< Previous38 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES CLARIFYING EFFECTS OF NEGATIVE MASS FLEET study clarifies understanding of negative mass A FLEET study in 2018 has helped to clarify understanding of mass effects in ultra-cold gases. When we think of ‘mass’, we usually consider the ‘inertial’ mass – the resistance of a body to acceleration due to an applied force. For a moving object, its mass is then a simple relationship between momentum applied to it and the velocity it acquires. However, in some situations, this relationship is not simply proportional and can depend on the impulse applied to the object. Physicists then talk about ‘effective’ mass, which can even be negative. In such a case, an object would move in a completely non-intuitive way when acted on by a force. “Imagine a soccer ball: you give it a first kick to get closer to the goal; you then give it an extra kick to score but, instead of accelerating, the ball slows down! You’re a bit puzzled, so you decide to kick the ball even harder, and it now moves towards your foot and not away from it!” explains the lead author of the study, Dr David Colas (University of Queensland). Dr David Colas (UQ) plans to name his next paper “Assisted negative-mass air soccer”. CASE STUDY39 FLEET 2018 ANNUAL REPORT The stability of funding allows for building a strong team for long-term projects. I like to think I do work of high quality, and it takes time for this to eventuate. Some very good stuff is coming in the next year or two. Prof Matthew Davis FLEET Chief Investigator, UQ Negative masses can be achieved experimentally in various systems; for example, in ultra-cold atomic gases. The UQ theoretical research expanded upon an earlier study at Washington State University that demonstrated a negative mass effect in the expansion of an ultra-cold atomic gas, nicely illustrating the versatility and great tunability of the UQ platform. The UQ researchers clarified the effects associated to the different types of negative mass and identified the striking ‘self-interfering effect’ in the atomic condensate. “To carry on with the soccer ball analogy, imagine that if you kick it too hard, you will squeeze it against your foot for a bit. When the ball leaves your boot, it re- expands and you see that the front part of the ball will eventually travel slower than its bottom part. The ball then interferes with itself,” continues Dr Colas. Negative mass effects can come out in different forms, such as self-interference. But one of the most striking is the backward propagation of a positive impulse: the hypothetical soccer ball that accelerates towards the kicker’s boot, not away from it. Clarification of the type of mass that is responsible for each observed phenomenon will avoid common misinterpretations about negative mass. Such clarification will help get negative mass research back on track. This addresses FLEET milestone 1.1, as well as working towards milestones 1.2 and 1.3; see p93. The study was published in Physical Review Letters in July 2018, vol. 121 (see publication 9, p104). COLLABORATING FLEET PERSONNEL: • Research Fellow David Colas (UQ) • Chief Investigator Matthew Davis (UQ) More at FLEET.org.au/negative-mass40 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES PROF XIAOLIN WANG Leader, Enabling technology A University of Wollongong “Novel materials are fascinating for both fundamental physics and their great practical applications in electronics.” Expertise: design/fabrication and electronic/spintronic/ superconducting properties of novel electronic or spintronic systems such as topological insulators, high spin-polarised materials, superconductors, multiferroic materials, single crystals, thin films, nanosize particles/ribbons/rings/wires Research outputs: 460+ publications, 9900+ citations, h-index 48 ENABLING TECHNOLOGY A: ATOMICALLY-THIN MATERIALS Each of FLEET’s three research themes is heavily enabled by the science of novel, atomically-thin, two-dimensional (2D) materials. These are materials that can be as thin as just one single layer of atoms, with resulting unusual and useful electronic properties. To provide these materials, from bulk crystals to thin films to atomically-thin layers, FLEET draws on extensive expertise in materials synthesis in Australia and internationally. The most well-known atomically-thin material is graphene, a 2D sheet of carbon atoms that is an extraordinarily-good electrical conductor. FLEET uses other atomically-thin materials, with its scientists seeking materials possessing the necessary properties for topological and exciton-superfluid states. 2018 HIGHLIGHTS • Collaborations between FLEET nodes to fabricate 2D materials for research • Achieved topological surface states robust to 50 degrees Kelvin in chemically-modified Sb 2 Se 3 • Discovery of new excitonic insulating state in atomically-thin antimony • Discovery of ferroelectricity in 2D semiconductor In 2 Se 3 • Successful fabrication of 2D oxides from liquid metals • Fabrication of high-quality perovskite oxide heterostructures, realising new oxides stable only in ultra-thin form with promise for new topological oxides • Discovery of first vdW hard ferromagnetic metal with near-square magnetic loop and strong perpendicular anisotropy • Thickness dependence of tungsten ditelluride (WTe 2 ) – see case study p44 below. IN 2019, FLEET WILL: • Further increase temperature for topological surface states in three-dimensional (3D) topological insulators • Continue to search for new magnetic systems for quantum anomalous Hall effect (QAHE) using modelling • Work on new magnetic doping for anomalous Hall effect or QAHE • Continue scanning tunnelling microscope study of atomically-thin systems • Continue angle-resolved photoemission spectroscopy (ARPES) study of electronic structures. DID YOU KNOW... FLEET scientists use materials that are ‘atomically thin’, ie, only one layer of atoms in thickness. These materials are also referred to as ‘two dimensional’ (2D). ENABLING TECHNOLOGY A41 FLEET 2018 ANNUAL REPORT DEFINITIONS graphene A single 2D layer of carbon atoms heterostructure A structure in which two dissimilar materials are brought together at a controlled interface molecular beam epitaxy (MBE) A method used to deposit thin films of single crystals monolayer A single layer of material quantum anomalous Hall effect (QAHE) A magnetic effect giving a material conducting edges carrying current in one direction only, completely without resistance van der Waals (vdW) material A material naturally made of 2D layers, which can be isolated individually or stacked with other materials to form new structures PhD student Wafa Afzal (UOW) presenting her work on the magnetic state of atomically-thin materials. FLEET’s research goals are at the extreme cutting edge, and are super challenging from a physics perspective, both in simply gaining understanding, and in their execution. I have the feeling that we are at the forefront of something massive. Dr Daniel Sandoo FLEET Research Fellow, UNSW42 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES RAPID NANOFILTER DEVELOPED FOR INSTANT CLEAN WATER Liquid metals the path to new nanofilter FLEET researchers have designed a rapid nanofilter that can clean dirty water over 100 times faster than current technology. Simple to make and simple to scale up, the technology harnesses naturally-occurring nanostructures of aluminium hydroxide, grown on liquid-metal gallium. This innovative technology can filter both heavy metals and oils from water at extraordinary speed. FLEET Research Fellow Dr Ali Zavabeti (RMIT) explains that water contamination remains a significant challenge globally – one in nine people have no clean water close to home. “Heavy metal contamination causes serious health problems and children are particularly vulnerable,” Dr Zavabeti says. “Our new nanofilter, made of stacked, atomically- thin sheets of aluminium hydroxide, is sustainable, environmentally friendly, scalable and low cost. “We’ve shown it works to remove lead and oil from water, but we also know it has potential to target other common contaminants, such as mercury, sulfates and phosphates. CASE STUDY Research Fellow Dr Ali Zavabeti (RMIT) prepares liquid metal samples used in nanofiltration.43 FLEET 2018 ANNUAL REPORT This new nanofilter could be a cheap and ultra-fast solution to the problem of dirty water. Dr Ali Zavabeti FLEET Research Fellow, RMIT The liquid-metal chemistry process developed by the researchers has potential applications across a range of industries, including electronics, membranes, optics and catalysis. “The technique is potentially of significant industrial value, since it can be readily upscaled, the liquid metal can be reused, and the process requires only short reaction times and low temperatures,” Dr Zavabeti says. Project leader FLEET CI Prof Kourosh Kalantar-zadeh (UNSW, RMIT) says the liquid-metal chemistry used in the process enabled differently shaped nanostructures to be grown, either as atomically-thin sheets or nanofibrous structures. “Growing these materials conventionally is power intensive, requires high temperatures and extensive processing times and uses toxic metals. Liquid-metal chemistry avoids all these issues so it’s an outstanding alternative.” Water is added to a drop of a liquid metal. The skin is delaminated by hydrogen bubbles to form 2D sheets in the water, forming a hydrogel. Experiments showed the nanofilter was efficient at removing lead from water that had been contaminated COLLABORATING FLEET PERSONNEL: • Research Fellow Ali Zavabeti (RMIT) • Alumnus Isabela Alves de Castro (now at Alcoa) • Associate Investigator Jian-zhen Ou (RMIT) • Alumnus Ben Carey (now at University of Munster) • Associate Investigator Torben Daeneke (RMIT) • Chief Investigator Kourosh Kalantar-zadeh (UNSW/RMIT) at over 13 times safe drinking levels, and was highly effective in separating oil from water. The process generates no waste and requires just aluminium and water, with the liquid metals reused for each new batch of nanostructures. The study was published in Advanced Functional Materials in September 2018, vol. 28 (see publication 58, p106). More at FLEET.org.au/nano-filter44 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES WHY 2D? FINDING THE 2D–3D TRANSITION POINT Measuring thickness-dependent electronic properties A FLEET UNSW/Wollongong collaboration found a key transition point from three-dimensional (3D) to two- dimensional (2D) properties in 2018. 2D materials are useful for FLEET because constraining the movement of charge carriers (such as electrons) to two dimensions unlocks unusual quantum properties and useful electronic properties. In essence, this means restricting electron movement to a range from a few nanometres to a few hundred nanometres. Much can be learned by observing precisely at what thickness such new quantum effects emerge. A 2018 FLEET study found this precise transition point in the promising material tungsten ditelluride (WTe 2 ) to be around 20 nanometres (that is, 20 millionths of a millimetre). FLEET Research Fellow Dr Feixiang Xiang prepared thin WTe 2 films of different thickness, cleaved from a single high-purity crystal. After studying WTe 2 thin films at the University of Wollongong (UOW), Dr Xiang used UNSW laboratories to CASE STUDY FLEET Research Fellow Feixiang Xiang (UNSW) studies 2D materials in collaboration with Centre colleagues at UOW.45 FLEET 2018 ANNUAL REPORT I love working in FLEET… It’s a broad scientific community with vastly different interests. This allows the bouncing around of some very crazy ideas. Dr Oleh Klochan FLEET Chief Investigator, UNSW fabricate the devices from thin-film samples and perform transport measurements using ultra-low-temperature and high-magnetic-field measurement facilities. Quantum oscillation measurements performed in FLEET CI Prof Alex Hamilton’s lab at UNSW showed how the material’s band structure changed with decreasing thickness, and indicated a 3D–2D crossover when the sample thickness was reduced below 26 nm. “This finding was very important,” says Dr Xiang, who led the study at both UOW and UNSW, “because it pins down two critical length scales of the thickness- dependent electronic structure in WTe 2 thin films”. Analysis indicated that the area of Fermi pockets decreases in thinner samples, suggesting the overlap between the conduction band and valence band is becoming smaller. This not only explains the measured decrease of carrier density in a thinner sample, it suggests it is possible to open a bandgap and realise the 2D topological insulator in even thin samples, as has been predicted by theory, and observed in related compounds. Constraining the movement of charge carriers to two dimensions results in very different electronic COLLABORATING FLEET PERSONNEL: • Research Fellow Feixiang Xiang (UNSW) • Chief Investigator Oleh Klochan (UNSW) • Chief Investigator Alex Hamilton (UNSW) • Chief Investigator Xiaolin Wang (UOW) properties compared to 3D ‘bulk’ materials. This also suggests that additional different physical properties could happen at the monolayer limit – the transition point from 3D to 2D. This addresses FLEET milestone 1.1; see p93. The study was published in Physical Review B in July 2018, vol. 98 (see publication 51, p106). More at FLEET.org.au/2D-transition46 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES ENABLING TECHNOLOGY B: NANO-DEVICE FABRICATION FLEET’s research sits at the very boundary of what is possible in condensed- matter physics. At the nano scale, nanofabrication of functioning devices will be key to the Centre’s success. Specialised techniques are needed to integrate novel atomically-thin, two- dimensional (2D) materials into high-quality, high-performance nanodevices. For example, atomically-thin topological insulators will need to be integrated with electrical gates to realise topological transistors. And atomically-thin semiconductors must be integrated with optical cavities to realise exciton- polariton condensate devices. Nano-device fabrication and characterisation links many of FLEET’s groups and nodes. Some groups bring expertise in device fabrication, while other groups are stronger in device characterisation. FLEET brings together Australian strength in microfabrication and nanofabrication with world-leading expertise in van der Waals (vdW) heterostructure fabrication to build the capacity for advanced atomically-thin device fabrication. PROF LAN WANG Leader, Enabling technology B RMIT University “FLEET is a great platform from which to establish collaborations with local and international researchers, allowing us to share ideas and work together.” Expertise: Low-temperature and high-magnetic field electron and spin transport; topological insulators; magnetic materials; spintronic and magneto- electronic devices; device fabrication; growth of single crystals, thin films and nanostructures Research outputs: 100+ papers, 2600+ citations, h-index 29 I love this research area because of the new ideas and new physics. This year we have got more ambitious in our research, and more motivated about our results. Cheng Tan FLEET PhD student, RMIT IN 2019, FLEET WILL: • Fabricate devices based on vdW heterostructures as a basis for quantum spin Hall effect (QSHE), quantum anomalous Hall effect (QAHE) and bi-layer exciton transistors • Expand large-scale synthesis of 2D materials towards thin nanosheets with desired electrical, topological and magnetic properties • Fabricate high-quality distributed Bragg reflector (DBR) microcavities. 2018 HIGHLIGHTS • Refined glove box fabrication of vdW heterostructures • Further developed liquid-metal synthesis of 2D materials, broadening the accessible range of 2D materials • Demonstrated patterning of 2D electron gases, a platform to realise quantum spin Hall systems in oxide heterostructures • Fabricated high-quality DBR microcavities, opening way to exciton-polariton condensation. ENABLING TECHNOLOGY B47 FLEET 2018 ANNUAL REPORT FLEET Research Fellow Golrokh Akhgar’s expertise in 2D vdW heterostructures bridges Research theme 1 and Enabling technology B. DEFINITIONS bandgap The energy gap that defines whether a material is a conductor, insulator or semiconductor; a large bandgap is required for a material to still be topological at room temperature distributed Bragg reflector (DBR) microcavity Layered, di-electric mirror used to reflect a particular wavelength glove box Sealed container allowing manipulation within a controlled atmosphere via gloves heterostructure A structure in which two dissimilar materials are brought together at a controlled interface quantum spin Hall effect (QSHE) The spin-orbit interaction driven effect that gives a non-magnetic material conducting edges, which can carry current without resistance, as long as no magnetic disorder is present quantum anomalous Hall effect (QAHE) A magnetic version of the QSHE (above), in which conducting edges carry currents in only one direction, and are completely without resistance van der Waals (vdW) material A material naturally made of 2D layers, held together by weak van der Waals forces van der Waals (vdW) heterostructure A structure made by stacking layers of different van der Waals materialsNext >