Experimental study of Ce-based heavy-fermion compounds

In the first part of the paper, a brief survey is given on the development of heavy-fermion physics with special emphasis on Ce-based intermetallic compounds. Subsequently, selected topics of current interest are discussed, e.g., heavy-fermion band magnetism, heavy-fermion superconductivity and its relationship to a novel kind of lattice instability.     

Evolution of the Fermi surface across a magnetic order-disorder transition in the two-dimensional Kondo lattice model: a dynamical cluster approach

We use the dynamical cluster approximation, with a quantum Monte Carlo cluster solver on clusters of up to 16 orbitals, to investigate the evolution of the Fermi surface across the magnetic order-disorder transition in the two-dimensional doped Kondo lattice model. In the paramagnetic phase, we observe the generic hybridized heavy-fermion band structure with large Luttinger volume. In the antiferromagnetic phase, the heavy-fermion band drops below the Fermi surface giving way to hole pockets centered around k=(pi/2,pi/2) and equivalent points. In this phase Kondo screening does not break down, but the topology of the resulting Fermi surface is that of a spin-density wave approximation in which the localized spins are frozen.     

Heavy fermions in a high magnetic field

We give an overview on experimental studies performed in the last 25 years on heavy-fermion systems in a high magnetic field. The properties of field-induced magnetic transitions in heavy-fermion materials close to a quantum antiferromagnetic-to-paramagnetic instability are presented. Effects of a high magnetic field to the Fermi surface, in particular the splitting of spin-up and spin-down bands, are also considered. Finally, we review on recent advances on the study of non-centrosymmetric compounds and ferromagnetic superconductors in a high magnetic field.     

Direct observation of dispersive Kondo resonance peaks in a heavy-fermion system

Ce 4d-4f resonant angle-resolved photoemission spectroscopy was carried out to study the electronic structure of strongly correlated Ce 4f electrons in a quasi-two-dimensional nonmagnetic heavy-fermion system CeCoGe1.2Si0.8. For the first time, dispersive coherent peaks of an f state crossing the Fermi level, the so-called Kondo resonance, are directly observed together with the hybridized conduction band. Moreover, the experimental band dispersion is quantitatively in good agreement with a simple hybridization-band picture based on the periodic Anderson model. The obtained physical quantities, i.e., coherent temperature, Kondo temperature, and mass enhancement, are comparable to the results of thermodynamic measurements. These results manifest an itinerant nature of Ce 4f electrons in heavy-fermion systems and clarify their microscopic hybridization mechanism.     

重费密子合金的Slave Boson平均场理论

本文从Yoshimori-Kasai模型出发,在Slave Boson平均场范围内采用相干势近似方法,给出一个描述重费密子合金正常态的理论。自洽计算了重费密子合金中传导电子和强关联f电子的态密度随合金浓度的变化。所得结果不仅明确揭示了态密度中赝隙形成的过程,还对重费密子合金中比热和热电功率的低温相干效应作出了合理的解释。     

Influence of exchange fields in hybrid pairing superconductors,I: Ground states

Hybrid pairing superconductors with two bands of localized and delocalized states in the presence of uniform molecular fields and a finite displacement of the bands at the Fermi level are analyzed in mean field theory.At zero temperature, in addition to the usual BCS like ground state, agapless superconducting state withfinite spin polarization can be realized. The appearance of this state is somewhat analogous to the occurrence of the Fulde Ferrell state in a single band superconductor with an exchange field.We comment on the relevance of this model for heavy-fermion superconductors.Work performed within the research program of Sonderforschungsbereich 125 Aachen-Jülich-Köln     

Electronic structure evolution accompanying heavy fermion formation in CeCu_2Si_2

The cooper pairs in the heavy-fermion superconductor CeCu_2Si_2 are formed of heavy fermions. Therefore, the heavy fermions are fundamental to the emergence of unconventional superconductivity and associated non-Fermi-liquid behavior in the normal state. The interplay between localization and itinerancy manifested on the electronic structure is key for understanding the heavyfermion behavior. Here, via the first-principle density functional theory(DFT) combined with single-site dynamical mean-field theory(DMFT), we investigate the temperature(T) evolution of the electronic structure of CeCu_2Si_2 in the normal state, focusing on the role of the 4f states in the low energy regime. Two characteristic temperature scales of this evolution, which accompanied the heavy-fermion formation, are established. The coherence onset temperature is around 130K, whereas the heavy-fermion band formation temperature is between 40 and 80K; both characteristic temperature scales are higher than the transport coherence temperature. Furthermore, the heavy-fermion formation is confirmed by calculating its effective mass variation with the temperature. Based on the calculated T-dependent evolution of the 4 f orbital occupancy and electronic structure, an explanation on the behavior of the temperature evolution of the correlation strength of CeCu_2Si_2 is provided. Our results offer a comprehensive microscopic picture of the heavy-fermion formation in CeCu_2Si_2, which is essential for further understanding the emergent superconducting pairing mechanism.     

