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“The influence of the 6s², configuration of Bi³+ on the structures of A ' BiNb₂O₇, (A ' = Rb, Na, Li) layered perovskite oxides”. Mallick S, Khalsa G, Kaaret JZ, Zhang W, Batuk M, Gibbs AS, Hadermann J, Halasyamani PS, Benedek NA, Hayward MA, Journal of the Chemical Society : Dalton transactions 50, 15359 (2021). http://doi.org/10.1039/D1DT02974F
Abstract: Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The 'hybrid improper' mechanism – in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure – offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb2O7, LiBiNb2O7 and NaBiNb2O7, which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi3+ cations which are often observed to stabilize acentric crystal structures due to their 6s(2) electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb2O7 and LiBiNb2O7 adopt polar crystal structures (space groups I2cm and B2cm respectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi3+ cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced by the observation that replacing the Bi3+ cations with Nd3+ cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb2O7 (space group P2(1)2(1)2(1)) differs significantly from the centrosymmetric structure of NaNdNb2O7, which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi3+ cations.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
DOI: 10.1039/D1DT02974F
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“Towards ductilization of high strength 7XXX aluminium alloys via microstructural modifications obtained by friction stir processing and heat treatments”. Lezaack MB, Hannard F, Zhao L, Orekhov A, Adrien J, Miettinen A, Idrissi H, Simar A, Materialia 20, 101248 (2021). http://doi.org/10.1016/J.MTLA.2021.101248
Abstract: High strength 7XXX aluminium series reach exceptional strength, higher than all other industrial aluminium alloys. However, they suffer from a lack of ductility compared to softer series. This work presents a procedure to improve the ductility of 7475 Al alloy in high strength condition, reaching a true fracture strain of 70% at full 500 MPa T6 yield strength. Using friction stir processing (FSP) and post-FSP heat treatments, 100% of industrial rolled material T6 yield stress is maintained but a 180% increase in fracture strain is measured for the processed material. This ductility improvement is studied by in-situ synchrotron X-ray tomography and is explained by the reduction of intermetallic particles size and the homogenization of their spatial distribution. Furthermore, the microstructure after FSP shows equiaxed refined grains which favour crack deviation as opposed to large cracks parallel to the elongated coarse grains in rolled plate. These results are paving the way to better formability and crashworthiness of 7XXX alloys.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
DOI: 10.1016/J.MTLA.2021.101248
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“Zitterbewegung of moiré, excitons in twisted MoS₂/WSe₂, heterobilayers”. Lavor IR, da Costa DR, Covaci L, Milošević, MV, Peeters FM, Chaves A, Physical review letters 127, 106801 (2021). http://doi.org/10.1103/PHYSREVLETT.127.106801
Abstract: The moire pattern observed in stacked noncommensurate crystal lattices, such as heterobilayers of transition metal dichalcogenides, produces a periodic modulation of their band gap. Excitons subjected to this potential landscape exhibit a band structure that gives rise to a quasiparticle dubbed the moire exciton. In the case of MoS2/WSe2 heterobilayers, the moire trapping potential has honeycomb symmetry and, consequently, the moire exciton band structure is the same as that of a Dirac-Weyl fermion, whose mass can be further tuned down to zero with a perpendicularly applied field. Here we show that, analogously to other Dirac-like particles, the moire exciton exhibits a trembling motion, also known as Zitterbewegung, whose long timescales are compatible with current experimental techniques for exciton dynamics. This promotes the study of the dynamics of moire excitons in van der Waals heterostructures as an advantageous solid-state platform to probe Zitterbewegung, broadly tunable by gating and interlayer twist angle.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); Condensed Matter Theory (CMT)
Times cited: 5
DOI: 10.1103/PHYSREVLETT.127.106801
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“A cross-European analysis of the impact of electricity pricing on battery uptake in residential microgrids with photovoltaic units”. Saviuc I, Milis K, Peremans H, Van Passel S, Journal of Sustainable Development of Energy, Water and Environment Systems 9, 1080368 (2021). http://doi.org/10.13044/J.SDEWES.D8.0368
Abstract: As decentralized electricity generation is supporting grid development into the prosumer era, this paper investigates the economic viability of adding batteries to residential microgrids powered by photovoltaic units, under various electricity pricing schemes. Batteries bring the benefits of grid-stabilization and congestion relief, and they are also becoming cheaper. The problem identified is that the main grid effectively acts as a lossless storage system, especially under the net-metering scheme, whereas using a battery involves investment costs and energy losses. This mismatch is addressed by analysing residential microgrid projects under seven tariff designs, each in seven countries of the European Union, and compare the economic viability of photovoltaic systems with and without batteries. The findings show that the conditions most favourable to batteries are given by a capacity tariff scheme allowing price arbitrage. Based on these findings, the paper discusses possibilities for further support in order to bring the economic viability of microgrids with batteries on par with that of microgrids without batteries.
