Projekt v okviru Načrta za okrevanje in odpornost financira Evropska unija – NextGenerationEU« www.gov.si/zbirke/projekti-in-programi/nacrt-za-okrevanje-in-odpornost 

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).