< Previous28 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES A/PROF ELENA OSTROVSKAYA Leader, Research theme 2 Australian National University “Research theme 2 highlights FLEET’s collaborative nature, involving cross- disciplinary input between nodes and with several Partner Investigators.“ Expertise: nonlinear physics, quantum degenerate gases, Bose- Einstein condensates, exciton-polaritons Research outputs: RESEARCH THEME 2: EXCITON SUPERFLUIDS FLEET’s second research theme uses 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, so 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 using 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, using atomically-thin semiconductors as the medium for the superfluid. IN 2019, FLEET WILL: • Fabricate microcavities with transition metal dichalcogenides (TMDs) and observe strong light-matter coupling • Characterise low-energy interactions in exciton systems • Investigate designs to support twin-layer excitons • Build on collaborations between FLEET nodes to design, fabricate and characterise heterostructures for theme 2 research. 2018 HIGHLIGHTS • Puzzling results explained: discovery of multiband mechanism for sign reversal of Coulomb drag in bi-layer graphene structures (see p30) • First ‘single-shot’ observation of exciton-polariton condensation; an insight into non-equilibrium, solid- state condensation (see p32) • Observation of hybrid exciton-polariton condensation in a quantum-well/TMD-monolayer microcavity; the first step towards condensation of exciton-polaritons in a TMD monolayer, a key FLEET goal. DID YOU KNOW... A superfluid is a quantum state in which particles flow without encountering any resistance to their motion. Both excitons and exciton–polaritons can flow in a superfluid. RESEARCH THEME 229 FLEET 2018 ANNUAL REPORT 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 Bose-Einstein condensate (BEC) A quantum state occurring at ultra-cold temperatures dissipationless current Electric current that flows without wasted dissipation of energy exciton Two strongly-bound charged particles: an electron and a ‘hole’ exciton-polariton Part matter and part light quasi-particle: an exciton bound to a photon microcavities A micrometre-scale structure; an optical medium sandwiched between ultra-reflective mirrors, used to confine light such that it forms exciton-polaritons monolayer A single 2D layer of material non-equilibrium state A state temporarily forced by the application of energy, such as light superfluid A quantum state in which particles flow without encountering any resistance to their motion; both excitons and exciton-polaritons can flow in a superfluid transition metal dichalcogenides (TMDs) Atomically-thin materials with useful physical properties for electronic and optoelectronic devices; used as the optical medium in microcavities Theme 2 researchers Dr David Colas (UQ) and Dr Eliezer Estrecho (ANU).30 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES PUZZLING RESULTS EXPLAINED IN EXCITON EXPERIMENT Taking a multiband approach explains electron-hole ‘reverse drag’ In 2018, a FLEET theoretical study finally unlocked a previously mysterious result that seemed to show coupled holes and electrons moving in the opposite direction to that predicted by theory. The FLEET study showed that this seemingly contradictory phenomenon is associated with the bandgap in dual- layer graphene structures, a bandgap which is very much smaller than in conventional semiconductors. The study authors, which included FLEET Partner Investigator Prof David Neilson at the University of Camerino (Italy) and FLEET CI Prof Alex Hamilton at UNSW, found that the new multiband theory fully explained the previously inexplicable experimental results. Exciton transport offers great promise to researchers, including the potential for ultra-low dissipation future electronics. In an indirect exciton, free electrons in one two-dimensional (2D) sheet can be electrostatically bound to ‘holes’ (effectively, absent electrons) in a neighbouring 2D sheet. Because the electrons and holes are each confined to their own 2D sheets, they cannot recombine, but they can electrically bind together if the two 2D sheets are very close (a few nanometres). CASE STUDY Left: An electron (e) accelerated in the top sheet causes a hole (h) in the lower sheet to be accelerated. Right: Device schematic: one sheet of graphene carries electrons, the other, separated by insulating hBN, carries holes.31 FLEET 2018 ANNUAL REPORT More at FLEET.org.au/puzzling-excitons This research area allows us to combine very deep, fundamental questions about the nature of quantum phase transitions in a solid state and, at