183 research outputs found
Phase diagram of the spin-1/2 -- Heisenberg model on the square lattice
We presents the results of an extensive numerical study of the phase diagram
of the spin-1/2, \protect{--} Heisenberg model on a square
lattice, for both ferromagnetic and antiferromagnetic nearest-neighbor
interactions , using exact diagonalization with periodic and twisted
boundary conditions. Comparison is made with published spin wave calculations.
We show that quantum fluctuations play a very important role, changing both the
extent and the wave vector of classical spiral phases, and leading to new
quantum phases where the classical spiral states have a high degeneracy. These
include a new phase with small or vanishing spin-stiffness, in addition to
known valence-bond-solid and bond-nematic phases.Comment: submitted for the International Conference on Magnetism to be held
26-31 July 2009 in Karlsruh
An investigation of the quantum model on the honeycomb lattice
We have investigated the quantum model on the honeycomb lattice
with exact diagonalizations and linear spin-wave calculations for selected
values of , and antiferromagnetic () or
ferromagnetic () nearest neighbor interactions. We found a variety of
quantum effects: "order by disorder" selection of a N{\'e}el ordered
ground-state, good candidates for non-classical ground-states with dimer long
range order or spin-liquid like. The purely antiferromagnetic Heisenberg model
is confirmed to be N{\'e}el ordered. Comparing these results with those
observed on the square and triangular lattices, we enumerate some conjectures
on the nature of the quantum phases in the isotropic models.Comment: 14 pages, 22 Postscript figures, uses svjour.cls and svepj.clo,
submitted to European Physical Journal B: condensed matter physi
Exact diagonalization Studies of Two-dimensional Frustrated Antiferromagnet Models
We describe the four kinds of behavior found in two-dimensional isotropic
quantum antiferromagnets. Two of them display long range order at T=0: the
N\'eel state and the Valence Bond Crystal. The last two are Spin-Liquids.
Properties of these different states are shortly described and open questions
are underlined.Comment: 7 pages; invited talk at "HFM 2000" (Waterloo, June 2000); submitted
to Can. J. Phy
Some remarks on the Lieb-Schultz-Mattis theorem and its extension to higher dimensions
The extension of the Lieb-Schultz-Mattis theorem to dimensions larger than
one is discussed. It is explained why the variational wave-function built by
the previous authors is of no help to prove the theorem in dimension larger
than one. The short range R.V.B. picture of Sutherland, Rokhsar and Kivelson,
Read and Chakraborty gives a strong support to the assertion that the theorem
is indeed valid in any dimension. Some illustrations of the general ideas are
displayed on exact spectra.Comment: 12 pages, LaTeX with 4 EPS figures embedded in the documen
Coupled frustrated quantum spin-1/2 chains with orbital order in volborthite Cu3V2O7(OH)2(H2O)2
We present a microscopic magnetic model for the spin-liquid candidate
volborthite Cu3V2O7(OH)2(H2O)2. The essentials of this DFT-based model are (i)
the orbital ordering of Cu(1) 3d 3z2-r2 and Cu(2) 3d 3x2-y2, (ii) three
relevant couplings J_ic, J_1 and J_2, (iii) the ferromagnetic nature of J_1 and
(iv) frustration governed by the next-nearest-neighbor exchange interaction
J_2. Our model implies magnetism of frustrated coupled chains in contrast to
the previously proposed anisotropic kagome model. Exact diagonalization studies
reveal agreement with experiments.Comment: 5 pages, 4 figures + supplementar
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