Strong interlayer coupling in two-dimensional PbSe with high thermoelectric performance.

It was generally believed that weak van der Waals interactions exist between neighboring layers in the two-dimensional group-IV chalcogenides. Using PbSe as a prototypal example, we find additional strong coupling between the Pb-Pb layers, as evidenced by detailed analysis of the differential charge density plot. The coupling is covalent-like and can be fine-tuned to obviously reduce the phonon thermal conductivity but slightly change the electronic transport of PbSe layer. As a consequence, a maximumZTvalue of 2.5 can be realized at 900 K for thep-type system. Our work also offers an effective and feasible design strategy to enhance the thermoelectric performance of similar layered structures.

Photoemission Spectroscopic Evidence for the Dirac Nodal Line in the Monoclinic Semimetal SrAs_{3}.

Topological nodal-line semimetals with exotic quantum properties are characterized by symmetry-protected line-contact bulk band crossings in the momentum space. However, in most of identified topological nodal-line compounds, these topological nontrivial nodal lines are enclosed by complicated topological trivial states at the Fermi energy (E_{F}), which would perplex their identification and hinder further applications. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide compelling evidence for the existence of Dirac nodal-line fermions in the monoclinic semimetal SrAs_{3}, which possesses a simple nodal loop in the vicinity of E_{F} without the distraction from complicated trivial Fermi surfaces. Our calculations revealed that two bands with opposite parities were inverted around Y near E_{F}, resulting in the single nodal loop at the Γ-Y-S plane with a negligible spin-orbit coupling effect. The band crossings were tracked experimentally and the complete nodal loop was identified quantitatively, which provide a critical experimental support for the existence of nodal-line fermions in the CaP_{3} family of materials. Hosting simple topological nontrivial bulk electronic states around E_{F} and without complication from the trivial states, SrAs_{3} is expected to be a potential platform for topological quantum state investigation and applications.

Circ-DB participates in the progression of gastric cancer by competitively binding microRNA-34a to regulate the expression of MET.

OBJECTIVE: To figure out the possible role and mechanism of circ-DB in the pathogenesis of gastric cancer (GCa). PATIENTS AND METHODS: 32 cases of postoperative GCa tissue samples and adjacent ones were collected and divided into groups of ≥ 5 cm and < 5 cm according to tumor diameter. Circ-DB, microRNA-34a, and MET expressions were detected by real time-quantitative PCR (RT-qPCR) in GCa tissues and adjacent tissues. To determine the main mode of action of circ-DB, the subcellular localization of circ-DB was examined by dividing the cells into the nucleus and cytoplasmic fractions. The binding of microRNA-34a to circ-DB was demonstrated by a Dual-Luciferase reporter gene assay. The expression of circ-DB in HGC-27 and AGS cells was overexpressed and knocked down to evaluate the migration function of the cells by transwell. The protein expression of MET, as well as the target gene of microRNA-34a, was detected by Western blot. RESULTS: The expression of circ-DB and MET in GCa tissues was significantly higher than that in the corresponding adjacent tissues. Circ-DB was positively correlated with MET expression, while microRNA-34a expression was negatively correlated with circ-DB and MET expression. Circ-DB was mainly located in the cytoplasm, and the Dual-Luciferase reporter gene demonstrated that microRNA-34a can bind to circ-DB. The down-regulation of circ-DB expression inhibited the migration of HGC-27 and AGS cells. In vitro cell experiments showed that low expression of circ-DB inhibited cell migration, which could be recovered by the co-transfection with microRNA-34a inhibitor. CONCLUSIONS: Circ-DB may regulate MET level through microRNA-34a and affect the proliferative ability and migration of GCa cells.

Unveiling the Superconducting Mechanism of Ba_{0.51}K_{0.49}BiO_{3}.

The mechanism of high superconducting transition temperatures (T_{c}) in bismuthates remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba_{0.51}K_{0.49}BiO_{3} reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions-the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics.Furthermore, we observe an isotropic superconducting gap with 2Δ_{0}/k_{B}T_{c}=3.51±0.05, and strong electron-phonon interactions with a coupling constant λ∼1.3±0.2. These findings solve a long-standing mystery-Ba_{0.51}K_{0.49}BiO_{3} is an extraordinary Bardeen-Cooper-Schrieffer superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high T_{c}. Such effects will also be critical for finding new superconductors.

Single Nodal Loop of Accidental Degeneracies in Minimal Symmetry: Triclinic CaAs_{3}.