Keywords: A1 Journal article; Engineering Management (ENM)
DOI: 10.13044/J.SDEWES.D8.0368
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“Agricultural technical efficiency of smallholder farmers in Ethiopia : a stochastic frontier approach”. Chekol Zewdie M, Moretti M, Tenessa DB, Ayele ZA, Nyssen J, Tsegaye EA, Minale AS, Van Passel S, Land 10, 246 (2021). http://doi.org/10.3390/LAND10030246
Abstract: In the past decade, to improve crop production and productivity, Ethiopia has embarked on an ambitious irrigation farming expansion program and has introduced new large- and small-scale irrigation initiatives. However, in Ethiopia, poverty remains a challenge, and crop productivity per unit area of land is very low. Literature on the technical efficiency (TE) of large-scale and small-scale irrigation user farmers as compared to the non-user farmers in Ethiopia is also limited. Investigating smallholder farmers' TE level and its principal determinants is very important to increase crop production and productivity and to improve smallholder farmers' livelihood and food security. Using 1026 household-level cross-section data, this study adopts a technology flexible stochastic frontier approach to examine agricultural TE of large-scale irrigation users, small-scale irrigation users and non-user farmers in Ethiopia. The results indicate that, due to poor extension services and old-style agronomic practices, the mean TE of farmers is very low (44.33%), implying that there is a wider room for increasing crop production in the study areas through increasing the TE of smallholder farmers without additional investment in novel agricultural technologies. Results also show that large-scale irrigation user farmers (21.05%) are less technically efficient than small-scale irrigation user farmers (60.29%). However, improving irrigation infrastructure shifts the frontier up and has a positive impact on smallholder farmers' output.
Keywords: A1 Journal article; Engineering Management (ENM)
DOI: 10.3390/LAND10030246
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“An attempt to develop ecotourism in an unknown area : the case of Nehbandan County, South Khorasan Province, Iran”. Ghorbani A, Mousazadeh H, Taheri F, Ehteshammajd S, Azadi H, Yazdanpanah M, Khajehshahkohi A, Tanaskovik V, Van Passel S, Environment, development and sustainability 23, 11792 (2021). http://doi.org/10.1007/S10668-020-01142-W
Abstract: This study is an attempt to develop ecotourism in Nehbandan County, in the South Khorasan Province, Iran, by representing unknown areas that have tourism potential. The most important obstacle in the development of tourism industry is the lack of strong marketing plans, tourism management, accommodations for overnight and long stays, and medical and health clinic in the desert. This is a qualitative, exploratory case study investigating the status of environmental, sociocultural, and economic impacts of ecotourism development on the native people's livelihood in Nehbandan County in Iran. The methodology is based on field observations, secondary data sources, and field survey. It is an interpretive approach that avoids numbers but focuses on issues linked to culture and nature. In the present study, the grounded theory was used to perform data analysis. Based on the findings, ecotourism has undoubtedly proven to be an effective environmental conservation tool in the study site and enables local people to increase their livelihood security through employment in activities relating to ecotourism and the development of rural livelihood.
Keywords: A1 Journal article; Engineering sciences. Technology; Engineering Management (ENM)
DOI: 10.1007/S10668-020-01142-W
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“Beyond focus : exploring variability of service provision of agricultural cooperatives”. Sebhatu KT, Taheri F, Berhanu T, Maertens M, Van Passel S, D'Haese M, Annals of public and cooperative economics 92, 207 (2021). http://doi.org/10.1111/APCE.12301
Abstract: The wide array of services provided by agricultural cooperatives for their members is often not considered in academic studies. Addressing this gap in the literature, our paper explores the wide array of services provided by agricultural cooperatives and how these extend beyond those they were initially intended to provide. We study the extent and characteristics of service portfolios from 511 agricultural cooperatives in the Tigray region of Ethiopia. Results from two-limit Tobit models confirm that government and NGO-initiated cooperatives have a wide service portfolio compared to member-initiated cooperatives. In many of the studied cooperatives, the services they provide and their portfolios are more diverse than expected. Cooperatives seem to go beyond their focal areas of intervention. Also, those cooperatives that are more outward-oriented and where the chair has contact with other cooperatives or businesses, have a wider service portfolio. These results may help to explain the mixed findings on the impact of cooperative membership.
Keywords: A1 Journal article; Economics; Engineering Management (ENM)
DOI: 10.1111/APCE.12301
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“Climate variability and macroeconomic output in Ethiopia : the analysis of nexus and impact via asymmetric autoregressive distributive lag cointegration method”. Berihun D, Van Passel S, Environment, development and sustainability (2021). http://doi.org/10.1007/S10668-021-01604-9
Abstract: Ethiopia showed a rapid, yet, a none resilient economic growth much threatened by climate variability. In Ethiopia, the adverse effects of climate variability are stipulated among the significant factors constraining its economic development. There are relatively few studies about the adverse effects of climate variability on the Ethiopian macroeconomy. In this context, little is known about the exact effects of the ongoing climate variability on Ethiopian macroeconomic growth. This study intends to examine whether climate variability factors, for instance rainfall and temperature, have an effect on the macroeconomic output of Ethiopia. An asymmetric autoregressive distributive lag cointegration method is used to investigate time-series data for the years 1950-2014. Diagnostic tests show the relevance of the applied method and robustness of our results. The study finds climate variability affects Ethiopia's economic growth in the long run. Rainfall and temperature fluctuation induce significant negative impacts. A percentage annual temperature variability for instance decreases the Ethiopian annual gross domestic yield (GDP) up to 4.5 percent. In the short run, climate variability particularly rainfall and temperature changes also have a profound effect on Ethiopia's economic output. Within such confirmed climate change impacts, Ethiopia should carry out more on adapting and mitigating the impacts as it is presented on its climate-resilient economic growth policies and strategies. In spite of the policy contribution of the results, the study will motivate further research and will also serve as a benchmark for the coming Ethiopian studies.
