Small Aircraft; Low Wing; 1 Eng; Retractable Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 3.9
Situational Awareness
Airspace Violation All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Human Factors
Aircraft X called airborne looking to pick up his flight plan.  I gave him a code and received a track and proceeded to clear him.  I then gave him the altimeter and thought nothing of it.  My d-side and I then realized that he was actually below the Minimum IFR Altitude (MIA) and that I had messed up.  At this point he was crossing into an area of lower MIAs and I felt that the damage had already been done and that there was no need to climb the aircraft.  This was a complete lack of focus on my part.  I should have noticed this sooner and cleared him to a higher altitude when issuing the clearance.  This is all my fault.  I would like to point out that in my opinion; the lack of traffic and complexity played a part in this.  We were chatting and just not engaged enough.Focus on combining sectors more to keep work force engaged.  I am not sure if anything could have been combined up or not in this instance.  I am commonly watching a small number of aircraft and losing focus on the sector because I have way too much time on my hands between transmissions.
ZDV Controller reports of discovering an aircraft that he was working; was below the MIA.