172 research outputs found
Orbiton-mediated multi-phonon scattering in LaSrMnO
We report on Raman scattering measurements of single crystalline
LaSrMnO (=0, 0.06, 0.09 and 0.125), focusing on the high
frequency regime. We observe multi-phonon scattering processes up to
fourth-order which show distinct features: (i) anomalies in peak energy and its
relative intensity and (ii) a pronounced temperature-, polarization-, and
doping-dependence. These features suggest a mixed orbiton-phonon nature of the
observed multi-phonon Raman spectra.Comment: 6pages, 6figures, submitted to PR
Existence of orbital polarons in ferromagnetic insulating LaSrMnO (0.110.14) evidenced by giant phonon softening
We present an inelastic light scattering study of single crystalline
(LaPr)SrMnO (, and
,). A giant softening up to 20 - 30 cm of the
Mn-O breathing mode has been observed only for the ferromagnetic insulating
(FMI) samples () upon cooling below the Curie
temperature. With increasing Pr-doping the giant softening is gradually
suppressed. This is attributed to a coupling of the breathing mode to orbital
polarons which are present in the FMI phase.Comment: 4 pages, 5 figure
Coexistence of Superconductivity and Magnetism in FeSe_1-x under Pressure
An extended investigation of the electronic phase diagram of FeSe up
to pressures of \,GPa by means of ac and dc magnetization, zero
field muon spin rotation (ZF SR), and neutron diffraction is presented. ZF
SR indicates that at pressures \,GPa static magnetic order
occurs in FeSe and occupies the full sample volume for \,GPa. ac magnetization measurements reveal that the superconducting volume
fraction stays close to 100% up to the highest pressure investigated. In
addition, above \,GPa both the superconducting transition temperature
and the magnetic ordering temperature increase
simultaneously, and both superconductivity and magnetism are stabilized with
increasing pressure. Calculations indicate only one possible muon stopping site
in FeSe, located on the line connecting the Se atoms along the
-direction. Different magnetic structures are proposed and checked by
combining the muon stopping calculations with a symmetry analysis, leading to a
similar structure as in the LaFeAsO family of Fe-based superconductors.
Furthermore, it is shown that the magnetic moment is pressure dependent and
with a rather small value of at \,GPa.Comment: 11 pages, 9 figure
Helical fluctuations in the Raman response of the topological insulator Bi2Se3
The topological insulator Bi2Se3 shows a Raman scattering response related to
topologically protected surface states amplified by a resonant interband
transition. Most significantly this signal has a characteristic Lorentzian
lineshape and spin-helical symmetry due to collision dominated scattering of
Dirac states at the Fermi level E_F on bulk valence states. Its resonance
energy, temperature and doping dependence points to a high selectivity of this
process. Its scattering rate (Gamma=40 cm-1=5 meV) is comparable to earlier
observations, e.g. in spin-polaron systems. Although the observation of
topological surface states in Raman scattering is limited to resonance
conditions, this study represents a quite clean case which is not polluted by
symmetry forbidden contributions from the bulkComment: 12 pages, 5 figures, 1 tabl
Influence of Co spin-state on optical properties of LaCoO and HoCoO
Optical properties of the isoelectronic compounds LaCoO and HoCoO has
been experimentally and theoretically investigated. We've measured the real
and imaginary parts of the dielectric
function, reflectance and optical conductivity at room temperature.
