1,864 research outputs found
Phonon density of states and heat capacity of La_(3−x)Te_4
The phonon density of states (DOS) of La_(3−x)Te_4 compounds (x=0.0,0.18,0.32) was measured at 300, 520, and 780 K, using inelastic neutron scattering. A significant stiffening of the phonon DOS and a large broadening of features were observed upon introduction of vacancies on La sites (increasing x). Heat-capacity measurements were performed at temperatures 1.85 ≤ T ≤ 1200 K and were analyzed to quantify the contributions of phonons and electrons. The Debye temperature and the electronic coefficient of heat capacity determined from these measurements are consistent with the neutron-scattering results, and with previously reported first-principles calculations. Our results indicate that La vacancies in La_(3−x)Te_4 strongly scatter phonons and this source of scattering appears to be independent of temperature. The stiffening of the phonon DOS induced by the introduction of vacancies is explained in terms of the electronic structure and the change in bonding character. The temperature dependence of the phonon DOS is captured satisfactorily by the quasiharmonic approximation
Investigation of the magnetic structure and crystal field states of pyrochlore antiferromagnet Nd2Zr2O7
We present synchrotron x-ray diffraction, neutron powder diffraction and
time-of-flight inelastic neutron scattering measurements on the rare earth
pyrochlore oxide Nd2Zr2O7 to study the ordered state magnetic structure and
cystal field states. The structural characterization by high-resolution
synchrotron x-ray diffraction confirms that the pyrochlore structure has no
detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals
long range all-in/all-out antiferromagnetic order below T_N ~ 0.4 K with
propagation vector k = (0 0 0) and an ordered moment of 1.26(2) \mu_B/Nd at 0.1
K. The ordered moment is much smaller than the estimated moment of 2.65
\mu_B/Nd for the local Ising ground state of Nd3+ (J=9/2) suggesting that
the ordering is partially suppressed by quantum fluctuations. The strong Ising
anisotropy is further confirmed by the inelastic neutron scattering data which
reveals a well-isolated dipolar-octupolar type Kramers doublet ground state.
The crystal field level scheme and ground state wavefunction have been
determined.Comment: 12 pages, 15 figures, 2 table
Nonharmonic phonons in MgB_2 at elevated temperatures
Inelastic neutron scattering was used to measure phonon spectra in MgB_2 and Mg_(0.75)Al_(0.25)B_2 from 7 to 750 K to investigate anharmonicity and adiabatic electron-phonon coupling. First-principles calculations of phonons with a linear response method were performed at multiple unit cell volumes, and the Helmholtz free energy was minimized to obtain the lattice parameters and phonon dynamics at elevated temperature in the quasiharmonic approximation. Most of the temperature dependence of the phonon density of states could be understood with the quasiharmonic approximation, although there was also significant thermal broadening of the phonon spectra. In comparison to Mg_(0.75)Al_(0.25)B_2, in the energy range of 60 to 80 meV the experimental phonon spectra from MgB_2 showed a nonmonotonic change with temperature around 500 K. This may originate from a change with temperature of the adiabatic electron-phonon coupling
Stabilization of Polar Nano Regions in Pb-free ferroelectrics
Formation of polar nano regions through solid-solution additions are known to
enhance significantly the functional properties of ferroelectric materials.
Despite considerable progress in characterizing the microscopic behavior of
polar nano regions, understanding their real-space atomic structure and
dynamics of formation remains a considerable challenge. Here, using the method
of dynamic pair distribution function, we provide direct insights into the role
of solid-solution additions towards the stabilization of polar nano regions in
the Pb-free ferroelectric of Ba(Zr,Ti)O3. It is shown that for an optimum level
of substitution of Ti by larger Zr ions, the dynamics of atomic displacements
for ferroelectric polarization are slowed sufficiently, which leads to
increased local correlation among dipoles below THz frequencies. The dynamic
pair distribution function technique demonstrates unique capability to obtain
insights into locally correlated atomic dynamics in disordered materials,
including new Pb-free ferroelectrics, which is necessary to understand and
control their functional properties
Effective One-Dimensional Coupling in the Highly-Frustrated Square-Lattice Itinerant Magnet CaCoAs
Inelastic neutron scattering measurements on the itinerant antiferromagnet
(AFM) CaCoAs at a temperature of 8 K reveal two
orthogonal planes of scattering perpendicular to the Co square lattice in
reciprocal space, demonstrating the presence of effective one-dimensional spin
interactions. These results are shown to arise from near-perfect bond
frustration within the - Heisenberg model on a square lattice with
ferromagnetic , and hence indicate that the extensive previous
experimental and theoretical study of the - Heisenberg model on
local-moment square spin lattices should be expanded to include itinerant spin
systems
Positive Vibrational Entropy of Chemical Ordering in FeV
Inelastic neutron scattering and nuclear resonant inelastic x-ray scattering were used to measure phonon spectra of FeV as a B2 ordered compound and as a bcc solid solution. The two data sets were combined to give an accurate phonon density of states, and the phonon partial densities of states for V and Fe atoms. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2 ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy by +0.22±0.03k_B/atom, which stabilizes the ordered phase to higher temperatures. First-principles calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites
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