Optical response of a line node semimetal

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• Distinguishing between different types of semimetals (line node, Dirac, Weyl) is crucial for understanding their electronic properties.
• The AC optical response is a sensitive probe of electronic band structure, particularly interband transitions.
• Graphene exhibits a universal optical background, serving as a reference for other 2D and 3D materials.

Purpose of the Study:

• To calculate and analyze the AC optical response of line node semimetals.
• To identify characteristic optical behaviors that differentiate line node semimetals from point node semimetals (Dirac and Weyl).
• To investigate the influence of temperature, chemical potential, and nodal circle radius on optical properties.

Main Methods:

• Theoretical calculation of the AC optical response, focusing on interband optical conductivity.
• Analysis of the energy dependence of the optical background at zero and finite temperatures.
• Calculation of optical spectral weight, Lorentz number, and thermopower as functions of temperature and energy scales.

Main Results:

• A flat optical background at low photon energies is observed, analogous to graphene but dependent on Fermi velocity and nodal circle radius (b).
• At high photon energies, the response becomes linear, consistent with 3D Dirac fermions.
• The optical spectral weight exhibits distinct low-temperature (linear in energy, proportional to b) and high-temperature (independent of b) regimes.
• The Lorentz number shows temperature-dependent plateaus (2.4L o and 4.2L o) characteristic of 2D-Dirac and 3D-Dirac behaviors, respectively.
• Thermopower evolution from 2D to 3D behavior with increasing temperature is demonstrated.

Conclusions:

• Line node semimetals possess unique AC optical responses that allow for their distinction from Dirac and Weyl semimetals.
• The nodal circle radius (b) acts as a critical energy scale defining the persistence of the flat optical background and influencing spectral weight.
• The temperature dependence of optical properties, including the Lorentz number and thermopower, reveals a transition from 2D-like to 3D-like behavior.