Gateable suppression of spin relaxation in semiconductors

Oscillatory spin-polarized tunnelling from silicon quantum wells controlled by electric field

Spin-dependent electronic transport is widely used to probe and manipulate magnetic materials and develop spin-based devices. Spin-polarized tunnelling, successful in ferromagnetic metal junctions, was recently used to inject and detect electron spins in organics and bulk GaAs or Si. Electric field control of spin precession was studied in III-V semiconductors relying on spin-orbit interaction, which makes this approach inefficient for Si, the mainstream semiconductor. Methods to control spin other than through precession are thus desired. Here we demonstrate electrostatic modification of the magnitude of spin polarization in a silicon quantum well, and detection thereof by means of tunnelling to a ferromagnet, producing prominent oscillations of tunnel magnetoresistance of up to 8%. The electric modification of the spin polarization relies on discrete states in the Si with a Zeeman spin splitting, an approach that is also applicable to organic, carbon-based and other materials with weak spin-orbit interaction.

Ultrafast spin dynamics including spin-orbit interaction in semiconductors

This Letter presents a theoretical investigation of ultrafast spin-dependent carrier dynamics in semiconductors due to strong spin-orbit coupling using holes in bulk GaAs as a model system. By computing the microscopic carrier dynamics in the anisotropic hole-band structure including spin-orbit coupling, we obtain spin-relaxation times in quantitative agreement with measured hole-spin relaxation times [Phys. Rev. Lett. 89, 146601 (2002)10.1103/PhysRevLett.89.146601]. We show that different optical techniques for the measurement of hole-spin dynamics yield different results, in contrast to the case of electron-spin dynamics.

Anisotropic spin transport in (110) GaAs quantum wells

Mobile piezoelectric potentials are used to coherently transport electron spins in GaAs (110) quantum wells (QW) over distances exceeding 60 microm. We demonstrate that the dynamics of mobile spins under external magnetic fields depends on the direction of motion in the QW plane. This transport anisotropy is an intrinsic property of moving spins associated with the bulk inversion asymmetry of the underlying GaAs lattice.

Spin relaxation in the presence of electron-electron interactions

The D'yakonov-Perel' spin relaxation induced by the spin-orbit interaction is examined in disordered two-dimensional electron gas. It is shown that, because of the electron-electron interactions, substantially different spin relaxation rates may be observed depending on the technique used to extract them. It is demonstrated that the relaxation rate of a spin population is proportional to the spin-diffusion constant D(s), while the spin-orbit scattering rate controlling the weak-localization corrections is proportional to the diffusion constant D, i.e., the conductivity. The two diffusion constants get strongly renormalized by the electron-electron interactions, but in different ways. As a result, the corresponding relaxation rates are different, with the difference between the two being especially strong near a magnetic instability or near the metal-insulator transition.

Gate control of dynamic nuclear polarization in GaAs quantum wells

Gate control of dynamic nuclear polarization under optical orientation is demonstrated in a Schottky-gated n-GaAs/AlGaAs (110) quantum well by time-resolved Kerr rotation measurements. Spin relaxation of electrons due to mechanisms other than the hyperfine interaction is effectively suppressed as the donor induced background electron density is reduced from metallic to insulating regimes. Subsequent accumulation of photoexcited electron spins dramatically enhances dynamic nuclear polarization at low magnetic field, allowing us to tune nuclear spin polarization by external gate voltages.

High temperature gate control of quantum well spin memory

Time-resolved optical measurements in (110)-oriented GaAs/AlGaAs quantum wells show a tenfold increase of the spin-relaxation rate as a function of applied electric field from 20 to 80 kV cm(-1) at 170 K and indicate a similar variation at 300 K, in agreement with calculations based on the Rashba effect. Spin relaxation is almost field independent below 20 kV cm(-1) reflecting quantum well interface asymmetry. The results indicate the achievability of a voltage-gateable spin-memory time longer than 3 ns simultaneously with a high electron mobility.

Manipulation of the spin memory of electrons in n-GaAs

We report on the optical manipulation of the electron spin relaxation time in a GaAs-based heterostructure. Experimental and theoretical study shows that the average electron spin relaxes through hyperfine interaction with the lattice nuclei, and that the rate can be controlled by electron-electron interactions. This time has been changed from 300 ns down to 5 ns by variation of the laser frequency. This modification originates in the optically induced depletion of an n-GaAs layer.

Spin-galvanic effect

There is much recent interest in exploiting the spin of conduction electrons in semiconductor heterostructures together with their charge to realize new device concepts. Electrical currents are usually generated by electric or magnetic fields, or by gradients of, for example, carrier concentration or temperature. The electron spin in a spin-polarized electron gas can, in principle, also drive an electrical current, even at room temperature, if some general symmetry requirements are met. Here we demonstrate such a 'spin-galvanic' effect in semiconductor heterostructures, induced by a non-equilibrium, but uniform population of electron spins. The microscopic origin for this effect is that the two electronic sub-bands for spin-up and spin-down electrons are shifted in momentum space and, although the electron distribution in each sub-band is symmetric, there is an inherent asymmetry in the spin-flip scattering events between the two sub-bands. The resulting current flow has been detected by applying a magnetic field to rotate an optically oriented non-equilibrium spin polarization in the direction of the sample plane. In contrast to previous experiments, where spin-polarized currents were driven by electric fields in semiconductor, we have here the complementary situation where electron spins drive a current without the need of an external electric field.