Penetration depth measurements of K2Cr3As3 and Rb2Cr3As3

•Penetration depth of K2Cr3As3 single crystals and polycrystalline Rb2Cr3As3.•Behavior of K2Cr3As3 crystals similar to polcrystals until downturn at 0.9–1.2 K.•Power law behavior of Rb2Cr3As3 similar to poorer quality polycrystals of K2Cr3As3.•Results are additional evidence for line nodes in K2Cr3As3.•Highlights importance of measuring high quality samples not exposed to air.

The newly discovered superconductors A2Cr3As3 (A=K, Rb, Cs), with a quasi-one-dimensional crystal structure have attracted considerable interest. The crystal structure consists of double-walled tubes of [Cr3As3]2− that extend along the c-axis. Previously we reported measurements of the change in London penetration depth of polycrystalline samples of K2Cr3As3 using a tunnel diode oscillator based technique, which show a linear temperature dependence at low temperatures, giving evidence for line nodes in the superconducting gap. Here we report similar measurements of the penetration depth for polycrystalline Rb2Cr3As3 and several single crystals of K2Cr3As3, prepared by two different research groups. The single crystal measurements show similar behavior to polycrystalline samples down to 0.9–1.2 K, where a downturn is observed in the frequency shift for all single crystal samples. These results give further evidence for nodal superconductivity in K2Cr3As3, which indicates that the superconducting pairing state is unconventional. The different low temperature behavior, observed in samples which have deteriorated after being exposed to air, emphasizes that it is necessary to properly handle the samples prior to being measured because the A2Cr3As3 compounds are extremely air sensitive and evidence for nodal superconductivity from penetration depth measurements is only observed in the samples which display a sharp superconducting transition. Therefore further work is required to improve the quality of single crystals and to identify the origin of the downturn.