Lan Qing

Impurity-assisted tunneling magnetoresistance under a weak magnetic field.

Injection of spins into semiconductors is essential for the integration of the spin functionality into conventional electronics. Insulating layers are often inserted between ferromagnetic metals and semiconductors for obtaining an efficient spin injection, and it is therefore crucial to distinguish between signatures of electrical spin injection and impurity-driven effects in the tunnel barrier. Here we demonstrate an impurity-assisted tunneling magnetoresistance effect in nonmagnetic-insulator-nonmagnetic and ferromagnetic-insulator-nonmagnetic tunnel barriers. In both cases, the effect reflects on-off switching of the tunneling current through impurity channels by the external magnetic field. The reported effect is universal for any impurity-assisted tunneling process and provides an alternative interpretation to a widely used technique that employs the same ferromagnetic electrode to inject and detect spin accumulation.

Anisotropy-Driven Spin Relaxation in Germanium

A unique spin depolarization mechanism, induced by the presence of g-factor anisotropy and intervalley scattering, is revealed by spin-transport measurements on long-distance germanium devices in a magnetic field longitudinal to the initial spin orientation. The confluence of electron-phonon scattering (leading to Elliott-Yafet spin flips) and this previously unobserved physics enables the extraction of spin lifetime solely from spin-valve measurements, without spin precession, and in a regime of substantial electric-field-generated carrier heating. We find spin lifetimes in Ge up to several hundreds of nanoseconds at low temperature, far beyond any other available experimental results.

Proximity effects of a symmetry-breaking interface on spins of photoexcited electrons.

We study reflection of optically spin-oriented hot electrons as a means to probe the semiconductor crystal symmetry and its intimate relation with the spin-orbit coupling. The symmetry breaking by reflection manifests itself by tipping the net-spin vector of the photoexcited electrons out of the light propagation direction. The tipping angle and the pointing direction of the net-spin vector are set by the crystal-induced spin precession, momentum alignment and spin-momentum correlation of the initial photoexcited electron population. We examine non-magnetic semiconductor heterostructures and semiconductor/ferromagnet systems and show the unique signatures of these effects.

g-factor anisotropy driven spin relaxation in germanium

In semiconductors possessing more than a single conduction band valley, g-factor anisotropy opens a new channel of electron spin relaxation. This unusual mechanism arises in a magnetic field because the effective Zeeman field is tilted along the valley axis, and is randomized when electrons undergo intervalley scattering. This fluctuation depolarizes electron spins [1], similar to the Dyakonov-Perel mechanism in noncentrosymmetric semiconductors where spin relaxation is driven by a wavevector dependent magnetic field. We study the unique nature of g-factor anisotropy spin relaxation by spin transport measurements from long-distance germanium devices in a magnetic field aligned to the initial spin orientation [2]. The confluence of electron-phonon scattering (leading to Elliott-Yafet spin flips) and this previously unobserved physics enables the extraction of spin lifetime solely from spin-valve measurements. We find spin lifetimes in Ge up to several hundreds of ns at low temperature, far beyond any other available experimental results. Electric field and magnetic field are used to manipulate the spin signal by accelerating the spin polarized electrons and generating carrier heating, or by inducing Hanle spin precession.