Giant photocurrent in asymmetric Weyl semimetals from the helical magnetic effect

We propose a type of photoresponse induced in asymmetric Weyl semimetals in an external magnetic field. In usual symmetric Weyl semimetals in a magnetic field, the particles and holes produced by an incident light in different Weyl cones have opposite helicities and hence move in opposite directions, canceling each other's contributions to the photocurrent. However this cancellation does not occur if the Weyl semimetal possesses both a broken particle-hole symmetry and a broken spatial inversion symmetry. We call the resulting generation of photocurrent the helical magnetic effect because it is induced by the helicity imbalance in a magnetic field. We find that due to the large density of states in a magnetic field, the helical magnetic effect induces a remarkable large photocurrent for incident THz frequency light. This suggests a potential application of asymmetric Weyl semimetals for creating THz photosensors.

Figure 1

Left- and right-handed tilted Weyl cones in aWSM with the Fermi sea represented by semitransparent region. The horizontal plane parametrizes the momentum space $(k_x,k_y)$ with fixed $k_z$ and the vertical axis parametrizes the energy $E\left(\mathit{k}\right)$. For a fixed frequency of external light, the states below Fermi surface (the lower red line) can be excited to above the surface (the upper red line) as indicated by the black arrows at zero temperature. The vertical distance between two red lines corresponds to the excitation energy provided by the external light. Since a part of the upper red line is below the Fermi surface in the tilted cone the excitation is highly suppressed by the Pauli blocking at low temperature.

Figure 2

The helical magnetic current as a function of the frequency of external light $\nu (=\omega /2\pi )$ for chemical potentials $\mu =1,5,10,25$ meV (solid blue, dashed orange, dotted green, and dot-dashed red, respectively), at the temperature $T=20\mathrm{K}$ (a) and $T=77\mathrm{K}$ (b). The other input parameters are $v_{\mathrm{F}}=c/300$, $v_{\mathrm{L}}/v_{\mathrm{F}}=0.1$, $B=1\mathrm{T}$, $\tau ={10}^{-11}\mathrm{s}$, and laser intensity $I={\epsilon }_{0}c{\left(2\pi \nu A\right)}^2=1.5\times {10}^6{\mathrm{W}\mathrm{m}}^{-2}$.