Encapsulation of phosphorus dopants in silicon for the fabrication of a quantum computer

The incorporation of phosphorus in silicon is studied by analyzing phosphorus delta-doped layers using a combination of scanning tunneling microscopy, secondary ion mass spectrometry and Hall effect measurements. The samples are prepared by phosphine saturation dosing of a Si(100) surface at room temperature, a critical annealing step to incorporate phosphorus atoms, and subsequent epitaxial silicon overgrowth. We observe minimal dopant segregation (5 nm), complete electrical activation at a silicon growth temperature of 250 degrees C and a high two-dimensional electron mobility of 100 cm2/Vs at a temperature of 4.2 K. These results, along with preliminary studies aimed at further minimizing dopant diffusion, bode well for the fabrication of atomically precise dopant arrays in silicon such as those found in recent solid-state quantum computer architectures.Comment: 3 pages, 4 figure

Magnetic Anisotropy of Single Mn Acceptors in GaAs in an External Magnetic Field

We investigate the effect of an external magnetic field on the physical properties of the acceptor hole states associated with single Mn acceptors placed near the (110) surface of GaAs. Crosssectional scanning tunneling microscopy images of the acceptor local density of states (LDOS) show that the strongly anisotropic hole wavefunction is not significantly affected by a magnetic field up to 6 T. These experimental results are supported by theoretical calculations based on a tightbinding model of Mn acceptors in GaAs. For Mn acceptors on the (110) surface and the subsurfaces immediately underneath, we find that an applied magnetic field modifies significantly the magnetic anisotropy landscape. However the acceptor hole wavefunction is strongly localized around the Mn and the LDOS is quite independent of the direction of the Mn magnetic moment. On the other hand, for Mn acceptors placed on deeper layers below the surface, the acceptor hole wavefunction is more delocalized and the corresponding LDOS is much more sensitive on the direction of the Mn magnetic moment. However the magnetic anisotropy energy for these magnetic impurities is large (up to 15 meV), and a magnetic field of 10 T can hardly change the landscape and rotate the direction of the Mn magnetic moment away from its easy axis. We predict that substantially larger magnetic fields are required to observe a significant field-dependence of the tunneling current for impurities located several layers below the GaAs surface.Comment: Non

Towards the fabrication of phosphorus qubits for a silicon quantum computer

The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices however requires atomic scale manipulation - an immense technological challenge. We demonstrate that it is possible to fabricate an atomically-precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a silicon-based quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth.Comment: To Appear in Phys. Rev. B Rapid Comm. 5 pages, 5 color figure

Pathways of patients with chronic haematological malignancies: a report from the UK’s population-based HMRN

