Magnetism,disorder and frustration in heavy-fermion compounds

We examine the magnetic properties of four strongly correlated electron compounds based upon the presence or absence of crystallographic disorder and geometric frustration. All four materials (URu2Si₂, CePd₂Al₃, UNi₄B and URh₂Ge₂) exhibit unusual and not fully understood behavior in their magnetic ordering that are surprising for stoichiometric intermetallic compounds. We summarize these behaviors and relate them to the appearance of disorder and frustration     

Pressure-induced superconductivity in noncentrosymmetric heavy-fermion CeRhSi3

We report the pressure-induced superconductivity in the noncentrosymmetric heavy-fermion CeRhSi3. The superconductivity emerges above about 12 kbar even though the antiferromagnetic ordering persists. Furthermore, another anomaly is observed in the superconducting phase. The anomalous magnetic field-temperature phase diagram with a high upper critical field suggests that an unconventional superconductivity is realized in CeRhSi3.     

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 (T _c) of the transition to the superconducting phase with the s symmetry of the order parameter. The temperature T _c 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 T _c values close to the experimental data.     

5f-electron localization in PuSe and PuSb

Thin layers of PuSb and PuSe were studied by photoelectron spectroscopy. X-ray photoelectron spectroscopy and high-resolution valence-band ultraviolet photoelectron spectroscopy spectra show localization of the 5f states and a low density of states at E(F) in PuSb. In PuSe, which can be classified as a heavy fermion system with low carrier density, we observed three narrow peaks in the valence band, which can be related to the 5f emission. These three features are very sensitive to stoichiometry deviations and disappear for PuSe prepared at T = 77 K.     

Magnetism and superconductivity in intermetallic uranium compounds

Superconductivity, spin-fluctuation effects and magnetic order are reviewed for intermetallic compounds of uranium with d-transition elements. Large values for the effective mass of the pairing electrons are deduced from critical field studies on the superconducting compounds. Magnetism in the 5f-electron compounds is strongly anisotropic even for paramagnetic materials. Magnetic order does not frequently occur in these intermetallics, spin fluctuations are often observed. A unique combination of spin-fluctuation phenomena and superconductivity is met in UPt₃.     

Mott transition and Kondo screening in f-electron metals

We study how a finite hybridization between a narrow correlated band and a wide conduction band affects the Mott transition. At zero temperature, the hybridization is found to be a relevant perturbation, so that the Mott transition is suppressed by Kondo screening. In contrast, a first-order transition remains at finite temperature, separating a local-moment phase and a Kondo-screened phase. The first-order transition line terminates in two critical end points. Implications for experiments on f-electron materials such as the cerium alloy Ce0.8La0.1Th0.1 are discussed.     

Dissymmetrical tunneling in heavy-fermion metals

A tunneling conductivity between a heavy-fermion metal and a simple metallic point is considered. We show that, at low temperatures, this conductivity can be noticeably dissymmetrical with respect to the change of voltage bias. The dissymmetry can be observed in experiments on heavy-fermion metals whose electronic system has undergone the fermion-condensation quantum phase transition.     

Magnetism and superconductivity in the extended periodic Anderson model

The Anderson model for a periodic array of magnetic ions has been extended by a BCS like interaction term for the conduction electrons. This extended model is studied by means of a generalized Hartree-Fock approximation. Five coupled self-consistency equations are considered for situations where ferromagnetic, superconducting, coexistent and normal phases can occur. Which of these phases actually are allowed for a given set of parameters strongly depends upon the band structure (especially upon the type of electrons near the Fermi energy). Whereas for the general case the self-consistency equations have to be solved numerically from the beginning, the situation is much simplified for the symmetric Anderson model (E _o=–U/2). Here, analytical results are obtained which explain the stability of the coexistent phase. We also use the symmetric model for an expansion of the Ginzburg-Landau type in the spatially homogeneous magnetic and superconducting order parameters. The onset of superconductivity and ferromagnetic order is well described within this approximation. However, at low temperatures we find drastically different solutions due to the strong temperature dependence of the expansion coefficients of the free energy. Therefore, it is difficult to obtain the correct transition temperatures within this approximation. These difficulties stem from general problems connected with the derivation of a Ginzburg-Landau theory for several order parameters from a microscopic theory.Work performed within the research program of the Sonderforschungsbereich 125, Aachen-Jülich-Köln     

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi₂B₂C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T _F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T _N, but no changes are seen at T _F.     

Melting of the rare earth metals and f-electron delocalization

Melting curves for Pr, Nd, Sm, Gd, and Y were measured in a diamond-anvil-cell to nearly 100 GPa and 4000 K. f-electron volume collapses are observed as triple points for Pr (24 GPa and 1400 K) and Gd (65 GPa and 3100 K). These pressures coincide with the volume collapses observed at room temperature. For Nd and Sm, the f-electron volume collapse has not been observed at room temperature but appears at approximately 2000-2500 K as a broad minimum in the melting curve, similar to that of Ce, near 50 GPa (Nd) and 70 GPa (Sm). The melting curve of Y goes smoothly along the entire rare earth sequence.     

Quantum effects on the competition between antiferromagnetism and superconductivity in heavy-fermion systems

We study a Ginzburg-Landau theory of two coupled fields describing superconductivity and antiferromagnetism in a metal. A coupling between the two-components superconductor and the antiferromagnetic (AF) field is included in the classical action. The classical results are improved calculating the quantum corrections to one-loop order with the method of the effective potential near the AF phase, but in the paramagnetic side. We discuss the influence of these corrections, including the possibility of fluctuation induced first order transitions. A scaling approach is used to obtain the critical and shift exponents at a quantum bicritical point.     

Evidence for unconventional s-wave superconductivity in the heavy-fermion compound UPt3

We have discussed the ultrasonic attenuation and specific heat in the superconducting state of the heavy-fermion compound UPt₃ in a quasiparticle picture. Coherence and pair-breaking effect have been taken into account for this system. Our results show that UPt₃ is most probably an unconventional s-wave superconductor.     

Three-dimensional MgB2-type superconductivity in hole-doped diamond

We substantiate by numerical and analytical calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB2, albeit in three dimensions. Holes at the top of the zone-centered, degenerate sigma-bonding valence-band couple strongly to the optical bond-stretching modes. The increase from two to three dimensions reduces the mode softening crucial for T(c) reaching 40 K in MgB2. Even if diamond had the same bare coupling constant as MgB2, which could be achieved with 10% doping, T(c) would be only 25 K. Superconductivity above 1 K in Si (Ge) requires hole doping beyond 5% (10%).     

Amorphous metals and their superconductivity

Quite a number of metals and alloys can be forced into the amorphous state by quenching. In particular, condensation onto a substrate cooled to He-temperature is very effective. The superconductivity and related properties of these metals are considered in this article. The influence of the amorphous structure on the electron and the phonon system is discussed. In simple metals the electrons show a free electron behaviour. The transverse phonons are softened but maintainn the ω³-density of states at low frequencies. The electron-phonon interaction does not conserve momentum due to the loss of translational invariance in the amorphous metal and yields a large contribution to α²F(ω) at low frequencies. Therefore the amorphous superconductors are strong coupling. The strong coupling character alters the temperature dependence of several superconducting parameters such as critical fields etc. The characteristic superconducting properties and superconducting fluctuations are discussed.