Phase instabilities in strongly correlated materials

In recent years, the standard model of metals, known as the Fermi liquid theory, has been challenged by the discovery of anomalous properties in an increasing large number of systems. Some of them are near a quantum critical point - a continuous phase transition in the limit of absolute zero between, typically, magnetically ordered and paramagnetic phases. Others are close to a total loss of the intrinsic magnetic moment. A quantum critical point is characterised by a high electronic degeneracy that may be lifted by complex interactions, giving rise to a wealth of novel states and physics of solids at low temperatures. To name just a few, the co-existence of unconventional superconductivity with ferro- or antiferromagnetic order, or the occurrence of non-Fermi-liquid phases are already well established. The goal of the proposed research is to carry out detailed studies comprising structure, transport, thermodynamic, nuclear probe and spectroscopic investigations in a wide parameter range on systems exhibiting distinct electron correlations as a precursor of anomalous features observed for such systems at low temperatures. Two different groups of materials have been identified and special emphasis is laid on the recently discovered heavy fermion superconductor CePt3Si [1], being the first magnetically ordered superconductor without -a centre of inversion. This fact has severe consequences for the order parameter and spins of those electrons (with significant spin - orbit coupling) building Cooper pairs might rotate around the momentum-space surface [2]. Ternary Yb systems (e.g. Yb2Pd2ln), as further focal point of the proposed project, representing the hole analogue to Ce compounds, are potentially rich in their ground state properties, however, are by far less well studied than Ce systems. Research directed to this field will help to draw conclusions and achieve common agreement predominantly about (i) the origin and characteristics of the unconventional superconducting phase of CePt3Si; (ii) the impact of non- centrosymmetry on the superconducting order parameter; iii) similarities and differences of the ground state physics of Yb compounds with respect to comparable Ce systems, particularly in the proximity of a quantum critical point.

The goal of the research carried out in the scope of the FWF project P18054 was a detailed study comprising structure, transport, thermodynamic, nuclear probe and spectroscopic investigations in a wide parameter range on systems exhibiting distinct electron correlations as a precursor of anomalous features observed for such systems at low temperatures. Two different groups of materials have been identified and special emphasis was laid on the heavy fermion superconductor CePt 3 Si, being the first magnetically ordered superconductor without a centre of inversion. This fact has severe consequences for the superconducting order parameter. Ternary Yb systems (e.g. Yb2 Pd2 In), as further focal point of the project work, representing the hole analogue to Ce compounds, are potentially rich in their ground state properties, however, are by far less well studied than Ce systems. The most important findings made in course of the project P18054 are: (a) Superconductivity in non-centrosymmetric CePt 3 Si and related compounds. Among materials with strong electron correlations, those exhibiting superconductivity have attracted the most intense interest because the rich variety of unconventional SC states. Cooper pairs in such systems are not formed by the interaction of electrons with phonons, resulting in general in the highly symmetric s-wave state, rather magnetic fluctuations are considered as the relevant pairing mechanism. If inversion symmetry in the crystal structure is missing, peculiar band splitting develops, which is detrimental to certain kinds of Cooper pairing channels. In particular, spin triplet pairing was claimed to become unlikely under these circumstances. Ternary CePt 3 Si is the first representative of heavy fermion superconductors without inversion symmetry. Macroscopic and microscopic studies carried out evidenced a coexistence of superconductivity Tc = 0.75 K and magnetic order, TN = 2.2 K on a microscopic scale. The absence of inversion symmetry favours a SC order parameter which may be composed of spin-singlet and spin-triplet components as indicated from a very unique NMR relaxation rate 1/T 1 and a linear temperature dependence of the penetration depth. Furthermore, a helical modification of the order parameter would explain the absence of substantial anisotropy of the upper critical field. Phase diagrams were established by i) the application of hydrostatic pressure and ii) by chemical substitution. The application of pressure as well as all the different substitutions reveals in any case the maximum superconducting transition temperature for undoped CePt 3 Si at ambient pressure. It is of importance to disentangle all the exciting superconducting features discovered in non- centrosymmetric heavy fermion compound CePt 3 Si among properties related to spin-orbit splitting due to the electric field gradient because of the missing inversion symmetry and those which are clearly associated with the strong electron correlations present in CePt 3 Si. A promising way to do this is to find non-centrosymmetric superconductors built up by simple, non-magnetic materials. In fact, two different superconducting materials without inversion symmetry have been identified in course of this FWF project, namely Al3Mg 2 and BaPtSi 3 . It turns out that both latter systems exhibit superconductivity entirely accountable in terms of the BCS theory, i.e., in the simplest form of superconductivity. (b) Two magnetic quantum critical points in Yb2 Pd2 Sn. Ce and Yb compounds have been proven as ideal playground to explore the principal features of competing electronic ground states and peculiarities associated with a quantum critical point (QCP). We have, for the first time discovered of two consecutive, pressure driven QCP`s. They emerge in a non-Fermi liquid environment at the origins of a dome-like, single magnetic phase in Yb2Pd2Sn at pressures pc1 1 GPa and pc2 4 GPa. This unique behaviour of Yb compounds is supposed to result from mutually competing, pressure modified energy scales, which in case of Yb2 Pd2 Sn cause a sign change of the pressure dependence of the Kondo temperature TK and magnetic ordering temperature TN . Our finding turns out to be inimitable for Yb compounds, unlikely occurring in any Ce system.