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Quantum Spectroscopy: exploring new states of matter out of equilibrium

Periodic Reporting for period 4 - QSpec-NewMat (Quantum Spectroscopy: exploring new states of matter out of equilibrium)

Reporting period: 2021-04-01 to 2021-09-30

The objective of the project was to discover the general principles that describe and control quantum systems far from equilibrium and orchestrate the behavior of many electrons and atoms to create new phenomena/states of matter. The long-term goal was to develop, implement and provide a novel theoretical and computational toolbox within the in-house theoretical framework of quantum electrodynamical density functional theory (QEDFT) applicable to complex materials. We addressed the following main research lines and challenges:

1. To establish, develop and implement the “first principles non-equilibrium QEDFT toolbox”.
2. To demonstrate new light induced “non-equilibrium states of matter” that have no equilibrium counterpart and identify the spectroscopic fingerprints of those new states
3. To investigate atoms and molecules with quantum optical fields. Which novel states arise in the strong light-matter coupling regime? Develop the field of QED-chemistry
4. Investigate how to imprint photon correlations into matter
5. To investigate a new class of polariton condensates in materials, addressing the cavity and Floquet materials engineering
6. To establish a new coherent quantum-control scheme within QEDFT
7. Describe quantum coherent time-resolved spectroscopy. How to generate a condensate of correlated photons
8. Develop a new theoretical framework for the simulation of non-adiabatic dynamics of molecules driven far from equilibrium by external electromagnetic fields.
9. To explore the possibility to construct more accurate and systematic xc-approximations for conventional DFT by directly use the fact that photons mediate the interaction between particles in QED

To fulfill the proposed ambitious goals of the project we made major methodological advances that can be divided in two types, one on the fundamentals of QEDFT and closely related theoretical frameworks and the second on the development of the framework to describe non-adiabatic dynamics of many-body systems driven far from equilibrium by external time-dependent fields. Those advances have set the ground for the development of two new fields of research QED-Chemistry and QED-materials that are now getting a lot of attention with many other groups implementing them. We envision that work made within this ERC project will have major implications for the development of new and more efficient materials and novel technologies for quantum information processing and energy-materials of relevance to our society.
The comprehensive understanding of materials phenomena provided by the novel theoretical tools develop in this project will enable its application on everyday processes, just to name a few:
• Control of the ground state of a material by strong light matter coupling in optical cavities
• Manipulation of chemical properties via photons, namely the improvement in efficiency of standard chemical reactions by strong coupling to cavity photons and do electronic structure engineering
• Ultimate control of the properties of materials and molecules at the nanoscale, enabling new ultrafast electronics for superfast computers in the future
• Establishment of new spectroscopic techniques to study quantum materials
• New facets to our modern views on material and reaction design that will provide with efficient tools to tackle modern-day problems such as energy harvesting and capturing
The tools developed within this project are immediately made available to the entire scientific community through the open-source Octopus project.
Following you find a summary on our most relevant findings in the 60 months of the project:
• We have developed a novel, unique and versatile QEDFT which is a reformulation of the Pauli-Fierz quantum field theory in the form of exact quantum Navier-Stokes equations. Besides working on fundamental theoretical and mathematical questions in connection with QEDFTs, we constructed approximations based on extensions of the Kohn-Sham scheme to coupled light-matter situations
• We explored how light-matter coupling produced a combined hybrid state of light and matter – a state which could be utilized in the design of new materials.
• We have demonstrated an ultrafast Lifshitz transition in the correlated type-II Weyl semimetal Td-MoTe2 by combining time-resolved multidimensional photoemission spectroscopy with our state-of-the-art TDDFT plus U simulations [118], possibly leading to the next generation of ultrafast electronic devices
• We have successfully extended electronic structure methods to include the coupling to the photons, showing how strong coupling to photons in an optical cavity changes chemical properties of molecules
• By means of Floquet theory and using high-level computational simulations of material properties we’ve shown how optical transformation via laser-driven can change the topology of a material
• Together with collaborators at CFEL, we showed the possibility of using a new knob to control and optimize the generation of high-order harmonics in bulk materials. We demonstrated that 2D materials can generate high-order harmonics with the same mechanism as atoms and molecules.The new technique might find intriguing applications in petahertz electronics and for spectroscopic studies of novel quantum materials
• We have demonstrated that phonons can be used to induce and control magnetic responses in non-magnetic 2D layers of transition metal dichalcogenides.
• Using state-of-the-art quantum dynamical methods to calculate the quantum of spin or charge Hall conductivity, we’ve successfully classified the intrinsic topological natures of insulators by means of their conductivity itself instead of the mathematically devised topological number.
• We have shown that the transfer of energy and charge between molecules can be drastically enhanced and controlled with virtual photons.
• We have discovered a significant new fundamental kind of quantum electronic oscillation, or plasmons, in atomically thin materials [100]. We found that plasmons behave in a peculiar universal way in all atomically thin materials. These findings have potential implications for novel imaging techniques and for the control of photochemical reactions at the nanoscale.
• In a very recent work [190] we have predicted that a unique laser source could produce highly controllable electric currents in any bulk material. These findings could contribute to the development of superfast, light-controlled electronic switches – the realm of petahertz electronics, where electronic motions need to be controlled both in time and space.
• Condensed Matter Quantum Simulator [138]: Applying the basic concept of moiré interference patterns to 2D crystals has given birth to the fast-growing field of engineering moiré van der Waals heterostructures. Different 2D materials are mixed and matched either at a relative twist angle or for varying chemical compositions to form such moiré interference patterns on the level of their microscopic lattice, which in many cases drastically alters their electronic band structures. This allows us to control their topological features or to promote correlation effects with unprecedented flexibility. The generality of the moiré engineering concept and has recently raised the hope that it provides a condensed matter-based quantum materials design platform of unparalleled versatility [99].
Collage of relevant figures
Interaction of a laser field (red) with a crystalline solid (white) @Harms/MPSD
Schematic of energy transfer between two molecules outside and inside a controlled vacuum @Harms/MPD
Strong coupling of light-vibrational waves to electrons in a 2D atomically thin material @Harms/MPSD