[Paper Review] Reply to Comment on "Tip induced unconventional superconductivity on Weyl semimetal TaAs"

He Wang, Huichao Wang+9 more|arXiv (Cornell University)|Jul 11, 2016

Topological Materials and Phenomena15 references4 citations

TL;DR

This paper responds to criticism questioning the unconventional superconductivity observed in TaAs via tip-induced point contact spectroscopy. The authors argue that the observed conductance plateau with double peaks, sharp dips, and a zero-bias peak—features inconsistent with conventional superconductors—arise from p-wave-like pairing and possible Majorana zero modes, not artifacts of conventional materials, and refute claims attributing the signal to critical current effects in Pb and Nb.

ABSTRACT

Recently, we presented a paper about the tip-induced superconducting phase on TaAs single crystal (arXiv:1607.00513). A conductance plateau with double conductance peaks at finite bias, sharp double dips and a zero bias conductance peak were observed in the point contact spectra (PCS). These three features in one PCS suggest the possibility of p-wave like superconductivity and Majorana zero modes in the surface. Soon after our paper posted on arXiv, Gayen el al. (arXiv:1607.01405) questioned the underlying mechanism of our observations based on their experimental measurements in conventional superconductors(Pb and Nb) and conclusively indicated that the superconductivity observed in our TaAs crystal is conventional. We disagree with their quick conclusion without any specific study on the TaAs materials. We will explain in the following that for the clean (Z=0) point contact (PC) in ballistic limit, the superconducting transition feature in temperature dependence of PC resistance is expected. And the critical current effect claimed by Gayen el al. cannot explain the observed PCS in our paper.

Motivation & Objective

• To counter the claim by Gayen et al. that the observed superconducting features in TaAs are due to conventional superconductivity.
• To demonstrate that the observed conductance features—double peaks, sharp dips, and zero-bias peak—are inconsistent with conventional superconductors like Pb and Nb.
• To argue that the critical current effect proposed by Gayen et al. cannot explain the point contact spectra in TaAs.
• To reaffirm the possibility of p-wave-like superconductivity and Majorana zero modes in Weyl semimetal TaAs based on ballistic point contact measurements.

Proposed method

• Conduct point contact spectroscopy (PCS) on TaAs single crystals under ballistic, clean (Z=0) conditions.
• Analyze temperature-dependent resistance to identify superconducting transition features.
• Compare observed PCS features with those from conventional superconductors (Pb and Nb) to rule out conventional mechanisms.
• Use theoretical analysis to show that the observed conductance plateau and double peaks are consistent with unconventional pairing in Weyl semimetals.
• Demonstrate that the critical current effect, cited by Gayen et al., fails to reproduce the observed spectral features in TaAs.
• Argue that the unique electronic structure of TaAs—specifically its Weyl fermion surface states—supports unconventional superconducting pairing.

Experimental results

Research questions

• RQ1 Can the observed conductance plateau with double peaks and a zero-bias peak in TaAs be explained by conventional superconductivity?
• RQ2 Is the critical current effect in conventional superconductors like Pb and Nb sufficient to explain the point contact spectra in TaAs?
• RQ3 Do the spectral features in TaAs point to p-wave-like pairing or Majorana zero modes rather than conventional BCS-like superconductivity?
• RQ4 Why do temperature-dependent resistance measurements in clean point contacts show superconducting transition features in TaAs despite the absence of bulk superconductivity?
• RQ5 What role does the Weyl semimetal surface state play in enabling unconventional superconducting pairing?

Key findings

• The observed conductance plateau with double peaks at finite bias and a sharp zero-bias peak in TaAs cannot be reproduced by conventional superconductors such as Pb and Nb.
• The critical current effect, proposed by Gayen et al. to explain the spectra, fails to account for the specific features observed in TaAs point contact measurements.
• In clean, ballistic point contacts, a superconducting transition feature in resistance is expected even without bulk superconductivity, supporting the validity of the observed signal.
• The spectral features in TaAs are consistent with p-wave-like pairing and the potential presence of Majorana zero modes on the surface.
• The electronic structure of TaAs, including its Weyl fermion surface states, provides a favorable environment for unconventional superconductivity.
• The authors conclude that the observed phenomena are not artifacts of conventional superconductivity but are intrinsic to the Weyl semimetal TaAs.