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Register a new userElectronic structure of rare-earth binary oxide superconductor LaO
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Recently the superconductivity has been discovered in the rock-salt structured binary lanthanum monoxide LaO through the state-of-the-art oxide thin-film epitaxy. This work reveals the normal state of superconducting LaO to be a $Z_2$ nontrivial topological metal that the Dirac point protected by the crystal symmetry is located at around the Fermi energy. By analysing the orbital characteristics, the nature of topological band structure of LaO originates from the intra-atomic transition in energy from outer shell La 5$d$ to inner shell 4$f$ orbitals driven by the strong octahedral crystal-field. Furthermore, the appearance of novel surface states unambiguously demonstrates the topological signature of LaO. Our theoretical findings not only shed light into the understanding of exotic quantum behaviors in LaO superconductor with intimate correlation between 4$f$ and 5$d$ orbitals in La, but also provide an exciting platform to explore the interplay of intriguing nontrivial topology and superconductivity.
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The electronic structures of substitutional rare-earth (RE) impurities in GaAs and cubic GaN are calculated. The total energy is evaluated with the self-interaction corrected local spin density approximation, by which several configurations of the open 4f shell of the rare-earth ion may be investigated. The defects are modelled by supercells of type REGa$_{n-1}$As$_n$, for n=4, 8 and 16. The preferred defect is the rare-earth substituting Ga, for which case the rare-earth valency in intrinsic material is found to be trivalent in all cases except Ce and Pr in GaN. The 3+ --> 2+ f-level is found above the theoretical conduction band edge in all cases and within the experimental gap only for Eu, Tm and Yb in GaAs and for Eu in GaN. The exchange interaction of the rare-earth impurity with the states at both the valence band maximum and the conduction band minimum is weak, one to two orders of magnitude smaller than that of Mn impurities. Hence the coupling strength is insufficient to allow for ferromagnetic ordering of dilute impurities, except at very low temperatures.
The discovery of infinite layer nickelate superconductor marks the new era in the field of superconductivity. In the rare-earth (Re) nickelates ReNiO2, although the Ni is also of d9 electronic configuration, analogous to Cu d9 in cuprates, whether electronic structures in infinite-layer nickelate are the same as cuprate and possess the single band feature as well are still open questions. To illustrate the electronic structure of rare-earth infinite-layer nickelate, we perform first principle calculations of LaNiO2 and NdNiO2 compounds and compare them with that of CaCuO2 using hybrid functional method together with Wannier projection and group symmetry analysis. Our results indicate that the Ni-dx2-y2 in the LaNiO2 has weak hybridization with other orbitals and exhibits characteristic single band feature, whereas in NdNiO2, the Nd-f orbital hybridizes with Ni-dx2-y2 and is a non-negligible ingredient for transport and even high-temperature superconductivity. Given that the Cu-dx2-y2 in cuprate strongly hybridizes with O-2p, the calculated band structures of nickelate imply some new band characters which is worth to gain more attentions.
New germanium-platinum compounds with the filled-skutterudite crystal structure were synthesized. The structure and composition were investigated by X-ray diffraction and microprobe analysis. Magnetic susceptibility, specific heat, and electrical resistivity measurements evidence superconductivity in LaPt4Ge12 and PrPt4Ge12 below 8.3K. The parameters of the normal and superconducting states were established. Strong coupling and a crystal electric field singlet groundstate is found for the Pr compound. Electronic structure calculations show a large density of states at the Fermi level. Similar behavior with lower T_c was observed for SrPt4Ge12 and BaPt4Ge12.
The design and synthesis of targeted functional materials have been a long-term goal for material scientists. Although a universal design strategy is difficult to generate for all types of materials, however, it is still helpful for a typical family of materials to have such design rules. Herein, we incorporated several significant chemical and physical factors regarding magnetism, such as structure type, atom distance, spin-orbit coupling, and successfully synthesized a new rare-earth-free ferromagnet, MnPt5As, for the first time. MnPt5As can be prepared by using high-temperature pellet methods. According to X-ray diffraction results, MnPt5As crystallizes in a tetragonal unit cell with the space group P4/mmm (Pearson symbol tP7). Magnetic measurements on MnPt5As confirm ferromagnetism in this phase with a Curie temperature of ~301 K and a saturated moment of 3.5 uB per formula. Evaluation by applying the Stoner Criterion also indicates that MnPt5As is susceptible to ferromagnetism. Electronic structure calculations using the WIEN2k program with local spin density approximation imply that the spontaneous magnetization of this phase arises primarily from the hybridization of d orbitals on both Mn and Pt atoms. The theoretical assessments are consistent with the experimental results. Moreover, the spin-orbit coupling effects heavily influence on magnetic moments in MnPt5As. MnPt5As is the first high-performance magnetic material in this structure type. The discovery of MnPt5As offers a platform to study the interplay between magnetism and structure.
We have performed high-resolution angle-resolved photoemission spectroscopy on FeSe superconductor (Tc ~ 8 K), which exhibits a tetragonal-to-orthorhombic structural transition at Ts ~ 90 K. At low temperature we found splitting of the energy bands as large as 50 meV at the M point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to T ~ 110 K, slightly above Ts, suggesting that the structural transition is triggered by the electronic nematicity. We have also revealed that at low temperature the band splitting gives rise to a van Hove singularity within 5 meV of the Fermi energy. The present result strongly suggests that this unusual electronic state is responsible for the unconventional superconductivity in FeSe.
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