Russia's ruble is the strongest currency in the world this year

Even as Russia marks a historic default on its debt, the nation's currency is gaining strength. The ruble hit a new high against the dollar this week, continuing its streak as the best-performing currency in the world this year.

Three months after the ruble's value fell to less than a U.S. penny amid the toughest economic sanctions imposed on a country in modern history, Russia's currency has mounted a stunning turnaround. The ruble has jumped 45% against the dollar since January. 

"It's an unusual situation," said Jeffrey Frankel, professor of capital formation and growth at the Harvard Kennedy School.