Tight-binding theory of the electronic structures for rhombohedral semimetals

Impact of inherent energy barrier on spin-orbit torques in magnetic-metal/semimetal heterojunctions

Spintronic devices are based on heterojunctions of two materials with different magnetic and electronic properties. Although an energy barrier is naturally formed even at the interface of metallic heterojunctions, its impact on spin transport has been overlooked. Here, using diffusive spin Hall currents, we provide evidence that the inherent energy barrier governs the spin transport even in metallic systems. We find a sizable field-like torque, much larger than the damping-like counterpart, in Ni81Fe19/Bi0.1Sb0.9 bilayers. This is a distinct signature of barrier-mediated spin-orbit torques, which is consistent with our theory that predicts a strong modification of the spin mixing conductance induced by the energy barrier. Our results suggest that the spin mixing conductance and the corresponding spin-orbit torques are strongly altered by minimizing the work function difference in the heterostructure. These findings provide a new mechanism to control spin transport and spin torque phenomena by interfacial engineering of metallic heterostructures.

β-Arsenene Monolayer: A Promising Electrocatalyst for Anodic Chlorine Evolution Reaction

Materials innovation plays an essential role to address the increasing demands of gaseous chlorine from anodic chlorine evolution reaction (CER) in chlor-alkali electrolysis. In this study, two-dimensional (2D) semiconducting group-VA monolayers were theoretically screened for the electrochemical CER by means of the density functional theory (DFT) method. Our results reveal the monolayered β-arsenene has the ultralow thermodynamic overpotential of 0.068 V for CER, which is close to that of the commercial Ru/Ir-based dimensionally stable anode (DSA) of 0.08 V @ 10 mA cm−2 and 0.13 V from experiments and theory, respectively. The change of CER pathways via Cl* intermediate on 2D β-arsenene also efficiently suppresses the parasitical oxygen gas production because of a high theoretical oxygen evolution reaction (OER) overpotential of 1.95 V. Our findings may therefore expand the scope of the electrocatalysts design for CER by using emerging 2D materials.

Two-Dimensional Pnictogen for Field-Effect Transistors

Two-dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group-VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next-generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field-effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.

Optical properties of monolayer bismuthene in electric fields

Optical excitations of monolayer bismuthene present very rich and unique absorption spectra. The optical energy gap corresponding to the threshold frequency is not equal to an indirect energy gap, and it becomes zero under the critical electric field. The frequency, number, intensity, and form of the absorption structures are dramatically changed when an external electric field is applied. The prominent peaks and the observable shoulders, respectively, arise from the constant-energy loop and the band-edge states of parabolic dispersions. These directly reflect the unusual electronic properties, being very different from those in monolayer graphene. The novel optical properties of bismuthine that are easily manipulated by electric fields may find a lot of various applications in optoelectronics, either combined with or complementary to those graphene-based systems.

Understanding the effect of nanowire orientation on time evolution of Raman spectra from laser irradiated InAs nanowire surface

We investigate differences observed in the time evolution of Raman spectra for differently oriented (in plane) InAs nanowires (NWs), using polarized Raman spectroscopy. Specially designed polarized Raman spectroscopy experiments elucidate that laser irradiation leads to the formation of an oriented crystalline oxide film on the InAs NW surface. Both the formation of oriented crystalline oxides and Raman selection rules leading to the presence/absence of oxide peaks in the unpolarized Raman spectra are uncommon occurrences and can lead to incorrect interpretations of the oxidation process, if not looked into carefully. Further, the specially designed heating and cooling experiments for a mixed phase (wurtzite + zinc blende) InAs NW revealed the formation of specific allotropes of elemental As, i.e. gray-As (rhombohedral) and black-As (orthorhombic: metastable) at low (700-950 K) and high simulated temperatures (1000-1300 K) on the InAs NW surface, respectively. Both have high electrical conductivity due to a layered structure and control over the growth of only a few layers using laser irradiation envisages properties similar to graphene. This kind of surface of InAs NWs has the potential for novel device applications, where a semiconductor-insulator-metal heterostructure is required.

Effects of trigonal deformation on electronic structure and thermoelectric properties of bismuth

First principles calculation and Boltzmann transport theory have been used to reveal the effects of trigonal deformation on electronic structure and thermoelectric properties of bulk bismuth. It is found that the semimetal-semiconductor transition would happen at the critical c/a points of 2.41 and 2.51, and that such a transition should be ascribed to the opposite changes of band edges at T and L points during trigonal deformation. Calculations also reveal that trigonal deformation has an important effect on various temperature-dependent thermoelectric properties, and that carrier density plays a decisive role in determining the magnitude of Seebeck coefficient and figure of merit. The semimetal  →  semiconductor transition as a result of trigonal compression with the decrease of c/a fundamentally induces the best performance of the thermoelectric properties of bismuth at the c/a ratio of 2.45. The present results agree well with experimental observations in the literature, and provide a deep understanding of the intrinsic relationship between trigonal deformation, band structure, and thermoelectric properties of bismuth.

