CeIrIn5 hides a secret since its discovery in 2001(1). The superconducting transition of this heavy fermion metal is clearly evidenced by a textbook-like specific heat anomaly and the sharp onset of Meissner shielding at Tc~400mK. At the same time, every high purity single crystal ever synthesized shows a transition to a zero-resistance state at substantially higher temperatures, up to Tc*~1.2K. This discrepancy has been extensively studied in the 2000’s, for example by Josephson tunneling(2) or systematic chemical disorder(3). The predominant explanations include local strain around extrinsic defects, akin to the 3K phase in Sr2RuO4, or intrinsic superconductivity confined to the crystal surface.
Our group specializes in the fabrication of mesoscale transport devices from high purity single crystals using Focused Ion Beam (FIB) machining, and thus we started to investigate the nature of the resistive anomaly on the micron-length scale. The overall transport properties of the devices quantitatively reproduce the overall resistance measured on macroscopic crystals in the normal state. In addition, the frequencies of the pronounced Shubnikov-de Haas oscillations measured in the microstructure agree quantitatively with the well-studied de Haas-van Alphen spectrum of macroscopic crystals. Thus we have no reason to believe that the microstructuring process has altered the crystals high purity, similar to related studies in CeRhIn5(4).
The superconducting state that unfolds in transport around Tc*, however, is bizarre. We have performed an extensive suite of experiments in the past year, including more than a dozen different structures and complementary experimental approaches. The robust experimental results leave us with no other choice than to postulate an exotic and potentially entirely novel type of superconductivity. The material consistently transitions to a robust, zero resistance state at temperatures around Tc*, however only for currents along the crystallographic c-direction. The in-plane transport, along the crystallographic a-direction, follows the typical T2 temperature dependence of the Fermi liquid without even the slightest anomaly at Tc*. Only at significantly lower temperatures, at the Tc~400mK observed by magnetic measurements, the material fully exhibits zero resistance along all directions.
Currently, the extensive experimental evidence we gathered defies our understanding within a BCS framework. I will present our unusual results, and invite you to discuss its origin.
1. C. Petrovic et al., A new heavy-fermion superconductor CeIrIn5: A relative of the cuprates? Europhys. Lett. 53, 354–359 (2001).
2. A. Sumiyama et al., Josephson effect in heavy-fermion superconductor CeTIn5 (T = Co, Ir). Phys. C Supercond. its Appl. 388–389, 545–546 (2003).
3. A. Bianchi et al., Origin of the zero-resistance anomaly in heavy fermion superconducting CeIrIn5: A clue from magnetic-field and Rh-doping studies. Phys. Rev. B. 64, 220504 (2001).
4. F. Ronning et al., Electronic in-plane symmetry breaking at field-tuned quantum criticality in CeRhIn5. Nature. 548, 313–317 (2017).