Keywords: A1 Journal article; Engineering sciences. Technology; Engineering Management (ENM)
DOI: 10.1007/S10668-021-01604-9
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“Determinants of commercial bank loan and advance disbursement : the case of private Ethiopian commercial banks”. Birhanu T, Deressa SB, Azadi H, Viira A-H, Van Passel S, Witlox F, International journal of bank marketing 39, 1227 (2021). http://doi.org/10.1108/IJBM-05-2021-0166
Abstract: Purpose This paper aimed to investigate the determinants of loans and advances from commercial banks in the case of Ethiopian private commercial banks. Design/methodology/approach The study randomly selected seven commercial banks to represent the population stratified on their asset, deposit and paid-up capital amounts. The study utilized an unbalanced panel data model as each bank started operation at a different period of time and considered the period 1995-2016 for secondary details. Findings The findings showed that the deposit size, credit risk, portfolio investment, average lending rate, real gross domestic product (GDP) and inflation rate had significant and optimistic effects on the lending and advancement of private commercial banks. On the contrary, liquidity ratio had significant and negative effects on private commercial bank loans and advances. Finally, the study forwarded a feasible recommendation for concerned organs to focus on deposit size, credit risk, portfolio investment, average lending rate, real GDP, inflation rate and liquidity ratio. The results of this study will help banking industry policymakers and planners understand how to minimize inflation and unemployment by improving development and sustainable economic growth. Originality/value The findings of this study can also affect the general attitudes of a society by increasing knowledge and improve the quality of life for the general public.
Keywords: A1 Journal article; Economics; Engineering Management (ENM)
DOI: 10.1108/IJBM-05-2021-0166
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“Exploring variability across cooperatives : economic performance of agricultural cooperatives in northern Ethiopia”. Sebhatu KT, Gezahegn TW, Berhanu T, Maertens M, Van Passel S, D'Haese M, The international food and agribusiness management review 24, 397 (2021). http://doi.org/10.22434/IFAMR2019.0215
Abstract: The number of agricultural cooperatives increased quickly in Ethiopia since the 1990s. While many papers studied the impact of membership of Ethiopian cooperatives, not much is known on their performance. This study takes a cooperative-level perspective which is unique in an African context. It compares the economic performance proxied by sales revenue and profit of a wide range of agricultural cooperatives in northern Ethiopia. Data were collected from 511 agricultural cooperatives in 12 districts of Tigray. The contributing factors of the performance are analyzed with Ordinary Least Squares regression (OLS) and Heckman selection models. Our results underscore the importance of membership size, total assets, presence of conflict among members, and union membership. Chairperson characteristics and the internal organization of a cooperative seem to be less correlated to performance.
Keywords: A1 Journal article; Engineering Management (ENM)
DOI: 10.22434/IFAMR2019.0215
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“Farmers' willingness to contribute to the restoration of an Ethiopian Rift Valley lake : a contingent valuation study”. Girma H, Huge J, Gebrehiwot M, Van Passel S, Environment, development and sustainability 23, 10646 (2021). http://doi.org/10.1007/S10668-020-01076-3
Abstract: Lakes provide considerable social, economic, and ecological benefits. However, lakes are shrinking and the water quality is declining, due to human pressures such as water withdrawal and land use change, particularly in the developing world. Despite this, information regarding the economic impact of lake level reduction and local willingness to support restoration programs is lacking. This study employed a contingent valuation method to estimate willingness to pay and to contribute labor to Lake Ziway restoration program, Ethiopia. Face-to-face interviews were administered to 259 randomly selected respondents. Our findings revealed that about one-third of the respondents are willing to pay and about two-third are willing to contribute labor to restore the lake. From the interval regression models, the annual mean willingness to pay was estimated about 21.0 USD for the status quo scenario (the program works to keep water levels constant at current levels) and 31.1 USD for the improvement scenario (the program works to increase the water levels permanently). The annual mean willingness to contribute labor was estimated about 27.7 man-days for the status quo and 39.3 man-days for the improvement scenarios. 'Farm income' positively influenced the willingness to pay together with 'farm plot area.' Similarly, labor contribution was positively influenced by 'farm plot area' and 'education' and negatively by 'farm plot distance.' The economic values derived from this study reflect societal preferences and can form a significant input for policymakers, in support of informed and evidence-based decision-making regarding lake management and restoration in developing countries like Ethiopia.
Keywords: A1 Journal article; Engineering sciences. Technology; Engineering Management (ENM)
DOI: 10.1007/S10668-020-01076-3
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“Leasing out unused meeting room capacity to reduce future office space needs : a case study of The Hague, Netherlands”. Peeters M, Compernolle T, Van Passel S, Journal of building engineering 44, 102953 (2021). http://doi.org/10.1016/J.JOBE.2021.102953
Abstract: Meeting rooms are reserved 30% of working hours but only used for 20% of that time. By implementing a strategy where the available capacity is leased to the wider market rather than just the building users, there is a positive impact on the economic, environmental, and social factors of the building and its surroundings. This study uses the building 'The Globe' in The Hague as a case study, and then projects the results to the entire city. In case of The Globe, implementing a lease out strategy achieves a reduction of 36% of the annual rent of the meeting rooms to the building's tenant. The owner benefits from a revenue increase of 12.5%, with the same operational expenses (except the reservation system), leading to a proportional higher valuation of the building. Annual energy consumption may be lowered by 6.2%. This study contributes to the literature by considering the total benefits that could be obtained by more efficient use of office space that is currently underused. The application of technology generates added value for economic, environmental, and social factors. These factors are important in real estate as they (among others) have a direct link to the Environmental, Social and Governance (ESG) analysis that investors make before proceeding with an investment.
