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: http://foroff.phys.msu.ru/eng/postdoc/neutr.htm
Дата изменения: Mon Nov 17 18:03:10 2003 Дата индексирования: Mon Oct 1 20:22:58 2012 Кодировка: |
Neutron diffraction researches of nuclear and magnetic structure of new materials and its correlations with physical properties
The information on structure of crystals and its changes at external influences is a basis for understanding of their physical properties. In researches of nuclear and magnetic structure of crystals with interesting or unusual properties diffraction of neutrons plays a basic role. With its help it is possible to carry out experiments with a wide interval of temperatures, at influence on a crystal of a magnetic field or a high pressure to reveal correlation in changes of nuclear structure with ordering the magnetic moments. Only with the help of diffraction of neutrons it was possible to receive the necessary information on structure of high-temperature superconductors, manganites with effect of the colossal magnetic resistance, new oxides vanadium, supersonics, relaxors etc. In FLNP JINR there are unique opportunities for neutronography researches of nuclear and magnetic structure of crystals. In particular, one of the best in the world neutron diffractometers – Fourier - diffractometer the high sanction which parameters allow to obtain the precision structural data operates. Experiments on diffractometer will be carried out with participation of experts from conducting crystallographic, the chemical and solid-state centers of Russia. During performance of degree work students receive fundamental knowledge on crystallography to the structural analysis of crystals, experimental methods of neutron physics, modern neutron difractometry, to methods of processing of experimental data.
The phenomenon of phase stratification in crystals complex oxides
The phenomenon of phase stratification in crystals or, in more general formulation, the tendency of formation of non-uniform structural conditions in the last some years were put forward on a first line theoretical and experimental researches of systems with strong electronic correlations. As a rule, such systems have interesting physical properties, for example, high-temperature superconductivity or colossal magnetic resistance. Neutron diffraction experiments conducted on diffractometers with high resolution, allow receiving the unique information on process of phase stratification in crystals, including about changes of an elementary cell arising at it and about the sizes of coherent areas of coexisted phases. The program on studying non-uniform structural conditions with the help of diffraction of neutrons is realized in FLNP JINR on High resolution Fourier diffractometer. For reception of data on a degree of uniformity of a magnetic condition of a crystal the method muon spectroscopy is in addition involved.
Research of structural changes in threefold systems of chalcohenides of mercury HgTe1-xSx, HgTe1-xSex (0.1 < x < 0.9) under influence of a high pressure
It is known, that in binary systems chalcogenides of mercury HgSe, HgTe with increase of pressure structural phase transitions from cubic structure sphalerite in hexagonal structure cinovary, and then in cubic structure such as NaCl is observed. As a result of these phase transitions there are also changes of an electronic condition of substance with semi-metallic on semi-conductor (at transition sphalerite - cinovary) and with semi-conductor on metal (at transition cinovary - NaCl). Influence of a high pressure on structure of threefold systems chalcogenides mercury HgTe1-xSx, HgTe1-xSex poorly studied. Measurements of electroresistance and magnetoresistance these systems have shown, that with increase of pressure in them as it is necessary to expect the structural phase transitions, similar themes which occur in binary systems chalcogenides of mercury.
Structure modulation in crystals
Structural and magnetic modulation is observed in magnetics, dielectrics, semiconductors and high-temperature superconductors and reflects complex character internuclear interactions in real crystals. In FLNP? it is executed full symmetrical analysis of a sequence of phase transitions high-symmetrical-non-proportional-proportional phase for crystal TlInS2 and are carried out preliminary neutron measurements on a mono-crystal. The measurements executed at temperatures 240?, 210?, 203?, 180?, 100? and 10?, have shown, that at all temperatures in TlInS2 there are structural changes. Structural modulation in TlInS2. Diffraction researches with the two-coordinate detector at various temperatures, calculation of vectors of modulation, comparison to theoretical calculations
The near order and magnetic structure in amorphous magnetics. Calculations on optimization of measurements, experimental researches without a magnetic field and in an external magnetic field
Amorphous materials - substances which structure consists from non-ordered atoms. In such structures there is no distant order, but there is a near order. Amorphous materials on the physical properties differ from crystal. So magnetic amorphous materials have high-quality magnetic characteristics and are widely applied in engineering. Their magnetic properties depend on structure and are formed by manufacturing techniques. Influence of structure and technology of processing is shown in structure of the near order. The near order is defined by interaction of the next atoms and depends on character of connection between them. The structure of the near order is described by radial function of distribution which defines average density of atoms from distance. Radial function of distribution is calculated from spectra of elastic coherent dispersion of neutrons
Research of fractal structure of aluminosilicates gels by a method of small angle scattering of neutrons and synchrotrons radiations
Gels name amorphous non-ordered systems with the liquid dispersive environment in which particles of a dispersive phase form a spatial structural grid. They represent the solids-like bodies, capable to keep the form, having elasticity and plasticity. Removal (distance) from gels of the liquid environment (drying) receive the thin-porous the connections widely used as sorbents. As a measure of the no ordered systems the size named fractal dimension which defines (determines) model of growth no ordered (fractal) structures serves and can be easily determined from curves neutron, x-ray (synchrotron) and light small-angle scattering ??????????? the structure of gels depends on many parameters, as that structure and concentration initial a component, a way of cultivation, temperature, a way of drying etc. In particular, aluminum-silicates the gels which have been dried up in supercritical conditions (aero-gel) and on air (xerogel), find out various behavior under influence of mechanical loadings or an electric field that is shown in occurrence in aerogels of significant piezoelectric effect in comparison with xerogels. Studying of interrelation of properties of the given gels with fractal organization of their structure represents an actual problem (task) which effectively can be solved methods of SANS and synchrotron radiations.
