Description du sujet de thèse

CONTEXT
The world we live in is increasingly influenced by the massive use of information and communication technologies (ICT). However, a major counterpart to the extraordinary progress of ICT is its energy consumption, which represents 10% of global electricity in 2021, with an annual growth of 9%. These enormous energy expenditures could be significantly reduced by changing the architecture of current computers, which operate according to the 75-year-old Von Neumann paradigm, to one based on hardware neural networks mimicking the human brain, potentially much more energy-efficient. But building such low-power neuromorphic electronics requires exploring new materials and new physical properties. One possible way to address this challenge is to turn to quantum materials, i.e. materials whose properties result from strong correlations and/or the topological nature of their electronic states [1,5]. In particular, insulator-metal transitions (IMTs) existing in the class of Mott insulators are currently attracting great interest worldwide, and the term "Mottronics" has been coined to represent this new electronics exploiting such Mott transitions [1,5]. The IMN team demonstrated that the electric field, a control parameter suitable for microelectronic applications, offered a simple way to control the IMT in Mott insulators [3, 4, 6, 7]. Furthermore, we established and patented that this "electric Mott transition" can be used to implement a new type of non-volatile memories, and new components for hardware neural networks [7, 8, 9, 10]. These studies, awarded the 2023 Félix Robin Prize of the SFP, are the basis of the project to create the startup "Mottronix".
However, the discovery of the electric Mott transition is still very recent. Its application potential and ultimate performance, particularly in terms of switching speed, remain to be evaluated. Our recent studies show that the electrical Mott transition is linked to the creation of hot electrons [11,12], which in turn lead to lattice compression [13.] The key role played by hot electrons that can be generated by laser has led to a collaboration with the Institut de Physique de Rennes (IPR) [14,15], where there is strong expertise in ultrafast pump-probe techniques [14-20]. IMN and IPR joined forces in the framework of the France - Japan International Associated Laboratory (IMLED, 2017-21), which has become an International Research Laboratory (IRL) focused on Dynamic Control of Materials (DYNACOM, 2022-26). In this context, we discovered that the electronic insulator-metal transition induced by light pulses in V2O3 (low temperature Mott insulator phase) propagates at the speed of sound, also causing a contraction of the lattice [2].

SCIENTIFIC LOCKS AND OBJECTIVES
The main objective of the thesis is to demonstrate the universality of the fundamental mechanisms involved in the out-of-equilibrium of Mott insulators by light or electric pulses. For this purpose, the MQ3E project aims to explore the temporal response of Mott insulator materials GaV4S8 and (V1-xCrx)2O3 when they are subjected, at room temperature, to electric or light pulses. The study of these Mott insulators by time-resolved pump-probe techniques represents an almost unexplored territory. Two types of innovative studies are therefore envisaged in the project: that of the photoinduced transition and that of the transition under electric pulse. Carrying out these studies requires a multidisciplinary approach at the border of chemistry and solid-state physics. It is based in particular on:
a. mastery of the synthesis and characterization of thin layers and Mott insulator crystals (IMN),
b. the production of microelectronic devices adapted to the observation of induced transitions under electrical pulses (IMN),
c. mastery of time-resolved pump-probe measurements, in particular using large instruments (IPR).

HISTORY OF THE COLLABORATION / CONTRIBUTION OF THE PARTICIPANTS
The two teams involved in the MQ3E project, one belonging to the Institut des Matériaux de Nantes Jean Rouxel (IMN) and the other to the Institut de Physique de Rennes (IPR), know each other well and have been working closely together for several years [2, 14, 15]. They have notably worked together in three ANR projects as well as in the framework of a LIA (2017-21) and an IRL (2022-2025) France-Japan. In the MQ3E project, the role of participants E. Janod and L. Cario for the IMN, and L. Guérin and M. Lorenc for the IPR will be very complementary. The IMN brings both the problem of the electric Mott transition, its expertise in the field of strongly correlated quantum materials, as well as its ability to synthesize well-characterized materials in the form of both single crystals and thin layers. The IPR (L. Guérin, M. Lorenc) brings to this collaborative project its very strong skills in phase transition physics [17-20], in particular out-of-equilibrium, as well as its great mastery of time-resolved pump-probe techniques, notably by X-ray diffraction [15, 16].

METHODOLOGIES TO BE IMPLEMENTED
The MQ3E project aims to study the out-of-equilibrium of Mott insulators GaV4S8 and (V1-xCrx)2O3 synthesized at IMN (crystals, thin films) using time-resolved reflectivity techniques mastered at IPR (reflectivity) or within the framework of IRL DYNACOM and to study the associated structural transformations using time-resolved X-ray diffraction techniques. The main tasks of the project are as follows:
- Production of thin films, crystals and devices: the various experiments envisaged will be carried out on crystals, thin films or devices whose preparation is mastered at IMN. The first task of this project will consist of producing the various samples/devices meeting the specificities of the various pump-probe experiments envisaged.
- Study of the photoinduced metal insulator transition: preliminary measurements carried out at Tohoku University using time-resolved reflectivity techniques on GaV4S8 have revealed a metal insulator transition under laser pulse. This is much faster than in the low-temperature V2O3 phase studied so far. This task will aim to carry out time-resolved X-ray diffraction (tr-XRD) measurements to elucidate the mechanism at work in GaV4S8 and in particular to explore the lattice response. IPR and IMN have extensive expertise in tr-XRD measurements [20-25] which will provide access to the large instruments necessary to carry out these experiments (5 experiments carried out during the first half of 2025).
-Study of the insulator-metal transition under electric pulses: this third task consists of studying the structural transformation of Mott insulators GaV4S8 and (V1-xCrx)2O3 when they are subjected to electric pulses generating a volatile resistive transition. For this it will be necessary to use a time-resolved micro diffraction technique in order to study the structural change within the conductive filament (with a diameter of a few hundred nanometers) that is created during the volatile resistive transition. The specific microelectronic devices made at the IMN will have to be used for these experiments in order to be able to locate the filament.

