Abstract
The discovery of Weyl semimetals (WSM) has brought forth the condensed matter realization of Weyl fermions, which were previously theorized as low energy excitations in high energy particle physics. Recently, transition metal monopnictides are under intense investigation for understanding properties of inversionsymmetry broken Weyl semimetals. Nontrivial Berry phase and chirality are important markers for characterizing topological aspects of Weyl semimetals. Most recently, theoretical calculations predict strong influence of the position of Weyl nodes with respect to Fermi surface and weak disorder that can drive WSMs into chirally symmetric Dirac semimetals. Using magnetotransport measurements in single crystals of WSM NbP, we observe an exceptionally large magnetoresistance at low temperature, which is nonsaturating and linear at high fields. The origin of linear transverse magnetoresistance is assigned to charge carrier mobility fluctuations. Negative longitudinal magnetoresistance is not seen, suggesting lack of welldefined chiral anomaly in NbP. Unambiguous Shubnikovde Haas oscillations are observed at low temperatures that are correlated to a trivial Berry phase corresponding to Fermi surface extrema at 30.5 Tesla. Our results are important towards identifying topological characteristics of Weyl semimetals and their experimental manifestations in the presence of weak disorder.
Introduction
The massless solutions of Dirac equations in relativistic particle physics have recently found material basis in topological phases of quantum condensed matter, and this has led to rediscovery of several semimetals with such exotic quantum field theory perspectives^{1,2,3}. From the experimental point of view, perhaps the starting point was the Quantum Hall effect wherein the role of magnetic field is replaced by strong spinorbit coupling in topological insulators of Bismuth based chalcogenides^{4,5}. The peculiar topology of the band structure of such tetradymites leads to a fully gapped insulating bulk and symmetryprotected conducting surface states^{5,6,7}. In the very recent past, similar properties are observed in a new class of topological quantum phase in the Weyl semimetals (WSMs)^{8}; the so called 3D analogues of Graphene^{9}. While WSMs are not gapped in bulk, they have gapless nodes (bandtouching points) distributed in three dimensional (3D) momentum (k) space. The linear dispersion at these nodes results in Dirac fermions, but with either time reversal or spatial inversion symmetry breaking, the degenerate Dirac point splits into two Weyl points. The nodes always appear in pairs with opposite chirality defined by their opposite Chern numbers in the Brillouin zone^{8,10}. In the electronic band structure, the consequence of this is the presence of Fermi arcs terminating at Weyl points^{11,12}, that conjures remarkable electromagnetic properties such as anomalous Hall effect^{13,14}, chiral magnetic effects^{15} and negative longitudinal magnetoresistance^{16}.
A major aspect of current debate concerning Weyl semimetals is the precise topological characteristics of the band structure in the Brillouin zone and its experimental manifestations. It has been suggested for long that the Berry curvature in momentum space leads to the topological classification of metallic systems. More precisely, the Weyl points in the band structure are construed analogous to monopoles of Berry curvature that constitute the sinks and sources of Berry flux^{17,18}. The Berry phase is introduced to account for the cyclotron motion of Weyl fermions in the presence of magnetic field. A zero Berry phase factor implies topologically trivial energy band while a nonzero πBerry phase suggests topologically nontrivial energy band. Experimentally the conclusion on Berry phase is assessed by Shubnikovde Haas (SdH) oscillations by mapping the LandauLevel (LL) fan diagram (LL index, n vs 1/H). Quite generally, a nontrivial πBerry phase that accounts for additional geometrical phase factor, along a closed trajectory enclosing a Weyl node, is predicted for several WSMs^{17,18}. However in NbP, the experimental reports are contradictory to one another. Recently, Wang et al.^{19} have reported negative longitudinal magnetoresistance due to chiral anomaly while results of Shekhar et al.^{20} indicate absence of chiral anomaly in NbP. Using quantum oscillation measurements, in this paper, we address these issues and investigate the origin of exceptional magnetoresistance in NbP and its dependence on chiral anomaly and Berry phase factor.
Most investigations of recent past have focused on the study of binary WSMs derived from transition metal monopnictides (NbAs, NbP, TaAs and TaP)^{10,11,19,21,22,23}. All these compounds possess noncentrosymmetric tetragonal structure with space group I4_{1}md. The Weyl nodes in these cases are created due to spatial inversion symmetry breaking. Amongst these compounds, NbP shows extremely large magnetoresistance (MR) and ultrahigh mobility^{20}. In essence, it combines electronic band structure features of WTe_{2} and Cd_{3}As_{2}type semimetals, that results in normal quadratic bands from hole pockets and linear Weyl bands from electron pockets. The search of chiral effect and negative longitudinal magnetoresistance in WSMs has collaterally led to the observation of extremely high positive transverse (H⊥I) magnetoresistance^{20}.
