Reference request about the parabolic Sobolev spaces

Question

I am looking for a reference where the following space is defined and studied : $$ L^p\left(0, T ; \mathbb{W}_0^{s, p}(U)\right), $$ where $T>0$, $p\geq 1$, $0<s<1$, $U$ is an open bounded subset of $\mathbb{R}^N$, and $\mathbb{W}_0^{s, p}(U)$ is the fractional Sobolev space defined as the completion of $\mathcal{C}^\infty_c(U)$ with respect to the norm $$ ||\phi||=\left(\int_U|\phi(x)|^p d x+\int_U\int_U \frac{|\phi(x)-\phi(y)|^p}{|x-y|^{N+p s}} d x d y\right)^{\frac{1}{p}} $$ Thank you very much in advance.