The boundaries the two curves drew
At Okayama University, Seigo Ogawa and colleagues measured the nuclear magnetic resonance of 75As nuclei in single crystals of K2Cr3As3, at magnetic fields of 3.7, 5.0, 6.0, 7.0 and 10 T with the field parallel to the c-axis. At 10 T the Knight shift stays constant as the sample cools through the transition temperature of 5.3 K and begins to fall only at 4.5 K. A spin-singlet superconductor leaves no such gap between the two temperatures; there the spin susceptibility drops just below the transition for every field direction.[1]
The nuclear spin-lattice relaxation rate gave the second curve. At 13 T it follows a T3 law below the transition, the behaviour expected when the energy gap closes along a whole line on the Fermi surface. At 10 T and 6 T the slope instead changes at 4.5 K and 5.4 K respectively and a T5 law takes over, the behaviour expected when the gap closes only at isolated points. At 10 T the two probes place the same boundary at the same temperature, so the paired-spin direction and the gap symmetry change together there. Below 7 T the boundary splits in two, and a third region opens between them.[1]
The phase the measurement does not name
Two of the three regions can be pinned to a symmetry with some confidence. The high-field region, where the relaxation rate follows T3 and the Knight shift does not fall, sits on a single order parameter allowed in the crystal's point group. The new low-field region does not narrow that far: the paper's own table lists three representations whose predicted node structure and spin susceptibility both match what was measured, and the data cannot separate them. The authors also note that nematic interactions such as local strain could promote the emergence of helical states from a chiral state, which means the ordering may depend on the individual crystal as much as on the field.[1]
The chirality attributed to the middle region is not established by these measurements either. Broken time reversal symmetry there is carried by an earlier muon spin rotation relaxation measurement, cited rather than repeated. Guo-qing Zheng's statement that the work definitively proved spin-triplet pairing reads best as a claim about the whole pattern: three phases that move with field and temperature have no ready explanation within singlet pairing. That is a strong argument, and it is drawn from the shape of a phase diagram rather than from a direct reading of the paired spins.[1]
Which measurement comes next?
The phase diagram published here covers only the field parallel to the c-axis. Ogawa and colleagues describe the perpendicular direction as an open task, and it is the measurement that puts the labels at risk. The same three phases, rotated, are what the assignments require in that geometry, and the quantised axis of the earlier zero-field relaxation data lies in that plane. A thin-film measurement is the other place where the topological reading becomes checkable: at the edge of a film thinner than the coherence length, the predicted gapless boundary states are either there or they are absent. Until one of those arrives, what stands is a phase diagram measured twice over, by two quantities that agree where they overlap, with one of its three rooms still unlabelled.[1]