Quantum materials

Arzhang Ardavan

Molecular magnets are a class of zero-dimensional strongly correlated electron systems exhibiting a highly coherent quantum spin at low temperatures. By varying the molecular structure, properties of the spin such as its moment, anisotropy, etc, can be manipulated in a controlled way, offering a beautiful playground for experiments in quantum magnetism.

This project will investigate ways of exploiting the quantum properties of molecular magnets with a range of objectives. Through close collaborations with chemists who synthesise the materials, we will design structures using molecular spins as qubit candidates. In simple multi-qubit structures, we will use state-of-the-art electron spin resonance equipment to generate entangled states and perform basic quantum information algorithms. We will also develop new ways of measuring electrical transport through single molecules using innovative methods for device construction, and scanning tunnelling microscopy (via collaboration with IBM).

Stephen Blundell

A muon is a spin-1/2 particle. When implanted in a solid, the muon behaves as a microscopic magnetometer. This is because its spin can precess in the local magnetic field. In various frustrated magnets it is possible for muons to probe low-frequency spin dynamics. By combining muon-spin rotation experiments and a.c. susceptibility one can learn a lot about these slow fluctuations which are known to persist to low temperature. But are these so-called “persistent spin dynamics” an intrinsic effect, or are they a highly subtle artefact of the muon implantation process? This project is designed to find out.

Stephen Blundell

The Nicholas Kurti High Magnetic Field Laboratory in the Clarendon Laboratory is capable of producing the highest magnetic fields for condensed matter physics experiments in the UK. This is achieved using pulsed magnetic fields which are short in duration and are a factor of 2 or 3 times higher than the ~20T limit set by superconducting magnets. This allows experiments in which quantum magnets are pushed into new regimes. Some of these quantum magnets are based on inorganic materials, but why not use molecules? This gives a much more flexible approach to understand low-dimensional quantum magnetism.

Stephen Blundell and Amalia Coldea

Magnetic fields are a unique tool to explore and tune quantum materials towards extreme experimental regimes in which new phases of matter can be stabilised. Additionally, magnetic fields are essential to characterise the phase diagram of novel superconductors to identify suitable candidates for practical applications. This is an experimental project to explore and develop new experimental techniques for studying quantum materials in ultra-high magnetic fields. The quantum materials to be explored will include novel iron-based superconductors and molecular magnets, as well as systems in which the spin, electronic and lattice degrees of freedom interact strongly and can be disturbed by a magnetic field.

The student will be combining transport and thermodynamic techniques using superconducting magnets and pulsed field magnets available in Oxford. The pulsed magnetic fields are short in duration but up to a factor of 3 times higher than the field produced by superconducting magnets. Oxford has a long tradition in high magnetic field research having the largest magnetic fields in the UK for condensed matter physics both using pulsed field in the Nicholas Kurti High Magnetic Field Laboratory as well as superconducting magnets up to 21T as part of the High Magnetic Field facilities and the new Oxford Centre for Applied Superconductivity.

A suitable candidate needs to have a good understanding of condensed matter physics and good computational skills as well the ability to work well in an experimental team. The student will be co-supervised by Professor Stephen Blundell and Professor Amalia Coldea.

Andrew Boothroyd

Systems of interacting electrons frequently exhibit subtle forms of order, examples being superconductivity and magnetism. In this project you will investigate electronic order and dynamics experimentally. Neutron and X-ray scattering will be the main techniques used, taking advantage of the new ISIS 2nd target station and the Diamond Light Source, plus other facilities in Europe.

A willingness to travel is essential. Magnetic, transport and thermal measurements will be performed in the Clarendon Laboratory, and there is scope for theoretical modelling and numerical analysis.

At a fundamental level, atomic magnetism originates from the intrinsic spin and the orbital motion of the electrons, which are connected by the spin-orbit coupling (SOC) interaction. This project concerns magnetism in oxides containing heavy metal atoms such as ruthenium, molybdenum and iridium. These atoms have partially filled 4d or 5d electronic orbitals with a very large SOC which strongly entangles the spin and orbital degrees of freedom, resulting in a diverse range of magnetic phenomena as well as the potential for applications in spintronics and energy harvesting.

