Spin-current switching
Ultrafast control over the orientation of magnetic matter represents a vital element of present day non-volatile memory, and so establishing switching at femtosecond time scales represents the key towards a next generation of spin control. However the large system sizes inherent in spin switching devices would appear to rule out the ab-initio approach. In this project we aim to circumvent this via augmentation of the fundamental equation of TD-DFT by an effective SU(2) potential designed to generate current flow of specified polarity within a magnetic system. This will then be employed to ab-initio design spin-current pulses that control magnetic order – e.g. reversible switching of spin direction – on femtosecond time scales.
Laser pump control over quasiparticles
In exploring the emerging world of pumped quantum materials excitons, magnons and phonons will play a vital role in the ultrafast dynamics, as can be seen, for example, in our recent work on femto-phono-magnetism that demonstrated the profound impact of phonons on early time spin-dynamics [1]. In this project our aim is to extend the ab-initio approach to treat the coupling of the quasiparticles to each other and to light. This will involve major extensions of present theory and will allow accurate modeling of the rich light-induced quasi-paricle physics in complex materials such as organic solids.
Light induced superconductivity
We have developed a new fully ab-initio method for calculating the superconducting state of solids by solving the Boglioubov equations for coupled electrons and phonons. The advantage of this is that they can be easily extended to the time domain, allowing the study transient super-conducting state of matter and how light – via the lattice – can control the superconducting state. One could imagine taking a material to a superconducting excited state via, for example, laser pumping and in this project we aim to explore the early time transient dynamics of light-matter coupling to the superconducting state.
Light control of excitons in two dimensional materials
Recent work by us has shown that ultrafast light pulses can, via interference physics, control valley states in 2d dichalcogenides [1], as well as offer unprecedented control of topological valley and spin currents, including lossless femtosecond current switching [2,3,4]. In this project we aim to tailor light pulses to create designed excitonic states and currents in two dimensional materials, including 2d magnets, and to design topological states by light in moire platforms of 2d heterostructures.
[1] Optica 9 (8), 947-952 (2022)
[2] Science Advances 9 (11), eadf3673 (2023)
Pulse design for valley states in two dimensional materials
Hybrid light pulses consisting of a weak envelope pulse coupling predominantly to the Bloch quasi-momentum, and a second component that generates inter-band excitations have been shown to provide a route towards rich control over valley states [1], including the generation of pure valley and spin currents [2,3]. In this project we aim to apply this protocol to a wide range of 2d materials searching for designed pulses that control both spin and valley charge as well as current.
[1] Applied Physics Letters 120 (3) (2022)
Virtual foundry for the production of metamaterials
Harvesting light control over matter requires not only microscopic understanding of the relation between light fields and locally induced currents, but the behaviour under intense laser fields of large scale sculpted metamaterials. Such length scales are dramatically beyond those that can be envisaged with any modern computation resource. We aim to overcome this seemingly insurmountable barrier, linking the development of the understanding of free carrier and excitonic currents to global behaviour via a trained neural network map linking ab-initio understanding of the microscale to the macroscale of metamaterials. This will represent a “virtual foundry” in which the computer design of materials for tailoring of designed matter response to light fields can be envisaged.
Transient response functions
In the field of ultrafast spin dynamics comparison between theory and experiment is itself a difficult challenge, a situation that arises as while theoretical calculations naturally yield spin and orbital moments as a function of time, the highly non-equilibrium nature of the systems studied means that in experiment these can only be accessed indirectly, from the optical response of the system such as the transient MOKE or MCD spectroscopy. Recently, we developed an approach that bridges this divide by directly extracting transient response functions (like MCD and MOKE) from TD-DFT, made possible by combining real-time and linear response formalism of TD-DFT, thus allowing direct comparison of theoretical and experimental spectroscopy [1-4]. In future work we aim to unify the transient response function formalism with a time dependent version of the long-range ansatz [5], new exchange-correlation functionals, coupled dynamics of electron and nuclear degrees of freedom, and Maxwell’s equations.
[1] Physical Review Letters 122 (21), 217202 (2019)
[2] Physical Review Letters 124 (7), 077203 (2020)
[3] Physical Review B 102 (10), 100405 (2020)