Search for unconventional superconductors among the YTE 2Si2 compounds (TE = Cr, Co, Ni, Rh, Pd, Pt).

Motivated by the recent discovery of exotic superconductivity in YFe2Ge2 we undertook reinvestigation of formation and physical properties of yttrium-based 1:2:2 silicides. Here we report on syntheses and crystal structures of the YTE 2Si2 compounds with TE = Cr, Co, Ni, Rh, Pd and Pt, and their low-temperature physical properties measurements, supplemented by results of fully relativistic full-potential local-orbital minimum basis band structure calculations. We confirm that most of the members of that family crystallize in a tetragonal ThCr2Si2-type structure (space group I4/mmm) and have three-dimensional Fermi surface, while only one of them (YPt2Si2) forms with a closely-related primitive CaBe2Ge2-type unit cell (space group P4/nmm) and possess quasi-two-dimensional Fermi surface sheets. Physical measurements indicated that BCS-like superconductivity is observed only in YPt2Si2 (T c = 1.54 K) and YPd2Si2 (T c = 0.43 K), while no superconducting phase transition was found in other systems at least down to 0.35 K. Thermal analysis showed no polymorphism in both superconducting phases. No clear relation between the superconductivity and the crystal structure (and dimensionality of the Fermi surface) was observed.

The influence of magnetic sublattice dilution on magnetic order in CeNiGe3 and UNiSi2.

Polycrystalline samples of the Y-diluted antiferromagnet CeNiGe(3) (T(N) = 5.5 K) and Th-diluted ferromagnet UNiSi(2) (T(C) = 95 K) were studied by means of x-ray powder diffraction, magnetization and specific heat measurements performed in a wide temperature range. The lattice parameters of the Ce(1-x)Y(x)NiGe(3) alloys decrease linearly with increasing Y content, while the unit cell volume of U(1-x)Th(x)NiSi(2) increases linearly with increasing Th content. The ordering temperatures of the systems decrease monotonically with increasing x down to about 1.2 K in Ce(0.4)Y(0.6)NiGe(3) and 26 K in U(0.3)Th(0.7)NiSi(2), forming a dome of long-range magnetic order on their magnetic phase diagrams. The suppression of the magnetic order is associated with distinct broadening of the anomalies at T(N,C) due to crystallographic disorder being a consequence of the alloying. Below the magnetic percolation threshold x(c) of about 0.68 and 0.75 in the Ce- and U-based alloys, respectively, the long-range magnetic order smoothly evolves into a short-range one, forming a tail on the magnetic phase diagrams. The observed behaviour of Ce(1-x)Y(x)NiGe(3) and U(1-x)Th(x)NiSi(2) is characteristic of diluted magnetic alloys.

Evolution from a localized to an intermediate valence regime in Ce2Cu(2-x)Ni(x)In.

Polycrystalline samples of the solid solution Ce2Cu(2-x)Ni(x)In were studied by means of x-ray powder diffraction, magnetic susceptibility and electrical resistivity measurements performed in a wide temperature range. Partial substitution of copper atoms by nickel atoms results in a quasi-linear decrease of the lattice parameters and the unit cell volume of the system. The lattice compression leads to an increase in the exchange integral and yields a reversal in the order of the magnetic 4f(1) and nonmagnetic 4f(0) states, being in line with the Doniach phase diagram. In the localized regime, where an interplay of the Kondo scattering and the crystalline electric field effect occurs, the rise in the hybridization strength is accompanied by a relative reduction in the scattering conduction electrons on excited crystal field levels.

Magnetic properties and electronic structures of intermediate valence systems CeRhSi2 and Ce2Rh3Si5.

The crystal structures and the physical (magnetic, electrical transport and thermodynamic) properties of the ternary compounds CeRhSi(2) and Ce(2)Rh(3)Si(5) (orthorhombic CeNiSi(2)- and U(2)Co(3)Si(5)-type structures, respectively) were studied over wide ranges of temperature and magnetic field strength. The results revealed that both materials are valence fluctuating systems, in line with previous literature reports. Direct evidence for valence fluctuations was obtained by means of Ce L(III)-edge x-ray absorption spectroscopy and Ce 3d core-level x-ray photoelectron spectroscopy. The experimental data were confronted with the results of ab initio calculations of the electronic band structures in both compounds.

Emergence of a superconducting state from an antiferromagnetic phase in single crystals of the heavy fermion compound Ce2PdIn8.

Single crystals of Ce2PdIn8 were studied by means of magnetic susceptibility, electrical resistivity, and specific heat measurements. The compound was found to be a heavy fermion clean-limit superconductor with Tc=0.68 K. Most remarkably, the superconductivity in this system emerges out of the antiferromagnetic state that sets in at TN=10 K, and both cooperative phenomena coexist in a bulk at ambient pressure conditions.

Kondo-cluster-glass state near a ferromagnetic quantum phase transition.

We report on a comprehensive study of CePd(1-x)Rh(x) (0.6 0, providing evidence for the absence of a quantum critical point. Instead, a peculiar "Kondo-cluster-glass" state is found for x >or= 0.65, and the non-Fermi-liquid effects in the specific heat, ac susceptibility, and magnetization are compatible with the quantum Griffiths phase scenario.