30 research outputs found
MCViNE -- An object oriented Monte Carlo neutron ray tracing simulation package
MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo
(MC) neutron ray-tracing program that provides researchers with tools for
performing computer modeling and simulations that mirror real neutron
scattering experiments. By adopting modern software engineering practices such
as using composite and visitor design patterns for representing and accessing
neutron scatterers, and using recursive algorithms for multiple scattering,
MCViNE is flexible enough to handle sophisticated neutron scattering problems
including, for example, neutron detection by complex detector systems, and
single and multiple scattering events in a variety of samples and sample
environments. In addition, MCViNE can take advantage of simulation components
in linear-chain-based MC ray tracing packages widely used in instrument design
and optimization, as well as NumPy-based components that make prototypes useful
and easy to develop. These developments have enabled us to carry out detailed
simulations of neutron scattering experiments with non-trivial samples in
time-of-flight inelastic instruments at the Spallation Neutron Source. Examples
of such simulations for powder and single-crystal samples with various
scattering kernels, including kernels for phonon and magnon scattering, are
presented. With simulations that closely reproduce experimental results,
scattering mechanisms can be turned on and off to determine how they contribute
to the measured scattering intensities, improving our understanding of the
underlying physics.Comment: 34 pages, 14 figure
Spin-dynamics of the low-dimensional magnet (CH3)2NH2CuCl3
Dimethylammonium copper (II) chloride (also known as DMACuCl3 or MCCL) is a
low dimensional S=1/2 quantum spin system proposed to be an alternating
ferro-antiferromagnetic chain with similar magnitude ferromagnetic (FM) and
antiferromagnetic (AFM) exchange interactions. Subsequently, it was shown that
the existing bulk measurements could be adequately modeled by considering
DMACuCl3 as independent AFM and FM dimer spin pairs. We present here new
inelastic neutron scattering measurements of the spin-excitations in single
crystals of DMACuCl3. These results show significant quasi-one-dimensional
coupling, however the magnetic excitations do not propagate along the expected
direction. We observe a band of excitations with a gap of 0.95 meV and a
bandwidth of 0.82 meV.Comment: 3 pages, 2 figures included in text, submitted to proceedings of
International Conference on Neutron Scattering, December 200
Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe3
We use elastic and inelastic neutron scattering (INS) to study the
antiferromagnetic (AF) phase transitions and spin excitations in the
two-dimensional (2D) zig-zag antiferromagnet FePSe. By determining the
magnetic order parameter across the AF phase transition, we conclude that the
AF phase transition in FePSe is first-order in nature. In addition, our INS
measurements reveal that the spin waves in the AF ordered state have a large
easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and
possible biquadratic magnetic exchange interactions. On warming across ,
we find that dispersive spin excitations associated with three-fold rotational
symmetric AF fluctuations change into FM spin fluctuations above . These
results suggest that the first-order AF phase transition in FePSe may arise
from the competition between symmetric AF and symmetric FM spin
fluctuations around , in place of a conventional second-order AF phase
transition
The dual nature of magnetism in a uranium heavy fermion system
The duality between localized and itinerant nature of magnetism in
electron systems has been a longstanding puzzle. Here, we report
inelastic neutron scattering measurements, which reveal both local and
itinerant aspects of magnetism in a single crystalline system of
UPtSi. In the antiferromagnetic state, we observe broad continuum
of diffuse magnetic scattering with a resonance-like gap of 7 meV,
and surprising absence of coherent spin-waves, suggestive of itinerant
magnetism. While the gap closes above the Neel temperature, strong dynamic spin
correlations persist to high temperature. Nevertheless, the size and
temperature dependence of the total magnetic spectral weight can be well
described by local moment with . Furthermore, polarized neutron
measurements reveal that the magnetic fluctuations are mostly transverse, with
little or none of the longitudinal component expected for itinerant moments.
