Terahertz conductivity measurements on chromium-vanadium alloys and photoexcited insulating cuprates

This thesis describes the application of terahertz (THz) spectroscopy to two physical systems, metallic Cr1-xVx alloys and insulating cuprates. In the first system, we use time-domain THz spectroscopy (THz-TDS) to measure the low-frequency dynamical conductivity of Cr1-xVx thin films. From the Drude model, we determine the plasma frequency of samples over concentrations with x=0-0.08, as the system undergoes a quantum phase transition at x~0.03 from a spin-density-wave state to a paramagnetic state. We compare these plasma frequency estimates to those inferred from the Hall resistance RH on the same samples. We find that while both techniques reveal the opening of the spin-density-wave gap, quantitative differences appear at low temperatures that we attribute to anisotropic scattering.In the second system, we use THz pulses to probe the conductivity of photoexcited carriers in insulating cuprates Sr2CuO2Cl2, YBa2Cu3O6, and La2CuO4. In all these compounds, photoconductivity appears promptly and decays non-exponentially in picoseconds. In the first few picoseconds after photoexcitation, the decay is characterized by a fast dynamics that is weakly dependent on material, temperature, and concentration in the range of 0.2 to 1.5% holes per unit cell. Assuming a quantum efficiency of unity, the estimated peak mobility for all three compounds falls in the range 0.1-0.5 cm2/V • s. This is lower than the Hall mobility in chemically doped systems with similar carrier concentrations, but orders of magnitude larger than earlier static photoconductivity results, leading us to identify it as the intrinsic mobility of photoexcited carriers in insulating cuprates. At about 50 ps after photoexcitation, the conductivity develops a relatively strong temperature dependence that indicates polaronic hopping transport.