  The effects of adsorption of Zn-phthalocyanine (ZnPc) molecules on the
superconductivity of the Si(111)-(root7xroot3)-In surface are studied through
transport measurements under ultrahigh vacuum environment. The ZnPc molecules
are found to increase the transition temperature Tc by 11% at maximum, which is
about 2.7 times the Tc increase previously reported using CuPc. By contrast,
angle-resolved photoemission spectroscopy measurements and ab initio
calculations show that charge transfer from the In atomic layers to ZnPc is
substantially smaller than that to CuPc. This clearly shows that charge
transfer should be excluded as the origin of the increase in Tc. The push-back
effect induced by physical adsorption of molecules is discussed as a possible
mechanism for the Tc enhancement.
