56 research outputs found
Spin-Lattice Coupling in K0.8Fe1.6Se2 and KFe2Se2: Inelastic Neutron Scattering and ab-initio Phonon Calculations
We report measurements of the temperature dependence of phonon densities of
states in K0.8Fe1.6Se2 using inelastic neutron scattering technique. While
cooling down to 150 K, a phonon peak splitting around 25 meV is observed and a
new peak appears at 31 meV. The measurements support the recent Raman and
infra-red measurements indicating a lowering of symmetry of K0.8Fe1.6Se2 upon
cooling below 250 K. Ab-initio phonon calculations have been carried out for
K0.8Fe1.6Se2 and KFe2Se2. The comparison of the phonon spectra as obtained from
the magnetic as well as non magnetic calculations show pronounced differences.
We show that in the two calculations the energy range of the vibrational
contribution from both Fe and Se are quite different. We conclude that Fe
magnetism is correlated to the phonon dynamics and it plays an important role
in stabilizing the structure of K0.8Fe1.6Se2 as well as that of KFe2Se2. The
calculations highlight the presence of low energy librational modes in
K0.8Fe1.6Se2 as compared to KFe2Se2.Comment: 22 pages, 3 Tables, 7 Figure
Phonons and Colossal Thermal Expansion Behavior of Ag3Co(CN)6 and Ag3Fe(CN)6
Recently colossal positive volume thermal expansion has been found in the
framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been
measured using the inelastic neutron scattering technique as a function of
temperature and pressure. The data has been analyzed using ab-initio
calculations. We find that the bonding is very similar in both compounds. At
ambient pressure modes in the intermediate frequency part of the vibrational
spectra in the Co compound are shifted to slightly higher energies as compared
to the Fe compound. The temperature dependence of the phonon spectra gives
evidence for large explicit anharmonic contribution to the total anharmonicity
for low-energy modes below 5 meV. We found that modes are mainly affected by
the change in the size of unit cell, which in turn changes the bond lengths and
vibrational frequencies. Thermal expansion has been calculated via the volume
dependence of phonon spectra. Our analysis indicates that Ag phonon modes in
the energy range from 2 to 5 meV are strongly anharmonic and major contributors
to thermal expansion in both compounds. The application of pressure hardens the
low-energy part of the phonon spectra involving Ag vibrations and confirms the
highly anharmonic nature of these modes.Comment: 19 pages, 14 figures and one tabl
Magnetic Lattice Dynamics of the Oxygen-Free FeAs Pnictides: How Sensitive are Phonons to Magnetic Ordering?
To shed light on the role of magnetism on the superconducting mechanism of
the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering
on phonon dynamics in the low-temperature orthorhombic parent compounds, which
present a spin-density wave. The study covers both the 122 (AFe2As2; A=Ca, Sr,
Ba) and 1111 (AFeAsF; A=Ca, Sr) phases. We extend our recent work on the Ca
(122 and 1111) and Ba (122) cases by treating computationally and
experimentally the 122 and 1111 Sr compounds. The effect of magnetic ordering
is investigated through detailed non-magnetic and magnetic lattice dynamical
calculations. The comparison of the experimental and calculated phonon spectra
shows that the magnetic interactions/ordering have to be included in order to
reproduce well the measured density of states. This highlights a
spin-correlated phonon behavior which is more pronounced than the apparently
weak electron-phonon coupling estimated in these materials. Furthermore, there
is no noticeable difference between phonon spectra of the 122 Ba and Sr,
whereas there are substantial differences when comparing these to CaFe2As2
originating from different aspects of structure and bonding
Raman Evidence for Superconducting Gap and Spin-Phonon Coupling in Superconductor Ca(Fe0.95Co0.05)2As2
Inelastic light scattering studies on single crystal of electron-doped
Ca(Fe0.95Co0.05)2As2 superconductor, covering the tetragonal to orthorhombic
structural transition as well as magnetic transition at TSM ~ 140 K and
superconducting transition temperature Tc ~ 23 K, reveal evidence for
superconductivity-induced phonon renormalization; in particular the phonon mode
near 260 cm-1 shows hardening below Tc, signaling its coupling with the
superconducting gap. All the three Raman active phonon modes show anomalous
temperature dependence between room temperature and Tc i.e phonon frequency
decreases with lowering temperature. Further, frequency of one of the modes
shows a sudden change in temperature dependence at TSM. Using first-principles
density functional theory-based calculations, we show that the low temperature
phase (Tc < T < TSM) exhibits short-ranged stripe anti-ferromagnetic ordering,
and estimate the spin-phonon couplings that are responsible for these phonon
anomalies
Effect of K Doping on Phonons in Ba1-xKxFe2As2
The lattice dynamics of Ba1-xKxFe2As2 (x = 0.00, 0.27) have been studied by
inelastic X-ray scattering measurement at room temperature. K doping induces
the softening and broadening of phonon modes in the energy range E = 10-15 meV.
Analysis with a Born-von Karman force-constant model indicates that the
softening results from reduced interatomic force constants around (Ba,K) sites
following the displacement of divalent Ba by monovalent K. The phonon
broadening may be explained by the local distortions induced by the K
substitution. Extra phonon modes are observed around the wave vector q =
(0.5,0,0) at E = 16.5 meV for the x = 0.27 sample. These modes may arise either
from the local disorder induced by K doping or from electron-phonon coupling.Comment: J. Phys. Soc. Jpn. (in press
Entrance channels and alpha decay half-lives of the heaviest elements
The barriers standing against the formation of superheavy elements and their
consecutive decay have been determined in the quasimolecular shape
path within a Generalized Liquid Drop Model including the proximity effects
between nucleons in a neck, the mass and charge asymmetry, a precise nuclear
radius and the shell effects given by the Droplet Model. For moderately
asymmetric reactions double-hump potential barriers stand and fast fission of
compact shapes in the outer well is possible. Very asymmetric reactions lead to
one hump barriers which can be passed only with a high energy relatively to the
superheavy element energy. Then, only the emission of several neutrons or an
particle can allow to reach an eventual ground state. For almost
symmetric heavy-ion reactions, there is no more external well and the inner
barrier is higher than the outer one
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