Structural and physical properties of the chiral antiferromagnet CeRhC₂

We report a study of the structural, magnetic, transport, and thermodynamic properties of polycrystalline samples of CeRhC2 and LaRhC2. CeRhC2 crystallizes in a tetragonal structure with space group P4(1), and it orders antiferromagnetically below T-N1 approximate to 1.8 K. Powder neutron diffraction measurements reveal a chiral magnetic structure with a single propagation vector Q(m) = (1/2, 1/2, 0.228(5)), indicating an antiferromagnetic arrangement of Ce magnetic moments in the ab plane and incommensurate order along the c axis with a root-mean-square ordered moment of mord = 0.68 mu B. A second antiferromagnetic phase (T-N2) becomes apparent in electrical resistivity, Hall, and heat capacity measurements in fields above 0.3 T. Electrical resistivity measurements reveal that LaRhC2 is a semiconductor with a band gap of E-g similar to 24 meV, whereas resistivity and Hall measurements indicate that CeRhC2 is a semimetal with a carrier concentration of n similar to 1020 cm(-3). With applied hydrostatic pressure, the zero-field antiferromagnetic transition of CeRhC2 is slightly enhanced, and CeRhC2 becomes notably more metallic up to 1.36 GPa. The trend toward metallicity is in line with density-functional calculations that indicate that both LaRhC2 and CeRhC2 are semimetals, but the band overlap is larger for CeRhC2, which has a smaller unit cell volume than that of its La counterpart.