Economic theory / papers on sovereign debt? I am an undergraduate student who does some research on sovereign debt and its sustainability. 

Which papers are essential to understand how sovereign debt works and which implication it has for the economic policy of the country?
Carmen Reinhart from Harvard Kennedy School works extensively on these questions. She has a recent paper forthcoming  in the Journal of the European Economic Association, 2016, entitled "Sovereign Debt Relief and its Aftermath" with Christoph Trebesch. They study sovereign debt relief in a long-term perspective, using two distinct samples: 1920-1939, and 1978-2010. They published a short Vox-CEPR column on this research.

In their book, with Kenneth Rogoff, "This time is different" they address this question. They have also other interesting and good papers:


Reinhart, Carmen, and Kenneth Rogoff. "Dealing with Debt." Journal of International Economics 96 (July 2015): S43-S55. 
Reinhart, Carmen, and Kenneth Rogoff. "Financial and Sovereign Debt
Crises: Some Lessons Learned and Those Forgotten." Financial Crises:
Causes, Consequences, and Policy Responses. Ed. Stijn Claessens, M.
Ayhan Kose, Luc Laeven, and Fabian Valencia. International Monetary
Fund, 2014.


Check also:


Reinhart, Carmen M., and M. Belen Sbrancia. "The Liquidation of
Government Debt." Economic Policy 30.82 (March 2015): 291-333.