Many Body Localisation in CeMnAsO_1-xF_x?

Transition metal oxypnictides are known to exhibit a range of exotic phenomena. For example, high temperature superconductivity in LnFeAsO_1-xF_x and colossal magnetoresistance in LnMnAsO_1-xF_x (Ln = Nd, Pr), where the resistivity reduces by 95% at 4 K in a 7 T field. We have been investigating CeMnAsO_1-<i>x</i>F<i>_x</i> (<i>x</i> = 0 – 0.075) and here we show how the electronic properties change with F^- doping. CeMnAsO is a Mott insulator. Upon electron doping, via substitution of O^2- with F^-, an unusual insulator – insulator transition is observed for <i>x</i> ≥ 0.035. The resistivity increases by more than two orders of magnitude over a 2 K temperature range and a superinsulating state is achieved below the transition temperature T_II. The superinsulating trnasition temperature can be tuned by increasing <i>x </i>in CeMnAsO_1-<i>x</i>F<i>_x</i>. Variable temperature synchrotron and neutron diffraction studies confirm that there is no change in the crystal or magnetic structure at T_II. Results from AC transport and Hall measurements suggest this transition could be the first observation of many body localisation (MBL) in the solid state. The most significant characteristic of MBL systems is that below a transition temperature they become perfect insulators, exhibiting zero electronic conductivity. The MBL phase also acts as a quantum memory and can be used to protect quantum memory allowing the tantalising possibility of performing topological quantum computation at finite temperatures.