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Semiconductors01:22

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There is variation in the electrical conductivity of materials - metals, semiconductors, and insulators that are showcased with the help of the energy band diagrams.
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The contact of metal and semiconductor can lead to the formation of a junction with either Schottky or Ohmic behavior.
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The vacuum level denotes the energy threshold required for an electron to escape from a material surface. It is usually positioned above the conduction band of a semiconductor and acts as a benchmark for comparing electron energies within various materials.
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When two or more atoms come together to form a molecule, their atomic orbitals combine and molecular orbitals of distinct energies result. In a solid, there are a large number of atoms, and therefore a large number of atomic orbitals that may be combined into molecular orbitals. These groups of molecular orbitals are so closely placed together to form continuous regions of energies, known as the bands.
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Optical response of a line node semimetal.

J P Carbotte1

  • 1Department of Physics, McMaster University, Hamilton, Ontario, L8S 4M1, Canada. Canadian Institute for Advance Research, Toronto, Ontario, M5G 1Z8, Canada.

Journal of Physics. Condensed Matter : an Institute of Physics Journal
|November 25, 2016
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Summary

We calculated the AC optical response of line node semimetals, distinguishing them from Dirac and Weyl semimetals. The study reveals unique optical behaviors dependent on nodal circle radius and Fermi velocity.

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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.