  Electrons in condensed matter have internal degrees of freedom, such as
charge, spin and orbital, leading to various forms of ordered states through
phase transitions. However, in individual materials, a charge/spin/orbital
ordered state of the lowest temperature is normally uniquely determined in
terms of the lowest-energy state, i.e., the ground state. Here, we summarize
recent results showing that under rapid cooling, this principle does not
necessarily hold, and thus, the cooling rate is a control parameter of the
lowest-temperature state beyond the framework of the thermo-equilibrium phase
diagram. Although the cooling rate utilized in low-temperature experiments is
typically 2*10^-3 - 4*10^-1 K/s, the use of optical/electronic pulses
facilitate rapid cooling, such as 10^2-10^3 K/s. Such an unconventionally high
cooling rate allows some systems to kinetically avoid a first-order phase
transition, resulting in a quenched charge/spin state that differs from the
ground state. We also demonstrate that quenched states can be exploited as a
non-volatile state variable when designing phase-change memory functions. The
present findings suggest that rapid cooling is useful for exploring and
controlling the metastable electronic/magnetic state that is potentially hidden
behind the ground state.
