Abstract
Although Weyl fermions have proven elusive in highenergy physics, their existence as emergent quasiparticles has been predicted in certain crystalline solids in which either inversion or timereversal symmetry is broken^{1,2,3,4}. Recently they have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention^{5,6,7}. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable electronic properties. The TMMPs are polar metals, a rare subset of inversionbreaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons^{8,9,10}. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversionsymmetry breaking, most notably secondorder nonlinear optical polarizability, χ^{(2)}, leading to phenomena such as optical rectification and secondharmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic χ^{(2)} in the TMMPs TaAs, TaP and NbAs. With the fundamental and secondharmonic fields oriented parallel to the polar axis, the value of χ^{(2)} is larger by almost one order of magnitude than its value in the archetypal electrooptic materials GaAs^{11} and ZnTe^{12}, and in fact larger than reported in any crystal to date.
Main
The past decade has witnessed an explosion of research investigating the role of bandstructure topology, as characterized for example by the Berry curvature in momentum space, in the electronic response functions of crystalline solids^{13}. While the best established example is the intrinsic anomalous Hall effect in timereversal breaking systems^{14}, several nonlocal^{15,16} and nonlinear effects related to Berry curvature generally^{17,18} and in Weyl semimetals (WSMs) specifically^{19,20} have been predicted in crystals that break inversion symmetry. Of these, the most relevant to this work is a theoretical formulation^{21} of SHG in terms of the shift vector, which is a quantity related to the difference in Berry connection between two bands that participate in an optical transition.
Figure 1a and its caption provide a schematic and description of the optical setup for measurement of SHG in TMMP crystals. Figure 1b, c shows results from a (112) surface of TaAs. The SH intensity from this surface is very strong, allowing for polarization rotation scans with signaltonoise ratio above 10^{6}. In contrast, SHG from a TaAs (001) surface is barely detectable (at least six orders of magnitude lower than the (112) surface). Below, we describe the use of the setup shown in Fig. 1a to characterize the secondorder optical susceptibility tensor, χ_{ijk}, defined by the relation, P_{i}(2ω) = ε_{0}χ_{ijk}(2ω)E_{j}(ω)E_{k}(ω).
As a first step, we determined the orientation of the highsymmetry axes in the (112) surface, which are the [1,−1,0] and [1,1,−1] directions. To do so, we simultaneously rotated the linear polarization of the generating light (the generator) and the polarizer placed before the detector (the analyser), with their relative angle set at either 0° or 90°. Rotating the generator and analyser together produces scans, shown in Fig. 1b, c, which are equivalent to rotation of the sample about the surface normal. The angles at which we observe the peak and the null in Fig. 1b and the null in Fig. 1c allow us to identify the principal axes in the (112) plane of the surface.
Having determined the highsymmetry directions, we characterize χ_{ijk} by performing three of types of scans, the results of which are shown in Fig. 2. In scans shown in Fig. 2a, b, we oriented the analyser along one of the two highsymmetry directions and rotated the plane of linear polarization of the generator through 360°. Figure 2c shows the circular dichroism of the SHG response—that is, the difference in SH generated by left and right circularly polarized light. For all three scans the SHG intensity as a function of angle is consistent with the secondorder optical susceptibility tensor expected for the 4mm point group of TaAs, as indicated by the high accuracy of the fits in Fig. 1b, c and Fig. 2a–c.
In the 4mm structure xz and yz are mirror planes but reflection through the xy plane is not a symmetry; therefore, TaAs is an acentric crystal with an unique polar (z) axis. In crystals with 4mm symmetry there are three independent nonvanishing elements of χ_{ijk}: χ_{zzz}, χ_{zxx} = χ_{zyy} and χ_{xzx} = χ_{yzy} = χ_{xxz} = χ_{yyz}. Note that each has at least one z component, implying null electric dipole SHG when all fields are in the xy plane. This is consistent with observation of nearly zero SHG for light incident on the (001) plane. Below, we follow the convention of using the 3 × 6 secondrank tensor d_{ij}, rather than χ_{ijk}, to express the SHG response, where the relation between the two tensors for TaAs is: χ_{zzz} = 2d_{33}, χ_{zxx} = 2d_{31} and χ_{xzx} = 2d_{15} (ref. 22) (see Methods and Supplementary Section A).
