Valence band ordering restored by the p-d exchange interaction in GaMnAs

In ferromagnetic GaMnAs, it is predicted that the Mn impurity level or impurity band (IB) is formed in the band gap by the strong $p$-$d$ exchange interaction with the hybridization [1], which is consistent with the recent result of x-ray photoemission unveiling the disordered IB [2]. Although the hybridization might be expected to result in the disordered or merged valence band (VB), it was found that VB is almost unchanged by Mn doping in GaAs [2-4]. In order to understand the bandstructure and ferromagnetism of GaMnAs induced by the strong exchange interaction, we measured the VB ordering by using the resonant tunneling spectroscopy in the GaMnAs quantum-well (QW) double-barrier hetrerostructures with the Mn content $x$ varied from 0.4\% to 2.3\%. In $x<1\%$ (paramagnetic), the $d^2I/dV^2$ oscillations weaken as $x$ increases, which shows that VB merges with the {\it paramagnetic} IB and becomes disordered. However, the oscillations are restored at the onset of the ferromagnetism ($x>1\%$) and become stronger as $x$ increases. Our results show that the strong exchange interaction does not fluctuate VB but forms the disordered IB. [1] Krstaji\'c et al., PRB (2004). [2] Kobayashi et al., arXiv:1302.0063 (2013). [3] Ohya et al., Nat. Phys. (2011). [4] Muneta et al., APL (2013).