These results suggest that a dual description of local and itinerant magnetism
is required to understand UPtSi, and by extension, other 5
systems in general.Comment: see supplementary material for more detail
An NCN-pincer ligand dysprosium single-ion magnet showing magnetic relaxation via the second excited state
Single-molecule magnets are compounds that exhibit magnetic bistability purely of molecular origin. The control of anisotropy and suppression of quantum tunneling to obtain a comprehensive picture of the relaxation pathway manifold, is of utmost importance with the ultimate goal of slowing the relaxation dynamics within single-molecule magnets to facilitate their potential applications. Combined ab initio calculations and detailed magnetization dynamics studies reveal the unprecedented relaxation mediated via the second excited state within a new DyNCN system comprising a valence-localized carbon coordinated to a single dysprosium(III) ion. The essentially C(2v) symmetry of the Dy(III) ion results in a new relaxation mechanism, hitherto unknown for mononuclear Dy(III) complexes, opening new perspectives for means of enhancing the anisotropy contribution to the spin-relaxation barrier
Intertwined magnetism and charge density wave order in kagome FeGe
Electron correlations often lead to emergent orders in quantum materials.
Kagome lattice materials are emerging as an exciting platform for realizing
quantum topology in the presence of electron correlations. This proposal stems
from the key signatures of electronic structures associated with its lattice
geometry: flat band induced by destructive interference of the electronic
wavefunctions, topological Dirac crossing, and a pair of van Hove singularities
(vHSs). A plethora of correlated electronic phases have been discovered amongst
kagome lattice materials, including magnetism, charge density wave (CDW),
nematicity, and superconductivity. These materials can be largely organized
into two types: those that host magnetism and those that host CDW order.
Recently, a CDW order has been discovered in the magnetic kagome FeGe,
providing a new platform for understanding the interplay between CDW and
magnetism. Here, utilizing angle-resolved photoemission spectroscopy, we
observe all three types of electronic signatures of the kagome lattice: flat
bands, Dirac crossings, and vHSs. From both the observation of a
temperature-dependent shift of the vHSs towards the Fermi level as well as
guidance via first-principle calculations, we identify the presence of the vHSs
near the Fermi level (EF) to be driven by the development of underlying
magnetic exchange splitting. Furthermore, we show spectral evidence for the CDW
order as gaps that open on the near-EF vHS bands, as well as evidence of
electron-phonon coupling from a kink on the vHS band together with phonon
hardening observed by inelastic neutron scattering. Our observation points to
the magnetic interaction-driven band modification resulting in the formation of
the CDW order, indicating an intertwined connection between the emergent
magnetism and vHS charge order in this moderately-correlated kagome metal.Comment: submitted on April 22, 202
Spin dynamics and transport in gapped one-dimensional Heisenberg antiferromagnets at nonzero temperatures
We present the theory of nonzero temperature () spin dynamics and
transport in one-dimensional Heisenberg antiferromagnets with an energy gap
. For , we develop a semiclassical picture of thermally
excited particles. Multiple inelastic collisions between the particles are
crucial, and are described by a two-particle S-matrix which has a
super-universal form at low momenta. This is established by computations on the
O(3) -model, and strong and weak coupling expansions (the latter using
a Majorana fermion representation) for the two-leg S=1/2 Heisenberg
antiferromagnetic ladder. As an aside, we note that the strong-coupling
calculation reveals a S=1, two particle bound state which leads to the presence
of a second peak in the T=0 inelastic neutron scattering (INS) cross-section
for a range of values of momentum transfer. We obtain exact, or numerically
exact, universal expressions for the thermal broadening of the quasi-particle
peak in the INS cross-section, for the magnetization transport, and for the
field dependence of the NMR relaxation rate of the effective
semiclassical model: these are expected to be asymptotically exact for the
quantum antiferromagnets. The results for are compared with the
experimental findings of Takigawa et al and the agreement is quite good. In the
regime we argue that a
complementary description in terms of semiclassical waves applies, and give
some exact results for the thermodynamics and dynamics.Comment: REVTEX, 53 pages and 23 postscript figures; added additional
reference and associated clarificatio