Low-energy quasiparticle excitations in the spin-triplet superconductor UTe₂

The new uranium-based superconductor UTe₂ attracts much interest as a paramagnetic analogue of ferromagnetic superconductors. The extremely high upper critical field along the magnetic hard axis, reentrant superconductivity, and only small reduction of the Knight shift in UTe₂ indicate the spin-triplet superconducting state likely mediated by ferromagnetic fluctuations. Furthermore, scanning tunneling spectroscopy and optical Kerr effect measurements suggest the time reversal symmetry breaking superconductivity with topologically nontrivial surface states. Focusing on the superconducting gap, previous experimental studies reported the gap structure with point nodes which is consistent with the spin-triplet superconductivity. However, whether the point nodes locate at the high-symmetry points or not is crucial to elucidate the superconducting properties, which is still unrevealed. Thus, we performed the magnetic penetration depth measurements using a tunnel diode oscillator with the ac magnetic field along each crystallographic axis. In addition, we also investigated the effects of weak dc magnetic field on the low-energy quasiparticle excitations. Based on our experimental results, we will discuss the possible superconducting gap functions in UTe₂.