\ua9 2024 Roman et al. Background: Arising in blood and lymph-forming tissues, haematological malignancies (leukaemias, lymphomas and myelomas) are the fifth most common group of cancers. Around 60% are currently incurable and follow a chronic, remitting–relapsing pathway often initially managed by ‘watch & wait’. This involves hospital-based monitoring, followed by treatment if the cancer progresses (which not all do) and then further observation, in a process that may continually repeat. New treatments are constantly emerging, survival is improving and prevalence is rising, but population-based data documenting entire care pathway are sparse. Hence, empirically-based incidence and prevalence estimates about various treatment states (watch and wait, first-line treatment, observation, second-line treatment, etc.) and patterns of healthcare activity are lacking. Likewise, despite complex trajectories, anxiety-provoking watch and wait, and therapies that impede quality of life and incur marked healthcare costs, evidence about patient preferences for information sharing and treatment decisions is scant. Objectives: Primary – to generate high-quality, evidence-based information about the care pathways of the general population of patients with chronic haematological malignancies. Secondary – to produce information resources suitable for testing in routine National Health Service practice. Design: Population-based cohort of ≈ 8000 patients with chronic haematological malignancies, incorporating five nested work packages, each with its own individual design: (1) exploration of patient experiences: information and treatment decisions; (2) population-based analyses; (3) health economics; (4) development of information resources to support decision-making; and (5) patient well-being and decision-making survey. Setting: This programme is predicated on the infrastructure of the United Kingdom’s Haematological Malignancy Research Network (www.hmrn.org); which provides ‘real-world’, robust, generalisable data to inform research and clinical practice, nationally and internationally. Set in Yorkshire and Humberside, the Haematological Malignancy Research Network’s catchment population of ≈ 4 million has a comparable sex, age, urban/rural, and area-based deprivation (Index of Multiple Deprivation, income domain) distribution to the United Kingdom as a whole; and in terms of ethnic diversity the region is centrally ranked, with around 80% of residents identifying as White British, 9% as Asian and 2% as black. Within the Haematological Malignancy Research Network, clinical practice adheres to national guidelines, and all patients with blood cancers are centrally diagnosed (≈ 2500 each year), tracked through their treatment pathways and linked to national databases (deaths, cancer registrations and Hospital Episode Statistics). Linked to the same national databases, the Haematological Malignancy Research Network also contains an age-and sex-matched general-population cohort. Participants: Patients aged ≥ 18 years, resident in the study region, and diagnosed with chronic lymphocytic leukaemia, follicular lymphoma or myeloma. Methods: Core Haematological Malignancy Research Network data were used to compare the hospital activity of patients with chronic lymphocytic leukaemia, follicular lymphoma and myeloma with that of the general population. Following additional linkages to genetic and clinical data, follicular lymphoma prognostic factors were examined. Two self-administered questionnaires addressing (1) quality of life and well-being and (2) decision-making were iteratively developed, piloted and deployed. Linkage to quality of life, clinical information and Hospital Episode Statistics enabled economic (myeloma) model development. In-depth interviews were conducted with 35 patients (10 alongside relatives). Results: Trajectories of ≈ 8000 patients were mapped, and patient-pathway visualisations summarising individual and aggregate information were developed. As expected, patients with chronic blood cancers experienced higher levels of hospital activity than their general population counterparts, the largest effects being for myeloma. Following survey deployment, 3153 patients were recruited across 14 hospitals, 1282 with chronic lymphocytic leukaemia, follicular lymphoma or myeloma. Over half of the questionnaires were completed by patients on watch and wait; the remainder were completed during treatment or post-chemotherapy monitoring. Information gathered, coupled with in-depth interviews, demonstrated patients’ marked anxiety and fluctuating preferences for information sharing and decision-making, contingent on complex, inter-related factors. In turn, prognostic and microsimulation economic models were used to predict individual-level trajectories across multiple treatment lines, examining associated overall survival, costs and quality-adjusted life-years. Limitations: Survey mapping to individual care pathways could not be completed because the COVID-19 pandemic delayed clinical data collection. Patients who attended clinics and participated in the survey were more likely than non-attenders to have had first-line chemotherapy, be slightly younger and live in more affluent areas. Conclusions: This programme collated high-quality, population-based evidence. Previously lacking, this, coupled with new findings on preferences for information sharing and treatment decisions, provides the foundation for future research. Future work: The translation of information accrued into resources suitable for testing in routine NHS practice is key. In this regard, COVID-19 has changed the communication landscape. The visualisations developed by this programme require further refinement/testing using participatory co-design with stakeholder groups. Underpinned by a suitable protocol applied within a single multidisciplinary team setting, prior to further evaluation within/outside the region, such outputs require testing in a cluster-randomised trial

Split-off dimer defects on the Si(001)2x1 surface

Dimer vacancy (DV) defect complexes in the Si(001)2x1 surface were investigated using high-resolution scanning tunneling microscopy and first principles calculations. We find that under low bias filled-state tunneling conditions, isolated 'split-off' dimers in these defect complexes are imaged as pairs of protrusions while the surrounding Si surface dimers appear as the usual 'bean-shaped' protrusions. We attribute this to the formation of pi-bonds between the two atoms of the split-off dimer and second layer atoms, and present charge density plots to support this assignment. We observe a local brightness enhancement due to strain for different DV complexes and provide the first experimental confirmation of an earlier prediction that the 1+2-DV induces less surface strain than other DV complexes. Finally, we present a previously unreported triangular shaped split-off dimer defect complex that exists at SB-type step edges, and propose a structure for this defect involving a bound Si monomer.Comment: 8 pages, 7 figures, submitted to Phys. Rev.

Reaction paths of phosphine dissociation on silicon (001)

Using density functional theory and guided by extensive scanning tunneling microscopy (STM) image data, we formulate a detailed mechanism for the dissociation of phosphine (PH3) molecules on the Si(001) surface at room temperature. We distinguish between a main sequence of dissociation that involves PH2+H, PH+2H, and P+3H as observable intermediates, and a secondary sequence that gives rise to PH+H, P+2H, and isolated phosphorus adatoms. The latter sequence arises because PH2 fragments are surprisingly mobile on Si(001) and can diffuse away from the third hydrogen atom that makes up the PH3 stoichiometry. Our calculated activation energies describe the competition between diffusion and dissociation pathways and hence provide a comprehensive model for the numerous adsorbate species observed in STM experiments

The Ruegeria pomeroyi acuI Gene Has a Role in DMSP Catabolism and Resembles yhdH of E. coli and Other Bacteria in Conferring Resistance to Acrylate

