For temperature-dependent Seebeck coefficient measurements, the Seebeck voltage V S (I heat , T ) and the resistance of the nearby four-probe platinum thermometer R Pt (I heat , T ) were measured as function of different heating currents I heat at bath temperature T . The chip's individual thermometer calibration is given by R Pt (I heat = 0, T ). Using this calibration, V S (low potential on the cold side) can be expressed as function of the temperature difference ∆T S . For all samples V S was proportional to I 2 heat and ∆T S , as expected and shown in Fig. ). The relative Seebeck coefficient with respect to the platinum metals lines was found as S Cu-P,Pt = S Cu-P -S Pt = -∂V S /∂∆T S . For calculation of the absolute Seebeck coefficient S Cu-P , reference data of Kockert et al. was used (sample PT3). 8 Some of the low-temperature data in Fig. (b) of the main article was discarded due to increasing contact resistance that hindered accurate voltage measurements.