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A 2D metallic phononic crystal (PnC), which is fabricated by drillingperiodic holes in a suspended niobium (Nb) film, is repeatedly cooled andwarmed in the temperature range of 2--300 K. During the first five temperaturecycles, the resistance of the metallic PnC gradually increases in accordancewith the Friedel sum rule, indicating that narrow Nb bridges between adjacentthru-holes are converted into $d$-orbital-filled Mott insulators. Theconsequent 2D Mott-insulating square lattice is a crystallographic analog of acopper oxide layer in high-temperature superconductors such as YBCO and BSCCO.Subsequent temperature cycles of the thus produced ideal Hubbard crystalrealize zero resistance at 60 K, and the zero-resistance state remains up to300 K, being retained in the atmosphere (i.e., at room temperature, in aterrestrial magnetic field, and under atmospheric pressure). The necessity ofthe latter temperature cycles remains unclear in this study; however, apossible transition mechanism involving the combined Josephson and chargingeffects is discussed. The critical current and critical field of theroom-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 specialinterface consisting of the p-type semiconducting and superconducting states isformed. Carriers diffuse because of the nonequilibrium charge concentration,but they are Andreev-reflected back. In the reversible, thermally isolatedsystem, the entropy never increases, and a simple experiment confirms alow-loss current generated from the interface spontaneously. That is, thisstudy challenges the forbidden perpetual motion machine which can be a solutionto the World energy crisis.

Room-temperature superconductivity in an artificial 2D Mott-insulating  square lattice and its advanced condensed phase that generates a low-loss  current in the atmosphere: A possible perpetual motion machine