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We report the effect of chemical composition variation on the superconductivity in noncentrosymmetric β-Mn-type high-entropy alloys (HEAs) studied in two very similar Ta5+xMo35−xW5Re35Ru20C20 and Ta5+yW35−yMo5Re35Ru20C20 series. It is found that both HEAs consist of a single cubic β-Mn-type phase for 0 ≤ x ≤ 6 and 0 ≤ y ≤ 9. Bulk fully gapped superconductivity is observed for all these HEAs and their zero-temperature upper critical fields approach the Pauli paramagnetic limit. While our analysis suggests that Tc of these HEAs is mainly controlled by the electron–phonon coupling strength, Tc of the Ta5+xMo35−xW5Re35Ru20C20 series is always higher than that of the Ta5+yW35−yMo5Re35Ru20C20 one even for (nearly) the same valence electron concentration and unit-cell volume. This difference is attributed to the different contributions of Mo and W to the electronic structure, which is supported by the first-principles calculations. Our study suggests that the chemical composition effect should be considered to better understand the superconducting behavior in noncentrosymmetric HEAs.

Abstract β-ThRhGe, the high-temperature polymorph of ThRhGe, is isostructural to the well-known ferromagnetic superconductor URhGe. However, contrary to URhGe, β-ThRhGe is nonmagnetic and undergoes an incomplete structural phase transition at 244 K, followed by a superconducting transition below 3.36 K. Here we show that the isovalent substitution of Ir for Rh leads to a strong enhancement of superconductivity by suppressing the structural transition. At x = 0.5, where the structural transition disappears, T c reaches a maximum of 6.88 K. The enhancement of superconductivity is linked to the proximity to a structural quantum critical point at this Ir concentration, as suggested by the analysis of thermodynamic as well as resistivity data. First principles calculations indicate that the Ir doping has little effect on the electronic band dispersion near the Fermi level. β-ThRh1−x Ir x Ge thus provides an excellent platform to study the interplay between superconductivity and structural quantum criticality in actinide-containing compounds.

We report a possible coexistence of nontrivial topology and antiferromagnetism in the newly discovered compounds EuCdBi2, with magnetic Eu layer locating above and below Bi square net. The X-ray diffraction on single crystals and powder indicats that this 112-type material crystalizes in space group of I4/mmm, the same as SrMnBi2 and EuMnBi2. Our combined measurements of magnetization, electrical transport and specific heat consistently reveal antiferromagnetic (AFM) transition of Eu2+ moments at TN = 20 K. The Eu moments are not saturated under a field of 7 T at 1.8 K. The anisotropic susceptibility suggests the Eu moments lie in the ab plane, and a metamagnetic (MM) transition is observed near 1 T below TN. Large positive magnetoresistance (MR) present for both H ‖ ab and H ‖ c, which are considered to contain part contributions from Dirac bands. Hall measurements show the electron-hole compensation effect is prominent above 100 K, with a crossover of Hall resistance from negative to positive values at ∼150 K. The fitted mobility of electrons is as high as 3250 cm2 V−1 S−1 at 1.8 K. Interestingly, the rapid increase of carrier density and suppression of mobility appear at around TN, indicating non-negligible interaction between Eu moments and electron/hole bands. EuCdBi2 may provide a new platform to investigate the interplay of topological bands and antiferromagnetic order.

Iron-pnictide superconductors share similar topological band structure with iron-chalcogenide superconductors, but no Majorana modes have been observed in the former. Here, the authors observe both the superconducting Dirac surface states and Majorana zero modes inside its vortex cores in CaKFe4As4.

The Z_{2} topological metals RV_{3}Sb_{5} (R = K, Rb, Cs) with a layered kagome structure provide a unique opportunity to investigate the interplay between charge order, superconductivity, and topology. Here, we report muon-spin relaxation/rotation (μSR) measurements performed on CsV_{3}Sb_{5} across a broad temperature range, in order to uncover the nature of the charge density wave order and superconductivity in this material. From zero-field μSR, we find that spontaneous magnetic fields appear below 50 K, which is well below the charge density wave transition (T^{*}∼93 K). We show that these spontaneous fields are dynamic in nature making it difficult to associate them with a hidden static order. The superconducting state of CsV_{3}Sb_{5} is found to preserve time-reversal symmetry, and the transverse-field μSR results are consistent with a superconducting state that has two fully open gaps.

The ground-state magnetic structures of the Eu^{2+} spins in recently discovered RbEu(Fe_{1−x}Ni_{x})_{4}As_{4} superconductors have been investigated by neutron powder diffraction measurements. It is found that as the superconductivity gets suppressed with the increase in Ni doping, the magnetic propagation vector of the Eu sublattice diminishes, corresponding to the decrease in the rotation angle between the moments in neighboring Eu layers. The ferromagnetic Eu layers are helically modulated along the c axis with an incommensurate magnetic propagation vector in both the ferromagnetic superconductor RbEu(Fe_{0.95}Ni_{0.05})_{4}As_{4} and the superconducting ferromagnet RbEu(Fe_{0.93}Ni_{0.07})_{4}As_{4}. Such a helical structure transforms into a purely collinear ferromagnetic structure for nonsuperconducting RbEu(Fe_{0.91}Ni_{0.09})_{4}As_{4}, with all the Eu^{2+} spins lying along the tetragonal (1 1 0) direction. The evolution from helical to collinear ferromagnetic order of the Eu^{2+} spins with increasing Ni doping is supported by first-principles calculations. The variation of the rotation angle between adjacent Eu^{2+} layers can be well explained by considering the change of magnetic exchange couplings mediated by the indirect Ruderman-Kittel-Kasuya-Yosida interaction.

