503 research outputs found
The spin 1/2 Heisenberg star with frustration II: The influence of the embedding medium
We investigate the spin 1/2 Heisenberg star introduced in J. Richter and A.
Voigt, J. Phys. A: Math. Gen. {\bf 27}, 1139 (1994). The model is defined by
; , . In extension to the Ref. we consider a more general
describing the properties of the spins surrounding the
central spin . The Heisenberg star may be considered as an essential
structure element of a lattice with frustration (namely a spin embedded in a
magnetic matrix ) or, alternatively, as a magnetic system with a
perturbation by an extra spin. We present some general features of the
eigenvalues, the eigenfunctions as well as the spin correlation of the model. For being a linear chain, a square
lattice or a Lieb-Mattis type system we present the ground state properties of
the model in dependence on the frustration parameter .
Furthermore the thermodynamic properties are calculated for being a
Lieb--Mattis antiferromagnet.Comment: 16 pages, uuencoded compressed postscript file, accepted to J. Phys.
A: Math. Ge
The square-kagome quantum Heisenberg antiferromagnet at high magnetic fields: The localized-magnon paradigm and beyond
We consider the spin-1/2 antiferromagnetic Heisenberg model on the
two-dimensional square-kagome lattice with almost dispersionless lowest magnon
band. For a general exchange coupling geometry we elaborate low-energy
effective Hamiltonians which emerge at high magnetic fields. The effective
model to describe the low-energy degrees of freedom of the initial frustrated
quantum spin model is the (unfrustrated) square-lattice spin-1/2 model in
a -aligned magnetic field. For the effective model we perform quantum Monte
Carlo simulations to discuss the low-temperature properties of the
square-kagome quantum Heisenberg antiferromagnet at high magnetic fields. We
pay special attention to a magnetic-field driven
Berezinskii-Kosterlitz-Thouless phase transition which occurs at low
temperatures.Comment: 6 figure
Enhanced magnetocaloric effect in frustrated magnetic molecules with icosahedral symmetry
We investigate the magnetocaloric properties of certain antiferromagnetic
spin systems that have already been or very likely can be synthesized as
magnetic molecules. It turns out that the special geometric frustration which
is present in antiferromagnets that consist of corner-sharing triangles leads
to an enhanced magnetocaloric effect with high cooling rates in the vicinity of
the saturation field. These findings are compared with the behavior of a simple
unfrustrated spin ring as well as with the properties of the icosahedron. To
our surprise, also for the icosahedron large cooling rates can be achieved but
due to a different kind of geometric frustration.Comment: 5 pages, 8 figures, more information at
http://obelix.physik.uni-osnabrueck.de/~schnack
Structured identification of business improvement opportunities using Life Cycle Assessment : A case study in the gas turbine industry
Part of:
Seliger, Günther (Ed.): Innovative solutions : proceedings / 11th Global Conference on Sustainable Manufacturing, Berlin, Germany, 23rd - 25th September, 2013. - Berlin: Universitätsverlag der TU Berlin, 2013. - ISBN 978-3-7983-2609-5 (online). - http://nbn-resolving.de/urn:nbn:de:kobv:83-opus4-40276. - pp. 205–209.In the last two decades the power sector has been adopting environmental conscious practices in several business areas and processes. Bridging the identification of environmental “hot-spots” in the product life cycle and the implementation and execution of an environmental management system requires an integrated approach starting with a Life Cycle Assessment to identify the improvement potentials; then analyzing the current management and product development systematics in use, and finally mapping the environmental practices against the improvement potential. The improvement tracking will be embedded in the management system as an Environmental Improvement Roadmap, mapping the efforts required to realize the goals. The methodology has been implemented in pilot studies, focusing on the processes performed in-house to enable further decisions on process alternatives and providing reliable information for strategic decisions within the Siemens Environmental Product Portfolio
Linear independence of localized magnon states
At the magnetic saturation field, certain frustrated lattices have a class of
states known as "localized multi-magnon states" as exact ground states. The
number of these states scales exponentially with the number of spins and
hence they have a finite entropy also in the thermodynamic limit
provided they are sufficiently linearly independent. In this article we present
rigorous results concerning the linear dependence or independence of localized
magnon states and investigate special examples. For large classes of spin
lattices including what we called the orthogonal type and the isolated type as
well as the kagom\'{e}, the checkerboard and the star lattice we have proven
linear independence of all localized multi-magnon states. On the other hand the
pyrochlore lattice provides an example of a spin lattice having localized
multi-magnon states with considerable linear dependence.Comment: 23 pages, 6 figure
A solvable model of a random spin-1/2 XY chain
The paper presents exact calculations of thermodynamic quantities for the
spin-1/2 isotropic XY chain with random lorentzian intersite interaction and
transverse field that depends linearly on the surrounding intersite
interactions.Comment: 14 pages (Latex), 2 tables, 13 ps-figures included, (accepted for
publication in Phys.Rev.B
Regularly alternating spin-1/2 anisotropic XY chains: The ground-state and thermodynamic properties
Using the Jordan-Wigner transformation and continued fractions we calculate
rigorously the thermodynamic quantities for the spin-1/2 transverse Ising chain
with periodically varying intersite interactions and/or on-site fields. We
consider in detail the properties of the chains having a period of the
transverse field modulation equal to 3. The regularly alternating transverse
Ising chain exhibits several quantum phase transition points, where the number
of transition points for a given period of alternation strongly depends on the
specific set of the Hamiltonian parameters. The critical behavior in most cases
is the same as for the uniform chain. However, for certain sets of the
Hamiltonian parameters the critical behavior may be changed and weak
singularities in the ground-state quantities appear. Due to the regular
alternation of the Hamiltonian parameters the transverse Ising chain may
exhibit plateau-like steps in the zero-temperature dependence of the transverse
magnetization vs. transverse field and many-peak temperature profiles of the
specific heat. We compare the ground-state properties of regularly alternating
transverse Ising and transverse XX chains and of regularly alternating quantum
and classical chains.
