888 research outputs found
Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNbO in a transverse field: Geometric frustration and quantum renormalization effects
The quasi-one-dimensional (1D) Ising ferromagnet CoNbO has recently
been driven via applied transverse magnetic fields through a continuous quantum
phase transition from spontaneous magnetic order to a quantum paramagnet, and
dramatic changes were observed in the spin dynamics, characteristic of weakly
perturbed 1D Ising quantum criticality. We report here extensive single-crystal
inelastic neutron scattering measurements of the magnetic excitations
throughout the three-dimensional (3D) Brillouin zone in the quantum
paramagnetic phase just above the critical field to characterize the effects of
the finite interchain couplings. In this phase, we observe that excitations
have a sharp, resolution-limited line shape at low energies and over most of
the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the
full bandwidth along the strongly dispersive chain direction and resolve clear
modulations of the dispersions in the plane normal to the chains,
characteristic of frustrated interchain couplings in an antiferromagnetic
isosceles triangular lattice. The dispersions can be well parametrized using a
linear spin-wave model that includes interchain couplings and further neighbor
exchanges. The observed dispersion bandwidth along the chain direction is
smaller than that predicted by a linear spin-wave model using exchange values
determined at zero field, and this effect is attributed to quantum
renormalization of the dispersion beyond the spin-wave approximation in fields
slightly above the critical field, where quantum fluctuations are still
significant.Comment: 11 pages, 6 figures. Updated references. Minor changes to text and
figure
Neutron inelastic scattering investigation of the magnetic excitations in Cu_2Te_2O_5X_2 (X=Br, Cl)
Neutron inelastic scattering investigations have been performed on the spin
tetrahedral system Cu_2Te_2O_5X_2 (X = Cl, Br). We report the observation of
magnetic excitations with a dispersive component in both compounds, associated
with the 3D incommensurate magnetic order that develops below =18.2
K and =11.4 K. The excitation in Cu_2Te_2O_5Cl_2 softens as the
temperature approaches , leaving diffuse quasi-elastic scattering
above the transition temperature. In the bromide, the excitations are present
well above , which might be attributed to the presence of a degree
of low dimensional correlations above in this compound
On the extension of the complex-step derivative technique to pseudospectral algorithms
Abstract The complex-step derivative (CSD) technique is a convenient and highly accurate strategy to perform a linearized ''perturbation'' analysis to determine a ''directional derivative'' via a minor modification of an existing nonlinear simulation code. The technique has previously been applied to nonlinear simulation codes (such as finite-element codes) which employ real arithmetic only. The present note examines the suitability of this technique for extension to efficient pseudospectral simulation codes which nominally use the fast Fourier transform (FFT) to convert back and forth between the physical and transformed representations of the system. It is found that, if used carefully, this extension retains the remarkable accuracy of the CSD approach. However, to perform this extension without sacrificing this accuracy, particular care must be exercised; specifically, the state (real) and perturbation (imaginary) components of the complexified system must be transformed separately and arranged in such a manner that they are kept distinct during the process of differentiation in the transformed space in order to avoid the linear combination of the large and small quantities in the analysis. It is shown that this is relatively straightforward to implement even in complicated nonlinear simulation codes, thus positioning the CSD approach as an attractive and relatively simple alternative to hand coding a perturbation (a.k.a. ''tangent linear'') code for determining the directional derivative even when pseudospectral algorithms are employed. Published by Elsevier Science B.V
Influence of static Jahn-Teller distortion on the magnetic excitation spectrum of PrO2: A synchrotron x-ray and neutron inelastic scattering study
A synchrotron x-ray diffraction study of the crystallographic structure of
PrO2 in the Jahn-Teller distorted phase is reported. The distortion of the
oxygen sublattice, which was previously ambiguous, is shown to be a chiral
structure in which neighbouring oxygen chains have opposite chiralities. A
temperature dependent study of the magnetic excitation spectrum, probed by
neutron inelastic scattering, is also reported. Changes in the energies and
relative intensities of the crystal field transitions provide an insight into
the interplay between the static and dynamic Jahn-Teller effects.Comment: 7 pages, 6 figure
Evidence for SrHo2O4 and SrDy2O4 as model J1-J2 zig-zag chain materials
Neutron diffraction and inelastic spectroscopy is used to characterize the
magnetic Hamiltonian of SrHo2O4 and SrDy2O4. Through a detailed computation of
the crystal-field levels we find site- dependent anisotropic single-ion
magnetism in both materials and diffraction measurements show the presence of
strong one-dimensional spin correlations. Our measurements indicate that
competing interactions of the zig-zag chain, combined with frustrated
interchain interactions, play a crucial role in stabilizing spin-liquid type
correlations in this series.Comment: 5 pages, 5 figure
Topological Magnon Bands in a Kagome Lattice Ferromagnet
There is great interest in finding materials possessing quasiparticles with topological properties. Such materials may have novel excitations that exist on their boundaries which are protected against disorder. We report experimental evidence that magnons in an insulating kagome ferromagnet can have a topological band structure. Our neutron scattering measurements further reveal that one of the bands is flat due to the unique geometry of the kagome lattice. Spin wave calculations show that the measured band structure follows from a simple Heisenberg Hamiltonian with a Dzyaloshinkii-Moriya interaction. This serves as the first realization of an effectively two-dimensional topological magnon insulator—a new class of magnetic material that should display both a magnon Hall effect and protected chiral edge modes.United States. Dept. of Energy. Office of Basic Energy Sciences (Grant DE-FG02-07ER46134)National Science Foundation (U.S.) (Grant CHE 1041863
Yb-Yb correlations and crystal-field effects in the Kondo insulator YbB12 and its solid solutions
We have studied the effect of Lu substitution on the spin dynamics of the
Kondo insulator YbB12 to clarify the origin of the spin-gap response previously
observed at low temperature in this material. Inelastic neutron spectra have
been measured in Yb1-xLuxB12 compounds for four Lu concentrations x = 0, 0.25,
0.90 and 1.0. The data indicate that the disruption of coherence on the Yb
sublattice primarily affects the narrow peak structure occurring near 15-20 meV
in pure YbB12, whereas the spin gap and the broad magnetic signal around 38 meV
remain almost unaffected. It is inferred that the latter features reflect
mainly local, single-site processes, and may be reminiscent of the inelastic
magnetic response reported for mixed-valence intermetallic compounds. On the
other hand, the lower component at 15 meV is most likely due to dynamic
short-range magnetic correlations. The crystal-field splitting in YbB12
estimated from the Er3+ transitions measured in a Yb0.9Er0.1B12 sample, has the
same order of magnitude as other relevant energy scales of the system and is
thus likely to play a role in the form of the magnetic spectral response.Comment: 16 pages in pdf format, 9 figures. v. 2: coauthor list updated; extra
details given in section 3.2 (pp. 6-7); one reference added; fig. 5 axis
label change
- …