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.     

Thermodynamic fingerprints of disorder in flux line lattices and other glassy mesoscopic systems

The internal friction Q-1 and the sound velocity deltav/v of vitreous silica were measured at very low temperatures using mechanical double paddle resonators operated at frequencies ranging from 0.33 to 14 kHz. Below approximately 40 mK the internal friction showed an unexpected temperature and frequency dependence, with absolute values of Q-1 clearly exceeding those predicted by the standard tunneling model. Even though the most plausible origin of the observed excess internal friction appears to be the mutual interaction between tunneling states, the results are difficult to reconcile quantitatively with present theories taking into account this interaction.     

Low frequency acoustic and dielectric measurements on glasses

The acoustic and dielectric properties of different glasses at audio frequencies and temperatures below 1 K have been investigated with the vibrating reed and a capacitance bridge technique. We found the temperature dependence of the absorption of vitreous silica (Suprasil W) to agree with the predictions of the tunneling model which is commonly used to explain the low temperature behaviour of amorphous materials. The variation of the sound velocity and of the dielectric constant, however, shows significant deviations from the expected behaviour which cannot be accounted for by a simple modification of the model. Instead, it seems to be necessary to introduce a temperature dependence of some relevant model parameters. Moreover, at very low temperatures (T < 0.1 K) the sound velocity strongly depends on the excitation levels. The absence of this effect at higher temperatures proves that it can be ascribed to a nonlinear response of tunneling systems. Similar results were found in sound velocity measurements on a cover glass and on a superconducting metallic glass (Pd₃₀Zr₇₀, T_c = 2.6 K), which indicates that these features are a general aspect of the dynamics of tunneling states in glasses. In contrast to the insulating glasses we found that in Pd₃₀Zr₇₀ also the internal friction is strain dependent.     

Thermodynamic properties of small amorphous and crystalline Silica particles at low temperatures

We have measured the low-temperature (K) specific heat and heat release of small amorphous and crystalline SiO₂ particles embedded in Teflon and of Vycor. The temperature and time dependence of these properties have been interpreted in terms of the tunneling model. We found that the particle size influences the density of states of tunneling systems of the composite. The smaller the size of the particles the larger is the density of states of tunneling systems P₀. Quartz grains with dimensions in the micrometer range show similar glass-like properties as vitreous silica. In comparison with bulk vitreous silica, Vycor shows a much larger P₀ in agreement with the behavior we found for small SiO₂ particles. We discuss the implication of our results on the origin of the universal low-temperature properties of glasses. Received 9 April 1998     

Low temperature ultrasonic properties of vapour condensed films of argon-xenon mixtures

Using surface acoustic waves we have studied the elastic properties of thin solid films of pure Argon and Xenon and of Ar–Xe mixtures condensed at 4.4 K. From our results we conclude that in all samples low energy excitations are present. They give rise to a relaxation absorption and to related changes of the sound velocity. An important difference to conventional glasses is, that even below Helium temperatures relaxation occurs not only via tunneling but also via thermally activated processes. The density of states of the low energy excitations is comparable to that of vitreous silica and increases continually with Xenon concentration.     

Slowly relaxing tunneling states in neutron-irradiated quartz: A study of the internal friction at audio-frequencies

The internal friction of two neutron-irradiated quartz samples was measured at audio-frequency as a function of temperature. This is an extension of acoustic measurements to much lower frequencies and thus to much longer time scales. A temperature independent behaviour has been observed and is discussed in the framework of the tunnelling model. One of the major assumptions of this model regarding the tunnelling states with long relaxation time could be verified for neutron-irradiated quartz.     

