I don't know the exact details of your issue, or if you've already handled everything, but I've had my own experiences with difficulties in interpersonal relationships in the workplace. One thing you can do is "address the issue". Sometimes taking the other person aside and discussing the issue can be the best solution, as hard as that is. As far as tension between co-workers though, time seems to be the best medicine. Anytime there is an overpowering tension at work because of something (someone went to HR, someone got fired, someone's in danger of getting fired, someone's upset with someone else), I've noticed that most of the major tension that makes your job suck is gone after about a month. Hope this helps.