Anomalous influence of an external magnetic field on spin fluctuations in magnetic semiconductors with strong p-d hybridization and the colossal magnetoresistance effect

The colossal magnetoresistance effect in magnetic semiconductors based on lanthanum manganites has been investigated in terms of the model allowing for the effects of p-d hybridization and electronelectron Coulomb correlations. The influence of an external magnetic field on spin fluctuations has been considered under the conditions where the chemical potential is in a narrow heavy-fermion band formed in the hybridization gap. It has been shown that, in the vicinity of the Curie point T _C, the strong spin anharmonicity leads to an anomalously strong suppression of spin fluctuations by the external magnetic field, a phenomenon contributing significantly to the formation of colossal negative magnetoresistance.     

Coherent heavy quasiparticles in a CePt5 surface alloy

We report on the results of a high-resolution angle-resolved photoemission study on the ordered surface alloy CePt(5). The temperature dependence of the spectra show the formation of the coherent low-energy heavy-fermion band near the Fermi level. These experimental data are supported by a multiband model calculation in the framework of the dynamical mean-field theory.     

一个可能的重费密子超导机制——电子-Slave Boson超导机制

采用Anderson晶格哈密顿量,在超出平均场的理论框架下,讨论了重费密子超导电性问题。发现在Slave Boson场的长波长极限情况下,同类准粒子之间通过交换Slave Boson而产生的相互作用是吸引的,而只有第一类准粒子——准f电子对重费密子超导电性有贡献。 关键词：     

Dynamics of Heavy Carriers in the Ferromagnetic Superconductor UGe<Subscript>2</Subscript>

Superconductivity and ferromagnetism in a number of uranium-based materials come from the same f-electrons with a relatively large effective mass, suggesting the presence of a band of heavy quasiparticles, whose nature is still a mystery. Here, UGe₂ dynamics in both ferromagnetic and paramagnetic phases is studied employing high-field μ⁺SR spectroscopy. The spectra exhibit a doublet structure characteristic to formation of subnanometer-sized magnetic polarons. This model is thoroughly explored here and correlated with the unconventional physics of UGe₂. The heavy-fermion behavior is ascribed to magnetic polarons; when coherent they form a narrow band, thus reconciling heavy carriers with superconductivity and itinerant ferromagnetism.     

Doped Mott insulator as the origin of heavy-fermion behavior in LiV2O4

We investigate the electronic structure of LiV2O4, for which heavy-fermion behavior has been observed in various experiments, by the combination of the local density approximation and dynamical mean field theory. To obtain results at zero temperature, we employ the projective quantum Monte Carlo method as an impurity solver. Our results show that the strongly correlated a 1g band is a lightly doped Mott insulator which, at low temperatures, shows a sharp (heavy) quasiparticle peak just above the Fermi level, which is consistent with recent photoemission experiments by Shimoyamada et al. [Phys. Rev. Lett. 96, 026403 (2006)10.1103/PhysRevLett.96.026403].     

Theoretical and experimental investigation of the influence of external effects on the electronic structure of Ce-based heavy-fermion systems

The electronic structure of Ce systems with different degrees of hybridization of f electrons with outer-shell electrons and the effect of pressure on the change in the electronic structure has been investigated. A correlation between the parameters of X-ray photoelectron spectra and the magnitude of the heavy-fermion state is established. The total and partial densities of states of atoms were calculated using the TB LMTO ASA method. X-ray photoelectron study of the effect of an X-ray induced vacancy in the inner levels on the shape of the Ce3d spectra for Ce-based systems with different degrees of covalence has been performed. It is shown that there is a relationship between the magnitude of the heavy-fermion state and the intensity of the shakedown satellites in X-ray photoelectron spectra and that the magnitude of the heavy-fermion state decreases under pressure.     

Electronic structure of cerium heavy-fermion systems

A number of cerium-based systems are investigated using x-ray photoelectron spectroscopy. It is revealed that there is a correlation between the parameters of the x-ray photoelectron spectra and the heavy-fermion state. This correlation makes it possible to investigate the relaxation processes, anomalously strong reorganization of orbitals, profound collective effects induced by a vacancy field, and hybridization of localized unoccupied f states with delocalized occupied states.     

