Dynamics and field-induced order in the layered spin $S=1/2$ dimer system (C$_5$H$_6$N$_2$F)$_2$CuCl$_4$.

The quasi-two-dimensional Heisenberg spin $S=1/2$ dimer system bis(2-amino-5-fluoro-pyridinium) tetrachlorocuprate(II) is studied by means of inelastic neutron scattering, calorimetry and nuclear magnetic resonance (NMR) experiments. In the absence of an applied magnetic field we find dispersive triplet excitations with a spin gap of $\Delta=1.112(15)$ meV and a bandwidth of $0.715(15)$ meV within the layers and $0.116(15)$ meV between the layers, respectively. In an applied magnetic field of $\mu_0H_c\approx 8.5$ Tesla the spin gap is closed and we find a field induced antiferromagnetically ordered phase.