Enhancement in conductivity and classical XMR in Perfect electron-hole compensated W${1-x}$Ni$x$Te$_2 $($x$=0, 0.1) Weyl Semimetal

The Magnetoresistance (MR) is an interesting property of a material which has immense technological importance in devices such as magnetic sensors, magnetic switches, magnetic valves and hard drives in electronics and spintronics, etc [1]. Usually, MR is small in simple metals and shows H$^2$ (H = magnetic field) dependence at low field and saturates at high fields. Some materials show huge MR at low temperatures. Different mechanisms have been proposed to explain the origin of such large MR [2-4], however, the exact origin of MR is still not fully understood. Recently, WTe$_2$ has been reported to exhibit extremely large magnetoresistance (XMR), as high as 1.3$\times10^7$% with no signature of saturation upto 60T at 0.53K [1]. Perfect electron-hole (e-h) compensation is pro-posed to be a origin of XMR in WTe$_2$ [1]. One pair of electron and hole pockets of nearly same size has been observed in a recent angle resolved photoemission spectroscopy (ARPES) experiment which supports the idea of carrier compensation for XMR [5]. Since, the e-h compensation is sensitive to the tuning of charge carrier, investigation of MR with doping may further clarify the origin of MR in these exotic materials.
In this work, we have studied the MR properties of W$_{1-x}$Ni$_x$Te$_2 $($x$=0, 0.1) single crystals. Enhancement in conductivity by one order is observed in Ni-doped compound relative to that in the parent compound. MR is found to be as high as 5.1$\times$10$^5$% at 2K and 12T in parent sample which decreased by one order in the doped sample. At low field, MR shows quadratic behavior but at high field it is nearly linear in both the samples. Kohler’s plot indicates presence of at least two types of charge carriers. Perfect e-h compensation has been observed by the H$^2$ fitting of MR at low field. High mobility up to 70000 and 15500 cm$^2$/VS have been found in the parent and doped samples, respectively. Such relatively lower value of mobility may be one possible reason for the reduced MR in the doped sample. Mobility controlled linear magnetoresistance has been observed at high field (H$\geq$9T) in both the samples which was consistent with classical model proposed by Parish and Littlewood. SdH oscillations have also been observed in both the samples. Fast Fourier transform of the background subtracted SdH oscillations shows single peak in both cases which further confirms the perfect e-h compensation in the parent and doped samples. Effective mass of the carriers has been also found to be increased in the doped sample. Change in effective mass of the charge carriers while maintaining the e-h compensation in the Ni-doped sample provides an interesting platform to realize technologically important materials.

[1] M.N. Ali et al., Nature 514, 205 (2014).
[2] M. Lee et al., Phys. Rev. Lett. 88, 066602 (2002).
[3] F.Y. Yang et al., Science 284, 1335 (1999).
[4] T. Liang et al., Nat. Mater. 14, 280 (2015).
[5] I. Pletikosi et al., Phys. Rev. Lett. 113, 216601 (2014).

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