Photoconductance of two-dimensional ballistic microstructures

We calculate the photoconductance of two-dimensional ballistic microstructures subject to a high-frequency electromagnetic field. First, we study a simple quantum point contact. Absorption of photons is due to electronic transitions between different modes. A transition between a propagating and a nonpropagating mode results effectively in a backscattering process, and gives a negative or positive contribution to the current, depending on the gate voltage; the total quantized conductance acquires an additional, quite pronounced step-structure. Then, we demonstrate a new effect where the electron-photon interaction in a structure of slightly more complex geometry plays the same role as impurity scattering does in a “dirty” system. All relevant photons of the external electromagnetic field are coherent and spatial interference effects in electron-photon scattering become possible in spite of the inelastic nature of the collisions. These interference effects can be controlled by the gate voltage or the frequency of the electromagnetic field. As an illustration we calculate the photoconductance of a double point-contact geometry.     

Two-photon-excited fluorescence in ultradisperse diamond powders

The spectra of two-photon-excited fluorescence in the near ultraviolet are studied in diamond powders prepared by detonation synthesis, with particle sizes of 20–40 Å. The spectra of ultradisperse diamond powders are compared with that of a diamond single crystal. It is shown that the proposed method can be used to detect ultradisperse diamonds in quantities as low as 10^?12 g.     

Stimulated infrared emission under excitation of condensed molecular dielectrics with giant pulses of a ruby laser

Stimulated infrared (IR) emission from a condensed dielectric medium under exposure to a giant pulse of a ruby laser is reported. This effect was predicted in the theoretical paper [1]. Experimental studies were carried out for a number of molecular liquids in two experimental geometries. In the first case (“in transmission” geometry) the propagation direction of the detected IR radiation coincided with that of the exciting radiation. In the second case IR radiation generated was detected in the opposite direction. The angle of divergence of IR radiation was found to be of 10⁻² rad, while the conversion efficiency with respect to the pumping intensity depended on the type of molecular liquid and varied in the range of 0.05–0.6%. Possible microscopic mechanisms of generation of IR radiation under pumping of the dielectric medium with visible or ultraviolet (UV) radiation are analyzed.     

Untersuchung des Hydroxyl-gruppengehalts auf der Oberfl?che der Makroporen und in den Ultraporen der por?sen Gl?ser

Ohne Zusammenfassung     

Bound and many-particle states in polariton Raman spectra of NH4Cl crystals

The properties of polariton Raman spectra for the internal vibration region of NH₄Cl crystal are discussed. These spectra have been recorded by means of a copper laser ( $\text{λ = 5105 and 5782}\text{A}\text{?}$) for sample temperature T=80 and 10°K.According to the theory a polariton singularities in the region of a polar bound state has been obtained. The two-particle bands have been studied and a very complicate structure inside of these bands has been revealed. Moreover, multiphonon singularities at polariton curves have been observed. The experimental polariton law has been compared to the theory and a conclusion has been made as to the necessity of a further development of the theory.     

Raman scattering in the disordered phase of ammonium halides

The Raman scattering spectra of pure and mixed ammonium halide crystals are presented. A correlation between the halogen concentration in the samples and the frequency of the outer translational mode is shown. Experimental results and theoretical models are compared.     

Quantum localization in one-dimensional quasi-random systems and magnetic breakdown

The magnetic breakdown in metals is shown to cause the appearance of a new class of one-dimensional quasi-random “incommensurable” systems where the electrons are localized due to quantum interference effects. At this time both absolute localization and phase transition of “metal- dielectric” type can be realized.