J. K. Furdyna

Diluted magnetic semiconductors

We review the physical properties of diluted magnetic semiconductors (DMS) of the type AII1−xMnxBVI (e.g., Cd1−xMnxSe, Hg1−xMnxTe). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the AII1−xMnxBVI alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic AIIBVI ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d5 shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the AII1−xMnxBVI alloys. In particular, we discuss the mechanism of the Mn++‐Mn++ exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susc...

The fractional a.c. Josephson effect in a semiconductor-superconductor nanowire as a signature of Majorana particles

Topological superconductors which support Majorana fermions are thought to be realized in one-dimensional semiconducting wires coupled to a superconductor [1–3]. Such excitations are expected to exhibit non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence [4, 5]. Here we report the observation of the fractional a.c. Josephson effect in a hybrid semiconductor/superconductor InSb/Nb nanowire junction, a hallmark of topological matter. When the junction is irradiated with a radio-frequency f0 in the absence of an external magnetic field, quantized voltage steps (Shapiro steps) with a height ∆V = hf0/2e are observed, as is expected for conventional superconductor junctions, where the supercurrent is carried by charge-2e Cooper pairs. At high magnetic fields the height of the first Shapiro step is doubled to hf0/e, suggesting that the supercurrent is carried by charge-e quasiparticles. This is a unique signature of Majorana fermions, elusive particles predicted ca. 80 years ago [6].

Formation of self‐assembling CdSe quantum dots on ZnSe by molecular beam epitaxy

We report the formation of self‐assembling CdSe quantum dots during molecular beam epitaxial growth on ZnSe and ZnMnSe. Atomic force microscopy measurements on specimens with uncapped dots show relatively narrow dot size distributions, with typical dot diameters of 40±5 nm, and with a diameter‐to‐height ratio consistently very close to 4:1. Uncapped CdSe dots are unstable with time: their density was observed to drop by an order of magnitude in 10 days, with clear evidence of ripening observed for some dots. Photoluminescence from capped dots indicates exciton localization much stronger than in ZnCdSe/ZnSe quantum wells, due to the additional lateral confinement.

Giant Zeeman splitting in nucleation-controlled doped CdSe:Mn2+ quantum nanoribbons

Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)(13) clusters with Mn(2+) ions governs the Mn(2+) incorporation during the nucleation stage. This highly efficient Mn(2+) doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of approximately 600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s-d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.

Growth of cubic (zinc blende) CdSe by molecular beam epitaxy

We report the growth of cubic (zinc blende) CdSe epilayers on [100] GaAs substrates by molecular beam epitaxy. The lattice constant of the CdSe epilayers is 6.077 A, and the energy gap is 1.75, 1.74, and 1.67 at 10, 80, and 300 K, respectively.

Spin-polarizable excitonic luminescence in colloidal Mn2+-doped CdSe quantum dots.

The photoluminescence of colloidal Mn2+-doped CdSe nanocrystals has been studied as a function of nanocrystal diameter. These nanocrystals are shown to be unique among colloidal doped semiconductor nanocrystals reported to date in that quantum confinement allows tuning of the CdSe bandgap energy across the Mn2+ excited-state energies. At small diameters, the nanocrystal photoluminescence is dominated by Mn 2+ emission. At large diameters, CdSe excitonic photoluminescence dominates. The latter scenario has allowed spin-polarized excitonic photoluminescence to be observed in colloidal doped semiconductor nanocrystals for the first time.

Molecular beam epitaxy of Zn1−xCdxSe epilayers and ZnSe/Zn1−xCdxSe superlattices

We have investigated the epitaxial growth of Zn1−x CdxSe epilayers and ZnSe/Zn1−x CdxSe superlattices (0≤x≤1) on (100)GaAs. Although thick epilayers of Zn1−x CdxSe are prone to defect formation with increasing Cd content, the structural and optical characteristics improve remarkably when Zn1−x CdxSe is  in the  form of  thin layers  within ZnSe/Zn1−x CdxSe  superlattices. High  quality superlattices  can be grown for x≤0.35. The characterization of these systems using transmission electron microscopy, x‐ray diffraction, reflectivity, and photoluminescence is reported.

Lattice parameters of Zn1−xMnxSe and tetrahedral bond lengths in AII1−xMnxBVI alloys

This paper reports the results of lattice parameter measurements on the ternary semiconductor alloy Zn1−xMnxSe over the range 0≤x≤0.57. We find that the mean cation‐cation distance increases linearly with manganese concentration x according to Vegard’s Law. It is also noted that this linear dependence occurs across the region in which the alloy changes crystal structure from zinc blende (x≤0.30) to wurtzite (0.33≤x). These observations are compared with the behavior of the crystal lattice as a function of composition in other AII1−xMnxBVI alloys. A fairly unifed pattern of behavior emerges, relating the lattice parameters and bond lengths for the entire family of these materials. In addition, this analysis provides an experimentally determined value of the tetrahedral radius of manganese.

Very Large Magnetoresistance in Lateral Ferromagnetic (Ga,Mn)As Wires with Nanoconstrictions

We have fabricated (Ga,Mn)As nanostructures in which domain walls can be pinned by sub-10 nm constrictions. Controlled by shape anisotropy, we can switch the regions on either side of the constriction to either parallel or antiparallel magnetization. All samples exhibit a positive magnetoresistance, consistent with domain-wall trapping. For metallic samples, we find a magnetoresistance up to 8%, which can be understood from spin accumulation. In samples where, due to depletion at the constriction, a tunnel barrier is formed, we observe a magnetoresistance of up to 2000%.

Magnetic properties of diluted magnetic semiconductors: A review (invited)

Diluted magnetic semiconductors (DMS) are semiconducting alloys containing a random distribution of substitutional magnetic ions (e.g., Mn++ in Cd1−xMnxSe). Magnetic properties of DMS—such as spin glass behavior due to lattice frustration, magnon excitations in a random system of spins, superexchange, and short range antiferromagnetic order—are of considerable interest in their own right. In addition, understanding these properties is important because they strongly influence the electronic phenomena in DMS via the sp‐d exchange interaction between the band electrons and the localized magnetic moments. We review the most recent results concerning magnetic susceptibility, Mn++‐Mn++ nearest‐neighbor exchange, and magnetic short‐range order in these materials. On this basis, we can present a fairly complete and unified picture of magnetic properties of DMS. In addition, we point out some of the issues and challenges that lie ahead.