S.M. Chudinov

Electric and magnetic properties of the Kondo-lattice compound CeCu2Si2

Electric, magnetic, and thermoelectric properties of CexLa1−xCu2Si2 (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 temperatureTM 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 inTM. 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 atTc⋍0.5 K. The variation of the magnetic properties of CexLa1−xCu2Si2 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 regimeTRKKY≫TK (in which the RKKY temperatureTRKKY exceeds the Kondo temperatureTK) to the nonmagnetic singlet ground state corresponding to the situation whenTK≫TRKKY. This crossover is accompanied by a sharp increase in the low-temperature Hall coefficientRH(T) in CexLa1−xCu2Si2 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≫TK toT≪TK. 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 theRH versusT curve. Further increase of the Kondo coupling constantJ in CeCu2Si2 under pressure induces an increase in (1) the Hall coefficientRH(T=4.2 K), (2) the superconducting transition temperatureTc, (3) the derivative of the upper critical fielddHc2/dTc, and (4) the low-temperature Seebeck coefficientS(T), which have maximum values at the same pressurepK1≈3 kbar, corresponding to the Kondo-lattice state with the maximum amplitude of the Abrikosov-Suhl resonance in CeCu2Si2 atp=pKL. At higher pressuresp>pKL, 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.

Superconductivity in CeCu2Si2

Abstract Resistivity measurements of CeCu 2 Si 2 are carried out under pressures p up to 12 kbars. Unlike polycrystalline samples, no traces of superconductivity have been observed in CeCu 2 Si 2 at ambient pressure. When pressure is applied, CeCu 2 Si 2 monocrystals become superconducting with anomalously large ratio H c2 (0)/T c (0) = 34 K0e/K and with the derivative dH c2 /dT(T=T c ) = 140 K0e/K

The appearance of the many-body resonance at the Fermi level in Kondo-lattices

Abstract Studing transport and magnetic properties of solid solutions Ce x La 1−x Cu 2 Si 2 (0≤ x ≤ 1) and using for CeCu 2 Si 2 pressure p as an external parameter, we have observed the continious transition from Kondo-impurity regime ( x « 1 ) to Kondo lattice ( x ⋍ 1, p = 0, ν Ce ≈ 3+ ) and then through the intermediate valence state (x = 1, 0 ν Ce ν Ce ⋍ 4+ . We have found that all the anomal ous properties of Kondo-lattices (CeCu 2 Si 2 , CeAl 3 , CeAl 2 ) are caused by the appearance at the Fermi level of the many-body resonance of Abrikosov-Suhl type.

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.

The electrophysical properties of silicon nitride at low temperatures

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.

Drift resonance in the quantum Hall effect

Abstract The integral quantum Hall effect (IQHE) has been investigated in GaAsue5f8GaAlAs heterostructures in the temperature range 0.1 ⩽ T ⩽ 4.2 with magnetic fields B ⩽ 8.5T. The source-drain conductivity has been measured in alternating current regime at frequencies from 0 up to 1 GHz. Distortion of the Hall-plateaux has been observed. The phenomenon is connected to a drift of electrons and holes cyclotron orbits in magnetic field along closed equipotential trajectories on the relief of a random potential.

Cluster superconductivity in diamond-like carbon-silicon nanocomposites containing tungsten

It is shown that the nonhomogeneous structure of the superconducting phase in the diamond-like carbon-silicon nanocomposites containing tungsten with concentration close to the metal-insulator transition is responsible for the perculiarities in the character of the superconducting resistive response. The observed nontrivial quasireentrant resistive transitions can be explained in terms of the phase coherence destruction between the superconducting grains and by the renewal of superconducting phase generation at lower temperatures defined by the presence of the two scales of inhomogeneity in the investigated films. The resistance-current characteristics in the vicinity to the superconducting transition and magnetic measurements do not contradict the qualitative model proposed.