The Escherichia coli YhdH polypeptide is in the MDR012 sub-group of medium chain reductase/dehydrogenases, but its biological function was unknown and no phenotypes of YhdH− mutants had been described. We found that an E. coli strain with an insertional mutation in yhdH was hyper-sensitive to inhibitory effects of acrylate, and, to a lesser extent, to those of 3-hydroxypropionate. Close homologues of YhdH occur in many Bacterial taxa and at least two animals. The acrylate sensitivity of YhdH− mutants was corrected by the corresponding, cloned homologues from several bacteria. One such homologue is acuI, which has a role in acrylate degradation in marine bacteria that catabolise dimethylsulfoniopropionate (DMSP) an abundant anti-stress compound made by marine phytoplankton. The acuI genes of such bacteria are often linked to ddd genes that encode enzymes that cleave DMSP into acrylate plus dimethyl sulfide (DMS), even though these are in different polypeptide families, in unrelated bacteria. Furthermore, most strains of Roseobacters, a clade of abundant marine bacteria, cleave DMSP into acrylate plus DMS, and can also demethylate it, using DMSP demethylase. In most Roseobacters, the corresponding gene, dmdA, lies immediately upstream of acuI and in the model Roseobacter strain Ruegeria pomeroyi DSS-3, dmdA-acuI were co-regulated in response to the co-inducer, acrylate. These observations, together with findings by others that AcuI has acryloyl-CoA reductase activity, lead us to suggest that YdhH/AcuI enzymes protect cells against damaging effects of intracellular acryloyl-CoA, formed endogenously, and/or via catabolising exogenous acrylate. To provide “added protection” for bacteria that form acrylate from DMSP, acuI was recruited into clusters of genes involved in this conversion and, in the case of acuI and dmdA in the Roseobacters, their co-expression may underpin an interaction between the two routes of DMSP catabolism, whereby the acrylate product of DMSP lyases is a co-inducer for the demethylation pathway

Evolution of breeding plumages in birds: A multiple-step pathway to seasonal dichromatism in New World warblers (Aves: Parulidae)

Ecology and Evolution published by John Wiley & Sons Ltd Many species of birds show distinctive seasonal breeding and nonbreeding plumages. A number of hypotheses have been proposed for the evolution of this seasonal dichromatism, specifically related to the idea that birds may experience variable levels of sexual selection relative to natural selection throughout the year. However, these hypotheses have not addressed the selective forces that have shaped molt, the underlying mechanism of plumage change. Here, we examined relationships between life-history variation, the evolution of a seasonal molt, and seasonal plumage dichromatism in the New World warblers (Aves: Parulidae), a family with a remarkable diversity of plumage, molt, and life-history strategies. We used phylogenetic comparative methods and path analysis to understand how and why distinctive breeding and nonbreeding plumages evolve in this family. We found that color change alone poorly explains the evolution of patterns of biannual molt evolution in warblers. Instead, molt evolution is better explained by a combination of other life-history factors, especially migration distance and foraging stratum. We found that the evolution of biannual molt and seasonal dichromatism is decoupled, with a biannual molt appearing earlier on the tree, more dispersed across taxa and body regions, and correlating with separate life-history factors than seasonal dichromatism. This result helps explain the apparent paradox of birds that molt biannually but show breeding plumages that are identical to the nonbreeding plumage. We find support for a two-step process for the evolution of distinctive breeding and nonbreeding plumages: That prealternate molt evolves primarily under selection for feather renewal, with seasonal color change sometimes following later. These results reveal how life-history strategies and a birds\u27 environment act upon multiple and separate feather functions to drive the evolution of feather replacement patterns and bird coloration

Methanethiol-dependent dimethylsulfide production in soil environments

Dimethylsulfide (DMS) is an environmentally important trace gas with roles in sulfur cycling, signalling to higher organisms and in atmospheric chemistry. DMS is believed to be predominantly produced in marine environments via microbial degradation of the osmolyte dimethylsulfoniopropionate (DMSP). However, significant amounts of DMS are also generated from terrestrial environments, for example, peat bogs can emit ~6 μmol DMS m−2 per day, likely via the methylation of methanethiol (MeSH). A methyltransferase enzyme termed ‘MddA’, which catalyses the methylation of MeSH, generating DMS, in a wide range of bacteria and some cyanobacteria, may mediate this process, as the mddA gene is abundant in terrestrial metagenomes. This is the first study investigating the functionality of MeSH-dependent DMS production (Mdd) in a wide range of aerobic environments. All soils and marine sediment samples tested produced DMS when incubated with MeSH. Cultivation-dependent and cultivation-independent methods were used to assess microbial community changes in response to MeSH addition in a grassland soil where 35.9% of the bacteria were predicted to contain mddA. Bacteria of the genus Methylotenera were enriched in the presence of MeSH. Furthermore, many novel Mdd+ bacterial strains were isolated. Despite the abundance of mddA in the grassland soil, the Mdd pathway may not be a significant source of DMS in this environment as MeSH addition was required to detect DMS at only very low conversion rates

Understanding psychiatric institutionalization: a conceptual review

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