833 research outputs found
Generic mixed columnar-plaquette phases in Rokhsar-Kivelson models
We revisit the phase diagram of Rokhsar-Kivelson models, which are used in
fields such as superconductivity, frustrated magnetism, cold bosons, and the
physics of Josephson junction arrays. From an extended height effective theory,
we show that one of two simple generic phase diagrams contains a mixed phase
that interpolates continuously between columnar and plaquette states. For the
square lattice quantum dimer model we present evidence from exact
diagonalization and Green's function Monte Carlo techniques that this scenario
is realised, by combining an analysis of the excitation gaps of different
symmetry sectors with information on plaquette structure factors. This presents
a natural framework for resolving the disagreement between previous studies.Comment: 4 pages, 5 figure
Coexistence of long-range and algebraic correlations for short-range valence-bond wave functions in three dimensions
We investigate nearest-neighbor valence-bond wave functions on bipartite
three-dimensional lattices. By performing large-scale Monte Carlo simulations,
we find that long-range magnetic order coexists with dipolar four-spin
correlations on the cubic lattice, this latter feature being reminiscent of the
Coulomb phase for classical dimers on the same lattice. Similar properties are
found for the lower-coordination diamond lattice. While this suggests that the
coexistence of magnetic order and dipolar four-spin correlations is generic for
bipartite three-dimensional lattices, we show that simple generalizations of
these wave functions can encode different ordering behaviors.Comment: 4+ pages, 5 figures. Updated version, to appear in Phys. Rev. Let
Ground state and low-lying excitations of the spin-1/2 XXZ model on the kagome lattice at magnetization 1/3
We study the ground state and low-lying excitations of the S=1/2 XXZ
antiferromagnet on the kagome lattice at magnetization one third of the
saturation. An exponential number of non-magnetic states is found below a
magnetic gap. The non-magnetic excitations also have a gap above the ground
state, but it is much smaller than the magnetic gap. This ground state
corresponds to an ordered pattern with resonances in one third of the hexagons.
The spin-spin correlation function is short ranged, but there is long-range
order of valence-bond crystal type.Comment: 2 pages, 1 figure included, to appear in Physica B (proceedings of
SCES'04
Slow relaxation and sensitivity to disorder in trapped lattice fermions after a quench
We consider a system of non-interacting fermions in one dimension subject to
a single-particle potential consisting of (a) a strong optical lattice, (b) a
harmonic trap, and (c) uncorrelated on-site disorder. After a quench, in which
the center of the harmonic trap is displaced, we study the occupation function
of the fermions and the time-evolution of experimental observables.
Specifically, we present numerical and analytical results for the post-quench
occupation function of the fermions, and analyse the time-evolution of the
real-space density profile. Unsurprisingly for a non-interacting (and therefore
integrable) system, the infinite-time limit of the density profile is
non-thermal. However, due to Bragg-localization of the higher-energy
single-particle states, the approach to even this non-thermal state is
extremely slow. We quantify this statement, and show that it implies a
sensitivity to disorder parametrically stronger than that expected from
Anderson localization.Comment: 15 pages, 11 figure
Symmetry Breaking on the Three-Dimensional Hyperkagome Lattice of Na_4Ir_3O_8
We study the antiferromagnetic spin-1/2 Heisenberg model on the highly
frustrated, three-dimensional, hyperkagome lattice of Na_4Ir_3O_8 using a
series expansion method. We propose a valence bond crystal with a 72 site unit
cell as a ground state that supports many, very low lying, singlet excitations.
Low energy spinons and triplons are confined to emergent lower-dimensional
motifs. Here, and for analogous kagome and pyrochlore states, we suggest finite
temperature signatures, including an Ising transition, in the magnetic specific
heat due to a multistep breaking of discrete symmetries.Comment: 4 pages, 3 figure
Disorder in a quantum spin liquid: flux binding and local moment formation
We study the consequences of disorder in the Kitaev honeycomb model,
considering both site dilution and exchange randomness. We show that a single
vacancy binds a flux and induces a local moment. This moment is polarised by an
applied field : in the gapless phase, for small the local susceptibility
diverges as ; for a pair of nearby vacancies on the same
sublattice, this even increases to . By
contrast, weak exchange randomness does not qualitatively alter the
susceptibility but has its signature in the heat capacity, which in the gapless
phase is power law in temperature with an exponent dependent on disorder
strength.Comment: 4 pages, 2 figure
Asymptotics of block Toeplitz determinants and the classical dimer model
We compute the asymptotics of a block Toeplitz determinant which arises in
the classical dimer model for the triangular lattice when considering the
monomer-monomer correlation function. The model depends on a parameter
interpolating between the square lattice () and the triangular lattice
(), and we obtain the asymptotics for . For we apply the
Szeg\"o Limit Theorem for block Toeplitz determinants. The main difficulty is
to evaluate the constant term in the asymptotics, which is generally given only
in a rather abstract form
Quasiparticle interference in iron-based superconductors
We systematically calculate quasiparticle interference (QPI) signatures for
the whole phase diagram of iron-based superconductors. Impurities inherent in
the sample together with ordered phases lead to distinct features in the QPI
images that are believed to be measured in spectroscopic imaging-scanning
tunneling microscopy (SI-STM). In the spin-density wave phase the rotational
symmetry of the electronic structure is broken, signatures of which are also
seen in the coexistence regime with both superconducting and magnetic order. In
the superconducting regime we show how the different scattering behavior for
magnetic and non-magnetic impurities allows to verify the symmetry of
the order parameter. The effect of possible gap minima or nodes is discussed.Comment: 19 pages, 7 figure
Possible Quantum Diffusion of Polaronic Muons in DyTiO Spin Ice
We interpret recent measurements of the zero field muon relaxation rate in
the frustrated magnetic pyrochlore DyTiO as resulting from the
quantum diffusion of muons in the substance. In this scenario, the plateau
observed at low temperature ( K) in the relaxation rate is due to coherent
tunneling of the muons through a spatially disordered spin state and not to any
magnetic fluctuations persisting at low temperature. Two further regimes either
side of a maximum relaxation rate at K correspond to a crossover
between tunnelling and incoherent activated hopping motion of the muon. Our fit
of the experimental data is compared with the case of muonium diffusion in KCl.Comment: 15 pages, 2 figure
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