  Ultra-thin layers of liquids on a surface behave differently from bulk
liquids due to liquid-surface interactions. Some examples are significant
changes in diffusion properties and the temperature at which the liquid-solid
phase transition takes place. Indeed, molecular dynamics simulations suggest
that thin layers of water on a diamond surface may remain solid even well above
room temperature. However, because of the small volumes that are involved, it
is exceedingly difficult to examine these phenomena experimentally with current
technologies. In this context, shallow NV centers promise a highly sensitive
tool for the investigation of magnetic signals emanating from liquids and
solids that are deposited on the surface of a diamond. Moreover, NV centers are
non-invasive sensors with extraordinary performance even at room-temperature.
To that end, we present here a theoretical work, complemented with numerical
evidence based on bosonization techniques, that predicts the measurable signal
from a single NV center when interacting with large spin baths in different
configurations. We therefore propose single NV centers as sensors capable to
resolve structural water features at the nanoscale and even sensitive to phase
transitions.
