Charge Density Wave Order in the Organic Metal α-(ET)₂ KHg(SCN)₄

The family of crystalline organic metals α-(ET)₂ XHg(SCN)₄, with X=K, NH₄, Tl, or Rb are of interest because three of the compounds have a coexisting charge density wave (CDW) and metallic phase at low temperatures, while one (X = NH₄) has a superconducting ground state. They are all triclinic crystals and have a quarter filled conduction band. To find the nature of the density wave state we studied α-(ET)₂ KHg(SCN)₄ using elastic x-ray scattering at the Advanced Photon Source at Argonne National Laboratory. We will present preliminary scattering data at 4 K to search for incommensurate CDW, below the thermodynamic phase transition at 8K, and contrast it with data at 104 K. We encountered serious challenges of radiation damage to these organic materials and employed a mitigation scheme by using 18 keV x-rays with reduced incident flux for extended period of time. There seems to be a weak broadening of Bragg peaks below 8 K, which would indicate some structural anomaly at this temperature.