Keywords: A1 Journal article; Economics; Engineering sciences. Technology; Economics; Engineering Management (ENM)
DOI: 10.1016/J.JOBE.2021.102953
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“Rapid economic valuation of ecosystem services in man and biosphere reserves in Africa : a review”. Azadi H, Van Passel S, Cools J, Global Ecology and Conservation 28, e01697 (2021). http://doi.org/10.1016/J.GECCO.2021.E01697
Abstract: Y Ecosystem services (ESs) include the benefits people receive from ecosystems that support the socio-economic requirements and human well-being. The Man and Biosphere (MAB) programs are a scientific plan in the African context that is linked to governments and attempts to create a systematic ground to enhance the relationship between individuals and their environments. Using economic valuation methods, the main objective of this study was to perform a systematic review on studies undertaken in African countries in order to expose the role of MAB programs in conserving ESs in these countries. Based on the results, MAB programs offer both the protection of cultural ecosystem services and the improvement of livelihoods. Moreover, the MAB programs enable biosphere reserves to help national governments find solutions to the pressing challenges in Africa. In conclusion, policymakers should be aware of the main goal of ESs valuation approaches. In other words, the main goal should focus on the creation of prevention rather than rehabilitation activities in order to avoid the loss of ecosystem services mainly at the cost of users. Overall, the findings of this study underlined that economic valuation methods should provide evidence to underpin the development of policy instruments for the conservation of ESs in African countries. More importantly, ESs conservation strategies should consider various measures to reduce human impacts while also considering their well-being. In this context, the study's main implication is that human well-being is determined not only by the quantity but also by the quality of ESs.
Keywords: A1 Journal article; Engineering Management (ENM)
DOI: 10.1016/J.GECCO.2021.E01697
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“Uncovering ecosystem services of expropriated land : the case of urban expansion in Bahir Dar, Northwest Ethiopia”. Admasu WF, Boerema A, Nyssen J, Minale AS, Tsegaye EA, Van Passel S, Land 9, 395 (2020). http://doi.org/10.3390/LAND9100395
Abstract: In Ethiopia, urban expansion happens at high rates and results in land expropriations often at the cost of agriculture and forests. The process of urban expansion does not include assessment of ecosystem services (ES). This has been causing unintended environmental problems. This study aims to uncover ES of three most important land use types (cropland, agroforestry, and grassland) that are threatened by land expropriation for urban expansion in Bahir Dar City. The study applied a participatory approach using community perception and expert judgments (N = 108). Respondents were asked to locate their perceptions on the use of 35 different ES, and then to evaluate the potential of the land use. Respondents were shown to have the ability to differentiate between ES and land use in terms of their potential to deliver ES. The results show that agroforestry is expected to have a high relevant potential to deliver 31% of all ES, but cropland 20% and grassland 14%. Food, fodder, timber, firewood, fresh water, energy, compost, climate regulation, erosion prevention, and water purification and treatment were identified as the ten most important services. It is not only the provisioning services that are being supplied by the land use types which are expropriated for urbanization, but also regulating, supporting and cultural services. To ensure sustainable urban land development, we suggest the consideration of the use of ES and the potential of the land use to supply ES when making land use decisions, including land expropriation for urban expansion.
Keywords: A1 Journal article; Engineering Management (ENM); Ecosystem Management
DOI: 10.3390/LAND9100395
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“Interfacial gliding-driven lattice oxygen release in layered cathodes”. Sun C, Liao X, Peng H, Zhang C, Van Tendeloo G, Zhao Y, Wu J, Cell reports physical science 3 (2022). http://doi.org/10.1016/J.XCRP.2021.100695
Abstract: The oxygen release of layered cathodes causes many battery failures, but the underlying mechanism in an actual working cathode is still elusive as it involves secondary agglomerates that introduce complicated boundary structures. Here, we report a general structure instability on the mismatch boundaries driven by interfacial gliding-it introduces a shear stress causing a distortion of the metal-oxygen octahedra framework that reduces its kinetic stability. The migration of cations and diffusion of oxygen vacancies continue to degrade the whole particle from the boundary to the interior, followed by the formation of nano-sized cracks on the fast-degrading interfaces. This work reveals a robust chemical and mechanical interplay on the oxygen release inherent to the intergranular boundaries of layered cathodes. It also suggests that radially patterned columnar grains with low-angle planar boundaries would be an efficient approach to mitigate the boundary oxygen release.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
DOI: 10.1016/J.XCRP.2021.100695
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“Computing Curie temperature of two-dimensional ferromagnets in the presence of exchange anisotropy”. Tiwari S, Vanherck J, Van de Put ML, Vandenberghe WG, Sorée B, Physical review research 3, 043024 (2021). http://doi.org/10.1103/PHYSREVRESEARCH.3.043024
