Colloquium: Hidden Order, Superconductivity, and Magnetism -- The   Unsolved Case of URu2Si2
  This Colloquium reviews the 25 year quest for understanding the continuous (second-order) mean-field-like phase transition occurring at 17.5 K in URu2Si2. About ten years ago, the term hidden order (HO) was coined and has since been utilized to describe the unknown ordered state, whose origin cannot be disclosed by conventional solid-state probes, such as x rays, neutrons, or muons. HO is able to support superconductivity at lower temperatures (Tc ~ 1.5 K), and when magnetism is developed with increasing pressure both the HO and the superconductivity are destroyed. Other ways of probing the HO are via Rh-doping and very large magnetic fields. During the last few years a variety of advanced techniques have been tested to probe the HO state and their attempts will be summarized. A digest of recent theoretical developments is also included. It is the objective of this Colloquium to shed additional light on the HO state and its associated phases in other materials. 
J. A. Mydosh and P. M. Oppeneer
cond-mat.str-el cond-mat.supr-con