Structural Transition in the Hidden Ordered Phase of CeCoSi

Abstract

We have performed X-ray diffraction experiments on a single crystalline\nCeCoSi to investigate the unresolved ordered phase below $T_0 \\sim 12$ K. We\nhave discovered that a triclinic lattice distortion takes place below $T_0$,\nwhich is further modified in the subsequent antiferromagnetic ordered phase.\nThe structural domains can be selected by applying a magnetic field, indicating\nthat some electronic ordering exists behind and affects the magnetic anisotropy\nin the hidden ordered phase below $T_0$. The transition at $T_0$, although the\norder parameter is still unknown, is associated with the maximum in the\n$c$-axis lattice parameter. In magnetic fields along $[1, 0, 0]$, the\nstructural transition temperature, named as $T_{\\text{s1}}$, deviates from\n$T_0$ and decreases with increasing the field, whereas $T_0$ increases. This\nshows that the hidden ordered phase without triclinic distortion exists between\n$T_{\\text{s1}}$ and $T_0$. The results for $H \\parallel [1, 1, 0]$ are also\nreported.\n