Electron spin resonance studies at the phase transition in YbInCu4

In the intermetallic compound YbInCu₄ the Gd³⁺-ESR and the static susceptibility were measured in the temperature range 1.5K–300 K, i.e. both below and above the valence phase transition which occurs in this material at T_ph≈50 K. The Gd³⁺ resonance is mainly determined by exchange interaction of Gd³⁺ ions with fluctuating Yb³⁺ ions via conduction electrons (RKKY coupling) both below and above this transition. Arguments are presented that at low temperature YbInCu₄ is a dense Kondo system (mixed valent state).     

An EPR study of a triplet state center in (NH4)2SO4

Heating ammonium sulphate to near its dissociation temperature and then quenching to room temperature produces a stable triplet state center that is identified as NH₂⁺. The symmetry of the EPR spectra indicates that the center is located in two non equivalent sites with each of these sites having slightly different EPR parameters and two measurably differents orientations. Thus the center is apparently occupying the positions of the NH₄⁺ groups in the perfect lattice. A study is made of this center both at room temperature and also well below the ferroelectric phase transition of (NH₄)₂SO₄. It is found that above and below the transition temperature (?50°C) there is very little temperature dependence of the EPR spectra but at the phase transition there is an abrupt change in the spectra. The number of detectable centers doubles below the phase transition indicating that the inversion symmetry of the NH₂⁺ sites is eliminated at the transition temperature.     

Raman study of squaric acid

Raman spectra of squaric acid have been measured at various temperatures below and above the transition temperature. Six low frequency bands below 300 cm^?1 are assigned to translational and librational modes of the squaric molecule on the basis of the recent crystal determination by X-ray diffraction. A discussion is given of the striking temperature dependence of these external modes in the ordered phase in connection with the phase transition.     

X-ray diffraction measurement of the Jahn-Teller distortion in TmVO4

Single crystal X-ray diffraction has been used to determine the magnitude and temperature dependence of the tetragonal-to-orthorhombic distortion below the cooperative Jahn-Teller phase transition occuring in TmVO₄ at 2.15 K. The results are in good agreement with mean field theory and with the results of a previous investigation of the soft acoustic mode above the transition. It can be concluded that the interaction of long wavelength acoustic phonons with the ground doublet of the Tm³⁺ ion is the dominant mechanism for the transition.     

Exciton-contribution to the reflection spectrum at high excitation intensities in CdS

We report measurements of changes in reflection spectrum of CdS due to increasing the density of photoexcited carriers at temperatures above the critical temperature for electron-hole liquid formation. The contribution of the exciton resonance is seen to decrease and analysis of the lineshape indicates that this decrease is due to exciton-exciton collision and a change in exciton polarizability. These results are consistent with a transition from exciton to free electron-hole plasma (Mott transition) at a density of n ～ 2.5 x 10¹⁷ cm^-3.     

Elastic property of organized lipid assembly — Effect of water incorporation and chain melting

The effect of electric field on the capacitance of lipid-water dispersion properly aligned on polyvinyl alcohol coated glass electrodes, shows a transition pattern where a rapid change in orientation of the organized lipid assembly takes place for an electric field above a threshold value (V_th), similar to that of Freedericksz’s transition as observed in the case of thermotropic liquid crystals. The splay elastic constant, k₁₁ and the bend elastic constant, k₃₃ have been calculated from the measured values of capacitance of the cell below and above V_th. These values change by several orders of magnitude depending upon the presence of water as well as temperature of chain melting of the lipid moiety.     

The effect of Ag substitution in the mixed conductor Cu7PSe6

Cu₇PSe₆ is a mixed conductor that crystallizes in the simple cubic structure at room temperature. Structural transitions above and below room temperature are accompanied by step-like changes in electrical conductivity. The substitution of Ag⁺ for Cu⁺ in Cu₇PSe₆ stabilized the simple cubic structure over a wider range of temperatures than is observed for the pure compound. A disproportionate decrease in electrical conductivity accompanies modest levels of silver substitution. The prominent step in electrical conductivity associated with the low-temperature crystallographic phase transition disappears in (Ag_xCu_1−x)₇PSe₆ solid solutions for a composition parameter x=0.20, replaced by two distinct changes in the slope of conductivity below room temperature.     

