Hosted by: Cedomir Petrovic

Heavy-fermion (HF) metals, i.e., intermetallic compounds of certain lanthanides and actinides, have been subject of intensive investigations over the last few decades. These research activities have furnished important discoveries, such as of unconventional superconductivity (SC) ("beyond BCS") and unconventional quantum criticality ("beyond Landau"). About fifty HF superconductors are currently known, more than half of which exhibiting a quantum critical point (QCP) where antiferromagnetic (AF) order is smoothly suppressed by tuning a non-thermal control parameter like pressure or magnetic field. Two variants of HF AF-QCPs have yet been established, i.e., a conventional ("3D SDW") and an unconventional, partial Mott ("Kondo destroying") QCP [1, 2]. In clean, stoichiometric HF metals, the huge entropy accumulated at such an AF QCP is commonly removed by forming an unconventional superconducting phase. The apparent validity of this 'quantum critical paradigm' will be illustrated in the first part of the talk by addressing exemplary quantum critical materials, i.e., the isostructural compounds YbRh2Si2 and CeCu2Si2. The former system exhibits a partial-Mott QCP as reflected by, e.g., an abrupt jump of the Fermi-surface volume [3- 5] and a violation of the Wiedemann-Franz law [6, 7]. For this compound, no SC had been detected down to 10 mK, the lowest temperature accessible in a commercial 3He-4He dilution refrigerator [8]. However, recent magnetic and specific-heat measurements performed in a nuclear demagnetization cryostat down to about 1 mK revealed HF, i.e., unconventional, SC below Tc = 2 mK [9]. CeCu2Si2, the first HF superconductor [10], exhibits SC at a 3D SDW-QCP and was considered a (one-band) d-wave superconductor until a few years ago, when its specific heat was found to exhibit two-gap behavior and exponential temperature dependence at very low temperatures [11]. Based on atomic substitution [12],