Variation of electronic properties and evidence for a reversible defect induced metal to semiconductor transition in PbMo6S8 observed by ion irradiation and subsequent annealing of thin films

The influence of lattice disorder varied by low temperature irradiation with 20 MeV³²S-ions and subsequent isochronal thermal annealing on Hall-effectR _H (T), resistivityp(T) and superconducting critical temperatureT _c of thin films of the Chevrel-phase PbMo₆S₈ is reported. It is found that the well known, unusual sensitivity ofT _c is correlated with drastic changes of normal state transport properties e.g.R _H (T) andp(T). In the low fluence regime (6·10¹³ cm^–2,T _c 2 K) annealing leads to a monotonous restoration of the initial properties with the main recovery occurring at temperatures as low as 500 K. Contrary to this, annealing of highly disordered samples (10¹⁵ cm^–2) creates semiconductor-like conduction behavior. This manifests itself by a strong increase of the electrical resistivity with decreasing temperature which becomes more pronounced at higher annealing temperaturesT _A . AfterT _A =800 K the resistivityp(15K) is enhanced by more than a factor of 1000 withp(15K)/p(280 K)=210 compared to as irradiated. Further annealing at 900 K and 1000K results in the reappearance of metallicp(T)-behavior and superconductivity (T _c >10K). The observed effects can be understood by systematic changes of the electronic density of states consistent with an earlier proposed defect model.