863 research outputs found
Fingerprints of Mott-Hubbard physics in optical spectra of antiferromagnetic LaTiO3
Magnetic properties of Mott-Hubbard insulators are determined by
superexchange interactions mediated by the high-spin (HS) and low-spin (LS)
intersite d-d charge excitations, which can be associated with the HS- and
LS-Hubbard subbands in optical experiments. To explore the Mott-Hubbard physics
in orthorhombic LaTiO_3 crystal exhibiting the G-type antiferromagnetic order
at the Neel temperature T_N = 146 K, we present a comprehensive spectroscopic
ellipsometry study in the spectral range 0.5-5.6 eV at temperatures 10 K < T <
300 K. We found that the complex dielectric function spectra of LaTiO_3 crystal
are almost featureless, nearly isotropic, and weakly temperature dependent in
the range of d-d optical transitions. Nonetheless, analyzing the difference
spectra below the T_N, we have identified the LS-state d^1d^1 - d^2d^0
excitations at ~3.7 and ~5.15 eV and estimated values of the on-site Coulomb
repulsion U ~4.2 eV and Hund's exchange constant J_H ~0.5 eV, which define the
energy of the HS-state d^1d^1 - d^2d^0 excitation at ~2.7 eV. In addition, we
discovered that the pronounced lowest-energy 1.3 eV optical band displays the
critical intensity behavior and anomalous broadening with decreasing
temperature below the T_N. The discovered properties indicate that the 1.3 eV
band in LaTiO_3 can be associated with a Mott-Hubbard exciton.Comment: 8 pages, 8 figures, 1 tabl
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
In-Plane Spectral Weight Shift of Charge Carriers in
The temperature dependent redistribution of the spectral weight of the
plane derived conduction band of the high
temperature superconductor (T_c = 92.7 K) was studied with wide-band (from 0.01
to 5.6 eV) spectroscopic ellipsometry. A superconductivity - induced transfer
of the spectral weight involving a high energy scale in excess of 1 eV was
observed. Correspondingly, the charge carrier spectral weight was shown to
decrease in the superconducting state. The ellipsometric data also provide
detailed information about the evolution of the optical self-energy in the
normal and superconducting states
Russian interferon <i>α</i>-2b drug in the treatment of chronic hepatitis C
Aim of investigation. To study efficacy and safety of domestic recombinant interferon α-2b Altevir® in combination to ribavirin in previously untreated patients with chronic hepatitis C (CHC).Material and methods. Retrospective treatment results rating of 85 patients (49 men (57%) and 36 women (43%)) was carried out. All patients received 3 million IU per day of Altevir® subcutaneously and ribavirin (Ribapeg). Treatment duration was 48 wks for patients with 1-st genotype of hepatitis C virus (HCV) and 24 wks — for patients with 2nd and 3а HCV genotypes. Results. Aviremia in 24 wks after the end of therapy (sustained virologic response — SVR) has been achieved in 100% of patients with 2-st HCV genotype, in 95,8% patients with 3а HCV genotype and in 65,6% patients with 1b HCV genotype. The registered adverse effects, well-known for interferon α-2b therapy, as a rule, were mild or moderate, AVT cancellation was not required in any patient.Conclusions. For patients with CHC with 2-st, 3а HCV genotypes and favorable prognosis of virologic response at combined AVT, including Altevir® and ribavirin, SVR frequency can reach 100 and 96% respectively. At AVT, including Altevir® and ribavirin to CHC patients with 1b genotype of HCV and favorable prognosis of virologic response, 66% SVR rate can be achieved
Infrared studies of a La_(0.67)Ca_(0.33)MnO_3 single crystal: Optical magnetoconductivity in a half-metallic ferromagnet
The infrared reflectivity of a La_(0.67)Ca_(0.33)MnO_3 single crystal is studied over a broad range of temperatures (78–340 K), magnetic fields (0–16 T), and wave numbers (20–9000cm^(-1)). The optical conductivity gradually changes from a Drude-like behavior to a broad peak feature near 5000cm-1 in the ferromagnetic state below the Curie temperature T_C=307K. Various features of the optical conductivity bear striking resemblance to recent theoretical predictions based on the interplay between the double exchange interaction and the Jahn-Teller electron-phonon coupling. A large optical magnetoconductivity is observed near T_C
Infrared Studies of a La_{0.67}Ca_{0.33}MnO_3 Single Crystal: Optical Magnetoconductivity in a Half-Metallic Ferromagnet
The infrared reflectivity of a single crystal
is studied over a broad range of temperatures (78-340 K), magnetic fields (0-16
T), and wavenumbers (20-9000 cm). The optical conductivity gradually
changes from a Drude-like behavior to a broad peak feature near 5000 cm
in the ferromagnetic state below the Curie temperature . Various
features of the optical conductivity bear striking resemblance to recent
theoretical predictions based on the interplay between the double exchange
interaction and the Jahn-Teller electron-phonon coupling. A large optical
magnetoconductivity is observed near .Comment: 4 pages, 4 figures, Latex, PostScript; The 7th Joint MMM-Intermag
Conference,San Francisco, January 6-9, 1998; The Int. Conf. on Strongly
Correlated Electron Systems, Paris, July 15-18,199
Low-energy Mott-Hubbard excitations in LaMnO_3 probed by optical ellipsometry
We present a comprehensive ellipsometric study of an untwinned, nearly
stoichiometric LaMnO_3 crystal in the spectral range 1.2-6.0 eV at temperatures
20 K < T < 300 K. The complex dielectric response along the b and c axes of the
Pbnm orthorhombic unit cell, \epsilon^b(\nu) and \epsilon^c(\nu), is highly
anisotropic over the spectral range covered in the experiment. The difference
between \epsilon^b(\nu) and \epsilon^c(\nu) increases with decreasing
temperature, and the gradual evolution observed in the paramagnetic state is
strongly enhanced by the onset of A-type antiferromagnetic long-range order at
T_N = 139.6 K. In addition to the temperature changes in the lowest-energy gap
excitation at 2 eV, there are opposite changes observed at higher energy at 4 -
5 eV, appearing on a broad-band background due to the strongly dipole-allowed O
2p -- Mn 3d transition around the charge-transfer energy 4.7 eV. Based on the
observation of a pronounced spectral-weight transfer between low- and
high-energy features upon magnetic ordering, they are assigned to high-spin and
low-spin intersite d^4d^4 - d^3d^5 transitions by Mn electrons. The anisotropy
of the lowest-energy optical band and the spectral weight shifts induced by
antiferromagnetic spin correlations are quantitatively described by an
effective spin-orbital superexchange model. An analysis of the multiplet
structure of the intersite transitions by Mn e_g electrons allowed us to
estimate the effective intra-atomic Coulomb interaction, the Hund exchange
coupling, and the Jahn-Teller splitting energy between e_g orbitals in LaMnO_3.
This study identifies the lowest-energy optical transition at 2 eV as an
intersite d-d transition, whose energy is substantially reduced compared to
that obtained from the bare intra-atomic Coulomb interaction.Comment: 10 pages, 14 figure
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