Possible Heavy-Fermion State in \(\mathcal{P}\mathcal{T}\)-Symmetric Antiferromagnet CeMnSi

The magnetic properties of CeMnSi with a tetragonal crystal structure were investigated for the first time using single crystals. CeMnSi exhibits an antiferromagnetic (AFM) ordering of Mn-3d electrons at TN = 242 K, whereas Ce-4f electrons remain paramagnetic down to the lowest temperature with the following characteristic behaviors: a moderately enhanced Sommerfeld coefficient, a steep decrease in electrical resistivity at low temperatures, and an almost temperature-independent magnetic susceptibility at low temperatures despite a Curie–Weiss increase at high temperatures. From these results, we propose that a noble correlated electron state is realized even in the Mn-AFM ordered state and is protected by the space–time (\(\mathcal{P}\mathcal{T}\)) inversion symmetry inherent in the Mn-AFM ordering. We also show systematic changes in the lattice parameters of RTSi (R = rare-earth elements, T = Mn, Fe, and Co), providing the basis for understanding the unique electronic properties of CeTSi.