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1.
A complete Raman scattering study of an oriented NH4Br single crystal has been carried out and new results are reported for temperature between 45 to 410°K. The study includes measurements and interpretations of the Raman spectral bandwidths, intensities and frequencies of the lattice and internal modes as a function of temperature. Because of the ability of generating a single domain crystal, unambiguous assignments of the Raman active modes are made. From the present data together with those presented previously significant information on the behavior of the ordering and crystal structure associated with the phase transitions in NH4Br is obtained. 相似文献
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Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions
in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure
dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T*AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the
quadrupolar nematic ordering Qij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity
of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with
pressure. 相似文献
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In the last few decades, significant progress has been achieved in the development of generators and detectors of terahertz
radiation (at frequencies in the range from ≈300 GHz to ≈3 THz). Different terahertz spectroscopic techniques have been widely
used now in investigating semiconductors, superconductors, molecular magnets, multiferroics, metamaterials, and other promising
objects. It has been demonstrated that terahertz spectroscopy offers wide but not completely realized possibilities for studying
quantum phase transitions in electron-correlated systems. 相似文献
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J.M. Guimarães Neto O.N. Oliveira Jr. R.M. Faria 《Applied Physics A: Materials Science & Processing》2000,71(3):267-270
Spontaneous voltages were observed in a structure made of poly(vinylidene fluoride-trifluoroethylene) copolymers films sandwiched
by metals evaporated onto both surfaces. Open voltages and short-circuit currents were measured as a function of the temperature
using different combinations of evaporated metals as electrodes: aluminum, gold, nickel and copper. The ferroelectric-to-paraelectric
transition was also observed in such structures for 60:40, 70:30 and 80:20 of VDF/TrFE molar ratios, including the temperature
hysteresis characteristic of the first-order transition.
Received: 9 February 2000 / Accepted: 28 March 2000 / Published online: 9 August 2000 相似文献
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The influence of grain size on the phase transitions of ferroelectric KNbO3 was studied by micro Raman spectroscopy. It was found that the three transitions observed are not sharp for small particles (∼50 μm), indicating that they do not behave like bulk particles. The transition temperatures depend on the size and all particles show hysteresis. From these experiments we have obtained some evidence that in small particles monodomains of the rhombohedral and orthorhombic phases coexist in a range of temperatures. 相似文献
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A single crystal neutron diffraction study of the low temperature structural phase transitions in LiKSO4 has been carried out. Detailed temperature dependence of the Bragg peak profiles and integrated intensities for a group of about 20 reflections has been investigated upon cooling and heating in the temperature range 300 K to 96 K. The crystal undergoes three clear phase transitions at 205 K (from sp. gr. P63 to sp. gr. P31c), at 189 K and at about 135 K on cooling. The corresponding temperatures are 250 K, 190 K and 138 K on the heating cycle. These transitions are extremely sluggish, the kinetics depending on the thermal treatment of the crystal. 相似文献
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A. A. Bokov 《Journal of Experimental and Theoretical Physics》1997,84(5):994-1002
A new model of ferroelectric phase transitions in disordered crystals is developed. The model takes into account the nonequivalence
of the structural environment of identical ions, which alters the balance of forces governing ferroelectric structural instability.
In contrast with its predecessors, the new model can be used for qualitative and, in many cases, quantitative predictions
of the temperature range of the diffuse ferroelectric phase transition and the value of the disorder-induced shift of the
average transition temperature as a function of the degree of disorder and chemical composition. This conclusion is confirmed
by a comparison of the theoretical results with an abundance of known experimental facts.
Zh. éksp. Teor. Fiz. 111, 1817–1832 (May 1997) 相似文献
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The role of temperature and anisotropy of the applied load in the pressure-induced transformations of alpha-cristobalite is investigated by means of first principles molecular dynamics combined with the metadynamics algorithm for the study of solid-solid phase transitions. We reproduce the transition to alpha-PbO2 as found in experiments and we observe that the transition paths are qualitatively different and yield different products when a nonhydrostatic load is applied, giving rise to a new class of metastable structures with mixed tetrahedral and octahedral coordination. 相似文献
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S. Achilli M. Allione A. M. Malvezzi M. Patrini A. Stella P. Cheyssac R. Kofman 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):223-226
In this work we use nonlinear optical properties of Ga
nanoparticles monolayers of different average sizes embedded in
dielectric matrices to investigate the liquid-solid phase
transitions in these materials. Ga nanoparticles, formed by
exploiting the partial wetting of liquid Ga over a
SiOx surface are irradiated with fs laser
pulses from a Ti:sapphire source. The resulting Second Harmonic
(SH) generated in the reflection and transmission directions is
measured along the phase transitions by cooling the sample from
320 K down to liquid nitrogen temperature. Hysteresis cycles are
observed in the nonlinear transmission, which exhibit a strong
amplification from the solid to liquid values as compared to the
linear optical results. A simple model for SH generation, based
on Mie scattering calculations which includes the effect of
surface plasmon resonance provides a fair key for the
interpretation of the observed effects. 相似文献
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V. L. Aksenov M. Bobeth N. M. Plakida J. Schreiber 《Zeitschrift für Physik B Condensed Matter》1987,69(2-3):393-397
A universal mechanism is proposed, which explains the appearance of the central peak by means of a transition from ergodic to nonergodic dynamical behaviour of the system at some temperatureT
g
above the phase transition. A detailed investigation of the scalar lattice 4-model with defects is given. The relation between the nonergodic state and the appearance of precursor clusters and Raman scattering experiments is discussed. 相似文献
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We report the infrared (IR) absorption spectra of different conformational isomers of gas phase amino acid molecules in the molecular fingerprint region of 330-1500 cm(-1). The IR absorption spectra for three conformers of the amino acid tryptophan show absorption bands that uniquely identify the conformational structure of the molecule and that are well matched by density functional theory calculations. The present observations hold great promise for future identification of conformational folding of larger molecules by means of their IR absorption characteristics. 相似文献
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A model involving nonlinear coupling between the overdamped phonon mode and temperature fluctuations is studied using the dynamic renormalization group method. It is shown that the behavior of the central peak in the dynamic form factor depends on the specific heat exponent α. For α < 0 and as T → Tc the central peak is found to merge with the over-damped phonon mode and the coupling goes to zero at Tc as (T ? Tc)?α. An argument on the intensity of the central peak in the critical region in presented and it is concluded that the hydrodynamic coupling may not be the dominant mechanism of the central peak in SrTiO3. 相似文献