We study the electronic states of quantum materials using angle-resolved photoemission spectroscopy (ARPES), and use molecular-beam epitaxy to fabricate new quantum materials, structured at the atomic scale, as a route to tailor their interacting electronic states. See below to read about some of our research interests, and our experimental approach and equipment. We also have a more general introduction to what we do for non-scientists.
One of the most active challenges of modern solid state physics and chemistry is harnessing the unique and varied physical properties of transition-metal oxides. Subtle collective quantum states underpin their diverse properties. These complicate their physical understanding but render them extremely sensitive to their local crystalline environment, offering enormous potential to tune their functional behaviour. They are therefore not only an exciting playground for study of the quantum many-body problem in solids, but also, if properly controlled, a powerful platform for future technologies. They provide almost unlimited scope to design new materials with near-arbitrary properties, exploiting powerful tuning parameters such as quantum confinement and epitaxial strain. Combining these via superlattice creation, forming new "metamaterials" which do not have any analogue in bulk form, holds enormous potential to stabilise new electronic, magnetic, and thermodynamic states and phases. Their complexity, however, means we are still far from able to dial-up a desired property at will.
As part of the Centre for Designer Materials, we perform growth of thin films and heterostructures of transition-metal oxides with near atomic precision, and study the electronic states and phases that we create with in situ angle-resolved photoemission spectroscopy. We also investigate the bulk and surface electronic structures of single-crystal oxides, with a particular current focus on Delafossite oxide metals. We are interested in the beautiful physics that these systems possess, but also motivated by the potential for future widespread technological impact exploiting the quantum many-body states and phases of transition-metal oxides.
2D Quantum Materials
Two-dimensional materials (2DMs), that can be as thin as a single atom, represent fundamental building blocks for creating new advanced materials. Widespread interest in these systems has been spurred by the remarkable properties of graphene, and more recently semiconductors such as MoS2. We are interested in developing methods to additionally exploit, and ultimately control, strong electronic interactions, to pave the way for a new generation of 2D quantum materials (2DQMs). Enormous strides in band structure engineering have already been achieved by stacking together different weakly-interacting 2D materials, realising functional devices
such as tunnelling transistors and chiral light emitters. Additionally harnessing strong correlations promises much greater tuneability: combining strongly-interacting 2DQMs in different configurations and environments will yield rich phase diagrams and dramatically-enhanced functional properties, but predicting their emergent properties is extremely challenging. To this end, we combine experimental screening of candidate building blocks, bottom-up atomic assembly of their custom heterostructures, and advanced spectroscopic feedback to develop an integrated approach to their targeted design.
Angle-resolved Photoemission Spectroscopy
Angle-resolved photoemission spectroscopy (ARPES) is a powerful probe of the momentum-dependent electronic structure and many-body interactions in solids. We operate the a lab-based ARPES system optimised for the study of quantum materials. The system's capabilities include:
A high-resolution Specs Phoibos 225 hemispherical electron analyser, combined with a high-intensity He plasma lamp with rotatable linear polarization. With this setup, we routinely achieve energy resolutions of 3-10 meV;
A tuneable laser source, based around the APE HarmoniXX harmonic generation, providing 190-210nm photons with 80MHz rep. rate;
A monochromatized Al K𝛼 X-ray source permitting X-ray photoemission spectroscopy measurements of core levels and valence band density of states with a resolution better than 300 meV;
An ultra-high vacuum (< 5 x 10-11 mbar) sample environment, with the sample held on a 6-axis manipulator where it can be cooled to 4 K;
Capabilities for in situ sample cleavage, preparation by sputter and annealing in a dedicated preparation chamber, and in situ transfer from the materials synthesis capabilities of the Centre for Designer Quantum Materials within an all-UHV sample transfer environment.
Spin- and Angle-resolved Photoemission Spectroscopy
We are currently developing a new capability for spin-resolved ARPES (the first of its kind in the UK), which we hope to become operational mid- to late-2019.
Spectroscopy at Central Facilities
We are regular users of major international facilities in the UK and abroad. In recent years, we have had regular beamtimes at: Diamond Light Source, Elettra, SOLEIL, and HiSOR synchrotrons, as well as the ARTEMIS facility of the UK central laser facility for time-resolved ARPES studies.
As part of the Centre for Designer Quantum Materials, we operate two DCA R450 molecular-beam epitaxy systems, optimised for the growth of transition-metal oxides and chalcogenides, respectively. They enable the atomically-precise growth of thin-films and heterostructure of a wide array of different compounds, including hybrid oxy-chalcogenides via a direct ultra-high vacuum connection between the growth modules. Core specifications include:
Distilled ozone as oxidant
10 differentially-pumped effusion cells, allowing for high-pressure ozone growths and rapid exchange of source materials
4-pocket e-beam evaporator with EIES flux control
In-situ RHEED monitoring
High-temperature ozone-resistant sample manipulator, taking substrates of up to 2" diameter
Up to 9 shuttered effusion cells
3-pocket e-beam evaporator
Valved cracker for Se/Te evaporation
In-situ RHEED monitoring
Sample manipulator, taking substrates of up to 2" diameter
Photo © Tricia Malley Ross Gillespie www.broaddaylightltd.co.uk
Both MBEs are connected via direct in-vacuum transfer to our spectroscopic tools for angle-resolved photoemission and, in due course, spin-resolved ARPES, and samples can be removed into a vacuum suitcase for transfer to a wide range of other experimental capabilities.
What we do - for non-scientists
The world around us is governed by materials, from the steel that we use to build bridges and buildings to the silicon that forms the basis of computer chips at the heart of so many modern technologies. Today, we are undoubtedly in the "silicon age", but just as the Stone Age gave way to the Bronze Age and then Iron Age, new materials will emerge that surpass the properties and use of Silicon. We are a group of scientists interested in understanding, and ultimately designing, such materials of the future. To do this, we build up new materials a single atom layer at a time, and study how their electrons behave using sensitive experimental probes that can build up road-maps for how the electrons move, and that can determine how large numbers of their electrons behave to give rise to some of the most spectacular, but still poorly understood, properties of materials that are known today. We work together with theorists, who model how materials behave and design new structures that could lead to devices of the future. The videos below, prepared with some of our collaborators, give a general introduction to what we do and why. You can also find some more information in recent features on our lab on the BBC, Scottish TV, and Scottish press.
Header photo © Tricia Malley Ross Gillespie www.broaddaylightltd.co.uk
We gratefully acknowledge support from: