< PreviousIN 2018, FLEET WILL: >Demonstrate transition from classical insulator to topological insulator in ultra-thin Na3Bi films >Encapsulate ultra-thin Na3Bi films for study outside ultra-high vacuum system >Study electrical transport properties of artificial graphene devices >Advance fundamental theoretical knowledge of topological materials’ electrical properties >Predict new 2D topological materials from first principle calculations >Investigate magnetic properties of atomically-thin van der Waals materials.PROF ALEX HAMILTONLeader, Research theme 1University of New South Wales“FLEET provides the resources and connections to tackle hard science problems”Expertise: electronic conduction in 2D and nanoscale transistors, spin-orbit interactions, behaviour of holes in semiconductor nanostructuresResearch outputs: 180+ papers, 2500+ citations, h-index 26RESEARCH THEME 1: TOPOLOGICAL MATERIALSThe first FLEET approach, to achieve electrical current flow with near-zero resistance, is based on a paradigm shift in the understanding of condensed-matter physics and materials science: the advent of topological insulators.Unlike conventional insulators, which do not conduct electricity at all, topological insulators conduct electricity, but only along their edges. Along those edge paths, they conduct electrons strictly in one direction, without the ‘back-scattering’ of electrons that dissipates energy in conventional electronics.FLEET’s challenge is to create topological materials that will operate as insulators in their interior, and have switchable conduction paths along their edges.For the new technology to form a viable alternative to traditional transistors, the desired properties must be achievable at room temperature – there’s no point in saving energy on transistor switching if you have to use even more energy to keep the system supercold.Topological transistors would ‘switch’, just as a traditional transistor does. Applying a controlling voltage would switch the edge paths of the topological material between being a topological insulator (‘on’) and a conventional insulator (‘off’). Approaches used are: >Magnetic topological insulators and quantum anomalous Hall effect (QAHE) >Topological Dirac semimetals >Artificial topological systems.18RESEARCH THEME 1Research fellow Daisy Wang studies artificially engineered topological systemsFLEET 2017 ANNUAL REPORT192017 HIGHLIGHTSThe year’s first two highlights are particularly important. They provide a possible route to making devices with topologically-protected properties from conventional semiconductors. >Demonstrating the first transistor made froma topological Dirac semimetal in thin-film form,a pathway to more-complex topological electronicdevices (see case study p20) >Fabricating prototype artificial-graphene devicesbased on conventional semiconductor materials,Research fellow Daniel Sando with oxide pulsed laser deposition (PLD) chamber used to fabricate thin-film samples with very high crystalline perfection, preparing interfaces required for FLEET Research theme 1understanding spin-orbit interactions that could convert artificial graphene into an artificial topological insulator >Nano-patterning of oxide structures, openingnew routes to fabrication of widely applicablenanoscale devices >Hosting Gordon Godfrey workshop at UNSW(see p41), bringing together experts in spin andstrong-electron correlation.DEFINITIONSartificial topological systems Artificial analogues of graphene and 2D topological insulatorsdissipationless current Electric current that flows without wasted dissipation of energyFloquet topological insulator A topological insulator created by applying light to a conventional insulatorquantum 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 presentquantum 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 resistancespin-orbit interaction The interaction between electrons’ movement and their inherent angular momentum, which drives topological effectstopological materials A relatively new class of material that is electrically insulating in its interior, but conducts along its edgestopological Dirac semimetal (TDS) Topological material at the boundary between conventional insulators (which don’t conduct) and topological insulators (which conduct along their edges)van der Waals (vdW) material A material naturally made of 2D layers, held together by weak van der Waals forces20ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIESI’m excited to see FLEET already leading the way, showing how new breakthroughs in electronics materials can be used to create practical opportunities in ICT.Dr Ellen William FLEET Advisory Committee University of MarylandCASE STUDYSWITCHING CONDUCTION MODE: A STEP TOWARDS TOPOLOGICAL TRANSISTORSApplying an electric field switches electronic conduction mode of a topological materialFLEET researchers achieved a significant landmark in the search for a functional topological transistor in 2017, using an applied electric field to switch the conduction mode of a topological material.A ‘gate’ electrode was used to switch the conduction mode in the topological material Na3Bi.Na3Bi is a topological Dirac semimetal (TDS), a material that has been referred to as ‘3D graphene’. “Electrons travelling within a TDS behave similarly to graphene, moving relativistically (ie, as if they have no mass),” explains FLEET associate investigator Dr Mark Edmonds, a co-author on the paper.Conduction mode in the TDS was switched between ‘n-type’ conduction (in which the current is carried by electrons) and ‘p-type’ conduction (in which the current is carried by ‘holes’ – which are effectively ‘missing electrons’).The work represented the first successful, simple, thin-film transistor made from a topological semimetal and the first transistor made from Na3Bi.Collaborator Jack Hellerstedt monitors growth of Na3Bi in scanning tunneling microscopeFLEET 2017 ANNUAL REPORT21As such a new field of physics, topological materials are ripe for development within the ARC Centre of Excellence program: This research would not have been feasible five years ago, whereas five years from now, everyone will be doing it. The time is right for Australia to lead the world in this area.Prof Michael Fuhrer Director, FLEETAs the first transistor made from any topological Dirac semimetal in a solid-state, thin-film form, this shows that the technology is amenable to processing into electronic devices over large areas.As the first demonstration that electronic properties can be successfully manipulated by an applied electric field, it was also a step on the way to more-complex, switchable topological transistors.In complex, switchable topological transistors, the key is the ability to switch a material between a conventional insulator, and the topological state. Ideally, such switching would be accomplished via an electric field induced by a voltage applied to the transistor’s gate electrode. Such technology would use a topological Dirac semimetal as the channel material, balanced between a conventional insulator and a topological insulator. “These results make the topological Dirac semimetal Na3Bi an incredibly fertile platform for exploring some very exciting new areas of physics,” says FLEET PhD student James Collins, a co-author on the study. “It means Na3Bi is an ideal starting point to realise control over the topological properties of a material.”This work is therefore a significant step towards two key goals for Research theme 1: >An atomically-thin topological insulatorwith bandgap greater than 77 degrees Kelvin >Successful switching from a conventionalinsulator to a topological insulator.The project represented a successful interdisciplinary collaboration between experts in thin-film growth and NEW PHYSICS AND THE 2016 NOBEL PRIZE IN PHYSICSelectronic characterisation at Monash University, and theoretical modelling led by FLEET Associate Investigator A/Prof Shaffique Adam at the National University of Singapore.The study was published in Physical Review Materials in October 2017, Vol. 1, issue 5 (see publication 8, p84).COLLABORATING FLEET PERSONNEL: >Associate Investigator Mark Edmonds(Monash University) >PhD student Chang Liu (Monash University) >PhD student James Collins (Monash University) >Associate Investigator Shaffique Adam (Yale-NUS) >CI Michael Fuhrer (Monash University)Topological materials represent a paradigm shift in material science, first proposed in 1987 and only demonstrated in the lab in the last decade.In 2004 the potential for topological materials to carry current with negligible dissipation was realised with the prediction of the quantum spin Hall effect (first demonstrated in the lab in 2007).The quantum anomalous Hall effect (QAHE) was achieved in the laboratory at Tsinghua University in 2013 by Prof Qi-Kun Xue, now a FLEET Partner Investigator and leading the Centre’s collaboration with Tsinghua University. QAHE showed that current could be carried with no measurable dissipation at all, and it was this 2013 discovery that opened up the field of topological electronics being investigated at FLEET.The importance of topological materials was recognised by the 2016 Nobel Prize in Physics, awarded to Michael Kosterlitz, Duncan Haldane and David Thouless. IN 2018, FLEET WILL: >Achieve strong light-matter coupling in novel microcavities hosting atomically-thin semiconductor monolayers such as the transition metal dichalcogenide MoSe2 >Investigate designs to support twin-layer excitons >Develop theoretical understanding of nonlinear interactions between exciton–polaritons and control of interactions via exciton-pair resonance >Build on collaborations between Monash (atomically-thin semiconductor synthesis and microcavity fabrication), UNSW (electrical characterisation of atomically-thin semiconductor devices) and ANU (design and characterisation of microcavities, optical probing of exciton–polaritons).A/PROF ELENA OSTROVSKAYALeader, Research theme 2Australian National University“Exciton–polariton research is entering its most active and exciting phase around the world”Expertise: nonlinear physics, quantum degenerate gases, Bose-Einstein condensates, microcavity exciton–polaritonsResearch outputs: 110+ papers, 3 book chapters, 3100+ citations, h-index 30RESEARCH THEME 2: EXCITON SUPERFLUIDSFLEET’s second research theme will use a quantum state known as a superfluid to achieve electrical current flow with minimal wasted dissipation of energy. In a superfluid, scattering is prohibited by quantum statistics, which means that charge carriers can flow without resistance. A superfluid is a quantum state in which all particles flow with the same momentum, and no energy is lost to other motion. Particles and quasi-particles, including both excitons and exciton–polaritons, can form a superfluid.Researchers are seeking to create superfluid flows following three approaches: >Exciton–polariton bosonic condensation in atomically-thin materials >Topologically-protected exciton–polariton flow >Exciton superfluids in twin-layer materials.If exciton-superfluid devices are to be a viable, low-energy alternative to conventional electronic devices, they must be able to operate at room temperature, without energy-intensive cooling.Thus, FLEET seeks to achieve superfluid flow at room temperature, which can be achieved using atomically-thin semiconductors as the medium for the superfluid.RESEARCH THEME 2Observing the atomic world – PhD student Fei Hou studying nanoscale properties of functional oxide materials using scanning probe microscopy2223FLEET 2017 ANNUAL REPORTFLEET 2017 ANNUAL REPORT2017 HIGHLIGHTS >Fabricating and characterising microcavities for stronglight-matter coupling, in-house (see case study, p24) >In world-first, single-shot imaging an exciton-polaritoncondensate in inorganic semiconductor microcavitiesat ANU, providing new insight into polaritoncondensation, with direct benefits for achievingcondensation in microcavities with embedded,atomically-thin semiconductors.Experimental setup for exciton-polariton condensation at cryogenic temperatures, ANUDEFINITIONSdissipationless current Electric current that flows without wasted dissipation of energyexciton Quasi-particle formed of two strongly-bound charged particles: an electron and a ‘hole’exciton–polariton Part matter and part light quasi-particle: an exciton bound to a photonmicrocavities A micrometre-scale structure; an optical medium sandwiched between ultra-reflective mirrors, used to confine light such that it forms exciton–polaritonsmonolayer A single 2D layer of materialsuperfluid A quantum state in which particles flow without encountering any resistance to their motion. Both excitons and exciton–polaritons can flow in a superfluid.van der Waals (vdW) material A material naturally made of 2D layers, held together by weak van der Waals forces24ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIESCASE STUDYPhD student Eliezer Estrecho awarded 1st Poster for ‘Single-shot imaging of exciton-polariton condensates’ at PLMCN18–international conference on physics of light-coupling in nanostructures. Pictured with PI Sven Höfling at University of Würzburg.TRAPPING LIGHT–MATTER PARTICLESFLEET collaboration traps light–matter particles FLEET’s Research theme 2 seeks to create near-zero resistance flow of exciton–polaritons, which are hybrid quasi-particles that are part matter and part light.The resistance-less flow relies on formation of an exciton–polariton condensate – a collective quantum state that behaves as a superfluid.In superfluids, particles flow without encountering any resistance to their motion. An exciton–polariton condensate is typically created within a semiconductor structure known as an optical microcavity, which enables strong coupling between the photons (light) and excitons (matter).These microcavities are micrometre-scale heterostructures with two highly-reflective mirrors on either side of an optical medium.This research took an important step forward in 2017 when FLEET researchers developed sufficiently high-quality microcavities to achieve a strong light–matter coupling regime and to ultimately achieve exciton-polariton condensation at room temperatures.FLEET’s Monash University engineering labs developed the high-quality optical microcavities, which were designed by FLEET researchers at the ANU, led by FLEET PhD student Eliezer Estrecho.25FLEET 2017 ANNUAL REPORTFollowing fabrication the structures were characterised in the exciton-polariton laboratory at ANU to assess their suitability for embedding the necessary exciton-hosting medium (an atomically-thin semiconductor monolayer) and creating exciton–polaritons.The main achievement of the experiment was the extended lifetime of the photon trapped in the microcavity, which was at least an order of magnitude larger than that achieved previously in microcavities of similar design. This vastly improves the chances of reaching the strong light-matter coupling regime necessary for observing exciton–polaritons in these structures.The ability to develop and characterise optical microcavities ‘in-house’ at FLEET is extremely important for future Centre research as it enables fabrication of highly customised semiconductor devices precisely tailored for studies of exciton–polariton condensation and superfluidity.The knowledge gained in design, fabrication and optical characterisation of the new microcavities will now be applied in creation of an optimised host structure for exciton–polaritons in atomically-thin semiconductor monolayers.Design, theory, and characterisation was done at the ANU in collaboration with RIKEN (Japan), while nanofabrication was performed by FLEET researchers at Monash University. Eliezer Estrecho’s two-months training with Centre partners at Würzburg University allowed him to take the lead of this project at the ANU.COLLABORATING FLEET PERSONNEL: >PhD student Eliezer Estrecho, ANU >CI Elena Ostrovskaya, ANU >PhD student Maryam Boozarjmehr, ANU >CI Qiaoliang Bao, Monash UniversityLeft: schematic of microcavity (red represents a TMD monolayer)Right: actual image of bottom DBR mirror composed of SiO2/Si3N4PROF KRIS HELMERSONLeader, Research theme 3Monash University“The physics of systems temporarily forced far from equilibrium could open the way to new dissipationless conduction mechanisms, a possible basis of future electronics”Expertise: ultra-cold collisions of atoms, matter–wave optics, nonlinear atoms dynamics, atomic gas superfluidity, atomtronics, non-linear atom opticsResearch outputs: 100+ papers, 5000+ citations, h-index 30RESEARCH THEME 326IN 2018, FLEET WILL: >Develop and characterise 2D materials with spatially uniform optical properties for Floquet band engineering >Control p-wave Feshbach resonance in 2D Fermi gas, generating a topological superfluid >Demonstrate spin-orbit coupling in periodically driven atomic system >Generate and optically characterise air-stable, metal–organic nanomaterials >Develop theoretical framework incorporating quantum correlations and thermal effects in dynamics of quantum many-body system far from equilibrium >Acquire and set-up femtosecond laser system for pump-probe spectroscopy of 2D materials >Design and acquire components for quantum-gas microscope (see p28).RESEARCH THEME 3: LIGHT-TRANSFORMED MATERIALSFLEET’s third research theme represents a paradigm shift in material engineering, in which materials are temporarily forced out of equilibrium.The zero-resistance paths for electrical current sought at FLEET can be created using two non-equilibrium mechanisms: >Short, intense bursts of light temporarily forcing matter to adopt a new, distinct topological state >Dynamically engineered dissipationless transport.Very short, intense pulses of light are used to force materials to become topological insulators (see Research theme 1, p18) or to shift into a superfluid state (see Research theme 2, p22).The forced state achieved is only temporary, but researchers learn an enormous amount about the fundamental physics of topological insulators and superfluids as they observe the material shifting between natural and forced states, over a period of several microseconds.By using ultrashort pulses to switch between the dissipationless-conducting and normal states, we can also create ultra-fast opto-electronic switching of this dissipationless current. Jeff Davis (top) and Chris Vale (bottom) study dynamic properties of topological materials via cold atoms in synthetic dimensions and ultrafast laser spectroscopy27FLEET 2017 ANNUAL REPORTThe understanding of atoms in equilibrium was one of the triumphs of 20th century physics, forming the basis of thermodynamics. But the behaviour of systems far from equilibrium is at the frontier of physics.Prof Kris Helmerson Research theme 3 leaderThe networking, brainstorming, and mixing of FLEET members between nodes was my highlight of working in the Centre this year. This motivates each researcher to increase their personal research output.Dr Shilpa Sanwali FLEET Research Fellow, Swinburne University of Technology2017 HIGHLIGHTS >Establishing new, shared infrastructure atMonash University (low-temperature scanningprobe microscopy) and Swinburne Universityof Technology (quantum-gas microscope,see case study p28) >Determining motion of vortices driven to non-equilibrium temperatures in a 2D superfluid atomic gas >Hosting visit of Nobel Laureate Prof WolfgangKetterle at FLEET’s ultra-cold laboratories, and hispresentation of public talks (see p64) >Beginning new collaborations between SwinburneUniversity of Technology and Colorado University,and between physics and engineering atMonash University.ULTRA-FAST PULSES OF LIGHTThe pulses of light FLEET uses to transform materials are intensely short. The period is measured in femtoseconds, which are millionths of billionths of a second. DEFINITIONS dissipationless current Electric current that flows without wasted dissipation of energyequilibrium state The state in which a material is in balance, unchanging with timenon-equilibrium state A state temporarily forced by the application of energy, such as lightmonolayer A single layer of materialnon-linear interactions Interactions in which forces acting on a system cause disproportionate resultssuperfluid A quantum state in which particles flow without encountering any resistance to their motion. Both excitons and exciton–polaritons can flow in a superfluid.Next >