  Unconventional symmetry-breaking phenomena due to nontrivial order parameters
attract increasing attention in strongly correlated electron systems. Here, we
predict theoretically the occurrence of nanoscale spontaneous spin-current,
called the spin loop-current (sLC) order, as a promising origin of the
pseudogap and electronic nematicity in cuprates. We reveal that the sLC is
driven by the odd-parity electron-hole condensation that are mediated by
transverse spin fluctuations around the pseudogap temperature $T^*$. At the
same temperature, odd-parity magnon pair condensation occurs. The sLC order is
"hidden" in that neither internal magnetic field nor charge density modulation
is induced, whereas the predicted sLC with finite wavenumber naturally gives
the Fermi arc structure. In addition, the fluctuations of sLC order work as
attractive pairing interaction between adjacent hot spots, which enlarges the
d-wave superconducting transition temperature $T_c$. The sLC state will be a
key ingredient in understanding the pseudogap, electronic nematicity as well as
superconductivity in cuprates and other strongly correlated metals.
