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Center for Nonlinear Studies |
As discussed in the first part of the talk, heavy-fermion (HF) metals, i.e., intermetallic compounds of certain lanthanide and actinide elements, have been the subject of intensive investigations over the last three to four decades. These research activities have furnished important discoveries, e.g., of unconventional superconductivity (“beyond BCS”) and unconventional quantum criticality (“beyond Landau”). Almost fifty HF superconductors are currently known, about 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 (“Kondo destroying”) QCP. To get rid of the huge entropy accumulated at a QCP, superconductivity (SC) very often develops in its vicinity.
The second part of the talk deals with the quantum critical paradigm according to which any AF-QCP in a clean, stoichiometric HF metal should generate unconventional SC. For this purpose, we address exemplary materials for the two different QCP scenarios, i.e., the isostructural compounds CeCu2Si2 and YbRh2Si2. CeCu2Si2, the first HF superconductor [1], exhibits SC at a 3D SDW-QCP and was considered a d-wave superconductor until very recently, when its specific heat was found to exhibit an exponential temperature dependence at very low temperatures [2], typical for a conventional BCS superconductor. However, based on inelastic neutron - scattering [3] and penetration - depth [4] results we shall argue that CeCu2Si2 most likely is a fully-gapped (two-band) d-wave superconductor.
Next, we address YbRh2Si2 whose unconventional (“Mott-type”) QCP behavior is reflected by, among others, an abrupt jump of the Fermi-surface volume [5, 6] and a violation of the Wiedemann-Franz law [7]. For this material, no SC has been detected down to 10 mK, the lowest temperature accessible for resistivity measurements in a 3He-4He dilution refrigerator [8]. On the other hand, 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]. This provides further evidence for the quantum critical paradigm to hold, regardless of the microscopic nature of the AF QCP.
Work performed in collaboration with O. Stockert, M. Brando, E. Schuberth, H.Q. Yuan, M. Smidman, E.M. Nica, R. Yu and Q. Si.
Host: Qimiao Si