An Ultra-Low-Temperature Diffractometer Based on an 3He–4He Dilution Refrigerator used for Synchrotron-Radiation X-ray Diffractometry and Topography

Abstract

The direct observation of crystal structure at ultra-low temperatures helps better understanding of phase transformations and phase stability. Two sets of 3He-4He dilution refrigerators with a top-loading facility were installed at BL-3C2 and 6C1 of the Photon Factory. The first is used exclusively for topography of quantum crystals of solid 3He and 4He for studies of lattice defects. Typical examples of topographs that indicate an annealing effect of solid helium, understood in terms of a quantum tunneling, are presented. The disappearance of downward diffraction Laue spots (2θB = 90°) is discussed on the basis of the temperature and other factors in solid helium. The second refrigerator is used for X-ray diffraction studies of crystalline solids. A splitting of the Laue spots from Cs2NaHoCl6 below 150 mK, giving evidence of a cooperative Jahn–Teller transition of the first kind at the lowest transition temperature ever observed, is briefly explained as an example of diffractometry.