353 research outputs found
Exotic phenomena in doped quantum magnets
We investigate the properties of the two-dimensional frustrated quantum
antiferromagnet on the square lattice, especially at infinitesimal doping. We
find that next nearest neighbor (N.N.) J2 and next-next N.N. J3 interactions
together destroy the antiferromagnetic long range order and stabilize a quantum
disordered valence bond crystalline plaquette phase. A static vacancy or a
dynamic hole doped into this phase liberates a spinon. From the profile of the
spinon wavefunction around the (static) vacancy we identify an intermediate
behavior between complete deconfinement (behavior seen in the kagome lattice)
and strong confinement (behavior seen in the checkerboard lattice) with the
emergence of two length scales, a spinon confinement length larger than the
magnetic correlation length. When a finite hole hopping is introduced, this
behavior translates into an extended (mobile) spinon-holon boundstate with a
very small quasiparticle weight. These features provide clear evidence for a
nearby "deconfined critical point" in a doped microscopic model. Finally, we
give arguments in favor of superconducting properties of the doped plaquette
phase.Comment: Submitted to J. of Phys. Condens. Matter (Proceedings of
International Conference "Highly Frustrated Magnets", Osaka (Japan), August
2006). 6 pages, 5 figures Display problems with Figure 2 fixe
Effect of local charge fluctuations on spin physics in the Neel state of LaCuO
We explore the effect of local charge fluctuations on the spin response of a
Mott insulator by deriving an effective spin model, and studying it using
Schwinger boson mean field theory. Applying this to LaCuO, we show that
an accurate fit to the magnon dispersion relation, measured by Coldea {\em et
al.} [Phys. Rev. Lett. {\bf 86}, 5377 (2001)] is obtained with Hubbard model
parameters , and . These parameters lead
to estimates of the staggered magnetization (), spin wave
velocity (-\AA), and spin stiffness (). In particular the staggered moment as well as the effective local moment
are renormalized to smaller values compared to the Heisenberg model due to
local charge fluctuations in the Hubbard model. The dynamical structure factor
shows considerable weight in the continuum along the zone boundary as well as
secondary peaks that may be observed in high resolution neutron scattering
experiments.Comment: Manuscript considerably revised following referee comments. Also
added a brief discussion of sum rules. 8 pages, 6 eps figure
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
Spin-orbital quantum liquid on the honeycomb lattice
In addition to low-energy spin fluctuations, which distinguish them from band
insulators, Mott insulators often possess orbital degrees of freedom when
crystal-field levels are partially filled. While in most situations spins and
orbitals develop long-range order, the possibility for the ground state to be a
quantum liquid opens new perspectives. In this paper, we provide clear evidence
that the SU(4) symmetric Kugel-Khomskii model on the honeycomb lattice is a
quantum spin-orbital liquid. The absence of any form of symmetry breaking -
lattice or SU(N) - is supported by a combination of semiclassical and numerical
approaches: flavor-wave theory, tensor network algorithm, and exact
diagonalizations. In addition, all properties revealed by these methods are
very accurately accounted for by a projected variational wave-function based on
the \pi-flux state of fermions on the honeycomb lattice at 1/4-filling. In that
state, correlations are algebraic because of the presence of a Dirac point at
the Fermi level, suggesting that the symmetric Kugel-Khomskii model on the
honeycomb lattice is an algebraic quantum spin-orbital liquid. This model
provides a good starting point to understand the recently discovered
spin-orbital liquid behavior of Ba_3CuSb_2O_9. The present results also suggest
to choose optical lattices with honeycomb geometry in the search for quantum
liquids in ultra-cold four-color fermionic atoms.Comment: 10 pages, 7 figure
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
Supersolid phase induced by correlated hopping in spin-1/2 frustrated quantum magnets
We show that correlated hopping of triplets, which is often the dominant
source of kinetic energy in dimer-based frustrated quantum magnets, produces a
remarkably strong tendency to form supersolid phases in a magnetic field. These
phases are characterized by simultaneous modulation and ordering of the
longitudinal and transverse magnetization respectively. Using Quantum Monte
Carlo and a semiclassical approach for an effective hard-core boson model with
nearest-neighbor repulsion on a square lattice, we prove in particular that a
supersolid phase can exist even if the repulsion is not strong enough to
stabilize an insulating phase at half-filling. Experimental implications for
frustrated quantum antiferromagnets in a magnetic field at zero and finite
temperature are discussed.Comment: 4 pages; 4 figures; published versio
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
The quadrupolar phases of the S=1 bilinear-biquadratic Heisenberg model on the triangular lattice
Using mean-field theory, exact diagonalizations and SU(3) flavour theory, we
have precisely mapped out the phase diagram of the S=1 bilinear-biquadratic
Heisenberg model on the triangular lattice in a magnetic field, with emphasis
on the quadrupolar phases and their excitations. In particular, we show that
ferroquadrupolar order can coexist with short-range helical magnetic order, and
that the antiferroquadrupolar phase is characterized by a remarkable 2/3
magnetization plateau, in which one site per triangle retains quadrupolar order
while the other two are polarized along the field. Implications for actual S=1
magnets are discussed.Comment: 4 pages, 5 figures, published versio
Quench dynamics and non equilibrium phase diagram of the Bose-Hubbard model
We investigate the time evolution of correlations in the Bose-Hubbard model
following a quench from the superfluid to the Mott insulating phase. For large
values of the final interaction strength the system approaches a distinctly
non-equilibrium steady state that bears strong memory of the initial
conditions. In contrast, when the final interaction strength is comparable to
the hopping, the correlations are rather well approximated by those at thermal
equilibrium. The existence of two distinct non-equilibrium regimes is
surprising given the non-integrability of the Bose-Hubbard model. We relate
this phenomena to the role of quasi-particle interactions in the Mott
insulating state
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