Temperature dependence of structure and density for D?O confined in MCM-41-S

Using neutron diffraction, we have tracked the temperature dependence of structural properties for heavy water confined in the nanoporous silica matrix MCM-41-S. By observing the correlation peak corresponding to the pore-pore distance, which is determined by the scattering contrast between the silica and the water, we monitored the density of the confined water. Concurrently, we studied the prominent first diffraction peak of D(2)O at ≈ 1.8 ?(-1), which furnishes information on the microscopic arrangement of the water molecules. The data show the presence of a density maximum at ≈ 275 K (± 10 K), a property similar to bulk water, and the occurrence of a density minimum at ≈ 180 K (± 10 K). The prominent diffraction peak of D(2)O is found to shift and sharpen over a wide T range from 200 to 270 K, reflecting structural changes that are strongly correlated with the changes in density. We also observe the continuous formation of external ice, arising from water expelled from the pores while expansion takes place within the pores. An efficient method for monitoring the density of the confined D(2)O using a triple-axis spectrometer is demonstrated.     

Observation of the boson peak at the surface of vitreous silica

The boson peak is an excess in the phonon density of states compared to the Debye model that appears in almost all glasses. It has been repeatedly measured in the bulk by a variety of methods, but its origin is still highly debated. Here we present first experimental evidence of the boson peak on the v-SiO2 surface. The measurements were obtained by helium atom scattering. The boson peak appears as a dispersionless mode of approximately 4 meV in the recorded time-of-flight spectra. It is clearly identified as an excess contribution to the low energy Debye-like region in the surface phonon spectral density which is extracted from the time-of-flight spectra using a straightforward theoretical model.     

Inelastic ultraviolet scattering from high frequency acoustic modes in glasses

The dynamic structure factor of vitreous silica and glycerol has been measured as a function of temperature and of the momentum transfer up to Q=0.105 nm(-1) using a novel experimental technique, the inelastic ultraviolet scattering. As in the case of Brillouin light scattering and ultrasonic measurements, the temperature dependence of the acoustic attenuation shows a plateau below the glass transition whose amplitude scales as Q2. Moreover, a slight temperature dependence of attenuation has been found in vitreous silica at about 130 K, which seems to be reminiscent of the peak measured at lower Qs. These two findings strongly support the idea that anharmonicity is responsible for sound attenuation at ultrasonic and hypersonic frequencies. Finally, we demonstrate that the attenuation mechanism should show a change of regime between 0.105 and 1 nm(-1).     

钽铌酸锂晶体的布里渊散射研究

研究了钽铌酸锂(LiTa_xNb_(1-x)O_3,x=0.0013,0.0052,0.015,0.06)晶体在室温的布里渊散射.给出了它们的弹性刚度常数和压电应力常数.测量了一种组份晶体的布里渊散射频移在50K～580K范围内的温度特性.发现某些声子散射谱中除三个正常峰外还存在一个异常峰.     

Boson peak measurements in neutron-irradiated quartz crystals

We have measured the low-frequency Raman scattering in neutron-irradiated quartz crystals with four different irradiation doses from 4.7×10¹⁹ n/cm² to 1×10²⁰ n/cm² and for 2 different crystallographic directions. For the used doses the range of density change of the investigated samples was 12% (the maximum change during amorphization is 14%) and the amorphous fraction varied from 35% to 100%. The same measurement was done in neutron-irradiated amorphous silica with a maximal dose 2×10²⁰ n/cm². In all cases we observed the boson peak in the Raman spectra. The position of the peak, at 67±3 cm^-1, was found to be the same for all the investigated samples independent of the dose. The shape of the peak for doses 6.8× 10¹⁹ n/cm² and higher was also found to be the same for 5 investigated samples (including irradiated vitreous silica). We found that the position of the boson peak in neutron-irradiated quartz crystals and vitreous silica corresponds to the Ioffe-Regel crossover frequency for phonons. The origin of the boson peak in neutron-irradiated quartz and vitreous silica can be attributed to local soft optic modes, which are analogous to the soft optic mode that drives the α–β transition in quartz.     

Specific heat of disordered superfluid 3He

The specific heat of superfluid 3He, disordered by a silica aerogel, is found to have a sharp discontinuity marking the thermodynamic transition to superfluidity at a temperature reduced from that of bulk 3He. The magnitude of the discontinuity is also suppressed. This disorder effect can be understood from the Ginzburg-Landau theory which takes into account elastic quasiparticle scattering suppressing both the transition temperature and the amplitude of the order parameter. We infer that the limiting temperature dependence of the specific heat is linear at low temperatures in the disordered superfluid state, consistent with predictions of gapless excitations everywhere on the Fermi surface.     

