254 research outputs found
Phonon Spectra in the Parent Superconducting Iron-tuned Telluride FeTe from Inelastic Neutron Scattering and Ab Initio Calculations
We report inelastic neutron scattering measurements of phonon spectra in the
parent superconductor iron-tuned chalcogenide FeTe, for two different x
contents (x 0.11), using neutron time-of-flight technique. Thermal
neutron spectroscopy allowed to collect the low-temperature Stokes spectra over
an extended Q-range, at 2, 40 and 120K - hence covering both the magnetic
monoclinic and the paramagnetic tetragonal phases. Whereas cold-neutrons
allowed to measure high-resolution anti-Stokes spectra at 140, 220 and 300K,
thus covering the tetragonal phase. Our results evidence a spin-phonon coupling
behaviour towards the observed noticeable temperature-dependent change of the
Stokes spectra across the transition temperatures. On the other hand, the
anti-Stokes spectra reveal a pronounced hardening of the low-energy, acoustic
region, of the phonon spectrum, upon heating, indicating a strong anharmonicity
and a subtle dependence of phonons on structural evolution within the
tetragonal phase. Experimental results are accompanied by ab initio
calculations of phonon spectra of the tetragonal stoichiometric phase for a
comparison with the high-resolution anti-Stokes spectra. Calculations included
different density functional methods. Spin polarization and van der Waals
interaction, were either considered or neglected, individually or
concomitantly, in order to study their respective effect on lattice dynamics
description. Our results suggest that including van der Waals interaction has
only a slight effect on phonon dynamics, however, phonon spectra are better
described when spin polarization is included, in a cooperative way with van der
Waals interactions
Negative thermal expansion in ZnF
We have investigated temperature dependence of the lattice parameters and the
unit cell volume of ZnF by neutron diffraction and have discovered negative
thermal expansion (NTE) at low temperature. To understand why this simple
compound exhibits NTE we performed first principle calculations. These
calculations reproduce qualitatively the experimental temperature dependence of
volume
Physics process of cosmogenics Li-9 and He-8 production on muons interactions with carbon target in liquid scintillator
Simulations were performed with GEANT4 to study the cosmogenics Li-9 and He-8
production by the interactions of the muons and their secondary shower
particles in the liquid scintillator. The photonuclear reactions seem dominate
their production. Their energy dependence were deduced. The comparisons with
available data show the reliability of the simulations results.Comment: 10 pages, 4 figures, 1 tabl
Remark on the studies of the muon-induced neutron background in the liquid scintillator detectors
This article gives a point of view on the studies of the muon-induced
background for the underground experiments using a liquid scintillator
detectors. The results obtained are in good agreement with the data, especially
for the neutron yield production. This is the first time, when a study of this
kind successes to get the same neutron yield as the one obtained from the
experiment.Comment: 5 pages, 2 figures, 1 tabl
Inelastic neutron scattering study of crystal field excitations of Nd<sup>3+</sup> in NdFeAsO
Inelastic neutron scattering experiments were performed to investigate the
crystalline electric field (CEF) excitations of Nd3+ (J = 9/2) in the iron
pnictide NdFeAsO. The crystal field level structures for both the
high-temperature paramagnetic phase and the low-temperature antiferromagnetic
phase of NdFeAsO are constructed. The variation of CEF excitations of Nd3+
reflects not only the change of local symmetry but also the change of magnetic
ordered state of the Fe sublattice. By analyzing the crystal field interaction
with a crystal field Hamiltonian, the crystal field parameters are obtained. It
was found that the sign of the fourth and sixth-order crystal field parameters
change upon the magnetic phase transition at 140 K, which may be due to the
variation of exchange interactions between the 4f and conduction electrons.Comment: 5 pages, 4 figure
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