All Optical Implementation of Multi-Spin Entanglement in a Semiconductor Quantum Well

We use ultrafast optical pulses and coherent techniques to create spin entangled states of non-interacting electrons bound to donors (at least three) and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle a large number of spins.Comment: 17 pages, 3 figure

Tunable magnetic exchange interactions in manganese-doped inverted core/shell ZnSe/CdSe nanocrystals

Magnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics. Central to these efforts is a drive to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms. In this respect, colloidal nanocrystal heterostructures provide great flexibility via growth-controlled `engineering' of electron and hole wavefunctions within individual nanocrystals. Here we demonstrate a widely tunable magnetic sp-d exchange interaction between electron-hole excitations (excitons) and paramagnetic manganese ions using `inverted' core-shell nanocrystals composed of Mn-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that, surprisingly, are tunable in both magnitude and sign. Effective exciton g-factors are controllably tuned from -200 to +30 solely by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystal materials can be utilized to manipulate carrier-Mn wavefunction overlap and the sp-d exchange parameters themselves.Comment: To appear in Nature Materials; 18 pages, 4 figures + Supp. Inf

Ferromagnetism in semiconductors and oxides: prospects from a ten years' perspective

Over the last decade the search for compounds combining the resources of semiconductors and ferromagnets has evolved into an important field of materials science. This endeavour has been fuelled by continual demonstrations of remarkable low-temperature functionalities found for ferromagnetic structures of (Ga,Mn)As, p-(Cd,Mn)Te, and related compounds as well as by ample observations of ferromagnetic signatures at high temperatures in a number of non-metallic systems. In this paper, recent experimental and theoretical developments are reviewed emphasising that, from the one hand, they disentangle many controversies and puzzles accumulated over the last decade and, on the other, offer new research prospects.Comment: review, 13 pages, 8 figures, 109 reference

Electric-field controlled ferromagnetism in MnGe magnetic quantum dots

Electric-field control of ferromagnetism in magnetic semiconductors at room temperature has been actively pursued as one of the important approaches to realize practical spintronics and non-volatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (Tc>300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Among various dilute magnetic semiconductors (DMSs), materials derived from group IV elements such as Si and Ge are the ideal candidates for such materials due to their excellent compatibility with the conventional complementary metal-oxide-semiconductor (CMOS) technology. Here, we review recent reports on the development of high-Curie temperature Mn0.05Ge0.95 quantum dots (QDs) and successfully demonstrate electric-field control of ferromagnetism in the Mn0.05Ge0.95 quantum dots up to 300 K. Upon the application of gate-bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer (i.e. the Mn0.05Ge0.95 quantum dots) was modulated as a function of the hole concentration. Finally, a theoretical model based upon the formation of magnetic polarons has been proposed to explain the observed field controlled ferromagnetism

Time-resolved differential reflection measurements on Ga1-xMnxAs

We measured the time-resolved differential reflectivity, Delta R, of Ga1-xMnxAs for x <= 0.05 for various excitation wavelengths and compared with the signals from semi-insulating GaAs substrates. The sign of AR from Ga1-xMnxAs (x = 0.015 and x = 0.03) was negative at 295 K for photon energies larger than bandgap, which was ascribed to defect-induced absorption or a reduction of exciton bleaching. We also discuss the screening of Mn alloy potential fluctuations by photocarriers in the time-resolved differential reflection of Ga1-xMnxAs

Temperature dependence of the band-edge photoluminescence of Zn1−xMnxSe films

We report a systematic investigation of band-edge photoluminescence of the diluted magnetic semiconductor alloy Zn1-xMnxSe for a series of compositions (0 0.20 the effect of inhomogeneous broadening Gamma(inh) on Gamma(T) was found to be stronger than that of the longitudinal optical phonon term Gamma(LO). Furthermore, the activation energies of thermal quenching were obtained for both the I-2 and the I-1 peaks from the temperature dependence of the bound exciton peaks, and were found to decrease with increasing Mn concentration. Finally, we show that at high temperature the broadening of the FWHM is dominated by the LO-phonon interaction.ope

Observation of type-I excitons and related confinement effects in type-II superlattices

CdSe/Zn1-xMnxTe superlattices are characterized by type-II band alignment. We have used these structures to demonstrate that type-II superlattices can exhibit type-I excitons, i.e., excitons which are confined in the same semiconductor layer (either in CdSe or in Zn1-xMnxTe in the present example). Such spatially direct excitons form in a type-II structure when one of the carriers (electron or hole) originates from a well, while the other (hole or electron) originates from a state localized in the barrier, which is typical for subbands at above-barrier energies. © 1993 The American Physical Society.link_to_subscribed_fulltex

Room-temperature ferromagnetism in highly Cr-doped II-Mn-VI magnetic semiconductor Cd1-x-yMnxCryTe

We describe structural, magnetic, and transport properties of the Cd1 - x-yMnxCryTe ferromagnetic semiconductor alloy. Bulk Cd1-x-yMnxCr yTe crystals containing up to 10% of Cr (y = 0.10) were grown by the Bridgman method and were found to be strongly p type and ferromagnetic, with the highest Curie temperature of 362 K observed for y = 0.10. Magnetotransport measurements reveal the presence of the anomalous Hall effect up to and above room temperature, providing evidence that the observed ferromagnetism is characteristic of bulk Cd1-x-yMnxCryTe. Our experiment data suggest that Cr ions play a crucial role in establishing ferromagnetism in this quaternary alloy.ope

Spin superlattice formation in ZnSe/Zn1-xMnxSe multilayers

We present magneto-optical evidence for the formation of a magnetic-field-induced spin superlattice in modulated ZnSe/Zn1-xMnxSe structures. In the samples studied, the offsets in both the conduction band and the valence band are very small at zero magnetic field. When a magnetic field is applied, the large Zeeman splitting of the Zn1-xMnxSe band edges overcomes the zero-field offsets and results in the formation of a spin superlattice in which spin states of both electrons and holes are spatially and periodically separated. © 1991 The American Physical Society.link_to_subscribed_fulltex