N. B. Brandt

Pressure spectroscopy of impurity states and band structure of bismuth telluride

The pressure and temperature dependences of the resistivity, the Hall effect and the Shubnikov-de Haas effect were investigated in p-Bi2Te3, p-Bi2Te3(Ge) and p-InxBi2-xTe3 for pressures up to 12 kbar and temperatures in the range 1.5 0.1 an increase of the upper hole band concentration of carriers under pressure and an increase of the resistivity and of the Hall coefficient Rx at T<10 K were observed. The results were interpreted as pointing to the existence of impurity states in In-doped samples.

Dynamics of the semiconductor-metal transition induced by infrared illumination in Pb1-xSnx Te(In) alloys

Abstract The photoconductivity and quantum oscillation effects induced by infrared illumination have been investigated in Pb 1- x Sn x Te(In) samples. The impurity monopolar character of the photoeffect is established. It is shown that the time-dependence of the residual photoconductivity is described by a power law, the relaxation time being ≈ 10 4 s at temperatures T ⩽ 20 K.

Investigation of the excitonic insulator phase in bismuth-antimony alloys

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 beΔ00∼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 temperatureTc is related to the EI gapΔ by the expressionTc⋍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.

Investigation of the gapless state in bismuth-antimony alloys

The results are presented from an experimental study of the gapless state produced in semiconducting alloysBi1−xSbx 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 pressurepk at which the gapless state is realized a “semiconductor-semimetal-semiconductor” transition is detected inBi1−xSbx alloys withx=0.070 and 0.071. The rates of change of the gap εgL 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 εgL → 0 is observed down to values of ∼ 10−4m0. It is shown that as εgL → 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.

Band edge motion in quantizing magnetic field and nonequilibrium states in Pb1?x Sn x Te alloys doped with In

Galvanomagnetic and oscillation effects in Pb1−xSnxTe 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 (Huql) for the metallic state of Pb1−xSnxTe(In) alloys, Fermi level pinning by high-density quasilocal states takes place. In a strong fieldH>Huql the equationEF = const is valid instead of the equationn = const which is usual for degenerate semiconductors (EF 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−xSnxTe 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.

SWITCHING EFFECTS IN THE DIELECTRIC PHASE OF THE PB1-XSNXTE(IN) COMPOUNDS

Abstract The current-voltage characteristics (CVC) and switching effects have been investigated in the monocrystalline Pb 1−x Sn x Te alloys doped with In at liquid helium temperatures. Experimental results are interpreted in terms of joule heat model.

Experiment and theory on the magnetic susceptibility of Bi-Sb alloys

The magnetic susceptibility of Bi1−xSbx 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.

Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

Abstract We have studied the stationary Josephson effect on YBa 2 Cu 3 O 7− δ ( T c =90 K ) and Bi 2 Sr 2 Ca 1 Cu 2 O 8 ( T c =80 K and 87 K for two samples of different origin ) ceramic based junctions. The temperature dependence of the critical current near T c has been found as I c ≈( T c - T ) 3 2 for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoffs theory for Bi 2 Sr 2 Ca 1 Cu 2 O 8 , the temperature dependence of the superconducting state gap Δ( T ) was calculated and it was evaluated that 1.45⩽2Δ(0)/ k B T c ⩽3.5.

Investigation of the gapless state induced by pressure in Hg1?x Cd x Te alloys

We report the results of an experimental study of the gapless state (GS) induced inp-type semimetallic alloys Hg1−xCdxTe (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 energyEF 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 gapEgis obtained. A significant role of scattering of electrons into the impurity band at liquid helium temperatures is revealed.

Pressure studies of the energy spectrum of irradiation-induced defects in Pb1-xSnxSe

The effect of high hydrostatic pressure (up to 18 kbar) on the galvanomagnetic properties of n- and p-Pb1-xSnxSe (x<or=0.06) alloys irradiated with electrons (T approximately=300 K, E=6 MeV, Phi <or=7*1017 cm-2) has been investigated. It is shown that irradiation creates a band of localized states Et1 the energy position of which relative to the valance band top L6+ depends on alloy composition and on pressure. A model of the energy spectrum of electron-irradiated Pb1-xSnxSe has been proposed assuming that electron irradiation generates two different types of defects and leads to the appearance of two bands of localized defect states situated within the conduction band and near the valence band top of Pb1-xSnxSe alloys.