T1 anomaly in AgSn proximity-effect bridges under phonon injection

We have observed that steady state heater phonon pumping into AgSn proximity-effect bridges causes a systematic reduction of the bridge critical current. Below T_λ, our results deviate anomalously from predictions of Parker's T¹ model though both above and below T_λ the injection-induced transition to the normal state appears continuous. Further, below T_λ, bridges underlaid with a thin-film of germanium exhibit a discontinuous injection induced transition to the normal state. We discuss possible implications.     

Studies of molecular motions in the three phases of 1,1′,2,2′-tetrachloroferrocene by Mössbauer and1H-NMR spectroscopy

The molecular motions in the three phases of 1,1′,2,2′-tetrachloroferrocene below m.p. were studied by using⁵⁷Fe-Mossbauer and¹H FT-NMR spectroscopy. In the plastic phase at the low temperature, a rotational mode around the pseudo C₅ molecular axis was found to be activated with activation energy of 15 kJ/mol and in the higher temperature phase the rotational mode of the pseudo C₅ axis itself was found to be activated with activation energy of 23 kJ/mol in addition to the rotational mode around the pseudo C₅ axis. On the⁵⁷Fe-Mossbauer spectra, a steep decrease of the areal intensity was observed above the first transition temperature. The areal intensity in the lower plastic phase was estimated to be less than a few percent relative to that at 298 K. The results were discussed in terms of different possibilities for the movement of the molecules.     

Infrared spectra of paraelectric and ferroelectric CsH2PO4

ir spectra are reported for polycrystalline CsH₂PO₄ above and below the transformation temperature, 153 K. Three absorption bands at 1760 cm^-1 2350 cm^-1 and 2800 cm^-1 are assigned to -OHO- vibrations. Whilst the 2350 cm^-1 band remains essentially unaltered, the 1760 cm^-1 band splits into multiplets and the 2800 cm^-1 band shifts to lower frequency on cooling below 153 K. A broad band centred at 3240 cm^-1 is found to exist in the ferroelectric phase and is associated with the -OHO- mode involving the H(2) proton ordered below the transition.     

Production of a bose einstein condensate of metastable helium atoms

We recently observed a Bose-Einstein condensate in a dilute gas of ⁴He in the 2³S₁ metastable state. In this article, we describe the successive experimental steps which led to the Bose-Einstein transition at 4.7 μK: loading of a large number of atoms in a MOT, efficient transfer into a magnetic Ioffé-Pritchard trap, and optimization of the evaporative cooling ramp. Quantitative measurements are also given for the rates of elastic and inelastic collisions, both above and below the transition. Received 15 October 2001     

Mössbauer studies of the charge-density-wave state of 57FeK0.30MoO3

Mössbauer studies have been performed between 6 K and 300 K on single crystals of ⁵⁷Fe-doped K_0.30MoO₃, which show a Peierls transition at 180 K towards on incommensurate charge density wave state. The spectra show three doublets. A strong line broadening below 120 K indicates that the Fe impurities are strong pinning centers for the charge density wave above ∼ 120 K and become progressively weak pinning ones below, in agreement with theoretical models.     

51V nuclear magnetic resonance in polycrystalline ferroelastic BiVO4

Nuclear magnetic resonance studies on polycrystalline ferroelastic BiVO₄ indicate that the ⁵¹V electric field gradient asymmetry parameter is an order parameter in the ferroelastic transition. Using ∩ = A(T?T_c)^B, B is found to be 0.48(5), in good agreement with earlier studies of this material. Near the phase transition above and below, the vanadium nuclear quadrupole coupling is constant with a value of 4.8(1) MHz.     

Mößbauer-Effekt im normalleitenden und supraleitenden Zustand von Sn119

The number of Moessbauer quanta from the 23.8 keV state of Sn¹¹⁹ in a tin metal source was measured after attenuation by a 16.1 mg/cm² absorber of tin metal, as a function of temperature above and below the transition point of superconductivity. Source and absorber were kept at the same temperature. The data give evidence of a discontinuous change at the transition temperature: The transmitted intensity is lowered by (1,3±0.15)·10^?3 in the superconducting state of source and absorber. This corresponds to an increase of the Debye-Waller-factor by about 0,4%.     

