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.

Pure spin currents are measured in micron-wide channels of GaAs two-dimensional electron gas (2DEG). Spins are injected and detected using quantum point contacts, which become spin polarized at high magnetic field. High sensitivity to the spin signal is achieved in a nonlocal measurement geometry, which dramatically reduces spurious signals associated with charge currents. Measured spin relaxation lengths range from 30µm to 50µm, much longer than has been reported in GaAs 2DEG's. The technique developed here provides a flexible tool for the study of spin polarization and spin dynamics in mesoscopic structures defined in 2D semiconductor systems.

Most quantum point contacts (QPCs) fabricated in high-mobility 2D electron gases show a zerobias conductance peak near pinchoff, but the origin of this peak remains a mystery. Previous experiments have primarily focused on the zero-bias peak at moderate conductance, in the range 1 − 2e 2 /h. Here, measurements are presented of zero-bias peaks that persist down to 10 −4 e 2 /h. Magnetic field and temperature dependencies of the zero-bias peak in the low-conductance limit are qualitatively different from the analogous phenomenology at higher conductance, with implications for existing theoretical models of transport in low-density QPCs.

Nuclear spin polarization is typically generated in GaAs quantum point contacts ͑QPCs͒ when an out-ofplane magnetic field gives rise to spin-polarized quantum-Hall edge states and a voltage bias drives transitions between the edge states via electron-nuclear flip-flop scattering. Here, we report a similar effect for QPCs in an in-plane magnetic field, where currents are spin polarized but edge states are not formed. The nuclear polarization gives rise to hysteresis in the dc transport characteristics with relaxation time scales around 100 s. The dependence of anomalous QPC conductance features on nuclear polarization provides a useful test of their spin sensitivity.QPCs are the simplest of all semiconductor nanostructures: short one-dimensional ͑1D͒ constrictions between regions of two-dimensional electron gas ͑2DEG͒ with conductance quantized in units of G =2e 2 / h at zero magnetic field and low temperature ͑the factor of 2 comes from spin degen-eracy͒ or 1e 2 / h at high magnetic field when spin degeneracy is broken. 1,2 Despite their apparent simplicity, the spin physics of QPCs has inspired a great deal of debate in the last ten years ever since it was pointed out that their lowconductance transport characteristics ͑G Ͻ 2e 2 / h͒ deviate from a simple noninteracting picture. [3][4][5] One of these anomalous characteristics is a zero-bias conductance peak ͑ZBP͒ observed for G Ͻ 2e 2 / h at low temperature. As the applied magnetic field is increased, some ZBPs collapse without splitting while others split into two peaks by 2g B B, with Landé g factor ranging from much less than the bulk value in GaAs, g = 0.44, to much greater than 0.44. 6-8 The complicated field dependence of ZBPs has given rise to controversial explanations, ranging from Kondo physics 6,9 to a nonspin-related phenomenological model. 8 Despite extensive experimental and theoretical work to understand the electron spin physics of QPCs below the 2e 2 / h plateau, the effects of nuclear spin on QPC conductance have only been studied deep in the quantum-Hall regime, where many of the conductance anomalies disappear. 10,11 Over the last decade, however, it has become increasingly clear that understanding the electron spin physics of semiconductor nanostructures requires a careful consideration of the influence of nuclear spin via the hyperfine interaction. 12,13 This is especially true for nanostructures defined in GaAs and other III-V materials, where large atomic masses lead to a large electron-nuclear coupling constant through the Fermi contact interaction. 14 The hyperfine interaction gives rise to an effective magnetic field acting on electron spin that is proportional to the local nuclear-spin polarization. Significant nuclear polarizations can be built up, also via the hyperfine interaction, when a nonequilibrium population of electron spins relaxes, flipping nuclear spins to conserve angular momentum: a process known as dynamic nuclear polarization ͑DNP͒. 15 For example, a large dc bias applied between spin-polarized edge states in the qua...

We consider electronic transport through laterally parallel double open quantum dots embedded in a quantum wire in a perpendicular magnetic field. The coupling modes of the dots are tunable by adjusting the strength of a central barrier and the applied magnetic field. Probability density and electron current density are calculated to demonstrate transport effects including magnetic blocking, magnetic turbulence, and a hole-like quasibound state feature. Fano to dip line-shape crossover in the conductance is found by varying the magnetic field.