Resonant inelastic x-ray scattering (RIXS) has emerged as a powerful spectroscopic technique and has been particularly influential in studying phenomena in compounds with strong SOC. In this collaboration with Dr. Ke-Jin Zhou at Beamline I21 in Diamond Light Source and Prof. Andrew Boothroyd at the University of Oxford, you will investigate strong SOC physics within two research themes. The first concerns excitonic Mott insulators with d4 electronic configuration, e.g., Ru4+ in Ca2RuO4 and Ca3Ru2O7. In the ideal case such d4 ions have a non-magnetic singlet ground state and a triplet excited state with inter-site interactions that create propagating excitations called triplons. The second theme explores ions with the d1 configuration in double perovskites, e.g. Ba2YMoO6, in which the spin and orbital angular momenta are highly entangled leading to strong orbital-dependent exchange and exotic magnetic order. We will utilise RIXS at the L-edges of transition-metal cations and K-edge of oxygen ligands to probe the low-energy magnetic excitations, the excitonic physics and crystal field splittings. Candidates are expected to have a good understanding of condensed matter physics and be motivated to develop advanced experimental skills and data modelling.

This project is jointly funded by the Diamond Light Source and Oxford University.

For more information, contact Prof Andrew Boothroyd or Dr Kejin Zhou.

Amalia Coldea

This project is to explore electronic and topological behaviour of quasi-two dimensional devices based on thin flakes of highly crystalline superconducting iron-based chalcogenides as well as Dirac and Weyl semimetals. The project will involve device preparation and a suite of physical properties measurements to study their electronic properties using high magnetic field and low temperatures. The aim is to search for quantum phenomena as well as for signature of topological matter in these highly tunable quantum material devices. The project will be hosted by the recently funded Oxford Centre for Applied Superconductivity (CfAS) in the Department of Physics. The student will investigate the phase diagrams of novel superconducting thin flake devices under different extreme conditions of high magnetic field, strain and pressure. Experiments using advanced techniques for transport will be performed using high magnetic field facilities available in Oxford and elsewhere. A suitable candidate needs to have a good understanding of condensed matter physics and good computing skills as well the ability to work well in an experimental team.

For more info, please contact Prof Amalia Coldea and check out the website

Amalia Coldea

Applied hydrostatic pressure is a unique tuning parameter to study the characteristics of a nematic quantum critical point in the absence of long-range magnetic order in a single material and to gives access to the electronic structure and correlations of new magnetic and structural phases. FeSe is an unique superconductor that show a nematic electronic phase in which absence of magnetism at ambient pressure. However, a magnetic phase is stabilised at high pressure and superconductivity is enhanced four-fold. By combining the chemical pressure with the hydrostatic pressure in the series FeSe1-xSx, it is possible to separate the nematic and magnetic phases. This project will aim to understand the evolution of the complex Fermi surfaces and electronic interactions across the nematic phase transitions using applied hydrostatic pressure in different iron-based superconductors. High magnetic field and low temperatures will be used to access directly the Fermi surface by detecting quantum oscillations in different ground states tuned by applied hydrostatic pressure. A suitable candidate needs to have a good understanding of condensed matter physics and good experimental and computational skills as well the ability to work well in an experimental team.

For more info, please contact Prof Amalia Coldea and check out the website

Amalia Coldea

This is an experimental project combining electronic transport and quantum oscillations to detect unusual signatures of the manifestation of topology in single crystals of quantum materials with Dirac dispersions. The student will perform a series of studies in high magnetic fields and at low temperatures to search for evidence of non-trivial Berry phases and low temperature quantum transport. Studies will be also extended under applied pressure and strain to identify proximity to new toplogical superconducting phase. The work will be combined with first-principle band structure calculations to compare with experiments and disentangle trivial from non-trivial effects. A suitable candidate needs to have a good understanding of condensed matter physics and good computing skills as well the ability to work well in an experimental team.

For more info, please contact Prof Amalia Coldea and check out the website

Radu Coldea

We explore experimentally materials where quantum correlation effects between electrons are important and often lead to novel forms of electronic order or dynamics dominated by quantum effects. Of particular interest is the phenomenon of "quantum frustration", ie how quantum systems resolve competing interactions, this is explored in frustrated spin-, orbital- and charge-ordered systems. Another focus is "quantum criticality" when the transition temperature to spontaneous magnetic order can be suppressed by high magnetic fields all the way down to zero temperature, thus realising a regime where all ~10^23 electron spins in the material fluctuate strongly, but in perfect unison, a new regime for quantum matter that is only now becoming accessible experimentally and we plan to measure directly the quantum spin fluctuations via neutron scattering. The DPhil project will involve a mix of thermodynamic measurements, xray and neutron scattering experiments, data analysis and modelling.

JC Séamus Davis

2 x DPhil studentships are available from ERC (European Research Council)

The studentships are part of an European Research Council funded project entitled "MILLIKELVIN VISUALIZATION OF TOPOLOGICAL ORDER (mVITO)" being undertaken by the University of Oxford, UK and University College Cork, IE under PI JC Séamus Davis. The objective of this project is to develop and apply new techniques for atomic-scale visualization of electronic and magnetic quantum matter. Per interest and availability, the student will focus on one of the following sub-projects:

Visualisation of electronic wavefunctions in topological Kondo insulators or heavy fermion superconductors, using millikelvin scanning tunnelling microscopy.

Visualisation of Cooper-pair condensates or pair density wave states using scanned Josephson tunnelling microscopy.

Visualisation of classical and quantum spin liquids using scanned magnetic-flux microscopy.

S/he will be a member of the Davis Group and will be supervised by Professor JC Séamus Davis. Further information on the group and its research can be found here.

Prospective candidates will be judged according to how well they meet the following criteria:

Demonstrates curiosity, creativity and courage in scientific research.

• A first class honours degree in Physics or equivalent
• Experience in low temperature physics at liquid-helium temperatures T=4.2K or below
• Experience in ultra-high vacuum scanned probe microscopy
• Experience in high-volume image-data management and analysis
• Excellent English or Irish written and spoken communication skills

The following skills are desirable but not essential:

• Ability to program in Matlab and/or Python
• Ability in Labview-based experiment design and management
• Ability in cryogenic operations with liquid helium, and ultra-high vacuum tech
• Ability with modern theoretical techniques of quantum matter research

John Gregg

The end of Moore's Law has long been prophesied, but its effects have been subtly present for over a decade: since 2004, computer processor clock speeds have been frozen circa 4GHz, as a desperation measure to prevent heat death of the chips. As increasing functionality is packed at ever higher density into semiconductor devices, the resulting heat dissipation yields multiple issues: device unreliability; inability of battery technology to keep pace with the power demands of portable devices such as phones and tablets; and the enormous heat generation of "server farms" - performing 30 Google searches is claimed to dissipate enough heat to boil a kettle.

Alternative computing technology based on magnons (waves of propagating angular momentum that exist in ordered magnetic materials) offers an elegant and viable room temperature solution to these problems. Magnonic processors use 1/1000 of the power of their silicon counterparts, are engineerable on the nanoscale and have clock speed ceilings that are potentially in the TeraHertz. Sophisticated logic devices such as XOR gates and half adders have already been demonstrated as has a magnonic equivalent of the field effect transistor. Moreover, magnonic computing paradigms offer functionality and economy of "real estate" that is impossible with silicon, such as the ability simultaneously to perform different operations on parallel datastreams using the same hardware. Recent work by our team demonstrates magnonic ability to perform the operations of time reversal and phase conjugation with a view to combining the speed of analogue computing with the versatility of digital.The D.Phil. project here described will involve further developing this new microwave science and its integration into conventional electronic hosts.

Thorsten Hesjedal

Overview of topolicxally non-trivial spin textures (Credit: Göbel et al., Phys. Reports 895, 1 (2021); CC BY 4.0).

Topological objects are localized in space and exist for extended periods of time. In magnets, there are many examples of such objects such as domain walls, skyrmions and antiskyrmions, and recently, many more have been predicted to exist, including hopfions, a 3D analogue of the skyrmionic vortex, as well as torons, merons and blochions. Noncollinear magnetic systems offer a rich spectrum of excitations across a huge range of timescales, some tied to the dynamics of ordered magnetic lattices and some to the internal dynamics of the magnetic textures themselves. Further, the 3D modulations present in such objects lead to emergent electric and magnetic fields acting on spin-polarised electrons and magnons, resulting in unusual transport phenomena.

Until now, conventional ferromagnetic resonance (FMR) has been used extensively to determine fundamental magnetic parameters in thin films using resonance frequencies (related to internal and applied fields) and relaxation (determined by damping of the resonance). Further, it has been used as an indirect identification of the internal dynamic skyrmion modes, e.g., clockwise and anticlockwise gyration modes or breathing modes. However, topological magnetic systems require the development of new techniques that give direct access to their dynamical properties. The novel technique of X-ray detected FMR (XFMR) enables us to study the element-selective magnetization dynamics via X-ray magnetic circular dichroism (XMCD). Time-dependent XFMR measures both the amplitude and phase of the spin precession of chemically distinct layers. The experimental challenge is that the precession frequency is on the order of GHz and the precession cone angle is <1°. The solution lies in stroboscopic measurements utilizing the time structure of the synchrotron (∼500 MHz). The radio frequency (RF) field that drives the spin precession is synchronized with the X-ray pulses using the clock of the synchrotron. Each X-ray pulse measures the magnetisation cone at precisely the same point in the cycle. In brief, XFMR combines FMR and XMCD as pump and probe, respectively.

The inverse spin-Hall effect (ISHE) is a process that converts a spin current into an electric current and can be investigated systematically in simple ferromagnetic/paramagnetic bilayer systems, whereby the spin pumping driven by ferromagnetic resonance injects a spin current into the paramagnetic layer. We aim to combine XFMR with ISHE, which detect the AC and DC components of the spin pumping, respectively, giving insight into the effects of emergent fields on the transport properties of topological magnetic objects.

The student will be making use of the following research techniques:

• Sample growth: Magnetron sputtering in Oxford.
• Ex-situ characterization (SQUID, XRD/XRR, MOKE) at Diamond Light Source (DLS) and Oxford.
• Photolithography at Oxford.
• ISHE measurements at Oxford and the Magnetic Spectroscopy Lab at DLS.
• XFMR and XAS/XMCD/XMLD: X-ray spectroscopies at DLS beamlines (and others).
• FMR measurements in the Magnetic Spectroscopy Lab at DLS.

The student will obtain the following training:

• Graduate courses, transferrable skills training at Oxford.
• Hesjedal group: Practical training in magnetron sputtering and MBE, structural and magnetic sample characterisation; photolithography; transport (ISHE).
• van der Laan team, DLS: Experimental synchrotron techniques (XAS, XMCD, XMLD), XFMR, as well as FMR and ISHE. Theoretical modelling and micromagnetic simulations.

Funding for the studentship has been applied for through the Diamond Doctoral Studentship Programme (funding decision early December 2022). Fully funded studentships will be available for UK/Home fee students only.

Email   Prof Thorsten Hesjedal at: thorsten.hesjedal@physics.ox.ac.uk

            Prof Gerrit van der Laan at: Gerrit.vanderlaan@diamond.ac.uk

Paolo G Radaelli

Fig. 1: (A-C) Spatial distribution of antiferromagnetic topological textures measured in 1nm Pt coated α-Fe₂O₃ thin films via polarized photo-emission electron microscopy (PEEM) which show that whirling in-plane vortices and antivortices (in B) entrap OOP cores at their respective centres (in A), proving that the textures we see are (C) topological merons and antimerons (|Q|=1/2), and their pairs form either bimerons (|Q|=1) or topologically trivial meron pairs, TTMPs (Q=0).

