Yes. It's kind of a "double-or-nothing" casino game.
The yield is determined by the free market, it's the amount that Greece has to (promise to) pay for anyone to take them. It's not determined by a formula afaik, just based on current events and the state of the country's finances.
I don't think you can invest (or rather, speculate, this is too crazy to call investing) in these as normal retail investor though.
To be clear, the yield is determined by the free market as the price they pay for the bond. If the bond was originally priced at $100, with a 1% coupon payment (the amount the government or bond issuer will pay you in interest), if the resale value goes down to $50 in the open market, then the yield has doubled to 2%, since a new owner of the bond only had to pay $50 to get the same $1 coupon payment while holding the bond.
Regardless of whether the bond has a coupon payment or not the face value of the bond is due at maturity. In your example the $1 coupon is still paid but in two years you also get $100 back on your $50 purchase (netting 20+%apy).
Of course if the government defaults on your bond you get maybe on $1 coupon and end with a net loss of $49.
The yield is determined by the free market, it's the amount that Greece has to (promise to) pay for anyone to take them. It's not determined by a formula afaik, just based on current events and the state of the country's finances.
I don't think you can invest (or rather, speculate, this is too crazy to call investing) in these as normal retail investor though.