A remarkable metal-insulator transition has been observed, for the first time, in hole-doped spin ladder compound La2Cu2O5 (4-leg) single crystals. Hole-doping was carried out by means of high-oxygen-pressure annealing at 600°C under a pressure of 400 atm (20%O2+80%Ar at a total pressure of 2000 atm) using a hot-isostatic-pressing (HIP) furnace. In order to realize metallicity, a prolonged HIP-treatment for about 22 days or more is required. However, this annealing period is reduced to about 5-6 days, when the crystals are grown from the starting charges containing Ag in the form of Ag2O. Even in the as-grown state, the measured room-temperature resistivity is about two to three orders of magnitude lower than that of La2Cu2O5 Crystals grown from charges without silver. The results show that the silver addition facilitates the oxygenation of the La2Cu2O5 crystals.
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