How to make a new iron-based superconductor

Superconductivity exerts a particular fascination, when it is caused by an electronic mechanism. Among the transition metal compounds, such unconventional superconductivity is extremely rare and has largely been confined to the copper oxide high temperature superconductors, Sr₂RuO₄, and to a diverse family of iron-pnictide and chalcogenides. In an earlier manuscript  [Chen et al., Physical Review Letters 116, 127001 (2016)], Malte's group had presented compelling evidence that the layered iron system YFe₂Ge₂ exhibits bulk superconductivity at low temperature.

Low temperature resistivity and heat capacity data in samples of YFe₂Ge₂ with different levels of impurity scattering, recently published in Physical Review B Rapid Comm.. As sample quality increases and residual resistance values greater than 100 are achieved, the low temperature heat capacity displays an anomaly indicative of bulk superconductivity.

In the normal state, YFe₂Ge₂ displays a strongly enhanced heat capacity Sommerfeld coefficient of nearly 100 mJ/(mol K²), an order of magnitude above band structure values, and a distinct non-Fermi liquid temperature dependence of the electrical resistivity at low temperature. Superconductivity is only observed in the purest samples, and the resistive T_c as well as the transition width depend strongly on the electronic mean free path. All of this suggests that superconductivity in YFe₂Ge₂ is unconventional.

Only high quality samples display bulk superconductivity, however, and it has taken the group another two years to work out how to grow such samples systematically, and what governs sample quality. A new paper in Physical Review B Rapid Comm. reports the main results of this study. By systematically scanning the composition phase diagram of YFe₂Ge₂, it was possible to determine the dominant source of disorder scattering, and to develop a reliable method for minimizing it. An analysis of the transport and thermodynamic signatures of superconductivity in dozens of samples with varying disorder level has revealed a strong correlation between disorder scattering and the transition temperature. Bulk superconductivity in YFe₂Ge₂ is suppressed when the scattering rate exceeds 2 k_BT_c, a strong indication that YFe₂Ge₂ is indeed that rare breed, an unconventional, transition-metal based superconductor. (4/1/19)