364 research outputs found
An application of the natural area concept to East London apartment areas
The world is faced with a population explosion, and cities are becoming ever larger. The world population will grow from its present 3500 million to more than 7 000 million by the year 2 000. The majority of cities are thus faced with the problem of housing vast numbers of people living in single family dwellings forming low density urban sprawl. Conditions are no different in South Africa where the present white population of about four million is expected to grow to between six and seven million by the year 2000. The present housing requirement (1970-75) for Whites, based on low and high population projections, is 32 732 and 40 150 houses respectively. From 1995-2000 the figures will have risen to 42 742 and 65 580 respectively. At that rate sprawl here will reach alarming proportions unless it can be curtailed by higher density housing. As the population trend does not seem likely to be reversed the problem lies in how to provide housing for an escalating population but at the same time to reduce urban sprawl and provide satisfactory living conditions
Electronic Ladders with SO(5) Symmetry: Phase Diagrams and Correlations at half-filling
We construct a family of electronic ladder models with SO(5) symmetry which
have exact ground states in the form of finitely correlated wave functions.
Extensions for these models preserving this symmetry are studied using these
states in a variational approach. Within this approach, the zero temperature
phase diagram of these electronic ladders at half filling is obtained,
reproducing the known results in the weak coupling (band insulator) and strong
coupling regime, first studied by Scalapino, Zhang and Hanke. Finally, the
compact form of the variational wave functions allows to compute various
correlation functions for these systems.Comment: RevTeX+epsf macros, 23 pp. including figure
Low Temperature Properties of Quantum Antiferromagnetic Chains with Alternating Spins S=1 and 1/2
We study the low-temperature properties of S=1 and 1/2 alternating spin
chains with antiferromagnetic nearest-neighbor exchange couplings using
analytical techniques as well as a quantum Monte Carlo method. The spin-wave
approach predicts two different low-lying excitations, which are gapped and
gapless, respectively. The structure of low-lying levels is also discussed by
perturbation theory in the strength of the Ising anisotropy. These analytical
findings are compared with the results of quantum Monte Carlo calculations and
it turns out that spin-wave theory well describes the present system. We
conclude that the quantum ferrimagnetic chain exhibits both ferromagnetic and
antiferromagnetic aspects.Comment: 13 pages, RevTeX, six figures, submitted to J. Phys. Cond. Ma
Exact symmetry breaking ground states for quantum spin chains
We introduce a family of spin-1/2 quantum chains, and show that their exact
ground states break the rotational and translational symmetries of the original
Hamiltonian. We also show how one can use projection to construct a spin-3/2
quantum chain with nearest neighbor interaction, whose exact ground states
break the rotational symmetry of the Hamiltonian. Correlation functions of both
models are determined in closed form. Although we confine ourselves to
examples, the method can easily be adapted to encompass more general models.Comment: 4 pages, RevTex. 4 figures, minor changes, new reference
Spin-dependent resonant tunneling through semimetallic ErAs quantum wells
Resonant tunneling through semimetallic ErAs quantum wells embedded in GaAs
structures with AlAs barriers was recently found to exhibit an intriguing
behavior in magnetic fields which is explained in terms of tunneling selection
rules and the spin-polarized band structure including spin-orbit coupling.Comment: 4 pages, figures supplied as self-unpacking figures.uu, uses
epsfig.sty to incorporate figures in preprin
Mixed Heisenberg Chains. I. The Ground State Problem
We consider a mechanism for competing interactions in alternating Heisenberg
spin chains due to the formation of local spin-singlet pairs. The competition
of spin-1 and spin-0 states reveals hidden Ising symmetry of such alternating
chains.Comment: 7 pages, RevTeX, 4 embedded eps figures, final versio
A new family of models with exact ground states connecting smoothly the S=1/2 dimer and S=1 Haldane phases of 1D spin chains
We investigate the isotropic two-leg S=1/2 ladder with general bilinear and
biquadratic exchange interactions between spins on neighboring rungs, and
determine the Hamiltonians which have a matrix product wavefunction as exact
ground state. We demonstrate that a smooth change of parameters leads one from
the S=1/2 dimer and Majumdar-Ghosh chains to the S=1 chain with biquadratic
exchange. This proves that these model systems are in the same phase. We also
present a new set of models of frustrated S=1/2 spin chains (including only
bilinear NN and NNN interactions) whose ground states can be found exactly.Comment: 4 pages, RevTeX, uses psfig.sty, submitted to Phys. Rev. Let
Magnetic excitations in SrCu2O3: a Raman scattering study
We investigated temperature dependent Raman spectra of the one-dimensional
spin-ladder compound SrCu2O3. At low temperatures a two-magnon peak is
identified at 3160+/-10 cm^(-1) and its temperature dependence analyzed in
terms of a thermal expansion model. We find that the two-magnon peak position
must include a cyclic ring exchange of J_cycl/J_perp=0.09-0.25 with a coupling
constant along the rungs of J_perp approx. 1215 cm^(-1) (1750 K) in order to be
consistent with other experiments and theoretical results.Comment: 4 pages, 3 figure
- …