30,921 research outputs found
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
Molecular dynamics simulations of oxide memristors: crystal field effects
We present molecular-dynamic simulations of memory resistors (memristors)
including the crystal field effects on mobile ionic species such as oxygen
vacancies appearing during operation of the device. Vacancy distributions show
different patterns depending on the ratio of a spatial period of the crystal
field to a characteristic radius of the vacancy-vacancy interaction. There are
signatures of the orientational order and of spatial voids in the vacancy
distributions for some crystal field potentials. The crystal field stabilizes
the patterns after they are formed, resulting in a non-volatile switching of
the simulated devices.Comment: 9 pages, 3 figure
Phonon and crystal field excitations in geometrically frustrated rare earth titanates
The phonon and crystal field excitations in several rare earth titanate
pyrochlores are investigated. Magnetic measurements on single crystals of
Gd2Ti2O7, Tb2Ti2O7, Dy2Ti2O7 and Ho2Ti2O7 are used for characterization, while
Raman spectroscopy and terahertz time domain spectroscopy are employed to probe
the excitations of the materials. The lattice excitations are found to be
analogous across the compounds over the whole temperature range investigated
(295-4 K). The resulting full phononic characterization of the R2Ti2O7
pyrochlore structure is then used to identify crystal field excitations
observed in the materials. Several crystal field excitations have been observed
in Tb2Ti2O7 in Raman spectroscopy for the first time, among which all of the
previously reported excitations. The presence of additional crystal field
excitations, however, suggests the presence of two inequivalent Tb3+ sites in
the low temperature structure. Furthermore, the crystal field level at
approximately 13 cm-1 is found to be both Raman and dipole active, indicating
broken inversion symmetry in the system and thus undermining its current
symmetry interpretation. In addition, evidence is found for a significant
crystal field-phonon coupling in Tb2Ti2O7. These findings call for a careful
reassessment of the low temperature structure of Tb2Ti2O7, which may serve to
improve its theoretical understanding.Comment: 13 pages, 7 figure
Crystal field effects on spin pumping
"Spin pumping" is the injection of spin angular momentum by a time-dependent
magnetization into an adjacent normal metal proportional to the spin mixing
conductance. We study the role of electrostatic interactions in the form of
crystal fields on the pumped spin currents generated by insulators with
exchange-coupled local moments at the interface to a metal. The crystal field
is shown to render the spin currents anisotropic, which implies that the spin
mixing conductance of insulator|normal metal bilayers depends on crystal cut
and orientation. We interpret the interface "effective field" (imaginary part
of the spin mixing conductance) in terms of the coherent motion of the
equilibrium spin density induced by proximity in the normal metal.Comment: 8 pages+, 7 figure
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