Iron-based superconductors contain layers of iron and a pnictogen – such as arsenic or phosphorus – or a chalcogen. This is currently the family with the second highest critical temperature, behind the cuprates. Interest in their superconducting properties began in 2006 with the discovery of superconductivity in LaFePO at 4 K (−269.15 °C) and gained much greater attention in 2008 after the analogous material LaFeAs(O,F) was found to superconduct at up to 43 K (−230.2 °C) under pressure. The highest critical temperatures in the iron-based superconductor family exist in thin films of FeSe, where a critical temperature in excess of 100 K (−173 °C) was reported in 2014.