Phase transitions of type the order - disorder, dynamics of amorphous materials and oriental glasses
On a spectrometer of high sanction NERA-PR there is an opportunity simultaneously to receive spectra of not elastic dispersion and diffraction of neutrons. It simultaneously allows receiving the information on dynamics and structure of researched substance, that it is essential at research of structural phase transitions and transitions of type the order - disorder
Magnetism on border of the unit (at the interface) of two layers: Fe/Gd, Fe/V, Fe/Co
Magnetic layered structures show the properties which are not observably in continuous magnetics: distribution of the magnetic order on a not magnetic layer, oscillat?ry character of exchange pairing, formation of the long-distance magnetic order in layered structure from layers with various magnetic anisotropy, inducted by the magnetic order. Their studying is important both from the fundamental point of view, and for the purposes of creation of elements of memory, recording and reading of the information. Polarizing reflectometry and diffractometry of neutrons is the effective tool for determination of spatial dependence of a vector of a magnetic induction vector in structure
Coexistence of ferromagnetism and superconductivity in bilayer Fe/V and Cr/V (superconducting)
Various types of interactions coexist near to interface of superconducting and ferromagnetic layers. It is reflected in an establishment of a non-uniform magnetic state. Spatial distribution of an induction of a magnetic field is studied by methods of neutron reflectometry and generation of a neutron field of standing waves.
Behavior of a vortex lattice in a superconductor depending on temperature and a magnetic field
The mixed state of a superconductor is characterized by complex behavior and presence of set of phases on the diagram a “magnetic field-temperature”. Methods of small-angle scattering and depolarization of polarized neutrons allow studying changes of structure of a vortex lattice and separate vortex. On formation of the mixed magnetic state of a film of a superconductor essential influence the renders pinning of vortex on a surface. The phenomenon of formation of a vortex lattice in an inclined magnetic field is studied by the polarizing analysis of the reflected and refracted neutrons in grazing geometry of the falling polarized neutron beam..
Research of structure of systems polymeric gels – SUS by method small-angle scattering of neutrons
Polyelectrolytes in water solutions form complexes with opposite charged surface-active substances. Similar complexes can be generated and gels from polymers. Thus aggregation occurs at concentration below, than critical concentration of micelle formation. This not trivial result demands detailed research.
Research of polyelectrolyte in a matrix of polymeric gel by SANS
Gels - the substances widely used in the industry. Interest to research of such objects is caused also by attempt of reception of thin manipulators. Except for that formation of units in solutions of polyelectrolyte simple enough model of aggregation of rigidly connected polymers of a biological origin
Studying mechanisms of crystallization, crystal structure of aluminous membranes
For understanding of mechanisms of action membranes fibers and enzymes it is necessary to know their high resolution structure. Recently together with Institute of physical and chemical biology of the Moscow State University was offered the new approach to crystallization of fiber with use lipids cubic phases as a matrix. At research of processes of crystallization and structure membranes fibers diffraction of synchrotron radiations and SANS is used.
Neutron spectroscopy of internal fluctuations of biologically active molecules and quantum-chemical modeling of their dynamic properties
As not elastic not coherent dispersion is the most sensitive to fluctuations of the molecular complexes connected to hydrogen NERA-PR is the unique tool for research rich protons of biological substances. In the field of quantum -chemical modeling dynamics of a lattice and rough progress in the field of computer engineering effective machine experiments, and with a complex with experiments with not elastic no-coherent scattering of neutrons allow to carry out a modern state of affairs to receive the unique information on "behavior" of biologically active molecules.