Starting with the symmetryconstrained d tensor, we derive expressions, specific to the (112) surface, for the angular scans with fixed analyser shown in Fig. 2a, b (Methods and Supplementary Section A). We obtain d_{eff}cos^{2}θ_{1} + 3d_{31} sin^{2}θ_{1}^{2}/27 and d_{15}^{2} sin^{2}(2θ_{1})/3 for analyser parallel to [1,1,−1] and [1,−1,0], respectively, where d_{eff} ≡ d_{33} + 2d_{31} + 4d_{15}. Fits to these expressions yield two ratios: d_{eff}/d_{15} and d_{eff}/d_{31} . Although we do not determine d_{33}/d_{15} and d_{33}/d_{31} directly, it is clear from the extreme anisotropy of the angular scans that d_{33}, which gives the SHG response when both generator and analyser are parallel to the polar axis, is much larger than the other two components. We can place bounds on d_{33}/d_{15} and d_{33}/d_{31} by setting d_{15} and d_{31} in and out of phase with d_{33}. We note that the observation of circular dichroism in SHG, shown in Fig. 2c, indicates that relative phase between d_{15} and d_{33} is neither 0° or 180°, but rather closer to 30° (Supplementary Section A).
The results of this analysis are plotted in Fig. 2d, where it is shown that d_{33}/d_{15} falls in the range ∼25–33 for all temperatures, and d_{33}/d_{31} increases from ∼30 to ∼100 with increasing temperature. Perhaps because of its polar metal nature, the anisotropy of the secondorder susceptibility in TaAs is exceptionally large compared with what has been observed previously in crystals with the same set of nonzero d_{ij}. For example, αZnS, CdS and KNiO_{3} have d_{31} ≅ d_{15} ≅d_{33}/2 (ref. 23), while in BaTiO_{3} the relative sizes are reversed, with d_{31} ≅ d_{15} ≈ 2 d_{33} (ref. 24).
Even more striking than the extreme anisotropy of χ_{ijk} is the absolute size of the SHG response in TaAs. The search for materials with large secondharmonic optical susceptibility has been of continual interest since the early years of nonlinear optics^{25}. To determine the absolute magnitude of the d coefficients in TaAs, we used GaAs and ZnTe as benchmark materials. Both crystals have large and wellcharacterized secondorder optical response functions^{11,12}, with GaAs regarded as having a SH susceptibility among the largest of any known crystal. GaAs and ZnTe are also ideal as benchmarks, because their response tensors have only one nonvanishing coefficient, d_{14} ≡ 1/2χ_{xyz}.
Figure 3a–c shows polar plots of SHG intensity as TaAs (112), ZnTe (110), and GaAs (111) are (effectively) rotated about the optic axis with the generator and analyser set at 0° and 90°. Also shown (as solid lines) are fits to the polar patterns obtained by rotating the χ^{(2)} tensor to a set of axes that includes the surface normal, which is (110) and (111) for our ZnTe and GaAs crystals, respectively (Methods and Supplementary Section A). Even prior to analysis to extract the ratio of d coefficients between the various crystals, it is clear that the SHG response of TaAs (112) is large, as the peak intensity in this geometry exceeds ZnTe (110) by a factor of 4.0(±0.1) and GaAs (111) by a factor of 6.6(±0.1). Figure 3d compares the parallel polarization data for TaAs shown in Fig. 3a with SHG measured under the same conditions in the (112) facets of two other TMMPs: TaP and NbAs. The strength of SHG from the three crystals, which share the same 4mm point group, is clearly very similar, with TaP and NbAs intensities relative to TaAs of 0.90(±0.02) and 0.76(±0.04), respectively. The SHG response in these compounds is also dominated by the d_{33} coefficient. Finally, we found that the SHG intensity of all three compounds does not decrease after exposure to atmosphere for several months.
