227 research outputs found
Quantum renormalization of high energy excitations in the 2D Heisenberg antiferromagnet
We find using Monte Carlo simulations of the spin-1/2 2D square lattice
nearest neighbour quantum Heisenberg antiferromagnet that the high energy peak
locations at (pi,0) and (pi/2,pi/2) differ by about 6%, (pi/2,pi/2) being the
highest. This is a deviation from linear spin wave theory which predicts equal
magnon energies at these points.Comment: Final version, Latex using iopart & epsfi
Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain
Assemblies of interacting quantum particles often surprise us with properties
that are difficult to predict. One of the simplest quantum many-body systems is
the spin 1/2 Heisenberg antiferromagnetic chain, a linear array of interacting
magnetic moments. Its exact ground state is a macroscopic singlet entangling
all spins in the chain. Its elementary excitations, called spinons, are
fractional spin 1/2 quasiparticles; they are created and detected in pairs by
neutron scattering. Theoretical predictions show that two-spinon states exhaust
only 71% of the spectral weight while higher-order spinon states, yet to be
experimentally located, are predicted to participate in the remaining. Here, by
accurate absolute normalization of our inelastic neutron scattering data on a
compound realizing the model, we account for the full spectral weight to within
99(8)%. Our data thus establish and quantify the existence of higher-order
spinon states. The observation that within error bars, the entire weight is
confined within the boundaries of the two-spinon continuum, and that the
lineshape resembles a rescaled two-spinon one, allow us to develop a simple
physical picture for understanding multi-spinon excitations.Comment: 22 pages, 4 figures, Supplementary material
Magnetic structure of Ba(TiO)Cu(PO) probed using spherical neutron polarimetry
The antiferromagnetic compound Ba(TiO)Cu(PO) contains square
cupola of corner-sharing CuO plaquettes, which were proposed to form
effective quadrupolar order. To identify the magnetic structure, we have
performed spherical neutron polarimetry measurements. Based on symmetry
analysis and careful measurements we conclude that the orientation of the
Cu spins form a non-collinear in-out structure with spins approximately
perpendicular to the CuO motif. Strong Dzyaloshinskii-Moriya interaction
naturally lends itself to explain this phenomenon. The identification of the
ground state magnetic structure should serve well for future theoretical and
experimental studies into this and closely related compounds.Comment: 9 pages, 4 figure
-cation control of magnetoelectric quadrupole order in (TiO)Cu(PO) ( = Ba, Sr, and Pb)
Ferroic magnetic quadrupole order exhibiting macroscopic magnetoelectric
activity is discovered in the novel compound (TiO)Cu(PO) with
= Pb, which is in contrast with antiferroic quadrupole order observed in
the isostructural compounds with = Ba and Sr. Unlike the famous lone-pair
stereochemical activity which often triggers ferroelectricity as in PbTiO,
the Pb cation in Pb(TiO)Cu(PO) is stereochemically inactive
but dramatically alters specific magnetic interactions and consequently
switches the quadrupole order from antiferroic to ferroic. Our first-principles
calculations uncover a positive correlation between the degree of -O bond
covalency and a stability of the ferroic quadrupole order.Comment: 7 pages, 4 figure
The J_{eff}=1/2 insulator Sr3Ir2O7 studied by means of angle-resolved photoemission spectroscopy
The low-energy electronic structure of the J_{eff}=1/2 spin-orbit insulator
Sr3Ir2O7 has been studied by means of angle-resolved photoemission
spectroscopy. A comparison of the results for bilayer Sr3Ir2O7 with available
literature data for the related single-layer compound Sr2IrO4 reveals
qualitative similarities and similar J_{eff}=1/2 bandwidths for the two
materials, but also pronounced differences in the distribution of the spectral
weight. In particuar, photoemission from the J_{eff}=1/2 states appears to be
suppressed. Yet, it is found that the Sr3Ir2O7 data are in overall better
agreement with band-structure calculations than the data for Sr2IrO4.Comment: 5 pages, 3 figure
Non-equilibrium hysteresis and spin relaxation in the mixed-anisotropy dipolar coupled spin-glass LiHoErF
We present a study of the model spin-glass LiHoErF using
simultaneous AC susceptibility, magnetization and magnetocaloric effect
measurements along with small angle neutron scattering (SANS) at sub-Kelvin
temperatures. All measured bulk quantities reveal hysteretic behavior when the
field is applied along the crystallographic c axis. Furthermore avalanche-like
relaxation is observed in a static field after ramping from the
zero-field-cooled state up to Oe. SANS measurements are employed to
track the microscopic spin reconfiguration throughout both the hysteresis loop
and the related relaxation. Comparing the SANS data to inhomogeneous mean-field
calculations performed on a box of one million unit cells provides a real-space
picture of the spin configuration. We discover that the avalanche is being
driven by released Zeeman energy, which heats the sample and creates positive
feedback, continuing the avalanche. The combination of SANS and mean-field
simulations reveal that the conventional distribution of cluster sizes is
replaced by one with a depletion of intermediate cluster sizes for much of the
hysteresis loop.Comment: 6 pages, 4 figure
- …