330 research outputs found
Vacancy in silicon revisited: structure and pressure effects
FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOThe structure of the single vacancy in silicon. one of the most common point defects and an important mediator of atomic diffusion, is examined through extensive first principles calculations. We find a hitherto unexpected result, namely that there exist two distinct distortions associated with the vacancy with essentially identical formation energies at zero pressure. The two distortions are distinguished by their different relaxations, volumes of formation, and :formation enthalpies. We discuss how, at finite pressure, one of the two distortions should become dominant, and suggest experimental tests of this effect.The structure of the single vacancy in silicon, one of the most common point defects and an important mediator of atomic diffusion, is examined through extensive first principles calculations. We find a hitherto unexpected result, namely that there exist two distinct distortions associated with the vacancy with essentially identical formation energies at zero pressure. The two distortions are distinguished by their different relaxations, volumes of formation, and formation enthalpies. We discuss how, at finite pressure, one of the two distortions should become dominant, and suggest experimental tests of this effect.811020882091FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOSem informaçãoSem informaçãoWe thank Professor M. J. Aziz for useful discussions throughout this work and helpful comments on the manuscript. A. A. wishes to thank Harvard University for its hospitality during the course of this work, and to acknowledge partial support from the Brazilian funding agencies Fundo de Apoio ao Ensino e à Pesquisa (FAEP/UNICAMP), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). Computer calculations were performed at the facilities of the Centro Nacional de Processamento de Alto Desempenho em São Paulo (CENAPAD-SP). The work at Harvard University was supported the Materials Science and Engineering Center, which is funded by the National Science Foundation
Recommended from our members
Microscopic Model of Heteroepitaxy of GaAs on Si(100)
A new microscopic model of heteroepitaxial growth is introduced using GaAs on Si(100) as a prototype. This model takes into account specific features of surface topology, predicts that in the prototype system conventional two-dimensional epitaxy should be inhibited, and provides a fundamental explanation for three-dimensional nature of the initial stages of growth. The ingredients of the model, which are supported by total-energy calculations, include new structural geometries for each state of growth and the chemical and rehybridization reactions linking these stages.Engineering and Applied Science
Recommended from our members
Hydrogenation of Semiconductor Surfaces: Si and Ge (111)
The relaxations of hydrogenated Si and Ge (111) surfaces are determined using ab initio self-consistent calculations in a slab configuration. The Si-H and Ge-H bonds are found to be considerably larger than the sum of covalent radii. The substrate relaxations are small and their physical origin can be explained in terms of electronic charge transfer which eliminates the surface dipole moment. The calculated frequencies of the hydrogen vibrational modes are in excellent agreement with experiment. A surface-atom vibrational mode is compared to similar modes in the amorphous hydrogenated materials. The comparison predicts that internal surfaces (microvoids) in the amorphous network are locally much softer than the corresponding crystalline surface configuration.Engineering and Applied Science
Modulating the electrochemical intercalation of graphene interfaces with -RuCl as a solid-state electron acceptor
Intercalation reactions modify the charge density in van der Waals (vdW)
materials through coupled electronic-ionic charge accumulation, and are
susceptible to modulation by interlayer hybridization in vdW heterostructures.
Here, we demonstrate that charge transfer between graphene and
-RuCl, which dopes the graphene positively, greatly favors the
intercalation of lithium ions into graphene-based vdW heterostructures. We
systematically tune this effect on Li ion intercalation, modulating the
intercalation potential, by using varying thicknesses of hexagonal boron
nitride (hBN) as spacer layers between graphene and -RuCl. Confocal
Raman spectroscopy and electronic transport measurements are used to monitor
electrochemical intercalation and density functional theory computations help
quantify charge transfer to both -RuCl and graphene upon Li
intercalation. This work demonstrates a versatile approach for systematically
modulating the electrochemical intercalation behavior of two-dimensional layers
akin to electron donating/withdrawing substituent effects used to tune
molecular redox potentials
Spinal cord compression by a solitary metastasis from a low grade leydig cell tumour: a case report and review of the literature
<p>Abstract</p> <p>Background</p> <p>Leydig tumour is rare and there are only three cases with metastatic disease reported.</p> <p>Case presentation</p> <p>A 52 year-old Caucasian male was admitted, on emergency basis to the Orthopaedic Department with six weeks history of increasing midthoracic back pain, change in gait, poor balance, subjective weakness and numbness of the lower trunk and legs. MRI scan showed change in the signal intensity of T4 and T5 vertebral body but their height were maintained. Urgent T4 and T5 corpectomies, decompression of the spinal cord and reconstruction of the vertebral bodies were performed followed by radiotherapy. Neurological status significantly improved with a mild residual numbness over the dorsum of the right foot. The histology of the excised tumour was identical to the primary. At 2 years follow-up visit the patient is neurologically stable and disease free without other organs metastases.</p> <p>Conclusion</p> <p>This is the first case in English literature, which shows that spinal metastases could occur even in the early stage of Leydig cell tumour, without other organs involvement. Aggressive surgical management of spinal metastases combined with post operative radiotherapy can give a better chance for long survivorship.</p
Variational finite-difference representation of the kinetic energy operator
A potential disadvantage of real-space-grid electronic structure methods is
the lack of a variational principle and the concomitant increase of total
energy with grid refinement. We show that the origin of this feature is the
systematic underestimation of the kinetic energy by the finite difference
representation of the Laplacian operator. We present an alternative
representation that provides a rigorous upper bound estimate of the true
kinetic energy and we illustrate its properties with a harmonic oscillator
potential. For a more realistic application, we study the convergence of the
total energy of bulk silicon using a real-space-grid density-functional code
and employing both the conventional and the alternative representations of the
kinetic energy operator.Comment: 3 pages, 3 figures, 1 table. To appear in Phys. Rev. B. Contribution
for the 10th anniversary of the eprint serve
Variational Hilbert space truncation approach to quantum Heisenberg antiferromagnets on frustrated clusters
We study the spin- Heisenberg antiferromagnet on a series of
finite-size clusters with features inspired by the fullerenes. Frustration due
to the presence of pentagonal rings makes such structures challenging in the
context of quantum Monte-Carlo methods. We use an exact diagonalization
approach combined with a truncation method in which only the most important
basis states of the Hilbert space are retained. We describe an efficient
variational method for finding an optimal truncation of a given size which
minimizes the error in the ground state energy. Ground state energies and
spin-spin correlations are obtained for clusters with up to thirty-two sites
without the need to restrict the symmetry of the structures. The results are
compared to full-space calculations and to unfrustrated structures based on the
honeycomb lattice.Comment: 22 pages and 12 Postscript figure
- …