We have measured upper-critical-field H c2 , specific heat C, and tunneling spectra of the intermetallic perovskite superconductor MgCNi 3 with a superconducting transition temperature T c Ϸ7.6 K. Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field H c (0), coherence length (0), penetration depth (0), lower-critical-field H c1 (0), and Ginzburg-Landau parameter (0). From the specific heat, we obtain the Debye temperature ⌰ D Ϸ284 K. We find a jump of ⌬C/␥T c ϭ2.1 at T c ͑where ␥ is the normal-state electronic specific coefficient͒, which is larger than the weak-coupling BCS value of 1.43, suggesting that MgCNi 3 may be a strong-coupling superconductor. In addition, we observed a pronounced zero-bias conductance peak ͑ZBCP͒ in the tunneling spectra. We discuss the possible physical origins of the observed ZBCP.

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Transition-metal oxides have proved to be particularly fruitful in this regard, especially for those materials with the perovskite structure, where the special characteristics of transition-metal-oxygen orbital hybridization determine their properties. Ruthenates have recently emerged as an important family of perovskites because of the unexpected evolution from high-temperature ferromagnetism in SrRuO3 to low-temperature superconductivity in Sr2RuO4 (refs 1, 2). Here we show that a ruthenate in a different structural family, La4Ru6O19, displays a number of highly unusual properties, most notably non-Fermi-liquid behaviour. The properties of La4Ru6O19 have no analogy among the thousands of previously characterized transition-metal oxides. Instead, they resemble those of CeCu6-xAux-a widely studied f-electron-based heavy fermion intermetallic compound that is often considered as providing the best example of non-Fermi-liquid behaviour. In the ruthenate, non-Fermi-liquid behaviour appears to arise from just the right balance between the interactions of localized electronic states derived from Ru-Ru bonding and delocalized states derived from Ru-O hybridization.

We have investigated depairing effects in Sr 2 RuO 4 , the unconventional superconductor in the layered perovskite structure. We prepared crystals of Sr 2 RuO 4 with very low levels of impurity elements, and systematically controled their superconducting transition temperature T c ranging from 1.5 to 0.6 K by adjustments of crystal growth conditions. The dependence of T c on the residual resistivity 0 in these crystals suggests that the defects are strong pair breakers, in addition to impurities. We further characterized the effects of pair breaking in this unconventional superconductor. We found that the in-plane coherence length ab (0) evaluated from H c2 (T) is inversely proportional to T c , and decreases with increasing mean free path l; the latter behavior is opposite to that of conventional superconductors. In addition, we examined the temperature dependence of H c2ʈc which substantially deviates from the BCS theory and even from a recent theory of a p-wave superconductor. ͓S0163-1829͑99͒11225-6͔

Please cite this article as: Y. Liu, Z-Q. Mao, Unconventional superconductivity in Sr 2 RuO 4 , Physica C (2015), doi: http://dx. AbstractSr 2 RuO 4 , featuring a layered perovskite crystalline and quasi-two-dimensional electronic structure, was first synthesized in 1957. Unconventional, p-wave pairing was predicted for Sr 2 RuO 4 by Rice and Sigrist and Baskaran shortly after superconductivity in this material was discovered in 1994. Experimental evidence for unconventional superconductivity in Sr 2 RuO 4 has obtained in the past two decades and reviewed previously. In this article, we will first discuss constraints on the pairing symmetry and the mechanism of superconductivity in Sr 2 RuO 4 and summarise experimental evidence supporting the unconventional pairing symmetry accumulated to date. We will then present several aspects of the experimental determination of the unconventional superconductivity in Sr 2 RuO 4 in some detail. In particular, we will discuss the phase-senistive measurements that have played an important role in the determination of the pairing symmetry in Sr 2 RuO 4 . The responses of superconductivity to the mechanical perturbations and their implications on the mechanism of superconductivity are discussed. A brief survey of various non-bulk Sr 2 RuO 4 is also included to illustrate the many unusual features resulted from the unconventional nature of superconductivity in this material system. Finally, we will discuss some outstanding unresolved issues on Sr 2 RuO 4 and provide an outlook of the future work on Sr 2 RuO 4 .