In our field, it is unpopular to publish a result where one finds an absence of a particular phenomenon. However, these results can be extremely valuable, as one can see what other authors have tried.
In the study of charge density wave (CDW) systems, which has been undergoing a renaissance of late, there is one particular null result I find quite fascinating. This result (sorry, paywall) was published by F. DiSalvo and R. Fleming in Solid State Communications, demonstrating the inability, even at high electric fields, for a charge density wave to depin and slide in two prototypical quasi-2D transition metal dichalcogenides, 1T-Tantalum Disulphide and 2H-Tantalum Diselenide.
In fact, I am unaware of any report of a sliding CDW in quasi-2D transition metal dichalcogenides. This has pretty vast implications for these materials, as it is difficult to probe the electronic subsystem alone due to the inability to divorce it from the ionic subsystem.
Any comments pointing me in the direction of observations of sliding CDWs in transition metal dichalcogenides are encouraged.