  The high-temperature superconducting state in cuprates appears if charge
carriers are doped into a Mott insulating parent compound. An unresolved puzzle
is the unconventional nature of the normal state above the superconducting
dome, and its connection to the superconducting instability. At weak
hole-doping, a "pseudo-gap" metal state with signatures of time-reversal
symmetry breaking is observed, which near optimal doping changes into a
"strange metal" with non-Fermi liquid properties. Qualitatively similar phase
diagrams are found in multi-orbital systems, such as pnictides, where the
unconventional metal states arise from a Hund coupling induced spin-freezing.
Here, we show that the relevant model for cuprates, the single-orbital Hubbard
model on the square lattice, can be mapped onto an effective multi-orbital
problem with strong ferromagnetic Hund coupling. The spin-freezing physics of
this multi-orbital system explains the phenomenology of cuprates, including the
pseudo-gap, the strange metal, and the d-wave superconducting instability. Our
analysis suggests that spin-freezing is the universal mechanism which controls
the properties of unconventional superconductors.
