2,598 research outputs found
High Order Perturbation Theory for Spectral Densities of Multi-Particle Excitations: S=1/2 Two-Leg Heisenberg Ladder
We present a high order perturbation approach to quantitatively calculate
spectral densities in three distinct steps starting from the model Hamiltonian
and the observables of interest. The approach is based on the perturbative
continuous unitary transformation introduced previously. It is conceived to
work particularly well in models allowing a clear identification of the
elementary excitations above the ground state. These are then viewed as
quasi-particles above the vacuum. The article focuses on the technical aspects
and includes a discussion of series extrapolation schemes. The strength of the
method is demonstrated for S=1/2 two-leg Heisenberg ladders, for which results
are presented.Comment: 21 pages, 14 figures included; to appear in Eur. Phys. J. B All
technical details for the computation of spectral densities by perturbative
CUTs Minor misprints removed, references update
Exact ground states of a spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice in a magnetic field
Exact ground states of a spin-1/2 Ising-Heisenberg model on the
Shastry-Sutherland lattice with Heisenberg intra-dimer and Ising inter-dimer
couplings are found by two independent rigorous procedures. The first method
uses a unitary transformation to establish a mapping correspondence with an
effective classical spin model, while the second method relies on the
derivation of an effective hard-core boson model by continuous unitary
transformations. Both methods lead to equivalent effective Hamiltonians
providing a convincing proof that the spin-1/2 Ising-Heisenberg model on the
Shastry-Sutherland lattice exhibits a zero-temperature magnetization curve with
just two intermediate plateaus at one-third and one-half of the saturation
magnetization, which correspond to stripe and checkerboard orderings of
singlets and polarized triplets, respectively. The nature of the remarkable
stripe order relevant to the one-third plateau is thoroughly investigated with
the help of the corresponding exact eigenvector. The rigorous results for the
spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice are compared
with the analogous results for the purely classical Ising and fully quantum
Heisenberg models. Finally, we discuss to what extent the critical fields of
SrCu2(BO3)2 and (CuCl)Ca2Nb3O10 can be described within the suggested
Ising-Heisenberg model.Comment: 15 pages, 6 figures, minor correction
Spectral Properties of Magnetic Excitations in Cuprate Two-Leg Ladder Systems
This article summarizes and extends the recent developments in the
microscopic modeling of the magnetic excitations in cuprate two-leg ladder
systems. The microscopic Hamiltonian comprises dominant Heisenberg exchange
terms plus an additional four-spin interaction which is about five times
smaller. We give an overview over the relevant energies like the one-triplon
dispersion, the energies of two-triplon bound states and the positions of
multi-triplon continua and over relevant spectral properties like spectral
weights and spectral densities in the parameter regime appropriate for cuprate
systems. It is concluded that an almost complete understanding of the magnetic
excitations in undoped cuprate ladders has been obtained as measured by
inelastic neutron scattering, inelastic light (Raman) scattering and infrared
absorption.Comment: 26 pages, 10 figures, review for Mod. Phys. Lett.
The Higgs Mode of Planar Coupled Spin-Ladders and its Observation in CHNCuBr
Polarized inelastic neutron scattering experiments recently identified the
amplitude (Higgs) mode in CHNCuBr, a two-dimensional
near-quantum-critical spin-1/2 two-leg ladder compound, which exhibits a weak
easy-axis exchange anisotropy. Here, we theoretically examine the dynamic spin
structure factor of such planar coupled spin-ladder systems using large-scale
quantum Monte Carlo simulations. This allows us to provide a quantitative
account of the experimental neutron scattering data within a consistent quantum
spin model. Moreover, we trance the details of the continuous evolution of the
amplitude mode from a two-particle bound state of coupled ladders in the
classical Ising limit all the way to the quantum spin-1/2 Heisenberg limit with
fully restored SU(2) symmetry, where it gets overdamped by the two-magnon
continuum in neutron scattering.Comment: 9 pages, 10 figure
Solids and supersolids of three-body interacting polar molecules in an optical lattice
We study the physics of cold polar molecules loaded into an optical lattice
in the regime of strong three-body interactions, as put forward recently by
B\"uchler [Nature Phys. 3, 726 (2007)]. To this end quantum Monte Carlo
simulations, exact diagonalization and a semiclassical approach are used to
explore hardcore bosons on the two-dimensional square lattice which interact
solely by long ranged three-body terms. The resulting phase diagram shows a
sequence of solid and supersolid phases. Our findings are directly relevant for
future experimental implementations and open a new route towards the discovery
of a lattice supersolid phase in experiment.Comment: 4+ pages, 4 figures, published versio
Multi-particle excitations and spectral densities in quantum spin-systems
The excitation spectrum of the 2-leg S=1/2 Heisenberg ladder is examined
perturbatively. Using an optimally chosen continuous unitary transformation we
expand the Hamiltonian and the Raman operator about the limit of isolated rungs
leading to high order series expansions allowing to calculate spectral
densities quantitatively. The 2-particle sector is examined for total momentum
k=0. We show that triplet-triplet interaction gives rise to a band splitting.Comment: 2 pages, 1 figure; submitted to the proceedings of the SCES2001
conference (Physica B
Excitations in one-dimensional S=1/2 quantum antiferromagnets
The transition from dimerized to uniform phases is studied in terms of
spectral weights for spin chains using continuous unitary transformations
(CUTs). The spectral weights in the S=1 channel are computed perturbatively
around the limit of strong dimerization. We find that the spectral weight is
concentrated mainly in the subspaces with a small number of elementary triplets
(triplons), even for vanishing dimerization. So, besides spinons, triplons may
be used as elementary excitations in spin chains. We conclude that there is no
necessity to use fractional excitations in low-dimensional, undoped or doped
quantum antiferromagnets.Comment: 4 pages, 1 figure include
Linear optical elements based on cooperative subwavelength emitter arrays
We describe applications of two-dimensional subwavelength quantum emitter
arrays as efficient optical elements in the linear regime. For normally
incident light, the cooperative optical response, stemming from emitter-emitter
dipole exchanges, allows the control of the array's transmission, its resonance
frequency, and bandwidth. Operations on fully polarized incident light, such as
generic linear and circular polarizers as well as phase retarders can be
engineered and described in terms of Jones matrices. Our analytical approach
and accompanying numerical simulations identify optimal regimes for such
operations and reveal the importance of adjusting the array geometry and of the
careful tuning of the external magnetic fields amplitude and direction.Comment: 12 pages main text + 4 Appendix, 8 figure
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