Structural and physical properties of the chiral antiferromagnet CeRhC${}_2$

Abstract

We report a study of the structural, magnetic, transport, and thermodynamic properties of polycrystalline samples of CeRhC${}_2$. CeRhC${}_2$ crystallizes in a tetragonal structure with space group $P{4}_1$ and it orders antiferromagnetically below $T_{\text{N1}}\approx$ 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 $m_{\text{ord}}$= 0.68 ${\mu }_{\text{B}}$/Ce. Applying a magnetic field suppresses the Néel temperature $T_{\text{N1}}$ to zero near ${\mu }_0{H}_{\text{c1}}\sim$0.75 T. A second antiferromagnetic phase ($T_{\text{N2}}$), however, becomes apparent in electrical resistivity, Hall and heat capacity measurements in fields above 0.5 T and extrapolates to zero temperature at ${\mu }_0{H}_{\text{c2}}\sim$ 1 T. Electrical resistivity measurements reveal that LaRhC${}_2$ is a semiconductor with a bandgap of $E_{\text{g}}\sim 24$ meV; whereas, resistivity and Hall measurements indicate that CeRhC${}_2$ is a semimetal with a low carrier concentration of $n\sim 10^{20}$ cm${}^{-3}$. With applied hydrostatic pressure, the zero-field antiferromagnetic transition of CeRhC${}_2$ is slightly enhanced and CeRhC${}_2$ becomes notably more metallic up to 1.36 GPa. The trend toward metallicity is in line with density-functional calculations that indicate that both LaRhC${}_2$ and CeRhC${}_2$ are semimetals, but the band overlap is larger for CeRhC${}_2$, which has a smaller unit cell volume that its La counterpart. This suggests that the bandgap closes due to a lattice contraction when replacing La with Ce in RRhC${}_2$ (R = rare-earth), in agreement with experimental results.