Is this defamation?

I gave at least a 2 week notice of resignation in January of 2008 i resigned from a nursing home where i was employed as a Registered Nurse. I gave a 2 week written notice and my last day with them was 02/01/09. Fast forward a few years,I applied for a job with the US federal govt and found out this month (12/2011) the nursing home has told the investigator that i was fired for negligence of patient and incorrect documentation on 02/20/2009. I was completely unaware of this. There was an incident that occurred with some nursing assistants and a patient and I wrote it up as soon as I was aware of it and gave the report to my supervisor. I asked her if there was anything else I needed to do and she said no and I heard nothing from my supervisior regarding the incident and went on to start my new job a few days later.

Can the nursing home say I was fired, when I gave notice and quit almost 2 weeks before they say I was fired?

I followed the guidelines for documentation and DID NOT negelect my patient as I was the one who ordered the xrays for the patient and contacted the patient's family and made them aware of the incident as well as the supervisor on duty that night.
I know I can omit this employer from my resume but most places do not want any gaps in employment on an application. There was no adverse actions made against my nursing license that I know of.
I would like to know if there is anything I can do so they won't report this "neglegience" to any future employers. Is this defamation? I am not sure if this has prevented me from being hired but I have been turned down for positions and my security clearance with the federal government is still pending.

Any advice would be helpful
TK