In 1879, Edwin H. Hall discovered that when a conductor carrying longitudinal current was placed in a vertical magnetic field, the carrier would be pressed against the transverse side of the conductor, which led to an observed transverse voltage. This is called the Hall effect (HE) [1] . After almost one century, the quantum Hall effect was discovered by K. von Klitzing in 1982 in a two-dimensional electron gas (2DEG) at low temperature and strong magnetic field [2] . However, in ferromagnetic metals like Fe, Co, and Ni, and newly discovered DMSs like Ga_1−xMn_xAs the Hall effect is anomalous and controlled more by magnetization than by Lorentz forces [3] [4] , called the anomalous Hall resistivity and the phenomenon is known as anomalous Hall effect (AHE). This phenomenon attracted both experimental and theoretical interest due to its potential application in emerging science of spintronics [5] . The origin of anomalous Hall effect is believed to be due to spin-orbit (SO) interaction in the presence of spin polarization [6] . On the other hand, DMS of type (III, Mn, V) spin polarization is due to exchange interaction between localized Mn²⁺ 3d⁵⁺ spins and holes introduced by substitution of Mn²⁺ by Ga³⁺ [7] . There are two popular theories to explain anomalous Hall effect seen in ferromagnetic system, named as intrinsic and extrinsic theories,both of these theories involve the SO interaction. The intrinsic theory was first time proposed by Karplus and Luttinger (KL) [8] . It required no impurity (the intrinsic scenario) and extrinsic theory, which was proposed by Smit and Berger; they pointed out the role of the impurity scatterings in the steady state equilibrium and hence in the AHE [9] [10] . Among others, the intrinsic theory dominantly plays a rule in dilute magnetic semiconductors of type (III, Mn, VI) and the current understanding on intrinsic theories of anomalous hall conductivity allows us to reformulate it with Berry curvature of quasi-particles [11] -[13] . Accordingly, intrinsic AHE results from curvature of electrons below the Fermi surface, as a consequence of the spin-orbit coupling induced topological properties in Bloch bands [13] . Although this anomalous Hall effect (AHE) has become a standard tool to determine the magnetization of ferromagnet and has been known for more than a century, its mechanism is still under debate. Particular attention has been paid to intrinsic mechanisms based on the Berry phase. According to recent experimental result, the intrinsic version of AHC is quantized [14] .

In this paper we theoretically study anomalous Hall conductivity. The paper is organized as follows. Firstly we develop model Hamiltonian on basis of above discussion, which obtains analytical expression for Berry potential and Berry curvature; secondly after applying Quantum Kubo formulism the connection between Berry curvature and intrinsic Anomalous Hall conductivity is established.

We consider two dimensional hole gas (2 DhG) in the presence of Spin-orbit coupling taking the form of the usual Rashba term, exchange field, kinetic energy of itinerant holes in the system.

where

and are usual kinetic energy of carriers (holes) and band mass of charge carriers (holes) respectively

Here, h_ex is exchange field resulting from exchange interaction between localized Mn 3d⁵ spins and valence band holes introduced by substitution of Mn²⁺ by Ga³⁺ Our approach is based on mean field treatment and magnetization along perpendicular to k_x and k_y plane or along z axis(along the direction of quantization). is spin-orbital interaction term in the form of Rash Hamiltonian which accounts that orbital motion of carriers coupled with its spin is given by

Here, and are Dirac spin operators along x and y direction respectively and and are wave vectors along x and y direction respectively and Rashba type of spin-orbit coupling constant. Using Equations (2), (3) and (4) into Equation (1), we rewrite the model Hamiltonian as

In 2D spin space, application of diagonalization procedures in Equation (5) generates two eigenvalues

And we have obtained the corresponding normalized eigenvectors for spinor part and its complex conjugate

From Equation (8) and Equation (9) we can observe that in absence of spin-orbital coupling (α_R = 0) Berry potential along x and y direction in k-space goes to zero and in similar manner the Berry curvature in Equation (24) also vanishes as (α_R = 0) Or/and (h_ex = 0). Therefore, the origin of Berry potential (connection) as well as Berry curvature in a DMS of type (III, Mn, V) is spin-orbital interaction and exchange field (h_ex). As seen in Figure 1, the Berry curvature is peaked at k = 0, and from Equation (27) the curvature is proportional to AHC; hence the maximum anomalous Hall conductivity can be attained for k = 0.

From Equation (30), if, for strong spin orbit coupling limit, we have approximate value anomalous Hall conductivity as

Equation (31) reveals that intrinsic anomalous Hall conductance is almost quantized which is also supported by recent experimental results [14] [17] [18] . On the other hand, from Equation (27), the intrinsic anomalous Hall conductance is simply the sum of the Chern numbers (the total Berry flux through the BZ) for all the occupied band.

From Case II, integrating Equation (29) from to upper empty band, we obtain anomalous Hall conductivity for lower occupied band is

Our analytical results obtained from Equation (30) are similar to result obtained for intrinsic contribution of anomalous hall conductance by classical approach [19] .

In Figure 2, we have plotted the variation anomalous Hall conductivity as function exchange field. As can be seen in Figure 1, when there is no magnetic interaction (), the spin lies in the xy plane but there is no spin polarization along z axis. As a result, intrinsic Hall conductivity is zero. However, As increases the spin is tilted out of the plane by larger amounts, increasing the phase acquired by the wave function (Berry curvature); as a result anomalous hall conductivity increases a until it reaches a maximum and gradually saturates.

In Figure 3, anomalous Hall conductivity increases monotonically as composition of magnetic dopant (x) increases. Hence, it is possible to control intrinsic anomalous Hall conductivity by changing the concentration of magnetic impurities.

In conclusion we say that, at low temperature, in the presence of strong spin-orbit interaction, the anomalous Hall conductivity is quantized. The interplay between spin-orbit interaction and exchange field introduces Berry curvature which is responsible for intrinsic anomalous Hall effect in dilute magnetic semiconductors (Ga_1−xMn_xAs). Anomalous Hall conductivity increases monotonically as composition of magnetic dopant (x) increases in mean field theory treatment

This work was supported by the school of Graduate studies of Addis Ababa University, Addis Ababa, Ethiopia and Arbaminch University, Arbaminch, Ethiopia.

SintayehuMekonnen,P.Singh, (2015) Berry Approach to Intrinsic Anomalous Hall Conductivity in Dilute Magnetic Semiconductors (Ga_1-xMn_xAs). World Journal of Condensed Matter Physics,05,179-186. doi: 10.4236/wjcmp.2015.53019