A recent experiment showed the superconducting transition temperature in the Ruddlesden-Popper bilayer La_{3}Ni_{2}O_{7} decreases monotonically with increasing pressure above 14 GPa. In order to unravel the underlying mechanism for this unusual dependence, we performed theoretical investigations by combining the density functional theory (DFT) and the unbiased functional renormalization group (FRG). Our DFT calculations show that the Fermi pockets are essentially unchanged with increasing pressure (above 14 GPa), but the bandwidth is enlarged, and particularly the interlayer hopping integral between the nickel 3d_{3z^{2}-r^{2}} orbitals is enhanced. From the DFT band structure, we construct the bilayer tight-binding model in terms of the nickel 3d_{3z^{2}-r^{2}} and 3d_{x^{2}-y^{2}} orbitals. On this basis, we investigate the superconductivity induced by correlation effects by FRG calculations. We find consistently s_{±}-wave pairing triggered by spin fluctuations, but the latter are weakened by pressure and lead to a decreasing transition temperature versus pressure, in qualitative agreement with the experiment. We emphasize that the itinerancy of the d orbitals is important and captured naturally in our FRG calculations, and we argue that the unusual pressure dependence would be unnatural, if not impossible, in the otherwise local-moment picture of the nickel d orbitals. This sheds light on the pertinent microscopic description, and more importantly the mechanism, of superconductivity in La_{3}Ni_{2}O_{7}.

The unique three-dimensional orthorhombic NbS(o-NbS)compound synthesized in 1969 has recently been experi-mentally confirmed to be a superconductor[Phys.Rev.B 108 174517(2023)].However,there is currently no theoretical research on its superconducting mechanism.In this work,we investigate the superconducting properties of o-NbS from first-principles calculations.Based on the Eliashberg equation,it is found that the superconductivity mainly originates from the coupling between the electrons of Nb-4d orbitals and the vibrations of Nb atoms in the low-frequency region and those of S atoms in the high-frequency region.A superconducting transition temperature(Tc)of 10.7 K is obtained,which is close to the experimental value and higher than most transition metal chalcogenides(TMCs).The calculated thermody-namic properties in the superconducting state,such as specific heat,energy gap,isotope coefficient,etc.,also indicate that o-NbS is a conventional phonon-mediated superconductor.These results are consistent with recent experimental reports and provide a good understanding of the superconducting mechanism of o-NbS.Furthermore,the TMCs of o-TaS and o-WS are also investigated;these belong to the same and neighboring groups of Nb,and we find that o-TaS and o-WS are also phonon-mediated superconductors with Tc of 8.9 K and 7.2 K,respectively.

The recent discovery of bulk superconductivity in trilayer nickelate La$_4$Ni$_3$O$_{10}$ with the critical temperature $T_c$ near $30$K under high pressure is attracting a new wave of research interest, after the breakthrough of bilayer La$_3$Ni$_2$O$_7$ with $T_c$ near $80$K. The similarities and differences of electronic structure and superconducting mechanism in these two systems are urgent theoretical issues. In this Letter, we study the electronic band structure and construct a minimal trilayer tight-binding model for the high-pressure phase of La$_4$Ni$_3$O$_{10}$ in terms of the nickel $3d_{x^2-y^2}$ and $3d_{3z^2-r^2}$ orbitals, and study the superconducting mechanism due to local Coulomb interactions by the unbiased functional renormalization group. We find antiferromagnetic correlations between the outer layers instead of neighboring ones, apart from the inplane correlations. The effective interaction induces Cooper pairing with the $s_\pm$-wave symmetry, which changes sign across the Fermi pockets. We find $T_c$ in La$_4$Ni$_3$O$_{10}$ is systematically lower than that in La$_3$Ni$_2$O$_7$, and electron doping can enhance $T_c$.

The recent discovery of bulk superconductivity in trilayer nickelate La4Ni3O10 under high pressure is attracting a new wave of research interest. In this Letter, we study its electronic band structure and construct a minimal trilayer tight-binding model in terms of the nickel 3dx2-y2 and 3d3z2-r2 orbitals, and investigate the superconducting mechanism due to local Coulomb interactions by the unbiased functional renormalization group. We find antiferromagnetic correlations between the outer layers instead of neighboring ones, apart from the in-plane correlations. The effective interaction induces Cooper pairing with the s +/--wave symmetry, which changes sign across the Fermi pockets. In addition, we find the Tc in La4Ni3O10 is systematically lower than that in La3Ni2O7, but electron doping can further enhance Tc.

