126 research outputs found
Lattice vibrations of alpha'-NaV_2O_5 in the low-temperature phase. Magnetic bound states?
We report high resolution polarized infrared studies of the quarter-filled
spin ladder compound alpha'-NaV_2O_5 as a function of temperature (5K <= T <=
300K). Numerous new modes were detected below the temperature T_c=34K of the
phase transition into a charge ordered nonmagnetic state accompanied by a
lattice dimerization. We analyse the Brillouin zone (BZ) folding due to lattice
dimerization at T_c and show that some peculiarities of the low-temperature
vibrational spectrum come from quadruplets folded from the BZ point (1/2, 1/2,
1/4). We discuss an earlier interpretation of the 70, 107, and 133cm-1 modes as
magnetic bound states and propose the alternative interpretation as folded
phonon modes strongly interacting with charge and spin excitations.Comment: 15 pages, 13 Postscript figure
Влияние различных удобрений на продуктивность яровой пшеницы и накопление тяжелых металлов (Cd, Zn, Cu) в зерне
Results of a 2-year greenhouse experiment have shown, that fertilizers differently effect on spring wheat productivity and heavy metals accumulation in the plant. Contaminated with Cd, Zn и Сu soddy-podzolic soil shown 10-45% lower wheat yield, compared with the soil without heavy metals. Zn effects more negative on growth and formation of wheat yield, especially in a variant with NPK-amended soil. In the second year, the effect is the same. Suprodit-amended Cd, Cu and Zn contaminated soil showed a 1,4-2,0 lower uptake of heavy metals in the plant than NPK and nitrophosphate-amended soils in the first year and a 1,4-2,2 - in the second year.Изучено влияние различных удобрений (NPK, нитрофоска, Супродит) на продуктивность пшеницы и переход тяжелых металлов в растения в 2-летнем вегетационном эксперименте. Результаты исследований показали, что внесение в дерново-подзолистую среднесуглинистую почву Cd, Zn и Сu приводит к снижению урожая зерна от 10 до 45% по сравнению с почвой, где тяжелые металлы не были внесены. Наибольшее отрицательное влияние на рост, развитие и формирование урожая яровой пшеницы оказывало присутствие в почве Zn, особенно в варианте с NPK. При загрязнении дерново-подзолистой среднесуглинистой почвы Cd, Zn и Cu Супродит ограничивает поступление токсикантов как в первый, так и во второй год исследований в 1,4-2,2 раза по сравнению с NPK и нитрофоской
High resolution infrared absorption spectra, crystal field, and relaxation processes in CsCdBr_3:Pr^3+
High resolution low-temperature absorption spectra of 0.2% Pr^3+ doped
CsCdBr_3 were measured in the spectral region 2000--7000 cm-1. Positions and
widths of the crystal field levels within the 3H5, 3H4, 3F2, and 3F3 multiplets
of the Pr^3+ main center have been determined. Hyperfine structure of several
spectral lines has been found. Crystal field calculations were carried out in
the framework of the semiphenomenological exchange charge model (ECM).
Parameters of the ECM were determined by fitting to the measured total
splittings of the 3H4 and 3H6 multiplets and to the observed in this work
hyperfine splittings of the crystal field levels. One- and two-phonon
relaxation rates were calculated using the phonon Green's functions of the
perfect (CsCdBr_3) and locally perturbed (impurity dimer centers in
CsCdBr_3:Pr^3+) crystal lattice. Comparison with the measured linewidths
confirmed an essential redistribution of the phonon density of states in
CsCdBr_3 crystals doped with rare-earth ions.Comment: 16 pages, 5 tables, 3 figure
Optical spectroscopic study of the interplay of spin and charge in NaV2O5
We investigate the temperature dependent optical properties of NaV2O5, in the
energy range 4meV-4eV. The symmetry of the system is discussed on the basis of
infrared phonon spectra. By analyzing the optically allowed phonons at
temperatures below and above the phase transition, we conclude that a
second-order change to a larger unit cell takes place below 34 K, with a
fluctuation regime extending over a broad temperature range. In the high
temperature undistorted phase, we find good agreement with the recently
proposed centrosymmetric space group Pmmn. On the other hand, the detailed
analysis of the electronic excitations detected in the optical conductivity,
provides direct evidence for a charge disproportionated electronic
ground-state, at least on a locale scale: A consistent interpretation of both
structural and optical conductivity data requires an asymmetrical charge
distribution on each rung, without any long range order. We show that, because
of the locally broken symmetry, spin-flip excitations carry a finite electric
dipole moment, which is responsible for the detection of direct two-magnon
optical absorption processes for E parallel to the a axis. The charged-magnon
model, developed to interpret the optical conductivity of NaV2O5, is described
in detail, and its relevance to other strongly correlated electron systems,
where the interplay of spin and charge plays a crucial role in determining the
low energy electrodynamics, is discussed.Comment: Revtex, 19 pages, 16 postscript pictures embedded in the text,
submitted to PRB. Find more stuff at
http://www.stanford.edu/~damascel/andreaphd.html or
http://www.ub.rug.nl/eldoc/dis/science/a.damascelli
Spin-phonon coupled modes in the incommensurate phases of doped CuGeO
The doping effect of the folded phonon mode at 98 cm was investigated
on the Si-doped CuGeO by magneto-optical measurements in far-infrared (FIR)
region under high magnetic field. The folded phonon mode at 98 cm
appears not only in the dimerized (D) phase but also in the
dimerized-anitiferromagnetic (DAF) phase on the doped CuGeO. The splitting
was observed in the incommensurate (IC) phase and the antiferromagnetically
ordered incommensurate (IAF) phase above . The split-off branches exhibit
different field dependence from that of the pure CuGeO in the vicinity of
, and the discrepancy in the IAF phase is larger than that in the IC
phase. It is caused by the interaction between the solitons and the impurities.Comment: 7 pages, 4 figures, resubmitted to Phys. Rev.
Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in -NaVO studied by far infrared spectroscopy
We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3
and 200cm-1 in polarizations of incident light parallel to a, b, and c
crystallographic axes in magnetic fields up to 33T. The triplet origin of an
excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The
magnitude of the spin gap at low temperatures is found to be magnetic field
independent at least up to 33T. All other infrared-active transitions appearing
below Tc are ascribed to zone-folded phonons. Two different dynamic
Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to
the oscillator strength of the otherwise forbidden singlet to triplet
transition. 1. The strongest singlet to triplet transition is an electric
dipole transition where the polarization of the incident light's electric field
is parallel to the ladder rungs, and is allowed by the dynamic DM interaction
created by a high frequency optical a-axis phonon. 2. In the incident light
polarization perpendicular to the ladder planes an enhancement of the singlet
to triplet transition is observed when the applied magnetic field shifts the
singlet to triplet resonance frequency to match the 68cm-1 c-axis phonon
energy. The origin of this mechanism is the dynamic DM interaction created by
the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated
for both mechanisms using the presented theory.Comment: 21 pages, 22 figures. Version 2 with replaced fig. 18 were labels had
been los
Using electronic structure changes to map the H-T phase diagram of alpha'-NaV2O5
We report polarized optical reflectance studies of \alpha'-NaV2O5 as a
function of temperature (4-45 K) and magnetic field (0-60 T). Rung directed
electronic structure changes, as measured by near-infrared reflectance ratios
\Delta R(H)=R(H)/R(H=0 T), are especially sensitive to the phase boundaries. We
employ these changes to map out an H-T phase diagram. Topological highlights
include the observation of two phase boundaries slightly below T_{SG}, enhanced
curvature of the 34 K phase boundary above 35 T, and, surprisingly, strong
hysteresis effects of both transitions with applied field.Comment: 4 pages, 3 figures, PRB accepte
Global, regional, and national incidence, prevalence, and years lived with disability for 328 diseases and injuries for 195 countries, 1990–2016: a systematic analysis for the Global Burden of Disease Study 2016
As mortality rates decline, life expectancy increases, and populations age, non-fatal outcomes of diseases and injuries are becoming a larger component of the global burden of disease. The Global Burden of Diseases, Injuries, and Risk Factors Study 2016 (GBD 2016) provides a comprehensive assessment of prevalence, incidence, and years lived with disability (YLDs) for 328 causes in 195 countries and territories from 1990 to 2016
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