X-ray backscattering study of crystal lattice distortion in hidden order of URu2Si2

We performed synchrotron X-ray diffraction measurements on a small single crystal of [GRAPHICS] with approximate dimensions of [GRAPHICS] . A backscattering diffraction set-up (2 [GRAPHICS] ) in combination with a Si(660) monochromator ensures a high instrumental resolution of [GRAPHICS] , where [GRAPHICS] denotes a lattice-plane spacing and [GRAPHICS] is its spread expressed as the full width at half maximum (FWHM). We carefully measured the (550) Bragg peak in the temperature range between 5 and 25K, and found it to show no signature of broadening and splitting when "Hidden Order (HO)" sets in below 17.5K. Temperature variation of the peak position closely follows the behaviour of the known linear thermal expansion coefficient. Detailed analysis of the peak width reveals that the tetragonal fourfold rotational symmetry of [GRAPHICS] is conserved in HO within the experimental accuracy of the orthorhombicity of [GRAPHICS] . We also found that the (550) peak profile exhibits a temperature-independent fine structure, which indicates that the tetragonal [GRAPHICS] -axis lattice parameter of the used crystal is distributed with a FWHM of [GRAPHICS] .