A novel mixed-anion oxychloride, KBi(SeO4)(IO3)Cl, has been hydrothermally synthesized and structurally characterized. The compound crystallizes in the monoclinic space group of P21/c with cell parameters of a = 7.8802(2) angstrom; b = 10.8088(2) angstrom, c =10.1364(2) angstrom, beta = 112.849(1)degrees, V = 795.62(3) angstrom 3, and Z = 4. KBi(SeO4)(IO3) Cl exhibits a three-dimensional tunnel framework that is composed of three types of atomic groups, i.e., tetrahedral SeO4, trigonal pyramidal IO3, and polyhedral BiO6Cl. Both IO3 pyramid and BiO6Cl polyhedron are in strongly acentric coordination due to the influence of I5+ and Bi3+ lone electron pairs. The measurement of UV-vis-NIR spectra for powder KBi(SeO4)(IO3)Cl indicates that the material is a wide band gap semiconductor with Eg = 3.9 eV. Theoretical analysis shows that the electron transition from Bi 6p to O 2p states in the BiO6Cl unit dominates the optical absorption edge. Thermal stability and IR spectrum were also analyzed for the material.

The multiple properties in two-dimensional (2D) materials have attracted widespread attention. Utilizing first -principles calculations, we theoretically predict four 2D transition metal Janus sulfide hydrides, named 2H/1T-W X H ( X = S, Se). The 2H-WSH and 2H - WSeH demonstrate superconductivity with critical temperatures ( T c ) of 13.4 K and 11.4 K, respectively. Whereas 1T-WSH and 1T-WSeH display charge density wave (CDW) properties arising from electron -phonon coupling (EPC). It is noteworthy that the CDW can be suppressed through biaxial compressive strain, leading to the emergence of superconductivity with T c of 12.2 K and 11.9 K for 1T-WSH (-2 %) and 1T-WSeH (-4 %), respectively. The superconductivity of the above materials originates from the coupling between W -3 d electrons and the lowfrequency vibrations of W. Interestingly, the 2H - WSeH and 1T-WSeH (-4 %) display nontrivial topological properties, as evidenced by topological invariant Z 2 and the presence of edge states. Our research not only provides theoretical guidance for further exploring 2D Janus materials, but also provides ideas for the competition of multiple orders in 2D materials.

Although hydrides such as LaH10 are experimentally confirmed to possess high superconducting critical temperature (T-c) of 250-260 K under 170-200 GPa, it is still a tough challenge to be applied. It is highly anticipated to find hydride superconductors with relatively high T-c at low or ambient pressure. Reducing the dimensionality of materials can induce unexpected properties that are distinct from their bulk counterparts, and whether it can modulate the superconducting properties deserves further investigation. Herein, a new 2D monolayer aluminum hydride h-AlH(2 )is theoretically predicted under ambient pressure based on the first-principles calculations. Since the electronic structures of h-AlH(2)reveal the metallicity, the electron-phonon coupling (EPC) and possible phonon-mediated superconductivity are investigated. Based on the isotropic Eliashberg equation, the calculated EPC constant lambda of h-AlH2 is 1.16, and the T-c is up to 42.6 K. The EPC mainly originates from the coupling between electrons of Al-s,p(x),p(y), and H-s orbitals and the in-plane vibration modes of H atoms. Especially, the T-c can be enhanced to 63.7 K by applying 3% biaxial tensile strain. Thus, the predicted h-AlH(2 )provides a new platform for finding hydride superconductors in low-dimensional materials at ambient pressure.

To explore phonon-mediated superconductors with critical temperature (Tc) exceeding the liquid-nitrogen temperature under ambient pressure, two pure carbon structures C6 and C10 with a carbon-cage-network are designed and their superconductivity is investigated. By shifting the Fermi level by hole doping, we successfully obtain the metallized sp3-hybridized covalent sigma-bonding bands, which generally show strong coupling with phonons. Based on first-principles calculations and the Wannier interpolation technique, the lattice dynamics, mechanical properties, electronic structures, and electron-phonon coupling of hole-doped C6 and C10 clathrates are investigated. Both the enlarged density of states at the Fermi level and the softened phonons play significant roles in the enhancement of electron-phonon coupling. By solving the anisotropic Eliashberg equations, we find that the Tc of hole-doped C6 and C10 clathrates can reach about 43 K and 150 K, respectively. Additionally, pristine C6 and C10 clathrates exhibit ultrahigh hardness simultaneously. This study suggests that a carbon-cagenetwork is another direction to explore materials simultaneously possessing high-temperature superconductivity and ultrahigh hardness.

