  We performed high-field magnetotransport experiments up to 50 T on strained
La1.85Sr0.15CuO4 films. Two films with different strain, one expansively
strained with Tc ~ 26 K and the other compressively strained with Tc ~ 44 K,
show a striking contrast in the normal-state transport: the former has a
prominent low-temperature upturn while the latter has not. Identifying the
low-temperature upturn as due to the Kondo effect, our experimental
observations indicate that Cooper and Kondo singlet formations are competing in
high- Tc cuprates: Kondo singlet formation hinders Cooper pair formation. Based
on these results, we propose a new scenario for the electronic phase diagram of
La2-xSrxCuO4. According to our scenario, underdoped insulating La2-xSrxCuO4 can
be regarded as a "Kondo insulator".
