FQHE in Cd1-xMnxTe: Composite Fermions in a diluted magnetic semiconductor

PAULA GIUDICI

Zurich

Conferencia; 31st. International Conference on Physics of Semiconductors; 2012

Since the discovery of the fractional Quantum Hall eect (FQHE)nearly 30 years ago experimental research was concentrated on inves-tigating fractional states in nonmagnetic materials. Typically GaAs-based systems have been studied where the small Zeeman energy EZcauses several fractional Quantum Hall (FQH) ground states to be spinunpolarized. More recently, the FQHE was reported in a CdTe quan-tum well (QW) [1] - a systems exhibiting spin-polarized FQH statesdue to their large g-factors.Magnetic manganese ions can easily be incorporated into CdTebased quantum structures, hence making this material system ideallysuited for the search of fractional states in Diluted Magnetic Semi-conductors (DMS). Cd1-xMnxTe is one of the most thoroughly in-vestigated members of the class with all its spin dependent propertiesbeing strongly enhanced due to exchange interaction between mobilecarriers and localized magnetic moments (s-d exchange). Therefore,the Zeeman splitting of electronic states can not only be gigantic butmay also be engineered in magnitude, sign and eld dependence.Here we report on the observation of FQH states in the lowestLandau level (LL) of a 2DEG formed in a high quality 30nm wideCd1􀀀xMnxTe quantum well. Our results present the rst experimen-tal observation of the FQHE in a diluted magnetic semiconductor ever.It thus demonstrates that the incorporation of magnetic ions into aquasi two-dimensional electron system does not inhibit the formationof FQH states.Standard magnetotransport experiments were performed up to 19Tin a tilted magnetic eld and at temperatures between 15mK and 1 K.Due to the presence of Mn impurities the Zeeman energy is extremelyhigh at low magnetic elds but is reduced when increasing the externaleld. We have designed our sample by adjusting the Mn concentrationsuch that the Zeeman splitting vanishes at about 13.5 T. This gave usthe unique opportunity to investigate spin eects in QH states emerg-ing at these elds. By measuring activation energies at tilted magneticelds we obtained the dependence on Zeeman energy of the excitationgaps at lling factors = 4/3, 5/3, 7/5 and 8/5. A simple modelwithin the framework of Composite Fermions (CFs) [2] provides anexcellent t of the gaps as a function of the total magnetic eld if theexchange interaction between CFs and Mn-spins is taken into account.Our analysis suggests that, similar to electrons in a DMS, CompositeFermions are subject to s-d exchange interaction with Mn-spins. Ananalysis of the extracted CF masses mCF in the vicinity of = 3/2will be discussed, as well as a quantitative comparison between modeland experiment considering deviations coming from disorder eects orLL broadening [3].[1] B. A. Piot et al., Phys. Rev. B 82, 081307 (2010)[2] J. K. Jain, Phys. Rev. Lett. 63, 199 (1989)[3] C. R. Dean et al., Phys. Rev. Lett. 100, 146803 (2008); R. R.Du et al., Phys. Rev. Lett. 70, 2944 (1993)