The report on near-ambient superconductivity in nitrogen-doped lutetium hydride is still under controversy. Here, guided by x-ray diffraction data of nitrogen-doped lutetium hydride, we choose a possible cubic superconducting phase named rocksalt-type LuH (RS-LuH) and study the superconductivity of pristine RS-LuH, nitrogen-doped RS-LuH named Lu4NH3, and lanthanide elements substitution of RS-LuH at pressures 0, 1, and 10 GPa by performing density functional theory and isotropic Eliashberg equation. As pressure increases from 0 to 10 GPa, all phonon spectra notably harden, resulting in the suppression of electron–phonon coupling. Moreover, the decrease in superconducting critical temperature (Tc) of Lu4NH3 is due to the reduction of electron–phonon coupling and the density of states at the Fermi level compared with pristine RS-LuH. Finally, our investigation reveals a monotonic increase in Tc with ascending atomic numbers via lanthanide element substitution. Notably, RS-LuH exhibits the highest Tc (Tc=19.7 K) among all compounds we studied. Therefore, our theoretical exploration enriches the understanding of the superconductivity in nitrogen-doped lutetium hydride under varying pressures.

In recent years, the superconducting properties of materials with diamond-like structures have attracted widespread attention. The superconducting transition temperature (Tc) of most diamond-like structures is relatively low, and there has been little research on the superconductivity of ternary diamond-like materials. Here, based on first-principles calculations, we predict B2CX (X = N, P) with P3m1 space group, and reveal that they all exhibit phonon-mediated superconductivity at ambient pressure, with Tc range from 44.3 to 46.1 K, which is higher than most diamond-like superconductors. The coupling between metallic sigma electrons and softened E phonon (B-C stretching modes) contributes greatly to their superconductivity. Our work provides an important theoretical basis for the experimental design and synthesis of ternary high-temperature superconductors with diamond-like structures.

Although hydrides such as are experimentally confirmed to possess high superconducting critical temperature () of 250–260 K under 170–200 GPa, it is still a tough challenge to be applied. It is highly anticipated to find hydride superconductors with relatively high at low or ambient pressure. Reducing the dimensionality of materials can induce unexpected properties that are distinct from their bulk counterparts, and whether it can modulate the superconducting properties deserves further investigation. Herein, a new 2D monolayer aluminum hydride h ‐ is theoretically predicted under ambient pressure based on the first‐principles calculations. Since the electronic structures of h ‐ reveal the metallicity, the electron–phonon coupling (EPC) and possible phonon‐mediated superconductivity are investigated. Based on the isotropic Eliashberg equation, the calculated EPC constant λ of h ‐ is 1.16, and the is up to 42.6 K. The EPC mainly originates from the coupling between electrons of Al‐ s ,,, and H‐ s orbitals and the in‐plane vibration modes of H atoms. Especially, the can be enhanced to 63.7 K by applying 3% biaxial tensile strain. Thus, the predicted h ‐ provides a new platform for finding hydride superconductors in low‐dimensional materials at ambient pressure.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Novel bismuth selenite iodate oxide BiSeIO6 was synthesized in a mild hydrothermal condition. BiSeIO6 was crystallized in the polar space group Pna21 of an orthorhombic system. The crystal structure features a three-dimensional framework composed of three types of lone pair cations with distorted BiO7 polyhedra, SeO3 pyramids, and IO3 pyramids in one structure. Interestingly, BiSeIO6 exhibits a strong and phase-matchable second-harmonic generation (SHG) of ∼6 times that of KH2PO4 (KDP). Dipole moment analysis shows that all three local acentric groups of BiO7, SeO3, and IO3 cooperatively contribute to the large macroscopic polarization and thereby strong SHG efficiency of BiSeIO6. In addition, BiSeIO6 has a broad transparency range from 0.35 to 11 μm, indicating its promising nonlinear optical applications from visible to mid-infrared bands.