Noise Spectroscopy as a Probe of Condensed Matter Systems

Darryl Smith, PhD
Los Alamos National Laboratory

Pump-probe techniques, which measure the response of a material to an external perturbation, are often used to study condensed matter systems.  Generally, the magnitude of the measured response decreases with decreasing system size.  Therefore measurements based on pump-probe techniques can become difficult for nanometer-scale structures.  The fluctuation-dissipation theorem guarantees that the properties of a system can also be determined from its intrinsic fluctuations.  Noise spectroscopies, which measure a system's fluctuations, can offer advantageous scaling with reduced system size and can also disturb the system less as compared with pump-probe measurements.  Thus noise spectroscopies can be attractive for the study of small scale systems.  I will discuss a study (experiments by Scott Crooker of the National High Magnetic Field Lab at LANL) of stochastic electron spin fluctuations—spin noise—in two physical systems: gases of alkali atoms, and electrons in n-type semiconductors.  Electron spin noise in classical alkali atomic gases will be discussed first.  Classical alkali atomic gases are well understood and these studies can be considered as a validation of the noise spectroscopy approach by applying it to a known system.  Based on the results for classical alkali atomic gases, I will discuss a theoretical study of the application of noise spectroscopy to ultracold fermionic gases of alkali atoms.  Information about the quantum state of these atomic systems is contained in the line shape of the noise peaks.  Lastly I will describe spin noise in lightly doped n-type GaAs.  Frequency spectra of electron spin noise are studied as a function of electron density, applied magnetic field, temperature, probe-laser wavelength and intensity, and interaction volume.  As an example of the results, the spin noise power increases linearly as a function of temperature at low temperature as expected for degenerate electrons, but with a zero-temperature offset that is not expected in the simplest theory.