| 31. |
- Gällström, Andreas, et al.
(författare)
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A defect center for quantum computing : Mo in SiC
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The electronic structure and vibrational properties of molybdenum (Mo) in SiC are analyzed and investigated in detail. Mo is considered as occupying the silicon-carbon divacancy in the so-called asymmetric split vacancy (ASV) configuration. Group-theoretical considerations within this model are used to explain the experimental results (optical properties and behavior in magnetic field). The vibrational properties of the defect are studied using simple the “defect molecule” model with parameters determined phenomenologically from the experimental data. The position of Mo in the ASV configuration deduced from this model is shown to be in good agreement with the earlier reported data from ab initio supercell calculations. The usefulness of molybdenum in SiC in quantum computing is investigated, and it shown that Mo is a highly promising candidate for quantum computing.
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| 32. |
- Gällström, Andreas, et al.
(författare)
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Electronic Configuration of Tungsten in 4H-, 6H-, and 15R-SiC
- 2012
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Ingår i: Materials Science Forum Vols 717 - 720. - : Trans Tech Publications Inc.. - 9783037854198 ; , s. 211-216
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Konferensbidrag (refereegranskat)abstract
- A commonly observed unidentified photoluminescence center in SiC is UD-1. In this report, the UD-1 center is identified to be tungsten related. The identification is based on (i) a W-doping study, the confirmation of W in the samples was made using deep level transient spectroscopy (DLTS), (ii) the optical activation energy of the absorption of UD-1 in weakly n-type samples corresponds to the activation energy of the deep tungsten center observed using DLTS. The tungsten-related optical centers are reported in 4H-, 6H-, and 15R-SiC. Further, a crystal field model for a tungsten atom occupying a Si-site is suggested. This crystal field model is in agreement with the experimental data available: polarization, temperature dependence and magnetic field splitting.
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| 33. |
- Gällström, Andreas, et al.
(författare)
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Optical identification and electronic configuration of tungsten in 4H-and 6H-SiC
- 2012
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Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 407:10, s. 1462-1466
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Tidskriftsartikel (refereegranskat)abstract
- Several optically observed deep level defects in SiC are still unidentified and little is published on their behavior. One of the commonly observed deep level defects in semi-insulating SiC is UD-1. less thanbrgreater than less thanbrgreater thanThis report suggests that UD-1 is Tungsten related, based on a doping study and previously reported deep level transient spectroscopy data, as well as photo-induced absorption measurements. The electronic levels involved in the optical transitions of UD-1 are also deduced. The transitions observed in the photoluminescence of UD-1 are from a Gamma(C3v)(4), to two different final states, which transform according to Gamma(C3v)(5)circle plus Gamma(C3v)(6) and Gamma(C3v)(4), respectively.
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| 34. |
- Gällström, Andreas, et al.
(författare)
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Optical properties and Zeeman spectroscopy of niobium in silicon carbide
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:7, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- The optical signature of niobium in the low-temperature photoluminescence spectra of three common polytypes of SiC (4H, 6H, and 15R) is observed and confirms the previously suggested concept that Nb occupies preferably the Si-C divacancy with both Si and C at hexagonal sites. Using this concept we propose a model considering a Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using first-principles calculation and the result is in very good agreement with the experimentally observed photon energy in 4H SiC at low temperature. The appearance of six Nb-related lines in the spectra of the hexagonal 4H and 6H polytypes at higher temperatures is tentatively explained on the grounds of the proposed model and the concept that the Nb center can exist in both C1h and C3v symmetries. The Zeeman splitting of the photoluminescence lines is also recorded in two different experimental geometries and the results are compared with theory based on phenomenological Hamiltonians. Our results show that Nb occupying the divacancy at the hexagonal site in the studied SiC polytypes behaves like a deep acceptor.
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| 35. |
- Hamdi, Hanen, et al.
(författare)
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Stone-Wales defects in hexagonal boron nitride as ultraviolet emitters
- 2020
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Ingår i: npj Computational Materials. - : Nature Research. - 2057-3960. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Many quantum emitters have been measured close or near the grain boundaries of the two-dimensional hexagonal boron nitride where various Stone-Wales defects appear. We show by means of first principles density functional theory calculations that the pentagon-heptagon Stone-Wales defect is an ultraviolet emitter and its optical properties closely follow the characteristics of a 4.08-eV quantum emitter, often observed in polycrystalline hexagonal boron nitride. We also show that the square-octagon Stone-Wales line defects are optically active in the ultraviolet region with varying gaps depending on their density in hexagonal boron nitride. Our results may introduce a paradigm shift in the identification of fluorescent centres in this material.
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| 36. |
- Isoya, J, et al.
