Electronic structure of realistically extended atomistically resolved disordered Si:P delta-doped layers

The emergence of scanning tunneling microscope (STM) lithography and low temperature molecular beam epitaxy (MBE) opens the possibility of creating scalable donor based quantum computing architectures. In particular, atomically precise Si:P monolayer structures (delta-doped layers) serve as crucial contact regions and in-plane gates in single impurity devices. In this paper we study highly confined delta-doped layers to explain the disorder in the P dopant placements in realistically extended systems. The band structure is computed using the tight-binding formalism and charge-potential self-consistency. The exchange-correlation corrected impurity potential pulls down subbands below the silicon valley minima to create impurity bands. Our methodology is benchmarked and validated against other theoretical methods for small ordered systems. The doping density is shown to linearly control the impurity bands. Disorder within the Si:P delta-doped layer is examined using an extended domain to describe the effects of experimentally unavoidable randomness through explicitly disordered dopant placement. Disorder in the delta-doped layer breaks the symmetry in the supercell and creates band splitting in every subband. Vertical segregation of dopants is shown to dramatically reduce valley splitting (VS). Such VS can be used as a measure of ideality of the fabricated Si:P delta-doped layer. Although the resulting disorder induces density of states fluctuations, this theoretical analysis shows that delta-doped layers can serve as quasimetallic 2D electron sources even in the presence of strong nonidealities

Effective mass theory of monolayer \delta-doping in the high-density limit

Monolayer \delta-doped structures in silicon have attracted renewed interest with their recent incorporation into atomic-scale device fabrication strategies as source and drain electrodes and in-plane gates. Modeling the physics of \delta-doping at this scale proves challenging, however, due to the large computational overhead associated with ab initio and atomistic methods. Here, we develop an analytical theory based on an effective mass approximation. We specifically consider the Si:P materials system, and the limit of high donor density, which has been the subject of recent experiments. In this case, metallic behavior including screening tends to smooth out the local disorder potential associated with random dopant placement. While smooth potentials may be difficult to incorporate into microscopic, single-electron analyses, the problem is easily treated in the effective mass theory by means of a jellium approximation for the ionic charge. We then go beyond the analytic model, incorporating exchange and correlation effects within a simple numerical model. We argue that such an approach is appropriate for describing realistic, high-density, highly disordered devices, providing results comparable to density functional theory, but with greater intuitive appeal, and lower computational effort. We investigate valley coupling in these structures, finding that valley splitting in the low-lying \Gamma band grows much more quickly than the \Gamma-\Delta band splitting at high densities. We also find that many-body exchange and correlation corrections affect the valley splitting more strongly than they affect the band splitting

A surface code quantum computer in silicon

The exceptionally long quantum coherence times of phosphorus donor nuclear spin qubits in silicon, coupled with the proven scalability of silicon-based nano-electronics, make them attractive candidates for large-scale quantum computing. However, the high threshold of topological quantum error correction can only be captured in a two-dimensional array of qubits operating synchronously and in parallel—posing formidable fabrication and control challenges. We present an architecture that addresses these problems through a novel shared-control paradigm that is particularly suited to the natural uniformity of the phosphorus donor nuclear spin qubit states and electronic confinement. The architecture comprises a two-dimensional lattice of donor qubits sandwiched between two vertically separated control layers forming a mutually perpendicular crisscross gate array. Shared-control lines facilitate loading/unloading of single electrons to specific donors, thereby activating multiple qubits in parallel across the array on which the required operations for surface code quantum error correction are carried out by global spin control. The complexities of independent qubit control, wave function engineering, and ad hoc quantum interconnects are explicitly avoided. With many of the basic elements of fabrication and control based on demonstrated techniques and with simulated quantum operation below the surface code error threshold, the architecture represents a new pathway for large-scale quantum information processing in silicon and potentially in other qubit systems where uniformity can be exploited

Human iPSC-hepatocyte modeling of alpha-1 antitrypsin heterozygosity reveals metabolic dysregulation and cellular heterogeneity

Individuals homozygous for the “Z” mutation in alpha-1 antitrypsin deficiency are known to be at increased risk for liver disease. It has also become clear that some degree of risk is similarly conferred by the heterozygous state. A lack of model systems that recapitulate heterozygosity in human hepatocytes has limited the ability to study the impact of a single Z alpha-1 antitrypsin (ZAAT) allele on hepatocyte biology. Here, we describe the derivation of syngeneic induced pluripotent stem cells (iPSCs) engineered to determine the effects of ZAAT heterozygosity in iPSC-hepatocytes (iHeps). We find that heterozygous MZ iHeps exhibit an intermediate disease phenotype and share with ZZ iHeps alterations in AAT protein processing and downstream perturbations including altered endoplasmic reticulum (ER) and mitochondrial morphology, reduced mitochondrial respiration, and branch-specific activation of the unfolded protein response in cell subpopulations. Our model of MZ heterozygosity thus provides evidence that a single Z allele is sufficient to disrupt hepatocyte homeostatic function.This work was supported by an Alpha-1 Foundation John W. Walsh Translational Research Award (to J.E.K.); a CJ Martin Early Career Fellowship from the Australian National Health and Medical Research Council (to R.B.W.); NIH grant R01HL095993 (to D.N.K.); and NIH grants R01DK101501 (to A.A.W.) and R01DK117940 (to A.N.H. and A.A.W.). iPSC distribution and disease modeling is supported by NIH grants U01TR001810 (to D.N.K. and A.A.W.) and N0175N92020C00005 (to D.N.K.); and by The Alpha-1 Project (TAP), a wholly owned subsidiary of the Alpha-1 Foundation (to D.N.K. and A.A.W.)

A survey of preferences for respiratory support in the intensive care unit for patients with acute hypoxaemic respiratory failure

Publisher Copyright: © 2023 The Authors. Acta Anaesthesiologica Scandinavica published by John Wiley & Sons Ltd on behalf of Acta Anaesthesiologica Scandinavica Foundation.Background: When caring for mechanically ventilated adults with acute hypoxaemic respiratory failure (AHRF), clinicians are faced with an uncertain choice between ventilator modes allowing for spontaneous breaths or ventilation fully controlled by the ventilator. The preferences of clinicians managing such patients, and what motivates their choice of ventilator mode, are largely unknown. To better understand how clinicians' preferences may impact the choice of ventilatory support for patients with AHRF, we issued a survey to an international network of intensive care unit (ICU) researchers. Methods: We distributed an online survey with 32 broadly similar and interlinked questions on how clinicians prioritise spontaneous or controlled ventilation in invasively ventilated patients with AHRF of different severity, and which factors determine their choice. Results: The survey was distributed to 1337 recipients in 12 countries. Of these, 415 (31%) completed the survey either fully (52%) or partially (48%). Most respondents were identified as medical specialists (87%) or physicians in training (11%). Modes allowing for spontaneous ventilation were considered preferable in mild AHRF, with controlled ventilation considered as progressively more important in moderate and severe AHRF. Among respondents there was strong support (90%) for a randomised clinical trial comparing spontaneous with controlled ventilation in patients with moderate AHRF. Conclusions: The responses from this international survey suggest that there is clinical equipoise for the preferred ventilator mode in patients with AHRF of moderate severity. We found strong support for a randomised trial comparing modes of ventilation in patients with moderate AHRF.Peer reviewe

Quantum Transport in Ultra-Scaled Phosphorous-Doped Silicon Nanowires

Highly phosphorous-doped nanowires in silicon (Si:P NW) represent the ultimate nanowire scaling limit of 1 atom thickness and a few atoms width. Experimental data are compared to an atomistic full-band model. Charge-potential self-consistency is computed by solving the exchange-correlation LDA corrected Schrödinger-Poisson equation. Transport through donor bands is observed in [110] Si:P NW at low temperature. The semi-metallic conductance computed in the ballistic regime agrees well with the experiment. Sensitivity of the NW properties on doping constant and placement disorder on the channel is addressed. The modeling confirms that the nanowires are semi-metallic and transport can be gate modulated