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Larmor diffraction from antiferromagnetic and hidden order in URu2Si2

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Following the border of antiferrmagnetism to zero temperature often turned out to be a promising route to unconventional metallic and superconducting phases. Even the simple d-electron system NiS2 revealed an extended non-Fermi-liquid regime dominated by spin-fluctuations above the quantum phase transition.[1] In the latter case, measurements of the electrical resistivity measurements were done in the framework of the clean and highly controlled technique of pressure tuning. The range of further quantitative experimental probes, which can be realised in a high-pressure environment is limited. However, advances have recently been made in neutron scattering, where elliptically shaped neutron guides can now increase the beam intensity sent to mm size sample for high pressure studies.[2] Neutron scattering also allows highly accurate measurements of the thermal expansion via the Larmor diffraction technique, which proved extremely useful in studying the high-pressure phase diagram of the itinerant helimagnet MnSi.[3] We now combined Larmor diffraction with conventional diffraction measurements to investigate the pressure-temperature phase diagram of URu2Si2 up to 20 kbar. URu2Si2 offers a further spectacular example for the presence of unconventional phases in the vicinity of antiferromagnetism. In this compound, antiferromagnetism (AF) is replaced below approximately 5 kbar by the mysterious “hidden order” (HO) and unconventional superconductivity. Our measurements allow the simultaneous observation of magnetic order and changes in the a- and c-axis lattice constants across the phase transitions. The results contain clear indications of a first-order transition and strong differences between the AF phase and the HO phase in the coupling to the lattice, questioning simple models of coupled order parameters.

[1] P. G. Niklowitz et al.; Phys. Rev. B 77 , 115135 (2008) [2] S. Mühlbauer, P. G. Niklowitz, et al.; Nucl. Inst & Methods in Phys. Res. A 586 , 77 (2008) [3] C. Pfleiderer, P. Böni, T. Keller, U. K. Rößler, and A. Rosch; Science 316, 1871 (2007)

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