Time-resolved terahertz spectroscopy and Hall measurement on chromium vanadium thin films

We have measured the low-frequency dynamical conductivity of Cr$_{1-x}$V$_x$ thin films through the quantum phase transition at $x\approx 0.35$ using terahertz time-domain spectroscopy. From Drude model fits we have determined the plasma frequency $\omega_p$ of samples over concentrations $x=0-0.1$ and temperatures 10--300 K. We have compared these to the Hall resistance R$_H$ on the same samples and found that both reveal the opening of the spin-density wave gap. At high temperatures $\omega_p^2\propto 1/R_H$, but as the temperature is lowered below 75 K, $1/R_H$ falls more rapidly than $\omega_p^2$. We will relate these observations to a theoretical model based on a realistic Fermi surface.