Larmor diffraction study of the thermal expansion of URu2Si2 under high pressure

The metallic rare-earth system URu2Si2 shows two transitions at zero pressure: at T0∼17.5 K a strong peak in the heat capacity indicates the transition to an unknown so-called hidden order (HO) state; the HO state might be relevant to explain the occurance of a superconducting state at Ts∼1.4 K. A key to the HO state might be the large-moment AF state, to which the HO state is transformed under pressure. The pressure dependence of T0 is surprisingly weak across the pressure induced phase change and it is interesting to follow the pressure evolution of thermodynamic quantities. We have studied the pressure evolution of the thermal expansion from zero pressure to 20 kbar and in a wide temperature range down to 3 K. Larmor diffraction experiments have been performed on a high quality single crystal of stoichiometric URu2Si2. In particular, we present in detail the pressure evolution of the signature of T0 in the thermal expansion of the a- and c-axis. Here, Larmor diffraction proves to be a very reliable technique for high precision measurements of the lattice constants. (orig.)