the same time, is very promising for future applications in low-energy electronics. A/Prof Elena Ostrovskaya FLEET Chief Investigator, ANU If electrons in the top (‘drive’) sheet are accelerated by an applied voltage, then each partnering hole in the lower (‘drag’) sheet can be ‘dragged’ by its electron. A goal in such a mechanism is for the exciton to remain bound, and to travel as a superfluid, a quantum state with zero viscosity, and thus without wasted dissipation of energy. To achieve this superfluid state, precisely-engineered 2D materials must be kept only a few nanometres apart. An insulating sheet between two sheets of atomically- thin (2D) graphene prevents recombination of electrons and holes. Passing a current through one sheet and measuring the drag signal in the other sheet allows experimenters to measure the interactions between electrons in one sheet and holes in the other, and to ultimately detect a clear signature of superfluid formation. However, experiments published in 2016 showed extremely puzzling results. Under certain experimental conditions, the Coulomb drag was found to be negative. That is, moving an electron in one direction caused the hole in the other sheet to move in the opposite direction! These results could not be explained by existing theories. COLLABORATING FLEET PERSONNEL: • Chief Investigator Alex Hamilton (UNSW) • Partner Investigator David Neilson (University of Camerino) The FLEET study explained these puzzling results using crucial multi-band processes that had not previously been considered in theoretical models. Bi-layer graphene has a very small bandgap, which can be changed by application of an electric field. The calculation of transport in multiple bands was the ‘missing link’ marrying theory to experimental results. The strange ‘negative drag’ happens when available thermal energy approaches the bandgap energy. This addresses FLEET milestone 1.2; see p93. The study was published in Physical Review Letters in July 2018, vol. 121 (see publication 57, p106) . I love this field of research because it gives us so much freedom and flexibility in designing the experiments and testing novel ideas in exciton- polariton superfluids. Dr Harley Scammell FLEET Research Fellow UNSW32 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES FIRST EXCITON SNAPSHOT First-ever ‘snapshot’ of Bose-Einstein condensation achieved in FLEET/ANU study Previously, observations of exciton-polaritons in a Bose- Einstein condensate have been limited to statistical averaging over millions of condensation events. ‘Snapshot’ imaging of polaritons forming a condensate in a typical inorganic semiconductor was considered impossible. In 2018, FLEET researchers at ANU led an international study imaging exciton-polaritons for the first time as a ‘single shot’, rather than averaging. “This offers a unique opportunity to understand the details of Bose-Einstein condensation of exciton- polaritons,” explains lead author Dr Eliezer Estrecho. Such fundamental advances also aid FLEET’s research on excitonic condensation and superfluidity as a mechanism for electronic conduction without wasted dissipation of energy. Exciton-polaritons are hybrid particles that are part matter and part light, bound together by strong coupling of photons and electron-hole pairs (excitons) within a semiconductor microcavity. However, because exciton-polariton lifetimes are measured in picoseconds (trillionths of a second), previously observations have always averaged over a million lifetimes of exciton-polaritons. CASE STUDY Single-shot condensation of polaritons. Theory (on the right) shows remarkable agreement with experiment (left).33 FLEET 2018 ANNUAL REPORT Single-shot imaging of a polariton condensate was thought impossible, but we still tried, and succeeded, finding interesting effects never observed in experiments before. Dr Eliezer Estrecho FLEET Research Fellow, ANU This is like taking a long exposure of moving objects: you get a blurred image. The ANU team made sure that their sensitive camera captures only one lifetime or ‘single shot’ of the condensate, enabling them to observe never-before-seen behaviour of exciton-polaritons. The single-shot imaging is performed by analysing photoluminescence caused by the decay of exciton- polaritons, a technique thought to be impossible in inorganic microcavities because emissions simply weren’t bright enough. Usually, the density of exciton-polaritons trapped in inorganic microcavities is too low to be detected in single-shot mode, partly because exciton-polaritons do not live long enough for the density to build up. To get a better signal, the team used ultra-high-quality samples designed