Abstract: We compare three first-principles methods of calculating the Curie temperature in two-dimensional (2D) ferromagnetic materials (FM), modeled using the Heisenberg model, and propose a simple formula for estimating the Curie temperature with high accuracy that works for all common 2D lattice types. First, we study the effect of exchange anisotropy on the Curie temperature calculated using the Monte Carlo (MC), the Green's function, and the renormalized spin-wave (RNSW) methods. We find that the Green's function method overestimates the Curie temperature in high-anisotropy regimes compared to the MC method, whereas the RNSW method underestimates the Curie temperature compared to the MC and the Green's function methods. Next, we propose a closed-form formula for calculating the Curie temperature of 2D FMs, which provides an estimate of the Curie temperature that is greatly improved over the mean-field expression for magnetic material screening. We apply the closed-form formula to predict the Curie temperature 2D magnets screened from the C2DB database and discover several high Curie temperature FMs, with Fe2F2 and MoI2 emerging as the most promising 2D ferromagnets. Finally, by comparing to experimental results for CrI3, CrCl3, and CrBr3, we conclude that for small effective anisotropies, the Green's-function-based equations are preferable, while for larger anisotropies, MC-based results are more predictive.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
DOI: 10.1103/PHYSREVRESEARCH.3.043024
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“Developing feature-rich electronic and magnetic properties in the beta-As monolayer for spintronic and optoelectronic applications by C and Si doping : a first-principles study”. Hoat DM, Duy Khanh Nguyen, Bafekry A, Vo Van On, Ul Haq B, Hoang D-Q, Cocoletzi GH, Rivas-Silva JF, Surfaces and interfaces 27, 101534 (2021). http://doi.org/10.1016/J.SURFIN.2021.101534
Abstract: In this work, the carbon (C) and silicon (Si) doping and codoping effects on beta-arsenene (As) monolayer structural, electronic, and magnetic properties have been comprehensively investigated using first-principles calculations. The studied two-dimensional (2D) materials exhibit good stability. Pristine beta-As single layer is an indirect gap semiconductor with a band gap of 1.867(2.441) eV as determined by PBE(HSE06) functional. Due to the difference in atomic size and electronic interactions, C and Si substitution induces a significant local structural distortion. Depending upon dopant concentration and doping sites, feature-rich electronic properties including non-magnetic semiconductor, magnetic semiconductor and half-metallicity may be obtained, which result from p-p interactions. High spin-polarization at the Fermi level vicinity and significant magnetism suggest As:1C, As:2C, As:1Si, As:2Si, and As:CSi systems as prospective spintronic 2D materials. While, the C-C, Si-Si, and C-Si dimer doping decreases electronic band gap, making the layer more suitable for applications in optoelectronic devices. Results presented herein may suggest an efficient approach to create novel multi-functional 2D materials from beta-As monolayer.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
DOI: 10.1016/J.SURFIN.2021.101534
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“Effect of mismatched electron-hole effective masses on superfluidity in double layer solid-state systems”. Conti S, Perali A, Peeters FM, Neilson D, Condensed Matter 6, 14 (2021). http://doi.org/10.3390/CONDMAT6020014
Abstract: Superfluidity has been predicted and now observed in a number of different electron-hole double-layer semiconductor heterostructures. In some of the heterostructures, such as GaAs and Ge-Si electron-hole double quantum wells, there is a strong mismatch between the electron and hole effective masses. We systematically investigate the sensitivity to unequal masses of the superfluid properties and the self-consistent screening of the electron-hole pairing interaction. We find that the superfluid properties are insensitive to mass imbalance in the low density BEC regime of strongly-coupled boson-like electron-hole pairs. At higher densities, in the BEC-BCS crossover regime of fermionic pairs, we find that mass imbalance between electrons and holes weakens the superfluidity and expands the density range for the BEC-BCS crossover regime. This permits screening to kill the superfluid at a lower density than for equal masses.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Times cited: 1
DOI: 10.3390/CONDMAT6020014
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“Electron-hole superfluidity in strained Si/Ge type II heterojunctions”. Conti S, Saberi-Pouya S, Perali A, Virgilio M, Peeters FM, Hamilton AR, Scappucci G, Neilson D, npj Quantum Materials 6, 41 (2021). http://doi.org/10.1038/S41535-021-00344-3
Abstract: Excitons are promising candidates for generating superfluidity and Bose-Einstein condensation (BEC) in solid-state devices, but an enabling material platform with in-built band structure advantages and scaling compatibility with industrial semiconductor technology is lacking. Here we predict that spatially indirect excitons in a lattice-matched strained Si/Ge bilayer embedded into a germanium-rich SiGe crystal would lead to observable mass-imbalanced electron-hole superfluidity and BEC. Holes would be confined in a compressively strained Ge quantum well and electrons in a lattice-matched tensile strained Si quantum well. We envision a device architecture that does not require an insulating barrier at the Si/Ge interface, since this interface offers a type II band alignment. Thus the electrons and holes can be kept very close but strictly separate, strengthening the electron-hole pairing attraction while preventing fast electron-hole recombination. The band alignment also allows a one-step procedure for making independent contacts to the electron and hole layers, overcoming a significant obstacle to device fabrication. We predict superfluidity at experimentally accessible temperatures of a few Kelvin and carrier densities up to similar to 6 x 10(10) cm(-2), while the large imbalance of the electron and hole effective masses can lead to exotic superfluid phases.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Times cited: 9
DOI: 10.1038/S41535-021-00344-3
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“Magnetic order and critical temperature of substitutionally doped transition metal dichalcogenide monolayers”. Tiwari S, Van de Put ML, Sorée B, Vandenberghe WG, npj 2D Materials and Applications 5, 54 (2021). http://doi.org/10.1038/S41699-021-00233-0
Abstract: Using first-principles calculations, we investigate the magnetic order in two-dimensional (2D) transition-metal-dichalcogenide (TMD) monolayers: MoS2, MoSe2, MoTe2, WSe2, and WS2 substitutionally doped with period four transition-metals (Ti, V, Cr, Mn, Fe, Co, Ni). We uncover five distinct magnetically ordered states among the 35 distinct TMD-dopant pairs: the non-magnetic (NM), the ferromagnetic with out-of-plane spin polarization (Z FM), the out-of-plane polarized clustered FMs (clustered Z FM), the in-plane polarized FMs (X-Y FM), and the anti-ferromagnetic (AFM) state. Ni and Ti dopants result in an NM state for all considered TMDs, while Cr dopants result in an anti-ferromagnetically ordered state for all the TMDs. Most remarkably, we find that Fe, Mn, Co, and V result in an FM ordered state for all the TMDs, except for MoTe2. Finally, we show that V-doped MoSe2 and WSe2, and Mn-doped MoS2, are the most suitable candidates for realizing a room-temperature FM at a 16-18% atomic substitution.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
DOI: 10.1038/S41699-021-00233-0
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“Nanoscale domain wall devices with magnetic tunnel junction read and write”. Raymenants E, Bultynck O, Wan D, Devolder T, Garello K, Souriau L, Thiam A, Tsvetanova D, Canvel Y, Nikonov DE, Young IA, Heyns M, Sorée B, Asselberghs I, Radu I, Couet S, Nguyen VD, Nature Electronics 4, 392 (2021). http://doi.org/10.1038/S41928-021-00593-X
Abstract: The manipulation of fast domain wall motion in magnetic nanostructures could form the basis of novel magnetic memory and logic devices. However, current approaches for reading and writing domain walls require external magnetic fields, or are based on conventional magnetic tunnel junctions (MTJs) that are not compatible with high-speed domain wall motion. Here we report domain wall devices based on perpendicular MTJs that offer electrical read and write, and fast domain wall motion via spin-orbit torque. The devices have a hybrid free layer design that consists of platinum/cobalt (Pt/Co) or a synthetic antiferromagnet (Pt/Co/Ru/Co) into the free layer of conventional MTJs. We show that our devices can achieve good tunnelling magnetoresistance readout and efficient spin-transfer torque writing that is comparable to current magnetic random-access memory technology, as well as domain wall depinning efficiency that is similar to stand-alone materials. We also show that a domain wall conduit based on a synthetic antiferromagnet offers the potential for reliable domain wall motion and faster write speed compared with a device based on Pt/Co. Domain wall devices based on perpendicular magnetic tunnel junctions with a hybrid free layer design can offer electrical read and write, and fast domain wall motion driven via spin-orbit torque.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
DOI: 10.1038/S41928-021-00593-X
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“Quasiparticle twist dynamics in non-symmorphic materials”. Juneja R, Thebaud S, Pandey T, Polanco CA, Moseley DH, Manley ME, Cheng YQ, Winn B, Abernathy DL, Hermann RP, Lindsay L, Materials Today Physics 21, 100548 (2021). http://doi.org/10.1016/J.MTPHYS.2021.100548
Abstract: Quasiparticle physics underlies our understanding of the microscopic dynamical behaviors of materials that govern a vast array of properties, including structural stability, excited states and interactions, dynamical structure factors, and electron and phonon conductivities. Thus, understanding band structures and quasiparticle interactions is foundational to the study of condensed matter. Here we advance a 'twist' dynamical description of quasiparticles (including phonons and Bloch electrons) in nonsymmorphic chiral and achiral materials. Such materials often have structural complexity, strong thermal resistance, and efficient thermoelectric performance for waste heat capture and clean refrigeration technologies. The twist dynamics presented here provides a novel perspective of quasiparticle behaviors in such complex materials, in particular highlighting how non-symmorphic symmetries determine band crossings and anti-crossings, topological behaviors, quasiparticle interactions that govern transport, and observables in scattering experiments. We provide specific context via neutron scattering measurements and first-principles calculations of phonons and electrons in chiral tellurium dioxide. Building twist symmetries into the quasiparticle dynamics of non-symmorphic materials offers intuition into quasi particle behaviors, materials properties, and guides improved experimental designs to probe them. More specifically, insights into the phonon and electron quasiparticle physics presented here will enable materials design strategies to control interactions and transport for enhanced thermoelectric and thermal management applications. (C) 2021 Published by Elsevier Ltd.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
DOI: 10.1016/J.MTPHYS.2021.100548
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“Semiconducting chalcogenide alloys based on the (Ge, Sn, Pb) (S, Se, Te) formula with outstanding properties : a first-principles calculation study”. Bafekry A, Shahrokhi M, Shafique A, Jappor HR, Fadlallah MM, Stampfl C, Ghergherehchi M, Mushtaq M, Feghhi SAH, Gogova D, ACS Omega 6, 9433 (2021). http://doi.org/10.1021/ACSOMEGA.0C06024
Abstract: Very recently, a new class of the multicationic and -anionic entropy-stabilized chalcogenide alloys based on the (Ge, Sn, Pb) (S, Se, Te) formula has been successfully fabricated and characterized experimentally [Zihao Deng et al., Chem. Mater. 32, 6070 (2020)]. Motivated by the recent experiment, herein, we perform density functional theory-based first-principles calculations in order to investigate the structural, mechanical, electronic, optical, and thermoelectric properties. The calculations of the cohesive energy and elasticity parameters indicate that the alloy is stable. Also, the mechanical study shows that the alloy has a brittle nature. The GeSnPbSSeTe alloy is a semiconductor with a direct band gap of 0.4 eV (0.3 eV using spin-orbit coupling effect). The optical analysis illustrates that the first peak of Im(epsilon) for the GeSnPbSSeTe alloy along all polarization directions is located in the visible range of the spectrum which renders it a promising material for applications in optical and electronic devices. Interestingly, we find an optically anisotropic character of this system which is highly desirable for the design of polarization-sensitive photodetectors. We have accurately predicted the thermoelectric coefficients and have calculated a large power factor value of 3.7 x 10(11) W m(-1) K-2 s(-1) for p-type. The high p-type power factor is originated from the multiple valleys near the valence band maxima. The anisotropic results of the optical and transport properties are related to the specific tetragonal alloy unit cell.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
DOI: 10.1021/ACSOMEGA.0C06024
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“Thermodynamic equilibrium theory revealing increased hysteresis in ferroelectric field-effect transistors with free charge accumulation”. Bizindavyi J, Verhulst AS, Sorée B, Vandenberghe WG, Communications Physics 4, 86 (2021). http://doi.org/10.1038/S42005-021-00583-7
Abstract: At the core of the theoretical framework of the ferroelectric field-effect transistor (FeFET) is the thermodynamic principle that one can determine the equilibrium behavior of ferroelectric (FERRO) systems using the appropriate thermodynamic potential. In literature, it is often implicitly assumed, without formal justification, that the Gibbs free energy is the appropriate potential and that the impact of free charge accumulation can be neglected. In this Article, we first formally demonstrate that the Grand Potential is the appropriate thermodynamic potential to analyze the equilibrium behavior of perfectly coherent and uniform FERRO-systems. We demonstrate that the Grand Potential only reduces to the Gibbs free energy for perfectly non-conductive FERRO-systems. Consequently, the Grand Potential is always required for free charge-conducting FERRO-systems. We demonstrate that free charge accumulation at the FERRO interface increases the hysteretic device characteristics. Lastly, a theoretical best-case upper limit for the interface defect density D-FI is identified. The ferroelectric field-effect transistor, which has attracted much attention for application as both a highly energy-efficient logic device and a non-volatile memory device, has often been studied within the framework of equilibrium thermodynamics. Here, the authors theoretically demonstrate the importance of utilizing the correct thermodynamic potential and investigate the impact of free charge accumulation on the equilibrium performance of ferroelectric-based systems.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
DOI: 10.1038/S42005-021-00583-7
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“Towards modelling active sound localisation based on Bayesian inference in a static environment”. McLachlan G, Majdak P, Reijniers J, Peremans H, Acta Acustica 5, 45 (2021). http://doi.org/10.1051/AACUS/2021039
Abstract: Over the decades, Bayesian statistical inference has become a staple technique for modelling human multisensory perception. Many studies have successfully shown how sensory and prior information can be combined to optimally interpret our environment. Because of the multiple sound localisation cues available in the binaural signal, sound localisation models based on Bayesian inference are a promising way of explaining behavioural human data. An interesting aspect is the consideration of dynamic localisation cues obtained through self-motion. Here we provide a review of the recent developments in modelling dynamic sound localisation with a particular focus on Bayesian inference. Further, we describe a theoretical Bayesian framework capable to model dynamic and active listening situations in humans in a static auditory environment. In order to demonstrate its potential in future implementations, we provide results from two examples of simplified versions of that framework.
Keywords: A1 Journal article; Engineering Management (ENM); Condensed Matter Theory (CMT)
DOI: 10.1051/AACUS/2021039
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“Vibrational properties and thermal transport in quaternary chalcogenides : the case of Te-based compositions”. Shi W, Pandey T, Lindsay L, Woods LM, Physical review materials 5, 045401 (2021). http://doi.org/10.1103/PHYSREVMATERIALS.5.045401
Abstract: Vibrational thermal properties of CuZn2InTe4, AgZn2InTe4, and Cu2CdSnTe4, derived from binary II-VI zinc-blendes, are reported based on first-principles calculations. While the chalcogenide atoms in these materials have the same lattice positions, the cation atom arrangements vary, resulting in different crystal symmetries and subsequent properties. The compositional differences have important effects on the vibrational thermal characteristics of the studied materials, which demonstrate that low-frequency optical phonons hybridize with acoustic phonons and lead to enhanced phonon-phonon scattering and low lattice thermal conductivities. The phonon density of states, mode Gruneisen parameters, and phonon scattering rates are also calculated, enabling deeper insight into the microscopic thermal conduction processes in these materials. Compositional variations drive differences among the three materials considered here; nonetheless, their structural similarities and generally low thermal conductivities (0.5-4 W/mK at room temperature) suggest that other similar II-VI zinc-blende derived materials will also exhibit similarly low values, as also corroborated by experimental data. This, combined with the versatility in designing a variety of motifs on the overall structure, makes quaternary chalcogenides interesting for thermal management and energy conversion applications that require low thermal conductivity.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
DOI: 10.1103/PHYSREVMATERIALS.5.045401
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“Boron structure evolution in magnetic Cr₂O₃, thin films”. Sun C, Street M, Zhang C, Van Tendeloo G, Zhao W, Zhang Q, Materials Today Physics 27, 100753 (2022). http://doi.org/10.1016/J.MTPHYS.2022.100753
