Failed superconducting transitions in organic charge transfer salts κ-[(BEDT-TTF)_1-x(BEDT-STF)_x]₂Cu₂(CN)₃

Previous measurements of transport and magnetism at ambient pressure and temperatures (T) above 1 K indicated onset of superconductivity on the charge transfer salt κ-[(BEDT-TTF)_1-x (BEDT-STF)_x] ₂Cu₂(CN)₃ (abbr. κ-(STF) _x) near x=0.11. In this study, we perform dc transport measurements at ambient pressure on κ-(STF)_0.11 and κ-(STF)_0.12 in magnetic fields (H) up to 18 T and temperatures (T) down to 20 mK. In zero magnetic field, despite a sharp drop of temperature-dependent resistivity due to the onset of superconducting fluctuations, the measured and the 0 K-extrapolated resistivities clearly remain finite. In finite fields, positive magnetoresistance and the associated scaling analysis indicate an underlying quantum “superconductor”-metal transition. In the high-field normal state, the resistivity exhibits a logarithmic temperature dependence, consistent with the Altshuler-Aronov effect. Surprisingly, universal conductance fluctuations (UCFs) are observed in these two macroscopic samples. Both phenomena indicate that the high-field normal state is metallic. Possible interpretations of the incomplete superconducting transitions and UCFs in macroscopic systems, and comparisons with “failed superconductors” will be discussed.