Acoustic properties of amorphous silica between 1 and 500 mK

We have made reliable measurements of the sound velocity delta v/v(0) and internal friction Q(-1) in vitreous silica at 1.03, 3.74, and 14.0 kHz between 1 mK and 0.5 K. In contrast with earlier studies that did not span as wide a temperature and frequency range, our measurements of Q(-1) reveal a crossover (as T decreases) only near 10 mK from the T(3) dependence predicted by the standard tunneling model to a T dependence predicted if interactions are accounted for. We find good fits at all frequencies using a single interaction parameter, the prefactor of the interaction-driven relaxation rate, in contrast to earlier claims of a frequency dependent power law. We also show that the discrepancy in the slopes d(delta v/v(0))/d(log(10)T) below and above the sound velocity maximum (1: -1 observed, 1: -2 predicted) can be resolved by assuming a modified distribution of tunneling states.     

Anomalous ultrasonic attenuation in vitreous silica at low temperatures

The attenuation of longitudinal sound waves of 750 and 970 MHz has been determined in vitreous silica down to 0.4 K. The unsaturated attenuation observed at the smallest incident acoustic intensities allows a quantitative comparison with the “tunneling model” for glasses.     

Spin-polarized inter-grain tunneling as the probable mechanism for low-field magnetoresistance in partially crystallized La0.5Sr0.5MnO3 thin films

Amorphous/crystalline mixed La_0.5Sr_0.5MnO₃ (LSMO) thin films on quartz wafers are prepared at different depositing temperatures using laser ablation and their low-field magnetoresistive property is investigated. It is argued that the insulating amorphous layers separating the magnetic microcrystalline grains may act as the barriers for electron tunneling. The rapid decay of magnetoresistance with increasing temperature is explained by the spin-polarized inter-grain tunneling. Given the spin-polarized inter-grain tunneling as the probable mechanism, it is believed that the spin flip during inter-grain tunneling reaches a minimum at the optimized depositing temperature of 600 °C and consequently the maximal low-field magnetoresistance is obtained. Received: 7 September 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001     

Low-temperature specific heat of compacted vitreous silica

We present low-temperature specific-heat measurements of two types of irreversibly compacted vitreous silica (Suprasil W and Suprasil I, differing in their OH⁻ content). In the whole temperature range investigated (0.1 K<T<35 K), the specific heatC is reduced by up to 50% compared to that of uncompacted samples but exhibits a similar temperature dependence, with the characteristic maximum inC/T ³ shifted to higher temperatures. This coherent change ofC, which is roughly the same for both types of vitreous silica, gives strong evidence that the low-energy excitations characteristic of amorphous materials have a common structural origin. Possible relations to the microscopic changes upon compaction are discussed, giving support to the coupled-rotation model of SiO₄ tetrahedra. Phenomenologically, the change ofC upon compaction can be understood by the soft-potential model. The relationship between height and position of theC/T ³ maximum observed in normal and compacted states for both types of vitreous silica and, surprisingly, also for α-quartz hints at a correlation between the mechanisms leading to theC/T ³ maximum for amorphous and crystalline materials with similar short-range order.     

Breakdown of the Planck blackbody radiation law at nanoscale gaps

The Planck theory of blackbody radiation imposes a limit on the maximum radiative transfer between two objects at given temperatures. When the two objects are close enough, near-field effects due to tunneling of evanescent waves lead to enhancement of radiative transfer above the Planck limit. When the objects can support electromagnetic surface polaritons, the enhancement can be a few orders-of-magnitude larger than the blackbody limit. In this paper, we summarize our recent measurements of radiative transfer between two parallel silica surfaces and between a silica microsphere and a flat silica surface that show unambiguous evidence of enhancement of radiative transfer due to near-field effects above the Planck limit.     

Ac conductivity in boron doped amorphous carbon films

Experimental results on frequency and temperature dependence of ac conduction in boron doped amorphous carbon films are analyzed in the framework of available microscopic models. Depending on the response, the conductivity plot is divided into three regimes (low frequency high temperature; moderate frequency intermediate temperature; high frequency low temperature) and the data in the respective regimes are corroborated with the various theoretical models accordingly. The conductivity data at high frequency and low temperature suggests that relaxation via quantum mechanical tunneling might be the dominant conduction mechanism. At intermediate temperatures and moderate frequencies, the conductivity data is in good agreement with extended pair approximation model with interaction correction. Signature of enhanced interaction effect is observed at low temperature.     

