Analyzing Spectrum of a signal?
Your signal has the form

$$x(n)=A[\cos(2\pi fn)+i\sin(2\pi fn)]=Ae^{i2\pi fn}$$

This is a complex exponential with normalized frequency $f$ (normalized by the sampling frequency). The bins of the signal's FFT of length $N$ correspond to normalized frequencies

$$f_i=\frac{i-1}{N},\quad i=1,2,\ldots, N$$

In your case $N=8$ and $f=6/8=f_7$ (using the Matlab indexation, which starts with index $1$). So the frequency of your signal $x(n)$ lies exactly on the FFT grid. This is why there is one peak at index $7$ and all other values are $0$ (apart from numerical inaccuracies).

If $x(n)$ were real-valued, its FFT would satisfy the following symmetry condition:

$$X_k=X^*_{N-k}$$

However, for complex-valued signals, such as yours, there is in general no symmetry.