Giant Planar Hall Effect of Unprecedented Amplitude in n-HgTe with Extremely Low Electron Density

We report the observation of a giant planar Hall effect in an n-HgTe single crystal with an extremely low electron density of 7.3×10¹⁴cm^-3. Its amplitude reaches an unprecedented value of 0.9  Ohm cm at T = 40 K and in a magnetic field of 10 T-to the best of our knowledge, the highest ever reported for this effect in any topological material. We attribute the strong magnetoresistance anisotropy underlying the giant planar Hall effect to the coexistence of a topological Weyl semimetal phase and a trivial gapless phase in the band spectrum of a bulk n-HgTe. The former gives rise to strong negative longitudinal magnetoresistance driven by chiral anomaly, whereas the latter leads to charge compensation-induced positive transverse magnetoresistance without saturation. We have established that it is precisely this magnetoresistance that provides the dominant contribution to the giant amplitude of the planar Hall effect. The observed giant planar Hall effect in this particular n-HgTe sample suggests that low-density mercury telluride merits further investigation as a candidate material for exploring chiral-anomaly transport and for potential sensor applications.