EXPECTED RESULTS
The MQ3E project proposes cutting-edge experiments that will allow exploring the spectroscopic and network responses during insulator-metal transitions induced by electric and optical pulses. The expected results are the highlighting of the ultrafast electronic and structural dynamics of insulator-metal transitions induced under electric or light pulses. These results will help establish the fundamental mechanisms involved in the out-of-equilibrium of Mott insulators and demonstrate the relationship between light and electrical pulses. The knowledge gained from these studies will help us better understand the destabilization of Mott insulators under electrical pulses and thus achieve better control of the microelectronic devices based on Mott insulators studied at IMN and at the origin of the Mottronix startup currently being incubated.

COMPLEMENTARITY OF TEAMS AND CONTRIBUTION OF PARTICIPANTS
In the MQ3E project, the role of participants L. Cario and E. Janod for the IMN, and L. Guérin and M. Lorenc for the IPR will be very complementary. The IMN brings both the problem of the electric Mott transition, its expertise in the field of strongly correlated quantum materials, as well as its ability to synthesize well-characterized materials and devices in the form of both single crystals and thin layers. The IPR (L. Guérin, M. Lorenc) brings to this collaborative project its very strong skills in phase transition physics [17-20], in particular out-of-equilibrium, as well as its great mastery of time-resolved pump-probe techniques, notably by X-ray diffraction [15, 16].

ROLE OF THE PhD STUDENT
The role of the doctoral student in this project will be essential. Indeed, he/she will be responsible on the one hand for the preparation and advanced characterization of the various Mott insulating compounds both in the form of single crystals and thin films. For this, he/she will benefit from both the expertise in the field of the IMN team [6-12] as well as access to the laboratory's synthesis and characterization platform. On the other hand, he/she will be heavily involved in the time-resolved measurements that will be carried out at the IPR. In addition, he/she will actively participate in measurement campaigns on Large Instruments (such as lines ID09 at ESRF-France, Bernina at SwissFEL-Switzerland and FEMTOMAX at MAX IV-Sweden, SACLA-Japan), on which our two laboratories regularly carry out several experiments per year. The geographical proximity between the two laboratories will greatly facilitate direct exchanges and the travel necessary for the completion of this thesis. The PhD student will be able to spend a few weeks in Japan for pump-probe experiments with ultimate time resolutions (less than 10 fs), within the framework of the IRL DYNACOM which involves the universities of Tohoku (Sendai) and Tokyo. The candidate will be required to present his results in national and international conferences and to write scientific articles related to his work. The profile sought for this thesis is that of a Master in condensed matter, with special attention given to candidates having received training in quantum materials and time-resolved manipulations.

REFERENCES
1. Tokura, Y., Kawasaki, M., Nagaosa, N. Emergent functions of quantum materials. Nature Physics 13, 1056–1068 (2017)
2. Amano, T. et al. Propagation of Insulator-to-Metal Transition Driven by Photoinduced Strain Waves in a Mott Material. Nat. Phys. 2024, 20 (11), 1778–1785. https://doi.org/10.1038/s41567-024-02628-
3. Janod, E. et al. Resistive Switching in Mott Insulators and Correlated Systems. Advanced Functional Materials 25, 6287–6305 (2015).
4. Cario, L. et al. Chapter 10 - Correlated Transition Metal Oxides and Chalcogenides for Mott Memories and Neuromorphic Applications. In Metal Oxides for Non-volatile Memory; Dimitrakis, P., Valov, I., Tappertzhofen, S., Eds.; Metal Oxides; Elsevier, 2022; pp. 307–360.
5. Basov, D. N., Averitt, R. D., Hsieh, D. Towards properties on demand in quantum materials. Nature Materials 16, 1077–1088 (2017).
6. Vaju, C. et al. Electric-Pulse-driven Electronic Phase Separation, Insulator–Metal Transition, and Possible Superconductivity in a Mott Insulator. Advanced Materials 20, 2760–2765 (2008).
7. Cario, L., Vaju, C., Corraze, B., Guiot, V., Janod, E. Electric-Field-Induced Resistive Switching in a Family of Mott Insulators: Towards a New Class of RRAM Memories. Advanced Materials 22, 5193–5197 (2010).
8. Stoliar, P. et al. A Leaky-Integrate-and-Fire Neuron Analog Realized with a Mott Insulator. Advanced Functional Materials 27, 1604740 (2017).
9. Tranchant, J. et al. Control of Resistive Switching in Mott Memories Based on TiN/AM4Q8/TiN MIM Devices. ECS Trans. 75, 3–12 (2017).
10. Adda, C. et al. Mott insulators: A large class of materials for Leaky Integrate and Fire (LIF) artificial neuron. Journal of Applied Physics 124, 152124 (2018).
11. Guiot, V. et al. Avalanche Breakdown in GaTa4Se8-xTex Narrow-Gap Mott Insulators. Nat Commun 2013, 4, 1722.
12. Diener, P. et al. How a dc Electric Field Drives Mott Insulators Out of Equilibrium. Phys. Rev. Lett. 121, 016601 (2018).

Contexte de travail

The PhD student will be recruited at the Nantes Institute of Materials (www.cnrs-imn.fr), a very well-equipped Materials Science laboratory with over 200 people, a large and very dynamic association of PhD students and post-docs. He or she will also spend time at the IPR in Rennes for time-resolved optical measurements and for discussions.

Contraintes et risques

The only risks identified are those related to the use of lasers during time-resolved measurements. Prior training on this topic will be provided.