  By making periodic thru-holes in a suspended film, the phonon system can be
modified. Motivated by the BCS theory, the technique -- so-called phonon
engineering -- was applied to a metallic niobium sheet. It was found that its
electrical resistance dropped to zero at 175 K, and the zero-resistance state
persisted up to 290 K in the subsequent warming process. Despite the initial
motivation, neither these high transition temperatures nor the phase transition
with thermal hysteresis can be accounted for by the BCS theory. Therefore, we
abandon the BCS theory. Instead, it turns out that the metallic holey sheet is
partly oxidized to form a niobium-oxygen square lattice, which has points of
resemblance to a copper-oxygen plane, the fundamental component of cuprate
high-$T_{c}$ superconductors. Therefore, the pairing mechanism underlying this
study should be related to that of cuprate high-$T_{c}$ superconductors, which
we may not yet understand. In addition to the electrical results of zero
resistance, the holey sheet exhibited a decrease in magnetization upon cooling,
i.e., the Meissner effect. Moreover, the remnant magnetization was clearly
detected at 300 K, which can only be attributed to persistent currents flowing
in a superconducting sample. Thus, this study meets the established criteria
for a conclusive demonstration of true superconductivity. Finally, the
superconducting transition with the unambiguous thermal hysteresis is
discussed. According to Halperin, Lubensky, and Ma, or HLM for short, any
superconducting transition must always be first order with thermal hysteresis
because of the intrinsic fluctuating magnetic field. The HLM theory is very
compatible with the highly oriented system harboring two-dimensional
superconductivity.
