Hidden order, collective excitations, and entropy loss in the heavy fermion superconductor URu2Si2

We have clarified the ‘hidden order’ of URu₂Si₂, and the spin density wave, and calculated the large entropy loss on Kondo channel transition to the one channel Kondo fixed point of the crystal. The three collective excitations, particularly the one discovered recently, have been either derived or explained. These and other findings, in excellent agreement with outstanding data, are of general applicability to the analogues of URu₂Si₂ including the nonstoichiometric cuprates. The results show the presence of multichannel Kondo effect in these crystals.     

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.     

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.     

CePd2Ga3: A new ferromagnetic Kondo lattice

Various temperature-, pressure- and field dependent investigations on CePd₂Ga₃ indicate this ternary compound as belonging to the group of ferromagnetically ordered Kondo lattices, with the Curie temperatureT _C =6K and the Kondo temperatureT _K =4K. The first excited crystal field level of this hexagonal compound is about 40 K above the crystal field ground state, while the overall splitting is much larger.     

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.     

Electrical resistivity and magnetoresistance of CeB6

Electrical resistivity and magnetoresistance of CeB₆ single crystal have been measured in the temperature range from 1.3 to 300 K under the magnetic field up to 85 kOe. Three characteristic phases are distinguished consistently with other measurements. The Kondo like behaviour in the resistivity observed in the high temperature phase is fitted by the conventional form for the dilute Kondo state with the Kondo temperature T_K = 5 ～ 10K and the unitarity limit resistivity $\text{?}{}_{\mathrm{u}}\text{? 110 μΩ}$ cm/Ce-atom. The negative magnetoresistance in the middle phase is stronger with increasing magnetic field and with decreasing temperature suggesting rapid destruction of the Kondo state. The magnetoresistance in the low temperature phase exhibits some anomalies suggesting sub-phases corresponding to several kinds of spin ordering.     

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.     

Volume collapse in the Kondo lattice

The α-γ transition of Ce and its compounds are explained within a compressible Kondo lattice model where the variation of |J|/D with volume is taken into account. We show that, contrary to the valence change model, the Kondo contribution is sufficient to induce a first order transition at low temperature from a magnetic to a Kondo phase. The disappearance of magnetism is then related to an extremely high Kondo temperature. Applications to Ce and CeAl₂ cases are given.     

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.     

Pressure-induced magnetic phase transitions in Yb Kondo lattice systems

In this paper we give a brief overview of the effect of pressure on the magnetic and electronic properties of Yb Kondo lattices using the ¹⁷⁰Yb Mössbauer technique, electrical resistance and X-ray diffraction. The selected materials were either nonmagnetic (YbCu₂Si₂ and Yb₂Ni₂Al) or ferromagnetic (YbNiSn). We show that pressure induces a first order transition to a magnetic ground state in both YbCu₂Si₂ and Yb₂Ni₂Al. In the former compound, the transition is accompanied by a valence change towards Yb³⁺ state. The behavior of both YbCu₂Si₂ and Yb₂Ni₂Al can be understood as resulting from a pressure enhancement of the RKKY interaction which finally dominates the Kondo effect. We demonstrate that the ground state properties of YbNiSn are governed by a volume dependent competition between anisotropic exchange interactions and crystal field anisotropy rather than by a direct competition between Kondo and RKKY interactions.     

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     

Crossover from weak to strong Kondo behaviour in CeAl2

High pressure electrical resistance experiments performed on a single crystal of CeAl₂ allow one to determine a characteristic line in the pressure-temperature plane where a continuous passage from weak to strong Kondo behaviour is observed. This line is not associated with any sizeable volume anomaly as shown by X-ray experiments.     

Influence of unstable valence of cerium ions on the crystal field in ReNi compounds

To examine the effect of hybridization of 4f electrons with conduction electrons on the crystal field potential using neutron spectroscopy, we studied the effects of the crystal electric field (CEF) in intermetallic compounds of the type ReNi, in which chemical substitution is followed by a transition of the cerium ions from an intermediate valence state to the Kondo state. Measurements were performed both on cerium ions in the compounds Ce_1−x La_xNi (x=0.5, 0.8), where they have a whole-number population of the 4f shell, and on the paramagnetic impurity ion Nd in the series of compounds Re_1−x Nd_xNi (Re=Ce, La, Y), in which the cerium ions are found either in an intermediate valence state or in the Kondo state. From the neutron inelastic magnetic scattering spectra on Nd ions, we have reconstructed the crystal field parameters in ReNi compounds and calculated the CEF level diagram of Ce ions in these compounds as functions of the interion distances Re-Ni. The results of our calculations are in good agreement with the experimentally determined splitting diagram of the ground-state multiplet of the Ce ions. We have determined that as the degree of hybridization with the conduction electrons grows the CEF potential varies considerably and the effective splitting of the 4f shell of the cerium ions increases. The estimated energy scale of the splitting of the ground-state multiplet of the Ce³⁺ ions in the ReNi CEF (Δ_CEF∼15 meV) turns out to be commensurate with the Kondo temperature (T _K ;140 K for CeNi. Analysis indicates that the CEF potential has a substantial effect on the formation of the valence-unstable ground state of the f shell in this compound. Zh. éksp. Teor. Fiz. 113, 1731–1747 (May 1998)     

