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hokhani
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When a transition from valence band to conduction band happens (in a direct-gap material), how the momentum is conserved while the group velocity of electron is changed considerably?
By this you mean that "group velocity times effective mass" is always conserved in the direct transition?Cthugha said:Away from k=0 there may be a different group velocity at the same k in the conduction and valence band, but at the same time the effective mass will also be different.
Momentum conservation is a fundamental law of physics that states that the total momentum of a closed system remains constant. This means that the total momentum before an event or interaction must equal the total momentum after the event or interaction, as long as no external forces are acting on the system.
In a direct transition, there is a change in the state or motion of a particle without any intermediate steps. Momentum conservation applies in a direct transition because the total momentum of the system must remain constant before and after the transition.
The mass and velocity of the particles involved in the direct transition are the main factors that affect momentum conservation. The total momentum will be conserved as long as the masses and velocities of the particles involved do not change during the transition.
Some examples of direct transitions where momentum conservation applies include collisions between billiard balls, the recoil of a gun after firing a bullet, and the movement of a rocket after releasing its boosters. In all of these examples, the total momentum of the system is conserved before and after the transition.
Momentum conservation is important in understanding direct transitions because it helps us predict the behavior of a system before and after the transition. It allows us to calculate the velocities of particles involved in the transition and understand how the momentum is distributed among them. Without momentum conservation, our understanding of the laws of motion and the behavior of particles would be incomplete.