Search

We study paramagnetic Ho(3+) centers in CaWO4, a promising material for applications in quantum electronics and quantum information devices. Oriented single crystals with nominal holmium concentrations 0.05, 0.5, and 1 at% were investigated at 4.2 K using EPR spectroscopy in the frequency range 37-850 GHz at temperatures 5-40 K and high-resolution optical transmission spectroscopy in the infrared and visible wave-length ranges. Along with the tetragonal Ho(3+) centers of the S4 point symmetry, four different types of low-symmetry centers were identified in the EPR spectra and their spectral parameters were determined. A well resolved hyperfine structure exhibiting holmium concentration dependent features was observed in optical spectra. Modeling of the spectra taking into account random lattice strains gave a possibility of reproducing satisfactorily the measured hyperfine structure of the EPR signals, in particular, at anticrossings of the electron-nuclear sublevels of the ground non-Kramers doublet, and the envelopes of the hyperfine structure of optical transitions. The widths of the probability distribution of random deformations related to the point lattice defects in the samples with different concentrations of the impurity Ho(3+) ions were estimated from a comparison of the simulated spectra with the experimental data.

Results of laboratory tests of polyurethane based material "Pentalur" conducted to determine its mechanical properties proved the material to meet basic requirements for removable dentures materials. The introduction of simple poluethers with certain molecular weight as well as 1,4-butanediol allows varying polyurethane properties in a fairly wide range. This range of polyurethane mechanical properties along with good biocompatibility opens new possibilities in creation of removable dentures with improved functional properties as well as maxillofacial prosthesis.

We have studied the Ho3+ spin dynamics for LiY0.998Ho0.002F4 via the positive muon (mu+) transverse field depolarization rate lambdaTF as a function of temperature and magnetic field. We find sharp minima in lambdaTF(H) at fields for which the Ho3+ ion system has field-induced (avoided) level crossings. The reduction scales with calculated level repulsions, suggesting that mu+ depolarization by slow fluctuations of nonresonant Ho3+ spin states is partly suppressed when resonant tunneling opens new fluctuation channels at frequencies much greater than the muon precession frequency.

The magnetic properties of single crystals of LiHo(x)Y(1-x)F(4) with x = 16.5% and x = 4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While this system is considered as the archetypal quantum spin glass, the detailed analysis of our magnetization data indicates the absence of a phase transition, not only in a transverse applied magnetic field, but also without field. A zero-Kelvin phase transition is also unlikely, as the magnetization seems to follow a noncritical exponential dependence on the temperature. Our analysis thus unmasks the true, short-ranged nature of the magnetic properties of the LiHo(x)Y(1-x)F(4) system, validating recent theoretical investigations suggesting the lack of phase transition in this system.

Multispin tunneling cross relaxations in an ensemble of weakly coupled Ho3+ ions, mediated by weak anisotropic dipolar interactions, can be evidenced by ac-susceptibility measurements in a high temperature regime. Based on a four-body representation, including the rare-earth nuclear spin, two-ion tunneling mechanisms can be attributed to both dipolar-biased tunneling and cotunneling processes. The coreversal involving entangled pairs of magnetic moments is discussed with a particular emphasis, giving new evidence to elucidate the many-body quantum dynamics in dipolar spin glasses.

Staircaselike hysteresis loops of the magnetization of a LiY0.998Ho0.002F4 single crystal are observed at subkelvin temperatures and low field sweep rates. This behavior results from quantum dynamics at avoided level crossings of the energy spectrum of single Ho3+ ions in the presence of hyperfine interactions. Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings. At faster sweep rates, nonequilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps.

Dillerent chicken tissues are shown to display a clearly pronounced specificity relative to [2-14C] orotic acid and [5-3H]uridine as precursors of synthesis of the pool and RNA pyrimidine nucleotides. The fraction of pyrimidine nucleotides synthetized relative to the reserve pathway (uridine utilization) decreases in the series: kidneys greater than duodenum mucosa greater than lungs greater than liver greater than pancreas greater than bone marrow greater than brain greater than spleen. The results of [2-14C]orotic acid and [53H]uridine incorporation into UMP and CMP of the liver and spleen tissues RNA are interpreted in terms of the concept on existence of separate pools of pyrimidine phosphates--RNA precursors.

The rate of total RNA synthesis determined by the method of Bukher and Sheffield in different tissues of rats and chickens decreases as follows: kidneys greater than liver greater than duodenum mucosa. The activity of CTP-synthetase (EC 6, 3, 4, 2) determined in vitro and in vivo by the method suggested correlates with the rate of total RNA synthesis in the kidneys and liver tissues of rats and chickens. It is shown that 34-35% of CTP synthetized de novo in the chicken tissues and 31-34% in the rat tissues are used for RNA synthesis. At the same time 75-90% of CTP incorporating into RNA is formed by salvage pathway.

Vitamin D3 induction of the vitamin-D3-dependent proteins biosynthesis and certain indexes characterizing the rachitic state in animals were studied as affected by potassium orotate. Introduction of potassium orotate to the rachitogenic diet in combination with vitamin D3 is shown to normalize the processess of bone mineralization and to increase intensity of chicken growth. In duodenum mucosa with rachitis orotate normalizes the activity of alkaline phosphatase, raises the content of calcium-binding protein and stimulates the latter formation after administration of vitamin D3; orotate favours redistribution of the purine and pyrimidine nucleotides content. Radioactive orotic acid is subjected to intensive metabolism in mucosa and is converted to nucleotides of pool and RNA.

The processes of pyrimidine nucleotides metabolism were analyzed in cells of chicken small intestine mucosa using the kinetic series-parallel model of the first order equations. The rate constants are calculated for absorption of [2-14C]orotic acid by mucosa cells, its accumulation in blood anc conversion to uridine nucleotides. Stages of uridine nucleotides conversion to cytidine ones and their incorporation into RNA are not described by the system of the first order equations based on the series-parallel model with the single-channel utilization of orotic acid through the pyrimidine pathway. The model into two pathways of free nucleotides metabolism and two individual flows of precursors into RNA corresponds qualitatively to the obtained experimental data. Metabolism of orotic acid along two separate channels might be typical of the chicken liver.