On the behaviour of the new Kondo lattice CeCuAl3

The temperature dependence of the electrical resistivity, thermopower, specific heat, susceptibility and magnetization of CeCuAl₃ are presented. CeCuAl₃ behaves as a Kondo lattice system with antiferromagnetic ground state properties (T _N 2.8 K). The valency of Ce in this tetragonal compound is close to 3 and the overall crystal field splitting found from our results is about 150 K. The Kondo temperatureT _K in the crystal field ground state, estimated from the magnetic susceptibility and the specific heat, is of the order of 8 K.     

Magnetic properties of CePd2Ga3

Field dependent specific heat measurements and a study of elastic neutron scattering experiments characterize CePd₂Ga₃ as a ferromagnetic Kondo compound with ordering in the basal plane belowT _C 6 K. The crystal field ground state of cerium in this hexagonal compound is the |±1/2> eigenstate. In the scope of a phenomenological model, the coupling constant and the Kondo temperature have been deduced.     

Magnetoresistivity of the Kondo-system (La,Ce)B6

The electrical resistivity of the Kondo system (La, Ce)B₆ has been measured in longitudinal and transversal magnetic fields up to 6 Tesla in the temperature range 0.04–20K. Corresponding to the strong increase of the resistivity with decreasing temperature the alloys show a very large negative magnetoresistivity with a Kondo temperatureT _K =1.05K and a Kondo magnetic fieldB _K =1.1 Tesla. The observed anisotropy of the resistivity due to the magnetic field direction cannot be explained well by existing theories.     

Interpretation of neutron magnetic scattering in the Kondo lattices YbPd2Si2 and YbAgCu4

We present an interpretation of published neutron inelastic scattering spectra in the Kondo lattices YbPd₂Si₂ and YbAgCu₄ obtained in terms of the Anderson impurity model, describing the hybridisation of the 4f Yb electrons with the band electrons, and also including the crystal electric field interaction. In YbPd₂Si₂, the tetragonal crystal field parameters were determined. In YbAgCu₄ the crystal field interaction was taken to exist by analogy with the isoelectronic compound YbAuCu₄ where it has been identified. Both compounds can be described by a Kondo temperature,T ₀=60 K and a Yb valency very close to 3.     

Field and pressure studies of Ce2Pd2In

Various measurements performed on the new ternary compound Ce₂Pd₂In indicate Kondo type interaction (T _K ≈1K) in the presence of strong crystal field splitting. Both an antiferromagnetic and a ferromagnetic transition were observed (T _N =4.4 K,T _C =3.8 K). Work supported by the Austrian FWF, P10269.     

Experimental study of electrical conduction in RuO2-based thick resistive films

We have found direct evidence for the dynamical instability of Pr, which could be observed in Pr_0.015 Pd by inelastic neutron scattering, and as such is first ever to be found for Pr ions. Crystalfield analysis of the measured dynamic susceptibility reveals that the spin instability is similar to that of Ce Kondo systems. We show that the previously observed resistivity increase belowT=25 K is a true Kondo effect and is not related to crystal field effects. We conclude that Pr diluted in Pd is a Kondo system withT _K =2.5 K.     

Specific heat of the spin-Peierls compound CuGeO3

The specific heat of the quasi-onedimensional magnetic compound CuGeO₃ shows a sharp anomaly at the spin-Peierls transition temperatureT _sp . The experimental decrease of the magnetic specific heat belowT _sp indicates the presence of a spin gap as observed previously with inelastic neutron scattering. A magnetic field of 6T suppressesT _sp only slightly but reduces the spin gap by a factor of two.     

First-order Kondo-frustated antiferromagnetic ordering in the heavy electron compound YbAs

¹⁷⁰Yb Mössbauer measurements show that the heavy electron compound YbAs undergoes a first order paramagnetic-antiferromagnetic transition centered at T_N=0.58K. The saturated 4f-shell magnetic moments are reduced by 47% relative to the value predicted by a crystal field model. A study of the line shapes below T_N in the presence of an external field shows that the RKKY exchange energy is about 20 times bigger than k_B T_N. These results show that YbAs presents a magnetic order frustrated by the Kondo effect.supported by CNPq     

