The orientation of the measured single crystal was performed and crystal structure of NbP was

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1 Extremely large magnetoresistance and ultrahigh mobility in the topological Weyl semimetal candidate NbP Chandra Shekhar 1, Ajaya K. Nayak 1, Yan Sun 1, Marcus Schmidt 1, Michael Nicklas 1, Inge Leermakers 2, Uli Zeitler 2, Yurii Skourski 3, Jochen Wosnitza 3, Zhongkai Liu 4, Yulin Chen 5, Walter Schnelle 1, Horst Borrmann 1, Yuri Grin 1, Claudia Felser 1, & Binghai Yan 1,6 1 Max Planck Institute for Chemical Physics of Solids, Dresden, Germany 2 High Field Magnet Laboratory (HFML - EMFL), Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands 3 Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum Dresden- Rossendorf, Dresden, Germany 4 Diamond Light Source, Harwell Science and Innovation Campus, Fermi Ave, Didcot, Oxford- shire, OX11 0QX, UK 5 Physics Department, Oxford University, Oxford, OX1 3PU, UK 6 Max Planck Institute for the Physics of Complex Systems, Dresden, Germany The orientation of the measured single crystal was performed and crystal structure of NbP was investigated using single-crystal X-ray diffraction. Plate-shaped single crystals were fixed with glue on the glass capillary. The diffraction data sets were collected on a Rigaku AFC7 diffractometer equipped with a Saturn 724+ CCD detector (monochromatic MoKα radiation, λ = Å). The intensities of the measured reflections were corrected for absorption using multi-scan technique. Structure refinement was performed by full-matrix least-squares on F within the program package WinCSD. The crystal structure of NbP is a non-centrosymmetric one. No indication of twinning was found in diffraction experiments. The orientation of the single crystal used for physical measurements is shown in Figure S1. Crystallographic data: space group I4 1 md, a = (3) Å, c = (1) Å, V = (4) Å 3, Z = 4, F(000) = 224, ρ calc = g/cm 3, µ(mokα) = mm 1, crystal dimensions NATURE PHYSICS 1

2 mm 3 (used for the structural characterization). Collected data: 2θ up to 84.09, 1597 measured reflections, 273 symmetry independent reflections, 271 observed reflections (I>2σ(I)), R int = 0.019, 9 parameters with anisotropic approximation of the atomic displacement, R(F) = Atomic coordinates in NbP are as follows: 4Nb in 4(a) 0 0 z, z = 0.0; 4P in 4(a) 0 0 z, z = (8). [100] [010] [001] Figure S1. Orientation of the single crystal NbP with the respective rotation diffraction patters. The rotation axis is in each case vertical. This crystal was used in all transport measurements. 2 NATURE PHYSICS

3 SUPPLEMENTARY INFORMATION Figure S2. Field dependence of Hall resistivity (ρ xy ) measured at different temperature. Figure S2 shows the field dependence of Hall resistivity, ρ xy, at various temperatures. The ρ xy are used to determine the Hall coefficient, mobility and charge carrier density. The sign of ρ xy gives the information about types of charge carriers (positive sign for hole and negative sign for electron). The ρ xy (H) behavior is quite linear both at low and high temperatures with change of sign. However, a nonlinear behavior is found at intermediate temperatures. This behavior is like a compensated semimetal, where both charge carriers contribute to the transport measurements. Therefore, NbP is proposed to show two sets of different electronic transport parameters; electron carrier density and mobility (n e and µ e ), and the hole carrier density and mobility (n h and µ h ). In the two-band Drude model, the matrix equation of the total resistivity tensor ρρ = ρρ!!!!!!! + ρρ! and the transverse component is NATURE PHYSICS 3

4 ρρ!" = 1 ee nn! μμ!! nn! μμ!! BB + μμ! μμ! nn! nn! BB! nn! μμ! + nn! μμ!! + μμ!! μμ!! nn! nn! BB! The cubic term in the numerator of ρ xy causes the slope changing in the Hall resistivity. Figure S3. Temperature dependence of Hall coefficient measured at 9 T. Figure S3 shows the temperature dependent of Hall coefficient, R H, at a field of 9 T. The observation of negative R H at low temperatures illustrates the presence of electrons as charge carrier. The presence of holes as charge carrier for T > 110 K is evident from the positive R H at higher temperatures. We found a lower charge-crossover temperature in the temperature dependent measurements in comparison to the field dependent measurements due to the contribution of the magnetoresistance in the former case. 4 NATURE PHYSICS

5 SUPPLEMENTARY INFORMATION Figure S4. Field dependence of longitudinal magnetoresistance measured at different temperatures. Figure S4 shows longitudinal magnetoresistance (current parallel to field) measured at different temperatures for magnetic field up to 9 T. The longitudinal magnetoresistance is of the same order of magnitude as the transverse MR (current perpendicular to field). This reflects that NbP is nearly isotropic, and hence the transport properties are three dimensional. NATURE PHYSICS 5