The Free University is located in Dahlem, which is in the south-west of Berlin. Coming by public transport, please take the subway U3 to the station "Dahlem-Dorf", from where it is a 3-minute walk to the Department of Physics (see street map below).
If you arrive at Tegel Airport, you would need to buy a single ticket Berlin AB and from Schönefeld Airport a single ticket Berlin ABC to get here by public transport.
On the following website of Berlin's public transport, you can plan your journey: www.bvg.de/en.
Research Group of Martin Weinelt
Department of Physics, FU Berlin