Multidimensional Schr\"odinger Operators Whose Spectrum Features a Half-Line and a Cantor Set

We construct multidimensional Schr\"odinger operators with a spectrum that has no gaps at high energies and that is nowhere dense at low energies. This gives the first example for which this widely expected topological structure of the spectrum in the class of uniformly recurrent Schr\"odinger operators, namely the coexistence of a half-line and a Cantor-type structure, can be confirmed. Our construction uses Schr\"odinger operators with separable potentials that decompose into one-dimensional potentials generated by the Fibonacci sequence and relies on the study of such operators via the trace map and the Fricke-Vogt invariant. To show that the spectrum contains a half-line, we prove an abstract Bethe--Sommerfeld criterion for sums of Cantor sets which may be of independent interest.