and made by their collaborators in the USA, extending the lifetime of polaritons by an order of magnitude and pushing the density high enough for the sensitive camera to detect. The imaging revealed that, contrary to the smooth condensate observed in averaged experiments, the condensate actually forms filaments whose orientation varies from shot to shot. COLLABORATING FLEET PERSONNEL: • Research Fellow Eliezer Estrecho (ANU) • Chief Investigator Elena Ostrovskaya (ANU) The study found remarkable agreement between experiment and numerical simulations, validating the background theory of exciton-polariton condensate dynamics. The work paves the way for further fundamental studies of quantum phase transitions and non-equilibrium condensation in solid-state systems. The single-shot experiments could prove critical for our understanding of the fundamental (and still debated) nature of the condensed phase in these systems. This addresses FLEET milestone 1.2; see p93. The study was published in Nature Communications in August 2018, vol. 9 (see publication 13, p104). More at FLEET.org.au/exciton-snapshot34 Because this research is so interdisciplinary, I am able to connect multiple research directions into one and see a bigger picture, making me a more-rounded scientist. Pavel Kolesnichenko FLEET PhD student, Swinburne ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES PROF KRIS HELMERSON Leader, Research theme 3 Monash University “FLEET puts us at the forefront of research and potential application of the non-equilibrium behaviour of materials” Expertise: ultra-cold collisions of atoms, matter–wave optics, nonlinear atoms dynamics, atomic gas superfluidity, atomtronics, non-linear atom optics Research outputs: 100+ papers, 5000+ citations, h-index 31 RESEARCH THEME 3: LIGHT-TRANSFORMED MATERIALS FLEET’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, p24) or to shift into a superfluid state (see Research theme 2, p28). 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. IN 2019, FLEET WILL: • Begin construction of quantum gas microscope facility at Swinburne University to study dipolar atoms in optical lattices • Engineer wave interactions via Feshbach resonances in a 2D Fermi gas to ultimately realise topological superfluidity • Further develop femtosecond band control in 2D solid-state material for engineering of Floquet topological insulators • Investigate, with ultracold atoms, the effect of interactions between particles in the quantum kicked rotor to test theories of many-body dynamical localisation and insulator behaviour of materials • Launch experimental infrastructure to study ultra-fast, light-induced dynamics and light-driven topological phase transitions in optically active materials • Develop theory of driven dissipative superfluid to improve understanding of non-equilibrium transport • Further develop a general framework for understanding behaviour of quasiparticles and low-energy excitation • Demonstrate spin-orbit coupling in periodically-driven atomic system • Investigate the utility of time crystals in the context of Floquet states. RESEARCH THEME 335 FLEET 2018 ANNUAL REPORT DEFINITIONS Bose-Einstein condensate (BEC) A quantum state occurring at ultra-cold temperatures dissipationless current Electric current that flows without wasted dissipation of energy equilibrium state The state in which a material is in balance, unchanging with time Floquet topological insulator A topological insulator created by applying light to a conventional insulator non-equilibrium state A state temporarily forced by the application of energy, such as light non-linear interactions Interactions in which forces acting on a system cause disproportionate results spin-orbit interaction The interaction between electrons’ movement and their inherent angular momentum, which drives topological effects superfluid A quantum state in which particles flow without encountering any resistance to their motion. Both excitons and exciton-polaritons can flow in a superfluid 2018 HIGHLIGHTS • Observation of quantum anomaly in an ultra-cold 2D Fermi gas (see case study p36) • Comprehensive explanation of the meaning of negative effective mass in spin-orbit coupled BECs (see case study p38), improving understanding of atom transport in materials due to transient applied forces or impulses • Realisation of negative absolute temperature distribution of vortices in a superfluid, verified in twin Monash/University of Queensland studies • Control of Floquet-Bloch bands with femtosecond laser pulses, indicating that Floquet-Bloch states are minimally affected by finite pulse duration (down to 30 fs): a step toward dynamic band-structure engineering