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Hall effect in the extremely large magnetoresistance semimetal WTe$_2$

We systematically measured the Hall effect in the extremely large magnetoresistance semimetal WTe$_2$. By carefully fitting the Hall resistivity to a two-band model, the temperature dependencies of the carrier density and mobility for both electron- and hole-type carriers were determined. We observed a sudden increase of the hole density below $\sim$160~K, which is likely associated with the temperature-induced Lifshitz transition reported by a previous photoemission study. In addition, a more pronounced reduction in electron density occurs below 50~K, giving rise to comparable electron and hole densities at low temperature. Our observations indicate a possible electronic structure change below 50~K, which might be the direct driving force of the electron-hole ``compensation'' and the extremely large magnetoresistance as well. Numerical simulations imply that this material is unlikely to be a perfectly compensated system.