Susceptibility: Imaginary Part = 0 - What Does it Mean?

[KFC]

I am reading a paper on applying an external field on a two-level system. And it turns out that at some frequency (of the field), the imaginary part of the susceptibility become very close to zero. I wonder what's the physical significance when imaginary part of the susceptibility is ZERO?

Thanks.

 

[Dr Transport]

I am reading a paper on applying an external field on a two-level system. And it turns out that at some frequency (of the field), the imaginary part of the susceptibility become very close to zero. I wonder what's the physical significance when imaginary part of the susceptibility is ZERO?

Thanks.

The imaginary part of the susceptibility is a related to the absorption, hence when it is zero, or nearly so, there isn't any absorption in the system.