The existence of closed loops of degeneracies in crystals has been intimately connected with associated crystal symmetries, raising the following question: What is the minimum symmetry required for topological character, and can one find an example? Triclinic CaAs_{3}, in the space group P1[over ¯] with only a center of inversion, has been found to display, without need for tuning, a nodal loop of accidental degeneracies with topological character, centered on one face of the Brillouin zone that is otherwise fully gapped. The small loop is very flat in energy, yet is cut four times by the Fermi energy, a condition that results in an intricate repeated touching of inversion related pairs of Fermi surfaces at Weyl points. Spin-orbit coupling lifts the fourfold degeneracy along the loop, leaving trivial Kramers pairs. With its single nodal loop that emerges without protection from any point group symmetry, CaAs_{3} represents the primal "hydrogen atom" of nodal loop systems.

Dynamical Mean-Field Theory Plus Numerical Renormalization-Group Study of Spin-Orbital Separation in a Three-Band Hund Metal.

We show that the numerical renormalization group is a viable multi-band impurity solver for dynamical mean-field theory (DMFT), offering unprecedented real-frequency spectral resolution at arbitrarily low energies and temperatures. We use it to obtain a numerically exact DMFT solution to the Hund metal problem for a three-band model on a Bethe lattice at 1/3 filling. The ground state is a Fermi liquid. The one-particle spectral function undergoes a coherence-incoherence crossover with increasing temperature, with spectral weight being transferred from low to high energies. Further, it exhibits a strong particle-hole asymmetry. In the incoherent regime, the self-energy displays approximate power-law behavior for positive frequencies only. The spin and orbital spectral functions show "spin-orbital separation": spin screening occurs at much lower energies than orbital screening. The renormalization group flows clearly reveal the relevant physics at all energy scales.

From antiferromagnetic insulator to correlated metal in pressurized and doped LaMnPO.

Widespread adoption of superconducting technologies awaits the discovery of new materials with enhanced properties, especially higher superconducting transition temperatures T(c). The unexpected discovery of high T(c) superconductivity in cuprates suggests that the highest T(c)s occur when pressure or doping transform the localized and moment-bearing electrons in antiferromagnetic insulators into itinerant carriers in a metal, where magnetism is preserved in the form of strong correlations. The absence of this transition in Fe-based superconductors may limit their T(c)s, but even larger T(c)s may be possible in their isostructural Mn analogs, which are antiferromagnetic insulators like the cuprates. It is generally believed that prohibitively large pressures would be required to suppress the effects of the strong Hund's rule coupling in these Mn-based compounds, collapsing the insulating gap and enabling superconductivity. Indeed, no Mn-based compounds are known to be superconductors. The electronic structure calculations and X-ray diffraction measurements presented here challenge these long held beliefs, finding that only modest pressures are required to transform LaMnPO, isostructural to superconducting host LaFeAsO, from an antiferromagnetic insulator to a metallic antiferromagnet, where the Mn moment vanishes in a second pressure-driven transition. Proximity to these charge and moment delocalization transitions in LaMnPO results in a highly correlated metallic state, the familiar breeding ground of superconductivity.

Kinetic frustration and the nature of the magnetic and paramagnetic states in iron pnictides and iron chalcogenides.

The iron pnictide and chalcogenide compounds are a subject of intensive investigations owing to their surprisingly high temperature superconductivity. They all share the same basic building blocks, but there is significant variation in their physical properties, such as magnetic ordered moments, effective masses, superconducting gaps and transition temperature (T(c)). Many theoretical techniques have been applied to individual compounds but no consistent description of the microscopic origin of these variations is available. Here we carry out a comparative theoretical study of a large number of iron-based compounds in both their magnetic and paramagnetic states. Taking into account correlation effects and realistic band structures, we describe well the trends in all of the physical properties such as the ordered moments, effective masses and Fermi surfaces across all families of iron compounds, and find them to be in good agreement with experiments. We trace variation in physical properties to variations in the key structural parameters, rather than changes in the screening of the Coulomb interactions. Our results also provide a natural explanation of the strongly Fermi-surface-dependent superconducting gaps observed in experiments.

Electron-hole symmetry and magnetic coupling in antiferromagnetic LaFeAsO.

When either electron or hole doped at concentrations x approximately 0.1, the LaFeAsO family displays remarkably high temperature superconductivity with Tc up to 55 K. In the most energetically stable Q-->M=(pi,pi,0) antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy arising from light carriers (m* approximately 0.25-0.33), and very strong magnetophonon coupling is uncovered. Doping (of either sign) beyond x approximately 0.08 results in heavy carriers per Fe (by roughly an order of magnitude) with a large Fermi surface. Calculated Fe-Fe transverse exchange couplings Jij(R) reveal that exchange coupling is strongly dependent on both the AFM symmetry and on the Fe-As distance.