No abstract

Strontium ruthenate (Sr 2 RuO 4) has long been thought to host a spin-triplet chiral p-wave superconducting state. However, the singletlike response observed in recent spin-susceptibility measurements casts serious doubts on this pairing state. Together with the evidence for broken time-reversal symmetry and a jump in the shear modulus c 66 at the superconducting transition temperature, the available experiments point towards an evenparity chiral superconductor with k z (k x ± ik y)-like E g symmetry, which has consistently been dismissed based on the quasi-two-dimensional electronic structure of Sr 2 RuO 4. Here, we show how the orbital degree of freedom can encode the two-component nature of the E g order parameter, allowing for a local orbital-antisymmetric spintriplet state that can be stabilized by on-site Hund's coupling. We find that this exotic E g state can be energetically stable once a complete, realistic three-dimensional model is considered, within which momentum-dependent spin-orbit coupling terms are key. This state naturally gives rise to Bogoliubov Fermi surfaces.

Twisted bilayer graphene has recently attracted a lot of attention for its rich electronic properties and tunability. Here we show that for very small twist angles, α 1 • , the application of a perpendicular electric field is mathematically equivalent to a new kind of artificial gauge field. This identification opens the door for the generation and detection of pseudo-Landau levels in graphene platforms within robust setups which do not depend on strain engineering and therefore can be realistically harvested for technological applications. Furthermore, this new artificial gauge field leads to the development of highly localized modes associated with flat bands close to charge neutrality which form an emergent Kagome lattice in real space. Our findings indicate that for tiny angles, biased twisted bilayer graphene is a promising platform which can realize frustrated lattices of highly localized states, opening a new direction for the investigation of strongly correlated phases of matter.

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 parameters FA(k) and FC (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 parameters can be employed as a guide to engineer normal state Hamiltonians in order to favour 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 non-centrosymmetric heavy fermion CePt3Si, the hole doped iron pnictide KFe2As2 and the doped topological insulator CuxBi2Se3.