Session P35: Focus Session: Spins in Semiconductors -- GaMnAs Electronic Structure

Atomic scale gate electrode formed by a charged defect on GaAs(110)

Electric-field control of spin-spin interactions at the atomic level is desirable for spintronics and spin-based quantum computation. Here we demonstrate the realization of an atomic-scale gate electrode formed by a single charged vacancy on the GaAs(110) surface[1]. A low temperature scanning tunneling microscope is used to position these vacancies with atomic precision. Tunneling spectroscopy suggests that the vacancies influence the in-gap resonance of Mn, Co and Zn acceptors via an interplay of quantum confinement, band bending and Coulomb electrostatics. We find that this electrostatic field can be used to tune the magnetic coupling between pairs of Mn acceptors. This suggests an avenue for controlling spin-spin interactions on the atomic scale. http://www.physics.ohio-state.edu/$\sim$jgupta \\[4pt] [1] D. Lee and J.A. Gupta (in preparation)   

High Speed Single Dopant Spin Manipulation with a Single Electrical Gate

The smallest semiconductor spintronic devices may involve single-spin control[1]. Mn ions with a bound hole in GaAs can be controlled electrically[2]. Through the spin-orbit interaction an oscillating electric field can manipulate the spin orientation of the spin-1 Mn ion-hole ground state. In an effort to create a scalable device design using a single gate we propose a configuration with fixed electric field direction. Static electric and magnetic fields are chosen to fix the lowest energy splitting at 5 GHz and increase the energy of the highest state, creating a nearly-degenerate doublet. Within this configuration, a static magnetic field of 2.5 T and an electric field that never exceeds 200 kV/cm, we predict Rabi periods on the order of picoseconds with visibilities near 90\%. This work was supported by NRI through WIN.\\[4pt] [1] D. D. Awschalom, N. Samarth, and D. Loss, eds., Semiconductor Spintronics and Quantum Computation (Springer Verlag, Heidelberg, 2002).\\[0pt] [2] J.-M Tang, Jeremy Levy, and M. E. Flatt\'e, Phys. Rev. Lett. 97, 106803 (2006).   

Infrared probe of charge density modification in GaMnAs films

We have explored the phase diagram of GaMnAs by modifying charge density and quantifying its effect on the electronic structure and dynamics via infrared spectroscopic measurements. Optical conductivity spectra from THz to the band gap of the GaAs host are found to have a similar lineshape consisting of finite far-infrared conductivity and a mid-infrared peak in the vicinity of the Mn acceptor state. The similarity of all spectra suggests that the electronic structure does not vary dramatically across either the insulator-to-metal or ferromagnetic transition. However, in all cases studied, the spectra show a persistent red-shift of the mid-infrared peak with an increase in charge density. In addition, temperature-dependent measurements reveal that ferromagnetic samples exhibit an enhanced spectral weight below T$_{C}$. This enhancement can be attributed to a reduction of the carrier mass, a feature which is not observed in paramagnetic GaMnAs.   

First-principle studies of magnetic impurities in GaAs in the presence of spin-orbit interaction

We report on density-functional theory studies of substitutional transition-metal magnetic impurities (Mn, Fe, Co, Ni) in GaAs. Our calculations include the effect of spin-orbit interaction, which has not been considered in detail so far. For two interacting magnetic impurities in bulk, we calculate the effective exchange interaction constant defined as the difference between the ground state energies for the ferromagnetic and antiferromagnetic configuration of the magnetic moments. We find that for Mn and Co impurities, ferromagnetic alignment is energetically favourable, while Ni impurities tend to order antiferromagnetically. For Fe pairs both configurations are possible, depending on the pair orientation with respect to the GaAs crystal structure. In all cases, the exchange constant is strongly anisotropic, as a function of both the pair orientation and, due to the spin-orbit interaction, the direction of the magnetic moment. We comment also on the case where the magnetic impurities substitute Ga atoms on the (110) surface of GaAs, which recently has been investigated experimentally by novel STM techniques.   

Spin relaxation of Mn + h complexes in III-V semiconductors

Splitting between heavy and light hole levels is known to results in long spin relaxation times of holes confined in compressively strained InAs quantum dots [1]. We show theoretically that $T_{1}$ can be elongated by orders of magnitudes if the hole resides on a Mn acceptor, as the $p-d$ exchange interaction introduces a magnetic anisotropy barrier for spin relaxation. In order to compare the magnitudes of thermally activated over-barrier spin relaxation with a competing non-stationary quantum tunnelling at level anticrossings we evaluate also the expected magnitude of the ground state splitting by various intrinsic and extrinsic effects, including random in-plane strains. The relevance of our results for optical [2] and transport studies [3] of Mn-containing InAs quantum dots and quantum wells, respectively is examined and shown to elucidate the origin of the observed anisotropies and hystereses. \\[4pt] [1] D. Heiss et al., \textit{Phys. Rev. B} 76, 241306(R) (2007). \\[0pt] [2] O Krebs et al., \textit{Phys. Rev. B} 80, 165315 (2009).\\[0pt] [3] U. Wurstbauer et al., \textit{J. Crystal Growth} 311, 2160 (2009); \textit{Phys. Rev. B }79, 155444 (2009); \textit{Phys. E} [doi:10.1016/j.physe.2009.11.012].   

