In layered semiconductors with spin-orbit interaction (SOI) a persistent spin helix (PSH) state with suppressed spin relaxation is expected if the strengths of the Rashba and Dresselhaus SOI terms, α and β, are equal. Here we demonstrate gate control and detection of the PSH in two-dimensional electron systems with strong SOI including terms cubic in momentum. We consider strain-free InGaAs/InAlAs quantum wells and first determine a ratio α/β 1 for nongated structures by measuring the spin-galvanic and circular photogalvanic effects. Upon gate tuning the Rashba SOI strength in a complementary magnetotransport experiment, we monitor the complete crossover from weak antilocalization via weak localization to weak antilocalization, where the emergence of weak localization reflects a PSH-type state. A corresponding numerical analysis reveals that such a PSH-type state indeed prevails even in presence of strong cubic SOI, however no longer at α = β. An electron moving in an electric field experiences, in its rest frame, an effective magnetic field pointing perpendicularly to its momentum. The coupling of the electron's spin to this magnetic field is known as spin-orbit interaction (SOI). The ability to control the corresponding magnetic field, and thereby spin states, all electrically in gated semiconductor heterostructures 1,2 is a major prerequisite and motivation for research towards future semiconductor spintronics. However, on the downside, the momentum changes of an electron moving through a semiconductor cause sudden changes in the magnetic field leading to spin randomization. Hence, suppression of spin relaxation in the presence of strong, tunable SOI is a major challenge of semiconductor spintronics.In III-V semiconductor heterostructures two different types of SOI exist: (i) Rashba SOI, 3 originating from structure inversion asymmetry (SIA), is linear in momentum k with a strength α that can be controlled by an electric gate.(ii) Dresselhaus SOI 4 is due to bulk inversion asymmetry (BIA), which gives rise to a band spin splitting, given by k-linear and k-cubic contributions. 5 The strength of the linear in k term β = γ k 2 z (where γ is a material parameter) can hardly be changed as it stems from crystal fields. These various spin-orbit terms in layered semiconductors are described by the Hamiltonian H SO = H R + H D with Rashba and Dresselhaus termswith σ x ,σ y the Pauli spin matrices. 7 If the k-cubic terms can be neglected, a special situation emerges if Rashba and Dresselhaus SOI are of equal strength: α = ±β.Then spin relaxation is suppressed. 8,9 A collinear alignment of Rashba and Dresselhaus effective magnetic fields gives rise to spin precession around a fixed axis, leading to spatially periodic modes referred to as persistent spin helix (PSH) and reflecting the underlying SU (2) symmetry in this case. 10The PSH is robust against all forms of spin-independent scattering. This favorable situation where spin relaxation is suppressed while the spin degree of freedom is still susceptible to electric...

We measured the Dresselhaus spin-orbit interaction coefficient β 1 for (001)-grown GaAs/Al 0.3 Ga 0.7 As quantum wells for six different well widths w between 6 and 30 nm. The varying size quantization of the electron wave vector z-component k 2 z ∼ (π/w) 2 influences β 1 = −γ k 2 z linearly. The value of the bulk Dresselhaus coefficient γ = (−11 ± 2) eVÅ 3 was determined. We discuss the absolute sign of the Landé g factors and the effective momentum scattering times.

Both structure and bulk inversion asymmetry in modulation-doped ͑001͒-grown GaAs quantum wells were investigated employing the magnetic field induced photogalvanic effect. We demonstrate that the structure inversion asymmetry ͑SIA͒ can be accurately tailored by the delta-doping layer position. Symmetrically doped structures exhibit a substantial SIA due to impurity segregation during the growth process. Tuning the SIA by the delta-doping position, we can grow samples with almost equal degrees of structure and bulk inversion asymmetry.

We report on the study of the linear and circular magnetogyrotropic photogalvanic effect (MPGE) in GaAs/AlGaAs quantum well structures. Using the fact that in such structures the Landé factor g * depends on the quantum well (QW) width and has different signs for narrow and wide QWs, we succeeded to separate spin and orbital contributions to both MPGEs. Our experiments show that, for most QW widths, the MPGEs are mainly driven by spin-related mechanisms, which results in a photocurrent proportional to the g * factor. In structures with a vanishingly small g * factor, however, linear and circular MPGE are also detected, proving the existence of orbital mechanisms.

We report on microwave (mw) radiation induced electric currents in (Cd,Mn)Te/(Cd,Mg)Te and InAs/(In,Ga)As quantum wells subjected to an external in-plane magnetic field. The current generation is attributed to the spin-dependent energy relaxation of electrons heated by mw radiation. The relaxation produces equal and oppositely directed electron flows in the spin-up and spin-down subbands yielding a pure spin current. The Zeeman splitting of the subbands in the magnetic field leads to the conversion of the spin flow into a spin-polarized electric current.