18 research outputs found
Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model
Slave boson calculations have been carried out in the three-band tJ model for
the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode
phonons. Phonon-induced Van Hove nesting leads to a phase separation between a
hole-doped domain and a (magnetic) domain near half filling, with long-range
Coulomb forces limiting the separation to a nanoscopic scale. Strong
correlation effects pin the Fermi level close to, but not precisely at the Van
Hove singularity (VHS), which can enhance the tendency to phase separation. The
resulting dispersions have been calculated, both in the uniform phases and in
the phase separated regime. In the latter case, distinctly different
dispersions are found for large, random domains and for regular (static)
striped arrays, and a hypothetical form is presented for dynamic striped
arrays. The doping dependence of the latter is found to provide an excellent
description of photoemission and thermodynamic experiments on pseudogap
formation in underdoped cuprates. In particular, the multiplicity of observed
gaps is explained as a combination of flux phase plus charge density wave (CDW)
gaps along with a superconducting gap. The largest gap is associated with VHS
nesting. The apparent smooth evolution of this gap with doping masks a
crossover from CDW-like effects near optimal doping to magnetic effects (flux
phase) near half filling. A crossover from large Fermi surface to hole pockets
with increased underdoping is found. In the weakly overdoped regime, the CDW
undergoes a quantum phase transition (), which could be obscured
by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes,
esp. in Sect. 3, Figs 1-4,6 replace
Postductal Coarctation of Aorta Causing Myocardial Ischaemia and Heart Failure in First Week of Life
Perceived environmental turbulence and its effect on selected entrepreneurship, marketing, and organizational characteristics in industrial firms
Entrepreneurship and marketing are approached as proactive corporate responses to an increasingly dynamic, threatening, and complex external environment. Both represent organizational orientations built around creativity, innovativeness, flexibility, and risk-taking. A conceptual model is proposed relating the levels of entrepreneurship, marketing activity, and marketing-related structure of a firm to the degree of perceived environmental turbulence confronting the firm. Results of a survey involving personal interviews with managers in 93 firms representing six industries are reported. Turbulence is found to have a significant causal impact on both the levels of entrepreneurship and the marketing orientation of the firm, but not on structural variables. © 1991 Academy of Marketing Science