Superconductivity In Heavy-fermion Compounds Research Articles

Superconducting gap and vortex lattice of the heavy-fermion compoundCeCu2Si2

The order parameter and pairing mechanism for superconductivity in heavy fermion compounds are still poorly understood. Scanning tunneling microscopy and spectroscopy at ultra-low temperatures can yield important information about the superconducting order parameter and the gap structure. Here, we study the first heavy fermion superconductor, CeCu_2Si_2. Our data show the superconducting gap which is not fully formed and exhibits features that point to a multi-gap order parameter. Spatial mapping of the zero bias conductance in magnetic field reveals the vortex lattice, which allows us to unequivocally link the observed conductance gap to superconductivity in CeCu_2Si_2. The vortex lattice is found to be predominantly triangular with distortions at fields close to \sim 0.7 H_{c2}.

On the s-type superconductivity in heavy-fermion compounds

The amplitude of scattering of f electrons has been calculated for the periodic Anderson model in the strong-correlation limit (U = ∞) in the Cooper channel. From the condition of the existence of a pole of this amplitude, an equation is derived for determining the critical temperature (Tc) of the transition to the superconducting phase with the s symmetry of the order parameter. The temperature Tc as a function of the electron density and hybridization parameter has been calculated by self-consistently solving the system of equations. The region of the existence of the superconducting phase is found to adjoin the region of the existence of the unsaturated ferromagnetic state and does not overlap it. The results can be used to describe the transition to the superconducting phase with the s symmetry of the order parameter in heavy-fermion skutterudite LaFe4P12. In this case, the inclusion of the scattering of fermions by spin fluctuations turns out to be substantial enough to obtain Tc values close to the experimental data.

Magnetism and superconductivity in heavy fermion compounds at high pressure

From high pressure transport measurements we compare the normal and superconducting properties of CeCu 2Ge 2 and CeCu 2Si 2. The pressure-induced enhancement of the superconducting transition temperature T c up to about 2 K is correlated to several characteristics of the normal phase which all point to the possibility that charge fluctuations are involved in superconductivity. We also examine whether the transport properties of CeRu 2Ge 2 and YbCu 2Si 2 show deviation of the Fermi liquid behaviour in the vicinity of the magnetic to non-magnetic transition.

Exotic Kondo effects in metals: Magnetic ions in a crystalline electric field and tunnelling centres

The ordinary single-channel Kondo model consists of one or more spin-½ local moments interacting antiferromagnetically with conduction electrons in a metal. This model has provided a paradigm for understanding many phenomena of strongly correlated electronic materials, ranging from the formation of heavyfermion Fermi liquids to the mapping of a one-band model in the cuprate superconductors. The simplest extension of this ordinary Kondo model in metals which yields exotic non-Fermi-liquid physics is the multichannel Kondo impurity model in which the conduction electrons are given an extra quantum label known as the channel or flavour index. In the overcompensated regime of this model, nonFermi-liquid physics is possible, in contrast with the single-channel model. We overview here the multichannel Kondo impurity model candidates most extensively studied for explaining real materials, specifically the two-level system Kondo model relevant for metallic glasses, nanoscale devices and some doped semiconductors, and the quadrupolar and magnetic two-channel Kondo models developed for rare-earth and actinide ions with crystal-field splittings in metals. We provide an extensive justification for the derivation of the theoretical models, noting that, whenever the local impurity degree of freedom is non-magnetic, a two-channel Kondo model must follow by virtue of the magnetic spin degeneracy of the conduction electrons. We carefully delineate all energy and symmetry restrictions on the applicability of these models. We describe the various methods used to study these models along with their results and limitations (multiplicative renormalization group, numerical renormalization group, non-crossing approximation, conformal field theory and Abelian bosonization), all of which provide differing and useful views of the physics. We pay particular attention to the role that scale invariance plays in all these theoretical approaches. We point out in each case how various perturbing fields (magnetic, crystalline electric, electric field gradients and uniaxial stress) may destabilize the non-Fermi-liquid fixed point. We then provide an extensive discussion of the experimental evidence for the relevance of the two-level system Kondo model to metallic glasses and nanoscale devices, and of the quadrupolar and magnetic two-channel models to a number of heavyfermion-based alloys and compounds. We close with a discussion of the extension of the single-impurity models which comprise the main focus of this review to other systems (Coulomb blockade), multiple impurities and lattice models. In the latter case, we provide an overview of the relevance of the two-channel Kondo lattice model to non-Fermi-liquid behaviour and exotic superconductivity in heavy-fermion compounds and to the theoretical possibility of odd-frequency superconductivity, which is realized (for the first time) in the limit of infinite spatial dimensions for this model.

Magnetically mediated superconductivity in heavy fermion compounds

In a conventional superconductor, the binding of electrons into the paired states that collectively carry the supercurrent is mediated by phonons — vibrations of the crystal lattice. Here we argue that, in the case of the heavy fermion superconductors CePd2Si2 and CeIn3, the charge carriers are bound together in pairs by magnetic spin–spin interactions. The existence of magnetically mediated superconductivity in these compounds could help shed light on the question of whether magnetic interactions are relevant for describing the superconducting and normal-state properties of other strongly correlated electron systems, perhaps including the high-temperature copper oxide superconductors.

Coexistence model of anisotropic electron hole pairing and unconventional superconductivity in Heavy Fermion compounds

Various Heavy Fermion compounds exhibit unconventional superconductivity together with another electronic instability like a spin density wave or possibly a more general type of anisotropic electron-hole pairing, e.g. a spin nematic state. The coexistence behaviour of these order parameters is studied within a simple weak coupling model. It is found that depeding on the symmetry of the order parameters coexistence or phase expulsion may occur. Whereas the former case is possibly realized for theU-based superconductors, CeCu2Si2 may be an example of the second case as observed and discussed in the context of elastic constant anomalies.

Competition, coexistence and interplay between magnetic order, Kondo effect and superconductivity in heavy-fermion compounds

The mean-field theory based on a new high-degeneracy model is derived to describe an interplay between magnetic order, the Kondo effect and superconductivity in heavy-fermion compounds. The competetion and coexistence of the Kondo effect and ferro- and antiferromagnetic orders of localized moments are considered in detail. It is shown that an antiferromagnetic order arising in the coherent Kondo state has an anomalously small antiferromagnetic moment per f-atom (∼10 -2μ B) and stimulates superconductivity with an anistropic superconducting gap which vanishes along lines on the Fermi surface.

Unconventional superconductivity in heavy fermion compounds

Theoretical aspects of unconventional superconductivity in heavy fermion metals are discussed and confronted with experiment. Theoretical predictions on the detectability of collective modes in the electromagnetic absorption, the dependence of thermodynamic and transport properties on the concentration and scattering strength of impurities and a two-phase model of U 1−xTh xBe 13 are presented.

Theory of superconductivity in heavy-fermion compounds

A single-order-parameter and two-gap model (a modified Suhl two-gap model) is proposed for heavy-fermion superconductors. A simple relation between two gaps is found. The thermodynamic quantities in the superconducting state are calculated in terms of this model. It is shown that the critical values of the physical quantities are quantitatively in agreement with experiment. The behavior of the specific heat, of the ultrasonic attenuation, and of the NMR rate are also in good agreement with those of CeCu2Si2 and UBe13 at all temperatures.