When banks and other lending agencies give up on a sovereign debt, they sometimes sell these debts to private debt holders often referred to as vulture funds.
Vultures "invest" in the sovereign debt of countries facing crisis – meaning they can buy debt cheap. They then "hold out" against any form of write-down on this debt. By doing this, they hope to get paid out in full. Given they paid a fraction of the value of the debt, getting full repayment represents an enormous profit.Vulture fund companies have a variety of methods, not always successful, of forcing payment from countries. Read all about it here.