I'll first start out by saying that I am 'mostly' atheist: agnostic with strong atheistic tendencies. Most of my friends consider me to be a flat-out atheist. And I am definitely a 'materialist' in the sense that I believe that *everything* ultimately depends on the physical aspects of existence. So, consciousness is a matter of brain states, and there are no such things (as far as i can tell) as ghosts, spirits, souls, or other non-physical aspects of existence.
So, if 'spiritual' means that you feel connected to a non-physical aspect of existence, then I am not spiritual at all. The whole range of such beliefs seems rather outlandish and strange to me. I don't see why otherwise intelligent people believe in such stuff.
But that is very far from being what *most* examples of 'spirituality' consist of. When do people usually report feeling 'spiritual'? Well, when they feel connected to others, when they feel connected to the universe at large, when they have feelings of awe and wonder, and when they are better able to grasp their place in this cosmic drama.
And *those* aspects of 'spirituality' are quite familiar to me.
To be out under a star-filled sky with someone I love, gazing up at the wonders around us: that is a time when what I feel seems to be exactly what others describe as 'spiritual'. And that is a feeling that goes deep and is important to my sense of connection and well-being.
When I see a beautiful mountain-scape, when I take an opportunity to help another, when I contemplate the wonders of this universe, those are the times I feel what, it seems to me, is usually called 'spiritual' by others.
So, while these emotions have *nothing* to do with spirits, or a non-physical aspect of existence, or anything to do with Gods, ghosts, or anything supernatural, they *are* a common human emotion and one that I, as an atheist, partake in and hold valuable. And they are often called 'spiritual'.
So, in that sense, yes, atheists can be spiritual.