Nb Brandt

CONCENTRATED KONDO SYSTEMS

Abstract This review considers the experimental and theoretical studies of concentrated Kondo systems (CKS), Kondo lattices, substitutional solid solutions and their transition from Kondo impurity to Kondo lattice, and ‘intermediate valence compounds’ which are, in fact, high T K CKS (T K is the Kondo temperature). The anomalous low temperature properties of CKS are related to the formation of the narrow (∼k B T K) high-amplitude Abrikosov-Suhl resonance E R in the vicinity of the Fermi level E F. This resonance is situated exactly at E F in low T K CKS with T K ΔCF (ΔCF is the crystal field splitting). In low T K ‘j=1/2’ CKS the condition E R=E F leads to an increase of the density of states at E F, which is large enough to induce heavy fermion superconductivity in CeCu2Si2, UBe13. We demonstrate that the transition from low T K (E R=E F) to high T K CKS (E R≠E F) might be what was formerly considered as a ‘Kondo-lattice-intermediate valence state’ transition...

Gap at the Fermi level in the intermetallic vacancy system RBiSn(R=Ti,Zr,Hf)

The new class of intermetallic compounds RNiSn(R=Ti,Zr,Hf) may be characterised by the presence of an ordered sublattice of Ni atom vacancies in comparison with normal metals RNi2Sn with no Ni vacancies. We report unusual transport and optical properties of the RNiSn system. The electrical resistivity of RNiSn is very high (3<p<100) mOhm*cm; the temperature coefficient of resistivity (TCR) is negative and strongly dependent on the annealing conditions. For some samples ZrNiSn and for a single crystal of TiNiSn the resistivity can be described by the Motts law at temperatures 0.1<T<20 K. A phase transition nearT=100 K without change of crystal structure was deduced from Hall effect data and the temperature dependence of the lattice constant. Preliminary data on transport phenomena in RPtSn and RPdSn(R=Ti,Zr,Hf) compounds are also reported. The unusual properties of RNiSn system might be related to a gap of the electron spectrum near the Fermi energy.

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.

High pressure studies of cerium hexaboride

Magnetic susceptibility and magnetoresistance of CeB 6 have been measured in magnetic fields H 6 and SmB 6 lattice parameters at pressures p⩽100 kbar.

Hall effect in CeAl3

Abstract We have studied temperature dependences of the Hall coefficient R H (T) and resistivity ϱ(T) in substitutional solid solutions Ce x La 1−x Al 3 (0 ≤ x ≤ 1) which realize the transition from Kondo impurity regime (X⪡1) to Kondo lattice ( X = 1). Lowering temperature results in the anomalous increase of R H (T). The ratio r = R H (3 K )/ R H (100 K ) has been found to increase with cerium content up to the r ≅ 15 in the marginal compound CeAl 3 . The specific features of R H (T) behaviour in Ce x La 1−x Al 3 alloys seem to be related to the existence at low temperatures of heavy fermions due to the formation of the narrow Abrikosov-Suhl resonance at the Fermi level.

SmB6 at high pressures: The transition from insulating to the metallic Kondo lattice

Abstract Temperature dependences of the resistivity have been studied in SmB 6 at pressures p ≲120 kbar. The suppression of the gap and the transition to the metallic conductivity observed at p ≳55 kbar is explained within the framework of the model which treats SmB 6 at p =0 as well as SmS at 6 kbar ≲ p ≲ 19 kbar as insulating Kondo lattices with JJ c and n m =1. We also report here preliminary results on SmB 6 current-voltage characteristics, and on magnetic ( H-T ) phase diagrams of CeB 6 at pressures p ≲11 kbar.

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.

Local unequilibrium states in Pb1−xSnxTe(In) (x=0.25)

Abstract It is shown that local long-life unequilibrium states are generated in Pb1−xSnxTe(In) alloys at the low temperatures under the action of the local light excitation. The character of the photoexcitation propagation in the darkened regions is discussed.