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We investigated the coexistence of superconductivity and antiferromagnetic order in the compound Er$_{2}$O$_{2}$Bi with anti-ThCr$_{2}$Si$_{2}$-type structure through resistivity, magnetization, specific heat measurements and first-principle calculations. The superconducting transition temperature $T_{\rm c}$ of 1.23 K and antiferromagnetic transition temperature $T_{\rm N}$ of 3 K are observed in the sample with the best nominal composition. The superconducting upper critical field $H_{\rm c2}$(0) and electron-phonon coupling constant $\lambda$$_{e-ph}$ in Er$_{2}$O$_{2}$Bi are similar to those in the previously reported non-magnetic superconductor Y$_{2}$O$_{2}$Bi with the same structure, indicating that the superconductivity in Er$_{2}$O$_{2}$Bi may have the same origin as in Y$_{2}$O$_{2}$Bi. The first-principle calculations of Er$_{2}$O$_{2}$Bi show that the Fermi surface is mainly composed of the Bi 6$p$ orbitals both in the paramagnetic and antiferromagnetic state, implying minor effect of the 4$f$ electrons on the Fermi surface. Besides, upon increasing the oxygen incorporation in Er$_{2}$O$_{x}$Bi, $T_{\rm c}$ increases from 1 to 1.23 K and $T_{\rm N}$ decreases slightly from 3 to 2.96 K, revealing that superconductivity and antiferromagnetic order may compete with each other. The Hall effect measurements indicate that hole-type carrier density indeed increases with increasing oxygen content, which may account for the variations of $T_{\rm c}$ and $T_{\rm N}$ with different oxygen content.

Coexistence of topological bands and charge density wave (CDW) in topological materials has attracted immense attentions because of their fantastic properties, such as axionic-CDW, three-dimensional quantum Hall effect, etc. In this work, a nodal-line semimetal InxTaS2 characterized by CDW and superconductivity is successfully synthesized, whose structure and topological bands (two separated Wely rings) are similar to In0.58TaSe2. A 2 x 2 commensurate CDW is observed at low temperature in InxTaS2, identified by transport properties and STM measurements. Moreover, superconductivity emerges below 0.69 K, and the anisotropy ratio of upper critical field [Gamma = H||ab c2(0)=H||c c2(0)] is significantly enhanced compared to 2H-TaS2, which shares the same essential layer unit. According to the Lawrence-Doniach model, the enhanced Gamma may be explained by the reduced effective mass in kx-ky plane, where Weyl rings locate. Therefore, this type of layered topological systems may offer a platform to investigate highly anisotropic superconductivity and to understand the extremely large upper critical field in the bulk or in the two-dimensional limit., Comment: Accepted for publication in PRB

Superconductivity in crystals without inversion symmetry has received extensive attention due to its unconventional pairing and possible nontrivial topological properties. Using first-principles calculations, we systemically study the electronic structure of noncentrosymmetric superconductors A2Cr3As3 (A = Na, K, Rb, and Cs). Topologically protected triply degenerate points connected by one-dimensional arcs appear along the C3 axis, coexisting with strong ferromagnetic (FM) fluctuations in the non-superconducting state. Within random phase approximation, our calculations show that strong enhancements of spin fluctuations are present in K2Cr3As3 and Rb2Cr3As3 and are substantially reduced in Na2Cr3As3 and Cs2Cr3As3. Symmetry analysis of pairing gap Δ(k) and spin–orbit coupling gk suggest that the arc surface states may also exist in the superconducting state, giving rise to possible nontrivial topological properties.

We investigate the electronic structure of nickelate superconductor NdNiO2 upon hole doping, by means of density-functional theory and dynamical mean-field theory. We demonstrate the strong intrinsic hybridization between strongly correlated states formed by Ni-3dx2-y2 orbital and itinerant electrons due to Nd-5d and Ni-3dz2 orbitals, producing a valence-fluctuating correlated metal as the normal state of hole-doped NdNiO2. The Hund's rule appears to play a dominating role on multi-orbital physics in the lightly doped compound, while its effect is gradually reduced by increasing the doping level. Crucially, the hole-doping leads to intricate effects on Ni-3d orbitals, such as a non-monotonic change of electron occupation in lightly doped level, and a flipping orbital configuration in the overdoped regime. Additionaly, we also map out the topology of Fermi surface at different doping levels. These findings render a preferred window to peek into electron pairing and superconductivity., Comment: 6 pages, 4 figures. 4 supply pages

