15-05-09 Super-efficient Transistor Material Predicted

New work by condensed-matter theorists at the Stanford Institute for Materials and Energy Science at SLAC National Accelerator Laboratory points to a material that could one day be used to make faster,more efficient computer processors.In a paper published online Sunday in Nature Physics,SIMES researchers Xiao-Liang Qi and Shou-Cheng Zhang,with colleagues from the Chinese Academy of Sciences and Tsinghua University in Beijing,predict that a room temperature material will exhibit the quantum spin Hall effect.In this exotic state of matter,electrons flow without dissipating heat,meaning a transistor made of the material would be drastically more efficient than anything available today.This effect was previously thought to occur only at extremely low temperatures.Now the race is on to confirm the room-temperature prediction experimentally.Zhang has been one of the leading physicists working on the quantum spin Hall effect;in 2006 he predicted its existence in mercury telluride,which experimentalists confirmed a year later.However,the mercury telluride had to be cooled by liquid helium to a frigid 30 millikelvins,much too cold for real-world applications.In their hunt for a material that exhibited the quantum spin Hall effect,Zhang and Qi knew they were looking for a solid with a highly unusual energy landscape.In a normal semiconductor,the outermost electrons of an atom prefer to stay in the valence band,where they are orbiting atoms,rather than the higher-energy conduction band,where they move freely through the material.Think of the conduction band as a flat plain pitted with small valence-band valleys.Electrons naturally "roll" down into these valleys and stay there,unless pushed out.But in a material that exhibits the quantum spin Hall effect,this picture inverts; the valence-band valleys rise to become hills,and the electrons roll down to roam the now lower-energy conduction band plain.In mercury telluride,this inversion did occur,but just barely;the hills were so slight that a tiny amount of energy was enough to push the electrons back up,meaning the material had to be kept extremely cold.When Zhang,Qi and their colleagues calculated this energy landscape for four promising materials,three showed the hoped-for inversion. In one,bismuth selenide,the theoretical conduction band plain is so much lower than the valence band hills that even room temperature energy can't push the electrons back up.In physics terms, the conduction band and valence band are now inverted,with a sizeable difference between them