5,137 research outputs found
Comment on ``Quantum Phase Transition of the Randomly Diluted Heisenberg Antiferromagnet on a Square Lattice''
In Phys. Rev. Lett. 84, 4204 (2000) (cond-mat/9905379), Kato et al. presented
quantum Monte Carlo results indicating that the critical concentration of
random non-magnetic sites in the two-dimensional antiferromagnetic Heisenberg
model equals the classical percolation density; pc=0.407254. The data also
suggested a surprising dependence of the critical exponents on the spin S of
the magnetic sites, with a gradual approach to the classical percolation
exponents as S goes to infinity. I here argue that the exponents in fact are
S-independent and equal to those of classical percolation. The apparent
S-dependent behavior found by Kato et al. is due to temperature effects in the
simulations as well as a quantum effect that masks the true asymptotic scaling
behavior for small lattices.Comment: Comment on Phys. Rev. Lett. 84, 4204 (2000), by K. Kato et al.; 1
page, 1 figur
Finite-size scaling and boundary effects in two-dimensional valence-bond-solids
Various lattice geometries and boundaries are used to investigate
valence-bond-solid (VBS) ordering in the ground state of an S=1/2
square-lattice quantum spin model---the J-Q model, in which 4- or 6-spin
interactions Q are added to the Heisenberg exchange J. Ground state results for
finite systems (with up to thousands of spins) are obtained using a projector
QMC method. Great care has to be taken when extrapolating the order parameter
to infinite size, in particular in cylinder geometry. Even though strong 2D VBS
order exists and is established clearly with increasing system size on L*L
lattices (or Lx* Ly lattices with a fixed Lx/Ly), only short-range VBS
correlations are observed on long cylinders (when Lx -> infinity at fixed Ly).
The correlation length increases with Ly, until long-range order sets in at a
"critical" Ly. This width is large even when the 2D order is strong, e.g, for a
system where the order parameter is 70% of the largest possible value, Ly=8 is
required for ordering. Correlation functions for small L*L lattices can also be
misleading. For a 20%-ordered system results for L up to 20 appear to
extrapolate to a vanishing order parameter, while for larger L the behavior
crosses over and extrapolates to a non-zero value (with exponentially small
finite size corrections). The VBS order also exhibits interesting edge effects
related to emergent U(1) symmetry, which, if not considered properly, can lead
to wrong conclusions for the thermodynamic limit. The finite-size behavior for
small L*L lattices and long cylinders is similar to that predicted for a Z2
spin liquid. The results raise concerns about recent works claiming Z2 spin
liquid ground states in frustrated 2D systems, in particular, the Heisenberg
model with nearest and next-nearest-neighbor couplings. VBS state in this
system cannot be ruled out.Comment: 26 pages, 28 figures. v2: final, published versio
Optimal taxation and normalisations
There still seems to be some confusion about the consequences of normalisations in the optimal taxation litterature. We claim that: 1) Normalisations do not matter for the real solution of optimal taxation problem. 2) Normalisations do matter for good characterisations of the solutions to optimal taxation problems. Whereas the first point is uncontroversial, the second one is less well understood. By postponing the normalisation of consumer prices, we detail how normalisations of consumer prices affect the characterisation of optimal commodity taxes, derive the preferred characterisation, and show how it depends on the normalisation.Optimal taxation; normalisation; marginal cost of public funds.
Dynamic scaling of the restoration of rotational symmetry in Heisenberg quantum antiferromagnets
We apply imaginary-time evolution, , to study relaxation
dynamics of gapless quantum antiferromagnets described by the spin-rotation
invariant Heisenberg Hamiltonian (). Using quantum Monte Carlo simulations,
we propagate an initial state with maximal order parameter (the
staggered magnetization) in the spin direction and monitor the expectation
value as a function of the time . Different
system sizes of lengths exhibit an initial size-independent relaxation of
toward its value the spontaneously symmetry-broken
state, followed by a size-dependent final decay to zero. We develop a generic
finite-size scaling theory which shows that the relaxation time diverges
asymptotically as where is the dynamic exponent of the low energy
excitations. We use the scaling theory to develop a way of extracting the
dynamic exponent from the numerical finite-size data. We apply the method to
spin- Heisenberg antiferromagnets on two different lattice geometries; the
two-dimensional (2D) square lattice as well as a site-diluted square lattice at
the percolation threshold. In the 2D case we obtain , which is
consistent with the known value , while for the site-dilutes lattice we
find . This is an improvement on previous estimates of . The scaling results also show a fundamental difference between the two
cases: In the 2D system the data can be collapsed onto a common scaling
function even when is relatively large, reflecting the
Anderson tower of quantum rotor states with a common dynamic exponent .
For the diluted lattice, the scaling works only for small , indicating a mixture of different relaxation time scaling
between the low energy states
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