  A 2D metallic phononic crystal (PnC), which is fabricated by drilling
periodic holes in a suspended niobium (Nb) film, is repeatedly cooled and
warmed in the temperature range of 2--300 K. During the first five temperature
cycles, the resistance of the metallic PnC gradually increases in accordance
with the Friedel sum rule, indicating that narrow Nb bridges between adjacent
thru-holes are converted into $d$-orbital-filled Mott insulators. The
consequent 2D Mott-insulating square lattice is a crystallographic analog of a
copper oxide layer in high-temperature superconductors such as YBCO and BSCCO.
Subsequent temperature cycles of the thus produced ideal Hubbard crystal
realize zero resistance at 60 K, and the zero-resistance state remains up to
300 K, being retained in the atmosphere (i.e., at room temperature, in a
terrestrial magnetic field, and under atmospheric pressure). The necessity of
the latter temperature cycles remains unclear in this study; however, a
possible transition mechanism involving the combined Josephson and charging
effects is discussed. The critical current and critical field of the
room-temperature superconductor (RTSC) are $I_{C}$ = 18.8 mA and
$\mu_{0}H_{\perp}$ = 12 T, respectively. Once a large current exceeding $I_{C}$
is applied to the RTSC, some of the $d$ electrons are forced out, and a special
interface consisting of the p-type semiconducting and superconducting states is
formed. Carriers diffuse because of the nonequilibrium charge concentration,
but they are Andreev-reflected back. In the reversible, thermally isolated
system, the entropy never increases, and a simple experiment confirms a
low-loss current generated from the interface spontaneously. That is, this
study challenges the forbidden perpetual motion machine which can be a solution
to the World energy crisis.