In this paper, we report synthesis and extensive characterization of single crystals of Weyl semimetal NbP through magnetotransport measurements. An extremely large MR is observed at low temperatures which is nonsaturating and linear at high fields. This linear MR scales well with the average mobility of charge carriers. In conjunction with Hall data the linear MR is attributed to the mobility fluctuations induced by scattering from low mobility inhomogeneous islands. Further, study of Shubnikovde Haas oscillations suggests trivial Berry phase in NbP.
Results
Figure 1 shows the refined diffraction pattern of powdered NbP crystals. The diffraction pattern is Rietveld refined using Fullprof software with both Nb and P occupying 4a Wyckoff positions (Nb (0, 0, 0) and P (0, 0, 0.4216)). The main panel of Fig. 1 shows the Rietveld refined diffraction pattern of NbP. The sample crystallizes in a noncentrosymmetric space group I4_{1}md. The Rietveld refinement using FULLPROF software gave lattice parameters a = 3.335 Å and c = 11.379 Å for NbP, in agreement with reported results^{20}. Inset (a) in Fig. 1 shows the optical image of NbP single crystalline sample in a millimeter scale. The sample was subjected to EDX analysis to confirm elemental composition (inset (b) of Fig. 1). The quantitative analysis by performing EDX at several points provided the molar ratio Nb:P = 1:0.997 (Nb (%) = 50.08 ± 5.5; P (%) = 49.92 ± 2.1) which confirms percentage composition of the sample close to its stoichiometric value. The schematic unit cell of the sample is also shown in the inset (c) of Fig. 1. Further, structural characterization of the sample was done using high resolution transmission electron microscope (HRTEM). Figure 2(a) shows the TEM image of the sample. The dvalues calculated from lattice fringes corresponding to planes (004) and (101) are 2.84 Å and 3.28 Å, respectively [Fig. 2(b)]. These values match well with the dspacing values obtained from XRD results. Figure 2(c) shows the selected area diffraction pattern (SAED) for NbP that confirms excellent single crystalline characteristics of as grown samples. Single crystalline nature of the sample is also confirmed from the spots [Fig. 2(d)] obtained using Bruker’s D8 QUEST single crystal Xray Diffractometer.
Figure 3 summarizes the temperature and field dependent magnetoresistivity measurements. Figure 3(a) shows temperature dependent resistivity measured in various transverse applied magnetic fields (H⊥I) ranging from 0 to 6 T. In this configuration, magnetic field is applied along caxis. In zero applied field, the resistivity of NbP shows metallic behavior with ρ(300 K) = 33 μΩcm and ρ(2.5 K) = 0.8 μΩcm. The residual resistivity ratio (RRR = ρ (300 K)/ρ(2.5 K)) is estimated to be 41. This value is smaller than that reported in ref. 20 and possibly our cystals have some disorder. For correlating disorder effects on magnetotransport data a quantitative estimation of disorder would be useful, but that would require full analysis of ADP(Atomic Displacement Parameter) that is little away from the main focus of the paper. Evidently, application of magnetic field induces extremely large magnetoresistance and changes the temperature dependence of resistivity significantly. An unambiguous broad shoulder in resistivity is also observed for fields above 1 T. This upturn in resistivity exhibits semiconductor like behaviour before metallicity takes over at lower temperatures. This is indicative of opening of a gap in the presence of magnetic field that becomes more prominent at higher magnetic fields. Moreover, the crossover temperature shifts to higher temperature with increasing magnetic field. At low temperatures, saturation in electrical resistivity is observed even in the presence of magnetic field that demonstrates that the resistivity of the bulk material is short circuited by the dominance of metallic topological Fermi arcs at low temperatures^{24,25}.