Abstract: B substituting O in antiferromagnetic Cr2O3 is known to increase the Ne ' el temperature, whereas the actual B dopant site and the corresponding functionality remains unclear due to the complicated local structure. Herein, A combination of electron energy loss spectroscopy and first-principles calculations were used to unveil B local structures in B doped Cr2O3 thin films. B was found to form either magnetic active BCr4 tetrahedra or various inactive BO3 triangles in the Cr2O3 lattice, with a* and z* bonds exhibiting unique spectral features. Identification of BO3 triangles was achieved by changing the electron momentum transfer to manipulate the differential cross section for the 1s-z* and 1s-a* transitions. Modeling the experimental spectra as a linear combination of simulated B K edges reproduces the experimental z* / a* ratios for 15-42% of the B occupying the active BCr4 structure. This result is further supported by first-principles based thermodynamic calculations.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 11.5
DOI: 10.1016/J.MTPHYS.2022.100753
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“Electronic and chemical properties of nickel oxide thin films and the intrinsic defects compensation mechanism”. Poulain R, Lumbeeck G, Hunka J, Proost J, Savolainen H, Idrissi H, Schryvers D, Gauquelin N, Klein A, ACS applied electronic materials 4, 2718 (2022). http://doi.org/10.1021/ACSAELM.2C00230
Abstract: Although largely studied, contradictory results on nickel oxide (NiO) properties can be found in the literature. We herein propose a comprehensive study that aims at leveling contradictions related to NiO materials with a focus on its conductivity, surface properties, and the intrinsic charge defects compensation mechanism with regards to the conditions preparation. The experiments were performed by in situ photo-electron spectroscopy, electron energy loss spectroscopy, and optical as well as electrical measurements on polycrystalline NiO thin films prepared under various preparation conditions by reactive sputtering. The results show that surface and bulk properties were strongly related to the deposition temperature with in particular the observation of Fermi level pinning, high work function, and unstable oxygen-rich grain boundaries for the thin films produced at room temperature but not at high temperature (>200 degrees C). Finally, this study provides substantial information about surface and bulk NiO properties enabling to unveil the origin of the high electrical conductivity of room temperature NiO thin films and also for supporting a general electronic charge compensation mechanism of intrinsic defects according to the deposition temperature.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
DOI: 10.1021/ACSAELM.2C00230
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“Exploring the role of graphene oxide as a co-catalyst in the CZTS photocathodes for improved photoelectrochemical properties”. Vishwakarma M, Batra Y, Hadermann J, Singh A, Ghosh A, Mehta BR, ACS applied energy materials 5, 7538 (2022). http://doi.org/10.1021/ACSAEM.2C01011
Abstract: The hydrogen evolution properties of CZTS heterostructure photocathodes are reported with graphene oxide (GO) as a co-catalyst layer coated by a drop-cast method and an Al2O3 protection layer fabricated using atomic layer deposition. In the CZTS absorber, a minor deviation from stoichiometry across the cross section of the thin film results in nanoscale growth of spurious phases, but the kesterite phase remains the dominant phase. We have investigated the band alignment parameters such as the band gap, work function, and Fermi level position that are crucial for making kesterite-based heterostructure devices. The photocurrent density in the photocathode CZTS/CdS/ZnO is found to be improved to -4.71 mAmiddotcm(-2) at -0.40 V-RHE, which is 3 times that of the pure CZTS. This enhanced photoresponse can be attributed to faster carrier separation at p-n junction regions driven by upward band bending at CZTS grain boundaries and the ZnO layer. GO as a co-catalyst over the heterostructure photocathode significantly improves the photocurrent density to -6.14 mAmiddotcm(-2) at -0.40 V-RHE by effective charge migration in the CZTS/CdS/ZnO/GO configuration, but the onset potential shifts only after application of the Al2O3 protection layer. Significant photocurrents of -29 mAmiddotcm(-2) at -0.40 V-RHE and -8 mAmiddotcm(-2) at 0 V-RHE are observed, with an onset potential of 0.7 V-RHE in CZTS/CdS/ZnO/GO/Al2O3. The heterostructure configuration and the GO co-catalyst reduce the charge-transfer resistance, while the Al2O3 top layer provides a stable photocurrent for a prolonged time (similar to 16 h). The GO co-catalyst increases the flat band potential from 0.26 to 0.46 V-RHE in CZTS/CdS/ZnO/GO, which supports the bias-induced band bending at the electrolyte-electrode interface.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 6.4
DOI: 10.1021/ACSAEM.2C01011
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“Photochemical production of hydrogen peroxide by digging pro-superoxide radical carbon vacancies in carbon nitride”. Ding Y, Maitra S, Arenas Esteban D, Bals S, Vrielinck H, Barakat T, Roy S, Van Tendeloo G, Liu J, Li Y, Vlad A, Su B-L, Cell reports physical science 3, 100874 (2022). http://doi.org/10.1016/J.XCRP.2022.100874
Abstract: Artificial photosynthesis of H2O2, an environmentally friendly oxidant and a clean fuel, holds great promise. However, improving its efficiency and stability for industrial implementation remains highly challenging. Here, we report the visible-light H2O2 artificial photosynthesis by digging pro-superoxide radical carbon vacancies in three-dimensional hierarchical porous g-C3N4 through a simple hydrolysis-freeze-drying-thermal treatment. A significant electronic structure change is revealed upon the implantation of carbon vacancies, broadening visible-light absorption and facilitating the photogenerated charge separation. The strong electron affinity of the carbon vacancies promotes superoxide radical (O-center dot(2)-) formation, significantly boosting the H2O2 photocatalytic production. The developed photocatalyst shows an H2O2 evolution rate of 6287.5 mM g(-1) h(-1) under visible-light irradiation with a long cycling stability being the best-performing photocatalyst among all reported g-C3N4-based systems. Our work provides fundamental insight into highly active and stable photocatalysts with great potential for safe industrial H2O2 production.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Times cited: 12
DOI: 10.1016/J.XCRP.2022.100874
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