Nature of the two superconducting phases of $\mathrm{CeRh_2As_2}$
In plain words
In $\mathrm{CeRh_2As_2}$ a magnetic field along one crystal axis switches the superconductor from one state to another, possibly from even-parity to odd-parity pairing, made possible because each cerium layer lacks a center of inversion although the whole crystal has one.
Precise statement
$\mathrm{CeRh_2As_2}$ ($T_{c} \sim 0.26\ \mathrm{K}$) shows for $H \mid\mid c$ a transition at $H* \sim 40 \mathrm{kG}$ between a low-field phase SC1 and a high-field phase SC2 extending to $H_c2 \sim 140 \mathrm{kG}$ (Khim et al., Science 2021). Determine whether SC1 and SC2 are the even- and odd-parity states of locally noncentrosymmetric Ce layers, or whether $H*$ is controlled by the quadrupole-density-wave or antiferromagnetic order found near or inside the superconducting phase. An answer identifies the order parameters of both phases by site-resolved NMR Knight shift, field-angle dependence of $H*$, and the relation of $H*$ to the normal-state order.
What would settle it
Site-resolved Knight shift in SC1 and SC2 plus a measurement showing whether $H*$ moves with the normal-state ordering field when that order is suppressed (by La substitution or pressure).
Status in the literature
Field-angle data (Landaeta et al., PRX 2022) support an even-to-odd parity transition, while a possible quadrupole density wave (Hafner et al., PRX 2022) and antiferromagnetism inside the superconducting state offer alternative explanations.
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