripe. The fixed point dynamics is derived,
showing besides free string behavior a variety of partially quantum disordered
phases, bearing relationships both with quantum spin-chains and surface
statistical physics.Comment: 22 page, 17 figure