New measurements of the Shubnikov-de Haas (SdH) effect in semimagnetic semiconductor (SMSC) Cd_{3-x-y}Zn_{x}Mn_{y}As_{2} with x = 0.09 and y = 0.018, using continuous magnetic fields up to 25 T, are presented. The observed modulation of the amplitudes of the first and second SdH harmonics as a function of magnetic field is interpreted in terms of the three band Kane model with the exchange interaction included and taking into account the spin-dependent scattering which appears to be particularly important at high magnetic fields.
An anisotropy of the electron effective g-factor in highly degenerated tetragonal semimagnetic semiconductors (SMSC) alloyed with manganese has been found by us and by other workers when studying the Shubnikov-de Haas (SdH) effect for different orientations of the magnetic field with respect to the crystal axes. The detailed analysis of the experimental data by using a model based on an infinite Hamiltonian matrix, provides evidence for the anisotropy of the p-d exchange interaction in the materials of interest.
We investigated magnetoresistance of p-type Ge-on-Si metal-oxide-semiconductor field-effect transistors in order to determine the hole mobility μ as a function of the gate polarization (V_{G}). Measurements were carried out at 4.2 K and magnetic fields up to 10 T. The signal measured was proportional to the derivative of the transistor resistance with respect to V_{G}. To determine the hole mobility we developed a method to treat the measured signal which is based on a numerical solution of a differential equation resulting from the theoretical description of the experimental procedure. As a result, we obtained a non-monotonic μ(V_{G}) dependence which is a characteristic feature of the carrier transport in gated two-dimensional structures.
We present the results of the low temperature electron paramagnetic resonance (EPR) and transport investigations of the crystals of Pb_{1-x}Cr_{x}Te (x ≤ 0.01). The samples with chromium concentrations x ≥ 0.0015 are all n-type. For these samples we observe the single EPR line with the g-factor decreasing from g = 1.97 till g = 1.93 with increasing carrier concentration. This resonance can be attributed to electrically and magnetically active Cr^{3+} ions. The crystals with Cr concentration x ≤ 0.0015 may be both n- and p-type. The EPR spectrum of these samples consists of two lines: the one discussed above and the other one with g = 1.99 observed only for samples with electron concentration n ≤ 10^{18} cm^{-3}.
We present the results of the experimental studies of the low temperature transport and magnetic properties of PbTe, Pb_{1-x}Sn_{x}Te (x ≤ 0.3) and SnTe crystals doped with 0.5 at.% of chromium. Cr was found to be a resonant donor in PbTe and PbSnTe. Magnetic susceptibility measurements revealed that PbTe:Cr and Pb_{1-x}Sn_{x}Te:Cr (x ≤ 0.2) are Curie paramagnets whereas SnTe:Cr exhibits van Vleck paramagnetism.
The anisotropic microwave absorption in the presence of alternative magnetic field has been studied for the first time in superconducting super-lattices based on PbTe-PbS, which are layered anisotropic systems similar to high T_{c} superconductors. A new method of study has been used. The microwave response was detected under broad-band conditions and compared with the results of synchronous detection. All the features which have been observed in high T_{c} materials are clearly seen here.
We report on the first experimental observations of an anomalous increase in the resistance of a semiconductor-super conductor contact in the vicinity of the critical temperature T_{c}. The effect is found in lead-doped InSb thin films having inclusions of lead of 1-3 μm in diameter: The observed effect can be a model that explains similar resistance increase observed sometimes in the high T_{c} superconductors.
The Shubnikov-de Haas oscillations were measured in Hg_{0.95}Mn_{0.05}Se at liquid helium temperatures, in magnetic fields up to 13 tesla. The analysis of the experimental data was performed by means of the wave shape method. The dependence of effective g-factor of the conduction electrons and difference Dingle temperature on magnetic field was determined. Non-zero difference Dingle temperature is a clear evidence of spin dependent scattering in this material. The sp-d exchange constants consistent with extracted spin splitting and difference Dingle temperature, α = -0.39 ± 0.06 eV, β = 0.95 ± 0.07 eV, are in good agreement with the values obtained from magnetooptics.
Here we present magnetic resonance study which confirms an existence of a phase separation picture in Eu_{0.7}Pb_{0.3}MnO_3 single crystals. The measurements were performed with a standard technique and with using unconventional schemes of the magnetic resonance method. The inhomogeneous paramagnetic-ferromagnetic state, which is sensitive to a magnetic field, takes place in 0.65T_c-1.15T_c temperature range, where the colossal magnetoresistance effect is observed. The behavior of spectra suggests that a scenario of the phase separation is realized and the mixed state is not related to a simple chemical inhomogeneity. The changes of the conductivity induced by the microwave resonance absorption are observed in a vicinity of T_c, too. The changes are not related to a trivial heating of the sample by a microwave radiation. We propose a mechanism connected with the change of equilibrium concentrations of the coexisting phases, this change takes place when the system is under magnetic resonance conditions. To analyze the experimental data we applied the effective-medium approximation for two-component system. Also we attracted the simplest phenomenological model qualitatively reproducing the essential features of the behavior of the two-phase system.
The resistance in an asymmetric double-well structure was measured as a function of magnetic fields oriented almost parallel to the plane of the electron layer. It was shown that the shape of the magnetoresistance curves is close to the in-plane magnetic-field-dependent density of states which we obtained by self-consistent numerical calculation. The novel feature is the negative magnetoresistance observed at low magnetic fields.
Hall effect, conductivity, electron paramagnetic resonance and magnetization of Pb_{1-x}Cr_{x}Se (x ≤ 0.01) crystals were measured as a function of Cr content and electron concentration. The experimental results suggest that Cr in PbSe creates a resonant donor state with the energy of about 125 meV above the bottom of the conduction band.