Peculiarities of crystal field effects in CeInCu2 based heavy-fermion compounds

We have studied the evolution of the inelastic neutron magnetic scattering spectra of a compound with cubic symmetry, CeInCu₂, in the temperature range 10–130 K, and also their transformation with variation of the Kondo temperature T _K due to substitution of cerium ions in the system Ce_1−x (La,Y)_xInCu₂ at T=10 K. It turns out that the energy of the transition between the ground state and excited state of the 4 f electrons (Δ_CF) in the crystal electric field in CeInCu₂ increases with growth of the population of the ground state as the temperature is reduced, with a slight change in its intensity. Such behavior is inconsistent with the notion of classical one-ion effects of the crystal electric field. We have found that the scale of the observed variations in the excitation spectra of the 4f electrons depends on the Kondo temperature T _K and is insensitive to disorder in the rare-earth sublattice. Thus, despite the fact that T _K ≪Δ_CF, hybridization with states in the conduction band has a substantial effect on all parameters of the excitation spectrum of the ground multiplet of the 4f electrons at low temperatures. Zh. éksp. Teor. Fiz. 115, 2197–2206 (June 1999) A. A. Baikov Institute of Metallurgy     

The effect of pressure on the resistivity of a gold-chromium Kondo alloy and on the low temperature phonon resistivity of pure gold

The electrical resistivity of the Kondo alloy Au (20ppm Cr) and of pure gold has been determined in the temperature range 1.3 – 20 K at pressures up to 80 kbar. For pure gold the pressure dependence of the temperature dependent part of the lattice resistivity can be explained fairly well by the Bloch-Grüneisen theory. Expressions for the volume dependence of the ideal lattice resistivity and of the Debye-temperature for gold are derived. — The Kondo temperatureT _K of Au(Cr) is found to increase with pressure to more than twice the value atp=0 kbar.Therefrom the volume dependence of the effective exchange constantJ is calculated. The results are similar as in other Kondo alloys described previously.     

Pressure dependence of the electrical resistivity of Ag(Mn), both in the dilute limit and in the spin glass regime

The electrical resistivity of the dilute spin glass alloys Ag(1,540 ppm Mn) and Ag(1,015 ppm Mn) has been measured from 1.3–30 K at pressures up to 70 kbars. The temperature of the resistance maximum,T _m , is found to shift to higher temperatures, this shift being stronger for the more concentrated alloys.Additionally an Ag(20 ppm Mn) Kondo-alloy has been investigated. Application of pressure causes an increase of the Kondo temperatureT _K from which the volume dependence of the effective exchange constantJ is derived.With this information theT _m -shift in the spin glass alloys can be understood in a qualitative and quantitative manner within Larsen's model.     

Magnetic order in a Kondo lattice: A neutron scattering study of CeCu2Ge2

Elastic and inelastic neutron scattering studies of the Kondo lattice CeCu₂Ge₂ were performed. AtT _N=4.1 K an incommensurate magnetic order develops with an ordering wave vectorq ₀=(0.28, 0.28, 0.54) and an ordered moment µ _s =0.74 µB. The crystalline electric field splits the 4f ¹-J-multiplet of the Ce ion into a ground state doublet and a quartet at 191 K. The wave function of the ground state yields an ordered moment of 1.54µB. Thus, due to the onset of the formation of a Kondo singlet the magnetic moment is considerably reduced. The magnetic relaxation rate was investigated via quasielastic neutron scattering. The temperature dependence of (T) is characteristic of heavy-fermion systems with a high temperature square root dependence and a limiting low temperature value, yielding a Kondo temperatureT _K10K. The quasielastic component of the scattered neutron intensities persists down to the lowest temperatures, well belowT _N. This quasielastic line is regarded as a characteristic feature of heavy-fermion systems and corresponds to the enhanced value of the linear term of the specific heat.     

On the theory of superconductivity in Kondo lattice systems

An attempt is made to explain the occurrence of superconductivity in Kondo lattice systems with special reference to CeCu₂Si₂. Starting point is the Fermi liquid approach. It is generalized from a Kondo impurity to the Kondo lattice by means of the Korringa-Kohn-Rostocker method. From it a hybridization model is derived and discussed in detail. Two electron-phonon mechanisms are investigated which appear in Kondo lattices. One results from the additional phase shifts caused by the Kondo ions while the other is responsible for the so-called Kondo volume collapse. It is shown that the latter is sufficiently strong in order to explain why CeCu₂Si₂ is a superconductor while LaCu₂Si₂ is not. An estimate for the superconducting transition temperatureT _c produces the right order of magnitude.     

