Shares in memory-chip giant Samsung Electronics closed 13.4% lower, notching their worst one-day fall in almost two decades, as investors questioned lofty valuations amid concerns over AI infrastructure financing and intensifying competition from China. A Historic Day for South Korean MarketsTuesday's decline was not confined to Samsung alone, with the broader South Korean market suffering one of its worst sessions in recent memory. Trading was temporarily halted at times as the Kospi dropped to its lowest level since April, closing 10.8% lower at 6,023.66. Shares of SK Hynix tumbled 14.7%, amplifying the Seoul market decline, with the two chipmakers together accounting for nearly half of the benchmark Kospi index. The index's decline marked its biggest one-day drop since the early days of the U.S.-Iran conflict in March.