To obtain the response of the TMMPs relative to the two benchmark materials we used the Bloembergen–Pershan formula^{25} to correct for the variation in specular reflection of SH light that results from the small differences in the index of refraction of the three materials at the fundamental and SH frequency. (See Methods. Details concerning this correction, which is less than 20%, can be found in Supplementary Section B.) Table 1 presents the results of this analysis, showing that d_{33} ≅3,600 pm V^{−1} at the fundamental wavelength 800 nm in TaAs exceeds the values in the benchmark materials GaAs^{11} and ZnTe^{12} by approximately one order of magnitude, even when measured at wavelengths where their response is largest. The d coefficient in TaAs at 800 nm exceeds the corresponding values in the ferroelectric materials BiFeO_{3} (ref. 26), BaTiO_{3} (ref. 24) and LiNbO_{3} (ref. 23) by two orders of magnitude. In the case of the ferroelectric materials, SHG measurements have not been performed in their spectral regions of strong absorption, typically 3–7 eV. However, ab initio calculations consistently predict that the resonanceenhanced d values in this region do not exceed roughly 500 pm V^{−1} (refs 27,28).
The results described above raise the question of why χ_{zzz} in the TMMPs is so large. Answering this question quantitatively will require further work in which measurements of χ^{(2)} as a function of frequency are compared with theory based on ab initio band structure and wavefunctions. For the present, we describe a calculation of χ^{(2)} using a minimal model of a WSM that is based on the approach to nonlinear optics proposed by Morimoto and Nagaosa (MN)^{21}. This theory clarifies the connection between bandstructure topology and SHG, and provides a concise expression with clear geometrical meaning for χ^{(2)}. Hopefully this calculation will motivate the ab initio theory that is needed to quantitatively account for the large SH response of the TMMPs and its possible relation to the existence of Weyl nodes.
The MN result for the dominant (zzz) response function is
In equation (1) the nonlinear response is expressed as a secondorder conductivity, σ_{zzz}(ω, 2ω), relating the current induced at 2ω to the square of the applied electric field at ω, that is, J_{z}(2ω) = σ_{zzz}E_{z}^{2}(ω). (The SH susceptibility is related to the conductivity through the relation χ^{(2)} = σ^{(2)}/2iωε_{0}). The indices 1 and 2 refer to the valence and conduction bands, respectively, ε_{21} is the transition energy, and v_{i, 12} is the matrix element of the velocity operator v_{i} = (1/ℏ)∂H/∂k_{i}. Bandstructure topology appears in the form of the ‘shift vector,’ R_{zz} ≡ ∂_{k}_{z}φ_{z, 12} + a_{z, 1} − a_{z, 2}, which is a gaugeinvariant length formed from the k derivative of the phase of the velocity matrix element, φ_{12} = Im{logv_{12}}, and the difference in Berry connection, a_{i} = −i〈u_{n} ∂_{k}_{i} u_{n}〉, between bands 1 and 2. Physically, the shift vector is the kresolved shift of the intracell wavefunction for the two bands connected by the optical transition.
We consider the following minimal model for a timereversal symmetric WSM that supports four Weyl nodes,
Here, σ_{i} and s_{i} are Pauli matrices acting on the orbital and spin degrees of freedom, respectively, t is a measure of the bandwidth, a is the lattice constant, m_{y} and m_{z} are parameters that introduce anisotropy, and inversion breaking is introduced by Δ. The Hamiltonian defined in equation (2) preserves twofold rotation symmetry about the zaxis and the mirror symmetries M_{x} and M_{y}. These symmetries form a subset of the 4mm point group which is relevant to the optical properties of TMMPs.
Figure 4 illustrates the energy levels, topological structure, and SHG spectra that emerge from this model. As shown in Fig. 4a, pairs of Weyl nodes with opposite chirality overlap at two points, k = (±π/2a, 0,0), in the inversionsymmetric case with Δ = 0. With increasing Δ the nodes displace in opposite directions along the k_{y} axis, with Δk_{y}≅Δ/a. The energy of electronic states in the k_{z} = 0 plane, illustrating the linear dispersion near the four Weyl points, is shown in Fig. 4b. Figure 4c shows the corresponding variation of v_{12}^{2}R_{zz}(k) for the s_{x} = +1 bands whose Weyl points are located at k_{y} < 0 (the variation of v_{12}^{2}R_{zz}(k) for the s_{x} = −1 bands is obtained from the transformation k → − k). The magnitude of σ^{(2)} derived from this model vanishes as Δ → 0, and is also sensitive to the anisotropy parameters m_{y} and m_{z}. Figure 4d shows that spectra corresponding to parameters t = 0.8 eV, Δ = 0.5, m_{z} = 5, and m_{y} = 1 can qualitatively reproduce the observed amplitude and large anisotropy of χ^{(2)}(ω, 2ω).
