Mid-infrared Photothermal Response in Magnetic Weyl Semimetal PrAlGe

Magnetic Weyl semimetal, PrAlGe shows a rich topological tunability of Weyl fermions at the low-energy band structure. We are particularly interested in two type-I Weyl bands near the fermi level that appeared by the ferromagnetic ordering at a temperature below 15 K, because measuring the photoresponse induced by circularly polarized mid-infrared light allows for the detection of the chiral nature in these two type-I Weyl bands. We investigated the temperature- and the frequency-dependent photothermal response in PrAlGe. We utilized a 9.2-micron mid-infrared quantum cascade laser (135 meV) with 18 mW power, which is low enough energy to excite electrons in Weyl bands and sufficient optical power to generate photothermal response. The sample with six gold contact wires is placed in a continuous-flow helium cryostat capable of reaching below 15 K. The photothermal response of a pair of contacts was precisely measured as a function of temperature and chopper frequency, with focused light on one of them. A temperature gradient across two contacts generates a photovoltaic effect. The photothermal response linearly decreased until a sharp increase near 15 K, coinciding with the Curie temperature of PrAlGe. The photothermal response also rolls off quickly over a few hundred Hz.