In case of Complex DFT spectrum, why the x axis range from N/2 to N point mean a negative frequency?
it's because the DFT is periodic with period $N$.

$$  X[k+N] = X[k] \qquad \forall k \in \mathbb{Z} $$

this periodic extension is also true about the discrete-time signal $x[n]$

$$  x[n+N] = x[n] \qquad \forall n \in \mathbb{Z} $$

doesn't matter if either time-domain or frequency-domain representations are complex or not.

that means for $-\frac{N}{2}\le k &lt; 0 $ (the negative frequencies), then $X[k] = X[k+N]$.  the explicit negative frequency component $X[k]$ has a copy $X[k+N]$, but that is in latter half of the DFT result.  

If  $-\frac{N}{2}\le k &lt; 0 $, then  $\frac{N}{2}\le k+N &lt; N $.