289 research outputs found
Ground-State Energy and Spin Gap of Spin-1/2 Kagome Heisenberg Antiferromagnetic Clusters: Large Scale Exact Diagonalization Results
We present a comprehensive list of ground state energies and spin gaps of
finite kagome clusters with up to 42 spins obtained using large-scale exact
diagonalization techniques. This represents the current limit of this exact
approach. For a fixed number of spins N we study several cluster shapes under
periodic boundary conditions in both directions resulting in a toroidal
geometry. The clusters are characterized by their side length and diagonal as
well as the shortest "Manhattan" diameter of the torii. A finite-size scaling
analysis of the ground state energy as well as the spin gap is then performed
in terms of the shortest toroidal diameter as well as the shortest "Manhattan"
diameter. The structure of the spin-spin correlations further supports the
importance of short loops wrapping around the torii.Comment: 4 pages, 4 figures, added one referenc
Dermatologische Proktologie: Eine Ăbersicht anhand des Pruritus ani
Zusammenfassung: Pruritus ani ist eines der hĂ€ufigsten Symptome, die den Patienten in die proktologische Sprechstunde fĂŒhren. Als Ursache können zahlreiche Krankheiten zugrunde liegen. Bei einem GroĂteil aller Patienten mit Pruritus ani finden sich dermatologische Ursachen. Fast jede Dermatose kann sich auch perianal manifestieren und zu Pruritus fĂŒhren. Eine Blickdiagnose ist selten möglich, da die klinische PrĂ€sentation hĂ€ufig atypisch ist. So sind fĂŒr die Diagnosestellung neben ausfĂŒhrlicher Anamnese und körperlicher Untersuchung mit Ganzkörperhautstatus mikrobiologische und allergologische AbklĂ€rungen sowie eine Hautbiopsie wichtige diagnostische MaĂnahmen. Begleitend zur Behandlung der Grundkrankheit empfiehlt sich als Basistherapie eine milde Reinigung mit seifenfreiem Wasser, anschlieĂendem vorsichtigen Trocknen sowie die Anwendung von rĂŒckfettenden oder zinkhaltigen Extern
Bond order wave instabilities in doped frustrated antiferromagnets: "Valence bond solids" at fractional filling
We explore both analytically and numerically the properties of doped t-J
models on a class of highly frustrated lattices, such as the kagome and the
pyrochlore lattice. Focussing on a particular sign of the hopping integral and
antiferromagnetic exchange, we find a generic symmetry breaking instability
towards a twofold degenerate ground state at a fractional filling below half
filling. These states show modulated bond strengths and only break lattice
symmetries. They can be seen as a generalization of the well-known valence bond
solid states to fractional filling.Comment: slightly shortened and reorganized versio
On product, generic and random generic quantum satisfiability
We report a cluster of results on k-QSAT, the problem of quantum
satisfiability for k-qubit projectors which generalizes classical
satisfiability with k-bit clauses to the quantum setting. First we define the
NP-complete problem of product satisfiability and give a geometrical criterion
for deciding when a QSAT interaction graph is product satisfiable with positive
probability. We show that the same criterion suffices to establish quantum
satisfiability for all projectors. Second, we apply these results to the random
graph ensemble with generic projectors and obtain improved lower bounds on the
location of the SAT--unSAT transition. Third, we present numerical results on
random, generic satisfiability which provide estimates for the location of the
transition for k=3 and k=4 and mild evidence for the existence of a phase which
is satisfiable by entangled states alone.Comment: 9 pages, 5 figures, 1 table. Updated to more closely match published
version. New proof in appendi
Effective Spin Couplings in the Mott Insulator of the Honeycomb Lattice Hubbard Model
Motivated by the recent discovery of a spin liquid phase for the Hubbard
model on the honeycomb lattice at half-filling, we apply both perturbative and
non-perturbative techniques to derive effective spin Hamiltonians describing
the low-energy physics of the Mott-insulating phase of the system. Exact
diagonalizations of the so-derived models on small clusters are performed, in
order to assess the quality of the effective low-energy theory in the
spin-liquid regime. We show that six-spin interactions on the elementary loop
of the honeycomb lattice are the dominant sub-leading effective couplings. A
minimal spin model is shown to reproduce most of the energetic properties of
the Hubbard model on the honeycomb lattice in its spin-liquid phase.
Surprisingly, a more elaborate effective low-energy spin model obtained by a
systematic graph expansion rather disagrees beyond a certain point with the
numerical results for the Hubbard model at intermediate couplings.Comment: 20 pages, 10 figure
A 2D quantum antiferromagnet with a four-fold degenerate valence-bond-solid ground state
We study the competition between antiferromagnetic order and valence bond
solid formation in a two-dimensional frustrated spin-1/2 model. The J1-J2 model
on the square lattice is further frustrated by introducing products of
three-spin projectors which stabilize four dimer-product states as degenerate
ground states. These four states are reminiscent of the dimerized singlet
ground state of the Shastry-Sutherland model. Using exact diagonalizations, we
study the evolution of the ground state by varying theratio of interactions.
For a large range of parameters (J2 > 0.25J1), our model shows a direct
transition between the valence-bond-solid phase and the collinear
antiferromagnetic phase. For small values of J2, several intermediate phases
appear which are also analyzed
Monte Carlo study of the critical properties of the three-dimensional 120-degree model
We report on large scale finite-temperature Monte Carlo simulations of the
classical or orbital-only model on the simple cubic lattice
in three dimensions with a focus towards its critical properties. This model
displays a continuous phase transition to an orbitally ordered phase. While the
correlation length exponent is close to the 3D XY value, the
exponent differs substantially from O(N) values. We also
introduce a discrete variant of the model, called -clock model,
which is found to display the same set of exponents. Further, an emergent U(1)
symmetry is found at the critical point , which persists for below
a crossover length scaling as , with an unusually small
.Comment: 13 pages, 6 figure
Two Step Restoration of SU(2) Symmetry in a Frustrated Ring-Exchange Magnet
We demonstrate the existence of a spin-nematic, moment-free phase in a
quantum four-spin ring exchange model on the square lattice. This unusual
quantum state is created by the interplay of frustration and quantum
fluctuations which lead to a partial restoration of SU(2) symmetry when going
from a four-sublattice orthogonal biaxial Neel order to this exotic uniaxial
magnet. A further increase of frustration drives a transition to a fully gapped
SU(2) symmetric valence bond crystal.Comment: 4 pages, 5 figure
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