[1] Department of Physics and Astronomy,
[2] University of North Carolina at Chapel Hill,undefined
[3] Chapel Hill,undefined
[4] NC 27599,undefined
[5] USA,undefined
[6] International Centre for Theoretical Physics,undefined
[7] P.O. Box 586,undefined
[8] Trieste 34100,undefined
[9] Italy,undefined
[10] National Laboratory of Solid State Microstructures,undefined
[11] Nanjing University,undefined
[12] Nanjing 210093,undefined
[13] P.R. China,undefined