Recent progress in 2D group-VA semiconductors: from theory to experiment

This review provides recent theoretical and experimental progress in the fundamental properties, electronic modulations, fabrications and applications of 2D group-VA materials.

Theoretical discovery of novel two-dimensional VA-N binary compounds with auxiticity

Novel two-dimensional V^A-nitride binary compounds with a large negative Poisson's ratio and a suitable band-gap are predicted based on first-principles calculations.

The generalized maximum hardness principle revisited and applied to solids (Part 2)

Building on Part 1 devoted to atoms and molecules (PCCP, in press 2017), we now focus on the crystal structure and electronic properties of solids as viewed from the Maximum Hardness Principle (MHP), first formulated by Pearson in 1987. The focus is on cases where nuclear potential acting on electrons does not remain constant and where substantial modifications of the nuclear geometry take place (Generalized MHP, GMHP). We present an overview of important manifestations of the (G)MHP for solids such as (i) a tendency of metals and doped-semiconductors to undergo superconducting transition at low temperatures, (ii) propensity of many types of alloys to develop a band gap or a pseudo-gap, (iii) preference for preserving the noble gas (octet, doublet) configuration of main block element ions in the solid state, (iv) preference of Jahn-Teller systems for band-gap-opening vibronic-coupling-related lattice distortions, (v) pressure phenomena leading to localization of the electronic density, (vi) tendency to annihilate the null band gap via phase separation (while preserving the nominal chemical composition), (vii) absence of a large number of families of high-T_C superconductors, (viii) resistance of most stable systems to chemical doping, etc. GMHP turns out to be an important qualitative guide in studies of solid state polymorphism and electronic phenomena. Exceptions from (G)MHP are discussed, and a more restrictive formulation of the principle is proposed.

Dramatically Enhanced Superconductivity in Elemental Bismuth from Excitonic Fluctuation Exchange

Motivated by the remarkable discovery of superconductivity in elemental Bismuth at ambient pressure, we study its normal state in detail using a combination of tight-binding (TB) band-structure supplemented by dynamical mean-field theory (DMFT). We show that a two-fluid model composed of preformed and dynamically fluctuating excitons coupled to a tiny number of carriers provides a unified rationalization of a range of ill-understood normal state spectral and transport data. Based on these, we propose that resonant scattering involving a very low density of renormalized carriers and the excitonic liquid drives logarithmic enhancement of vertex corrections, boosting superconductivity in Bi. A confirmatory test for our proposal would be the experimental verification of an excitonic semiconductor with electronic nematicity as a ‘competing order’ on inducing a semi-metal-to semiconductor transition in Bi by an external perturbation like pressure

Semiconducting Group 15 Monolayers: A Broad Range of Band Gaps and High Carrier Mobilities

Optoelectronic applications require materials both responsive to objective photons and able to transfer carriers, so new two-dimensional (2D) semiconductors with appropriate band gaps and high mobilities are highly desired. A broad range of band gaps and high mobilities of a 2D semiconductor family, composed of monolayer of Group 15 elements (phosphorene, arsenene, antimonene, bismuthene) is presented. The calculated binding energies and phonon band dispersions of 2D Group 15 allotropes exhibit thermodynamic stability. The energy band gaps of 2D semiconducting Group 15 monolayers cover a wide range from 0.36 to 2.62 eV, which are crucial for broadband photoresponse. Significantly, phosphorene, arsenene, and bismuthene possess carrier mobilities as high as several thousand cm(2)  V(-1)  s(-1) . Combining such broad band gaps and superior carrier mobilities, 2D Group 15 monolayers are promising candidates for nanoelectronics and optoelectronics.

Fermi surface and anisotropic spin-orbit coupling of Sb(111) studied by angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission spectroscopy has been performed on Sb(111) to elucidate the origin of anomalous electronic properties in group-V semimetal surfaces. The surface was found to be metallic despite the semimetallic character of bulk. We clearly observed two surface-derived Fermi surfaces which are likely spin split, demonstrating that the spin-orbit interaction plays a dominant role in characterizing the surface electronic states of group-V semimetals. The universality or dissimilarity of the electronic structure in Bi and Sb is discussed in relation to the granular superconductivity, electron-phonon coupling, and surface charge or spin density wave.