Spectroscopy of spontaneous spin noise as a non-perturbative probe of spin dynamics and magnetic resonance

Scott Crooker, LANL-NHMFL

Noise in experimental measurements is historically and generally disregarded as being an unwelcome background, or at best, a nuisance to be minimized. However, certain types of noise contain a wealth of information about the system itself —a classic example being the small, inherent fluctuations of electrical current (current shot noise), which both demonstrates the discrete nature of the current carriers, and also directly yields the electron charge. In magnetic systems, the intrinsic random fluctuations of N paramagnetic spins will generate measurable noise of order sqrt{N} spins, even in zero magnetic field. Here we address precisely these same sqrt{N} spin fluctuations, using off-resonant Faraday rotation to passively „listen‰ to the magnetization noise in an equilibrium ensemble of paramagnetic alkali atoms [1]. These random fluctuations generate spontaneous spin coherences which precess and decay with the same characteristic energy and time scales as the macroscopic magnetization of an intentionally polarized or driven ensemble. Correlation spectra of the measured spin noise reveals g-factors, nuclear spin, isotope abundance ratios, hyperfine splittings, nuclear moments, and spin coherence lifetimes -- without having to excite, optically pump, or otherwise drive the system away from thermal equilibrium. These noise signatures scale inversely with interaction volume, suggesting routes towards non-perturbative, sourceless magnetic resonance of small systems, particularly in the solid state.

As part of ongoing efforts to measure the intrinsic spin fluctuations of conduction electrons in semiconductors, we use related optical techniques (scanning Kerr microscopy) to directly acquire two-dimensional images of (intentionally) spin-polarized electrons that are flowing laterally in bulk epilayers of n:GaAs [2]. Optical injection provides a local dc source of polarized electrons, whose subsequent drift and/or diffusion is controlled with electric, magnetic, and - in particular - strain fields. Spin precession induced by controlled uniaxial stress along the <110> axes demonstrates the direct k-linear spin-orbit coupling of electron spin to the shear (off-diagonal) components of the strain tensor.

1. S.A. Crooker, D. Rickel, A. Balatsky, D.L. Smith, Nature v431, p49 (2004).
2. S.A. Crooker, D.L. Smith, cond-mat/0411461.

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