ESR spectroscopy is used in various branches of science as a sensitive tool allowing for detection of unpaired electrons in in a given material. It can be applied not only for detection of free radicals, transition metals or rare earth metals, but also defects in materials or conduction electrons in metals and semiconductors.
In general, ESR spectroscopy entails the measurement of changes in the microwave-cavity quality factor (Q) as a function of magnetic field. The quality factor of the resonator can vary for different reasons. In case of a classical ESR measurement, changes in the cavity Q-factor due to energy absorption (A) by spins are recorded. The absorption of microwave radiation leads to magnetization change and in a typical ESR spectrometer the signal is proportional to dA/dB.
The Q-factor of the cavity can also change due to variation of the sample resistivity. In this case, the signal recorded by the ESR spectrometer is proportional to dσ/dB, where σ is the sample conductance. Therefore, with the use of the ESR spectrometer it is also possible to measure contactless magnetoconductance, including Shubnikov-de Haas oscillations or weak (anti-)localization features.
The basic phenomena that stands behind this technique is to modulate electric field present in the structure and to measure the change of reflectance during this process. The benefits this technique gives are that there are very sharp lines even in room temperature and that they are very sensitive to the electric field present in the structure in the way that it is possible to measure the value of electric field directly.
Modulation of electric field can be performed on several ways. For example one of the methods to do this is to put on the sample contacts and apply ac voltage, this is called Electroreflectance. The other method is Photoreflectance when we light the sample impulsively by laser.
Electromodulation reflectance gives such an information as the value of energy gap or the value of electric field near the surface or interfaces. When additionally to the contacts there is applied constant voltage it is possible to obtain the charge distribution in the structure.
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