The observed extremely large transverse MR can be better studied by isothermal measurement of field dependent resistivity. The magnetoresistance at temperature T is calculated using the relation: MR = [(ρ(H) − ρ(0)/ρ(0)] 100%, where ρ(H) is the resistivity at field H and ρ(0) is the zero field resistivity. Figure 3(b) shows the field dependent MR at temperatures varying from 2.5 K to 100 K. Inset in Fig. 3(b) shows MR at 200 K and 300 K as well. Clear signatures of Shubnikovde Haas quantum oscillations in electrical resistivity are observed. At 2.5 K, MR reaches 5.4 × 10^{4}% at 6 T without any trace of saturation. Even at room temperature large MR = 151% is observed at 6 T. The high value of MR is comparable to recent reports of other Weyl semimetals^{22,26,27}. In fact, MR shows weak temperature dependence upto 10 K and thereafter decreases sharply. Intriguingly, at low temperatures, the field dependent MR shows crossover from parabolic to linear dependence. At temperatures above 100 K, the field dependent MR shows parabolic behaviour. An interesting aspect of the data is the observation of almost linear magnetoresistance at high fields in both temperature regimes.
Towards understanding the role of scattering in temperature dependent magnetoresistance, in Fig. 3(c) we apply Kohler’s scaling to transverse MR data. According to Kohler’s rule^{28}: Δρ/ρ(0) = A(μ_{0}H/ρ(0))^{b}, where Δρ ( = ρ(H) − ρ(0)) is the change in resistivity in transverse applied field and A and b are constants. In a semimetal, the charge carriers could show different relaxation time scales and effective masses with a spread in their velocities. Kohler’s rule predicts that when the charge carriers possess same scattering rate, the plot between MR vs μ_{0}H/ρ(0) curves measured at different temperatures merge into a single line. This is confirmed in Fig. 3(c) where for temperature up to 100 K, all the MR curves collapse into a single curve. This indicates that same scattering rate is followed by all the charge carriers in the sample. Moreover, at temperatures 200 K and 300 K, the curves do not fall on the same curve which can be associated with the change in concentration, sign and mobilities of compensated charge carriers. Further, a change in slope between high field and low field MR is observed for the collapsed curves upto 100 K. From the fit to Kohler’s relation in these two regimes, the values of parameter b is found to be 1.5 at low fields and 0.9 at high fields. This justifies a crossover from nearly parabolic behavior at low fields to linear behavior at high fields. From the plot between MR and H, shown in the inset of Fig. 3(c), we observe that the crossover field increases on increasing temperature (∼1 T at 2.5 K to ∼2 T at 25 K). This suggests different scattering mechanisms at low and high magnetic fields are present at low temperatures and this feature has been seen in several other WSMs^{29,30}. The consequent changes in power law dependence of MR at different temperature are theoretically concluded in Weyl semimetals due to the presence of disordered environment in the sample^{31,32}.
The study of longitudinal MR in Weyl semimetals is of substantial current interest as it brings forth the chirality aspects of electronic transport^{21,22,33,34,35}. Figure 3(d) shows the comparative plots for both transverse and longitudinal field direction (with respect to applied current direction) at T = 2.5 K. At 5 T, MR(H⊥I)/MR(HI) = (4.75 × 10^{4})/(0.27 × 10^{4}) = 17.3, which indicates that the MR in this material is relatively isotropic in comparison to layered semimetal WTe_{2}^{36}. Further, in Weyl semimetals, negative longitudinal MR (LMR) is expected due to chiral anomaly effect^{37,38}. Clearly, no negative longitudinal MR is observed in NbP (see Fig. 3(d)). In recent theoretical studies, it is reported that there are two essential factors affecting the experimental observation of chiral anomaly effect in Weyl monopnictides^{21,24,39}. First, it is required that the Weyl points lie close to the Fermi level and secondly, the chirality of the Weyl points needs to be well defined, i.e, each Weyl point of opposite chirality are enclosed by well separated Fermi pockets. Based on these facts, it is reported that in case of NbP and TaP, the experimental observation of chiral anomaly effect is unlikely^{21,39}. This is in contrast to LMR results in TaAs^{21,24,39}. Negative LMR is also reported in NbP, TaP and NbAs^{19,40,41}. However, the origin of the negative LMR is controversial as the geometry of voltage and current contacts on the sample can lead to current jetting effects which is not related to chiral anomaly^{21,35}. In case of NbP, it is known that there are two pairs of Weyl points, W1 and W2. W1 lies in k_{z} = 0 plane, ∼57 meV below Fermi level while W2 lies in k_{z} = π/c plane ∼5 meV above Fermi level. W1 and W2 are enclosed within the electron and hole pockets^{39}. Since the Weyl point W1 is far away from the Fermi surface it may not contribute to chiral anomaly. Moreover, W2 is enclosed within the hole pocket and therefore it leads to the cancellation of chirality and Berry flux. Our null results on the absence of chiral anomaly therefore supports recent theoretical analysis on the improbability of experimental observation of chiral anomaly effect in NbP^{21,24,39}.
