No abstract

No abstract

We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional MnGa dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the midinfrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mndoping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band.PACS numbers: 74.20. Mn, 74.25.Nf, 74.72.Bk, 74.76.Bz The discovery of ferromagnetism in (Ga,Mn)As above 100 K [1] opened an attractive prospect for exploring the physics of magnetic phenomena in doped semiconductors and for developing advanced concepts for spintronics. Assessment of a wide range of magnetic and transport properties of the material [2][3][4] showed that in ferromagnetic (Ga,Mn)As with Mn dopings x > 1%, disorderbroadened and shifted host Bloch bands represent a useful one-particle basis for describing this mixed-crystal degenerate semiconductor. The common kinetic-exchange model implementation of this valence band theory and the more microscopic tight-binding Anderson model or ab-initio density functional theory can all be shown [5] to be mutually consistent on the level of atomic and orbital resolved band structure. The main utility of valence band theories have been in providing a qualitative and often semi-quantitative description of phenomena originating from the exchange split and spin-orbit coupled electronic structure and in assisting the development of prototype spintronic devices [4]. Other basic physical properties of (Ga,Mn)As, namely those reflecting the vicinity of the metal-insulator transition and localization and electronelectron interaction effects, remain to be fully understood and require to go beyond the commonly employed perturbative or disorder averaged Bloch-band theories.In the insulator non-magnetic regime (x 1%), the system is readily described by localized Fermi level states residing inside a narrow impurity band separated from the valence band by an energy gap of magnitude close to the isolated Mn Ga impurity binding energy. Recently, a debate has been stirred by proposals, based in particular on optical spectroscopy measurements [6], that the narrow impurity band persists in high-doped (Ga,Mn)As with metallic conduction. Several phenomenological variants of the impurity band model have been proposed for the high-doped regime [6][7][8][9][10] which are mutually inconsistent from the perspective of the assumed atomic orbital nature of the impurity band states [5]. Further theoretical inconsistencies arise when recreating ...

No abstract

The nonlinear magnetoresistance of the two-dimensional electron gas measured in GaAs/ Al~aaq As heterostructures is attributed to the breakdown of edge-state electrons into the partly filled Landau level at the bulk part of the sample. Based on a simplified model, the transition between edge and bulk states is described with use of the electron mean free path at the edge channels. It was found that the mean free path scales exponentially with the product of the Hall voltage and the number of edge channels.High-mobility samples of GaAs/Al Gaq As heterostructures generally exhibit the strongly asymmetric line shapes of Shubnikovde Haas (SdH) oscillations in p(B) at high magnetic fields. Published data on this effect show a reduced resistivity (or conductivity) on the low-energy (i.e., high-magnetic-field) side of the peaks.~S uch an asymmetry can be suppressed or even reversed by several factors, including an increase of the width of the conducting channel in very narrow samples, enhancement of the measuring current and/or temperature, and the application of a backgate voltage to the structure. 2 It seems to be evident from the published data that there are two distinct types of asymmetry observed. The first of them concerns the form of individual subpeaks corresponding to one particular orientation of the spin within a Landau level. In extreme conditions (very narrow samples, low current density and temperature) these spin-resolved peaks take a sawtooth form. Several alternative explanations of this asymmetry have been suggested s depending on the particular conditions of the experiment and we will not deal with this effect here.The second type of asyrrimetry concerns the line shape of the double peak connected with one Landau level that consists of two more-or-less overlapping spin-resolved subpeaks. It has been observed that the relative heights of the two subpeaks depend strongly on the measuring current density j (Ref. 2) due to their widely different current dependence. This is the effect we want to address in this paper.In Fig. 1 we present the p(B) dependence for three different values of the measuring current I. The data have been taken at T = 1.3 K on a GaAs/Al Gaq As heterostructure in the form of a common Hall bar with the overall width of' the conducting channel n = 400 pm and the distance between voltage contacts along the sample I, = 1100 pm. The basic parameters of the 1.2 0. 8-C: 4g 0. 6-0. 4-2T l t I s IJ& 1«& iliad(I 10 12 B (T) FIG. 1. Dependence of the Shubnikovde Haas oscillations at T = 1.3 K on the measuring dc current I: I=0.7 pA (full line), I= 7 pA (dashed line), and I= 70 pA (dotted line), respectively.two-dimensional electron gas (2DEG) in the samplethe carrier density and mobilityhave been found to be n, = 5.5 x 10t~c m~and p = 3.04 x 10s cm2/V s, respectively. The most striking effect is a remarkable increase with I of the peak value for the spin-up subpeak (as compared to the spin-down one) for all Landau levels where the spin splitting has been observed. It is worth noting that with...