Letter

1. Institute for Solid State Research, IFW-Dresden, PO Box 270116, 01171 Dresden, Germany
   
2. Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany
   
3. Institute of Metal Physics of National Academy of Sciences of Ukraine, 03142 Kyiv, Ukraine
   
4. Department of Physics, Loughborough University, Loughborough, LE11 3TU, UK
   
5. Elektronenspeicherring BESSY II, Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, 12489 Berlin, Germany

Correspondence to: S. V. Borisenko¹ Correspondence and requests for materials should be addressed to S.V.B. (Email: s.borisenko@ifw-dresden.de).

The distribution of valence electrons in metals usually follows the symmetry of the underlying ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors^{1, 2, 3}, and are often considered to be essential for superconductivity to exist⁴. Recent measurements^{5, 6, 7, 8, 9} seem to show that the properties of the iron pnictides^{10, 11} are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials^{12, 13, 14, 15}. Here we report that the electronic structure of Ba_1-xK_xFe₂As₂ is in sharp disagreement with those band structure calculations^{12, 13, 14, 15}, and instead reveals a reconstruction characterized by a (, ) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K).