To gain more insight into charge transport in NbP, and to correlate the MR data with Fermi surface analysis, in Fig. 4(a) we show the field dependence of Hall resistance, R_{xy}, at temperatures ranging from 2.5 K to 300 K. We observe that R_{xy} is negative in the temperature range 2.5 K to 100 K. This indicates that the dominant quasiparticles on the Fermi surface are electronlike at low temperatures. At temperature above 100 K, R_{xy}(H) becomes positive, reflecting the nature of the quasiparticles has switched to holelike behaviour with increasing temperature. A simple model to account for magnetotransport in semimetals would be to assume two types of carriers near the Fermi level. The electronlike quasiparticles come from the Weyl fermions that follow linear dispersion, with the Weyl cones placed slightly below the Fermi level. The holelike quasiparticles on the other hand have a simple quadratic dispersion. Expanding the Hamiltonian near the Weyl cones in the lowenergy limit, we can write the corresponding Hamiltonians as
here ν is the inplane Fermi velocity, and ν_{z} is the same but along the caxis. t_{z} is the hoping integrals between the nearest neighbour lattice sites along the zdirection. Momentum k is measured with respect to the Weyl nodes. We notice that the Weyl Hamiltonian (H_{e}) is linear in momentum in all three directions with anisotropic Fermi velocities. The first term stems from the usual Weyllike Hamiltonian as seen in other Weyl semimetals, while sin(k_{z}) (∼k_{z} in the lowenergy region) is required to include due to the fact that Weyl nodes are present on both k_{z} = 0 and k_{z} = π/c planes. Therefore, unlike other Weyl/Dirac semimetals, in NbP one can expect chiral anomaly in all three directions. However the Weyl points are far away from the Fermi level, so the unequivocal confirmation of chiral anomaly from magnetotransport in NbP is challenging.
The Hall data of Fig. 4(a) can be understood from the coexistence of relativistic electrons (linear dispersion) and nonrelativistic holes (parabolic dispersion) with extremely high effective mass. Recalling that Hall resistivity is proportional to the density of states (DOS), in the following, we examine how the DOS qualitatively evolves with temperature for both Weyl electrons and heavy holes. Evidently, the DOS of Weyl electrons is linear in energy, while that of the hole is quadratic, i.e.,
here, a and b are constants. Since N_{e} is linear in T, it dominates at low temperature regime. Above some threshold value of T, the quadratic contributions from the hole part (N_{h}) becomes dominant. This behaviour is schematically shown in the inset of Fig. 4(a). This qualitatively explains the sign reversal of the Hall resistivity data.
Our attempt to fit a two band model to Hall data did not succeed and in the following we discuss the results of simplistic single band analysis. The Hall data for singlecarrier Drude model is used to calculate charge carrier density (N_{e,h}) and charge carrier mobility (μ_{e,h} = R_{H}(T)/ρ(T)). Here R_{H} is the Hall coefficient, N_{e,h} is electron/hole charge carrier density, μ_{e,h} is the average charge carrier mobility corresponding to electrons or holes, respectively, and ρ(T) is the resistivity of the sample. To calculate R_{H}(T), we have used slope of Hall resistance R_{xy}(H) at high fields. The value of R_{H}, and N_{e,h} thus calculated are shown in Fig. 4 (b). The value of electron charge carrier density at 2.5 K is found to be, N_{e} = 3.5 × 10^{18} cm^{−3} which increases with increase in temperature demonstrating typical semimetallic behavior of the sample. The electron carrier mobility at 2.5 K is found to be very high, μ_{e} = 2 × 10^{6} cm^{2} V^{−1}s^{−1}. This value of electron charge carrier mobility is close to the reported results on NbP^{19,20}.
