Antiferromagnetic s-d exchange coupling in GaMnAs

Measurements of coherent electron spin dynamics in Ga(1-x)Mn(x)As/Al(0.4)Ga(0.6)As quantum wells with 0.0006% < x < 0.03% show an antiferromagnetic (negative) exchange bewteen s-like conduction band electrons and electrons localized in the d-shell of the Mn2+ impurities. The magnitude of the s-d exchange parameter, N0 alpha, varies as a function of well width indicative of a large and negative contribution due to kinetic exchange. In the limit of no quantum confinement, N0 alpha extrapolates to -0.09 +/- 0.03 eV indicating that antiferromagnetic s-d exchange is a bulk property of GaMnAs. Measurements of the polarization-resolved photoluminescence show strong discrepancy from a simple model of the exchange enhanced Zeeman splitting, indicative of additional complexity in the exchange split valence band.Comment: 5 pages, 4 figures and one action figur

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

Large negative magnetoresistance in a ferromagnetic shape memory alloy : Ni_{2+x}Mn_{1-x}Ga

5% negative magnetoresistance (MR) at room temperature has been observed in bulk Ni_{2+x}Mn_{1-x}Ga. This indicates the possibility of using Ni_{2+x}Mn_{1-x}Ga as magnetic sensors. We have measured MR in the ferromagnetic state for different compositions (x=0-0.2) in the austenitic, pre-martensitic and martensitic phases. MR is found to increase with x. While MR for x=0 varies almost linearly in the austenitic and pre-martensitic phases, in the martensitic phase it shows a cusp-like shape. This has been explained by the changes in twin and domain structures in the martensitic phase. In the austenitic phase, which does not have twin structure, MR agrees with theory based on s-d scattering model.Comment: 3 pages, 3 figures, Appl. Phys. Lett 86, 202508 (2005

Surface state scattering by adatoms on noble metals

When surface state electrons scatter at perturbations, such as magnetic or nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized at the adatom site, can develop below the bottom of the surface state band for both spin channels. In the case of adatoms, these states have been found very recently in scanning tunneling spectroscopy experiments\cite{limot,olsson} for the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried out a systematic theoretical investigation of the electronic structure of these surface states in the presence of magnetic and non-magnetic atoms on Cu(111). We found that Ca and all 3$d$ adatoms lead to a split-off state at the bottom of the surface band which is, however, not seen for the $sp$ elements Ga and Ge. The situation is completely reversed if the impurities are embedded in the surface: Ga and Ge are able to produce a split-off state whereas the 3$d$ impurities do not. The resonance arises from the s-state of the impurities and is explained in terms of strength and interaction nature (attraction or repulsion) of the perturbing potential.Comment: 6 pages, 5 figure

Grand canonical Gutzwiller approximation for magnetic inhomogeneous systems

The Gutzwiller approximation (GA) for Gutzwiller-projected grand canonical wave functions with fugacity factors is investigated in detail. Our systems in general contain inhomogeneity and local magnetic moments. In deriving renormalization formulae, we also derive or estimate terms of higher powers of intersite contractions neglected in the conventional GA. We examine several different constraints, i.e., local/global spin-dependent/independent particle-number conservation. Out of the four, the local spin-dependent constraint seems the most promising at present. An improved GA derived from it agrees with the variational Monte Carlo method better than the conventional GA does. The corrections to the conventional GA can be interpreted as two-site correlation including the phase difference of configurations. Furthermore, projected quasi-particle excited states are orthogonal to each other within the GA. Using these states, spectral weights are calculated. We show that asymmetry between electron addition and removal spectra can appear by taking into account the higher powers of the intersite contractions in the case of the d-wave superconductors and the Fermi sea; the addition is smaller than the removal. However, the asymmetry is quite weak especially near the Fermi level. In contrast, projected s-wave superconductors can have the opposite asymmetry (addition larger than removal) especially near the Fermi level. In addition, formulae from the other three constraints are also derived, which may be useful depending on purposes.Comment: 18 pages. 8 figures added. Sec.II B, Sec.III F G H, Sec.IV G mainly revise

Electronic and structural properties of GaN by the full-potential LMTO method : the role of the dd electrons

The structural and electronic properties of cubic GaN are studied within the local density approximation by the full-potential linear muffin-tin orbitals method. The Ga $3d$ electrons are treated as band states, and no shape approximation is made to the potential and charge density. The influence of $d$ electrons on the band structure, charge density, and bonding properties is analyzed. It is found that due to the energy resonance of the Ga 3$d$ states with nitrogen 2$s$ states, the cation $d$ bands are not inert, and features unusual for a III-V compound are found in the lower part of the valence band and in the valence charge density and density of states. To clarify the influence of the Ga $d$ states on the cohesive properties, additional full and frozen--overlapped-core calculations were performed for GaN, cubic ZnS, GaAs, and Si. The results show, in addition to the known importance of non-linear core-valence exchange-correlation corrections, that an explicit description of closed-shell repulsion effects is necessary to obtain accurate results for GaN and similar systems. In summary, GaN appears to be somewhat exceptional among the III-V compounds and reminiscent of II-VI materials, in that its band structure and cohesive properties are sensitive to a proper treatment of the cation $d$ bands, as a result of the presence of the latter in the valence band range.Comment: ( 20 REVTEX-preprint pages (REVTEX macros are included) 8 figures available upon reques

Excitonic giant Zeeman effect in GaN:Mn^3+

We describe a direct observation of the excitonic giant Zeeman splitting in (Ga,Mn)N, a wide-gap III-V diluted magnetic semiconductor. Reflectivity and absorption spectra measured at low temperatures display the A and B excitons, with a shift under magnetic field due to s,p-d exchange interactions. Using an excitonic model, we determine the difference of exchange integrals between Mn^3+ and free carriers in GaN, N_0(alpha-beta)=-1.2 +/- 0.2 eV. Assuming a reasonable value of alpha, this implies a positive sign of beta which corresponds to a rarely observed ferromagnetic interaction between the magnetic ions and the holes.Comment: 4 pages, 4 figure