Radio-frequency (14.6 MHz) AC magnetic susceptibility, $chi^{prime}_{AC}$, of dytio was measured using a self-oscillating tunnel-diode resonator. Measurements were made with the excitation AC field parallel to the superimposed DC magnetic field up 5
T in a wide temperature range from 50 mK to 100 K. At 14.6 MHz a known broad peak of $chi^{prime}_{AC}(T)$ from kHz - range audio-frequency measurements around 15~K for both [111] and [110] directions shifts to 45~K, continuing the Arrhenius activated behavior with the same activation energy barrier of $E_a approx 230$~K. Magnetic field dependence of $chi^{prime}_{AC}$ along [111] reproduces previously reported low-temperature two-in-two-out to three-in-one-out spin configuration transition at about 1~T, and an intermediate phase between 1 and 1.5~T. The boundaries of the intermediate phase show reasonable overlap with the literature data and connect at a critical endpoint of the first-order transition line, suggesting that these low-temperature features are frequency independent. An unusual upturn of magnetic susceptibility at $T to 0$ was observed in magnetic fields between 1.5~T and 2~T for both magnetic field directions, before fully polarized configuration sets in above 2~T.
Low temperature ($sim$ 20~K) electron irradiation with 2.5 MeV relativistic electrons was used to study the effect of controlled non-magnetic disorder on the normal and superconducting properties of the type-II Dirac semimetal PdTe$_2$. We report mea
surements of longitudinal and Hall resistivity, thermal conductivity and London penetration depth using tunnel-diode resonator technique for various irradiation doses. The normal state electrical resistivity follows Matthiessen rule with an increase of the residual resistivity at a rate of $sim$0.77$ mu Omega$cm/$(textrm{C}/textrm{cm}^2)$. London penetration depth and thermal conductivity results show that the superconducting state remains fully gapped. The superconducting transition temperature is suppressed at a non-zero rate that is about sixteen times slower than described by the Abrikosov-Gorkov dependence, applicable to magnetic impurity scattering in isotropic, single-band $s$-wave superconductors. To gain information about the gap structure and symmetry of the pairing state, we perform a detailed analysis of these experimental results based on insight from a generalized Anderson theorem for multi-band superconductors. This imposes quantitative constraints on the gap anisotropies for each of the possible pairing candidate states. We conclude that the most likely pairing candidate is an unconventional $A_{1g}^{+-}$ state. While we cannot exclude the conventional $A_{1g}^{++}$ and the triplet $A_{1u}$, we demonstrate that these states require additional assumptions about the orbital structure of the disorder potential to be consistent with our experimental results, e.g., a ratio of inter- to intra-band scattering for the singlet state significantly larger than one. Due to the generality of our theoretical framework, we think that it will also be useful for irradiation studies in other spin-orbit-coupled multi-orbital systems.
Low-temperature (22~K) irradiation with 2.5~MeV electrons was used to study the competition between stripe ${rm C_2}$ and tetragonal ${rm C_4}$ antiferromagnetic phases which exist in a narrow doping range around $x=$0.25 in hole-doped Ba$_{1-x}$K$_x
$Fe$_2$As$_2$. In nearby compositions outside of this range, at $x=$0.22 and $x=$0.19, the temperatures of both the concomitant orthorhombic/stripe antiferromagnetic transition $T_{rm C2}$ and the superconducting transition $T_{rm c}$ are monotonically suppressed by added disorder at similar rates of about 0.1~K/$mu Omega$cm, as revealed through using resistivity variation as an intrinsic measure of scattering rate. In a stark contrast, a rapid suppression of the ${rm C_4}$ phase at the rate of 0.24 K/$mu Omega cdot$cm is found at $x=$0.25. Moreover, this suppression of the ${rm C_4}$ phase is accompanied by unusual disorder-induced stabilization of the ${rm C_2}$ phase, determined by resistivity and specific heat measurements. The rate of the ${rm C_4}$ phase suppression is notably higher than the suppression rate of the spin-vortex phase in the Ni-doped CaKFe$_4$As$_4$ (0.16 K/$mu Omega$cm).
We report the experimental details of how mechanical detwinning can be implemented in tandem with high sensitivity nuclear magnetic resonance measurements and use this setup to measure the in-plane anisotropy of the spin-lattice relaxation rate in un
derdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ with $x=0.048$. The anisotropy reaches a maximum of 30% at $T_{N}$, and the recovery data reveal that the glassy behavior of the spin fluctuations present in the twinned state persist in the fully detwinned crystal. A theoretical model is presented to describe the spin-lattice relaxation rate in terms of anisotropic nematic spin fluctuations.