Transmission phase shift of a quantum dot with kondo correlations

We study the effects of Kondo correlations on the transmission phase shift of a quantum dot in an Aharonov-Bohm ring. We predict in detail how the development of a Kondo resonance should affect the dependence of the phase shift on transport voltage, gate voltage, and temperature. This system should allow the first direct observation of the well-known scattering phase shift of pi/2 expected (but not directly measurable in bulk systems) at zero temperature for an electron scattering off a spin- 1 / 2 impurity that is screened into a singlet.     

Low temperature penetration depth of strongly coupled superconductors

We have made in the clean limit a systematic exploration within strong coupling theory of the low temperature dependence of the London penetration depth for isotropic superconductors. We have found that strong coupling effects can reasonably simulate a power law dependence for an important range of parameters characterizing the boson spectrum responsible for pairing. Sometimes the precision is excellent. In such cases it would be quite difficult to distinguish experimentally between a pure power law and the strong coupling result. The physical origin of this temperature dependence is the quasi elastic scattering of electrons by low frequency bosons. Rather wide boson spectra are required in order to have these low frequency bosons at low temperature and a fairly strong coupling is necessary for their scattering to be effective. Laboratoire associé au Centre National de la Recherche Scientifique et aux Universités Paris 6 et Paris 7     

NMR of mercury in porous carbon and silica gel

The temperature dependences of the integrated intensity and of the Knight shift of ¹⁹⁹Hg NMR signals are measured for liquid and solid mercury introduced into porous carbon and silica gel. A decrease in the temperature of completion of crystallization and a small temperature hysteresis (from 4 to 9 K) between melting and crystallization are observed. The melting temperature of mercury in pores coincides with that in the bulk. The ¹⁹⁹Hg NMR signal from crystalline mercury under the condition of restricted geometry is observed for the first time. It is established that the Knight shift for liquid and crystalline mercury in pores is smaller than in the bulk.     

Temperature effects on primary bragg peaks in sulphur-copper and lead-nickel surfaces

The primary Bragg peak height, width and position in the intensity versus voltage plot (300–800 °K) is measured as a function of temperature for Ni (110) and Cu(111) samples with an adsorbed single layer of lead and sulphur respectively. Little peak's position dependence of temperature is found as if surface thermal expansion did not occur. The results are interpreted in terms of the difference in amplitudes of vibration of bulk and surface layers and a mechanism for quantitative analysis is proposed.     

Resonant Raman scattering in germanium and zincblende-type semiconductors temperature dependence

We have measured the resonance in the Raman scattering near the E₁ gaps of InAs and of a Ge_0.77 Si_0.23 alloy at 77, 300 and 594°K. In contrast to the E₁ gap determined in absorption and transmission measurements, the coresponding peak in the spectral dependence of the scattering cross section shifts very little with temperature; it occurs at all temperatures very near the energy of the absorption peak measured at low temperatures (∼ 77°K).     

永磁性大块金属玻璃Nd60Al10Fe20Co10低温磁性的研究

研究了Nd6 0 Al1 0 Fe2 0 Co1 0 大块金属玻璃磁性随温度的变化关系 ,结果表明Nd6 0 Al1 0 Fe2 0 Co1 0 在室温下表现为永磁性 ,随着温度的降低 ,矫顽力和磁滞回线形状都有很大的变化 .交流磁化率在 18K左右出现尖峰而且峰值温度随频率变化 ,表明该大块非晶体系中存在自旋玻璃态     

Rapid tarnishing of silver nanoparticles in ambient laboratory air

Silver has useful surface-plasmon-resonance properties for many potential applications. However, chemical activity in silver nanoparticles exposed to laboratory air can make interpretation of optical scattering and extinction spectra problematic. We have measured the shift of the plasmon polariton wavelength of arrays of silver nanoparticles with increasing exposure to ambient laboratory air. The resonance peak wavelength shifts 65 nm in 36 h (1.8 nm/h). We show by scanning Auger spectroscopy that the shift is due to contamination from sulfur, most likely chemisorbed on the surface. The rate of corrosion product growth on the nanoparticles is estimated to be 3 nm per day, 7.5 times higher than that of bulk Ag under the same conditions.     