(författare)
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Pulsed EPR studies of phosphorus shallow donors in diamond and SiC
- 2006
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Ingår i: Physica B, Vols. 376-377. - : Elsevier BV. ; , s. 358-361
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Konferensbidrag (refereegranskat)abstract
- Phosphorus shallow donors having the symmetry lower than T-d are studied by pulsed EPR. In diamond:P and 3C-SiC:P, the symmetry is lowered to D-2d and the density of the donor wave function on the phosphorus atom exhibits a predominant p-character. In 4H-SiC:P with the site symmetry of C-3v, the A(1) ground state of the phosphorus donors substituting at the quasi-cubic site of silicon shows an axial character of the distribution of the donor wave function in the vicinity of the phosphorus atom. (c) 2005 Elsevier B.V. All rights reserved.
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| 37. |
- Ivády, Viktor, et al.
(författare)
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Ab initio theory of the negatively charged boron vacancy qubit in hexagonal boron nitride
- 2020
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Ingår i: npj Computational Materials. - : Nature Publishing Group. - 2057-3960. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Highly correlated orbitals coupled with phonons in two-dimension are identified for paramagnetic and optically active boron vacancy in hexagonal boron nitride by first principles methods which are responsible for recently observed optically detected magnetic resonance signal. Here, we report ab initio analysis of the correlated electronic structure of this center by density matrix renormalization group and Kohn-Sham density functional theory methods. By establishing the nature of the bright and dark states as well as the position of the energy levels, we provide a complete description of the magneto-optical properties and corresponding radiative and non-radiative routes which are responsible for the optical spin polarization and spin dependent luminescence of the defect. Our findings pave the way toward advancing the identification and characterization of room temperature quantum bits in two-dimensional solids.
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| 38. |
- Ivady, Viktor, et al.
(författare)
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Asymmetric Split-Vacancy Defects in SiC Polytypes: A Combined Theoretical and Electron Spin Resonance Study
- 2011
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 107:19, s. 195501-
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal defects were studied in different polytypes of silicon carbide (SiC) by ab initio supercell calculations. We found asymmetric split-vacancy (ASV) complexes for these defects that preferentially form at only one site in hexagonal polytypes, and they may not be detectable at all in cubic polytype. Electron spin resonance study demonstrates the existence of ASV complex in niobium doped 4H polytype of SiC.
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| 39. |
- Ivády, Viktor
(författare)
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Development of theoretical approaches for post-silicon information processing
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Despite knowing the fundamental equations in most of the physics research areas, still there is an unceasing need for theoretical method development, thanks to the more and more challenging problems addressed by the research community. The investigation of post-silicon, non-classical information processing is one of the new and rapidly developing areas that requires tremendous amount of theoretical support, new understanding, and accurate theoretical predictions. My thesis focuses on theoretical method development for solid-state quantum information processing, mainly in the field of point defect quantum bits (qubits) in silicon carbide (SiC) and diamond. Due to recent experimental breakthroughs in this field, there are diverse theoretical problems, ranging from functional development for accurate first principles description of point defects, through complete theoretical characterization of qubits, to the modeling and simulation of actual quantum information protocols, that are needed to be addressed. The included articles of this thesis cover the development of (i) hybrid-DFT+Vw approach for the first principles description of mixed correlated and uncorrelated systems, (ii) zero-field-splitting tensor calculation for solid-state quantum bit characterization, (iii) a comprehensive model for dynamic nuclear spin polarization of solid-state qubits in semiconductors, and (iv) group theoretical description of qubits and novel twodimensional materials for topologically protected states.
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| 40. |
- Ivády, Viktor, et al.
(författare)
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First principles calculation of spin-related quantities for point defect qubit research
- 2018
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Ingår i: npj Computational Materials. - : SPRINGERNATURE. - 2057-3960. ; 4
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Forskningsöversikt (refereegranskat)abstract
- Point defect research in semiconductors has gained remarkable new momentum due to the identification of special point defects that can implement qubits and single photon emitters with unique characteristics. Indeed, these implementations are among the few alternatives for quantum technologies that may operate even at room temperature, and therefore discoveries and characterization of novel point defects may highly facilitate future solid state quantum technologies. First principles calculations play an important role in point defect research, since they provide a direct, extended insight into the formation of the defect states. In the last decades, considerable efforts have been made to calculate spin-dependent properties of point defects from first principles. The developed methods have already demonstrated their essential role in quantitative understanding of the physics and application of point defect qubits. Here, we review and discuss accuracy aspects of these novel ab initio methods and report on their most relevant applications for existing point defect qubits in semiconductors. We pay attention to the advantages and limitations of the methodological solutions and highlight additional developments that are expected in the near future. Moreover, we discuss the opportunity of a systematic search for potential point defect qubits, as well as the possible development of predictive spin dynamic simulations facilitated by ab initio calculations of spin-dependent quantities.
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