Very often the order parameter which characterized an electronic order can be accessed by diffraction and spectroscopic techniques such as optical, electron and X-ray scattering which enable the identification of the broken symmetry. In many correlated electron systems however, the situation is not as simple. Because competing electronic orders involving different symmetry breaking and order parameters can exist in close proximity, a rich diversity of possible ground states can be found. The most mysterious examples are the exotic order states whose order parameters are still unknown. A famous case is certainly the hidden order state of URu2Si2 which remains a mystery after 30 years of intense experimental and theoretical studies. Other examples have emerged, like the electronic order associated with the metal to insulator transition in the skutterudite PrRu4P12.
URu2Si2 and its Hidden Order :
After 30 years of research, the Hidden Order of URu2Si2 remains one of the most complex challenges of condensed matter. URu2Si2 presents an exotic order at T0=18K whose order parameter is still not identified and so named “Hidden Order”. This order is easily detected by many bulk measurements such as specific heat and resistivity. But it doesn’t show any clear microscopic static signal for probes like neutrons or x-ray scattering. Far below the transition temperature of this “Hidden Order”, a superconducting state appears at Tsc =1.2 K. For pressure above a critical pressure of 0.5 GPa, the “hidden order” state is replaced by an antiferromagnetic structure and the superconducting state vanishes.
Recenlty using Raman scattering, we provided new results on the hidden order state. As shown in the figure, the hidden order os characterized by the emergence of a peak at 14cm-1 and a gap below 55cm-1 in the pure A2g symmetry. While the gap is consistent with optical conductivity measurements, we have associated the peak to the Q0 neutron resonance. Most of all, the A2g symmetry it-self is very peculiar and its observation clearly puts new strong constrain on the theoretical models.
References :
1- J. Mydosh, R and P. Oppenner, Reviews of Modern Physics, 83 (2011) 1301-1322.
2- J. Mydosh, Hidden order in URu2Si2, Philosophical Magazine 94:32-33, 3640 (2014).
3- J. Buhot, M.-A. Méasson, et al. Symmetry of the Excitations in the Hidden Order State of URu2Si2 , Phys. Rev. Lett. 113, 266405 (2014).
4- J. Buhot, M.A. Méasson, et al. Lattice dynamics of the heavy fermion compound URu2Si2, Phys. Rev. B 91, 035129 (2015).