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Related Concept Videos

Fermi Level01:18

Fermi Level

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The Fermi-Dirac function is represented by an S-shaped curve indicating the probability of an energy state being occupied by an electron at a given temperature. The Fermi level is the energy level at which there is a fifty percent chance of finding an electron, and it is positioned between the lower-energy valence band and the higher-energy conduction band.
At absolute zero temperature, electrons fill all energy states up to the Fermi level, leaving upper states empty. As the temperature rises,...
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Types of Semiconductors01:20

Types of Semiconductors

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Intrinsic semiconductors are highly pure materials with no impurities. At absolute zero, these semiconductors behave as perfect insulators because all the valence electrons are bound, and the conduction band is empty, disallowing electrical conduction. The Fermi level is a concept used to describe the probability of occupancy of energy levels by electrons at thermal equilibrium. In intrinsic semiconductors, the Fermi level is positioned at the midpoint of the energy gap at absolute zero. When...
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Semiconductors01:22

Semiconductors

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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.
Metals such as copper (Cu), zinc (Zn), or lead (Pb) have low resistivity and feature conduction bands that are either not fully occupied or overlap with the valence band, making a bandgap non-existent. This allows electrons in the highest energy levels of the valence band to easily transition to the conduction band upon gaining...
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Band Theory

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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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Theory of Metallic Conduction01:17

Theory of Metallic Conduction

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The conduction of free electrons inside a conductor is best described by quantum mechanics. However, a classical model makes predictions close to the results of quantum mechanics. It is called the theory of metallic conduction.
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Fermi Level Dynamics01:12

Fermi Level Dynamics

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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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Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving
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Population Inversion and Dirac Fermion Cooling in 3D Dirac Semimetal Cd3As2.

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  • 1State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, P. R. China.

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Ultrafast dynamics of 3D Dirac fermions were revealed in Cd3As2. A long-lived population inversion was observed, contrasting with 2D graphene, offering insights for future device applications.

Keywords:
3D Dirac semimetalpopulation inversiontime-resolved ARPESultrafast dynamics

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Ultrafast Spectroscopy

Background:

  • Understanding the ultrafast dynamics of three-dimensional (3D) Dirac fermions is crucial for fundamental science and technological applications.
  • Key unanswered questions involve the distinct cooling mechanisms of 3D Dirac fermions compared to their two-dimensional (2D) counterparts and the existence of population inversion.

Purpose of the Study:

  • To investigate the ultrafast dynamics of Dirac fermions in the model 3D Dirac semimetal cadmium arsenide (Cd3As2).
  • To determine the cooling mechanisms and explore the possibility of population inversion in 3D Dirac fermions.

Main Methods:

  • Employed time- and angle-resolved photoemission spectroscopy (TARPES) with a tunable probe photon energy.
  • Analyzed energy- and momentum-resolved relaxation rates of photoexcited carriers.

Main Results:

  • Observed a linear energy dependence of the relaxation rate, indicating Dirac fermion cooling via intraband relaxation.
  • Reported a population inversion with accumulated photoexcited carriers in the conduction band, exhibiting a lifetime of 3.0 picoseconds (ps).
  • This long-lived population inversion in 3D Dirac semimetals contrasts sharply with the much shorter lifetimes observed in 2D graphene.

Conclusions:

  • Provided direct experimental evidence for a long-lived population inversion in a 3D Dirac semimetal.
  • The findings highlight differences in ultrafast dynamics between 3D and 2D Dirac systems.
  • The observed phenomena have implications for the development of novel electronic devices based on 3D Dirac semimetals.