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We study the influence of symmetry-breaking perturbations on superconductivity in multiorbital materials, with a particular focus on an external magnetic field. We introduce the field-fitness function which characterizes the pair-breaking effects of the perturbation on a given superconducting state. For even parity superconductors we find that this field-fitness function for an external magnetic field is one, implying that the paramagnetic response is controlled only by a generalized effective $g$-factor. For odd parity superconductors, the interplay of the effective $g$-factor and the field-fitness function can lead to counter-intuitive results. We demonstrate this for $p$-wave pairing in the effective $j=3/2$ electronic states of the Luttinger-Kohn model.

Recently, evidence has emerged for a field-induced even- to odd-parity superconducting phase transition in CeRh$_2$As$_2$ [S. Khim et al., Science 373 1012 (2021)]. Here we argue that the P4/nmm non-symmorphic crystal structure of CeRh$_2$As$_2$ plays a key role in enabling this transition by ensuring large spin-orbit interactions near the Brillouin zone boundaries, which naturally leads to the required near-degeneracy of the even- and odd-parity channels. We further comment on the relevance of our theory to FeSe, which crystallizes in the same structure., 9 pages, 9 figures

We investigate the influence of general forms of disorder on the robustness of superconductivity in multiband materials. Specifically, we consider a general two-band system where the bands arise from an orbital degree of freedom of the electrons. Within the Born approximation, we show that the interplay of the spin-orbital structure of the normal-state Hamiltonian, disorder scattering, and superconducting pairing potentials can lead to significant deviations from the expected robustness of the superconductivity. This can be conveniently formulated in terms of the so-called ``superconducting fitness.'' In particular, we verify a key role for unconventional $s$-wave states, permitted by the spin-orbital structure and which may pair electrons that are not time-reversed partners. To exemplify the role of Fermi-surface topology and spin-orbital texture, we apply our formalism to the candidate topological superconductor ${\mathrm{Cu}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, for which only a single band crosses the Fermi energy, as well as models of the iron pnictides, which possess multiple Fermi pockets.

Microwave conductivity experiments can directly measure the quasiparticle scattering rate in the superconducting state. We show that this, combined with knowledge of the Fermi surface geometry, can distinguish between closely related superconducting order parameters, e.g., d(x2-y2) and d(xy) superconductivity. We benchmark this method on YBa2Cu3O7-(delta) and, unsurprisingly, confirm that this is a d(x2-y2) superconductor. We then apply our method to kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br, which we discover is a d(xy), superconductor.

We consider the nuclear spin-lattice relaxation rate, $1/T_1T$ in superconductors with accidental nodes. We show that a Hebel-Slichter-like peak occurs even in the absence of an isotropic component of the superconducting gap. The logarithmic divergence found in clean, non-interacting models is controlled by both disorder and electron-electron interactions. However, for reasonable parameters, neither of these effects removes the peak altogether., Comment: 10 pages, 5 figures