Exchange-inversion interaction and acoustical magnon soft mode in Mn2-xCrxSb

A magnon soft mode in Mn_2-xCr_xSb in the vicinity of the Brillouin zone boundary in c¹ direction, for the first order antiferro-ferrimagnetic phase transition, and the magnetization curve of the antiferromagnetic phase were measured. It was found that the soft mode energy squared is a linear function of temperature over a wide temperature range below and above the phase transition point.     

有限数目捕获原子的玻色-爱因斯坦凝聚

研究了各向异性谐振子捕获势中有限数目原子的玻色-爱因斯坦凝聚(BEC),结果表明：对有限数目捕获原子来说,它的BEC临界温度比热力学极限下捕获原子的BEC临界温度要低,原子间的相互作用也将提高（散射长度为负）或降低（散射长度为正）原子BEC临界温度．临界温度以下体系的比热随温度的变化行为不再是T^3/2,而近似为T³．在定域密度近似下,还研究了各向同性捕获势中原子的空间分布,对有限数目的原子来说,它的密度比势力学极限下的要小,原子间的相互作用也降低原子的空间分布密度． 关键词：     

Magnetic properties of magnetite thin films close to the Verwey transition

Temperature dependence of the magnetic properties of magnetite thin film across the Verwey transition has been investigated. As the temperature is decreased, the magnetization of the film in a fixed field showed a sharp decrease close to the Verwey transition temperature (T_v). The M–H loops of the film have been recorded at various temperatures below and above T_v. It is found that film does not saturate at any temperature and saturation becomes more difficult below T_v. While cooling through T_v, the extrapolated value of magnetization to infinite field (Q), calculated from the numerical fit 4πM=Q [1−(H*/H)^1/2], does not show a drop, but the coefficient indicating difficulty in saturation (H*) shows a sharp rise as does the coercivity.     

Temperature evolution of the infrared spectrum of Ni(ClO4)2·6H2O

Temperature evolution of the infrared spectrum of the title compound confirms the phase transition temperatures 223, 311 and 355 K reported earlier and suggests a new phase transition at 180 K. From the spectral evidence, the transitions below the room temperature (～300 K) are attributed to tumbling motion of the metal aquo-complex, while those above the room temperature are attributed to reorientational motion of the water molecule. The space group in low temperature phases is suggested to be C²_s.     

Mössbauer studies of polymers in the glassy state and near their glass transition

Mössbauer absorption spectra of glassy block and copolymers with lowT _G containing ferrocene have been recorded in a wide temperature range (4–330 K) to investigate the dynamical processes of the system, in the glassy state and around the glass transition. An inhomogeneous broadening due to conformational substates with different hyperfine parameters found at lower temperatures becomes motionally narrowed above 40 K. Whereas in the glassy state both the block and the copolymers reveal the same vibrational behaviour, we found differences in the vicinity of their respective glass transitions. For the block polymer an anomalous decrease of the recoilfree fractionf _A was found above the static glass transition. In agreement with neutron data on other polymers we attribute this behaviour to an onset of motions of the ferrocene units of higher than 10¹⁰ Hz. For the copolymers, however, a departure of lnf _A from linearity was detected already far below the staticT _G. We propose that this is caused by residual solvents which allow an easier cooperative rearrangement of the segments even belowT _G. First attempts to interpret the Mössbauer data by means of the predictions of the mode coupling theory are reported.     

Effect of some statin group of drugs on the phase profile of liposomal membrane – a fluorescence anisotropy study

Using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene, we have investigated the effect of some statin group of drugs, widely used in hyperlipidemia, on the phase transition of model membranes such as dipalmitoyl phosphatidylcholine (DPPC) liposomes. The nature of the changes in the fluorescence anisotropy values suggested that the drugs simvastatin and mevastatin fluidized the membrane both before and after the phase transition temperature (T _m), whereas atorvastatin fluidized the membrane below T _m but rigidified the same above T _m, i.e., introduced an intermediate fluid condition within the lipid matrix. We have calculated the changes in van’t Hoff enthalpy values associated with the phase transitions due to chain melting in all the cases, and observed that the values of enthalpy decreased with increase in drug concentrations. In order to get a better insight, the fraction of motionally restricted lipid molecules was determined.