Available online at www.sciencedirect.com Physica E 22 (2004) 135–137 www.elsevier.com/locate/physe Upshift of the fractional quantum Hall plateaux: evidence for repulsive scattering for composite fermions Hong Syuan Wang a , Yi Hsing Chiu b , Gil-Ho Kim b , C.-T. Liang a ; * , M.Y. Simmons c; d , D.A. Ritchie c a Department of Physics, National Taiwan University, Taipei 106, Taiwan b Department of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon 440-746, South Korea c Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK d School of Physics, University of New South Wales, Sydney 2052, Australia Abstract It is now established that if there are an equal number of positively and negatively charged impurities, the quantum Hall plateaux in xy (B) are expected to be centred about the classical value xy = B= (en) where n is the 2D carrier density. It has also been shown that when repulsive or attractive scattering centres are deliberately introduced close to a two-dimensional electron gas (2DEG), the centres of the quantum Hall plateaux are shifted to higher or lower magnetic eld, respectively. In this paper, we present magneto transport measurements on a high-quality GaAs electron system. We extend this concept to the composite fermions where each electron is bound to two magnetic ux quanta. We provide compelling evidence that the dominant scattering for the composite fermions is dierent from that for electrons. This is supported by studying the upshift of the = 1 quantum Hall plateau within the composite fermion picture. ? 2003 Elsevier B.V. All rights reserved. PACS: 73:40 - c; 73:40 - f Keywords: Fractional quantum Hall eects; Composite fermion; Repulsive scattering The fractional quantum Hall eect (FQHE) is observed in high-mobility two-dimensional electron systems in the low-temperature, high-magnetic-eld regime [1]. It is believed that the FQHE arises from strong electron–electron interactions, causing the two-dimensional electrons to condense into a frac- tional quantum Hall liquid [2]. Jain [3] introduced the concept of “composite fermion” where each elec- tron is bound to two magnetic ux quanta, and in this picture the FQHE can then be understood as a * Corresponding author. Tel.: +886-2-23697238; fax: +886- 2-23639984. E-mail address: ctliang@phys.ntu.edu.tw (C.-T. Liang). manifestation of the integer quantum Hall eect of composite fermions. At a Landau level lling factor = 1 2 , a two-dimensional electron system can be trans- formed into a composite fermion system interacting withaChern–Simonsgaugeeld[4,5].Todate,awide variety of experiments have demonstrated that at = 1 2 the eective magnetic eld acting on the composite fermions is zero [6–13]. Away from = 1 2 , the eec- tive magnetic eld acts on the composite fermions is given by B e = B ext - B( = 1 2 ) where B ext is the ap- plied external magnetic eld. At = 1 2 , each electron is bound to two magnetic ux quanta, and thus the density of the composite fermion system is equal to that of the electron system. 1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.234