Spin Coulomb Drag Research Articles

Coulomb drag, magnetoresistance, and spin-current injection in magnetic multilayers

We study the effect of spin Coulomb drag on the magnetoresistance and the spin-current injection efficiency of a layered structure consisting of a nonmagnetic semiconductor sandwiched between two ferromagnetic electrodes of spin polarization p. The calculations are done within the framework of the drift–diffusion theory, which we generalize to include the spin trans-conductivity σ ↑↓. We find that for p close to 100% the spin drag enhances the magnetoresistance, while for smaller values of p it reduces it. A new approach to the measurement of σ ↑↓ is suggested.

Spin Coulomb drag in the two-dimensional electron liquid

We calculate the spin-drag transresistivity $\rho_{\uparrow \downarrow}(T)$ in a two-dimensional electron gas at temperature $T$ in the random phase approximation. In the low-temperature regime we show that, at variance with the three-dimensional low-temperature result [$\rho_{\uparrow\downarrow}(T) \sim T^2$], the spin transresistivity of a two-dimensional {\it spin unpolarized} electron gas has the form $\rho_{\uparrow\downarrow}(T) \sim T^2 \ln T$. In the spin-polarized case the familiar form $\rho_{\uparrow\downarrow}(T) =A T^2$ is recovered, but the constant of proportionality $A$ diverges logarithmically as the spin-polarization tends to zero. In the high-temperature regime we obtain $\rho_{\uparrow \downarrow}(T) = -(\hbar / e^2) (\pi^2 Ry^* /k_B T)$ (where $Ry^*$ is the effective Rydberg energy) {\it independent} of the density. Again, this differs from the three-dimensional result, which has a logarithmic dependence on the density. Two important differences between the spin-drag transresistivity and the ordinary Coulomb drag transresistivity are pointed out: (i) The $\ln T$ singularity at low temperature is smaller, in the Coulomb drag case, by a factor $e^{-4 k_Fd}$ where $k_F$ is the Fermi wave vector and $d$ is the separation between the layers. (ii) The collective mode contribution to the spin-drag transresistivity is negligible at all temperatures. Moreover the spin drag effect is, for comparable parameters, larger than the ordinary Coulomb drag effect.

Intrinsic Decay of Spin Currents: The Spin Coulomb Drag Effect

We review the properties of spin Coulomb drag, which describes the effects of the “friction” arising between different spin-polarized carrier populations when they travel with different average velocities. We compare this effect with the ordinary Coulomb drag between separate slabs underlining some important differences related to the form of the Coulomb interaction in the two cases. We show that the spin-transresistivity, a measure of the spin Coulomb drag effect, can become as high as 10−2–10−3 Ω cm in three dimensions and of the order of several kiloohms in two dimensions. We finally underline that, in some realistic systems, the spin transresistivity can become comparable to the usual Drude resistivity.

Coulomb interaction effects in spin-polarized transport

We study the effect of the electron-electron interaction on the transport of spin polarized currents in metals and doped semiconductors in the diffusive regime. In addition to well-known screening effects, we identify two additional effects, which depend on many-body correlations and exchange and reduce the spin diffusion constant. The first is the "spin Coulomb drag" - an intrinsic friction mechanism which operates whenever the average velocities of up-spin and down-spin electrons differ. The second arises from the decrease in the longitudinal spin stiffness of an interacting electron gas relative to a noninteracting one. Both effects are studied in detail for both degenerate and non-degenerate carriers in metals and semiconductors, and various limiting cases are worked out analytically. The behavior of the spin diffusion constant at and below a ferromagnetic transition temperature is also discussed.

Spin Coulomb drag and spin diffusion in doped semiconductors

We investigate the effect of many-body correlations and exchange on spin-polarized transport in the diffusive regime. We show that the spin diffusion constant of a spin packet is reduced by the combined effect of the “spin Coulomb drag”—an intrinsic friction mechanism which operates whenever the average velocities of up- and down-spin electrons differ—and by the Coulomb enhancement of the spin susceptibility. The behavior of the spin diffusion constant at and below the ferromagnetic transition temperature is also discussed.

Theory of spin Coulomb drag in spin-polarized transport

We introduce a distinctive feature of spin-polarized transport, the spin Coulomb drag: there is an intrinsic source of friction for spin currents due to the Coulomb interaction between spin ``up'' and spin ``down'' electrons. We calculate the associated ``spin trans-resistivity'' in a generalized random-phase approximation and show that, to the leading order in the interactions, it has no contribution from correlated impurity scattering. We show that, in an appropriate range of parameters, such resistivity is measurable, and we propose an experiment to measure it.