Anything that is truly atonal and lacks any sort of centricity is preferred. A few studies confirm that true atonality in music relaxes the listener and promotes ethical and stronger learning behaviors. Don't think Eric Whitacre or Debussy, as their music does have a tonal center, think more like Gyorgi Ligeti. Also, make sure it's not an "agitating" atonal piece like Threnody for the victims of Hiroshima (Penderecki)... I recommend Ligeti's Lux Aeterna. Or anything Ligeti for that matter, it's all gorgeous.
Make sure you're not listening to things with backbeats or lyrics in any language, as that can and will distract you. This is also a question that is highly depending on a few factors including your own study habits and what you are studying, so take it with a grain of salt!