Magnetic relaxation in UCu4 + xAl8 - x

A quasielastic neutron scattering study has been performed on UCu_4+xAl_8-x, a system which reveals an alloying-induced transition from magnetic order to heavy-fermion behavior. The magnetic response can be described by a broad quasielastic Lorentzian for all concentrations. No crystal field excitations could be detected. Remarkably, the linewidth decreases with increasing hybridization strength and seems to be no measure of the Kondo lattice temperature T*. In addition, electron spin resonance (ESR) experiments were performed which revealed that, even in the heavy-fermion regime, spin fluctuations play an important role. Both experimental findings are in distinct contrast to cerium-based heavy fermion systems.     

Non-Fermi liquid behavior in polycrystalline Ce2PdIn8

We report on the formation of a novel ternary compound Ce₂PdIn₈ that is isostructural with the heavy-fermion superconductors Ce₂CoIn₈ and Ce₂RhIn₈. Its magnetic, electrical transport and thermodynamic properties were studied on polycrystalline samples in wide ranges of temperature and magnetic field strength. The results revealed Ce₂PdIn₈ to be a paramagnetic Kondo lattice with a coherence temperature of about 12 K. The C/T ratio of the specific heat reaches at 350 mK a strongly enhanced magnitude of about per Ce-atom, thus clearly indicating a heavy-fermion nature of this material. Moreover, a logarithmic divergence of C/T vs. T, observed below 3 K, which is accompanied by a linear temperature dependence of the electrical resistivity below 6 K, hint at a non-Fermi liquid character of the electronic ground state in the new compound reported.     

Distorted vortex lattice in a tetrahedral superconductor

The equilibrium shape and the orientation of the vortex lattice are studied for an s-wave tetrahedral superconductor in the vicinity of the upper critical field. The phase diagram, which includes transitions between rhombic and rectangular lattices, is constructed in the parameter space of the Ginzburg-Landau functional. The developed theory is applied to the heavy-fermion superconductor PrOs₄Sb₁₂. In a wide range of parameters, the shape of the vortex lattice is only weakly dependent on the temperature. The neutron scattering measurements of the vortex lattice in PrOs₄Sb₁₂ can be explained by particularities of the tetrahedral symmetry group and are further supported by analysis of the appropriate band structure calculations. The text was submitted by the authors in English.     

Cross-over effects in heavy-fermion superconductors

The influence of correlation effects on superconductivity in lanthanide and actinide compounds is investigated theoretically. As driving mechanism for the formation of heavy-fermion Cooper pairs different electron-phonon interactions are considered and discussed within the Anderson lattice model. The elimination of the bare electron-phonon interactions and a BCS-like truncation procedure lead to the appearance of different types of Cooper pairs. The stability of these Cooper pairs in the Kondo lattice state is studied by using a semi-phenomenological parameter which effectively describes the correlation off electrons. It is found that hybrid pairing is less important and that superconductivity of conduction andf electrons always occurs simultaneously. Furthermore, in the case of strongly correlatedf electrons a cross-over takes place from mainly heavy-fermion superconductivity to conduction electron superconductivity.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

双导带Anderson晶格的Slave-Boson平均场理论

应用Slave-Boson技术和泛函积分方法,处理了双导带Anderson晶格模型。在鞍点近似下,求得了系统的电子态密度和一些低温热力学量。结果表明,由于系统的相干性和f-c混合作用,电子态密度在费密面附近出现赝能隙结构。系统的低温热力学量具有重费密子特性。 关键词：     

Field induced magnetic ordering transition in Kondo insulators

We study the 2D Kondo insulators in a uniform magnetic field using quantum Monte Carlo simulations of the particle-hole symmetric Kondo lattice model and a mean field analysis of the Periodic Anderson model. We find that the field induces a transition to an insulating, antiferromagnetically ordered phase with staggered moment in the plane perpendicular to the field. For fields in excess of the quasi-particle gap, corresponding to a metal in a simple band picture of the periodic Anderson model, we find that the metallic phase is unstable towards the spin density wave type ordering for any finite value of the interaction strength. This can be understood as a consequence of the perfect nesting of the particle and hole Fermi surfaces that emerge as the field closes the gap. We propose a phase diagram and investigate the quasi-particle and charge excitations in the magnetic field. We find good agreement between the mean-field and quantum Monte Carlo results.Received: 17 December 2003, Published online: 8 June 2004PACS: 71.27. + a Strongly correlated electron systems; heavy fermions - 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.30. + h Metal-insulator transitions and other electronic transitions - 75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena - 75.30.Fv Spin-density waves