Superconducting order parameter of Sr2_2RuO4_4: a microscopic perspective

The character of the superconducting phase of Sr$_2$RuO$_4$, is topic of a longstanding discussion. The classification of the symmetry allowed order parameters has relied on the tetragonal symmetry of the lattice and on cylindrical Fermi surfaces, usually taken to be featureless, not including the non-trivial symmetry aspects related to their orbital content. Here we show how the careful account of the orbital degree of freedom in Sr$_2$RuO$_4$, leads to a much richer classification of order parameters. We analyse the stability and degeneracy of these new order parameters from the perspective of the concept of superconducting fitness and propose a new best order parameter candidate.Comment: 13 page

Superconductivity-induced improper orders

The study of improper phases in the context of multiferroic materials has a long history, but superconductivity has yet to be connected to the network of ferroic orders. In this work, we highlight an overlooked mechanism that couples superconducting order parameters to odd-parity orders in the charge or spin sectors such that the latter emerge as improper orders. For that, we explore a novel perspective of nonsymmorphic symmetries based on extended symmetry groups in real space. We highlight how nonsymmorphic symmetries can generate rather nonintuitive couplings between order parameters. In particular, we find that a bilinear in the superconducting order parameter can couple linearly to odd-parity orders in centrosymmetric systems. Our findings can account for the unusual phenomenology of CeRh$_2$As$_2$, a recently discovered heavy fermion superconductor, and open the door for exploring nonsymmorphic symmetries in the broader context of improper orders with potential applications to functional materials.Comment: 8 pages, 3 figure

Superconductivity in disordered locally noncentrosymmetric materials : an application to CeRh2As2

Layered three-dimensional centrosymmetric crystals can exhibit characteristics of noncentrosymmetric mate rials. This happens when each individual layer alone lacks inversion but, when combined, inversion symmetry is restored; hence the designation locally noncentrosymmertic superconductors (LNCSs). In LNCSs, the effects of impurities and subdominant magnetic field induced pairing channels remain unexplored. Using a minimal model, we examine all pairing channels and show that there is always a subdominant superconducting instability that is favored at high magnetic fields, which can substantially alter the magnetic field–temperature phase diagram. Also, we find that the phase diagram responds to disorder in a nonmonotonic way, which can be subjected to experimental verification. We apply these ideas to the recently unveiled two-phase superconducting phase diagram of CeRh2As2. We identify the two phases as singlet-triplet mixed even- and odd-parity states at low and high-fields, respectively. Furthermore, we predict the presence of two superconducting phases also for in-plane magnetic fields in cleaner samples, since a high-field phase could have been so far hindered by impurity effects

Superconductivity in disordered locally noncentrosymmetric materials : an application to CeRh2As2

Layered three-dimensional centrosymmetric crystals can exhibit characteristics of noncentrosymmetric mate rials. This happens when each individual layer alone lacks inversion but, when combined, inversion symmetry is restored; hence the designation locally noncentrosymmertic superconductors (LNCSs). In LNCSs, the effects of impurities and subdominant magnetic field induced pairing channels remain unexplored. Using a minimal model, we examine all pairing channels and show that there is always a subdominant superconducting instability that is favored at high magnetic fields, which can substantially alter the magnetic field–temperature phase diagram. Also, we find that the phase diagram responds to disorder in a nonmonotonic way, which can be subjected to experimental verification. We apply these ideas to the recently unveiled two-phase superconducting phase diagram of CeRh2As2. We identify the two phases as singlet-triplet mixed even- and odd-parity states at low and high-fields, respectively. Furthermore, we predict the presence of two superconducting phases also for in-plane magnetic fields in cleaner samples, since a high-field phase could have been so far hindered by impurity effects