We report on the influence of doping on vortex dynamics in 3 MeV proton-irradiated single crystals of CaK(Fe1-xNix)(4)As-4 (1144, x = 0.015, 0.025, and 0.03) and Ba(Fe1-xCox)(2)As-2 (x = 0.04, 0.062, 0.066 and 0.074). Non-irradiated crystals of the 1144 system display superconducting critical temperatures ranging from 31 K for x = 0.015-20.5 K, as doping increases to 0.03. On the other hand, pristine crystals of the 122 system show T-c values between 14.6 and 23.6 K, with the maximum T-c occurring at intermediate doping levels. The fluence was set at 3 x 10(16) p cm(-2), resulting in a decrease in the T-c by around 1.5 K for all samples and significantly affecting the vortex dynamics by reducing the flux creep relaxation compared to previously reported values for unirradiated crystals. Parameters such as vortex pinning energy U-0 and the glassy exponent mu dependencies on doping and magnetic field strength are identified. For the 1144 system, U-0 reaches values approaching 500 K for small fields in samples with T-c= 29.3 K (x = 0.015), systematically decreasing to around 200 K as T-c falls below 20 K. Furthermore, U-0 decreases as the field increases to 3 T for the same sample, varying from approximately 250 K to 100 K as T-c decreases. These changes are typically accompanied by modifications in mu, gradually increasing from values around 1 towards 1.5, corresponding to small bundle relaxation in the collective creep theory. Despite differences in the substitutional disorder and magnetic phase diagram with respect to the 1144 system, the results for 122 single crystals follow a similar tendency in which U-0 usually reduces and mu increase rise as the applied magnetic field is increased. Due to moderate U-0 in these systems (few hundreds of kelvins), the resulting decay of persistent current at liquid helium temperatures is primarily determined by a balance between U-0 and bundle size contribution. These findings provide valuable insights for potential applications of these systems, particularly in the context of intrinsic superconducting parameters and the resulting pinning landscape.

The effect of pressure on the low-temperature states of the Re3Ge7 is investigated by both electrical and Hall resistance and magnetization measurements. At ambient pressure, the temperature-dependent resistance of Re3Ge7 behaves quasilinearly from room temperature down to 60 K, then undergoes a two-step metal-toinsulator transition (MIT) at temperatures T1 = 59.4 K and T2 = 58.7 K, which may be related to a structural phase transition or occurrence of charge-density wave ordering. Upon applying pressure, the two-step (T1, T2) MIT splits into three steps (T1, T2. and T3) above 1 GPa, and all traces of MITs are fully suppressed by -8 GPa. Subsequently, the onset of bulk superconductivity (SC) occurs between 10.8 and 12.2 GPa and persists to our highest pressure of 26.8 GPa. At 12.2 GPa the superconducting transition temperature, Tc, and upper critical field, Hc2 reach the maximum of Tc (onset) -5.9 K and Hc2 (1.8 K) -14 kOe. Our results not only present the observation of SC under high pressure in Re3Ge7 but also delineate the interplay between SC and other competing electronic states by creating a T-p phase diagram for this potentially topologically nontrivial system Re3Ge7.

Although SrNi2P2 adopts the common ThCr2Si2 structure for T >= 325 K, being in an uncollapsed tetragonal state rather than a collapsed tetragonal version, it is a special case for the ThCr2Si2 class: on cooling below 325 K it adopts a one-third collapsed orthorhombic phase where one out of every three P rows bonds across the Sr layers. On the other hand, SrCo2P2 only exhibits the uncollapsed ThCr2Si2 structure from room temperature down to 1.8 K. Regardless of their low-temperature structures, neither SrNi2P2 nor SrCo2P2 manifests magnetic transitions down to 50 mK and 2 K, respectively. In this work we report the effects of Co substitution in Sr(Ni1-xCox)(2)P-2, which allows for tuning the transition between the one-third collapsed and the uncollapsed structure. We find a rapid decrease of the one-third collapsed structural transition temperature with increasing Co fraction, until reaching full suppression for x >= 0.1. Substitution levels in the range 0.11 <= x <= 0.58 show no signs of any transition down to 1.8 K in the magnetization or resistance measurements in the range 1.8 <= T <= 300 K. However, different magnetically ordered states emerge for x >= 0.65, and disappear for x >= 0.99, recovering the known paramagnetic properties of the parent compound SrCo2P2. These results are summarized in a phase diagram, built upon the characterization by energy dispersive x-ray spectroscopy, x-ray diffraction, temperature-dependent resistance, and field-and temperature-dependent magnetization measurements done on single crystals with different Co fraction. Both the magnetic and structural properties are compared to other systems with ThCr2Si2 structure that exhibit magnetic ordering and collapsed tetragonal transitions. The magnetic ordering and moment formation are well described by Takahashi's spin fluctuation theory of itinerant electron magnetism [Y. Takahashi, J. Phys. Soc. Jpn. 55, 3553 (1986)].

M. Xu,1, 2 J. Schmidt,1, 2 E. Gati,1, 2, 3 L. Xiang,1, 2, 4 W. R. Meier,1, 2, 5 V. G. Kogan,1 S. L. Bud’ko,1, 2 and P. C. Canfield1, 2 Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA Department of Materials Science and Engineering, University of Tennessee Knoxville, Knoxville, Tennessee, 37996, USA (Dated: April 26, 2022)

We demonstrate that the anisotropy in the critical current densities, J c , of CaK(Fe 1− x Ni x ) 4 As 4 ( x = 0, 0.015, 0.025, and 0.030) single crystals reduces with increasing chemical and irradiation-induced disorder. The magnetic field dependences of J c are analyzed by performing magnetization measurements with H applied parallel and perpendicular to the crystallographic c- axis. The results show that undoped crystals display large anisotropies in J c due to an enhancement of the vortex pinning with H applied parallel to the crystallographic ab -planes. This anisotropy reduces substantially as Ni addition increases. Moreover, we found that random disorder introduced by proton irradiation enhances mainly the vortex pinning for H parallel to the c -axis. Consequently, using adequate fluencies, the vortex pinning at low temperatures in both undoped and doped samples becomes close to isotropic. These results make the CaKFe 4 As 4 system promising for applications that require isotropic J c under magnetic fields.