嵌入单量子点Aharonov-Bohm环中的近藤效应

使用单杂质的Ansderson模型, 从理论上研究了一个嵌入单量子点Aharonov-Bohm环系统处在 近藤区时的基态性质, 并用slave-boson平均场方法求解了该模型.结果表明：在零温, 当介 观环内电子平均能级间隔大于近藤关联能时, 系统内仍然存在一个被减弱了的近藤效应；系 统的基态性质依赖于系统的宇称和环的大小；而尺寸效应和近藤屏蔽效应的共存导致了系统 丰富的物理性质.同时, 可以通过测量介观环中的持续电流和杂质磁化率, 达到探测近藤屏 蔽云的目的. 关键词： 持续电流 杂质磁化率 宇称效应 近藤效应 近藤屏蔽云     

Kondo state of Co impurities at noble metal surfaces

We use scanning tunneling microscopy and spectroscopy to study the properties of magnetic Co adatoms on noble metal surfaces at 6 K. Due to spin-flip scattering of the substrate electrons at the impurity the many-body Kondo state forms. This state is characterized by an energy, the Kondo temperature T_K. We measure T_K of adatom systems and a resonant scattering phase shift locally and are thus able to discuss the coupling of the Co adatom to the metal electronic system. From the resonant scattering phase shift of the surface-state electrons scattering off a Co adatom on Ag(111), we find that the coupling to the surface state is rather weak. On the other hand, increasing the number of nearest neighbor substrate atoms increases the coupling of a Co adatom to the host metal and increases T_K. This shows the dominant character of the coupling of the Co atom to the bulk states of the substrate crystal. PACS 72.10.Fk; 68.37,Ef; 72.15.Qm     

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).     

Self-consistent theory of pressure on doped Kondo insulators

The effects of pressure on doped Kondo insulators are studied in the framework of slave-boson mean-field theory under the coherent potential approximation. A unified picture is presented for both the h-type Kondo insulators (SmB₆ and YbB₁₂) and the e-type Kondo insulators (Ce₃Bi₄Pt₃). The f-electron's density of states within the whole range of the concentration of nonmagnetic atoms are calculated self-consistently under various pressures. The variation of the energy gap and the Kondo temperature with pressure are also obtained. These theoretical results can be considered as a possible approach to study systematically the development of the gap behavior by doping and by mechanical pressure.     

Magnetic and non-magnetic ground states of the Kondo lattice

We use the variational method to investigate the ground state phase diagram of the Kondo lattice Hamiltonian for arbitraryJ/W, and conduction electron concentrationn _c (J is the Kondo coupling andW the bandwidth). We are particularly interested in the question under which circumstances the globally singlet (collective Kondo) Fermi liquid type ground state becomes unstable against magnetic ordering. For the collective Kondo singlet we use the lattice generalization of Yosida's wavefunction which implies the existence of a large Fermi volume, in accordance with Luttinger's theorem. Using the Gutzwiller approximation, we derive closed-form results for the ground state energy at arbitraryJ/W andn _c, and for the Kondo gap atn _c=1. We introduce simple trial states to describe ferromagnetic, antiferromagnetic, and spiral ordering in the small-J (RKKY) regime, and Nagaoka type ferromagnetism at largeJ/W. We study three particular cases: a band with a constant density of states, and the (tight binding) linear chain, and square lattice periodic Kondo models. We find that the lattice enhancement of the Kondo effect, which is described in our theory of the Fermi liquid state, pushes the RKKY-to-nonmagnetic phase boundary to much smaller values ofJ/W than it was previously thought. In our study of the square lattice case, we also find a region of itinerant, Nagaoka-type ferromagnetism at largeJ/W forn _c 1/3.     

Effect of substitution of Ce by La in the CeNixPt1 − x dense Kondo ferromagnets

The effects of substitution of Ce by La in the orthorhombic CeNi_{1 − x}Pt_x dense Kondo ferromagnets are studied by means of magnetization and electrical resistivity measurements. A decrease of the exchange RKKY interactions leads to a decrease of the Curie temperature T_c as a function of the La content and hence to an enhancement of the Kondo character in the thermal dependence of the resisitivity. However, the Ce moment is almost independent of the La amount. The Kondo temperature being also independent, this surprising result seems in contradiction with the available Kondo lattice models.