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Optical conductivity and band gap in the double-Weyl candidate SrSi2 at ambient pressure

L. Z. Maulana, A. A. Tsirlin, E. Uykur, Y. Saito, M. Dressel, M. Imai, A. V. Pronin

TL;DR

The paper addresses whether cubic SrSi2, previously proposed as a double-Weyl semimetal, is actually a narrow-gap semiconductor at ambient pressure and how this fate depends on the exchange-correlation functional used in DFT. It employs broadband optical conductivity from $70$ to $22{,}000$ cm$^{-1}$ across $10$–$295$ K and compares the interband response with DFT calculations using LDA, PBE, and the mBJ functional, including a small lattice-parameter adjustment to test the band-topology. The results show the interband conductivity is best described by the mBJ band structure with a lattice expansion of about 1.2%, yielding a direct optical gap of roughly $40$ meV and an indirect gap near $1$ meV, indicating SrSi2 is a gapped material at ambient pressure and not a Weyl semimetal. This provides a robust bulk benchmark for ab initio methods in narrow-gap semiconductors and clarifies the conditions under which a Weyl state might emerge (e.g., under pressure).

Abstract

We probe the possible double-Weyl state in cubic SrSi2 using optical spectroscopy. The complex optical conductivity was measured in a frequency range from 70 to 22 000 cm-1 at temperatures down to 10 K at ambient pressure. The optical response of SrSi2 can be well separated into the intraband (free carriers) and interband contributions. Additionally, four infrared-active phonons are detected. As follows from the optical spectra, the free-carrier density decreases with decreasing temperature, consistent with an activation behaviour. Experimental interband conductivity juxtaposed with ab initio calculations shows that conventional density-functional theory fails to describe the electronic structure of SrSi2 in the vicinity of the Fermi level. A semi-local exchange-correlation potential allows a much better agreement with the experiment, resulting in the trivial (gapped) band structure of SrSi2. The direct gap estimated from the measurements is approximately 40 meV.

Optical conductivity and band gap in the double-Weyl candidate SrSi2 at ambient pressure

TL;DR

The paper addresses whether cubic SrSi2, previously proposed as a double-Weyl semimetal, is actually a narrow-gap semiconductor at ambient pressure and how this fate depends on the exchange-correlation functional used in DFT. It employs broadband optical conductivity from to cm across – K and compares the interband response with DFT calculations using LDA, PBE, and the mBJ functional, including a small lattice-parameter adjustment to test the band-topology. The results show the interband conductivity is best described by the mBJ band structure with a lattice expansion of about 1.2%, yielding a direct optical gap of roughly meV and an indirect gap near meV, indicating SrSi2 is a gapped material at ambient pressure and not a Weyl semimetal. This provides a robust bulk benchmark for ab initio methods in narrow-gap semiconductors and clarifies the conditions under which a Weyl state might emerge (e.g., under pressure).

Abstract

We probe the possible double-Weyl state in cubic SrSi2 using optical spectroscopy. The complex optical conductivity was measured in a frequency range from 70 to 22 000 cm-1 at temperatures down to 10 K at ambient pressure. The optical response of SrSi2 can be well separated into the intraband (free carriers) and interband contributions. Additionally, four infrared-active phonons are detected. As follows from the optical spectra, the free-carrier density decreases with decreasing temperature, consistent with an activation behaviour. Experimental interband conductivity juxtaposed with ab initio calculations shows that conventional density-functional theory fails to describe the electronic structure of SrSi2 in the vicinity of the Fermi level. A semi-local exchange-correlation potential allows a much better agreement with the experiment, resulting in the trivial (gapped) band structure of SrSi2. The direct gap estimated from the measurements is approximately 40 meV.

Paper Structure

This paper contains 5 sections, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Materials and Methods
  3. Results and Discussion
  4. Conclusions
  5. Acknowledgements

Figures (4)

  • Figure 1: (a) Powder XRD patterns of the studied SrSi$_{2}$ samples, (b) their temperature-dependent dc resistivity, and (c) a photograph of one of the samples featuring the surface used for the reflectivity measurements.
  • Figure 2: (a, b) Reflectivity and (c, d) the real part of optical conductivity for samples 1 (left-hand panels) and 2 (right-hand panels).
  • Figure 3: Band structures of SrSi$_{2}$ calculated using different approximations for the exchange-correlation potential, as indicated. Experimental lattice parameter of $a=6.535~\textrm{\AA}$ is used evers1978.
  • Figure 4: (a, b) Experimental conductivity spectra and their Drude-Lorentz fits at 10 K for both samples. (c) Interband part of the experimental conductivity for both samples and the conductivity spectra calculated using three different exchange-correlation functionals (LDA, PBE, mBJ). The increased conductivity above $\sim 0.5$ eV is due to transitions involving multiple (non-parabolic) bands present at higher energies, see Fig. \ref{['bands']}. (d) Band structures in the vicinity of the possible band crossing obtained within the mBJ calculations for two lattice parameters $a$.