Making use of the corresponding unitary transformations we extend the
elaborated approach to the study of thermodynamics of regularly alternating
spin-1/2 anisotropic XY chains without field. We use the exact expression for
the ground-state energy of such a chain of period 2 to discuss how the exchange
interaction anisotropy destroys the spin-Peierls dimerized phase
Dynamic and thermodynamic properties of the generalised diamond chain model for azurite
The natural mineral azurite Cu3(CO3)2(OH)2 is an interesting spin-1/2 quantum
antiferromagnet. Recently, a generalised diamond chain model has been
established as a good description of the magnetic properties of azurite with
parameters placing it in a highly frustrated parameter regime. Here we explore
further properties of this model for azurite. First, we determine the inelastic
neutron scattering spectrum in the absence of a magnetic field and find good
agreement with experiments, thus lending further support to the model.
Furthermore, we present numerical data for the magnetocaloric effect and
predict that strong cooling should be observed during adiabatic
(de)magnetisation of azurite in magnetic fields slightly above 30T. Finally,
the presence of a dominant dimer interaction in azurite suggests the use of
effective Hamiltonians for an effective low-energy description and we propose
that such an approach may be useful to fully account for the three-dimensional
coupling geometry.Comment: 19 pages, 6 figures; to appear in: J. Phys.: Condens. Matter (special
issue on geometrically frustrated magnetism
High spin cycles: topping the spin record for a single molecule verging on quantum criticality
The cyclisation of a short chain into a ring provides fascinating scenarios in terms of transforming a finite array of spins into a quasiinfinite
structure. If frustration is present, theory predicts interesting quantum critical points, where the ground state and thus lowtemperature
properties of a material change drastically upon even a small variation of appropriate external parameters. This can be
visualised as achieving a very high and pointed summit where the way down has an infinity of possibilities, which by any parameter
change will be rapidly chosen, in order to reach the final ground state. Here we report a mixed 3d/4f cyclic coordination cluster that
turns out to be very near or even at such a quantum critical point. It has a ground state spin of S = 60, the largest ever observed for
a molecule (120 times that of a single electron). [Fe10Gd10(Me-tea)10(Me-teaH)10(NO3)10]\ub720MeCN forms a nano-torus with
alternating gadolinium and iron ions with a nearest neighbour Fe\u2013Gd coupling and a frustrating next-nearest neighbour Fe\u2013Fe
coupling. Such a spin arrangement corresponds to a cyclic delta or saw-tooth chain, which can exhibit unusual frustration effects. In
the present case, the quantum critical point bears a \u2018flatland\u2019 of tens of thousands of energetically degenerate states between
which transitions are possible at no energy costs with profound caloric consequences. Entropy-wise the energy flatland translates
into the pointed summit overlooking the entropy landscape. Going downhill several target states can be reached depending on the
applied physical procedure which offers new prospects for addressability
Influence of quantum fluctuations on zero-temperature phase transitions between collinear and noncollinear states in frustrated spin systems
We study a square-lattice spin-half Heisenberg model where frustration is
introduced by competing nearest-neighbor bonds of different signs. We discuss
the influence of quantum fluctuations on the nature of the zero-temperature
phase transitions from phases with collinear magnetic order at small
frustration to phases with noncollinear spiral order at large frustration. We
use the coupled cluster method (CCM) for high orders of approximation (up to
LSUB6) and the exact diagonalization of finite systems (up to 32 sites) to
calculate ground-state properties. The role of quantum fluctuations is examined
by comparing the ferromagnetic-spiral and the antiferromagnetic-spiral
transition within the same model. We find clear evidence that quantum
fluctuations prefer collinear order and that they may favour a first order
transition instead of a second order transition in case of no quantum
fluctuations.Comment: 6 pages, 6 Postscipt figures; Accepted for publication in Phys. Rev.
- …