Research of structure of phase transitions modeling phospholipid membranes at cooling at presence cryoprotectors
The purpose of work is creation of the program (Pascal) which will allow to process in a mode of scanning some hundreds spectra written down at synchrotron at cooling membranes down to temperature – 600?. The program should expect position of diffraction peaks (centre of gravity), half - width and intensity as function of temperature. The structure of new phases arising after formation of ice should be indexed and investigated as function of lipid concentration. Results for various phospholipids and cryoprotectors should be analyzed and submitted for the publication
Research of micelle to vesicles transitions by X-rays diffraction f and small-angle scattering of X-rays and neutrons for system phospholipids/ionic surfactant
A research of of self-assembly membrane process (dissolution) is the important scientific and applied task. The adequate theory of process for today does not exist. Progress is possible on the basis of self-organizing systems theory. Process of membrane dissolution under action of surfactant consists of consecutive steps (return process - self-assembly): ?) swelling of multilayer structure (it is studied by diffraction of X-rays), disappearance between membrane interactions and transition to no interacting bilayers (lamellares or vesicles). b) Dissolution bilayer surfactant (transition from lamellar structures to micelles) the Purpose of work is: 1. to describe the forces working between membranes during swelling (a stage ?) on the basis of results received on synchrotron DCI. 2. A stage b. To carry out modeling calculations of spectra small-angle scattering of neutrons received on spectrometer PAXY (LLB) and spectra small-angle scattering of X-rays received on synchrotron DCI. Results of application of neutrons and x-ray radiation to compare and present as article.
The near order and magnetic structure in amorphous magnetics. Calculations on optimization of measurements, experimental researches without a magnetic field and in an external magnetic field
Amorphous materials - substances which structure consists from non-ordered atoms. In such structures there is no distant order, but there is a near order. Amorphous materials on the physical properties differ from crystal. So magnetic amorphous materials have high-quality magnetic characteristics and are widely applied in engineering. Their magnetic properties depend on structure and are formed by manufacturing techniques. Influence of structure and technology of processing is shown in structure of the near order. The near order is defined by interaction of the next atoms and depends on character of connection between them. The structure of the near order is described by radial function of distribution which defines average density of atoms from distance. Radial function of distribution is calculated from spectra of elastic coherent dispersion of neutrons
Neutron diffraction researches of internal pressure in volumetric products and materials
Among numerous methods of studying of internal pressure in crystal materials the method of diffraction of neutrons stands on a special place as, in comparison with the others, it is the most informative. In particular, as against traditional methods, neutrons can penetrate into a material on depth up to 2-3 cm for still and up to 10 cm for aluminum. Besides neutrons provide: preservation of integrity of researched object (not destroying control), the high spatial resolution (up to 1-2 mm in any measurement), definition of crystallographic anisotropies of mechanical deformations, the information on distribution of pressure for everyone components of a multiphase material separately. In FLNP JINR on neutron diffractometers the high resolution experiments with studying residual pressure phase structure and mechanical properties of new perspective materials will be carried out: gradient structures Cu/W, MoSi2/SiC, composites Al/Al2O3, Ni3Al/Al2O3 etc. Results of neutron experiments are used for optimization of technology of reception of such materials with necessary mechanical properties. Except for new materials, internal pressureare studied in industrial products, including at influence of high temperature or external mechanical loading. Fundamental crystallographic aspects of occurrence and distribution of internal pressure are studied in crystals - ferroelastics at phase transitions with change of symmetry.
Modeling texture-formation in polycrystals at plastic deformation
At performance of degree work addition of Tailor method by the equations which are taking into account high-temperature deformation of polycrystalline geomaterials, a spelling of the program for computer modeling of reorientation of crystallites is offered. Primary goal of work is the establishment of connection between types of a structure and orientation textural a component with tensor of the deformations, working at formation of a structure
Research is intense the deformed condition of blocks of lithosphere of the Earth on the basis of the data of the neutron-graphic textural analysis
In work use of the thermodynamic and kinematics approach for an establishment of dependence between primary orientation of minerals and orientation of the main axes of a matrix of mechanical pressure is offered. At performance of work the task of reconstruction of polio-pressures should be solved on the basis of the data of the neutronographic textural analysis.
Coherent fields of waves of neutron and synchrotron radiations in layered nanostructures
There is a big interest to researches on generation and detecting high coherent wave fields of neutron and synchrotron radiations intended for researches of structural and magnetic properties multilayered nanomaterials. These, so-called fields of standing waves can be used for characterization of the used in microelectronics and for systems of record and reading of the information, and also for creation high-monochromatic and high-collimated beams of radiations. The coherent structures of radiations are also a basis for phase-shifting elements (interferometer), which used for investigations with neutron and synchrotron radiations. Themes of works:
EXAFS spectroscopy on a beam of synchrotron radiations, its application for research of the condensed environments, x-ray optics of the deformed crystals, influence of parameters of the monochromator and a beam (SI) on effects of a spherical aberration in polychromatic focus, registration of x-ray radiation by the positional - sensitive detector, a choice of an operating conditions, processing of experimental data of EXAFS-spectra.
