Projects

The new Marie-Curie doctoral network SPARKLE will build a mixed experimental and theoretical network of 14 PhD students to develop the necessary tools and techniques to exploit light-matter coupling to manipulate materials functionalities. SPARKLE will bridge the gap between photonics on the one hand and materials science on the other by advancing the understanding of light-matter interaction via two distinct yet complementary approaches: (i) the utilisation of tailored laser fields and (ii) the confinement of light in optical environments, called cavities. The network will connect leading academic and industrial experts to push the development of novel detection techniques, innovative light sources, state-of-the-art ab initio simulation methods and innovative designs of light-matter hybrid solid-state systems for devices at the interface of photonics, electronics and quantum technology.  More information about the project, other groups involved and possibilities to join the project is available at https://sparkle-dn.eu/.

Description of the project performed at IJS. 

Exciton-polaritons in strongly correlated materials 

Objectives

In semiconductors, the interaction between light and excitons leads to new hybrid quasiparticles, named exciton-polaritons, which exhibit remarkable collective phases of matter, including Bose-Einstein condensation and superfluidity and could serve as future quantum Hamiltonian simulators.  Much less is understood about analogous exciton-polariton in strongly correlated quantum materials, including Mott and charge transfer insulators. In these systems, the enhanced interaction between quasiparticles should lead to much higher critical temperatures for condensation, and an intrinsic strong coupling with active degrees of freedom (spin, orbital, lattice) enables manipulation of collective phenomena, including magnetism [1] and ferroelectricity [2]. For instance, our recent study in charge-transfer insulators revealed a strong coupling between excitons and magnetic order, allowing for unprecedented photo manipulation, either by classical or quantum light [3]. In recent years, PI has shed light on the interaction between classical light and excitons in quantum materials, like nickelates [4], employing powerful non-perturbative methods based on the Dynamical mean-field theory. These preliminary steps will be extended toward two main objectives:

A)    Extend the non-perturbative description of excitons from Mott to charge transfer insulators using Dynamical mean-field theory as is relevant for experimentally studied materials, including NiPS3 [5,6] and NiO [3].

B)    Describe the coupling of strongly correlated excitons with quantum light and its interplay with magnetism.

Tasks

Excitons in charge transfer insulators:

·      Extend Dynamical mean-field theory to charge transfer insulators [9] by explicitly treating p and d orbitals, allowing for nonperturbative treatment of exciton formation.

·      Understand exciton coupling with magnetic order and how to use resonant excitation to manipulate magnetic order.

·      Merge ab initio determined parameters for NiPS3 and NiO with model calculations.

Strongly correlated exciton-polariton:

·      Include light-matter fluctuation and polariton formation by extending the Dynamical mean-field theory with the two-particle self-consistency [10].

·      Obtain a detailed understanding of exciton-polariton properties, including their effective interaction and how light fluctuations couple with magnetic excitations.

Scientific training goals

The PhD student will learn:

·      The basics of exciton-polariton physics in semiconductors and strongly correlated materials, including Mott and charge transfer insulators.

·      Modern non-perturbative approaches for strongly correlated systems, including (time-dependent) Dynamical mean-field theory, Hubbard-I approximation and exact-diagonalization of small clusters.

·      Using extended Dynamical mean-field theory to evaluate susceptibilities and two-particle self-consistent schemes in strongly correlated systems [9, 10].

Expected outcomes:

·  Understanding the fundamental difference between exciton-polariton in semiconductors and strongly interacting quantum materials.

·  Detailed understanding of the interplay between strongly correlated exciton-polariton and magnetism.

·  Proposing manipulation protocols for collective phenomena using cavities on concrete materials (NiPS3 or NiO).

In Action MINIMUM, we employ powerful tools based on dynamical mean-field theory to investigate time-dependent fluctuations in realistic multi-band systems. Our focus is on charge-transfer insulators, which exhibit some of the most fascinating phenomena in condensed matter physics, including high-temperature superconductivity. We will apply this framework to the study of metastability in transition metal dichalcogenides, with a particular emphasis on the hidden phase in 1T-TaS₂.