Electronic structure of ferromagnetic Ga$_{1-x}$Mn$_{x}$As probed by infrared to visible magneto-optical spectroscopy

We present a study of the electronic band structure of ferromagnetic Ga$_{1-x}$Mn$_{x}$As determined from the spin dependent optical transitions that are manifested in the Faraday and Kerr spectra in the infrared to visible range (100-2000 meV). The data are compared to predictions from conventional mean field p-d exchange model. The spin-split band structure is probed as function of the exchange interaction strength and carrier concentration by exploring a series of optimally annealed Ga$_{1-x}$Mn$_{x}$As films with a broad range of magnetic doping (x=0.01- 0.14)~ and Curie temperatures (up to 190K). Large shifts in the magneto-optical spectra are observed with varying Mn concentration, consistent with valence band models (Acbas et al., Phys. Rev. Lett. 103, 137201 (2009)).   

Investigation on the valence-band structure of ferromagnetic-semiconductor GaMnAs using spin-dependent resonant tunneling spectroscopy

We investigate the valence-band (VB) structure of ferromagnetic-semiconductor GaMnAs by analyzing the resonant tunneling levels of a GaMnAs quantum well (QW) in double-barrier heterostructures. The resonant level from the heavy-hole first state (HH1) is clearly observed in the metallic GaMnAs QW with the Curie temperature of 60 K, which indicates that no holes reside in the VB of GaMnAs in the equilibrium condition. Clear enhancement of tunnel magnetoresistance induced by resonant tunneling is demonstrated. We find that the resonant levels formed in the GaMnAs QW are well explained by using the transfer matrix method with the 6x6 Luttinger-Kohn \textit{kp} Hamiltonian, $p-d$ exchange Hamiltonian, and Bir-Pikus strain Hamiltonian. The VB structure of GaMnAs is well reproduced by that of GaAs with a small exchange splitting energy of 3-5 meV and with the Fermi level lying at $\sim $30 meV higher than HH1 in the bandgap. This work was partly supported by Grant-in-Aids for Scientific Research, the Special Coordination Programs for Promoting Science and Technology, R{\&}D for Next-generation Information Technology by MEXT, and PRESTO of JST.   

Magnetic Circular Dichroism (MCD) studies on GaMnAs

Although there is general consensus that the ferromagnetic coupling between Mn spins in GaMnAs is mediated by itinerant holes, the nature of the hole wavefunctions is still under debate. We studied MCD on a series of Ga$_{1-x}$Mn$_x$As layers grown by MBE, with x ranging from 0.02 to 0.06 to address this issue. We compare the MCD spectra taken on those samples with spectra taken on Ga$_{0.98}$Mn$_{0.02}$As samples co-doped with Be. We observe that the MCD signal disappears in the vicinity of the energy gap for samples with Be concentration higher than $1 \times10^{20}$ cm$^{-3}$ while in the undoped samples (even for x = 0.06) MCD rises sharply at the band gap. As was shown by Berciu et al [1], the MCD signal in GaMnAs arises primarily from a difference in the density of spin-up and spin-down states in the valence band. In the case of Be-doped samples the Fermi level lies in the valence band and consequently interband transitions at the band gap disappear. By contrast, strong MCD signal observed at the band gap in the undoped samples indicates a difference in the density of spin-up and spin-down states at the top of the band, indicating that the Fermi level must lie in the IB. \newline [1] M. Berciu et al., Phys. Rev. Lett. 102, 247202 (2009).   

