Affiliation: | (1) Department of Physics, North Bengal University, Siliguri, Dist. Darjeeling, 734 013, India;(2) Department of Physics, Nankai University, Tianjin, 300071, People’s Republic of China;(3) BK21 Division of Advanced Research and Education in Physics, Hanyang University, Seoul, 133-791, Korea;(4) Present address: Khoribari High School (H.S.), Khoribari, Dist. Darjeeling, 734 427, India |