See the data reflecting the yields to maturity on accrued principal for the 30-Year 2-1/8% Treasury Inflation-Indexed Note, Due 2/15/2041.  Treasury Inflation-Protected Securities (TIPS) have principals that increase with inflation and decrease with deflation since they are tied to the consumer price index (CPI).  When the security matures, the U.S. Treasury pays either the original or adjusted principal, whichever is greater.