The application of external pressure on a solid can be used to continuously tune structural (tolerance factors, bond angles, …) or electronic (hopping amplitudes, nonlocal Coulomb interactions, …) parameters. In particular in superconductors pressure can drive the compounds towards or away from lattice instabilities by varying relevant parameters for the superconducting properties and thus tuning the critical transition temperature Tc.
Typically conventional low temperature superconductors show a linear decrease of Tc under pressure due to changes of the density of states at the Fermi energy and the characteristic phonon frequency that alter the effective electron phonon coupling.
More interesting is the behavior of the cuprate high temperature superconductors where the Tc increases as pressure is applied in certain ranges. Models explaining this behavior are based on the change of the effective charge reservoir between the CuO2 layers and the exchange interactions due to structural parameters.
Diamond Anvil Cell
For generating high static pressures and having optical access we use a Diamond Anvil Cell (DAC).
A typical DAC has a structure like the one reported in Fig. 1. Pressure is applied by squeezing the sample inbetween the two diamonds. Quantitative information about the pressure is extracted in situ by measuring the shift of the fluorescence doublet of a ruby chip placed next to the sample.
Optical properties of diamond, the anvils can be also used as a reference for the determination of the optical properties
Given the well-known. In time domain spectroscopy we are able to distinguish reflections from diamond-air and sample-diamond interfaces. Their ratio gives then the absolute reflection coefficient for the sample under investigation. In Fig. 2 is shown a sketch of the experimental configuration and the reflected fields at the two interfaces as measured in a THz time domain experiment.
Figure 2:
(left) Electric fields reflected from the diamond and the sample surfaces.
(right) Reflected THz Fields as measured in a time-domain experiment.
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