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In the d-electron system YFe2Ge2, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ∼ 100 mJ/(mol K2) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close-by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe2Ge2 superconduct below 1.8 K. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

We examine the low temperature states of two transition metal compounds: (i) NbFe 2 is poised on the threshold of ferromagnetism and can be pushed into a spin-aligned state at low temperature by modifying the composition slightly. Stoichiometric NbFe 2 has been reported as a rare example of low-temperature spin density wave order in a d-metal system. We have used pressure, field and composition tuning to examine the phase diagram of NbFe 2 . Near the quantum critical point, we find distinct non-Fermi liquid forms of the resistivity and heat capacity, whereas we observe strong, hysteretic magnetoresistance effects deep in the ordered phase. (ii) Ca 3 Ru 2 O 7 undergoes first a magnetic transition (T N = 56 K) and then a structural transition (T s = 48 K) on cooling. Most of the Fermi surface is gapped out at low temperature, leading to a very low carrier density and small Fermi surface pockets. Pressure suppresses both T N and T S and, for p > 3.5 GPa, induces a third low temperature state, which is robust up to at least 7.5 GPa.

We report the presence of two disconnected superconducting domes in the pressure-temperature phase diagram of partially germanium-substituted CeCu2Si2. The lower density superconducting dome lies on the threshold of antiferromagnetic order, indicating magnetically mediated pairing, whereas the higher density superconducting regime straddles a weakly first-order volume collapse, suggesting a pairing interaction based on spatially extended density fluctuations. Two distinct pairing mechanisms thus appear to operate in the single, wide, superconducting range of stoichiometric CeCu2Si2, both of which might apply more generally to other classes of correlated electron systems.