London penetration depth in Half- Heusler YPdBi and TbPdBi compounds

Using the combination of a tunnel diode oscillator (TDO) and a commercial dilution refrigerator, we measured temperature dependent magnetic penetration depth △λ(T) in single crystals of YPdBi and TbPdBi, down to temperatures as low as 0.1K. We found that penetration depths of both compounds do not show exponential temperature dependence and saturation at low temperatures, as expected for conventional BCS superconductors. Instead, in both materials, the penetration depth can be described by a power law △λ(T)=A×Tⁿ . The coeficient A was found to be about 50% smaller in TbPdBi, but the exponents are very similar, n = 2.76 ± 0.04 in YPdBi and n = 2.6 ± 0.3 in TbPdBi, respectively. Our results suggest unconventional superconductivity in both YPdBi and TbPdBi.