Final answer:
The unitary STM is a limitation of the MSM because it assumes that the tunneling current is solely determined by the surface properties of the sample, disregarding other factors such as the electronic structure of the tip and the interaction between the tip and the sample.
Step-by-step explanation:
The unitary STM is a limitation of the MSM because it assumes that the electron tunneling current from the tip to the sample is solely determined by the surface properties of the sample. In reality, the tunneling current is also affected by other factors such as the electronic structure of the tip and the interaction between the tip and the sample.
For example, if the tip and sample have different work functions, there will be a barrier to electron tunneling that can significantly decrease the tunneling current. Additionally, if the tip and sample are chemically reactive, there can be chemical reactions at the interface that affect the tunneling current.
Therefore, the unitary STM simplifies the system by assuming that the tunneling current is solely determined by the surface properties of the sample, but this assumption can lead to inaccurate measurements and limited understanding of the system.