We report synthesis, crystal structure, and physical properties of Sr2Cr2AsO3. The new compound crystallizes in a Sr2GaO3CuS-type structure with two distinct Cr sites, Cr(1) in the perovskite-like block layers of “Sr3Cr2O6” and Cr(2) in the ThCr2Si2-type layers of “SrCr2As2”. An inter-block-layer charge transfer is explicitly evidenced, which dopes electrons in the CrO2 planes and simultaneously dopes holes into the CrAs layers. Measurements of electrical resistivity, magnetization, and specific heat, in combination with density-functional theoretical calculations, indicate that the title material is an antiferromagnetic metal. The Cr(2) magnetic moments in the CrAs layers order at 420 K, while the Cr(1) spins in the CrO2 planes show quasi-two-dimensional magnetism with long-range ordering below 80 K. Both Néel temperatures are significantly reduced, compared with those of the cousin material Sr2Cr3As2O2, probably due to the intrinsic charge-carrier doping. Complex re-entrant magnetic transitions with a huge magnetic hysteresis were observed at low temperatures.

A microwave technique suitable for investigating the AC magnetic susceptibility of small samples in the GHz frequency range is presented. The method—which is based on the use of a coplanar waveguide resonator, within the resonator perturbation approach—allows one to obtain the absolute value of the complex susceptibility, from which the penetration depth and the superfluid density can be determined. We report on the characterization of several iron-based superconducting systems, belonging to the 11, 122, 1144, and 12442 families. In particular, we show the effect of different kinds of doping for the 122 family, and the effect of proton irradiation in a 122 compound. Finally, the paradigmatic case of the magnetic superconductor EuP-122 is discussed, since it shows the emergence of both superconducting and ferromagnetic transitions, marked by clear features in both the real and imaginary parts of the AC susceptibility.

In this study, we measure the in-plane transport properties of high-quality Ba(Fe0.914Co0.086)2As2 single crystals. Signatures of vortex unbinding Berezinskii–Kosterlitz–Thouless (BKT) transition are shown from both the conventional approach and the Fisher–Fisher–Huse dynamic scaling analysis, in which a characteristic Nelson–Kosterlitz jump is demonstrated. We also observe a non-Hall transverse signal exactly at the superconducting transition, which is explained in terms of guided motion of unbound vortices.

We report the discovery of bulk superconductivity (SC) at 6.1 K in a quasi-one-dimensional chromium pnictide K_{2}Cr_{3}As_{3} that contains [(Cr_{3}As_{3})^{2−}]_{∞} double-walled subnanotubes with face-sharing Cr_{6/2} (As_{6/2}) octahedron linear chains in the inner (outer) wall. The material has a large electronic specific-heat coefficient of 70–75 mJ K^{−2} mol^{−1}, indicating significantly strong electron correlations. A linear temperature dependence of resistivity in a broad temperature range from 7 to 300 K is observed, which suggests non-Fermi liquid behavior of the material. Unconventional SC is preliminarily manifested by the estimated upper critical field exceeding the Pauli limit by a factor of 3–4. The title compound represents a rare example that possibly unconventional SC emerges in a quasi-1D system with strong electron correlations.

We report electrical and magnetic measurements of an optimally doped Eu(Fe _0.88 Ir _0.12 ) _2 As _2 crystal which shows superconductivity at T _sc = 22 K and magnetic ordering of the Eu ^2+ spins below 20 K. The results suggest that the Eu ^2+ spins lie flat ferromagnetically in the ab plane at T ^ab _m = 20 K, and then tilt toward the c -axis at T ^tilt _m = 17.4 K. The isothermal magnetization loop at low temperatures shows both ferromagnetic behavior and superconducting characteristics, unambiguously demonstrating the coexistence of ferromagnetism and superconductivity. The upper critical field measured is remarkably reduced, as compared to other Fe-based superconductors with the same T _sc , and it exhibits abnormal temperature dependence featured by the existence of an inflection point around T ^ab _m , where the anisotropy ratio γ (≡ H ^∥ ^ab _c2 / H ^∥ ^c _c2 ) shows a minimum value smaller than 1.0. These observations can be explained by a ferromagnetic exchange field of ∼30 T which tilts its direction toward the c -axis below T ^tilt _m . The strong internal field, much higher than the intrinsic lower critical field expected, leads to the absence of Meissner state, which is confirmed by the magnetic measurements under ultra-low fields.