From antidot to dot superlattices: Magnetoresistance anomalies

For low Fermi energies and large antidot diameters an antidot superlattice changes into a dot superlattice. A negative magnetoresistance (NMR) for low magnetic fields coexisting with a commensurability peak is observed in this transition regime. We reproduce these features for both classical and quantum-mechanical calculations in a model, which traces the NMR back to saddle points with varying heights between the antidots. 1997 Elsevier Science B.V. All rights reserved.     

Elastic anomalies in strontium titanate

The anomalies of the second and third-order elastic constants have been considered for the phase transition of strontium titanate within the framework of Landau’s theory. All the anomalies of the second-order elastic constants have been obtained in a single formula using Kronecker delta functions and relations among them have been established. The real parts ofC*₁₁ andC*₄₄ decrease steeply across the transition temperature and thereafter flatly tend to their asymptotic values in the low temperature phase agreeing qualitatively with experimental observations. We have also derived expressions for the third-order elastic anomalies and discussed the temperature variation of the real part ofC*₁₁₁. We have derived expressions for the attenuation of the longitudinal and transverse waves along certain simple symmetry directions and have shown that there is nearly good agreement with experimental observations.     

Elastic anomalies in compressible Ising models

The renormalization-group-study indications of critical elastic anomalies (softening) inducing first order transitions in compressible Ising systems are examined here and are compared with the results obtained by employing other methods. The indications are concluded to be spurious.     

Elastic anomalies at phase transitions in multiferroics

The temperature dependences of the elastic modulus in multiferroics-magnetoelectrics are analyzed, in which magnetic and ferroelectric orderings appear as the result of two successive phase transitions. The analytical relationships for the elastic modulus near the phase transitions to ordered states are obtained for the cases of either linear-quadratic or biquadratic contributions to magneto- and electroelastic coupling. The explicit dependence of the elastic modulus in the multiferroic phase on the magnetoelectric coupling constant was found. It is shown that the characteristic elastic properties in multiferroics can be treated using the Landau theory without taking into account fluctuations. The analysis includes changes in the phase diagrams due to the magneto- and electroelastic coupling.     

Elastic anomalies near a surface-induced phase transition

An analysis is made of the influence of elastic degrees of freedom on a surface-induced phase transition. It is found that elastic interactions give rise to a nonlinear correction to the boundary conditions for the order parameter, with the result that the surface transition is of the second order despite the fact that a first order phase transition occurs in the bulk. The propagation of Rayleigh waves near such a transition is discussed.     

Elastic anomalies in 1T-TaS2

The temperature dependence of the resonant period in 1T-TaS₂ was measured using the resonant flexural vibration technique. From the close study on the effect of thermal cycles, it is suggested that the anomalous regions observed both on cooling down (183–193 K) and on warming up (218–282 K) were associated with the nearly commensurate-commensurate charge density waves (CDW's) phase transition.     

Elastic properties of semiconductor a-Si:H/a-SiNx:H quasiperiodic and periodic superlattices

Surface Rayleigh acoustic waves and related elastic properties of quasiperiodic and periodic modulated a-Si:H/a-SiN_x:H superlattices have been studied by means of a light-scattering technique. Changes in the phase velocity of the surface Rayleigh acoustic waves as a function of sublayer (a-Si:H) thickness are interpreted using an effective medium model. Despite the existence of structural mismatch in the bond-length and coordination number and of structural disorder in the bond-angle for Si-N and Si-Si, no appreciable discrepancy between the measured results and the theoretical predictions has been found.     

对“磁力小火车”的原理探究及实验验证

根据磁铁在不均匀磁场中受力公式和洛伦兹力公式推导出磁力小火车现象中,小车的平衡速度表达公式,并通过MATLAB和COMSOL等软件结合实验测量数据得出表达式中相关参数数值,利用MATLAB做出小车平衡速度v与电池电压U_0,v与线圈半径R,v与线圈匝数密度n,v与加入磁铁个数N的理论图像,并通过实验探究v与U_0,v与n,v与N的关系,得到的实验图像与理论图像趋势吻合程度较好.     

Elastic and photoelastic anomalies at the phase transition of chloranil

Brillouin scattering experiments on chloranil reveal important elastic and photoelastic anomalies associated to the displacive transition at 93 K. The kind of anomaly is characteristic of a quadratic coupling of the order parameter with the elastic strains; a maximum of this coupling is found for e₁.     

手摇交流发电机演示现象的解释

发现了手摇交流发电机演示实验中出现脉动直流电的奇异现象并对此作出了合理的解释.提出了演示实验的改进方案.     

An explanation of optical phenomena in non-crystalline semiconductors

Barrier model of a non-crystalline semiconductor is described in this article. The most important optical phenomena, which are typical for this group of materials, are explained on the base of this model. The model assumes that in non-crystalline semiconductors the potential barriers exist, which separate certain microscopic areas from each other, assuming barriers possess a parabolic profile. This conception explains the rise of exponential tails of optical absorption at the end of optical edge as well as electroabsorption, photoelectric conductivity, photoluminescence, and others. Using this model, many electric transport properties of non-crystalline semiconductors can be explained successfully.     

Elastic surface wave anomalies near the cluster-to-layer transition of Au on NaCl

The cluster-to-layer transition of Au on NaCl(001) has been investigated by means of Brillouin scattering from elastic surface wave excitations. In the cluster regime the softening of the Rayleigh surface mode with increasing cluster size is paralleled by that of a new acoustic surface mode, which appears in the frequency range above the transverse acoustic (TA) phonon mode of NaCl. The frequency crossing of the new mode and the TA mode marks the percolation threshold.     

An explanation of the phenomenon of polytypism

Based on the analogy between polytypes and spin-half Ising chains, polytypes can be considered as different phases of a spin-half Ising system with competing nearest neighbour and next nearest neighbour interactions operating in a single direction. It is known that such an Ising system exhibits extremely rich and complicated phase behaviour. This behaviour is shown to be very similar to the phase behaviour exhibited by polytypes.     

An explanation of the beam dump experiment

The production of prompt neutrinos in the beam dump experiment is explained in a cluster model, by postulating the emission of strange clusters in hadron collisions, besides the usual non-strange clusters. The low mass strange clusters can decay only by the weak interaction. The leptonic and semi-leptonic decay modes of these clusters give rise to prompt neutrinos. A prediction of the model is that the ratios <v _e>/π ⁺ at PS energies would be the same as SPS energies.