15 research outputs found
One-point functions in massive integrable QFT with boundaries
We consider the expectation value of a local operator on a strip with
non-trivial boundaries in 1+1 dimensional massive integrable QFT. Using finite
volume regularisation in the crossed channel and extending the boundary state
formalism to the finite volume case we give a series expansion for the
one-point function in terms of the exact form factors of the theory. The
truncated series is compared with the numerical results of the truncated
conformal space approach in the scaling Lee-Yang model. We discuss the
relevance of our results to quantum quench problems.Comment: 43 pages, 20 figures, v2: typos correcte
Strong-coupling expansion and effective hamiltonians
When looking for analytical approaches to treat frustrated quantum magnets,
it is often very useful to start from a limit where the ground state is highly
degenerate. This chapter discusses several ways of deriving {effective
Hamiltonians} around such limits, starting from standard {degenerate
perturbation theory} and proceeding to modern approaches more appropriate for
the derivation of high-order effective Hamiltonians, such as the perturbative
continuous unitary transformations or contractor renormalization. In the course
of this exposition, a number of examples taken from the recent literature are
discussed, including frustrated ladders and other dimer-based Heisenberg models
in a field, as well as the mapping between frustrated Ising models in a
transverse field and quantum dimer models.Comment: To appear as a chapter in "Highly Frustrated Magnetism", Eds. C.
Lacroix, P. Mendels, F. Mil
Efimov effect in quantum magnets
Physics is said to be universal when it emerges regardless of the underlying
microscopic details. A prominent example is the Efimov effect, which predicts
the emergence of an infinite tower of three-body bound states obeying discrete
scale invariance when the particles interact resonantly. Because of its
universality and peculiarity, the Efimov effect has been the subject of
extensive research in chemical, atomic, nuclear and particle physics for
decades. Here we employ an anisotropic Heisenberg model to show that collective
excitations in quantum magnets (magnons) also exhibit the Efimov effect. We
locate anisotropy-induced two-magnon resonances, compute binding energies of
three magnons and find that they fit into the universal scaling law. We propose
several approaches to experimentally realize the Efimov effect in quantum
magnets, where the emergent Efimov states of magnons can be observed with
commonly used spectroscopic measurements. Our study thus opens up new avenues
for universal few-body physics in condensed matter systems.Comment: 7 pages, 5 figures; published versio
Scattering and duality in the 2 dimensional OSP(2|2) Gross Neveu and sigma models
We write the thermodynamic Bethe ansatz for the massive OSp(2|2) Gross Neveu
and sigma models. We find evidence that the GN S matrix proposed by Bassi and
Leclair [12] is the correct one. We determine features of the sigma model S
matrix, which seem highly unconventional; we conjecture in particular a
relation between this sigma model and the complex sine-Gordon model at a
particular value of the coupling. We uncover an intriguing duality between the
OSp(2|2) GN (resp. sigma) model on the one hand, and the SO(4) sigma (resp. GN
model) on the other, somewhat generalizing to the massive case recent results
on OSp(4|2). Finally, we write the TBA for the (SUSY version of the) flow into
the random bond Ising model proposed by Cabra et al. [39], and conclude that
their S matrix cannot be correct.Comment: 41 pages, 27 figures. v2: minor revisio
Bose-Einstein Condensation in Magnetic Insulators
The elementary excitations in antiferromagnets are magnons, quasiparticles
with integer spin and Bose statistics. In an experiment their density is
controlled efficiently by an applied magnetic field and can be made finite to
cause the formation of a Bose-Einstein condensate (BEC). Studies of magnon
condensation in a growing number of magnetic materials provide a unique window
into an exciting world of quantum phase transitions (QPT) and exotic quantum
states.Comment: 17 pages, 3 figure
The role of environmental factors in promoting and limiting biological invasions in South Africa
CITATION: Wilson, J.R. et al. 2020. The role of environmental factors in promoting and limiting biological invasions in South Africa. In: Biological Invasions in South Africa. van Wilgen, B.W., Measey, J., Richardson, D.M., Wilson, J.R. and Zengeya, T.A. (eds.). Springer, Cham. pp. 355-385. doi:10.1007/978-3-030-32394-3_13The original publication is available at https://link.springer.com/book/10.1007/978-3-030-32394-3This chapter provides an overview of the researchers and research initiatives relevant to invasion science in South Africa over the past 130 years, profiling some of the more recent personalities, particularly those who are today regarded as international leaders in the field. A number of key points arise from this review. Since 1913, South Africa has been one of a few countries that have investigated and implemented alien plant biological control on a large scale, and is regarded as a leader in this field. South Africa was also prominent in the conceptualisation and execution of the international SCOPE project on the ecology of biological invasions in the 1980s, during which South African scientists established themselves as valuable contributors to the field. The development of invasion science benefitted from a deliberate strategy to promote multi-organisational, interdisciplinary research in the 1980s. Since 1995, the Working for Water programme has provided funding for research and a host of practical questions that required research solutions. Finally, the establishment of a national centre of excellence with a focus on biological invasions has made a considerable contribution to building human capacity in the field, resulting in advances in all aspects of invasion science—primarily in terms of biology and ecology, but also in history, sociology, economics and management. South Africa has punched well above its weight in developing the field of invasion science, possibly because of the remarkable biodiversity that provided a rich template on which to carry out research, and a small, well-connected research community that was encouraged to operate in a collaborative manner.https://link.springer.com/chapter/10.1007%2F978-3-030-32394-3_13Publisher’s versio