Transfer of Spin Polarization

Spin Transfer from an Optically Pumped Alkali Vapor to a Solid

K. Ishikawa, B. Patton, Y.Y. Jau, and W. Happer

PRL 98, 183004 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e183004

This editor recommendation was a very well-written paper.

As the title indicates, the authors demonstrated the transfer of spin from optically pumped cesium vapor to cesium salt on the surface of a sealed glass cylinder. The authors deposited a layer of CsH salt on the walls of the container about 10 microns thick. Next, they put in pure cesium and nitrogen at various pressures and sealed the cylinders.

They used a laser tuned to a specific atomic transition in the cesium vapor to pump the system using both right and left handed circularly polarized light. To see the effects of pumping the vapor on the salt layers, the authors measured the free induction decay of the sample --- a common technique in NMR. They were able to discriminate between the vapor and the salt because the resonance of the vapor is at 52.39 MHz while the salt is at 53.15 MHz. They found that in optically pumped samples, the polarization of the salt was 4 times its value in the unpumped samples. They take this as demonstration of transfer of spin polarization from the vapor to the salt.

Charles Marcus spoke at Penn a couple months ago and discussed his work on one and two electron quantum dots. He makes artificial atoms on solid state chips and inserts electrons, makes them interact, and reads out their final states. It's one of the first approaches to quantum computing that I would consider feasible. Anyway, Marcus and his group observed that when they sent spin polarized electrons through the system, they could polarize the nuclear spins in the dot. I thought he called it a "nuclear zamboni," but I can't track down that reference right now. The idea is that you can use an electron beam to smooth out the nuclear environment and make it less likely that nuclear disorder will destroy your carefully prepared electron qbits.

The authors of the current paper discuss a similar phenomenon, and they give a nice theoretical description of the process. They argue that nuclear and electronic spins obey a set of coupled diffusion equations (and give very clear descriptions of where all the terms in the equation come from). They give approximate solutions to this set of equations, then show that these approximate and easy-to-understand solutions reproduce the major features of a much more detailed numerical model. Their simple model explains why the electron current scales inversely with pressure, and why the nuclear current is proportional to the pressure for small pressure, and constant at large pressures.

Despite the qualitative agreement between the diffusion model and experimental data, the authors imply the theory of spin transfer is not understood all that well. It's quite an interesting theoretical problem --- one I'm interested in myself. Usually in the analysis of collisions (as in particle physics calculations), one averages over the initial and final spin polarizations to compute the cross section. For a spin polarized sample, this is not the right approach.

In addition, atomic physics offers possibilities not allowed in particle physics. Suppose an atom in a singlet state scatters off a magnetic impurity. There is some probability for an interaction that would leave the atom in a triplet state and flip a spin in the magnetic impurity.

Another interesting situation is Coulomb drag. The Coulomb interaction does not allow for spin flips. This results in scattering processes that conserve charge current but reduce the spin current. I have an article on the phenomenon somewhere. I should write that up.