Experimental methods of optical spectroscopy in research of nanostructure systems and objects
Spectroscopy of combinational dispersion of the condensed environments, fluorescent spectroscopy of biological objects, semi-conductor laser systems for excitation, methods of construction of optical systems with scanning the image from objects, research of a structure of Raman spectrum with the spatial resolution 100 nm for ferroelectric and magnetic domains
The project of small-angle scattering X-ray spectrometer for DELSY synchrotron source
The purpose of work is designn: 1. of installation with focusing a beam on a sample (bent monochromator) 2. for installation with monochromator as double crystal. It is necessary on the basis of D22 and D24 installations of the source DCI LURE to make calculation of function of the resolution, intensity at the given resolution and speed of the account for three variants of detectors: gas, image - Plate, Gruner type. On the basis of the done analysis to offer more effective variant for investigation of lipid based drug delivery systems.
Research of dynamics luminescence a crystal of sulfide of zinc
The purpose of research is the establishment of key parameters of a luminescence of a crystal in structure of the scintillation screen. Allocation of the basic components of a luminescence, definition of their specific capacity and constants of time. Development of mathematical model for allocation of a signal of capture of a neutron. A method of researches: the program statistical analysis of signals of a photodetector. Development of mathematical algorithm of division of events an effect / background in a sequence of pulses
The analysis of the data of small-angle scattering of neutrons and processing of mistakes of measurements
The purpose of work is consideration of existing algorithms and the computer programs, allowing carrying out an estimation of computing mistakes at the analysis of the anisotropic data on installations on small-angle scattering of neutrons. Development of effective algorithm of an estimation of mistakes. Methods: the comparative analysis of techniques of data processing on an example of the same sample. Use of statistical methods of an estimation of mistakes
Creation of the stand of calibration low temperature gauges
The purpose of work is development of a technique of calibration low temperature gauges on the basis of a regulator of temperature, the helium refrigerator and the personal computer. Automation of process of calibration.
Development of methods of designing and adjustment of positional - sensitive detectors for x-ray and neutron sources
The purpose of work is comparison of settlement and experimental characteristics and x-ray and neutron positional - sensitive detectors for improvement of key parameters. A method: calculation and modeling by the program "Garfield" developments of electronic avalanches in gases, at the certain configurations of electric fields and structure of a gas mix. Receiving of characteristics of the detector
The proportional detector of thermal neutrons
Acquaintance to a principle of action of the proportional gas detector ionic radiations; research of characteristics of proportional detectors on existing devices; development, manufacturing of the detector of neutrons; testing for a beam of a reactor.
The proportional detector of thermal neutrons with the increased speed of light
Acquaintance to a principle of action of the proportional gas detector of ionic radiations; research of characteristics of the gas detector ionic radiations; research of characteristics of multiwire proportional detectors on existing devices; research of dependence of speeds of electron drift from structure of a working mix; development, manufacturing of the detector of neutrons, testing on a beam of a reactor.
Research of dynamics of a pulse reactor of periodic action on fast neutrons Research of self-oscillations of a pulse reactor of periodic action on fast neutrons
Studying the phenomenon spontaneous self heating of cold moderators of neutrons
At low temperatures and at influence of a neutron stream in the substances serving as a moderator of neutrons, the latent energy (radicals, defects) collects. At achievement of critical density of defects (radicals) there is temperature instability, and the substance is spontaneously quickly heated up. Now in FLNP installation (URAM-2) for detailed studying this phenomenon which is interesting not only as preventing factor at work cold moderators is created, but also matters for understanding of processes of evolution of organic substance in the universe. The specialist will work in the scientists conducting experiments with installation and to participate in the analysis of results.
Measurement of intensity of a luminescence at an irradiation of firm methane
Work is connected to the phenomenon spontaneous self heating firm methane under action of an irradiation. At low temperatures and at influence of a neutron stream in the substances serving as a moderator of neutrons, the latent energy (radicals, defects) collects. At achievement of critical density of defects (radicals) there is temperature instability, and the substance is spontaneously quickly heated up. Installation (PIM) for detailed studying this phenomenon on the tandem - accelerator of protons in Juilich (Germany) now is created. The data on intensity of a luminescence can give the valuable information on a nature of the latent energy in methane and about kinetics of defects of recombination. The entrant should understand work of photo multipliers to have some experience in electronics; it is good to know a computer, English or German.