Identifying the Electronic Character and Role of the Mn States in the Valence Band of (Ga,Mn)As

We report high-resolution hard x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of Mn doping. Supported by theoretical calculations we identify, for both low (1%) and high (13%) Mn doping values, the electronic character of the states near the top of the valence band. Magnetization and temperature-dependent core-level photoemission spectra reveal how the delocalized character of the Mn states enables the bulk ferromagnetic properties of (Ga,Mn)As.

Figure 1

(a)–(c) HAXPES spectra ($h\nu =5953\mathrm{eV}$) for different Mn doping in GaAs. The GaAs spectra have been aligned to the (GaMn)As spectra using the BE position of the As $4s$ core level. No background subtraction was applied. (a) Extended valence band PES ($T=20\mathrm{K}$), including the As $4s$, Ga $4s$, and As $4p$ shallow core levels of GaAs(100) and 13% Mn-doped GaAs. (b) Zoom of the valence band region ($T=100\mathrm{K}$), showing the spectra from pure GaAs (black dots), 1% and 13% Mn-doped GaAs (red and blue dots, respectively). (c) High-resolution spectra measured in the vicinity of $E_{\mathrm{F}}$. Difference spectra, corresponding to the Mn contribution only, are shown in orange [13% Mn spectrum (blue dots) minus pure GaAs spectrum (black dots)] and grey [1% Mn spectrum (red dots) minus pure GaAs spectrum (black dots)]. The reference Fermi level of Au is displayed, offset. (d) Calculated angle-integrated PES (including matrix elements) for photon energy and geometry ($p$ polarization) as used in the experiment. (e) Calculated valence-band spectra of GaAs(100) using LDA (black curve) and (Ga,Mn)As (13%) using both LDA (red curve) and DMFT (blue curve). (f) Zoom of the vicinity of $E_{\mathrm{F}}$ with calculated difference spectra, as in (c), for LDA (violet curve) and DMFT (green curve).

Figure 2

(a) HAXPES wide energy scan ($h\nu =7940\mathrm{eV}$, linear polarization, $T=20\mathrm{K}$) of 13% Mn-doped GaAs. Main lines positions are indicated. The inset shows a zoom of the Mn $2p$ core level with the $2p_{1/2}$ and $2p_{3/2}$ multiplet structure, where the well-screened and poorly screened peaks are indicated. For a definition of well-screened and poorly screened peaks in (GaMn)As see Ref. 25. (b) MCD in HAXPES. Polarization-dependent Mn $2p$ spectra (blue and red dots for right- and left-circular polarization, respectively), measured at $T=20\mathrm{K}$, i.e., below $T_{\mathrm{C}}$; the dashed thin line represents the background. In the lower panel the difference spectrum (MCD), representing the magnetic signal (open white circles) is shown; the green line is a smoothed curve. The value of the maximum magnetic asymmetry, defined as $(I^{+}-I^{-})/(I^{+}+I^{-})$, where $I^{+}(I^{-})$ is the intensity measured with $\mathrm{Circ}+$ ($\mathrm{Circ}-$), is indicated. (c) Calculations for an Anderson impurity model with varying hybridization $V$, as well as a fully localized $d^5$ state, and an itinerant electron model. The calculated curves are offset for clarity. The best agreement is for a hybridization parameter $2.0\le \mathrm{V}\le 2.5\mathrm{eV}$.

Figure 3

Temperature dependence of the (Ga,Mn)As (13% Mn) PES. (a) Relative BE shift of the GaAs host valence band, Ga $2p$ core level, Mn $2p_{3/2}$ low energy feature, and Mn $2p_{3/2}$ broad peak; the BEs at 20 K are used as reference. (b) Mn $2p_{3/2}$ core level measured at 20 K and at RT.

Figure 4

Schematics of the electronic structure of (Ga,Mn)As. (a) Experimental valence band (smoothed) at $h\nu =5953\mathrm{eV}$. GaAs host band (blue curve) and (Ga,Mn)As band with 13% Mn (red curve). The difference between both spectra identifies the Mn-derived states (dark green). The dashed vertical bar marks the position of $E_{\mathrm{F}}$; the center of the Mn band is located at $\sim 250\mathrm{meV}$ BE. (b) Zoom of the region near $E_{\mathrm{F}}$. The GaAs host band does not cross $E_{\mathrm{F}}$ (energy separation of 50 meV); the Mn-induced states have a finite nonzero density at $E_{\mathrm{F}}$. The color shading illustrates the difference in electronic character of the Mn $d$ states as derived from our theoretical calculations: near $E_{\mathrm{F}}$ a $3d^{5}h$ localized character is found, whereas $3d^6{h}^2$ delocalized character is assigned to the dark green region. The latter corresponds to the hybridization responsible for mediating the ferromagnetism.