The shift of the most pronounced spectral features to the high energy region on
0.3 eV associated with larger distortions due to the smaller rare earth ionic
radii in HoCoO in comparison with LaCoO was observed. Also there was
found an enhancement of absorption intensity in the range 1.3-2.3 eV in all
kinds of spectra in HoCoO, which can be attributed basing on the results of
LDA+U calculations to the different spin-states of Co ion in these
compounds. The shift of the onset of the absorption from less than 0.1 eV in
LaCoO to 0.7 eV in HoCoO and an absorption intensity enhancement in a
narrow spectral range 1.2-2.6 eV in HoCoO are clearly seen from the
calculated convolution of partial densities of states obtained in the LDA+U
approach. Such changes are assumed to be induced by the different Co
spin-state in these compounds at room temperature.Comment: 10 pages, 3 figure
Bond-Stretching-Phonon Anomalies in Stripe-Ordered La(1.69)Sr(0.31)NiO(4)
We report a neutron scattering study of bond-stretching phonons in
La(1.69)Sr(0.31)NiO(4), a doped antiferromagnet in which the added holes order
in diagonal stripes at 45 deg to the Ni-O bonds. For the highest-energy
longitudinal optical mode along the bonds, a softening of 20% is observed
between the Brillouin zone center and zone boundary. At 45 deg to the bonds, a
splitting of the same magnitude is found across much of the zone. Surprisingly,
the charge-ordering wave vector plays no apparent role in the anomalous
dispersions. The implications for related anomalies in the cuprates are
discussed.Comment: 4 two-col pages, including 4 figures (2 in color); references added
and updated Final version, accepted for publication in PR
Magnetic ordering in Co2+-containing layered double hydroxides via the low-temperature heat capacity and magnetisation study
The low-temperature heat capacity and the magnetisation of Co2+
n Al3+ layered double hydroxides (LDH) with the
cobalt-to-aluminium ratio n = 2 and 3 and intercalated with different anions have been studied in a wide range of
magnetic fields up to 50 kOe. The heat capacity, C(T), was found to demonstrate a Schottky-like anomaly
observed as a broad local maximum in the temperature dependence below 10 K. The effect is caused by a
splitting of the ground-state Kramers doublet of Co2+ in the internal exchange field and correlates with magnetic
ordering in these LDH. In low applied fields, the temperature-dependent dc magnetic susceptibility demonstrates
a pronounced rise, which is associated with an onset of magnetic ordering. Both the heat capacity anomaly and
the magnetic susceptibility peak are more pronounced for the LDH with n = 2 than for those with n = 3. This
feature is associated with an excess of the honeycomb-like Co–Al coordination (which corresponds to a 2:1
Co–Al ordering) over the statistical cation distribution in Co2Al LDH, while a rather random cobalt-aluminium
distribution is typical for Co3Al LDH. The temperature of the Schottky-like anomaly measured in a zero field is
independent of the interlayer distance. Application of the magnetic field results in a widening of the anomaly
range and a shift to higher temperatures. The observed experimental data are typical for a cluster spin glass
ground state.publishe
Adiabatic description of nonspherical quantum dot models
Within the effective mass approximation an adiabatic description of
spheroidal and dumbbell quantum dot models in the regime of strong dimensional
quantization is presented using the expansion of the wave function in
appropriate sets of single-parameter basis functions. The comparison is given
and the peculiarities are considered for spectral and optical characteristics
of the models with axially symmetric confining potentials depending on their
geometric size making use of the total sets of exact and adiabatic quantum
numbers in appropriate analytic approximations
An approach to verification and validation of MHD codes for fusion applications
We propose a new activity on verification and validation (V&V) of MHD codes presently employed by the fusion community as a predictive capability tool for liquid metal cooling applications, such as liquid metal blankets. The important steps in the development of MHD codes starting from the 1970s are outlined first and then basic MHD codes, which are currently in use by designers of liquid breeder blankets, are reviewed. A benchmark database of five problems has been proposed to cover a wide range of MHD flows from laminar fully developed to turbulent flows, which are of interest for fusion applications: (A) 2D fully developed laminar steady MHD flow, (B) 3D laminar, steady developing MHD flow in a non-uniform magnetic field, (C) quasi-two-dimensional MHD turbulent flow, (D) 3D turbulent MHD flow, and (E) MHD flow with heat transfer (buoyant convection). Finally, we introduce important details of the proposed activities, such as basic V&V rules and schedule. The main goal of the present paper is to help in establishing an efficient V&V framework and to initiate benchmarking among interested parties. The comparison results computed by the codes against analytical solutions and trusted experimental and numerical data as well as code-to-code comparisons will be presented and analyzed in companion paper/paper
- …