Experimental probing of the emergence of magnetic order at the insulator-to-metal transition in (Ga,Mn)As

The question whether the Anderson-Mott localization enhances or reduces magnetic correlations is central to the physics of magnetic alloys. Particularly intriguing is the case of (Ga,Mn)As the canonical diluted magnetic semiconductors in which the spin-spin coupling is mediated by holes. In order to find out how magnetism evolves when the carrier density is diminished, magnetisation changes induced by an electric field in metal/insulator/(Ga,Mn)As structures were probed directly by SQUID magnetometry [1]. Our findings show that the channel depletion results in a monotonic decrease of the Curie temperature and spontaneous magnetic moment, with no evidence for the maximum expected within the impurity-band models but explained theoretically in terms of the appropriately modified p-d Zener model [1]. We have found that this transformation proceeds via the emergence of a hitherto non-revealed superparamagnetic-like spin arrangement, which points to a fragmentation of long range spin order into ferromagnetic and nonmagnetic regions, which are driven by critical fluctuations in the local density of states, specific to the Anderson-Mott quantum transition. Finally, our studies provide a direct magnetic evidence for spontaneous 90 deg switching of the in-plane uniaxial easy axis upon gate-voltage-induced reduction of the hole density in the channel [2]. The work was done in collaboration with D. Chiba, Y. Nishitani, F. Matsukura, and H. Ohno in Sendai and with A. Korbecka, J.A. Majewski, and T. Dietl in Warsaw. The support from Japanese: Grant-in-Aids from MEXT/JSPS, the GCOE program, the Research and Development for Next-Generation Information Technology Program (MEXT), and EU: FunDMS Advanced Grant of ERC and InTechFun (POIG.01.03.01-00-159/08) is gratefully acknowledged. \\[4pt] [1] M. Sawicki, D.Chiba, A. Korbecka, Y. Nishitani, J.A. Majewski, F. Matsukura, T. Dietl, H. Ohno, Nature Phys., DOI: 10.1038/NPHYS1455. \\[0pt] [2] D. Chiba, M. Sawicki, Y. Nishitani, Y. Nakatani, F. Matsukura, and H. Ohno, Nature \textbf{455}, 515 (2008).   

Visualizing Critical Spatial Correlations for Electronic States near the Metal-Insulator Transition in Ga$_{1-x}$Mn$_{x}$As

Semiconductors have long been used to study critical phenomena near the disorder-induced (Anderson) metal-insulator transition (MIT). We studied the dilute magnetic semiconductor Ga$_{1-x}$Mn$_{x}$As with dopings near the MIT using low temperature cross sectional scanning tunneling microscopy (STM). This allows us to visualize the electronic states near the Fermi level (E$_{F})$ which display unique critical properties. Suppression of the density of states (DOS) around E$_{F}$ due to electron-electron interactions is observed. In this energy range, the electronic states show a diverging correlation length approaching E$_{F}$, where the suppression of the DOS is strongest. The distance dependence of the correlations at E$_{F}$ is consistent with a power law decay, expected for states near criticality, while away from E$_{F}$ the correlations fall off exponentially. These results highlight the importance of electron-electron interactions and represent some of the first experimental observations of states near the Mott-Anderson MIT, where both disorder and interactions are equally important for the localization of electronic states.   

Multifractal Behavior in Ga$_{1-x}$Mn$_{x}$As Close to the Metal-Insulator Transition

We have used a low temperature scanning tunneling microscope (STM) to study the metal-insulator transition in Ga$_{1-x}$Mn$_{x}$As. STM topographies reveal a variety of electronic modulations indicating the presence of a high level of disorder. Furthermore, spectroscopic mapping reveals that the local density of states (LDOS) near E$_{F}$ is strongly influenced by e-e correlations. Therefore, Ga$_{1-x}$Mn$_{x}$As provides an opportunity to examine critical behavior of electronic states close to the MIT in a correlated system. We have studied the probability distribution and the multifractal character of the spatial distribution of the LDOS. Near the MIT, LDOS shows a transition from a Gaussian to a lognormal distribution. The multifractal character which was studied by calculating the singularity spectrum shows a trend towards strong multifractality with decreasing Mn doping. Our work demonstrates the fractal nature of wavefunctions in a correlated system, and constitutes the first experimental results to map the critical behavior of states near this quantum phase transition   

Codoping of (Ga,Mn)As as a route to higher T$_{c}$

We will present a theoretical study of diluted magnetic semiconductors (DMS) with focus on (Ga,Mn)As. Earlier studies have revealed that the critical temperatures should rapidly increase with Mn concentration and could potentially reach room temperature if no compensating defects are present. Using Li interstitial atoms as codoping elements in (Ga,Mn)As, we have found a promising way to boost the Mn concentration to large values ($>$ 20 {\%}). However, the Li interstitials destroy the ferromagnetic properties of (Ga,Mn)As and in order to have a functional material, the Li atoms needs to be removed from the system using annealing techniques, similar to what is used to remove Mn interstitials. Therefore we have performed a detailed study of the diffusion dynamics such as migration barriers etc of Li interstitials using very large scale supercell calculations employing special quasirandom structures and compare this with the case of Mn interstitials. Moreover, several defect complexes consisting of Li (Mn) interstitials and Mn substitutionals have been considered. It is found that the migration barrier for a Li interstitial is typically lower than for a Mn interstitial, however the migration barrier for the latter case is very dependent on local environment effects.