2,013,508 research outputs found
High-energy spectroscopic study of the III-V nitride-based diluted magnetic semiconductor GaMnN
We have studied the electronic structure of the diluted magnetic
semiconductor GaMnN ( = 0.0, 0.02 and 0.042) grown on Sn-doped
-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn
-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 core
level and the Mn 3 partial density of states were analyzed using
configuration-interaction calculation on a MnN cluster model. The deduced
electronic structure parameters reveal that the - exchange coupling in
GaMnN is stronger than that in GaMnAs.Comment: 6pages, 10figures. To be published to Phys. Rev.
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 =~ -2.8 eV. This is smaller
than that obtained with LSDA, which overestimates the exchange coupling between
Mn ions and the As holes.
In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the
double-exchange mechanism, since a hole of strong 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 type
GaN:Mn and type (Ga,Mn)As, type (Ga,Mn)P. In type DMS with
Mn impurities the exchange mechanisms is rather close to the
kinematic exchange proposed by Zener for mixed-valence Mn ions. In 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 impurities. Using the molecular field approximation the
Curie temperatures 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
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 - exchange scatterings, are explicitly included. The
Elliot-Yafet mechanism is also incorporated. From this approach, we study the
spin relaxation in both -type and -type Ga(Mn)As quantum wells. For
-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
-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 - 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
On varieties in multiple-projective spaces
AbstractIn this paper, mP will denote a projective space of dimension m, and (m,n)P will denote a doubly-projective space of dimension m+n, namely the space of all pairs of points (x¦y), where x varies in mP and y in nP. Just so, (m,n,s)P will denote a triply-projective space of dimension m+n+s, and so on.A variety V of dimension d in mP has just one degree g, namely the number of points of intersection of V with d generic linear hyperplanes (ux)=0, where (ux) means ∑uixi. On the other hand, a variety V' of dimension d in (m,n)P has several degree ga,b (a+b=d), defined as follows: ga,b is the number of points of intersection of V' with a hyperplanes (ux)=0 and b hyperplanes (vy)=0.Let xo, …, xm, yo, …, yn be the homogeneous coordinates of a point in m+n+1P. It sometimes happens that the equations of a variety V in m+n+1P are not only homogeneous in all variables x and y together, but even homogeneous in the x's and in the y's. In this case the same set of equations also defines a variety V' in the doubly-projective space (m+n)P. If d is the dimension of V', the dimension of V is d+1, for to every point (x¦y) of V' corresponds a whole straight line of points (xα, yβ) in V.In some cases it is easier to determine the degrees ga,b of V' than to determine the degree g of V. For this reason, it is desirable to have a rule that enables us to calculate g from the ga,b's. Such a rule will be proved here. It says:The degree g is the sum of all ga,b with a+b=d.In the case of multiply-projective spaces (m,n,…)P the same rule holds: g is the sum of the ga,b,… with a+b+…=d.Examples of applications of this rule will be given at the end
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