ALBERT

Notes: The conductivity of the metal-silicon nitride-silicon dioxide-semiconductor structure has been studied over a wide range of temperatures, 2–450 K, in a high electric field up to 8 MV/cm and magnetic field up to 70 kG. A strong positive magnetoresistance has been found at temperatures below 30 K. At temperatures of 30〈T〈250 K a weak negative magnetoresistance is observed. The existence of a positive magnetoresistance and its weak dependence on the magnetic field direction concerning that of current through a structure shows a hopping conductivity mechanism for silicon nitride film at temperatures below 30 K. Different hopping conductivity mechanisms in a high electric field have been discussed. The influence of a degradation process at T=4.2 K on the structure's conductivity has been investigated. Qualitative changes in the structure's conductivity during a degradation process have been found.

Notes: Abstract The electrothermal instabilities in high-resistance single crystals of PbTe(Ga) irradiated by infrared light are described phenomenologically. Instabilities show up in the form of periodic oscillations in the current in the sample circuit and in its temperature in sufficiently high electric fields. For observation of these phenomena it is important that the energy spectrum of the semiconductor contain metastable electronic states lying about 20 meV below the bottom of the conduction band.

Notes: Abstract Pressure-induced metal-semiconductor transitions in bismuth-antimony alloys in a strong magnetic field (up to 70 kOe) at helium temperatures have been investigated. It is found that for values of the “overlap-gap” |G|≲1 meV the alloy forms an excitonic insulator (EI) in magnetic fields above a certain “threshold” (30–40 kOe). It is inferred that the EI energy gapΔ increases with the magnetic field. The maximum gap observed in fields of ∼70 kOe turns out to beΔ 00∼7.5 K. An analysis of the results shows that transitions to the EI phase are observed from both the semimetal and the semiconducting states. The critical transition temperatureT c is related to the EI gapΔ by the expressionT c⋍0.7Δ. Arguments are advanced in support of the fact that the formation of the EI phase involves the pairing of electrons at theL point with holes at theT point.

Notes: Abstract Galvanomagnetic and oscillation effects in Pb1−x Sn x Te single crystals doped with 0.5 at % In have been studied in magnetic fields up to 60 kOe at temperatures from 4.2 to 30 K under hydrostatic pressure up to 18 kbar. Beyond the ultraquantum magnetic field limit (H uql) for the metallic state of Pb1−x Sn x Te(In) alloys, Fermi level pinning by high-density quasilocal states takes place. In a strong fieldH〉H uql the equationE F = const is valid instead of the equationn = const which is usual for degenerate semiconductors (E F is the electron or hole Fermi energy, andn is their concentration). This makes it possible to determine the direction of the band edge motion in the Pb1−x Sn x Te energy spectrum in a quantizing magnetic field in the direct and inverse spectral regions. It is found that the charge carrier transitions between quasilocal and band states are of anomalously long duration (∼105 sec atT=4.2 K). By the application of a quantizing magnetic field we obtained a nonequilibrium metallic state of the system with a frozen or slowly diminishing Fermi surface. The characteristic time of the transition was found as a function of temperature and pressure. The relaxation kinetics of the nonequilibrium states induced by a quantizing magnetic field and infrared irradiation is discussed.

Notes: Abstract The results are presented from an experimental study of the gapless state produced in semiconducting alloysBi 1−x Sb x by pressure-induced band inversion. The magnetoresistance properties of the alloys have been investigated both in weak magnetic fields (μH « 1) and in strong fields (H≤75 kOe) at liquidhelium temperatures in the Sb concentration interval 0.06≤x≤0.15 and pressure interval 1 bar ≤p〈20 kbar. At pressuresp close to the pressurep k at which the gapless state is realized a “semiconductor-semimetal-semiconductor” transition is detected inBi 1−x Sb x alloys withx=0.070 and 0.071. The rates of change of the gap ε gL before and after inversion are determined: −(2.5±0.5)×10−6 eV/bar and (1.5±0.5)×10−6 eV/bar, respectively. A reduction in the carrier effective mass as ε gL → 0 is observed down to values of ∼ 10−4 m 0. It is shown that as ε gL → 0 the carrier mobilities in the alloys increase abruptly, the effect being a maximum in the purest alloys, where forT=4.2 K the mobility along the binary axis attains the record-high value of ∼ 3×108 cm2/V · sec.

