Colloquium: Hidden order, superconductivity, and magnetism: The unsolved case of URu2Si2

This Colloquium reviews the 25 year quest to understand 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. The HO is able to support superconductivity at lower temperatures (T-c approximate to 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 large magnetic fields. During the last few years a variety of advanced techniques have been tested to probe the HO state and these 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. DOI: 10.1103/RevModPhys.83.1301 PACS numbers: 71.27.+a, 74.70.Tx, 75.30.Mb