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.

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Single-crystalline, one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics. Elucidation of the vapour-liquid-solid growth mechanism has already enabled precise control over nanowire position and size, yet to date, no reports have demonstrated the ability to choose from different crystallographic growth directions of a nanowire array. Control over the nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization, and bandgap may be used to tune the physical properties of nanowires made from a given material. Here we demonstrate the use of metal-organic chemical vapour deposition (MOCVD) and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties. Epitaxial growth of wurtzite gallium nitride on (100) gamma-LiAlO(2) and (111) MgO single-crystal substrates resulted in the selective growth of nanowires in the orthogonal [1\[Evec]0] and [001] directions, exhibiting triangular and hexagonal cross-sections and drastically different optical emission. The MOCVD process is entirely compatible with the current GaN thin-film technology, which would lead to easy scale-up and device integration.

High-temperature superconductivity in cuprates arises from an electronic state that remains poorly understood. We report the observation of a related electronic state in a noncuprate material, strontium iridate (Sr2IrO4), in which the distinct cuprate fermiology is largely reproduced. Upon surface electron doping through in situ deposition of alkali-metal atoms, angle-resolved photoemission spectra of Sr2IrO4 display disconnected segments of zero-energy states, known as Fermi arcs, and a gap as large as 80 millielectron volts. Its evolution toward a normal metal phase with a closed Fermi surface as a function of doping and temperature parallels that in the cuprates. Our result suggests that Sr2IrO4 is a useful model system for comparison to the cuprates.

High-temperature superconductivity in cuprates emerges out of a highly enigmatic 'pseudogap' metal phase. The mechanism of high-temperature superconductivity is probably encrypted in the elusive relationship between the two phases, which spectroscopically is manifested as Fermi arcs-disconnected segments of zero-energy states-collapsing into d-wave point nodes upon entering the superconducting phase. Here, we reproduce this distinct cuprate phenomenology in the 5d transition-metal oxide Sr 2 IrO 4 . Using angle-resolved photoemission, we show that the clean, low-temperature phase of 6-8% electron-doped Sr 2 IrO 4 has gapless excitations only at four isolated points in the Brillouin zone, with a predominant d-wave symmetry of the gap. Our work thus establishes a connection between the low-temperature d-wave instability and the previously reported high-temperature Fermi arcs in electron-doped Sr 2 IrO 4 (ref. 1). Although the physical origin of the d-wave gap remains to be understood, Sr 2 IrO 4 is the first non-cuprate material to spectroscopically reproduce the complete phenomenology of the cuprates, thus o ering a new material platform to investigate the relationship between the pseudogap and the d-wave gap.Sr 2 IrO 4 is a single-electron-band magnetic insulator with pseudospin-1/2 moments 2,3 on a square lattice 4 . Despite very strong spin-orbit coupling inherent to 5d transition-metal elements, magnetic interactions between iridium pseudospins are predominantly of Heisenberg type 5 , with a large energy scale 6,7 as reflected in their magnon bandwidth of ∼200 meV. This one-to-one correspondence between Sr 2 IrO 4 and high-temperature superconducting (HTSC) cuprates in their lattice, electronic and magnetic structures allows us to present, in Fig. 1a,b, angle-resolved photoemission (ARPES) intensity maps of approximately 7% electron-doped Sr 2 IrO 4 recorded at the Fermi level following the standard notations used in the cuprate literature. A sign difference between the two systems in one of the tight-binding parameters 8 characterizing the hopping of an electron in a quasi-two-dimensional (2D) square lattice renders Fermi surfaces shifted by (π, π) with respect to each other in their non-interacting electron descriptions. This sign of the next-nearest hopping can be reversed by electron-hole conjugation, which means that our results on electron-doped Sr 2 IrO 4 can be directly compared to those of hole-doped cuprates 9 .