about susceptibility

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.

KFC	about susceptibility Tweet 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.