Kondo effect in spin glasses: Electrical resistivity

We investigate the influence of the Kondo effect on the electrical resistivity of spin glasses. Our approximation reduces in the limit of vanishing impurity concentrations to the Suhl-Nagaoka theory for the Kondo effect. The magnetic impurity interactions are taken into account in the form of time dependent two-spin correlation functions which can be measured by neutron scattering. The dynamics of the impurity spins leads to a partial destruction of the Kondo effect. For the resistivity this can be described by a temperature dependent effective spinS _eff ² (T) withS _eff ² 0 forT0 andS _eff ² S(S+1) forT, and by a reduction of the Kondo temperatureT _K. Sufficiently strong interactions lead toT _K=0. We obtain a resistivity maximum at a temperatureT _m due to the interplay of the Kondo effect and the spin dynamics;T _m depends onT _K and on the excitation spectrum, and therefore on parameters such as impurity concentration or pressure. The ratioT _m/T_f (T _f is the freezing temperature) is calculated for a single relaxation time and for a square density of relaxation modes and is compared with experimental data forAuCr,AuMn,AgMn, andCuMn. The influence of other possible modes on various spin glass properties is discussed.SFB 125 Aachen-Jülich-Köln     

Anomalous thermoelectric power behaviour in PrSn3 and NdSn3

Results of the thermoelectric power (TEP) measurements done on monocrystalline samples of RESn₃ compounds (RE=La, Pr, Nd, and Gd) are presented for the temperature range of 5.5-300 K. It was found that the TEP is positive and weakly temperature dependent at temperatures T>100 K. For T<100 K pronounced anomalies have been observed for the PrSn₃ and the NdSn₃ compounds in the vicinity of 10 K.We argue that the Kondo and crystal field effects cause these anomalies. A shape of the TEP anomaly found for PrSn₃ resembles very much that observed in the electrical resistivity.     

On the low-temperature thermal properties and low-energy Fermi excitations in CeNiSn

The thermal expansion of a single crystal of the intermetallic compound CeNiSn has been measured at low temperatures 0.3 K<T<12 K and in a magnetic field up to 8 T. A large anisotropy of the linear expansion is observed which is strongly influenced by the magnetic field. These data are interpreted within the theory explaining the origin of the quasigap in the heavy fermion spectrum of CeNiSn by the interplay between the heavy fermions and low-energy excitations in non-cubic Kondo lattices.     

The S-J exchange interaction under the influence of the crystalline field

A magnetic impurity, having the s-j interaction with conduction electrons, under the crystalline field of host metals is investigated. The Hamann integral equation for the t-matrix can be derived and solved by the method of Zittartz and Müller-Hartmann for the cases of (1) small splittings, yielding the Kondo anomaly and decrease of the Kondo temperatureT _K compared with no splittings, (2) a large splitting from the crystalline ground doublet state, producing the higherT _K by the exicited level, and (3) a large splitting from the ground singlet state giving no Kondo effect. The macroscopic properties are calculated for (1) and (2).     

Inhomogenous magnetic ground state in CeAgGa

CeAgGa crystallizes in CeCu₂ Imma structure with Ag and Ga atoms randomly distributed at 8h sites. The magnetic and transport properties of the orthorhombic CeAgGa compound have been obtained from the analysis of ac magnetic susceptibility χ _ac , magnetization M vs. magnetic field, specific heat C and electrical resistivity ρ. The results provide evidence for the formation of a spin-glass state with a freezing temperature T _f = 5.1 K. The randomness in the Ce-Ce magnetic exchange interactions seem to arise from a statistical distribution of Ag and Ga atoms on a crystallographic site of the CeAgGa crystal lattice. The results provide also evidence for the formation of ferromagnetic-like order at the temperature T _C ≈ 3.6 K. Band structure calculations for a disordered system give magnetic moment similar to saturation moment obtained from magnetization measurements, however, its calculated value is insensitive on Ga/Ag off-stoichiometry in the 8h position. Complex behavior of CeAgGa Kondo-lattice compound is discussed in terms of interplaying Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, Kondo effect and structural disorder.