Impurity screening and stability of Fermi arcs against Coulomb and magnetic scattering in a Weyl monopnictide

We present a quasiparticle interference study of clean and Mn surface-doped TaAs, a prototypical Weyl semimetal, to test the screening properties as well as the stability of Fermi arcs against Coulomb and magnetic scattering. Contrary to topological insulators, the impurities are effectively screened in Weyl semimetals. The adatoms significantly enhance the strength of the signal such that theoretical predictions on the potential impact of Fermi arcs can be unambiguously scrutinized. Our analysis reveals the existence of three extremely short, previously unknown scattering vectors. Comparison with theory traces them back to scattering events between large parallel segments of spin-split trivial states, strongly limiting their coherence. In sharp contrast to previous work [R. Batabyal et al., Sci. Adv. 2, e1600709 (2016)], where similar but weaker subtle modulations were interpreted as evidence of quasiparticle interference originating from Femi arcs, we can safely exclude this being the case. Overall, our results indicate that intra- as well as inter-Fermi arc scattering are strongly suppressed and may explain why—in spite of their complex multiband structure—transport measurements show signatures of topological states in Weyl monopnictides.

Figure 1

Structural and electronic properties of TaAs. (a) Photography of the TaAs single crystals used in the present study. (b) Crystal structure of TaAs. (c) Schematic illustration of the bulk-to-surface correspondence leading to the emergence of Fermi arcs. (d) Topographic image of pristine TaAs (inset: atomic resolution image). (e) LDOS as inferred by STS on the pristine surface and (f) theoretically calculated LDOS for an As-terminated surface. (g) QPI map and its Fourier transformed of clean TaAs ($E-E_{\mathrm{F}}=+50$ meV). (h) Topographic image of the Mn surface-doped TaAs. The inset shows the Mn adsorption site. (i) Experimental LDOS as measured after Mn deposition. (k) Difference in the experimentally measured LDOS before and after Mn deposition. (m) QPI map and its Fourier transformed obtained on the Mn-doped surface ($E-E_{\mathrm{F}}=+50$ meV). The central region within the dashed box will be analyzed in detail in Fig. 2.

Figure 2

Comparison of theoretical and experimental scattering maps. (a) Theoretical constant energy cut for the As-terminated surface revealing the existence of multiple energy contours. (b) Theoretical ($E-E_{\mathrm{F}}=-100$ meV) and (c) experimental FT-QPI ($E-E_{\mathrm{F}}=-90$ meV). The good agreement between theory and experiment allows us to identify a first subset of the dominant scattering mechanisms and to link them to their corresponding initial and final states (see labeled arrows). The central region within the dashed box will be analyzed in detail in Fig. 4.

Figure 3

Fermi arc contribution to the Fermi surface and scattering vectors expected for TaAs. (a) Theoretically calculated Fermi surface containing the Fermi arcs only and (b) the QPI interference pattern expected from the arcs displayed in (a). None of the scattering vectors is observed experimentally. The region within the hatched box will be discussed in detail in Fig. 4.

Figure 4

Short scattering vectors and theoretical analysis of their origin. (a) Detail of the FT-QPI data for short scattering vectors $\mathbf{q}$. (b)–(g) Theoretical constant-energy contours (red sections contribute strongly to QPI signal) and calculated QPI maps that identify the origin of scattering vectors ${\mathbf{q}}_5, {\mathbf{q}}_6$, and ${\mathbf{q}}_7$.