43 research outputs found
Holons on a meandering stripe: quantum numbers
We attempt to access the regime of strong coupling between charge carriers
and transverse dynamics of an isolated conducting ``stripe'', such as those
found in cuprate superconductors. A stripe is modeled as a partially doped
domain wall in an antiferromagnet (AF), introduced in the context of two
different models: the t-J model with strong Ising anisotropy, and the Hubbard
model in the Hartree-Fock approximation. The domain walls with a given linear
charge density are supported artificially by boundary conditions. In both
models we find a regime of parameters where doped holes lose their spin and
become holons (charge Q=1, spin S_z=0), which can move along the stripe without
frustrating AF environment. One aspect in which the holons on the AF domain
wall differ from those in an ordinary one-dimensional electron gas is their
transverse degree of freedom: a mobile holon always resides on a transverse
kink (or antikink) of the domain wall. This gives rise to two holon flavors and
to a strong coupling between doped charges and transverse fluctuations of a
stripe.Comment: Minor revisions: references update
Quantum magnetism in two dimensions: From semi-classical N\'eel order to magnetic disorder
This is a review of ground-state features of the s=1/2 Heisenberg
antiferromagnet on two-dimensional lattices. A central issue is the interplay
of lattice topology (e.g. coordination number, non-equivalent nearest-neighbor
bonds, geometric frustration) and quantum fluctuations and their impact on
possible long-range order. This article presents a unified summary of all 11
two-dimensional uniform Archimedean lattices which include e.g. the square,
triangular and kagome lattice. We find that the ground state of the spin-1/2
Heisenberg antiferromagnet is likely to be semi-classically ordered in most
cases. However, the interplay of geometric frustration and quantum fluctuations
gives rise to a quantum paramagnetic ground state without semi-classical
long-range order on two lattices which are precisely those among the 11 uniform
Archimedean lattices with a highly degenerate ground state in the classical
limit. The first one is the famous kagome lattice where many low-lying singlet
excitations are known to arise in the spin gap. The second lattice is called
star lattice and has a clear gap to all excitations.
Modification of certain bonds leads to quantum phase transitions which are
also discussed briefly. Furthermore, we discuss the magnetization process of
the Heisenberg antiferromagnet on the 11 Archimedean lattices, focusing on
anomalies like plateaus and a magnetization jump just below the saturation
field. As an illustration we discuss the two-dimensional Shastry-Sutherland
model which is used to describe SrCu2(BO3)2.Comment: This is now the complete 72-page preprint version of the 2004 review
article. This version corrects two further typographic errors (three total
with respect to the published version), see page 2 for detail
The suitability of SROI to assess Social Marketing Campaigns
The objective of this study is to demonstrate how an existing cost based analysis framework can be
utilised to assess either the potential or actual impact of a social marketing campaign. It is based on a case study with People Matters (Leeds) CIC who commissioned the author to analyse the
effectiveness of its social marketing campaigns so that it could demonstrate value and gain
additional funding from the Big Lotter
Ion cyclotron instabilities driven by the nearly perpendicular neutral beam injection in the Wendelstein VII-A stellarator
SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman