Black phosphorus consists of stacked layers of phosphorene, a two-dimensional semiconductor with promising device characteristics. We report the realization of a widely tunable band gap in few-layer black phosphorus doped with potassium using an in situ surface doping technique. Through band structure measurements and calculations, we demonstrate that a vertical electric field from dopants modulates the band gap, owing to the giant Stark effect, and tunes the material from a moderate-gap semiconductor to a band-inverted semimetal. At the critical field of this band inversion, the material becomes a Dirac semimetal with anisotropic dispersion, linear in armchair and quadratic in zigzag directions. The tunable band structure of black phosphorus may allow great flexibility in design and optimization of electronic and optoelectronic devices.

Strong charge-spin coupling is found in a layered transition-metal trichalcogenide NiPS_{3}, a van der Waals antiferromagnet, from studies of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption, photoemission spectroscopy, and density functional calculations. NiPS_{3} displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission, and optical spectra support a self-doped ground state in NiPS_{3}. Our work demonstrates that layered transition-metal trichalcogenide magnets are useful candidates for the study of correlated-electron physics in two-dimensional magnetic materials.

Topological insulators (TI's) are a new class of quantum matter with extraordinary surface electronic states, which bear great potential for spintronics and error-tolerant quantum computing. In order to put a TI into any practical use, these materials need to be fabricated into devices whose basic units are often p-n junctions. Unique electronic properties of a 'topological' p-n junction were proposed theoretically such as the junction electronic state and the spin rectification. However, the fabrication of a lateral topological p-n junction has been challenging because of materials, process, and fundamental reasons. Here, we demonstrate an innovative approach to realize a p-n junction of topological surface states (TSS's) of a three-dimensional (3D) topological insulator (TI) with an atomically abrupt interface. When a ultrathin Sb film is grown on a 3D TI of Bi 2 Se 3 with a typical n-type TSS, the surface develops a strongly p-type TSS through the substantial hybridization between the 2D Sb film and the Bi 2 Se 3 surface. Thus, the Bi 2 Se 3 surface covered partially with Sb films bifurcates into areas of n-and p-type TSS's as separated by atomic step edges with a lateral electronic junction of as short as 2 nm. This approach opens a different avenue toward various electronic and spintronic devices based on well defined topological p-n junctions with the scalability down to atomic dimensions.Keywords topological insulator, topological p-n junction, angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, ultrathin Sb film Surface states of topological insulators, 1,2 called topological surface states (TSS's), are robustly protected by the bulk topological nature and form necessarily a Dirac band with their spins locked helically with momentum. 1-6 These unique properties find obvious merits in spintronic applications and can yield a Majorana Fermion in proximity with supercon-2 ductivity. 7,8 However, there has been a huge barrier in making devices based on TSS's. The challenge is closely related to the notorious issue of controlling impurities or dopants in a TI crystal. While quite a few works tried to control the chemical potential of a TSS by impurity doping, 4,9-14 the deterioration of the surface channel and the inclusion of bulk channels were inevitable in many cases. Especially, the tunability of the chemical potential was often not enough to make a good p-type TSS. Fabricating a well defined topological p-n junction is even more challenging, 15-18 which represents one of the most important technological issues in staging applications of TI's. Nevertheless, a topological p-n junction, defined as an electronic junction of a p-and a n-type TSS, features unique properties, which are not shared by conventional p-n junctions of semiconductors but promise attractive new applications. 7,8 At a topological p-n junction, the electron scattering and transport are largely governed by the spin polarization of TSS's involved. This property provides the spin rectification eff...

Melatonin is a possible protective agent in postburn gut pathophysiological dynamics. We investigated the role of endogenously-produced versus exogenously-administered melatonin in a major thermal injury rat model with well-characterized gut inflammatory complications. Our rationale is that understanding in vivo melatonin mechanisms in control and inflamed tissues will improve our understanding of its potential as a safe anti-inflammatory/antioxidant therapeutic alternative. Towards this end, we tested the hypothesis that the gut is both a source and a target for melatonin and that mesenteric melatonin plays an anti-inflammatory role following major thermal injury in rats with 3rd degree hot water scald over 30% TBSA. Our methods for assessing the gut as a source of melatonin included plasma melatonin ELISA measurements in systemic and mesenteric circulation as well as rtPCR measurement of jejunum and terminal ileum expression of the melatonin synthesizing enzymes arylalkylamine N-acetyltransferase (AA-NAT) and 5-hydroxyindole-O-methyltransferase (HIOMT) in sham versus day-3 postburn rats. Our melatonin ELISA results revealed that mesenteric circulation has much higher melatonin than systemic circulation and that both mesenteric and systemic melatonin levels are increased three days following major thermal injury. Our rtPCR results complemented the ELISA data in showing that the melatonin synthesizing enzymes AA-NAT and HIOMT are expressed in the ileum and jejunum and that this expression is increased three days following major thermal injury. Interestingly, the rtPCR data also revealed negative feedback by melatonin as exogenous melatonin supplementation at a dose of 7.43 mg (32 μmole/kg), but not 1.86 mg/kg (8 μmole/kg) drastically suppressed AA-NAT mRNA expression. Our methods also included an assessment of the gut as a target for melatonin utilizing computerized immunohistochemical measurements to quantify the effects of exogenous melatonin supplementation on postburn gut mucosa barrier inflammatory profiles. Here, our results revealed that daily postburn intraperitoneal melatonin administration at a dose of 1.86 mg/kg (8 μmole/kg) significantly suppressed both neutrophil infiltration and tyrosine nitrosylation as revealed by Gr-1 and nitrotyrosine immunohistochemistry, respectively. In conclusion, our results provide support for high mesenteric melatonin levels and dynamic de novo gut melatonin production, both of which increase endogenously in response to major thermal injury, but appear to fall short of abrogating the excessive postburn hyper-inflammation. Moreover, supplementation by exogenous melatonin significantly suppresses gut inflammation, thus confirming that melatonin is protective against postburn inflammation.

We investigated the electronic reconstruction in IrTe2 across the q 1/5 = (1/5 0 1/5) charge order phase transition using the angle-resolved photoemission spectroscopy and the ab initio calculation. The complicated shadow bands are attributed to the band folding effect of the charge ordered unit cell, while the valence transition causes the Fermi level shift as well as the size change of the Fermi surface. Implementation of a band unfolding scheme enables us to resolve the complicated folded band features, and successfully provides us information on the electronic reconstruction involving the transition. As a result, we could identify a flattened band and a segmented dispersion near the Γ and A point. We also identified the surface state contributions of additional bands crossing the Fermi level and a Dirac-cone-like band. The observed electronic reconstruction implies that the transition is induced by the strong correlation between the itinerant electrons and the spin-orbit coupled charge ordered states.