  We report a high-pressure study of simultaneous low-temperature electrical
resistivity and Hall effect measurements on high quality single-crystalline
KFe2As2 using designer diamond anvil cell techniques with applied pressures up
to 33 GPa. In the low pressure regime, we show that the superconducting
transition temperature T_c finds a maximum onset value of 7 K near 2 GPa, in
contrast to previous reports that find a minimum T_c and reversal of pressure
dependence at this pressure. Upon applying higher pressures, this T_c is
diminished until a sudden drastic enhancement occurs coincident with a
first-order structural phase transition into a collapsed tetragonal phase. The
appearance of a distinct superconducting phase above 13 GPa is also accompanied
by a sudden reversal of dominant charge carrier sign, from hole- to
electron-like, which agrees with our band calculations predicting the emergence
of an electron pocket and diminishment of hole pockets upon Fermi surface
reconstruction. Our results suggest the high-temperature superconducting phase
in KFe2As2 is substantially enhanced by the presence of nested electron and
hole pockets, providing the key ingredient of high-T_c superconductivity in
iron pnictide superconductors.
