Empty $4{\mathit{f}}^1$-State Magneto-Optical Spectroscopy in LaSe

We report the first observation of the magneto-optical signal of an empty spin-polarized $4f$ state in a paramagnetic metal. At 1.5 K and in a magnetic field of 10 T the Kerr rotation spectrum of LaSe shows a remarkably sharp positive peak of 2.1° at 2.77 eV and a broader and smaller structure at 3.01 eV. A comparison with CeSe, which displays a similar sharp peak with opposite sign but no second peak, gives evidence that the LaSe signal is due to $5d\to 4f$ $(J=5/2\mathrm{})$ and $5d\to 4f$ $(J=7/2\mathrm{})$ transitions, while the magneto-optical signal in CeSe corresponds to a $4f(J=5/2\mathrm{})\to 5d$ transition.