of materials • Development of a new theoretical approach for finite- temperature dynamics, improving understanding of impurity effects at non-zero temperature, a phenomena common to all materials • Theoretical study of quantum battery, indicating that quantum effects can improve charging of a spintronic battery with implications for new approaches of driven, dissipationless conduction, as well as faster switching of magnetic materials. DID YOU KNOW... FLEET researchers cool atomic gases to only a few nanoKelvins above Absolute Zero, which is a billion times colder than interstellar space. FLEET PhD student Marina Castelli examines samples in scanning tunnelling microscope (Monash).36 ARC CENTRE OF EXCELLENCE IN FUTURE LOW-ENERGY ELECTRONICS TECHNOLOGIES BREAKING A CLASSICAL SYMMETRY WITH ULTRA-COLD ATOMS Scaling symmetry in a 2D Fermi gas breaks down with strong interactions between particles A 2018 FLEET study of ultra-cold atomic gases – a billionth the temperature of outer space – unlocked new, fundamental quantum effects. In this study, a simple, classical theory of atomic interaction is shown to be break down, and a more- sophisticated quantum treatment is required. The researchers at Swinburne University of Technology studied collective oscillations in ultra-cold atomic gases – identifying where quantum effects occur to ‘break’ symmetries predicted by classical physics. They also observed the transition between two- dimensional (2D) behaviour and three-dimensional (3D) behaviour. “Fundamental discoveries made from such observations will inform FLEET’s search for electronic conduction without wasted dissipation of energy,” explained study author Prof Chris Vale. Two-dimensional materials exhibit many novel physical properties and are keenly studied for their potential uses; for example, in ultra-low energy electronics. However, strong correlations and imperfections within 2D materials make them difficult to understand Laser equipment (532nm) at Swinburne University of Technology, used to confine the atomic gas. An oscillating magnetic field is applied to an atomic gas (shown upper right), causing it’s size to oscillate in two dimensions. CASE STUDY37 FLEET 2018 ANNUAL REPORT theoretically. Quantum gases of ultra-cold atoms help unlock the fundamental physics of 2D materials, as well as uncovering new phenomena not readily accessible in other systems. Experiments performed on quantum gases of ultra-cold neutral atoms enhance our understanding of phase transitions and the effects of interactions between particles. This improved ability, understanding and control of phase transitions will have a direct application in FLEET’s development of future low-energy, topologically-based electronics. ‘Symmetries’ are an essential ingredient in the formulation of many physics theories, allowing simplified descriptions by identifying which factors don’t modify a system’s underlying physical properties. For example, in a ‘scale invariant’ system, changing the distances between its particles doesn’t alter the behaviour of a material but merely ‘scales’ it by an appropriate factor. Gases of ultra-cold atoms confined to a two- dimensional plane allowed FLEET researchers to explore regimes where that ‘scaling symmetry’ can be broken by quantum effects. COLLABORATING FLEET PERSONNEL: • Associate Investigator Paul Dyke (Swinburne) • Research Fellow Ivan Herrera (Swinburne) • Research Fellow Sasha Hoinka (Swinburne) • Chief Investigator Chris Vale (Swinburne) • Chief Investigator Meera Parish (Monash) • Associate Investigator Jesper Levinson (Monash) FLEET inspires me to do new research and to seek new collaborations. Dr Jesper Levinson FLEET Scientific AI, Monash University Researchers studied a strongly-interacting 2D gas of lithium atoms, measuring the frequency of a radial oscillation known as the ‘breathing mode’, which is a window to the gas’s thermodynamic equation of state and whose frequency is set by the gas’s compressibility. The breathing mode is the gas’s lowest energy collective oscillation, and as long as scaling symmetry exists, the breathing mode should always occur at the same frequency (exactly twice the harmonic confinement frequency). The study confirmed that scaling symmetry is broken in the presence of strong interactions between particles, affecting the thermodynamic relation between the pressure and density. This is called a quantum anomaly, being something that occurs when a symmetry that is present in a classical theory is broken in the corresponding quantum theory. 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 September 2018, vol. 121 (see publication 32, p105). More at FLEET.org.au/breaking-symmetryNext >