4,272 research outputs found
Frustrated magnetism and resonating valence bond physics in two-dimensional kagome-like magnets
We explore the phase diagram and the low-energy physics of three Heisenberg
antiferromagnets which, like the kagome lattice, are networks of corner-sharing
triangles but contain two sets of inequivalent short-distance resonance loops.
We use a combination of exact diagonalization, analytical strong-coupling
theories, and resonating valence bond approaches, and scan through the ratio of
the two inequivalent exchange couplings. In one limit, the lattices effectively
become bipartite, while at the opposite limit heavily frustrated nets emerge.
In between, competing tunneling processes result in short-ranged spin
correlations, a manifold of low-lying singlets (which can be understood as
localized bound states of magnetic excitations), and the stabilization of
valence bond crystals with resonating building blocks.Comment: Published versio
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China’s rise in a liberal world order in transition – introduction to the FORUM
In a time of great uncertainty about the future and resilience of the liberal world order this Forum focuses on China’s rise and interplay with the foundations of that liberal order. The key question is the extent to and variegated ways in which China - with its (re)ascendance to power and potential global leadership – is adapting to and perhaps even strengthening liberal institutions and rules of the game, confronting them, or developing alternative paths. In this introduction to the Forum we advance three key points based on the contributions. First, contrasting the orthodox binary scenarios of either inevitable conflict or co-optation offered in the mainstream IR debate, the Forum highlights the possibility of a third scenario of China’s interplay with the liberal world and its key actors, institutions, and rules. A hybrid and variegated scenario that entails both conflict and adaptation, differently entangled in different issue areas. Second, it stresses the need to conceptualize and empirically comprise the essentially interlinked nature of domestic state-society models and the global political economy. Third, we argue for a perspective that incorporates underlying economic and social structures and the power relations embedded therein
Orbital effects in manganites
In this paper I give a short review of some properties of the colossal
magnetoresistance manganites, connected with the orbital degrees of freedom.
Ions Mn{3+}, present in most of these compounds, have double orbital degeneracy
and are strong Jahn-Teller ions, causing structural distortions and orbital
ordering. Mechanisms leading to such ordering are shortly discussed, and the
role of orbital degrees of freedom in different parts of the phase diagram of
manganites is described. Special attention is paid to the properties of
low-doped systems (doping 0.1 - 0.25), to overdoped systems (x > 0.5), and to
the possibility of a novel type of orbital ordering in optimally doped
ferromagnetic metallic manganites.Comment: 28 pages, 7 figures, to be published in J. Mod. Phys.
Towards the M(atrix) model action in an arbitrary 11D supergravity background. Progress report
We are searching for the action principle for multiple M0-brane (multiple
M-wave or mM0) system starting from the mM0 equations of motion obtained in the
frame of superembedding approach. Surprisingly, the way from these equations to
the action happens to be hampered by a problem which suggests a possible
generalization of the action principle which we call "hierarchical action
principle".Comment: 4 pages, iopams style, contribution to the ERE 2010 Proceedings, to
be published in J.phys. Conf. Se
Spin-orbital frustrations and anomalous metallic state in iron-pnictide superconductors
We develop an understanding of the anomalous metal state of the parent
compounds of recently discovered iron based superconductors starting from a
strong coupling viewpoint, including orbital degrees of freedom. On the basis
of an intermediate-spin (S=1) state for the Fe^{2+} ions, we derive a
Kugel-Khomskii spin-orbital Hamiltonian for the active t_{2g} orbitals. It
turns out to be a highly complex model with frustrated spin and orbital
interactions. We compute its classical phase diagrams and provide an
understanding for the stability of the various phases by investigating its
spin-only and orbital-only limits. The experimentally observed spin-stripe
state is found to be stable over a wide regime of physical parameters and can
be accompanied by three different types of orbital orders. Of these the
orbital-ferro and orbital-stripe orders are particularly interesting since they
break the in-plane lattice symmetry -- a robust feature of the undoped
compounds. We compute the magnetic excitation spectra for the effective spin
Hamiltonian, observing a strong reduction of the ordered moment, and point out
that the proposed orbital ordering pattern can be measured in resonant X-ray
diffraction.Comment: 16 pages, 12 figure
A Potts model for the distortion transition in LaMnO
The Jahn-Teller distortive transition of \lmo is described by a modified
3-state Potts model. The interactions between the three possible orbits depends
both on the orbits and their relative orientation on the lattice. Values of the
two exchange parameters which are chosen to give the correct low temperature
phase and the correct value for the transition temperature are shown to be
consistent with microscopy theory. The model predicts a first order transitions
and also a value for the entropy above the transition in good agreement with
experiment. The theory with the same parameters also predicts the temperature
dependence of the order parameter of orbital ordering agreeing well with
published experimental results. Finally, the type of the transition is shown to
be close to one of the most disordered phases of the generalised Potts model.
The short range order found experimentally above the transition is investigated
by this model.Comment: 16 pages, 7 figures and no tables. Re-submitted to Phys. Rev.
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