Strong and nonmonotonic temperature dependence of Hall coefficient in superconducting K$_x$Fe$_{2-y}$Se$_2$ single crystals

摘要

In-plane resistivity, magnetoresistance and Hall effect measurements havebeen conducted on quenched K$_x$Fe$_{2-y}$Se$_2$ single crystals in order toanalysis the normal-state transport properties. It is found that the Kohler'srule is well obeyed below about 80 K, but clearly violated above 80 K.Measurements of the Hall coefficient reveal a strong but non-monotonictemperature dependence with a maximum at about 80 K, in contrast to any otherFeAs-based superconductors. With the two-band model analysis on the Hallcoefficient, we conclude that a gap may open below 65 K. The data above 65 Kare interpreted as a temperature induced crossover from a metallic state at alow temperature to an orbital-selective Mott phase at a high temperature. Thisis consistent with the recent data of angle resolved photoemissionspectroscopy. These results call for a refined theoretical understanding,especially when the hole pockets are absent or become trivial inK$_x$Fe$_{2-y}$Se$_2$ superconductors.