Isolated Susceptibility and its applicability to state populations in LiY_1-xHo_xF_4.

The (quasi-static) isolated susceptibility is a fundamental component of any susceptibility measurement but can generally be neglected. Yet when cleanly measured, it would be a quantity of great interest as it is essentially a `spectroscopic' probe of quantum states and their populations, but one that is inaccessible to standard techniques like neutron scattering. We have realised that driving at 10⁴ - 10⁶ Hz is sufficient to reveal the isolated susceptibility in many systems. If the states are known (e.g. through knowledge of the spin hamiltonian) then populations can be measured, giving a direct characterisation of far-from equilibrium behaviour, while if populations are near to equilibrium, then quantum states can be visualised. In the latter case, the isolated susceptibility has the most structure when spin states are superposed: hence the probe can be used to quantify superposed states and to measure their state concurrence. Here we demonstrate the applicability of this model to LiY_1-xHo_xF_4., demonstrate that in the dilute we are in the isolated regime and discuss the implications of this observation to experimental interpretations of the low temperature behaviour.