Symmetry of Hidden Ordering and Supercondutivity in URu₂Si₂

Owing to recent investigations, understanding of the symmetry of hidden order and superconductivity in URu₂Si₂ is fairly progressed. It seems that we are not far from the answer. Following issues and perspectives will be discussed.<br/><b>Hidden order</b>: Possible space groups of hidden order and corresponding multipolar order parameters are restricted on following conditions which are considered to be reasonable at present, 1) The ordering wave vector is <i>Q = (001)</i>, 2) The 4-fold local symmetry at U, Ru and Si sites is preserved . There are controversial experimental results about the time-reversal symmetry (TRS), so far. Actually, expected behavior of physical quantities for higher magnetic multipolar ordered states without usual dipolar ordering may be not straightforward. Thus, on the conditions 1) and 2), possible TRS preserved and TRS broken cases are addressed. Experimental efforts to distinguish those space groups will be discussed.<br/><b>Superconductivity</b>: Spin-singlet state is concluded based on Knight shift measurement in the superconducting state. This is consistent with the proposed chiral <i>d</i>-wave state [basis function: <i>kz(kx + iky)]</i>, which is quite likely at present. In addition, the highly anisotropic spin susceptibility χc &gt;&gt; χa is indicated as well as dHvA effect measurement. As next challenge, unified understanding of superconducting, antiferromagnetic, and hidden ordered states will be discussed.