The observation of linear and nonsaturating MR under high magnetic fields in WSMs is an interesting feature. Usually, MR varies as H^{2} at low fields (<1 T) due to Lorentz force deviation of charge carriers and shows saturation at high fields. The existence of linear nonsaturating MR is highly intriguing. In metals, it may originate due to open Fermi surfaces (e.g. Au). In Weyl semimetals, however, this is not the case. Both quantum and classical origin of linear MR in metallic systems are proposed^{42,43,44}. Quantum interpretation of linear nonsaturating MR at high field was proposed by Abrikosov^{42} at ultraquantum limit. In this limit, only the first landau level (LL) is filled, which happens when ħω_{c} > E_{F} and E_{F} ≫ κ_{B}T, where ħω_{c} = eB/m^{*} is the cyclotron frequency. It gives the relation between carrier concentration and magnetic field as: n < (eB/ħ)^{3/2}. From our Hall measurements, the value of carrier concentration n is ∼10^{18} cm^{−3} (T = 2.5 K) which gives the condition for ultraquantum limit to be B > 7 T. However, our sample starts showing linear MR in field as low as 1 T which rules out the possibility of quantum origin of linear MR in our sample. The appearance of linear MR is therefore assigned to classical behaviour applicable to inhomogeneous systems with high charge carrier mobility. Parish and Littlewood (PL)^{43,44} proposed that linear MR originates from fluctuations in mobility in a strongly disordered system. According to PL model, MR strongly depends on fluctuations in mobility which is the ratio between width of mobility disorder (Δμ) and average mobility (〈μ〉). At high fields the PL equations for MR are as follows: MR ∝ 〈μ〉 for Δμ/〈μ〉 < 1, and MR ∝ Δμ for Δμ/〈μ〉 > 1. As seen in Fig. 4(c), the transverse MR varies linearly with average charge carrier mobility. This implies: MR ∝ 〈μ〉. Figure 4(d) shows correlation between temperature dependent mobility and slope of MR in the high field region. From the plot, we observe that the change in MR follows the average mobility of charge carriers in the entire temperature range. This further confirms that MR ∝ 〈μ〉. The deviation between 100 K and 200 K are associated with change in type of charge carriers from electrons to holes. In summary, the linear MR in NbP is due to mobility fluctuations induced by scattering from low mobility inhomogeneous regions in the system. This behaviour has also been evidenced in single crystals of topological insulator Bi_{2}Se_{3}^{45}.
As is strikingly evident in Fig. 3(b), in the presence of high transverse magnetic field, clear Shubnikov–de Haas (SdH) oscillations in MR are observed at low temperatures. The SdH oscillations are extracted by subtracting mean polynomial fitting to MR data (measured at 2 K in applied magnetic field upto 12 T). We wish to emphasize that 3 different crystals of the same batch were used to check reproducibility of the experimental data. The oscillatory component (Δρ) thus extracted is shown in Fig. 5 (inset (a)) as a function of 1/μ_{0}H. The oscillations show periodicity with 1/μ_{0}H. The spectrum is substantially complex due to the contribution from various subbands on the Fermi surface. The SdH oscillations are further analysed using the following expression for the oscillatory component of magnetoresistance in a 3D system:
where 〈ρ(0)〉 is the nonoscillatory component of transverse ρ, F is the frequency of oscillation, γ is the Onsager phase and δ (= ± 1/8) is the phase shift introduced by the threedimensionality of the Fermisurface. By performing FastFourier transformation (FFT) of Δρ vs 1/μ_{0}H data, we have extracted three oscillations corresponding to frequencies: F_{α} = 6.8 T, F_{β} = 13.6 T and F_{γ} = 30.5 T with its harmonics at 61.1 T and 91.6 T, (Fig. 5(b)). Frequencies F_{α}, F_{β} and F_{γ} are associated with the electron pockets near the Fermi energy in NbP^{39}. In the present case, the period of oscillations corresponding to F_{γ} (1/μ_{0}H) is 0.033 T^{−1}. Further, the frequency of quantum oscillations is proportional to the crosssectional Fermisurface area, A_{F}, that follows the LifshitzOnsager relation: F = (Φ_{0}/(2π^{2}))A_{F}. Here, Φ_{0} = h/2e is the magnetic flux quantum and e is the electronic charge. The calculated crosssectional area of the Fermisurfaces corresponding to frequencies F_{α}, F_{β} and F_{γ} are 0.18 × 10^{−3} Å^{2}, 0.41 × 10^{−3} Å^{2} and 3 × 10^{−3} Å^{2}, respectively. The crosssectional Fermisurface for F_{γ} matches well with the reported data^{20}. These areas are very small, only about 0.005%, 0.01% and 0.08% of the total area of the Brillouin zone in the k_{x} − k_{y} plane, respectively. The Fermi