546 research outputs found
Giant phonon anomalies in the pseudo-gap phase of TiOCl
We report infrared and Raman spectroscopy results of the spin-1/2 quantum
magnet TiOCl. Giant anomalies are found in the temperature dependence of the
phonon spectrum, which hint to unusual coupling of the electronic degrees of
freedom to the lattice. These anomalies develop over a broad temperature
interval, suggesting the presence of an extended fluctuation regime. This
defines a pseudo-gap phase, characterized by a local spin-gap. Below 100 K a
dimensionality cross-over leads to a dimerized ground state with a global
spin-gap of about 2~430 K.Comment: 4 pages, 3 figures, for further information see
http://www.peter-lemmens.d
Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study
Magnetic materials are usually divided into two classes: those with localised
magnetic moments, and those with itinerant charge carriers. We present a
comprehensive experimental (spectroscopic ellipsomerty) and theoretical study
to demonstrate that these two types of magnetism do not only coexist but
complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material
the itinerant charge carriers interact with large localised magnetic moments of
Tb(4f) states, forming complex magnetic lattices at low temperatures, which we
associate with self-organisation of magnetic clusters. The formation of
magnetic clusters results in low-energy optical spectral weight shifts, which
correspond to opening of the pseudogap in the conduction band of the itinerant
charge carriers and development of the low- and high-spin intersite electronic
transitions. This phenomenon, driven by self-trapping of electrons by magnetic
fluctuations, could be common in correlated metals, including besides
Kondo-lattice metals, Fe-based and cuprate superconductors.Comment: 30 pages, 6 Figure
Preparation of crystalline Mg(OH)2 nanopowder from serpentinite mineral
In this paper we describe a route to produce crystalline Mg(OH)2 nanopowders from serpentinite ore distributed in the Halilovskiy array (Russia, Orenburg region). An efficient extraction route consisting of treatment on serpentinite in 40% HNO3 at 80 C followed by NH4OH titration for Mg(OH)2 precipitation was demonstrate
Signatures of Electronic Correlations in Optical Properties of LaFeAsOF
Spectroscopic ellipsometry is used to determine the dielectric function of
the superconducting LaFeAsOF ( = 27 K) and undoped LaFeAsO
polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 350 K. The free charge carrier response in both samples is heavily
damped with the effective carrier density as low as 0.0400.005 electrons
per unit cell. The spectral weight transfer in the undoped LaFeAsO associated
with opening of the pseudogap at about 0.65 eV is restricted at energies below
2 eV. The spectra of superconducting LaFeAsOF reveal a
significant transfer of the spectral weight to a broad optical band above 4 eV
with increasing temperature. Our data may imply that the electronic states near
the Fermi surface are strongly renormalized due to electron-phonon and/or
electron-electron interactions.Comment: 4 pages, 4 figures, units in Fig.2 adde
Ferromagnetism and Lattice Distortions in the Perovskite YTiO
The thermodynamic properties of the ferromagnetic perovskite YTiO are
investigated by thermal expansion, magnetostriction, specific heat, and
magnetization measurements. The low-temperature spin-wave contribution to the
specific heat, as well as an Arrott plot of the magnetization in the vicinity
of the Curie temperature K, are consistent with a
three-dimensional Heisenberg model of ferromagnetism. However, a magnetic
contribution to the thermal expansion persists well above , which
contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a
comparison with the corresponding model system EuS. The pressure dependences of
and of the spontaneous moment are extracted using thermodynamic
relationships. They indicate that ferromagnetism is strengthened by uniaxial
pressures and is weakened by uniaxial
pressures and hydrostatic pressure.
Our results show that the distortion along the - and -axes is further
increased by the magnetic transition, confirming that ferromagnetism is favored
by a large GdFeO-type distortion. The c-axis results however do not fit
into this simple picture, which may be explained by an additional
magnetoelastic effect, possibly related to a Jahn-Teller distortion.Comment: 12 pages, 13 figure
Spin-controlled Mott-Hubbard bands in LaMnO_3 probed by optical ellipsometry
Spectral ellipsometry has been used to determine the dielectric function of
an untwinned crystal of LaMnO_3 in the spectral range 0.5-5.6 eV at
temperatures 50 K < T < 300 K. A pronounced redistribution of spectral weight
is found at the Neel temperature T_N = 140 K. The anisotropy of the spectral
weight transfer matches the magnetic ordering pattern. A superexchange model
quantitatively describes spectral weight transfer induced by spin correlations.
This analysis implies that the lowest-energy transitions around 2 eV are
intersite d-d transitions, and that LaMnO_3 is a Mott-Hubbard insulator.Comment: 4 pages, 4 figure
The PCA-seq method applied to analyze of the dynamics of COVID-19 epidemic indicators
In time series analysis using the SSA method, a univariate series is converted into the multivariate one by shifts. The resulting trajectory matrix is subjected to principal component analysis (PCA). However, the principal components can also be computed using the PCA-Seq method if segments of the original series are selected as objects. The matrix of Euclidean distances between the objects can be obtained using any method, which offers additional opportunities for time series analysis compared to the conventional SSA. In this study, the PCA-Seq method was used to analyze the dynamics of COVID-19 epidemic indicators
A systematic review and cost effectiveness analysis of reusable vs. single-use flexible bronchoscopes
Dipole-active optical phonons in YTiO_3: ellipsometry study and lattice-dynamics calculations
The anisotropic complex dielectric response was accurately extracted from
spectroscopic ellipsometry measurements at phonon frequencies for the three
principal crystallographic directions of an orthorhombic (Pbnm) YTiO_3 single
crystal. We identify all twenty five infrared-active phonon modes allowed by
symmetry, 7B_1u, 9B_2u, and 9B_3u, polarized along the c-, b-, and a-axis,
respectively. From a classical dispersion analysis of the complex dielectric
functions \tilde\epsilon(\omega) and their inverses -1/\tilde\epsilon(\omega)
we define the resonant frequencies, widths, and oscillator strengths of the
transverse (TO) and longitudinal (LO) phonon modes. We calculate
eigenfrequencies and eigenvectors of B_1u, B_2u, and B_3u normal modes and
suggest assignments of the TO phonon modes observed in our ellipsometry spectra
by comparing their frequencies and oscillator strengths with those resulting
from the present lattice-dynamics study. Based on these assignments, we
estimate dynamical effective charges of the atoms in the YTiO_3 lattice. We
find that, in general, the dynamical effective charges in YTiO_3 lattice are
typical for a family of perovskite oxides. By contrast to a ferroelectric
BaTiO_3, the dynamical effective charge of oxygen related to a displacement
along the c-axis does not show the anomalously large value. At the same time,
the dynamical effective charges of Y and ab-plane oxygen exhibit anisotropy,
indicating strong hybridization along the a-axis.Comment: 8 pages, 7 figure
- …