Unconventional superconductivity in Sr2RuO4

Abstract

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 .

Unconventional superconductivity

“…Liu and Mao, in their recent review [534] of Sr2RuO4, state that whether or not the order parameter (k) has nodes remains unresolved. If one -using the presence of the three bands at the Fermi energy in Sr2RuO4 -argues that the 5 -pairing state can have an admixture of another state which brings in kz dependence (for example the triplet f-wave pairing state d(k) = z(kx  iky)coskzc -see Hasegawa, Machida and Ozaki [550]) , then one can have horizontal nodes [534].…”

“…If one -using the presence of the three bands at the Fermi energy in Sr2RuO4 -argues that the 5 -pairing state can have an admixture of another state which brings in kz dependence (for example the triplet f-wave pairing state d(k) = z(kx  iky)coskzc -see Hasegawa, Machida and Ozaki [550]) , then one can have horizontal nodes [534]. Or, indeed, the multiple bands (just as discussed above for the IBS for the field dependence of the specific heat ) can mimic nodal behavior without there being any nodes.…”

“…However, Balian and Werthamer proposed the use of a three component complex vector d(k) for triplet pairing. Discussing now specifically (Liu and Mao [534]) tetragonal Sr2RuO4 believed to be in the spin triplet state, there are five possible spin triplet pairing states (five possible d(k)) allowed. As we will see after further discussion of the experimental data (in particular the time reversal symmetry breaking, section 7.6) below, the preferred one of these pairing states for Sr2RuO4 is the Eu (5 -), with d(k) = z(kx  iky) -the only one of the five with d(k) along the c-axis.…”

“…The 5 -pairing state in a single band superconductor (which Sr2RuO4 -just like the IBS -is not) would be a priori fully gapped, with (Liu and Mao [534]) gapless chiral edge states. The 5 -pairing state has the possibility of domains (kx + iky or kx -iky) and chiral surface currents.…”

“…Sr2RuO4 has been reviewed numerous times, the most recent review was by Liu and Mao [534] in 2015. Some other important reviews, in addition to those mentioned above, include those by Maeno et al [537], Maeno, Rice and Sigrist [538], and Bergemann et al [539].…”