Bulk and nano GaN: Role of Ga d states

We have studied the role of Ga 3d states in determining the properties of bulk as well as nanoparticles of GaN using PAW potentials. A significant contribution of the Ga d states in the valence band is found to arise from the interaction of Ga 4d states with the dominantly N p states making up the valence band. The errors arising from not treating the Ga 3d states as a part of the valence are found to be similar, ~ 1%, for bulk as well as for nanoclusters of GaN.Comment: 17 pages, 7 figure

Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N

The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is extensively studied over a vast range of Mn concentrations. We calculate the electronic structures of these materials using density functional theory in both the local spin density approximation and the LDA+U scheme, that we have now implemented in the code SIESTA. For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the ferromagnetism, with an exchange constant $N\beta$ =~ -2.8 eV. This is smaller than that obtained with LSDA, which overestimates the exchange coupling between Mn ions and the As $p$ holes. In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the double-exchange mechanism, since a hole of strong $d$ character is found at the Fermi level in both the LSDA and the LDA+U approaches. In this case the coupling between the Mn ions decays rapidly with the Mn-Mn separation. This suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a robust ferromagnetic phase at large Mn concentration coexisting with a diluted weak ferromagnetic phase.Comment: 12 pages, 11 figure

Double exchange mechanisms for Mn doped III-V ferromagnetic semiconductors

A microscopic model of indirect exchange interaction between transition metal impurities in dilute magnetic semiconductors (DMS) is proposed. The hybridization of the impurity d-electrons with the heavy hole band states is largely responsible for the transfer of electrons between the impurities, whereas Hund rule for the electron occupation of the impurity d-shells makes the transfer spin selective. The model is applied to such systems as $n-$type GaN:Mn and $p-$type (Ga,Mn)As, $p-$type (Ga,Mn)P. In $n-$type DMS with Mn$^{2+/3+}$ impurities the exchange mechanisms is rather close to the kinematic exchange proposed by Zener for mixed-valence Mn ions. In $p-$type DMS ferromagnetism is governed by the kinematic mechanism involving the kinetic energy gain of heavy hole carriers caused by their hybridization with 3d electrons of Mn$^{2+}$ impurities. Using the molecular field approximation the Curie temperatures $T_C$ are calculated for several systems as functions of the impurity and hole concentrations. Comparison with the available experimental data shows a good agreement.Comment: Submitted to PR

High-energy spectroscopic study of the III-V nitride-based diluted magnetic semiconductor Ga1−x_{1-x}Mnx_{x}N

We have studied the electronic structure of the diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$N ($x$ = 0.0, 0.02 and 0.042) grown on Sn-doped $n$-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn $L$-edge x-ray absorption have indicated that the Mn ions are in the tetrahedral crystal field and that their valence is divalent. Upon Mn doping into GaN, new state were found to form within the band gap of GaN, and the Fermi level was shifted downward. Satellite structures in the Mn 2$p$ core level and the Mn 3$d$ partial density of states were analyzed using configuration-interaction calculation on a MnN$_{4}$ cluster model. The deduced electronic structure parameters reveal that the $p$-$d$ exchange coupling in Ga$_{1-x}$Mn$_{x}$N is stronger than that in Ga$_{1-x}$Mn$_{x}$As.Comment: 6pages, 10figures. To be published to Phys. Rev.

Observation of strong-coupling effects in a diluted magnetic semiconductor (Ga,Fe)N

A direct observation of the giant Zeeman splitting of the free excitons in (Ga,Fe)N is reported. The magnetooptical and magnetization data imply the ferromagnetic sign and a reduced magnitude of the effective p-d exchange energy governing the interaction between Fe^{3+} ions and holes in GaN, N_0 beta^(app) = +0.5 +/- 0.2 eV. This finding corroborates the recent suggestion that the strong p-d hybridization specific to nitrides and oxides leads to significant renormalization of the valence band exchange splitting.Comment: 4 pages, 2 figure

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells is studied via the fully microscopic kinetic spin Bloch equation approach where all the scatterings, such as the electron-impurity, electron-phonon, electron-electron Coulomb, electron-hole Coulomb, electron-hole exchange (the Bir-Aronov-Pikus mechanism) and the $s$-$d$ exchange scatterings, are explicitly included. The Elliot-Yafet mechanism is also incorporated. From this approach, we study the spin relaxation in both $n$-type and $p$-type Ga(Mn)As quantum wells. For $n$-type Ga(Mn)As quantum wells where most Mn ions take the interstitial positions, we find that the spin relaxation is always dominated by the DP mechanism in metallic region. Interestingly, the Mn concentration dependence of the spin relaxation time is nonmonotonic and exhibits a peak. This behavior is because that the momentum scattering and the inhomogeneous broadening have different density dependences in the non-degenerate and degenerate regimes. For $p$-type Ga(Mn)As quantum wells, we find that Mn concentration dependence of the spin relaxation time is also nonmonotonic and shows a peak. Differently, this behavior is because that the $s$-$d$ exchange scattering (or the Bir-Aronov-Pikus) mechanism dominates the spin relaxation in the high Mn concentration regime at low (or high) temperature, whereas the DP mechanism determines the spin relaxation in the low Mn concentration regime. The Elliot-Yafet mechanism also contributes the spin relaxation at intermediate temperature. The spin relaxation time due to the DP mechanism increases with Mn concentration due to motional narrowing, whereas those due to the spin-flip mechanisms decrease with Mn concentration, which thus leads to the formation of the peak.... (The remaining is omitted due to the space limit)Comment: 12 pages, 8 figures, Phys. Rev. B 79, 2009, in pres