Notes: Abstract Electric, magnetic, and thermoelectric properties of Ce x La1−x Cu2Si2 (0⩽x⩽1) compounds have been studied over a wide temperature interval 0.04 ⩽T⩽300 K in magnetic fieldsH⩽40 kOe. The paramagnetic-magnetic ordering transition temperatureT M is found to rise from ∼0.32 K for cerium concentrationx=0.2 to 1.6 K forx=0.6. A further increase inx from ∼0.8 to 1.0 leads to a decrease inT M . Simultaneously, the susceptibility kink is smeared out and atx≈1.0 it is transformed into temperature-independent enhanced Pauli paramagnetism. The magnetic phase diagram has been found to be similar to that proposed by Doniach for the one-dimensional Kondo-necklace model. The Kondo-lattice compound CeCu2Si2 exhibits a superconducting transition atT c ⋍0.5 K. The variation of the magnetic properties of Ce x La1−x Cu2Si2 from magnetic ordering at 0.2≲x≲0.8 to the nonmagnetic superconducting state atx → 1.0 is caused by the crossover from the magnetic regimeT RKKY≫T K (in which the RKKY temperatureT RKKY exceeds the Kondo temperatureT K) to the nonmagnetic singlet ground state corresponding to the situation whenT K≫T RKKY. This crossover is accompanied by a sharp increase in the low-temperature Hall coefficientR H(T) in Ce x La1−x Cu2Si2 compounds atx → 1. At the same time, a minimum of the negative Seebeck coefficient with a high amplitude appears at 10〈T〈100 K. The anomalous low-temperature properties of Kondo lattices have been shown to be due to the rise of the narrow Abrikosov-Suhl resonance in the vicinity of the Fermi level εF as the temperature is lowered fromT≫T K toT≪T K. This resonance has a giant amplitude in concentrated Kondo systems and is responsible for the existence in CeCu2Si2 of heavy fermions with extremely low degeneracy temperatureT*F estimated to be 10 K from theR H versusT curve. Further increase of the Kondo coupling constantJ in CeCu2Si2 under pressure induces an increase in (1) the Hall coefficientR H(T=4.2 K), (2) the superconducting transition temperatureT c , (3) the derivative of the upper critical fielddH c2/dT c , and (4) the low-temperature Seebeck coefficientS(T), which have maximum values at the same pressurep K1≈3 kbar, corresponding to the Kondo-lattice state with the maximum amplitude of the Abrikosov-Suhl resonance in CeCu2Si2 atp=p KL. At higher pressuresp〉p KL, a continuous transition from the Kondo lattice to the intermediate valence state is observed, which is accompanied by a complete smearing out of the resonance near the Fermi level. Therefore the Kondo lattices represent a new class of solids, which can be characterized as the link between stable magnetism of metals with a deep 4f level and unstable magnetism associated with fluctuating valence. This novel state can be described by a set of anomalous low-temperature properties related to the giant Abrikosov-Suhl resonance near the Fermi level.

Notes: Abstract The magnetic susceptibility of Bi 1−x Sb x alloys is investigated for the concentration range 0 ⩽x≲22 at%, the temperature range 4.2–270 K, and magnetic fields up to 60 kOe. The experimental results are compared with the theoretical dependences of susceptibility on the band parameters, temperature, and magnetic field.

Notes: Abstract We report the results of an experimental study of the gapless state (GS) induced inp-type semimetallic alloys Hg 1−x Cd x Te (0〈x〈0.15) by pressure. Galvanomagnetic effects in weak magnetic fields (2≤T〈300 K) and the Shubnikov-de Haas effect have been investigated in the pressure interval 1≤p〈15 kbar. Direct evidence for the existence inp-type semimetallic alloys of an impurity hole band overlapping with the conduction band by 3–4 me V is obtained. At liquid helium temperatures the Fermi level is located in the impurity band, so that two groups of carriers take part in transport effects: “light” electrons in the conduction band and “heavy” holes in the impurity band. The electron Fermi energyE F is proved to be essentially constant during the transition to the GS. A linear dependence of the electron effective mass at the band edgem* (0) upon the gapE g is obtained. A significant role of scattering of electrons into the impurity band at liquid helium temperatures is revealed.