2-dimensional arrays of Co- and Pd-clusters embedded in carbon films were fabricated by means of heat-treatment method of carboxylated cellulose films after the exchange of COOH-group protons by Co- and Pd-cations. The sizes of metal clusters within range 10 nm-1μm were obtained in dependence on the heat-treatment temperature. The dependencies of the resistance on temperature and magnetic field for the samples annealed at T=700ºC and 900ºC were measured. The R(T) dependencies both for carbon films with Co- and Pd-clusters can be fitted by expression R=R_0 exp(T_0/T)^{1/n} inherent for variable-range hopping. In the whole range of investigated magnetic field and temperature magnetoresistance was negative and can be related to quantum interference in the variable range hopping transport along neighboring alternative paths.
We present contactless surface photovoltage spectroscopy and photoreflectance studies of 10 nm wide, p-type doped asymmetric GaAs/InGaAs/AlGaAs quantum well structures. The MBE grown structures differ in spacer thickness between the quantum well and the reservoir of holes. The doping causes that quantum well is placed in electric field. The surface photovoltage spectroscopy measurements gave us detailed information about the optical transitions between confined states and between confined and unconfined states. The comparison of experimental and numerical analysis allows us to identify all features present in the surface photovoltage spectroscopy and photoreflectance spectra. It has been found that spacer layer thickness has significant influence on surface photovoltage spectroscopy spectra.
Recombination of excitons and positive trions is studied by two-beam photoluminescence of a two-dimensional hole gas in a high magnetic field. The singlet, dark-triplet and bright-triplet states of a free trion are resolved, and their binding energies are determined. Recombination of acceptor-bound trions is also detected, including a low-energy cyclotron replica, corresponding to a hole shake-up process. Identification of all these different transitions was possible by analysis of optical selection rules and the comparison of experimental spectra with realistic numerical calculations.
The electrical properties of polycrystalline tin dioxide films were investigated by impedance spectroscopy in the frequency range 100 Hz-1 MHz at temperatures 4.2 K, 77 K and 300 K. Analysis of the experimental data by means of complex nonlinear least squares method made it possible to divide the contributions of grain bulk and grain boundaries to the conductivity. It was found that at room temperature charge transport processes are mainly determined by the grain volume while at the low temperatures contribution from the grain boundaries to the impedance of the system prevails.
The lateral motion of excitons in GaAs quantum wells is studied by means of spatially resolved photoluminescence. We show that at low temperatures (4.2 K) the exciton motion evolves from localised excitons (zero mobility) in thin quantum wells to extremely high mobilities in wide wells. We find that for the widest quantum well investigated the observed motion cannot be explained by simple exciton diffusion and must be explained by the propagation of ballistic exciton polaritons.
Magnetotransport properties of the nanogranular SnO_2 films were invesigated. Non-linear current-voltage (I-V) characteristics were observed at low temperatures. The temperature dependence of the resistance and non-ohmic I-V curves can be well approximated by fluctuation-induced tunnelling model, indicating importance of the contacts barriers between SnO_2 grains. Magnetoresistance was measured within temperature range 2-15.3 K and could be consistent with the variable-range hopping conduction mechanism due to existence of localized states on the surface of SnO_2 grains.
Positively charged excitons in a two-dimensional hole gas in symmetric and asymmetric GaAs/Ga_{1-x}Al_{x}As quantum wells are studied in polarization-resolved photoluminescence experiments in high magnetic fields B (up to 23 T) and low temperatures (down to 300 mK). The experiments are accompanied by numerical calculations of a real structure. The whole family of trions (the singlet and a pair of triplets) are observed. The Coulomb energies crossing of singlet and triplet is found: hidden in symmetric and visible in asymmetric structures.
The diodes manufactured on the wafers of single-crystalline silicon uniformly doped with phosphorus are studied. The wafer resistivity was 90 Ω cm. Krypton ions are implanted to the side of the p^{+}-region of diodes (energy 107 MeV, fluence Φp from 5 × 10^7 to 4 × 10^9 cm^{-2}). It is shown that recovery charge Q_{rr} is inversely proportional to the square root of the irradiation fluence value Φp. When the fluence increases, the part of the recovery charge Q_{rrA}, due to the high reverse conductance phase, decreases faster than the value Q_{rr}.
In magneto-photoluminescence spectra of a two-dimensional hole gas in a GaAs quantum well we observe coupling of two different radiative states. The pair of coupled states are an acceptor-bound trion AX^{+} and an essentially free (only weakly localized by a shallow lateral potential) trion X^{+}, brought into resonance by an additional cyclotron excitation controlled by the magnetic field. The coupling mechanism is the exciton transfer, and the optical signature is a clear anticrossing of the emission lines of an X^{+} and a cyclotron replica of the AX^{+}.
Diodes manufactured on the wafers of single-crystalline silicon uniformly doped with phosphorus are studied. The wafer resistivity was 90 Ω cm. Xenon ions were implanted into the diodes from the side of the p^{+}-region (implantation energy 170 MeV, fluence Φp from 5 × 10^7 to 10^9 cm^{-2}). It is shown that the formation of a continuous irradiation damaged layer with the thickness of the order of magnitude of the average projective range creates prerequisites for the negative differential resistance in the current-voltage characteristics of the irradiated diodes.
Photothermal radiometry was applied to investigate the influence of a rolling process on the thermal properties of NiTi foils. The thermal diffusivity and thermal effusivity of the foils were determined at room temperature from the frequency variation of the photothermal signals in thermal transmission. Measurements were conducted on unrolled, rolled and thermally annealed samples. The thermal diffusivity and thermal effusivity are found to decrease considerably on rolling which is attributed to the influence of the rolling induced lattice defects.
Photothermal radiometry was applied to investigate the effect of abrasion by cavitation on steel which was supplied with a wear protecting NiTi film. Phase and amplitude data from areas on the sample which where damaged differently by the cavitation effect indicate the appearance of a three layer structure with the cavitation treatment. With cavitational impact the thermal diffusion time of the first layer decreases and an interfacial layer emerges which is identified as a surface region of the steel substrate which has undergone a stress induced transformation from an austenite to a martensite structure.
Silicon diodes irradiated with helium ions with energies of 4.1, 6.8 and 8.9 MeV are studied. It is shown that the mechanism determining the behaviour of frequency dependence of complex electric module and correspondingly the behavior of impedance of diodes irradiated with helium ions in the frequency region 3-200 kHz is a recharging of vacancy complexes localized in the space charge region.
Static and dynamic properties of electron spins in self-assembled (In,Ga)As/GaAs quantum dots which contain on average a single electron per dot were studied by pump-probe Faraday rotation. Examples given here are the g-factor tensor components as well as the dephasing time T*_2 within a dot ensemble.
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