101,983 research outputs found
Analytical approach to the 4D-resolution function of three axes neutron spectrometers with focussing monochromators and analysers
No full textThree axes neutron spectrometers explore excitations in condensed matter at almost arbitrary points within the 4-dimensional (Q-omega)-space of momentum and energy transfers. The knowledge of resolution is important for both, the preparation and interpretation of experiments. In order to meet the requirements of modem focussing techniques in direct and reciprocal spaces, an analytical algorithm is presented which allows the detailed calculation of the 4D-resolution function. This approach takes into account all essential features of state-of-the-art three axes spectrometers. Complementary to existing numerical simulation methods, the analytical treatment provides neutron users with almost instantaneous information about resolution effects of ongoing experiments. Moreover, it can be used as an efficient tool for the quantitative interpretation of experimental results. Several examples are shown which demonstrate the performance and the accuracy of the present approach. (C) 2014 Elsevier B.V. All rights reserved.German Federal Ministry for Education and Research (BMBF) [05K10MG2
Imaging chemical concentration pattern and early stages of spinodal decomposition in the AgxNa1-xBr system by scanning force microscopy
No full textIn quasi binary systems like AgBr-NaBr the demixing from the homogeneous high temperature phase into separated phases is a complex process that depends on the ageing temperature as well as on the quench rate. Spinodal decomposition and nucleation processes can be distinguished which lead, however, to the same final equilibrium state. Using frequency modulated scanning force microscopy we aimed to distinguish the demixed phases and their morphologies on a nm-scale at room temperature for samples with different concentration. While no contrast between the different phases are observed in the usual topographic mode, a modified evaluation of Kelvin experiments allows the distinction of silver-and sodium enriched phases. Moreover, direct evidence is found for concentration fluctuations which are characteristic for spinodal decomposition
Small-angle neutron scattering from tetradecyltrimethylammonium bromide in NaBr aqueous solutions
No full textSmall angle neutron scattering was used to characterise quantitatively the micelles of tetradecyltrimethylammonium bromide at a concentration of about 50 mM in aqueous (D2O) solutions with added salt NaBr. It is shown that the shape of the micelles changes from spheres to rod-like particles with increasing electrolyte concentration. The interparticle interaction becomes effectively screened if the salt concentration exceeds 0.1 M leading to a remarkable growth of the cylindrical micelles. Moreover, it is demonstrated that there is a gradual dehydration of the micelles due to the addition of NaBr. (C) 2001 Elsevier Science B.V. All rights reserved
The new thermal triple-axis spectrometer PUMA at the Munich neutron-source
No full textThe instrumentation of the new German neutron-source FRM II is currently making great progress. In this contribution we present the main and unique features of the new thermal triple-axis spectrometer PUMA, which is under construction now and is expected to be operational by the end of 2001. (C) 2000 Elsevier Science B.V. All rights reserved
Transnational Networking and Joint Research Activities of NMI3 – First 14 months
No full textNMI3 (Neutron and Muon Integrated Infrastructure Initiatives) – General Assembl
Time-evolution of phonon spectra during decomposition in AgxNa1-xBr ionic crystals
No full textReal-time inelastic neutron scattering has been used to characterise the mechanism of decomposition in AgxNa1-xBr mixed single crystals well below the critical temperature on a microscopic scale. The chemical demixing process is associated with a splitting of acoustic phonon branches, which is observed to take place within some 100 s. Interestingly, the Bragg reflections exhibit a completely different time behaviour: the adaptation of the equilibrium lattice parameters occurs on a much longer time scale. Hence, decomposition is governed by large coherency strains which persist over a long period of time. Only in a second stage does the crystal lattice relax into the final equilibrium state
Kinetics of decomposition in ionic solids II : neutron scattering study of the system AgCl-NaCl
No full textThe kinetic of decomposition in the quasi-binary ionic system AgCl-NaCl has been studied by means of time-resolved neutron diffraction and small-angle neutron scattering. The coherent critical point is estimated as 447 +/- 3 K, 24 K below the upper critical binodal temperature. The chemical decomposition is almost completed in the first 200 s irrespective of the temperature. The mechanical relaxation of the lattice, however, takes place on a much longer timescale and is dominated by coherency strains leading to metastable intermediate states
Kinetics of decomposition in ionic solids: IV: Transition between nucleation and spinodal decomposition observed by time-resolved phonon spectroscopy
No full textReal-time inelastic neutron scattering has been used to study the kinetics of the demixing process in AgBr-NaBr mixed single crystals. The variation of transverse acoustic phonon spectra provides most direct information about the microscopic non-equilibrium behaviour. The phase separation is monitored by a well defined splitting of phonon peaks that reflects the different dynamical behaviours of the constituents. As a function of temperature, time evolution of the phonon spectra changes drastically, thus proving unambiguously that the demixing process is dominated by nucleation in the high-temperature regime and by concentration fluctuations at low temperatures
Electric field induced anomalies in ferroelectric K2SeO4
No full textThe lock-in transition of K2SeO4 at 93.2 K was studied using high-resolution. gamma-ray diffraction, in situ dielectric measurements and neutron scattering. The phase transition shows a coexistence regime of 1.0 K with clearly resolved incommensurate and commensurate satellite reflections. Within a temperature interval of several K above the lock-in transition, strong third order satellites are observed, indicating a pronounced squaring of the modulation wave. Under the influence of high electric fields an intermediate phase is observed, which is characterized by a diffuse intensity distribution. In addition, the dielectric permittivity exhibits two maxima, indicating the existence of two separate phase transitions. This behaviour is attributed to distortions of the discommensuration lattice and pinning of the discommensurations by accumulated charged defects. It seems to be characteristic for modulated ferroelectrics of the A(2)BX(4) group, since a similar behaviour is also found in isostructural Rb2ZnCl4 and K2ZnCl4. The soft-mode dynamics close to T-c is found to be essentially independent of the electric field and not affected by the discommensuration lattice.DFG [EC153/2-2
Kinetics of decomposition in ionic solids: III. Time evolution of phonons during spinodal decomposition in AgCl-NaCl
No full textReal-time inelastic neutron scattering has been used to study the kinetics of the demixing process in AgCl-NaCl mixed single crystals. The variation of transverse acoustic phonon spectra provides the most direct information about the microscopic non-equilibrium behaviour. It is shown that a well defined splitting of phonon peaks corresponding to the formation of the product phases takes place on a timescale of seconds. Along with the observation of satellite reflections these findings clearly demonstrate that the phase separation is governed by the mechanism of spinodal decomposition in this ionic system. During the entire process the lattice parameter keeps almost unchanged, thus leading to a metastable state characterized by large coherency strains
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