ORCID Profile
0000-0003-3476-2348
Current Organisations
University of Melbourne
,
CSIRO Clayton
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Publisher: arXiv
Date: 2022
Publisher: Springer Science and Business Media LLC
Date: 25-05-2023
Publisher: Wiley
Date: 11-02-2013
Publisher: American Chemical Society (ACS)
Date: 22-04-2013
DOI: 10.1021/JP311012M
Publisher: American Physical Society (APS)
Date: 22-10-2012
Publisher: Springer New York
Date: 2013
Publisher: Wiley
Date: 12-02-2021
Abstract: Metalated phthalocyanines (Pc's) are robust and versatile molecular complexes, whose properties can be tuned by changing their functional groups and central metal atom. The electronic structure of magnesium Pc (MgPc)—structurally and electronically similar to chlorophyll—adsorbed on the Ag(100) surface is investigated by low‐temperature scanning tunneling microscopy and spectroscopy, non‐contact atomic force microscopy, and density functional theory. Single, isolated MgPc's exhibit a flat, fourfold rotationally symmetric morphology, with doubly degenerate, partially populated (due to surface‐to‐molecule electron transfer) lowest unoccupied molecular orbitals (LUMOs). In contrast, MgPc's with neighbouring molecules in proximity undergo a lift of LUMOs degeneracy, with a near‐Fermi local density of states with reduced twofold rotational symmetry, indicative of a long‐range attractive intermolecular interaction. The latter is assigned to a surface‐mediated two‐step electronic hybridization process. First, LUMOs interact with Ag(100) conduction electrons, forming hybrid molecule‐surface orbitals with enhanced spatial extension. Then, these delocalized molecule‐surface states further hybridize with those of neighbouring molecules. This work highlights how the electronic structure of molecular adsorbates—including orbital degeneracies and symmetries—can be significantly altered via surface‐mediated intermolecular hybridization, over extended distances (beyond 3 nm), having important implications for prospects of molecule‐based solid‐state technologies.
Publisher: IOP Publishing
Date: 09-01-2014
DOI: 10.1088/0957-4484/25/5/055207
Abstract: Tailoring of electronic and optical properties of self-assembled InAs quantum dots (QDs) is a critical limit for the design of several QD-based optoelectronic devices operating in the telecom frequency range. We describe how fine control of the strain-induced surface kinetics during the growth of vertically stacked multiple layers of QDs allows for the engineering of their self-organization process. Most noticeably, this study shows that the underlying strain field induced along a QD stack can be modulated and controlled by time-dependent intermixing and segregation effects occurring after capping with a GaAs spacer. This leads to a drastic increase of the TM/TE polarization ratio of emitted light, not accessible from conventional growth parameters. Our detailed experimental measurements, supported by comprehensive multi-million atom simulations of strain, electronic and optical properties, provide in-depth analysis of the grown QD s les allowing us to give a clear picture of the atomic scale phenomena affecting the proposed growth dynamics and consequent QD polarization response.
Publisher: American Physical Society (APS)
Date: 09-12-2011
Publisher: AIP Publishing
Date: 15-05-2011
DOI: 10.1063/1.3587167
Abstract: The design of some optical devices, such as semiconductor optical lifiers for telecommunication applications, requires polarization-insensitive optical emission at long wavelengths (1300–1550 nm). Self-assembled InAs/GaAs quantum dots (QDs) typically exhibit ground state optical emissions at wavelengths shorter than 1300 nm with highly polarization-sensitive characteristics, although this can be modified by the use of low growth rates, the incorporation of strain-reducing capping layers, or the growth of closely-stacked QD layers. Exploiting the strain interactions between closely stacked QD layers also affords greater freedom in the choice of growth conditions for the upper layers, so that both a significant extension in their emission wavelength and an improved polarization response can be achieved due to modification of the QD size, strain, and composition. In this paper, we investigate the polarization behavior of single and stacked QD layers using room temperature sub-lasing-threshold electroluminescence and photovoltage measurements, as well as atomistic modeling with the NEMO 3-D simulator. A reduction is observed in the ratio of the transverse electric (TE) to transverse magnetic (TM) optical mode response for a GaAs-capped QD stack as compared to a single QD layer, but when the second layer of the two-layer stack is InGaAs-capped, an increase in the TE/TM ratio is observed, in contrast to recent reports for single QD layers.
Publisher: Elsevier BV
Date: 09-2012
Publisher: Royal Society of Chemistry (RSC)
Date: 2015
DOI: 10.1039/C5NR04710B
Abstract: An electric field transforms molecular states to atomic states.
Publisher: Springer New York
Date: 2009
Publisher: Springer Science and Business Media LLC
Date: 06-06-2016
Abstract: Scaling of Si-based nanoelectronics has reached the regime where device function is affected not only by the presence of in idual dopants, but also by their positions in the crystal. Determination of the precise dopant location is an unsolved problem in applications from channel doping in ultrascaled transistors to quantum information processing. Here, we establish a metrology combining low-temperature scanning tunnelling microscopy (STM) imaging and a comprehensive quantum treatment of the dopant-STM system to pinpoint the exact coordinates of the dopant in the Si crystal. The technique is underpinned by the observation that STM images contain atomic-sized features in ordered patterns that are highly sensitive to the STM tip orbital and the absolute dopant lattice site. The demonstrated ability to determine the locations of P and As dopants to 5 nm depths will provide critical information for the design and optimization of nanoscale devices for classical and quantum computing applications.
Publisher: AIP Publishing
Date: 14-09-2009
DOI: 10.1063/1.3226108
Abstract: Time-resolved photoluminescence measurements performed on proton implanted and annealed GaN layers have shown that carrier lifetime can be tuned over two orders of magnitude and, at implantation dose of 1×1015 cm−2, decreases down to a few picoseconds. With annealing at temperatures between 250 and 750 °C, carrier lifetime, contrary to electrical characteristics, is only slightly restored, indicating that electrical compensation and carrier dynamics are governed by different defects. Ga vacancies, free and bound at threading dislocations, are suggested as the most probable defects, responsible for electrical compensation and carrier lifetime quenching.
Publisher: Wiley
Date: 15-08-2047
Publisher: IEEE
Date: 06-2011
Publisher: IEEE
Date: 13-09-2021
Publisher: IOP Publishing
Date: 09-08-2023
Abstract: Quantum computing is poised to solve practically useful problems which are computationally intractable for classical supercomputers. However, the current generation of quantum computers are limited by errors that may only partially be mitigated by developing higher-quality qubits. Quantum error correction (QEC) will thus be necessary to ensure fault tolerance. QEC protects the logical information by cyclically measuring syndrome information about the errors. An essential part of QEC is the decoder, which uses the syndrome to compute the likely effect of the errors on the logical degrees of freedom and provide a tentative correction. The decoder must be accurate, fast enough to keep pace with the QEC cycle (e.g. on a microsecond timescale for superconducting qubits) and with hard real-time system integration to support logical operations. As such, real-time decoding is essential to realize fault-tolerant quantum computing and to achieve quantum advantage. In this work, we highlight some of the key challenges facing the implementation of real-time decoders while providing a succinct summary of the progress to-date. Furthermore, we lay out our perspective for the future development and provide a possible roadmap for the field of real-time decoding in the next few years. As the quantum hardware is anticipated to scale up, this perspective article will provide a guidance for researchers, focusing on the most pressing issues in real-time decoding and facilitating the development of solutions across quantum, nano and computer science.
Publisher: IEEE
Date: 05-2009
Publisher: AIP Publishing
Date: 11-2011
DOI: 10.1063/1.3657783
Abstract: The design of optical devices such as lasers and semiconductor optical lifiers for telecommunication applications requires polarization insensitive optical emissions in the region of 1500 nm. Recent experimental measurements of the optical properties of stacked quantum dots have demonstrated that this can be achieved via exploitation of inter-dot strain interactions. In particular, the relatively large aspect ratio (AR) of quantum dots in the optically active layers of such stacks provide a two-fold advantage, both by inducing a red shift of the gap wavelength above 1300 nm, and increasing the TM001-mode, thereby decreasing the anisotropy of the polarization response. However, in large aspect ratio quantum dots (AR & 0.25), the hole confinement is significantly modified compared with that in lower AR dots—this modified confinement is manifest in the interfacial confinement of holes in the system. Since the contributions to the ground state optical intensity (GSOI) are dominated by lower-lying valence states, we therefore propose that the room temperature GSOI be a cumulative sum of optical transitions from multiple valence states. This then extends previous theoretical studies of flat (low AR) quantum dots, in which contributions arising only from the highest valence state or optical transitions between in idual valence states were considered. The interfacial hole distributions also increases in-plane anisotropy in tall (high AR) quantum dots (TE110 ≠ TE−110), an effect that has not been previously observed in flat quantum dots. Thus, a directional degree of polarization (DOP) should be measured (or calculated) to fully characterize the polarization response of quantum dot stacks. Previous theoretical and experimental studies have considered only a single value of DOP: either [110] or [−110].
