Superconducting Fitness and Symmetry Classification

external pageSuperconducting order parameter of Sr₂RuO₄: A microscopic perspectivecall_made

Aline Ramires and Manfred Sigrist

The character of the superconducting phase of Sr₂RuO₄ is the 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 nontrivial symmetry aspects related to their orbital content. Here we show how the careful account of the orbital degree of freedom and a three-dimensional description lead to a much richer classification of order parameters. We analyze the stability and degeneracy of these new order parameters from the perspective of the concept of superconducting fitness and propose new order parameter candidates which can systematically account for the observed phenomenology in this material.

 

external pageTailoring T_c by symmetry principles: The concept of superconducting fitnesscall_made

Aline Ramires, Daniel F. Agterberg and Manfred Sigrist

We propose a generalization of the concept of superconducting fitness, which allows us to make statements analogous to Anderson's theorems concerning the stability of different superconducting states. This concept can be applied to complex materials with several orbital, layer, sublattice, or valley degrees of freedom. The superconducting fitness functions F_A(k) and F_C(k) give a direct measure of the robustness of the weak-coupling instability and of the presence of detrimental terms in the Hamiltonian, respectively. These two functions can be employed as a guide to engineer normal state Hamiltonians in order to favor or suppress superconducting order parameters with different symmetries and topological properties. To illustrate the applicability and power of this concept we study three cases: the noncentrosymmetric heavy fermion CePt₃Si, the hole-doped iron pnictide KFe₂As₂, and the doped topological insulator Cu_xBi₂Se₃.

 

external pageA note on the upper critical field of Sr2RuO4 under straincall_made

Aline Ramires and Manfred Sigrist

In the light of the recently discussed mechanism for suppression of superconductivity in multi-orbital systems called inter-orbital effect, here we extend our analysis of the upper critical field in Sr₂RuO₄ now under strain. We show that the presence of the standard orbital effect and the new mechanism can consistently account for the qualitative changes observed in the upper critical field for the strained system, in particular, combining the overall enhancement of the critical field with the reduction in the anisotropy. The proposed picture holds for a triplet superconducting state, showing that a singlet state is not the only possibility to account for the observations. We suggest further experiments in order to clarify our understanding about the superconducting phase of Sr₂RuO₄.

external pageIdentifying detrimental effects for multiorbital superconductivity: Application to Sr₂RuO₄call_made

Aline Ramires and Manfred Sigrist

We propose a general scheme to probe the compatibility of arbitrary pairing states with a given normal state Hamiltonian by the introduction of a concept called superconducting fitness. This quantity gives a direct measure of the suppression of the superconducting critical temperature in the presence of key symmetry-breaking fields. A merit of the superconducting fitness is that it can be used as a tool to identify nontrivial mechanisms to suppress superconductivity under various external influences, in particular, magnetic fields or distortions, even in complex multiorbital systems. In the light of this concept we analyze the multiband superconductor Sr₂RuO₄
and propose a new mechanism for the suppression of superconductivity in multiorbital systems, which we call interorbital effect, as a possible explanation for the unusual limiting feature observed in the upper critical field of this material.