Novel States of Electronic Matter
Strong electronic correlations can cause the emergence of entirely different states of electronic matter. A long-standing mystery in this area has been the nature of the so-called 'hidden order' phase in the heavy-fermion compound URu2Si2. The onset of the hidden order phase is signalled by a large peak in the specific heat. Assuming antiferromagnetic order the ordered moment is expected to be 0.4 B per U atom. However, only a sample dependent tiny average moment of less then 0.04 B per U atom has been found in experiment. If tuned by hydrostatic pressure, URu2Si2 maintains its peak in the specific heat but develops the expected size of magnetic order.
Using the technique of neutron Larmor diffraction we have not only mapped out the phase diagram of URu2Si2 (figure), but shown that any residual magnetic moment in the hidden-order phase can be understood to arise from local lattice distortions. This reinforces the notion that the hidden-order state is a much more exotic new state of electonic matter.
There are other compounds with recently discovered exotic states of electonic matter, whose nature is already better understood. One famous examples is the transition-metal compound MnSi, for whose A-phase there is now strong evidence that this can be understood as a Skyrmion phase with spins twisted in three dimensions.
 PG Niklowitz, C Pfleiderer, T Keller, M. Vojta, Y-K Huang, JA Mydosh, ”Parasitic small-moment antiferromagnetism and nonlinear coupling of hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction”, Phys. Rev. Lett. 104 (2010) 106406
 A Neubauer, C Pfleiderer, B Binz, A Rosch, R Ritz, PG Niklowitz, P Böni, ”Topological Hall Effect in the A Phase of MnSi”, Phys. Rev. Lett. 102 (2009) 186602