Ballistic spin resonance

Abstract

The phenomenon of spin resonance has had far-reaching influence since its discovery 70 years ago. Electron spin resonance driven by high-frequency magnetic fields has enhanced our understanding of quantum mechanics, and finds application in fields as diverse as medicine and quantum information. Spin resonance can also be induced by high-frequency electric fields in materials with a spin-orbit interaction; the oscillation of the electrons creates a momentum-dependent effective magnetic field acting on the electron spin. Here we report electron spin resonance due to a spin-orbit interaction that does not require external driving fields. The effect, which we term ballistic spin resonance, is driven by the free motion of electrons that bounce at frequencies of tens of gigahertz in micrometre-scale channels of a two-dimensional electron gas. This is a frequency range that is experimentally challenging to access in spin resonance, and especially difficult on a chip. The resonance is manifest in electrical measurements of pure spin currents-we see a strong suppression of spin relaxation length when the oscillating spin-orbit field is in resonance with spin precession in a static magnetic field. These findings illustrate how the spin-orbit interaction can be harnessed for spin manipulation in a spintronic circuit, and point the way to gate-tunable coherent spin rotations in ballistic nanostructures without external alternating current fields.

Energy spectra for quantum wires and two-dimensional electron gases in magnetic fields with Rashba and Dresselhaus spin-orbit interactions

“…Spin orbit interaction has also been investigated experimentally 6,7 in parallel quantum wires, where universal conductance fluctuations are suppressed. More recently, ballistic spin resonance due to an intrinsically oscillating spin-orbit field has been experimentally realized in a quantum wire 8 . The observation of the 'one-dimensional spin-orbit gap' in quantum wires has also been reported recently 9 .…”

Ballistic spin resonance in multisubband quantum wires

“…This is in contrast with semiclassical Monte Carlo simulations [6,13] where the electron moves in a 2D electron gas undergoing momentum randomizing scattering events and bouncing off the walls of the channel. In this case, each BSR dip has a well-defined value for the external magnetic field and depends on the electron Fermi velocity and the channel width [14].…”

“…According to the experimental setup used to detect the BSR [14], electrons are injected using a voltage applied through a QPC (injector) and diffuses along the multisubband quantum wire until their detection by another QPC (detector). Both QPCs are fully spin polarized (conductances equal to e 2 /h) with quantization axis defined by the external magnetic field B ext .…”

“…A proper geometry for the split gate allows a pure spin current injection via a spin-selective quantum point contact (QPC). Similarly, the corresponding spin accumulation due to this spin current can be detected using a spatially separated QPC [14]. Considering a square wire confinement with width L, the Hamiltonian describing the system reads…”

“…Then, the randomized electron spin can be detected using another spin-selective QPC. A nonlocal voltage, measured between the detector QPC and the reservoir [10,14], quantifies the spin accumulation along the channel and it is suppressed whenever the resonance condition is fulfilled.…”