We address the origin of the magnetic-field independent −|A| T 1/2 term observed in the lowtemperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ZrAs1.58Se0.39, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the non-magnetic version of the twochannel Kondo effect, whose origin we ascribe to a dynamic Jahn-Teller effect operating at the vacancy-carrying As layer with a C4 symmetry. The pair-breaking nature of the dynamical defects in the square nets of As explains the low superconducting transition temperature Tc ≈ 0.14 K of ZrAs1.58Se0.39, as compared to the free-of-vacancies homologue ZrP1.54S0.46 (Tc ≈ 3.7 K). Our findings should be relevant to a wide class of metals with disordered pnictogen layers.

Single crystals of HfAs(1.7)Se(0.2) are grown by chemical transport reaction and their chemical composition characterized in detail by various analytical methods. Chemical analyses and crystal structure investigations by single-crystal X-ray diffraction as well as powder diffraction with synchrotron radiation reveal a tetragonal PbFCl structure type with strong disorder caused by a significant arsenic deficiency (As(0.9)) on the 2a site and mixed occupancy of the 2c site (As(0.8)Se(0.2)). HfAs(1.7)Se(0.2) is a diamagnetic metal which transforms into a superconducting state at T(c)=0.52 K. Similar to other PbFCl-type arsenide selenides, the title compound displays a magnetic-field-independent -AT(1/2) term in the low-temperature electrical resistivity. This unusual term presumably originates from the electron scattering of structural two-level systems. According to the experimental results, HfAs1.7Se0.2 appears to be a rare example of a nonmagnetic Kondo material.

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Thermodynamic and transport properties of the tetragonal M-As-Se (M ¼ Zr, Hf, Th) phases demonstrate that these materials are disordered metals with the electronic-specific-heat coefficient as small as, e.g., 0.3 mJ K À2 mol À1 for Th-As-Se.Remarkably, all the M-As-Se systems display a magneticfield-independent AT 1/2 term in the low-temperature electrical resistivity. As observed for ZrAs 1.595 Se 0.393 , the magnitude of this extra term may vary by a factor of nearly 3 as well as the resistivity minimum is shifted from 10.8 to about 15.0 K, dispite virtually the same electron-diffusion constant. On the other hand, a magnetic-field (B 14 T) dependence of the electron transport in arsenide selenides is strongly material dependent: both the positive and negative sign of the magnetoresistivity being proportional to either B 2 or B 1/2 were found. These observations point to a non-magnetic Kondo effect due to a random distribution of structural two-level systems in the disordered As/Se sublattice.