Top 20 US stock transactions: Nvidia receives anti-monopoly investigation summons, stock price drops 9.5%. On Tuesday, chip giant Nvidia, ranked first in US stock transactions, closed down 9.53%, falling below its 100 day moving average and trading at $51.3 billion. On Tuesday, it was reported that the antitrust investigation against Nvidia has escalated and the company has received a subpoena from the US Department of Justice. The US Department of Justice, which is collecting evidence of Nvidia's violation of antitrust laws, has issued subpoenas to the chipmaker and other companies, upgrading its investigation into the tech giant that dominates the field of artificial intelligence computing.