Concentration of small ring structures in vitreous silica from a first-principles analysis of the Raman spectrum

Using a first-principles approach, we calculate Raman spectra for a model structure of vitreous silica. We develop a perturbational method for calculating the dielectric tensor in an ultrasoft pseudopotential scheme and obtain Raman coupling tensors by finite differences with respect to atomic displacements. For frequencies below 1000 cm(-1), the parallel-polarized Raman spectrum of vitreous silica is dominated by oxygen bending motions, showing a strong sensitivity to the intermediate range structure. By modeling the Raman coupling, we derive estimates for the concentrations of three- and four-membered rings from the experimental intensities of the Raman defect lines.     

Luminescence enhancement of ZnO nanoparticles on metal surface

We have studied luminescence enhancement of zinc oxide (ZnO) nanoparticles with the average size of 30 nm on several metal surfaces at low temperatures. Bandedge luminescence originated from bound exciton (BE) annihilation is observed at 3.360 eV, and strongly depends on the kind and surface roughness of metal. The luminescence intensity is about 10 times larger for Ag surface than that for quartz surface. Furthermore, the luminescence increases remarkably when the roughness of Ag surface is almost the same as the particle size. The intensity ratio of the fast decay component to the slow one decreases for Ag surface compared with quartz. These results suggest that the luminescence enhancement is partially attributed to suppressing of the nonradiative recombination process in ZnO nanoparticles on metal surface.     

Absorption of 35 GHz phonons in vitreous silica between 50 k and 300 k at pressures up to 9 Kbar

Abstract

We have studied the acoustic absorption of vitreous silica under high pressure by high resolution Brillouin scattering. Using a sample 1.0 mm in diameter and 0.6 mm in height, mounted in a sapphire anvil cell, the volume was large enough to avoid parasitic broadening of the Brillouin line by boundaries. An absorption peak typical for glasses is observed around 100 K. It is caused by structural relaxation of defect states. Under pressure the peak is strongly enhanced and shifted to higher temperatures. This behaviour indicates that number and activation energies of the structural defects are increased. These observations can be explained with the same model that describes the anomalous properties of glasses at very low temperatures.     

Temperature-dependent theory of tunneling in the fractional quantum Hall effect

Recent experiments have studied the tunneling current between two edges of the same fractional quantum Hall liquid as a function of temperature and voltage. The experimental findings for low temperatures are at odds with the model where the edges are described as chiral Luttinger liquids, while the data at high temperatures are quite consistent with the same model. Here, we argue that a temperature dependence of the tunneling amplitude, not foreseen in previous works, can explain this discrepancy.     

Effects of structural disorders on sequential tunneling in multiple quantum wells

Based on an isotropic random distribution model, the effects of structural disorders embedded in the barriers on the sequential electron tunneling in multiple quantum wells were studied at low temperatures. By using a sequential tunneling model [Stievenard et al., Appl. Phys. Lett. 61 (1992) 1582], the transmission coefficient through a single barrier was calculated using a finite-difference method and averaged over random configurations of disorders. To compute the tunneling current, a self-consistent calculation for the electronic states was performed, including the Hartree and exchange interactions and non-parabolic energy dispersion. Both disorder-assisted and disorder-impeded electron tunneling phenomena were found as a function of the activation energy. The effects of electric field, barrier width, and temperature were also studied. The predicted resonant disorder-assisted electron tunneling should be large enough to be observable at low temperatures in an experiment.     

非晶态石英的变电荷分子动力学模拟

基于迭代变电荷方法,用分子动力学模拟了非晶态石英的结构与振动特征.首先利用熔化-猝火方法得到了非晶石英的平衡结构.在此基础上获得了体系不同原子对之间的对关联函数、键角分布函数和振动频谱等,结果与实验数据均符合较好.变电荷方法的计算结果表明,非晶石英体系内粒子的电荷与石英晶体内粒子电荷显著不同,并且出现了较大的涨落. 关键词： 分子动力学 变电荷 非晶石英     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000