In spite of its extraordinary success in fuelling the IT revolution, silicon (CMOS) technology is intrinsically energy-inefficient, because it relies on the movement of electrical charge, which is associated with Joule heating. One of the front runners among ‘beyond-CMOS’ technologies is spintronics, which relies on spins rather than charges to transfer and process information; however, much of the energy efficiency of spintronics is lost if spin flipping – the elementary spintronic operation – must in itself be performed by electrical currents. For this reason, voltage control of magnetic components is widely considered to be the key to large-scale commercialisation of spintronics [1-4]. The field of oxide electronics emerges precisely from the consideration that oxides, especially those containing magnetic transition metal ions such as Co, Mn and Fe, can display a multitude of intriguing electrically-controlled multi-functional properties in their insulating states, whilst integration with CMOS is already a reality. The potential of oxide electronics can be further enhanced by exploiting the power of topology, which involves, quite literally, tying spins into ‘magnetic knots’. In work published Nature [5] an international team of collaborators lead by Professor Paolo G. Radaelli (Oxford Physics) presented a major breakthrough in this field: they created, for the first time, a wide family of nanoscale antiferromagnetic topological spin textures – (anti)merons and bimerons at room temperature in iron oxide (α-Fe₂O₃). This follows their recently published work in Nature Materials [6,7], where they observed that these topological textures couple strongly with ferromagnetic metallic cobalt (Co). One particularly appealing feature of this system is that it employs cheap and readily available materials (α-Fe₂O₃ is the most abundant constituent of common rust!) and relatively simple fabrication, raising hopes that these systems could be deployed on a commercial scale in the future (for extended lay descriptions, see the Oxford Physics Newsletter – Autumn 2018 and this Diamond Research Highlight(link is external)).

This EPSRC-funded DPhil project will give the successful candidate the opportunity to develop this line of research in different directions, both fundamental and applied:
• Identify and grow new oxides with topological magnetic states, study their fundamental properties and image the topological structures at the nanoscale using state-of-the-art microscopy techniques.
• Experiment with novel ways to control topological magnetic states, exploiting either intrinsic magnetoelectric properties or interactions with active substrates
• Design and test prototype devices, built using electron beam lithography and other clean room processes.
This project is likely to involve a combination of experimental techniques, such as:
• Growth of thin films and devices – currently in collaboration with the groups of Prof. Thorsten Hesjedal (Oxford Physics), Prof. Venky Venkatesan (National University of Singapore), Prof. Chang-Beom Eom (Univ. of Wisconsin – Madison).
• Advanced microscopy. To image multi-functional domains, which are the fundamental unit of information storage in oxides, we employ spectral microscopy (PEEM – we invented and continue to develop many of the relevant data analysis methods at the Diamond synchrotron), Magnetic Force Microscopy (MFM – in house) Magneto-Optical Kerr Effect magnetometry/microscopy (new in house development) and Nitrogen Vacancy Centre Microscopy (new collaboration with the Max Planck Institute for Solid State Research in Stuttgart)
• X-ray imaging: this includes both established techniques such as XMLD-PEEM and  new direction, such as coherent imaging.  Students with a particular interest in the X-ray imaging should apply to the Diamond-funded studentship intitled “Imaging of antiferromagnetic domains by soft X-ray microscopy”, co-supervised by Prof. S. Dhesi at Diamond.
• Dielectric and transport measurements (in house)
• Elastic neutron scattering. We will perform experiments on bulk and films samples predominantly at the ISIS facility at Rutherford Appleton Laboratory.
• X-ray scattering, including resonant and non-resonant magnetic X-ray diffraction with hard and soft X-rays. We run state-of-the-art laboratory instrumentation in the Clarendon Laboratory, but we perform most of our high-end experiment at the Diamond Light source.
• Nanofabrication. In collaboration with National University of Singapore, we will be using electron beam lithography and other clean-room methods to design and build prototype oxide quantum materials devices.