As discussed above, our minimal model of an inversionbreaking WSM is intended mainly to motivate further research into the mechanism for enhanced SHG in the TMMPs. However, the model does suggest universal properties of χ^{(2)} that arise from transitions near Weyl nodes between bands with nearly linear dispersion. According to bulk bandstructure measurements^{7}, such transitions are expected at energies below approximately 100 meV in the TaAs family, corresponding to the farinfrared and terahertz regimes. In these regimes, where the interband excitation is within the Weyl cones, the momentumaveraged v_{12}^{2}R_{zz}(k) tends to a nonzero value, 〈v^{2}R〉, leading to the prediction that σ^{(2)} → g(ω)〈v^{2}R〉/ω^{2} as ω → 0. Because g(ω), the joint density of states for Weyl fermions, is proportional to ω^{2}, we predict that σ^{(2)} approaches a constant (or alternatively χ^{(2)} diverges as 1/ω) as ω → 0, even as the linear optical conductivity vanishes in proportion to ω (ref. 29). The 1/ω scaling of SHG and optical rectification is a unique signature of a WSM in lowenergy electrodynamics, as it requires the existence of both inversion breaking and point nodes. In real materials, this divergence will be cut off by disorder and nonzero Fermi energy. Disorderinduced broadening, estimated from transport scattering rates^{30}, and Pauli blocking from nonzero Fermi energy, estimated from optical conductivity^{30} and band calculation^{3}, each suggest a lowenergy cutoff in the range of a few meV.
We conclude by observing that the search for inversionbreaking WSMs has led, fortuitously, to a new class of polar metals with unusually large secondorder optical susceptibility. Although WSMs are not optimal for frequencydoubling applications in the visible regime because of their strong absorption, they are promising materials for terahertz generation and optoelectronic devices such as farinfrared detectors because of their unique scaling in the ω → 0 limit. Looking forward, we hope that our findings will stimulate further investigation of nonlinear optical spectra in inversionbreaking WSMs for technological applications and in order to identify the defining response functions of Weyl fermions in crystals.
Methods
Crystal growth and structure characterization.
Single crystals of TaAs, TaP and NbAs were grown by vapour transport with iodine as the transport agent. First, polycrystalline TaAs/TaP/NbAs was produced by mixing stoichiometric amounts of Ta/Nb and As/P and heating the mixture to 1,100/800/700 °C in an evacuated quartz ampule for two days. 500 mg of the resulting powder was then resealed in a quartz ampoule with 100 mg of iodine and loaded into a horizontal twozone furnace. The temperatures of the hot and cold ends were held at 1,000 °C and 850 °C, respectively, for TaAs and 950 °C and 850 °C for TaP and NbAs. After four days, wellfaceted crystals up to several millimetres in size were obtained. Crystal structure was confirmed using singlecrystal Xray microLaue diffraction at room temperature at beamline 12.3.2 at the Advanced Light Source.
Optics setup for secondharmonic generation.
The optical setup for measuring SHG is illustrated in Fig. 1a. Generator pulses of 100 fs duration and centre wavelength 800 nm pass through a mechanical chopper that provides amplitude modulation at 1 kHz and are focused at nearnormal incidence onto the sample. Polarizers and waveplates in the beam path are used to vary the direction of linear polarization and to generate circular polarization. Both the specularly reflected fundamental and the secondharmonic beam are collected by a pickoff mirror and directed to a shortpass, bandpass filter combination that allows only the secondharmonic light to reach the photomultiplier tube (PMT) photodetector. Another wiregrid polarizer placed before the PMT allows for analysis of the polarization of the secondharmonic beam. Temperaturedependence measurements were performed by mounting the TaAs sample in a coldfinger cryostat on an xyzmicrometer stage. Benchmark measurements on TaAs, TaP, NbAs, ZnTe and GaAs were performed at room temperature in atmosphere with the samples mounted on an xyzmicrometer stage to maximize the signal.