Significance The 3D nodal points in Dirac and/or Weyl semimetals are in the vanguard of quantum materials research. A hallmark of these systems is the linear band dispersion. This latter electronic stricture gives rise to unconventional transport and optical phenomena. Here, we demonstrate that solids with dispersive nodal lines in the electronic structure share many common aspects with the response of 3D nodal-points systems. We investigated N b A s 2 using a combination of optical and magneto-optical techniques and have identified electromagnetic signature of dispersive nodal lines. This particular compound has allowed us to inquire the impact of spin-orbit coupling on the universal characteristic of nodal metals.

The relationship between magnetism and superconductivity has been one of the most discussed topics in iron-based superconductors. Using first-principles calculations, we have studied the electronic structure of 1144-type iron-based superconductor EuRbFe4As4. We find the crystal field splitting of EuRbFe4As4 is unique, such that the $$d_{z^2}$$ d z 2 orbitals are closer to the Fermi level $$\epsilon _{\mathrm F}$$ ϵ F than the dxy orbitals. The Ruderman−Kittel−Kasuya−Yosida (RKKY) interaction strength is approximately 0.12 meV in pristine EuRbFe4As4. Upon Ni-doping on the Fe site, the RKKY interaction strength is barely changed upon Ni-doping due to the highly anisotropic Fermi surfaces and multiband effect, despite the drastically reduced dzx(y) density of state at $$\epsilon _{\mathrm F}$$ ϵ F . Finally, in both pristine and doped compounds, the RKKY interaction is primarily mediated through bands due to Fe-$$d_{z^2}$$ d z 2 orbitals. Our calculations suggest the RKKY interaction mediated by $$d_{z^2}$$ d z 2 orbital is probably responsible for the magnetism in EuRbFe4As4 and doesn’t change upon Ni-doping.

We report a topological semimetal ${\mathrm{W}}_{2}{\mathrm{As}}_{3}$ with a space group $C2/m$. Based on the first-principles calculations, band crossings are partially gapped when spin-orbit coupling is included. The ${Z}_{2}$ indices at the electron filling are [1;111], characterizing a strong topological insulator and topological surface states. From the magnetotransport measurements, nearly quadratic field dependence of magnetoresistance (MR) ($B\ensuremath{\parallel}[200]$) at 3 K indicates an electron-hole compensated compound whose longitudinal MR reaches 11 500% at 3 K and 15 T. In addition, multiband features are detected from the high-magnetic-field Shubnikov--de Haas (SdH) oscillation, Hall resistivity, and band calculations. A nontrivial $\ensuremath{\pi}$ Berry's phase is obtained, suggesting the topological feature of this material. A two-band model can fit well the conductivity and Hall coefficient. Our experiments manifest that the transport properties of ${\mathrm{W}}_{2}{\mathrm{As}}_{3}$ are in good agreement with the theoretical calculations.

The electronic structures and topological properties of transition metal dipnictides $XP{n}_{2}$ ($X\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{Ta}$, Nb; $Pn\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{P}$, As, Sb) have been systematically studied using first-principles calculations. In addition to small bulk Fermi surfaces, the band anticrossing features near the Fermi level can be identified from band structures without spin-orbit coupling, leading to nodal lines in all these compounds. Inclusion of spin-orbit coupling gaps out these nodal lines, leaving only a pair of disentangled electron/hole bands crossing the Fermi level. Therefore, the low-energy physics can be in general captured by the corresponding two-band model with several isolated small Fermi pockets. Detailed analysis of the Fermi surfaces suggests that the arsenides and ${\mathrm{NbSb}}_{2}$ are nearly compensated semimetals while the phosphorides and ${\mathrm{TaSb}}_{2}$ are not. Based on the calculated band parities, the electron and hole bands are found to be weakly topological nontrivial, giving rise to surface states. As an example, we presented the surface-direction-dependent band structure of the surfaces states in ${\mathrm{TaSb}}_{2}$.