A power of attorney expires when the person who created it, the principal, dies. Attempting to cash a bond using a power of attorney after the principal is deceased is illegal. Although you may succeed in cashing the bond if the principal's death is not yet public knowledge, the consequences for this improper behavior can be serious.
Power of Attorney
A power of attorney, or POA, is a legal document. The person making the POA -- the principal -- uses the POA to appoint an agent to act for her if she is unable to act for herself. The authority granted the agent can be general or very specific, and the agent can do no more than is outlined in the document.
A principal can terminate the POA at any time while she is competent. A general POA terminates automatically if the principal becomes incompetent, while a durable POA does not. All POAs, however, terminate upon the death of the principal.
Possible Criminal Penalties
The agent acting under a POA is a trusted fiduciary, held to the highest duty of care recognized under the law. An attempt to act under a power of attorney after the principal's death is a blatant misuse of authority, regardless of your motivation. Violation of your duties under a POA is a crime and also can lead to civil penalties.