Temperature (T) dependence of conductivity (σ) was studied in semi-insulating GaAs as a function of the magnetic field (B) for 1.8 K < T < 40 K for high electric fields. An infrared illumination of a sample and application of an electric field caused a non-equilibrium distribution of electrons in the conduction band. An increase in B caused a localization transition which manifested itself by a gradual disappearance of the impact ionization of shallow bound states. The transition was connected with a change from a non-activated to an activated conductivity only if T > 4 K, otherwise σ showed only a non-activated character. It is proposed that for T < 4 K the electron distribution function is mostly determined by optical and electric field excitations, which results in a non-activated conductivity. For T > 4 K thermal excitations become dominant which leads to an activated character of σ.
On the example of the PbTe and Pb_{0.77}Sn_{0.23}Te on BaF_{2} the possibility of using the weak magnetic field resistance technique for the evaluation of mismatch-thermally induced strains in semiconductors with multivalley band structure is discussed.
Ζn_{1-x}Mg_{x}Se mixed crystals with x ranging from 0 to 0.56 were obtained by high pressure Bridgman method. It has been found that a phase transition from sphalerite structure to wurtzite one occurs at x = 0.185 ± 0.03. The crystals exhibit blue-violet and yellow-green (depending on x) luminescence in the temperature range from 40 K to room temperature. An attempt has been also made to dope Ζn_{1-x}Μg_{x}Se crystals with Al. The incorporation of Al produces a strong green photoluminescence in the temperature range from 40 K to 300 K but almost completely quenches the near-band-edge emission.
In this paper we show that intersubband scattering can lead to apparent inconsistency of the experimental results obtained by means of classical and quantum transport measurements and this discrepancy is entirely connected with the usage of classical formulae to describe magnetic field dependence of a conductivity tensor. We prove that there is no contradiction in our observations and that the models describing quantum oscillations and magnetic-field dependence of the conductivity tensor, which are present in the literature, complement each other.
The temperature dependence of the hole-phonon energy relaxation time τ_{hph} (T) under hot-hole conditions was studied in SiGe p-type quantum wells. The hot-hole temperature T_{h} was estimated through three different experimental methods: (i) from a comparison of the amplitude of the Shubnikov-de Haas oscillations changed by current and temperature; (ii) from a comparison of the phase relaxation time in the effect of weak localization obtained either at different temperatures and minimum current or at different current at a fixed temperature; (iii) from a comparison of the temperature and current dependences of the sample resistance. The values of T_{h} obtained by all three different methods were used to calculate, from the heat balance equation, the temperature dependence of the hole-phonon energy relaxation time τ_{hph} (T). All three temperature dependences τ_{hph} (T) were almost identical and demonstrated transition of the 2D system from "partial inelasticity" to small angle scattering at lower temperatures.
The electrical conductivity tensor components σ_{xx}(H) and σ_{xy}(H) have been transformed into mobility spectrum. The resolution of the mobility spectrum has been discussed. As an example the electrical conduction in HgTe and HgCdTe samples have been analyzed using resistivity tensor components, conductivity tensor components and using the mobility spectrum.
Cyclotron resonance and interband measurements are reported in magnetic fields up to 160 T for InAs/InAs_{1-x}Sb_{x} superlattices, one of which is a "self-organised" or "natural" superlattice, and InAs_{1-x}Sb_{x} epilayers. The samples were grown by MBE at temperatures between 370°C to 500°C. No dependence of the band gap, effective masses or g-values on the growth temperature was detected. Anomalous tilt behaviour was observed for the superlattices. Inversion asymmetry induced spin-splitting of the subbands in gated InAs quantum wells is investigated by means of the Shubnikov-de Haas effect and cyclotron resonance. The non-parabolicity was well fitted by Kane theory, although the measured values of effective mass were substantially higher than predictions. Infrared life time measurements at wavelengths between 6 and 85 microns are undertaken with free electron lasers on InAs/InAs_{1-x}Sb_{x} superlattices and InAs/AlSb quantum wells. Suppression of the Auger recombination times is demonstrated with the superlattices.
Phase equilibria in Gd_2O_3-CeO_2-CuO system were studied based on samples obtained from appropriate mixtures of the starting components, prepared by a conventional solid state reaction in air. The phase relations prevalent in this system have been deduced based on X-ray diffraction of well-equilibrated samples. An isothermal cross-section diagram through the system at 980°C is presented and chemical constitution of the Ce-doped Gd_2CuO_4 phase is proposed.
Two samples of nominal composition Gd_{1.80}Ce_{0.15}CuO_{4.0} and Gd_{1.85}Ce_{0.15}CuO_{4.075} were subjected for a long-term treatment in air at 1100°C, comprised with a precise control of their weights vs. time of the treatment applied. The gravimetric analysis and the iodometric titration, both showed a deficiency on Cu-site, steadily increasing vs. time with a slight tendency to a saturation. The X-ray analysis of the resulting products showed that even after 272 h of sintering, the samples remained single phase of the "214" structure type. Refinement of their crystal structures as well as density measurements, both allowed confirming Cu deficient structure constitution of these samples. Results of dc-magnetization measurements, performed prior to reduction of the sample's overall oxygen content, did not show any sign of superconductivity down to 4.2 K. These preliminary results indicate that copper deficiency, developing during synthesis, may occur to be a decisive factor for the reported lack of superconductivity in Ce-doped Gd_2CuO_4 phase. Oxygen content reduction and further tests on the samples obtained are in progress.
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