wavevectors are then calculated from the observed A_{F} values using the relation: k_{F} = (A_{F}/π)^{1/2}. The calculated k_{F} corresponding to F_{α}, F_{β} and F_{γ} are 8 × 10^{−3} Å^{−1}, 0.011 Å^{−1} and 0.031 Å^{−1}, respectively. From the quantum oscillation data, in the following we discuss the topological signatures of these electron and hole pockets by studying the Berry phase accumulated around the Weyl nodes. Identifying the Berry phase (Φ_{B}) corresponding to Fermi pockets in Weyl semimetal is challenging because of its multi band characteristics. The Onsager phase γ = 1/2 − Φ_{B}/2π + δ, relates to Φ_{B}. In case of topologically trivial band, Φ_{B} = 0, which implies γ = 1/2 + δ while in case of topologically nontrivial band, Φ_{B} = π, which implies γ = δ. To identify the topological nature of the Fermisurface associated with the most prominent frequency in our data, F_{γ}, we have plotted the Landau fan diagram (a plot between Landau Level (LL) index n and 1/μ_{0}H) in main panel of Fig. 5 (T = 2 K). The linear fitting gives Onsager phase of γ = 0.30. For electron pockets, δ = −1/8 and therefore the Berry phase parameter, indicating a topologically trivial Berry phase in NbP corresponding to frequency F_{γ}. This is in disagreement with reported conclusion on nontrivial Berry phase in NbP^{19}. Towards understanding the implication of our experimental results, it is worthwhile to mention that such conclusions are expected based on nonlinear sigma model^{46}. In realistic situations, the experimental observation of the topological properties is affected by presence of weak disorder in the system. Theoretical analysis^{46} on the effect of disorder in timereversal invariant WSMs predicts possibility of Anderson localization leading to the disappearance of chirality. In essence, the absence of chiral anomaly in WSM NbP can be assigned majorly to two factors; significant separation of Weyl points from the Fermi level^{39} and suppression of chiral anomaly in presence of weak disorder.
In conclusion, we provide evidence for extremely high, nonsaturating linear transverse magnetoresistance in high quality single crystals of Weyl semimetal NbP. Kohler scaling to MR data shows nearly quadratic field dependence at low fields and confirms linear behavior at high fields. Different scaling behavior at low and high fields also indicate different scattering mechanisms coexisting in NbP. Clear Shubnikov–de Haas quantum oscillations are observed at high fields and low temperatures with three main frequencies. The Berry phase calculated for frequency = 30.5 T of SdH oscillations shows null additional phase factor. No evidence for longitudinal magnetoresistance is seen that rules out chiral anomaly in NbP. This is ascribed to large separation between Fermi level and Weyl nodes and lack of well defined chirality. Combined Hall and magnetotransport data suggest that the linear MR observed in NbP is due to charge carrier mobility fluctuations.
Methods
Millimeter size single crystals of NbP were synthesized using vapour transport technique. In the first step, polycrystalline samples of NbP were synthesized using solid state reaction method. Nb powder (Sigma Aldrich, 99.8%) and Phosphorus chips (Sigma Aldrich, 99.999%) were ground together and pressed into pellets using a hydraulic press. The pellets were then vacuum sealed in quartz tubes and sintered at 850 °C for 48 hours. The polycrystalline NbP was then sealed with iodine (13 mg/cm^{3}) (Sigma Aldrich, >99.99%) in a quartz tube of length 12 cm and ID = 12 mm. The tube was put in a tubular furnace which is calibrated to have a temperature gradient of 100 °C over a distance of 12 cm when set at 950 °C. The samples were sintered for 2 weeks. The obtained crystals were ground and characterized with powder Xray diffraction at room temperature using a Rigaku Xray powder diffractometer (Miniflex600, CuK_{α}). Energy Dispersive Xray analysis (EDX) was performed using Bruker AXS microanalyzer. High resolution transmission electron microscopy (HRTEM) measurements were performed using JEOL (JEM2100F) transmission electron microscope. Magnetoresistivity and Hall measurements were done using a Cryogenic cryogen Free Magnet (CFM) system.
Additional Information
How to cite this article: Sudesh. et al. Evidence for trivial Berry phase and absence of chiral anomaly in semimetal NbP. Sci. Rep. 7, 46062; doi: 10.1038/srep46062 (2017).
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References
 1
Hasan, M. Z., Xu, S.Y. & Bian, G. Topological insulators, topological superconductors and Weyl fermion semimetals: discoveries, perspectives and outlooks. Phys. Scr. T164, 014001 (2015).