Depending on the candidate's interests, the project may involve development of one or more of the above techniques (particularly the new techniques such as MOKE and N-V microscopy and X-ray coherent imaging) and may also include a computational element. We also employ micromagnetic simulations to study [8] the formation and dynamics of topological structures such as vortices, merons, antimerons, bimerons, skyrmions, skyrmionium and others. In collaboration with Prof. Feliciano Giustino at the Oden Institute, Univ. of Texas, we employ Density Functional Theory methods and other computational techniques to model the functional properties of oxides and to predict their behaviour in different architectures. There will also be an opportunity of extended stays at one of the collaborating institutes to acquire new skills.

For more information email Prof. Paolo Radaelli and visit the group webpages.

[1] Matsukura, F., Tokura, Y. & Ohno, H. Control of magnetism by electric fields. Nature Nanotechnology 10, 209–220 (2015).

[2] Eerenstein, W., Mathur, N. D. & Scott, J. F. Multiferroic and magnetoelectric materials. Nature 442, 759–65 (2006).

[3] Ramesh, R. & Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nature Materials 6, 21–9 (2007).

[4] S. Manipatruni, et al, Nature 565, 35 (2019).

[5] H. Jani et al., Nature 590, 74 (2021), arxiv.org/abs/2006.12699

[6] F. Chmiel et al., Nature Materials 17, pages 581–585 (2018)

[7] M. Fiebig, Nature Materials 17, pages 567–568 (2018)

[8] Radaelli P.G. et al., Phys. Rev B 101, 144420 (2020)

Paolo G Radaelli

A collaboration between Professor Paolo G. Radaelli and Professor Andrea Cavalleri, who holds a joint appointment between the Clarendon Laboratory and the Max Planck Institute for the Structure and Dynamics of Matter, (Hamburg).

The use of light to control the structural, electronic and magnetic properties of solids is emerging as one of the most exciting areas of condensed matter physics. One promising field of research, known as femto-magnetism, has developed from the early demonstration that magnetic ‘bits’ in certain materials can be ‘written’ at ultra-fast speeds with light in the visible or IR range [1]. More radically, it has been shown that fundamental materials properties such as superconductivity can be ‘switched on’ transiently under intense illumination [2]. Recently, the possibilities of manipulating materials by light have been greatly expanded by the demonstration of mode-selective optical control, whereby pumping a single infrared-active phonon mode results in a structural/electronic distortion along the coordinates of a second, anharmonically coupled Raman mode – a mechanism that was termed ‘nonlinear phononics’. Crucially, the Raman distortion is partially rectified, meaning that it oscillates around a different equilibrium position than in the absence of illumination. Recently it was realised that, under appropriate conditions, the rectified Raman distortion can transiently break the structural and/or magnetic symmetry of the crystal and hypothesised that such symmetry breaking would persist for a time corresponding to the carrier envelope of the pump, which can be less than a picosecond, and can give rise to the ultra-fast emergence of ferroic properties such as ferromagnetism and ferroelectricity [4]. Even more recently, this effect was experimentally demonstrated for the first time in our collaborators’ laboratory in Hamburg. Surprisingly, photo-ferroicity persisted for a significantly longer time than the carrier envelope (100s of ps). Although this is not yet fully understood, the most likely explanation is that magnetisation is being transferred to slower electronic/magnonic excitations.

This DPhil project will give the successful candidate the opportunity to pioneer this new field of research. Initial experiments on the ‘photo-ferroic’ materials that we have already characterised will be performed at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany. As a mode-selective pump, we are employing coherent laser radiation in the THz or far-IR range with sub-ps carrier envelopes, while the transient emergence of the ferroic properties will be probed with second-harmonic generation (SHG), Faraday rotation and dichroic absorption of visible/near-infra-red light. Later on in the project, changes in the crystal and magnetic structures of the materials will be probed with X-rays at free electron laser sources such as the European XFEL in Hamburg. Meanwhile, the candidate will develop search strategies for new classes of ‘photo-ferroic’ materials, based on symmetry and time-dependent density functional theory calculations. He/she will develop the materials specifications in collaborations with crystal growers in Oxford and elsewhere, and will be involved hands on in all aspects of the design and realisation of the experiments and the data analysis.

The experimental part of this project will be predominantly based in Hamburg, so it is essential for the candidate to be willing and able to be based in Germany for extended periods during the DPhil.

References
[1] “Femtomagnetism: Magnetism in step with light”. Uwe Bovensiepen, Nature Physics 5, 461 - 463 (2009).
[2] See for example M. Mitrano,et al., “Possible light-induced superconductivity in K3C60 at high temperature”, Nature, 530, 461–464 (2016).
[3] “Nonlinear phononics as an ultrafast route to lattice control”, M. Först et. al., Nature Physics, 7, 854–856 (2011).
[4] “Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling”, P. G. Radaelli, Phys. Rev. B 97, 085145 (2018).
[5] Disa, A.S., Fechner, M., Nova, T.F., Tobia F. Nova, Biaolong Liu, Michael Först, Dharmalingam Prabhakaran, Paolo G. Radaelli & Andrea Cavalleri, “Polarizing an antiferromagnet by optical engineering of the crystal field”, Nat. Phys. 16, 937–941 (2020).

Paolo G Radaelli

A new generation of devices, based on integrating novel antiferromagnetic (AFM) oxides with existing CMOS architectures has immense potential for low-energy electronics. The AFM oxide acts as a robust ‘write’ layer which is immune to perturbations of magnetic fields, produces no stray-fields, can be switched at ultrafast THz speeds, and is capable of generating large magneto-transport effects[1-4]. To develop this technology, it is essential to image and control the AFM state reliably. Indeed, the difficulty in imaging the AFM state has been up to now a serious obstacle in this emerging field, and has been particularly acute for topological object with complex spin textures. Techniques such as XMLD-PEEM, which rely on images acquired with linearly polarised X-rays of different polarisations and/or energies, do provide some contrast for antiferromagnets, but single-shot images are typically rather uninformative. Recently, we develop a new ‘AFM vector mapping’ approach in which many images acquired with different X-ray polarisations, energies and sample orientation are combined to provide exquisite maps of topological textures in AFM, such as domain boundaries and vortices [5-8]. This technique was applied on the I06 PEEM to great effect, first on BiFeO3 and later on α-Fe2O3. In particular, we discovered that AFM α-Fe2O3 thin films host topological (anti)merons,[6] wherein the primary ‘whirling’ parameter is the staggered magnetization. Later, we found that these textures can be generated and destroyed by thermally cycling through a spin-reorientation ‘Morin’ transition (TM) present in this material system. By applying a large ex-situ magnetic field, these textures can be wiped out and then regenerated at will. This breakthrough was recently published in Nature[8].

Although original implemented with X-PEEM, the technique we developed is equally suitable to be deployed with other X-ray microscopy techniques. Methods working in transmission (such as STXM) have obvious geometrical advantages with respect to PEEM because contrast can be produced primarily by rotating the X-ray polarisation rather than the sample. Moreover, one can apply in situ magnetic fields and image devices in operando without the difficulties associated with large electric fields. Lensless imaging employing coherent diffraction with reconstruction techniques such as HERALDO[9,10] offers a complementary set of advantages because of the fixed field of view (ideal for image reconstructions) and the greater space available for sample environment. Both STXM and HERALDO require thin samples: very recently, we were able to grow single-crystal membranes of α-Fe2O3 using a revolutionary new technique based on epitaxial growth on top of a sacrificial layer. [11] Using these samples, we obtained excellent image reconstructions of α-Fe2O3 topological textures using both STXM (at PolLux, PSI, Switzerland and HERALDO (at SEXTANT, Soleil, France). The HERALDO experiment was particularly successful, because we were able to measure the evolution of the topological textures in situ as a function of temperature (through the Morin transition) and magnetic field.