The effect of pressure on lattice parameter,magnetic susceptibility and reflectivity of SmS

The lattice parameter of SmS single crystals has been measured under hydrostatic pressure up to 70 kbar. The fit for the metallic phase by means of the Birch equation gives an unusually large pressure derivative of the bulk modulus B' = 11 and hence a large pressure dependence of B. No change in the nonmagnetic intermediate valence state of Sm is observed up to 24 kbar as concluded from our magnetic susceptibility measurements. Reflectivity measurements under hydrostatic pressure p > 6.5 kbar show a small plasma edge shift (～0.08 eV) towards lower energy upon cooling from 77 to 4.2 K. Hence the low temperature resistivity anomaly is attributed to an enhanced conduction electron scattering.     

Structure and dynamics of water confined in silica hydrogels: X-ray scattering and dielectric spectroscopy studies

We have used a sol-gel technique to obtain optically transparent hydrogels in which water is confined within a 3D silica matrix. In this work we report X-ray scattering and dielectric spectroscopy measurements on samples having different aging times and compare them with previously obtained results with near-infrared (NIR) absorption spectroscopy. X-ray scattering at room temperature enables to characterize the structure and size of the matrix pores and the non-uniform distribution of water inside the hydrogel. Broad band dielectric spectroscopy in the temperature range 130-280 K enables to study water dynamics. In aged hydrogels two relaxations are clearly evident and show characteristic temperature dependence. The faster relaxation has an Arrhenius behavior in the whole temperature range investigated with an activation enthalpy of approximately 50 kJ/mol; it is attributed to water molecules strongly interacting with the silica matrix. The slower relaxation has a markedly non-Arrhenius behavior which can be fitted with a Vogel-Fulcher-Tamman (VFT) relation with critical temperature of approximately 100 K and activation enthalpies of 35 and 95 kJ/mol at 300 and 170 K respectively; it is attributed to water molecules within the pores that do not interact strongly with the matrix and behave collectively. The VFT temperature dependence of the dielectric relaxation time suggests that this water does not crystallize, in agreement with previous results from NIR spectroscopy.     

Ultra-fast diagnosis of shock waves and plasma at front and rear surfaces in the bulk of fused silica induced by an Nd:YAG pulse laser

We investigate the dynamic processes of the Nd:YAG pulse laser ablation of fused silica by ultrafast timeresolved optical diagnosis with a nanosecond time resolution. The evolution process of plasma expansion in air and shock waves propagation in the bulk are both obtained with spatial and temporal resolutions.Laser-induced damage in the bulk of fused silica with filaments and shock waves are observed. Thermoelastic wave,mechanical wave,and shock wave dependence on the laser fluence and intensity of the plasma are analyzed. The shock pressure P and temperature T calculated through the measured shock velocity D and the Hugoniot data of fused silica are measured.     

Temperature dependence of Raman linewidth and shift in CuI

The temperature dependence of the optical phonon linewidth and frequency shift in CuI has been measured in the temperature range of 4.2 ～ 300 K. Utilizing phonon dispersion curves obtained from neutron scattering measurements, the linewidths and frequency shifts are calculated in terms of three-phonon interactions proposed by Pine and Tannenwald. The experimental results for the change in linewidth and frequency with temperature are in good agreement with this theory.     

Exchange bias with Fe substitution in LaMnO<Subscript>3</Subscript>

We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field in case of 30% of Fe substitution in LaMnO₃. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility measurements show a sharp peak (T_p) at 51 K and a shoulder (T_f) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process, which is in consistent with the cluster-glass compound.     

Surface and bulk normal state transport properties in K(3)C(60)

The temperature dependence of the surface resistivity for a metallic K(3)C(60) ordered film in the nonsuperconducting state has been obtained by reflection electron energy loss spectroscopy. We demonstrate that the normal state electronic and transport properties of the top molecular layer of K(3)C(60) are similar to the corresponding properties measured with bulk sensitive techniques. These observations strengthen and give a general character to the experimental results obtained with surface sensitive techniques on fullerene compounds. In addition, the transport properties may deviate from the Fermi-liquid behavior above 500 K.     

Low bias electron scattering in structure-identified single wall carbon nanotubes: role of substrate polar phonons

We have performed temperature-dependent electrical transport measurements on known structure single wall carbon nanotubes at low bias. The experiments show a superlinear increase in nanotube resistivity with temperature, which is in contradiction with the linear dependence expected from nanotube acoustic-phonon scattering. The measured electron mean free path is also much lower than expected, especially at medium to high temperatures (>100 K). A theoretical model that includes scattering due to surface polar phonon modes of the substrates reproduces the experiments very well. The role of surface phonons is further confirmed by resistivity measurements of nanotubes on aluminum nitride.