Question

Carbon nanotube shows magneto-resistive effects:

• A. at high temperature
• B. at low temperature
• C. at room temperature
• D. at very high temperature

Hint:

As a general rule in condensed matter physics, most quantum coherence effects (like the Aharonov-Bohm effect, quantum Hall effect, superconductivity) are best observed, or only observable, at very low temperatures.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Magnetoresistance is the property of a material to change its electrical resistance in the presence of an external magnetic field. In carbon nanotubes (CNTs), this effect is primarily due to quantum mechanical phenomena, such as the Aharonov-Bohm effect.
Step 2: Detailed Explanation:
The magnetoresistive effects in CNTs are quantum interference effects. For such effects to be observable, the electrons must maintain their quantum coherence as they travel through the material.
At high temperatures, the increased thermal energy leads to significant lattice vibrations (phonons). Collisions between electrons and phonons (electron-phonon scattering) become frequent and intense. These scattering events destroy the phase coherence of the electron wave function.
Therefore, to observe quantum phenomena like magnetoresistance, it is necessary to reduce thermal scattering by cooling the material to very low temperatures. At these low temperatures, electron coherence is preserved over longer distances, allowing the quantum interference effects to become dominant.
Step 3: Final Answer:
The magneto-resistive effects in carbon nanotubes are quantum phenomena that are only prominent when thermal scattering is minimized, which occurs at low temperatures.