Suppression of low-frequency noise in two-dimensional electron gas at degenerately doped Si:P \delta-layers

We report low-frequency 1/f noise measurements of degenerately doped Si:P \delta-layers at 4.2K. The noise was found to be over six orders of magnitude lower than that of bulk Si:P systems in the metallic regime and is one of the lowest values reported for doped semiconductors. The noise was found to be nearly independent of magnetic field at low fields, indicating negligible contribution from universal conductance fluctuations. Instead interaction of electrons with very few active structural two-level systems may explain the observed noise magnitudeComment: 4 pages, 4 figure

Fragility of the Dirac Cone Splitting in Topological Crystalline Insulator Heterostructures

The 'double Dirac cone' 2D topological interface states found on the (001) faces of topological crystalline insulators such as Pb$_{1-x}$Sn$_{x}$Se feature degeneracies located away from time reversal invariant momenta, and are a manifestation of both mirror symmetry protection and valley interactions. Similar shifted degeneracies in 1D interface states have been highlighted as a potential basis for a topological transistor, but realizing such a device will require a detailed understanding of the intervalley physics involved. In addition, the operation of this or similar devices outside of ultra-high vacuum will require encapsulation, and the consequences of this for the topological interface state must be understood. Here we address both topics for the case of 2D surface states using angle-resolved photoemission spectroscopy. We examine bulk Pb$_{1-x}$Sn$_{x}$Se(001) crystals overgrown with PbSe, realizing trivial/topological heterostructures. We demonstrate that the valley interaction that splits the two Dirac cones at each $\bar{X}$ is extremely sensitive to atomic-scale details of the surface, exhibiting non-monotonic changes as PbSe deposition proceeds. This includes an apparent total collapse of the splitting for sub-monolayer coverage, eliminating the Lifshitz transition. For a large overlayer thickness we observe quantized PbSe states, possibly reflecting a symmetry confinement mechanism at the buried topological interface

Disentangling Electron-Boson Interactions on the Surface of a Familiar Ferromagnet

We report energy renormalizations from electron-phonon and electron-magnon interactions in spin minority surface resonances on Ni(111). The different interactions are disentangled and quantified in strength $\lambda$, based on the characteristic shapes of their complex self-energies, and the largely different binding energies at which they occur. The observed electron-magnon interactions reveal a strong dependence on momentum and energy band position in the bulk Brillouin zone. In contrast, electron-phonon interactions from the same bands are observed to be practically momentum- and symmetry-independent. Additionally, a moderately strong ($\lambda>0.5$) electron-phonon interaction is observed from a `buried', near-parabolic spin majority band that does not cross the Fermi level.Comment: QuSpin 202

Bottom-Up Growth of Monolayer Honeycomb SiC

The long theorized two-dimensional allotrope of SiC has remained elusive amid the exploration of graphenelike honeycomb structured monolayers. It is anticipated to possess a large direct band gap (2.5 eV), ambient stability, and chemical versatility. While sp2 bonding between silicon and carbon is energetically favorable, only disordered nanoflakes have been reported to date. Here we demonstrate large-area, bottom-up synthesis of monocrystalline, epitaxial monolayer honeycomb SiC atop ultrathin transition metal carbide films on SiC substrates. We find the 2D phase of SiC to be almost planar and stable at high temperatures, up to 1200 \ub0C in vacuum. Interactions between the 2D-SiC and the transition metal carbide surface result in a Dirac-like feature in the electronic band structure, which in the case of a TaC substrate is strongly spin-split. Our findings represent the first step towards routine and tailored synthesis of 2D-SiC monolayers, and this novel heteroepitaxial system may find diverse applications ranging from photovoltaics to topological superconductivity

The sub-band structure of atomically sharp dopant profiles in silicon

This work was partly supported by the Research Council of Norway through its Centres of Excellence funding scheme, Project Number 262633, ‘QuSpin’, and through the Fripro program, Project Numbers 250985 ‘FunTopoMat’, 262339 ‘NEAT’, and by the Villum Fonden through the Centre of Excellence for Dirac Materials (Grant No. 11744). J.A.M. acknowledges funding support from the Danish Council for Independent Research, Natural Sciences under the Sapere Aude program (Grant No. DFF-6108-00409) and the Aarhus University Research Foundation. P.D.C.K. acknowledges financial support from The Royal Society.The downscaling of silicon-based structures and proto-devices has now reached the single-atom scale, representing an important milestone for the development of a silicon-based quantum computer. One especially notable platform for atomic-scale device fabrication is the so-called Si:P δ-layer, consisting of an ultra-dense and sharp layer of dopants within a semiconductor host. Whilst several alternatives exist, it is on the Si:P platform that many quantum proto-devices have been successfully demonstrated. Motivated by this, both calculations and experiments have been dedicated to understanding the electronic structure of the Si:P δ-layer platform. In this work, we use high-resolution angle-resolved photoemission spectroscopy to reveal the structure of the electronic states which exist because of the high dopant density of the Si:P δ-layer. In contrast to published theoretical work, we resolve three distinct bands, the most occupied of which shows a large anisotropy and significant deviation from simple parabolic behaviour. We investigate the possible origins of this fine structure, and conclude that it is primarily a consequence of the dielectric constant being large (ca. double that of bulk Si). Incorporating this factor into tight-binding calculations leads to a major revision of band structure; specifically, the existence of a third band, the separation of the bands, and the departure from purely parabolic behaviour. This new understanding of the band structure has important implications for quantum proto-devices which are built on the Si:P δ-layer platform.Publisher PDFPeer reviewe