 2
Singh, B. et al. Topological electronic structure and Weyl semimetal in the TlBiSe2 class of semiconductors. Phys. Rev. B 86, 115208 (2012).
 3
Turner, A. M. & Vishwanath, A. Beyond band insulators: Topology of semimetals and interacting phases. arXiv:1301.0330v1 (2013).
 4
Xia, Y. et al. Observation of a largegap topologicalinsulator class with a single Dirac cone on the surface Nat. Phys. 5, 398 (2009).
 5
Zhang, H. et al. Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface. Nat. Phys. 5, 438 (2009).
 6
Chen, Y. L. et al. Experimental Realization of a ThreeDimensional Topological Insulator, Bi2Te3 . Science 325, 178 (2009).
 7
Gibson Q. D. et al. Threedimensional Dirac semimetals: Design principles and predictions of new materials. Phys. Rev. B 91, 205128 (2015).
 8
Wan, X., Turner, A. M., Vishwanath, A. & Savrasov, S. Y. Topological semimetal and Fermiarc surface states in the electronic structure of pyrochlore iridates. Phys. Rev. B 83, 205101 (2011).
 9
Novoselov, K. S. et al. Twodimensional gas of massless Dirac fermions in graphene. Nature 438, 197 (2005).
 10
Weng, H., Fang, C., Fang, Z., Bernevig, B. A. & Dai, X. Weyl Semimetal Phase in Noncentrosymmetric TransitionMetal Monophosphides. Phys. Rev. X 5, 011029 (2015).
 11
Yang, L. X. et al. Weyl semimetal phase in the noncentrosymmetric compound TaAs. Nat. Phys. 11, 728 (2015).
 12
Liu, Z. K. et al. Evolution of the Fermi surface of Weyl semimetals in the transition metal pnictide family. Nat. Mater. 15, 27 (2016).
 13
Yang, K. Y., Lu, Y. M. & Ran, Y. Quantum Hall effects in a Weyl semimetal: Possible application in pyrochlore iridates. Phys. Rev. B 84, 075129 (2011).
 14
Burkov, A. A. & Balents, L. Weyl semimetal in a topological insulator multilayer. Phys. Rev. Lett. 107, 127205 (2011).
 15
Fukushima, K., Kharzeev, D. E. & Warringa, H. J. Chiral Magnetic Effect. Phys. Rev. D 78, 074033 (2008).
 16
Son, D. T. & Spivak, B. Z. Chiral anomaly and classical negative magnetoresistance of Weyl metals. Phys. Rev. B 88, 104412 (2013).
 17
Taskin, A. A. & Ando, Y. Berry phase of nonideal Dirac fermions in topological insulators. Phys. Rev. B 84, 035301 (2011).
 18
Hu, J. et al. π Berry phase and Zeeman splitting of Weyl semimetal TaP. Sci. Rep. 6, 18674 (2016).
 19
Wang, Z. et al. Helicityprotected ultrahigh mobility Weyl fermions in NbP. Phys. Rev. B 93, 121112(R) (2016).
 20
Shekhar, C. et al. Extremely large magnetoresistance and ultrahigh mobility in the topological Weyl semimetal candidate NbP. Nat. Phys. 11, 645 (2015).
 21
Arnold, F. et al. Negative magnetoresistance without welldefined chirality in the Weyl semimetal TaP. Nat. Commun. 7, 11615 (2016).
 22
Ghimire, N. J., Luo, Y., Neupane, M., Williams, D. J., Bauer, E. D. & Ronning, F. Magnetotransport of single crystalline NbAs. J. Phys.: Condens. Matter. 27, 152201 (2015).
 23
Huang, S.M. et al. A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class. Nat. Commun. 6, 7373 (2015).
 24
Arnold, F. et al. Chiral Quasiparticles at the Fermi Surface of the Weyl Semimetal TaAs. arXiv:1603.08846v1 (2016).
 25
Chang, G. et al. Signatures of Fermi Arcs in the Quasiparticle Interferences of the Weyl Semimetals TaAs and NbP. Phys. Rev. Lett. 116, 066601 (2016).
 26
Shekhar, C., Süβ, V. & Schmidt, M. Mobility induced unsaturated high linear magnetoresistance in transitionmetal monopnictides Weyl semimetals. arXiv:1606.06649 (2016).
 27
Ghimire, N. J., Botana, A. S., Phelan, D., Zheng, H. & Mitchell, J. F. Magnetotransport of single crystalline YSb. arXiv:1604.04232v1 (2016).