As we demonstrated previously, [5-8] Diamond provides an ideal source of soft X-rays for XMLD-based X-ray imaging of AFM domains. However, many of the aforementioned techniques and others such as soft-X-ray ptychography, tomography and laminography have not been implemented yet at Diamond. Doing so is extremely important for the magnetism user community, and would enable a wealth of experiments to be performed both on the I06 aberration-corrected PEEM, on I08 (where we have already performed a successful STXM test) and on the future CSXID beamline (currently in the design phase). The strong support for CSXID demonstrated both the interest of the community and the need to develop appropriate methodology in advance of its commissioning.

The central aim of this joint Oxford-Diamond studentship is to implement XMLD-based X-ray microscopy techniques at Diamond. We will develop these techniques to image topological structures in our α-Fe2O3| overlayers platform; however, these techniques have very wide applicability and, once deployed, will be available to all Diamond users. This is a 4-year funded studentship, partly supported by the Diamond Light Source, focused on the development of new imaging techniques for antiferromagnets with potential applications in IT technology. The successful applicant will be co-supervised by Prof. Paolo G. Radaelli at Oxford and Prof. Sarnjeet Dhesi at Diamond, and will spend approximately 50% of their time at each institution.

[1] P. Wadley et al., Science 351 6273 (2016)
[2] T. Ashida et al., Appl. Phys. Lett. 106 134207 (2015)
[3] O. Bezencenet, et. al., Phys. Rev. Lett. 106, 107201 (2011)
[4] X. Zhou, et al., Phys. Rev. B 92, 060402(R) (2015)
[5] N. W. Price, et al., Phys. Rev. Lett. 117, 177601 (2016)
[6] F. Chmiel, et al., Nat. Mater. 17, 581 (2018)
[7] P. G. Radaelli, et al., PRB. 101 144420 (2020)
[8] H. Jani, et al., Nature 590, 74 (2021)
[9] M. Guizar-Sicairos et al., Opt Exp 17592 (2007)
[10] M. Guizar-Sicairos et al., Opt Lett 33 (2008)
[11] D. Lu, D. J. Baek, S. S. Hong, L. F. Kourkoutis, Y. Hikita, and H. Y. Hwang, Nat. Mater. 15, 1255 (2016).
[12] T. Eggebrecht, M. Möller, J. G. Gatzmann, et al., Phys. Rev. Lett. 118, 097203 (2017).

Dr Mustafa Bakr (PI) & Dr Peter Leek (Co-supervisor)

Over the last two decades, superconducting circuits have developed to become one of the leading technologies for quantum computing, where devices at 10s of qubits scale and quantum logic gates with low errors have been demonstrated [1]. The community is now transitioning to focus on the challenges for building a viable quantum computer at scale. Specifically, for superconducting circuits, microwave engineering expertise can dramatically help in the next stage of development, particularly, for addressing improved performance, and efficient use of microwave infrastructure for superconducting quantum computers [2]. The aim of this project to develop engineering solutions inspired by microwave filtering techniques for reliable quantum circuits at scale – based on our high-coherence superconducting quantum processor in Oxford [3]. This may include the utilisation of innovative microwave engineering design for improving coherence and relaxation times and gate speed in multiqubit circuits as well as developing frequency multiplexing methods to enable control and readout of large qubit arrays.  

The project will involve both experimental and theoretical work, including: 

• Developing and applying new theoretical tools to suppress unwanted interactions in many-qubit couplings 
• Developing engineering solution of 3D multiplexing of readout and control of quantum processers 
• Building superconducting devices using these developed techniques for high-fidelity many-qubit circuits 
• Device measurement and data analysis 

[1] Kjaergaard, M. et al. Superconducting qubits: Current state of play. Annual Review of CMP 11, 369-395 (2020).  

[2] Krinner, S. et al. Engineering cryogenic setups for 100-qubit scale superconducting circuit systems. EPJ Quantum Technology 6, 2 (2019). 

[3] arXiv.2107.11140v1 "

For more information about the project please contact mustafa.bakr@physics.ox.ac.uk. 
For more information about how to apply please contact cmpgradadmin@physics.ox.ac.uk

Below is a list of DPhil projects for 2022 in quantum materials; please address all informal enquiries to the named supervisor.