Publisher: Springer New York
Date: 08-09-2013
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 2023
Publisher: AIP
Date: 2007
DOI: 10.1063/1.2730156
Publisher: IOP Publishing
Date: 24-08-2023
Abstract: Machine learning is among the most widely anticipated use cases for near-term quantum computers, however there remain significant theoretical and implementation challenges impeding its scale up. In particular, there is an emerging body of work which suggests that generic, data agnostic quantum machine learning (QML) architectures may suffer from severe trainability issues, with the gradient of typical variational parameters vanishing exponentially in the number of qubits. Additionally, the high expressibility of QML models can lead to overfitting on training data and poor generalisation performance. A promising strategy to combat both of these difficulties is to construct models which explicitly respect the symmetries inherent in their data, so-called geometric quantum machine learning (GQML). In this work, we utilise the techniques of GQML for the task of image classification, building new QML models which are equivariant with respect to reflections of the images. We find that these networks are capable of consistently and significantly outperforming generic ansatze on complicated real-world image datasets, bringing high-resolution image classification via quantum computers closer to reality. Our work highlights a potential pathway for the future development and implementation of powerful QML models which directly exploit the symmetries of data.
Publisher: SAGE Publications
Date: 07-2013
Abstract: This article researches the influence of IJEEE on electrical engineering and electrical engineering education as a discipline. For this purpose, the history of this journal has been presented from a citation perspective. To identify leading and evolving research areas within IJEEE the authors conducted keyword analysis, which additionally showed how IJEEE contains both educational and technical contributions. The authors also studied the temporal evolution and distribution of keywords. Word co-occurrence was analysed to discover the main context in which the keywords have been used. The analysis also revealed the prominent contributors within the community of IJEEE based on various authorship and citation criteria. It was observed that the influential authors appear in multiple ways, i.e. most of the authors who were influential by one criterion also made to the top list of other criteria. The authors concluded that the single-author pattern is quite prominent within this community, and very little work has been done between the same co-authors. Therefore, there is a need to encourage IJEEE authors to write more collaborative publications so that the authorship/co-authorship network may grow.
Publisher: Royal Society of Chemistry (RSC)
Date: 2022
DOI: 10.1039/D2NA00385F
Abstract: Non-covalent intermolecular hybridization in a 2D molecular self-assembly gives rise to a narrow electronic energy band, a promising prospect for organic nanoelectronics.
Publisher: IEEE
Date: 08-2008
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D1NR04858A
Abstract: Scanning tunnelling microscopy (STM) images are shown to depend on the momentum space features of s le electronic states, with important implications for material design and characterisation, and applications in quantum computing, quantum sensing and photonics.
Publisher: American Physical Society (APS)
Date: 27-08-2018
Publisher: American Physical Society (APS)
Date: 02-05-2018
Publisher: American Physical Society (APS)
Date: 25-06-2015
Publisher: arXiv
Date: 2022
Publisher: Royal Society of Chemistry (RSC)
Date: 2019
DOI: 10.1039/C9NR07237C
Abstract: Million-atom atomistic simulations propose custom-designed GaBi x As 1−x /GaAs core–shell nanowires with the possibility of myriad applications in nanoelectronic and nanophotonic technologies.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 05-2009
Publisher: AIP Publishing
Date: 04-2013
DOI: 10.1063/1.4799149
Abstract: High brightness light emissions from single InAs/GaAs quantum dots (QDs) epitaxially grown directly on silicon substrates are realized for the first time. The grown structures contain a low quantum dot density of about 108 cm−2. Low-temperature spatially resolved photoluminescence (PL) measurements illustrate bright single QD emissions with relatively sharp excitonic lines comparable to PL spectra of near-surface QDs grown on GaAs substrates.
Publisher: American Physical Society (APS)
Date: 23-06-2023
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 09-2007
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 2023
Publisher: American Physical Society (APS)
Date: 04-11-2014
Publisher: American Physical Society (APS)
Date: 23-09-2011
Publisher: Springer Science and Business Media LLC
Date: 18-03-2015
Publisher: IOP Publishing
Date: 08-07-2011
DOI: 10.1088/0957-4484/22/31/315709
Abstract: Atomistic electronic structure calculations are performed to study the coherent inter-dot couplings of the electronic states in a single InGaAs quantum dot molecule. The experimentally observed excitonic spectrum by Krenner et al (2005) Phys. Rev. Lett. 94 057402 is quantitatively reproduced, and the correct energy states are identified based on a previously validated atomistic tight binding model. The extended devices are represented explicitly in space with 15-million-atom structures. An excited state spectroscopy technique is applied where the externally applied electric field is swept to probe the ladder of the electronic energy levels (electron or hole) of one quantum dot through anti-crossings with the energy levels of the other quantum dot in a two-quantum-dot molecule. This technique can be used to estimate the spatial electron-hole spacing inside the quantum dot molecule as well as to reverse engineer quantum dot geometry parameters such as the quantum dot separation. Crystal-deformation-induced piezoelectric effects have been discussed in the literature as minor perturbations lifting degeneracies of the electron excited (P and D) states, thus affecting polarization alignment of wavefunction lobes for III-V heterostructures such as single InAs/GaAs quantum dots. In contrast, this work demonstrates the crucial importance of piezoelectricity to resolve the symmetries and energies of the excited states through matching the experimentally measured spectrum in an InGaAs quantum dot molecule under the influence of an electric field. Both linear and quadratic piezoelectric effects are studied for the first time for a quantum dot molecule and demonstrated to be indeed important. The net piezoelectric contribution is found to be critical in determining the correct energy spectrum, which is in contrast to recent studies reporting vanishing net piezoelectric contributions.
Publisher: American Physical Society (APS)
Date: 09-10-2018
Publisher: AIP
Date: 2010
DOI: 10.1063/1.3295541
Publisher: AIP Publishing
Date: 17-06-2019
DOI: 10.1063/1.5088824
Abstract: An atomistic band anticrossing (BAC) model is developed and used to study “unusual bowing” in energy bandgap and its dependence on the material composition in minority O anion-alloyed ZnS (ZnS1−xOx) and minority S anion-alloyed ZnO (ZnO1−xSx) highly mismatched alloys. For dilute O in ZnS1−xOx, it is found that the bandgap decreases as the O composition is increased. A “down-shift” in the conduction band edge (CBE) of host ZnS, which arises from an interaction between the CBE and the localized O defect state, is identified as the root cause. However, the reduction in bandgap as a function of dilute S composition in the ZnO1−xSx alloy follows an “up-shift” in the valence band edge (VBE) of host ZnO, which arises from an interaction between the VBE and the localized S defect state. The BAC model captures the E+ and E− splitting in the sub-bands, which are found to be an admixture of the extended CBE (VBE) of ZnS (ZnO) and the localized O (S) state. A fully atomistic 8-band sp3-spin tight-binding basis set is used to construct the Hamiltonian for the wurtzite host materials as well as their alloy supercells. For alloy supercells, a strain is computed via the valence force-field formalism using Keating potentials. The O and S energy states are found to be approximately 199 meV below the CBE of ZnS and 190 meV above the VBE of ZnO, respectively. Overall, the calculated energy bandgaps using the BAC model are in good agreement with corrected local density approximation (LDA+U) calculations and experimental results.
Publisher: Springer New York
Date: 2014
Publisher: American Physical Society (APS)
Date: 25-02-2022
Publisher: Wiley
Date: 19-11-2012
Publisher: AIP Publishing
Date: 17-11-2015
DOI: 10.1063/1.4865827
Abstract: Large-supercell tight-binding calculations are presented for GaBixAs1−x/GaAs single quantum wells (QWs) with Bi fractions x of 3.125% and 12.5%. Our results highlight significant distortion of the valence band states due to the alloy disorder. A large full-width-half-maximum (FWHM) is estimated in the ground state interband transition energy (≈33 meV) at 3.125% Bi, consistent with recent photovoltage measurements for similar Bi compositions. Additionally, the alloy disorder effects are predicted to become more pronounced as the QW width is increased. However, they are less strong at the higher Bi composition (12.5%) required for the design of temperature-stable lasers, with a calculated FWHM of ≈23.5 meV at x = 12.5%.
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/D0NR04728G
Abstract: Addition of bismuth in GaAs core-shell nanowires allows an exquisite control over their band-gap energies and light polarisation properties, paving the way for new possibilities for future technologies in the areas of photonics, quantum electronics, and solar energy harvesting.
Publisher: American Physical Society (APS)
Date: 26-07-2018
Publisher: Wiley
Date: 07-10-2021
DOI: 10.1002/SPE.3039
Abstract: Quantum computing (QC) is an emerging paradigm with the potential to offer significant computational advantage over conventional classical computing by exploiting quantum‐mechanical principles such as entanglement and superposition. It is anticipated that this computational advantage of QC will help to solve many complex and computationally intractable problems in several application domains such as drug design, data science, clean energy, finance, industrial chemical development, secure communications, and quantum chemistry. In recent years, tremendous progress in both quantum hardware development and quantum software/algorithm has brought QC much closer to reality. Indeed, the demonstration of quantum supremacy marks a significant milestone in the Noisy Intermediate Scale Quantum (NISQ) era—the next logical step being the quantum advantage whereby quantum computers solve a real‐world problem much more efficiently than classical computing. As the quantum devices are expected to steadily scale up in the next few years, quantum decoherence and qubit interconnectivity are two of the major challenges to achieve quantum advantage in the NISQ era. QC is a highly topical and fast‐moving field of research with significant ongoing progress in all facets. A systematic review of the existing literature on QC will be invaluable to understand the state‐of‐the‐art of this emerging field and identify open challenges for the QC community to address in the coming years. This article presents a comprehensive review of QC literature and proposes taxonomy of QC. The proposed taxonomy is used to map various related studies to identify the research gaps. A detailed overview of quantum software tools and technologies, post‐quantum cryptography, and quantum computer hardware development captures the current state‐of‐the‐art in the respective areas. The article identifies and highlights various open challenges and promising future directions for research and innovation in QC.
Publisher: arXiv
Date: 2022
Publisher: American Physical Society (APS)
Date: 10-04-2018
Publisher: IOP Publishing
Date: 18-07-2019
Publisher: American Physical Society (APS)
Date: 10-02-2014
Publisher: Springer Science and Business Media LLC
Date: 30-11-2020
DOI: 10.1038/S41467-020-19835-1
Abstract: Tunneling is a fundamental quantum process with no classical equivalent, which can compete with Coulomb interactions to give rise to complex phenomena. Phosphorus dopants in silicon can be placed with atomic precision to address the different regimes arising from this competition. However, they exploit wavefunctions relying on crystal band symmetries, which tunneling interactions are inherently sensitive to. Here we directly image lattice-aperiodic valley interference between coupled atoms in silicon using scanning tunneling microscopy. Our atomistic analysis unveils the role of envelope anisotropy, valley interference and dopant placement on the Heisenberg spin exchange interaction. We find that the exchange can become immune to valley interference by engineering in-plane dopant placement along specific crystallographic directions. A vacuum-like behaviour is recovered, where the exchange is maximised to the overlap between the donor orbitals, and pair-to-pair variations limited to a factor of less than 10 considering the accuracy in dopant positioning. This robustness remains over a large range of distances, from the strongly Coulomb interacting regime relevant for high-fidelity quantum computation to strongly coupled donor arrays of interest for quantum simulation in silicon.
Publisher: IOP Publishing
Date: 06-04-2022
Abstract: Strain is extensively used to controllably tailor the electronic properties of materials. In the context of indirect band-gap semiconductors such as silicon, strain lifts the valley degeneracy of the six conduction band minima, and by extension the valley states of electrons bound to phosphorus donors. Here, single phosphorus atoms are embedded in an engineered thin layer of silicon strained to 0.8% and their wave function imaged using spatially resolved spectroscopy. A prevalence of the out-of-plane valleys is confirmed from the real-space images, and a combination of theoretical modelling tools is used to assess how this valley repopulation effect can yield isotropic exchange and tunnel interactions in the xy -plane relevant for atomically precise donor qubit devices. Finally, the residual presence of in-plane valleys is evidenced by a Fourier analysis of both experimental and theoretical images, and atomistic calculations highlight the importance of higher orbital excited states to obtain a precise relationship between valley population and strain. Controlling the valley degree of freedom in engineered strained epilayers provides a new competitive asset for the development of donor-based quantum technologies in silicon.
Publisher: arXiv
Date: 2022
Publisher: American Physical Society (APS)
Date: 05-03-2013
Publisher: Unpublished
Date: 2021
Publisher: Springer Science and Business Media LLC
Date: 16-03-2020
DOI: 10.1038/S41524-020-0282-0
Abstract: Atomic-level qubits in silicon are attractive candidates for large-scale quantum computing however, their quantum properties and controllability are sensitive to details such as the number of donor atoms comprising a qubit and their precise location. This work combines machine learning techniques with million-atom simulations of scanning tunnelling microscopic (STM) images of dopants to formulate a theoretical framework capable of determining the number of dopants at a particular qubit location and their positions with exact lattice site precision. A convolutional neural network (CNN) was trained on 100,000 simulated STM images, acquiring a characterisation fidelity (number and absolute donor positions) of % over a set of 17,600 test images including planar and blurring noise commensurate with experimental measurements. The formalism is based on a systematic symmetry analysis and feature-detection processing of the STM images to optimise the computational efficiency. The technique is demonstrated for qubits formed by single and pairs of closely spaced donor atoms, with the potential to generalise it for larger donor clusters. The method established here will enable a high-precision post-fabrication characterisation of dopant qubits in silicon, with high-throughput potentially alleviating the requirements on the level of resources required for quantum-based characterisation, which will otherwise be a challenge in the context of large qubit arrays for universal quantum computing.
Publisher: IOP Publishing
Date: 22-08-2012
Publisher: American Chemical Society (ACS)
Date: 12-06-2020
Publisher: IOP Publishing
Date: 18-03-2015
DOI: 10.1088/0953-8984/27/15/154207
Abstract: Atomistic tight-binding (TB) simulations are performed to calculate the Stark shift of the hyperfine coupling for a single arsenic (As) donor in silicon (Si). The role of the central-cell correction is studied by implementing both the static and the non-static dielectric screenings of the donor potential, and by including the effect of the lattice strain close to the donor site. The dielectric screening of the donor potential tunes the value of the quadratic Stark shift parameter (η2) from -1.3 × 10(-3) µm(2) V(-2) for the static dielectric screening to -1.72 × 10(-3) µm(2) V(-2) for the non-static dielectric screening. The effect of lattice strain, implemented by a 3.2% change in the As-Si nearest-neighbour bond length, further shifts the value of η2 to -1.87 × 10(-3) µm(2) V(-2), resulting in an excellent agreement of theory with the experimentally measured value of -1.9 ± 0.2 × 10(-3) µm(2) V(-2). Based on our direct comparison of the calculations with the experiment, we conclude that the previously ignored non-static dielectric screening of the donor potential and the lattice strain significantly influence the donor wave function charge density and thereby leads to a better agreement with the available experimental data sets.
Publisher: IEEE
Date: 2007
Publisher: IOP Publishing
Date: 20-11-2013
Publisher: Elsevier BV
Date: 06-2021
Publisher: Royal Society of Chemistry (RSC)
Date: 2017
DOI: 10.1039/C7NR05081J
Abstract: High-precision physics modeling at the atomic scale indicates potential for direct observation of central-cell-effects in scanning tunnelling microscope images of single dopant wave functions.
No related grants have been discovered for Muhammad Usman.