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We demonstrate visible-light electroluminescence due to d-d transitions in GaAs:Mn based light emitting diodes (LEDs) [1][2]. We prepared p+n junctions with a p+GaAs:Mn layer. At a reverse bias voltage (-3 to -6V), holes are injected from the n-type layer to the depletion layer and accelerated by the intense electric field, and excite the d electrons of Mn in the p+GaAs:Mn layer by impact excitations. We observe visible-light emission E1 = 1.89eV and E2 = 2.16eV, which are exactly the same as the 4T1 -> 6A1 and 4A2 -> 4 T1 transition energy of Mn. Furthermore, by utilizing optical transitions between the p-d hybridized orbitals of Mn atoms doped in Si, we demonstrate Si-based LEDs that continuously emit reddish-yellow visible light at room temperature. The Mn p-d hybrid states are excited by hot holes that are accelerated in the depletion layers of reverse biased Si pn junctions. Above a threshold reverse bias voltage of about -4V, our LEDs show strong visible light emission with two peaks at E1 = 1.75eV and E2 = 2.30eV, corresponding to optical transitions from the t-a (spin-down anti-bonding) states to the e- (spin-down non-bonding) states, and from the e- to the t+a (spin-up anti-bonding) states. The internal quantum efficiency of the E1 and E2 transitions is 3-4 orders of magnitude higher than that of the indirect band-gap transition [3]. [1] P. N. Hai, et al., APL 104, 122409 (2014). [2] P. N. Hai, et al., JAP 116, 113905 (2014). [3] P. N. Hai, et al., submitted.

We investigate the crystal structure, transport and magnetic properties of Fe-doped ferromagnetic semiconductor (Al1-x,Fex)Sb thin films up to x = 14% grown by molecular beam epitaxy. All the samples show p-type conduction at room temperature and insulating behavior at low temperature. The (Al1-x,Fex)Sb thin films with x lower or equal to 10% maintain the zinc blende crystal structure of the host material AlSb. The (Al1-x,Fex)Sb thin film with x = 10% shows intrinsic ferromagnetism with a Curie temperature (TC) of 40 K. In the (Al1-x,Fex)Sb thin film with x = 14%, a sudden drop of mobility and TC was observed, which may be due to microscopic phase separation. The observation of ferromagnetism in (Al,Fe)Sb paves the way to realize a spin-filtering tunnel barrier that is compatible with well-established III-V semiconductor devices., Manuscript and Supplemental Material, all 18 pages

In order to investigate the mechanism of ferromagnetic ordering in the new $n$-type magnetic semiconductor (In,Fe)As codoped with Be, we have performed x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of ferromagnetic and paramagnetic samples. The spectral line shapes suggest that the ferromagnetism is intrinsic, originating from Fe atoms incorporated into the zinc-blende-type InAs lattice. The magnetization curves of Fe measured by XMCD were well reproduced by the superposition of a Langevin function representing superparamagnetic (SPM) behavior of nanoscale ferromagnetic domains and a $T$-linear function representing Curie-Weiss paramagnetism even much above the Curie temperatures. The data at 20 K showed a deviation from the Langevin behavior, suggesting a gradual establishment of macroscopic ferromagnetism on lowering temperature. The existence of nanoscale ferromagnetic domains indicated by the SPM behavior suggests spatial fluctuations of Fe concentration on the nanoscale.

The formation of a magnetic dead layer at the interfaces of the perovskite oxide La0.67Sr0.33MnO3 (LSMO) is one of the crucial issues for its spintronic applications. In this letter, we report the reduction of the dead layer by growing LSMO on a LaMnO3 (LMO) layer. Furthermore, we detect tunneling magnetoresistance (TMR) in an LSMO/LMO/LSMO heterostructure. The obtained sign of the TMR was negative, but it changed to positive after annealing. This unusual negative TMR can be attributed to the intrinsic structural difference between the upper and lower interfaces of LMO and can be understood by a weak antiferromagnetic metallic thin layer formed at the upper LSMO/LMO interface. This layer is thought to be formed by diffused Sr atoms and oxygen vacancies in the LMO barrier. Our results indicate that control of intermixing of atoms at the interfaces is a key to controlling the TMR.

A new n-type magnetic semiconductor (In1− x ,Co x )As (x = 3–18%) has been successfully grown by low-temperature molecular beam epitaxy (LT-MBE) on GaAs(001) substrates. Reflection high energy electron diffraction (RHEED) patterns during the MBE growth and transmission electron microscopy (TEM) images indicate that (In,Co)As layers have zinc-blende crystal structure with a small fraction of embedded CoAs nanoclusters. The electron concentration of the (In,Co)As layers can be changed in the range of 1.9 × 1018–2.4 × 1019 cm−3 by changing the Co concentration. The metal–insulator transition (MIT) is observed at x = 5%. Large negative magnetoresistance (up to −17.5% at 0.95 T) is observed at low temperature and can be attributed to spin-disorder scattering in the (In,Co)As matrix.

Using coherent potential approximation we study zero-temperature Mott transition of the half-filled Hubbard model in a two-dimensional square lattice with geometrical frustration. It turns out that the geometrical frustration reduces the gap between the Hubbard bands. As a result the metallic phase is stabilized up to a fairly large value of the on-site Coulomb interaction. We found that the critical value UC for the Mott transition is enhanced by the geometrical frustration. Our results are in good agreement with the ones obtained by the single-site dynamical mean-field theory. Mott-insulator; Hubbard model; geometrical frustration; coherent potential approx- imation.

We show high-temperature ferromagnetism in heavily Fe-doped ferromagnetic semiconductor (Ga1−x,Fex)Sb (x = 23% and 25%) thin films grown by low-temperature molecular beam epitaxy. Magnetic circular dichroism spectroscopy and anomalous Hall effect measurements indicate intrinsic ferromagnetism of these samples. The Curie temperature reaches 300 K and 340 K for x = 23% and 25%, respectively, which are the highest values reported so far in intrinsic III-V ferromagnetic semiconductors.

The half-filled isotropic degenerate two-band Hubbard model is studied within coherent potential approximation. The model is characterized by an Ising-type Hunds exchange coupling, intraand inter-orbital Coulomb parameters. We found that the band degeneracy slightly reduces the Mott-Hubbard critical value UC . We reveal that the system can have two distinct critical values for Mott-Hubbard transitions.

We show that by introducing isoelectronic iron (Fe) magnetic impurities and Beryllium (Be) double-donor atoms into InAs, it is possible to grow a n-type ferromagnetic semiconductor (FMS) with the ability to control ferromagnetism by both Fe and independent carrier doping by low-temperature molecular-beam epitaxy. We demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS. This achievement opens the way to realize novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps understand the mechanism of carrier-mediated ferromagnetism in FMSs., arXiv admin note: substantial text overlap with arXiv:1106.0561