One-dimensional confinement and width-dependent bandgap formation in epitaxial graphene nanoribbons

The ability to define an off state in logic electronics is the key ingredient that is impossible to fulfill using a conventional pristine graphene layer, due to the absence of an electronic bandgap. For years, this property has been the missing element for incorporating graphene into next-generation field effect transistors. In this work, we grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. Angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling spectroscopy measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. Furthermore, ARPES demonstrates an ideal one-dimensional electronic behavior that is realized in a graphene-based environment, consisting of well-resolved subbands, dispersing and non-dispersing along and across the ribbons respectively. Our experimental findings, coupled with theoretical tight-binding calculations, set the grounds for a deeper exploration of quantum confinement phenomena and may open intriguing avenues for new low-power electronics. © 2020, The Author(s)

Origin of the π -band replicas in the electronic structure of graphene grown on 4H -SiC(0001)

The calculated electronic band structure of graphene is relatively simple, with a Fermi surface consisting only of six Dirac cones in the first Brillouin zone-one at each (K) over bar. In contrast, angle-resolved photoemission measurements of graphene grown on SiC(0001) often show six satellite Dirac cones surrounding each primary Dirac cone. Recent studies have reported two further Dirac cones along the (Gamma) over bar-(K) over bar line, and argue that these are not photoelectron diffraction artifacts but real bands deriving from a modulation of the ionic potential in the graphene layer. Here we present measurements using linearly polarized synchrotron light which show all of these replicas as well as several additional ones. Using information obtained from dark corridor orientations and angular warping, we demonstrate that all but one of these additional features-including those previously assigned as real initial-state bands-are possible to explain by simple final-state photoelectron diffraction

Spectroscopic Evidence for a Three-Dimensional Charge Density Wave in Kagome Superconductor CsV3_3Sb5_5

The recently discovered AV3Sb5 (A=K, Rb, Cs) family, possessing V kagome nets, has received considerable attention due to the topological electronic structure and intriguing correlated phenomena, including an exotic charge density wave (CDW) and superconductivity. Detailed electronic structure studies are essential to unravel the characteristics and origin of the CDW as well as its interplay with superconductivity. Here, we present angle-resolved photoemission spectroscopy (ARPES) measurements for CsV3Sb5 at multiple temperatures and photon energies to reveal the nature of the CDW from an electronic structure perspective. We present evidence for a three-dimensional (3D) CDW order. In the process we also pinpoint a surface state attributed to a Cs terminated surface. This state was previously attributed to band folding band due to a CDW along the c direction or a quantum well state from quantum confinement. The CDW expected 2-fold lattice reconstruction along c axis is observed to be a quadrupling of the unit cell, thus for the first time directly demonstrating the 3D nature of the CDW from the electronic structure perspective. Moreover, this 3D CDW configuration originates from two distinct types of distortions in adjacent kagome layers. These present results not only provide key insights into the nature of the unconventional CDW in CsV3Sb5 but also provides an important reference for further studies on the relationship between the CDW and superconductivity.Comment: 19 pages, 4 figure

Designing a water leasing market for the Mimbres River, New Mexico.

The objective of this study is to develop a conceptual framework for establishing water leasing markets in New Mexico using the Mimbres River as a test case. Given the past and growing stress over water in New Mexico and the Mimbres River in particular, this work will develop a mechanism for the short term, efficient, temporary transfer of water from one user to another while avoiding adverse effects on any user not directly involved in the transaction (i.e., third party effects). Toward establishing a water leasing market, five basic tasks were performed, (1) a series of stakeholder meetings were conducted to identify and address concerns and interests of basin residents, (2) several gauges were installed on irrigation ditches to aid in the monitoring and management of water resources in the basin, (3) the hydrologic/market model and decision support interface was extended to include the Middle and Lower reaches of the Mimbres River, (4) experiments were conducted to aid in design of the water leasing market, and (5) a set of rules governing a water leasing market was drafted for future adoption by basin residents and the New Mexico Office of the State Engineer

What future/s for outdoor and environmental education in a world that has contended with COVID-19?

This is an unusual article in that it brings together the perspectives of many on this journal’s editorial board, around the issue of contending with COVID-19. Twenty statements showcase a range of thoughts and experiences, highlighting the differences and similarities in the way the pandemic is impacting on the educational practice of outdoor and environmental education. The future is not yet written, of course, so it is worth thinking about how the current moment may impact on the months and years to come. The aim of this article is to influence and support such thinking