 28
Ziman, J. M. Principles of the Theory of Solids. (Cambridge university press, 1972).
 29
Zhang, C. et al. Large magnetoresistance over an extended temperature regime in monophosphides of tantalum and niobium. Phys. Rev. B 92, 041203 (2015).
 30
Zhang, C. et al. Tantalum Monoarsenide: an Exotic Compensated Semimetal. arXiv:1502.00251 (2015).
 31
Ziegler, K. Quantum transport in 3D Weyl semimetals: Is there a metalinsulator transition?arXiv:1501.00268v1 (2015).
 32
Ramakrishnan, N., Milletari, M. & Adam, S. Transport and magnetotransport in threedimensional Weyl semimetals. Phys. Rev. B 92, 245120 (2015).
 33
Huang, X. et al. Observation of the chiralanomalyinduced negative magnetoresistance: In 3D Weyl semimetal TaAs. Phys. Rev. X 5, 031023 (2015).
 34
Li, C.Z. et al. Giant negative magnetoresistance induced by the chiral anomaly in individual Cd3As2 nanowires. Nat. Commun. 6, 10137 (2015).
 35
Reis, R. D. dos et al. On the search for the chiral anomaly in Weyl semimetals: The negative longitudinal magnetoresistance. New J. Phys. 18, 085006 (2016).
 36
Zhao, Y. et al. Anisotropic magnetotransport and exotic longitudinal linear magnetoresistance in Wte2 crystals. Phys. Rev. B 92, 041104(R) (2015).
 37
Adler, S. L. Axialvector vertex in spinor electrodynamics. Phys. Rev. 177, 2426 (1969).
 38
Bell,J. S. & Jackiw, R. A PCAC puzzle: π0 → γγ in the σmodel. Nuov Cim A 60, 47 (1969).
 39
Klotz, J. et al. Quantum oscillations and the Fermi surface topology of the Weyl semimetal NbP. Phys. Rev. B 93, 121105(R) (2016).
 40
Du, J. et al. Unsaturated both large positive and negative magnetoresistance in Weyl Semimetal TaP. Sci. China Phys. Mech. Astron. 59, 657406 (2016).
 41
Yang, X., Liu, Y., Wang, Z., Zheng, Y. & Xu, Z. Chiral anomaly induced negative magnetoresistance in topological Weyl semimetal NbAs. arXiv:1506.03190v1 (2015).
 42
Abrikosov, A. A. Quantum magnetoresistance. Phys. Rev. B 58, 2788 (1998).
 43
Parish, M. M. & Littlewood, P. B. Nonsaturating magnetoresistance in heavily disordered semiconductors. Nature 426, 162 (2003).
 44
Parish, M. M. & Littlewood, P. B. Classical magnetotransport of inhomogeneous conductors. Phys. Rev. B 72, 094417 (2005).
 45
Kumar, D. & Lakhani, A. Large linear magnetoresistance from neutral defects in Bi2Se3 . arXiv:1606.09059v1 (2016).
 46
Imam Makhfudz, Nonperturbative & Berry Phase Effects in Disordered TimeReversal Invariant Weyl Semimetals.arXiv:1608.07194v1 (2016)
Acknowledgements
Sudesh, P. Kumar and P. Neha acknowledge UGCD. S. Kothari fellowship, JNU (New Delhi) and UGCBSR, respectively for financial support. Authors are thankful to AIRF (JNU) for access to the PPMS, EDAX and TEM facilities. We thank Prof A. K. Rastogi and Dr. Chandra Shekhar for useful discussions. Low–temperature high magnetic field at JNU is supported under the FIST program of Department of Science and Technology, Government of India. T. Das’s work is supported by SERB young scientist Startup research grant. We thank Dr. Dinabandhu Das for advice on single crystal diffraction data analysis.
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The single crystals used for the study are synthesized by Sudesh and P.K. Sudesh and P.K. conducted structure characterization and high field magnetotransport measurements. Sudesh, P.K., P.N. and T.D. analysed the results. Sudesh, T.D. and S.P. wrote the manuscript. All authors reviewed the manuscript. This work was supervised by S.P.
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Sudesh, Kumar, P., Neha, P. et al. Evidence for trivial Berry phase and absence of chiral anomaly in semimetal NbP. Sci Rep 7, 46062 (2017). https://doi.org/10.1038/srep46062
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