Mean-boson field theory for heavy fermion uranium compounds

A mean-field theory for heavy fermion Uranium compounds is presented along the lines of previous functional integral approaches to the Anderson lattice in the boson representation. Two boson fields are necessary provided both occupations are magnetic states. It is shown that the mean-field solution also results in a strongly renormalised quasi-particle band at the Fermi level, the quasi-particle renormalisation factor being determined by the product of the squares of the classical expectation values for the two boson fields.     

Solution of the two-channel Anderson impurity model: implications for the heavy fermion UBe13

We solve the two-channel Anderson impurity model using the Bethe-ansatz. We determine the ground state and derive the thermodynamics, obtaining the impurity entropy and specific heat over the full range of temperature. We show that the low-temperature physics is given by a line of fixed points describing a two-channel non-Fermi-liquid behavior in the integral valence regime associated with moment formation as well as in the mixed valence regime where no moment forms. We discuss the relevance for the theory of UBe13.     

Coexistence of heavy fermion and intermediate valence behaviour in Ce24Co11

Low temperature specific heat, magnetic susceptibility andL _III-XAS measurements on Ce₂₄Co₁₁ are reported. The electronic specific heat can be decomposed into two contributions: a low temperature one (arising atT<5 K), with a maximum of 2.2 J/Kmole at 0.9 K, and another one becoming dominant atT>5 K, characterized by a _HT = 1.8 J/K² mole coefficient. The ·T product changes continuously with temperature, from 0.48 emu K/mole at 2 K to 7.85 emu K/mole at 200 K. TheL _III-mean valence is =3.15.As the 24 Ce atoms are distributed in ten unequivalent crystalline sites, we interpret the experimental results as due to: one atom which behaves as a Heavy Fermion and the rest as Kondo and Intermediate Valent, with a distribution of characteristic energies governed by nine different environments. Qualitatively, the specific heat behaviour under applied field and the =3.15 value are in agreement with this interpretation.     

Neutron scattering of fragile antiferromagnetic phases in heavy fermion compounds

Results of neutron scattering and specific heat measurements are reported for two kinds of weak antiferromagnetic compounds corresponding to (i) the situation at the quantum critical point and (ii) the double magnetic transition observed in CeRu₂Ge₂.     

Anderson lattice with explicit Kondo coupling revisited: metamagnetism and the field-induced suppression of the heavy fermion state

We apply the extended (statistically consistent, SCA) Gutzwiller-type approach to the periodic Anderson model (PAM) in an applied magnetic field and in the strong-correlation limit. The finite-U corrections are included systematically by transforming the PAM into the form with the Kondo-type interaction and the residual hybridization, both appearing at the same time and on equal footing. This effective Hamiltonian represents the essence of our Anderson-Kondo lattice model. We show that in ferromagnetic phases the low-energy single-particle states are strongly affected by the presence of the applied magnetic field. We also find that for large values of hybridization strength the system enters the so-called locked heavy fermion state introduced earlier. In this state the chemical potential lies in the majority-spin hybridization gap and, as a consequence, the system evolution is insensitive to further increase of the applied field. However, for a sufficiently strong magnetic field, the system transforms from the locked state to the fully spin-polarized phase. This is accompanied by a metamagnetic transition, as well as by a drastic reduction of the effective mass of the quasiparticles. In particular, we observe no effective mass enhancement in the fully polarized state. The findings are in overall agreement with experimental results for the Ce compounds in high magnetic fields. The mass enhancement for the spin-minority electrons may also diminish with the increasing field, unlike for the quasiparticle states in a single narrow band in the same limit of strong correlations.     

Tunneling into clean heavy fermion compounds: origin of the Fano line shape

Recently observed tunneling spectra on clean heavy-fermion compounds show a lattice periodic Fano line shape similar to what is observed in the case of tunneling to a Kondo ion adsorbed at the surface. We show that the translation symmetry of a clean surface in the case of weakly correlated metals leads to a tunneling spectrum which shows a hybridization gap but does not have a Fano line shape. By contrast, in a strongly correlated heavy-fermion metal the heavy quasiparticle states will be broadened by interaction effects. The hybridization gap is completely filled in this way, and an ideal Fano line shape of width ～2TK results. In addition, we discuss the possible influence of the tunneling tip on the surface, in (i) leading to additional broadening of the Fano line and (ii) enhancing the hybridization locally, hence adding to the impurity type behavior. The latter effects depend on the tip-surface distance.     

Valence fluctuation and heavy fermion behaviour in rare earth and actinide based compounds

A brief historical review of some of the experimental work reported on the mixed valent (MV) or valence fluctuating (VF) and heavy fermion (HF) systems is given. The characteristic physical properties of MV and HF systems are discussed. The salient features of the theoretical models are outlined. Results on some systems are presented.     

重电子金属CeCu(6-x)Nix低温电阻与比热的研究

测量了重电子金属CeCu6-xNix(x=0,005,01,015,02)01K—250K的低温电阻和5K—70K低温比热,发现样品电阻的极大值温度随着掺Ni含量的增大而急剧下降,这一现象反映少数与Ni邻近的Ce离子在极低温下磁矩的加强和整个Ce离子点阵对导电电子相干散射的减弱.与此相反,低温电子比热系数γ在较低温度下近于常数,而在8K附近因有效质量变大而明显上升,但γ明显上升的温度,对Ni的含量却不敏感,表明绝大部分Ce离子的状况并未受到影响 关键词： 重费米子系统 低温比热 低温电阻     

Thermal conductivity and Lorenz number of the heavy electron in Ce compounds

The thermal conductivity and the Lorenz number are investigated using the Anderson lattice model in the framework of the single-site approximation. The obtained results are qualitatively in agreement with the experimental findings of CeAl₃. The characteristic features of the transport properties in the presence of a magnetic field are also discussed in connection with the gap structure above the Fermi level.     

ESR of Gd-impurities in the heavy fermion compounds CeCu2Si2 and CeAl3

The heavy fermion systems CeCu₂Si₂ and CeAl₃ are characterized by a huge quasiparticle density of states responsible for the large electronic specific heat. The observation of a Gd³⁺ electron spin resonance (ESR) in single crystals CeCu₂Si₂ and in polycrystalline CeAl₃ clearly demonstrates the local character of these quasiparticles. Nevertheless, the Gd-spin relaxation shows remarkable anomalies with respect to the isostructural compounds LaCu₂Si₂ and LaAl₃: Probably via RKKY coupling, Ce 4f-spin fluctuations give rise to an enhanced Gd-spin relaxation resulting in an unusual non-linear thermal broadening around the Kondo temperature. From this we obtain information about the temperature dependence of the Ce 4f-spin correlation time.     

Multiple superconducting phases in heavy fermion compounds PrOs4Sb12 and CeCoIn5

In recently discovered heavy fermion compounds, quasi-two-dimensional CeCoIn₅ and skutterudite PrOs₄Sb₁₂, multiple superconducting phases with different symmetries manifest themselves belowT _c. The angle-resolved magnetothermal transport measurements revealed that in PrOs₄Sb₁₂ a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state. The ultrasound velocity measurements revealed that in CeCoIn5 the Fulde-Ferrel-Larkin-Ovchinikov (FFLO) phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices, appears at low temperature and high field. These results open up a new realm for the study of the superconductivity with multiple phases.