Fermi Energy

In Physics, the concept of energy is tricky because it has different meanings depending on the context. For example, in atoms and molecules, energy comes in different forms such as light energy, electrical energy, heat energy, etc. In quantum mechanics, it gets even trickier. In this branch of physics, scientists rely on concepts like Fermi energy which refers to the energy of the highest occupied quantum state in a system of fermions at absolute zero temperature.

What is Fermi Energy?

Named after the physicist, Enrico Fermi, a Fermi level is the measure of the energy of least tightly held electrons within a solid. It is important in determining the thermal and electrical properties of solids. The value of the Fermi level at absolute zero temperature (−273.15 °C) is known as the Fermi energy. It is also the maximum kinetic energy an electron can attain at 0K. Fermi energy is a constant for each solid.

Calculating Fermi Energy

To determine the lowest possible Fermi energy of a system, we first group the states with equal energy into sets and arrange them in an increasing order of energy. We then add particles one at a time, successively filling up the unoccupied quantum states with the lowest energy.

When all the particles are arranged accordingly, the energy of the highest occupied state is the Fermi energy. In Spite of the extraction of all possible energy from a metal by cooling it to near absolute zero temperature (0 Kelvin), the electrons in the metal still move around. The fastest ones move at a velocity corresponding to a kinetic energy equal to the Fermi energy.

Fermi Energy

The Fermi level and Fermi energy are usually confusing terms and often used interchangeably to refer each other. Although, both the terms are equal at absolute zero temperature, different at other temperature.

Fermi energy is applied in determining the electrical and thermal characteristics of the solids. It is one of the important concepts in superconductor physics and quantum mechanics. It is used in semiconductors and insulators.

Fermi Level

The highest energy level that an electron can occupy at the absolute zero temperature is known as the Fermi Level. The Fermi level lies between the valence band and conduction band because at absolute zero temperature the electrons are all in the lowest energy state. Due to lack of sufficient energy at 0 Kelvin, the Fermi level can be considered as the sea of fermions (or electrons) above which no electrons exist. The Fermi level changes as the solid is warmed and as electrons are added to or withdrawn from the solid.

Fermi Energy Level

The reason for the existence of this energy level is due to Pauli’s exclusion principle which states two fermions cannot occupy that same quantum state. So, if a system has more than one fermions, each fermion has a different set of magnetic quantum numbers associated with it.

The Fermi Temperature can be defined as the energy of Fermi level divided by the Boltzmann’s constant. It is also the temperature at which the energy of the electron is equal to the Fermi energy. It is the measure of the electrons in the lower states of energy in metal.

Applications of Fermi Energy

It is one of the important concepts in quantum mechanics and condensed matter physics. It is used in semiconductors and insulators. It is used to describe insulators, metals, nd semiconductor. Fermi energy is applied in determining the electrical and thermal characteristics of the solids. It also important in nuclear physics to understand the stability of white dwarfs. White dwarfs are stars that have ma ass comparable to the un but have about a hundredth of its radius.


Practise This Question

Laser beams are used to measure long distances because