Calculation and fitting procedure for SHG.
In noncentrosymmetric materials the secondorder term, P_{i}^{(2)} = ε_{0}χ_{ijk}E_{j}E_{k}, which gives rise to SHG and optical rectification, is allowed^{31,32}. These two phenomena arise from excitation with a single frequency; therefore, there is an automatic symmetry with respect to permutation of the second and third indices in χ_{ijk}. This motivates the use of a 3 × 6 secondrank tensor d_{ij} instead of χ_{ijk}, and the former is more often used in SHG. The relation between d_{ij} and χ_{ijk} is as follows: the first index i = 1,2,3 in d_{ij} corresponds to i′ = x, y, z, respectively, in χ_{i′j′k′} and the second index j = 1,2,3,4,5,6 in d_{ij} corresponds to j′k′ = xx, yy, zz, yz/zy, zx/xz, xy/yx in χ_{i′j′k′}. Further discussion is provided in the Supplementary Information.
To fit the SHG polar pattern of TMMPs, we first fit data obtained with a fixed analyser at 90°, which is the [1,−1,0] crystal axis, because there is only one free parameter (d_{15}) in this configuration:
With this value for d_{15}, we then fit data in the three other types of scans discussed in the text, with d_{15} and d_{31} set in and out of phase with d_{33}. The angular dependences in the parallel and perpendicular configurations are
When the analyser is fixed along 0°, the angular dependence is
This procedure yields upper and lower bounds on the anisotropy ratios d_{15}/d_{33} and d_{31}/d_{33}, which are shown with error bars in Fig. 2d.
In the circular dichroism experiment, to lowest order in d_{ij}, the angular dependence is
In the case of GaAs and ZnTe, all scans are fitted accurately by the only symmetryallowed free parameter, d_{14} . The angular dependences for ZnTe (110) in the parallel and perpendicular configurations are
The angular dependences for GaAs (111) in the parallel and perpendicular configurations are
See Supplementary Information for a full derivation.
Bloembergen–Pershan correction.
When measuring in the reflection geometry, one needs to consider the boundary condition to calculate χ^{(2)} from χ_{R}^{(2)}, which was directly measured, where ‘R’ stands for reflection geometry. The correction was worked out by Bloembergen–Pershan (BP)^{33}:
where ε is the relative dielectric constant and T(ω) = (2/(n(ω) + 1)) is the Fresnel coefficient of the fundamental light. In the current experiment, performed at 800 nm, the BP correction is fairly small (less than 20%). See Supplementary Information for more details.
Data availability.
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
We thank B. M. Fregoso, T. R. Gordillo, J. Neaton and Y. R. Shen for helpful discussions and B. Xu for sharing refractive index data of TaAs. Measurements and modelling were performed at the Lawrence Berkeley National Laboratory in the Quantum Materials program supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under Contract No. DEAC0205CH11231. J.O., L.W. and A.L. received support for performing and analysing optical measurements from the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4537 to J.O. at UC Berkeley. Sample growth was supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative Grant GBMF4374 to J.A. at UC Berkeley. T.M. is supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative Theory Center Grant GBMF4307 to UC Berkeley. J.E.M. received support for travel from the Simons Foundation. The authors would like to thank Nobumichi Tamura for his help in performing crystal diffraction and orientation on beamline 12.3.2 at the Advanced Light Source. N. Tamura and the ALS are supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DEAC0205CH11231. J. A. and N. N. acknowledge support by the Office of Naval Research under the Electrical Sensors and Network Research Division, Award No. N000141512674.
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L.W. and J.O. conceived the project. L.W. and S.P. performed and contributed equally to the SHG measurements with assistance from E.T. and A.L. L.W. and J.O. analysed the data. T.M. and J.E.M. performed the model calculation. L.W., T.M. and J.O. performed the frequency scaling analysis. N.L.N. and J.G.A. grew the crystals and characterized the crystal structure. L.W., T.M. and J.O. wrote the manuscript. All authors commented on the manuscript.
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Wu, L., Patankar, S., Morimoto, T. et al. Giant anisotropic nonlinear optical response in transition metal monopnictide Weyl semimetals. Nature Phys 13, 350–355 (2017). https://doi.org/10.1038/nphys3969
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