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Principle of Raman scattering

Fig. 1: Schematic drawing of an inelastic light scattering experiment. The incident photon (blue) creates an excitation (red) in the sample (yellow) and a scattered photon with shifted energy (green).

Inelastic light scattering is a photon in-photon out process and was discovered simultaneously by Raman and Krishnan in liquids and by Landsberg and Mandelstam in quartz in 1928. In these days it is a widely used tool in analytics and basic research. At the WMI, Raman scattering is mainly used for the study of electronic properties of highly correlated electron systems and superconductors. In addition, metallic glasses, charge density wave (CDW) systems, fullerenes, carbon nanotubes, hydrogen, and perovskites are investigated.

As shown in Fig. 1, an incoming polarized photon (blue) is absorbed by the sample (yellow) and another one (green) with different energy, momentum, and polarization is emitted. In the course of the scattering process, an elementary excitation (red) is created in the material with the energy and momentum corresponding to the energy and momentum difference of the two photons. Since light can be scattered by molecular vibrations and rotations, lattice modes (phonons), conduction electrons, spin waves, and orbital excitations a host of information can be obtained. Fig. 2 shows a state-of-the-art light scattering setup with a He-flow cryostat (center), a double monochromator (center right) and a CCD detector (right).

Due to the freedom to independently adjust the polarizations of two photons, selection rules arise which are widely used for, e.g., the interpretation of vibrational spectra. In the case of conduction electrons different regions of the Fermi surface can be accessed independently using appropriate combinations of photon polarizations. In the copper-oxygen compounds (cuprates) and in the Fe-based materials (pnictides), the diagonal parts and the principle axes and, respectively, the electron and hole bands can be projected out separately, as shown in Fig. 3.

This allowed us to map out the energy gap and to determine dynamic properties or normal electrons as a function of momentum in cuprate and Fe-based superconductors hence providing information beyond infrared (optical) spectroscopy.
Since recently, properties can be studied with applied pressure and, simultaneously, in magnetic fields. Using the CDW amplitude mode as a probe the pressure-temperature phase diagram of tri-telluride systems could be studied.

Setup for light scattering
Setup for light scattering

Fig. 2: Light scattering setup. The incident laser light (lower left) has a well-defined polarization when entering the cryostat (center). The scattered light is collected along the surface normal of the sample. After the selection of the desired polarization state the photons enter the spectrometer (center right). The transmitted photons of a given energy are detected with a CCD camera (right).

Fig. 3: Selection rules for conduction electrons in copper-oxygen and Fe-based compounds. The light polarizations are indicated symbolically. (a) In the cuprates, electrons with momenta along the diagonal and principle axes of the Brillouin zone can be selected for the B2g and the B1g symmetry, respectively. (b) In the pnictides, the hole and the electron bands have dominant contributions to the A1g and B1g spectra, respectively.

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