Spin-transistor designs relying on spin-orbit interaction suffer from low signal levels resulting from low spin-injection efficiency and fast spin decay. Here, we present an alternative approach in which spin information is protected by propagating this information adiabatically. We demonstrate the validity of our approach in a cadmium manganese telluride diluted magnetic semiconductor quantum well structure in which efficient spin transport is observed over device distances of 50 micrometers. The device is turned "off" by introducing diabatic Landau-Zener transitions that lead to a backscattering of spins, which are controlled by a combination of a helical and a homogeneous magnetic field. In contrast to other spin-transistor designs, we find that our concept is tolerant against disorder.

We report on optical properties of CdTe self-assembled quantum dots ͑SADs͒ grown by molecular beam epitaxy on ZnTe. Formation of SADs was achieved by deposition of 1.5-2.5 monolayers of CdTe at a substrate temperature of 420°C and by applying growth interrupts for few seconds in Cd flux. The resulting dots have a typical diameter of 2 nm and a sheet density of 10 12 cm Ϫ2 . At Tϭ2 K the photoluminescence ͑PL͒ spectra consist of two emission lines. The high-energy line originates from excitonic recombination in a wetting layer while the low-energy emission PL band is assigned to recombination in SADs. The increase in temperature up to 70 K does not affect the SADs-related emission intensity. It shifts, however, the PL peak energy towards low energies and causes a significant narrowing of the PL linewidth, from 80 meV at 1.9 K to 50 meV at 130 K. The activation energy of the thermal quenching of SADs-related PL emission was found to be equal to 47 meV. This value is three times greater than the one observed in CdTe/ZnTe quantum wells, that is, 12-17 meV.

We report the observation of the fractional quantum Hall effect in the lowest Landau level of a two-dimensional electron system (2DES), residing in the diluted magnetic semiconductor Cd 1−x Mn x Te. The presence of magnetic impurities results in a giant Zeeman splitting leading to an unusual ordering of composite fermion Landau levels. In experiment, this results in an unconventional opening and closing of fractional gaps around the filling factor ν = 3/2 as a function of an in-plane magnetic field, i.e., of the Zeeman energy. By including the s-d exchange energy into the composite Landau level spectrum the opening and closing of the gap at filling factor 5/3 can be modeled quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES provides a means to manipulate fractional states. The fractional quantum Hall effect (FQHE) is a collective high-magnetic field phenomenon, originating from Coulomb repulsion of electrons confined in two dimensions. At certain fractional fillings, ν = p/q, of the Landau levels (LLs) (ν = filling factor, p,q = integers), quantized plateaus in the Hall resistance ρ xy and the vanishing longitudinal resistance ρ xx herald the presence of peculiar electron correlations [1,2]. Here, the electrons condense into a liquidlike ground state that is separated by a gap from the excited states. Most experiments to date have been carried out on GaAsbased systems, being still the cleanest material system with the highest electron mobilities [3]. When the direction of the magnetic field B is tilted, the orbital LL splitting is given by the field component B ⊥ normal to the two-dimensional electron system (2DES) while the total field strength B determines the Zeeman splitting E Z . Early experiments on GaAs revealed that the ν = 4/3, 5/3, and 8/5 states behaved differently upon tilting the sample [4,5]: While the ν = 4/3 and 8/5 states were undergoing a transition from a spinunpolarized state to a polarized one, the ν = 5/3 state was always fully spin polarized.Although the FQHE has been reported in quite a number of different materials [6][7][8][9][10][11][12], the FQHE has never been observed in a diluted magnetic semiconductor in which atoms with magnetic moment (e.g., Mn 2+ ) are placed in a 2DES. Then, the localized spins in the magnetic impurities' d orbitals interact with the correlated electron system via the quantum mechanical s-d exchange interaction, causing giant Zeeman splitting [13] which is tunable in magnitude, sign, and field dependence [14]. The constant αN 0 specifies the s-d exchange strength and is the largest energy scale in the system. It hence has remained unclear whether FQHE states survive in the presence of magnetic impurities. Below we demonstrate that (i) the FQHE indeed exists in magnetic 2DESs and (ii) the opening and closing of gaps in an in-plane field can be described within a modified composite fermion (CF) picture, in which the s-d exchange is taken into account.Let us first recall the CF model which maps the FQHE onto the integer quantum Hall effe...

The fractional quantum Hall ͑FQH͒ effect is reported in a high mobility CdTe quantum well at millikelvin temperatures. Fully developed FQH states are observed at filling factor 4/3 and 5/3 and are found to be both spin-polarized ground state for which the lowest energy excitation is not a spin flip. This can be accounted for by the relatively high intrinsic Zeeman energy in this single valley two-dimensional electron gas. FQH minima are also observed in the first excited ͑N =1͒ Landau level at filling factor 7/3 and 8/3 for intermediate temperatures. In contrast, the 5/2 FQH state remains absent down to T ϳ 10 mK. Interacting carriers in certain fractional quantum Hall ͑FQH͒ ground states can have opposite spins provided the Zeeman energy is sufficiently small. This is typically observed in GaAs-based two-dimensional electron gases ͑2DEGs͒, where an increase in the Zeeman energy induces a change in the spin polarization of the ground state from unpolarized to fully spin polarized. This transition has been reported for the FQH states at filling factor =4/ 3, =8/ 5, =2/ 3, or =2/ 5, 1-4 as well as in a GaAs 2D hole gas. 5 Subsequently, this behavior was elegantly interpreted within the composite fermions ͑CFs͒ model 6 for the FQH effect by invoking Zeeman energy-induced crossings between spinsplit composite fermion Landau levels ͑LLs͒, leading to possible changes in the spin configuration of the ground state. 7 More recently, the =4/ 3 FQH state was investigated in a strained Si quantum well, 8 where the associated resistance minimum was found to maintain its strength with increasing Zeeman energy, which was interpreted as the consequence of a spin-polarized ground state. The latter work addresses the interesting question of how the FQH effect manifests itself in a 2D system with an intrinsically larger Zeeman energy than in GaAs. However, the influence of the valley degeneracy inherent in Si is another degree of freedom that may also interfere with the FQH physics.In the present work, we study the evolution of FQH states under relatively high intrinsic Zeeman energy in a single valley electron system. This is made possible by investigating the FQH effect in a high-quality 2D electron gas in CdTe, a single valley, direct gap, semiconductor in which the bare electronic g factor is about four times larger than in GaAs. A fundamental asset of this system is the possibility to incorporate magnetic ions to form a so-called diluted magnetic semiconductor, which offers possible applications in the fields of spintronics and quantum computing. The transport measurements performed at millikelvin temperature reveal fully developed FQH states ͑i.e., zero longitudinal resistance and exact quantization of the Hall resistance͒ in the upper spin branch of the lowest ͑N =0͒ LL, which constitutes to our knowledge the first observation of the FQH effect in a II-VI semiconductor. Tilted magnetic fields experiments up to 28 T show no significant changes in the FQH gap both at filling factor 4/3 and 5/3, a behavior typical of spin-pola...