By combining these advanced theoretical tools with direct comparisons to experimental probes—such as time-resolved optical experiments and scanning tunneling spectroscopy—we aim to gain unique insights into the microscopic nature of the metastable phase. Specifically, we will explore the dynamical interplay of Mott physics, charge-density waves, and polaronic effects to understand the formation of microscopic domain structures. The ability to simulate material responses on electronic time scales will provide crucial guidance for controlling materials and developing ultra-fast, all-electronic resistance-switching memory devices.

To achieve these goals, we will pursue three key objectives:

• Objective i focuses on developing a non-equilibrium multiband solver that treats orbitals within appropriate approximations and applying this formalism to charge-transfer insulators. Using this framework, we will simulate two distinct experimental approaches for material control: photo-excitation and periodic driving.

• Existing theoretical tools for determining the electron-phonon coupling function from temporal evolution fail to accurately describe experiments on charge-transfer insulators. Objective ii will address this limitation by modeling the time evolution of the electronic system coupled to lattice vibrations. We will extract the bosonic coupling function and provide experimental feedback to refine theoretical predictions.

• While model calculations offer valuable insights into material properties, the extreme sensitivity of complex materials necessitates realistic parameters for meaningful comparisons with experiments. In Objective iii, we will integrate multiple theoretical approaches to develop a first-principles description of electronic properties under non-equilibrium conditions. Applying this formalism to 1T-TaS₂, we aim to provide direct guidance for future experiments, helping to disentangle the vast array of intertwined fluctuations.

• Through Action MINIMUM, we seek to establish a robust theoretical framework for understanding ultrafast material responses, providing essential insights for both fundamental research and technological applications.

Highlights of the research:

• Nonthermal metal-insulator transition in Ca2RuO4

In collaboration with experimental colleagues from Cornell and Columbia University have demonstrated that the crystal Ca₂RuO₄ can be rapidly transformed from an insulator to a metal using an ultrafast laser pulse.

The biggest surprise came when they analyzed the properties of the metallic state and discovered that it differs from the states found in the equilibrium phase diagram. Such exotic states of matter are called hidden states, and this new confirmation has been published in Nature Physics.

The theoretical breakthrough of this research lies in linking the laser-induced transition to a thermodynamic phase transition. The latter is similar to the transition between water and ice, as it is of first order, allowing the system to be trapped in a metastable state.

They demonstrated that the analogy with supercooled water can be directly applied to the dynamics of the insulator-to-metal transition, enabling them to track the transition trajectories between the two phases (see figure).

The vision of this project is to provide a firm theoretical framework for a description of ultrafast material response and to enhance and simplify the transfer of theoretical ideas to the experimental community. During the last years, PI has developed powerful tools based on numerical solutions of non-equilibrium Keldysh theory allowing for an advanced description of material responses, while still relying on model simplifications. This project aims to push the theory to the level where material-specific properties of strongly correlated systems out of equilibrium are taken into account and hence provide an ab initio,parameter-free theory. We will apply the description to the question of metastability in transition metal dichalcogenides and in particular to the question of the hidden phase in 1T-TaS2.

Applications of these powerful theoretical tools and a direct comparison with experimental probes, like time-resolved optical experiments or scanning tunneling spectroscopy will provide a unique insight into the microscopical nature of the metastable phase. We will complement the microscopical description with phenomenological approaches to understand the global topological properties of domain wall structures, like chiral or amorphous-like state. The ability to simulate material responses on electronic time scales will provide crucial guidance for the manipulation of materials and their applications for ultra-fast all-electronic resistance-switching memory devices.

Stages of research:

1. Development of multiscale modelling in time. We have developed a new methods based on hierarchical approach to nonequilibrium Greens functions. Publication: https://www.scipost.org/SciPostPhys.10.4.091?acad_field_slug=chemistry

2. Application of nonequilibrium dynamics on quantum materials with focus on Ta2NiSe5. Publications: 

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.106.L121106

https://www.science.org/doi/full/10.1126/sciadv.abd6147

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.144304

3. Merging real time dynamics with ab-initio modelling. Work in progress.

Team:

Dr. Banhi Chatterjee [https://cris.cobiss.net/ecris/si/sl/researcher/52451 ]

Dr. Madhumita Sarkar [https://cris.cobiss.net/ecris/si/sl/researcher/53683]

Jože Gašperlin [https://cris.cobiss.net/ecris/si/sl/researcher/54082]