Fluctuations de charge et transition métal-isolant dans les sulfures de métaux de transition bidimensionnels // Charge fluctuations and metal-insulator transition in 2D transition metal sulfides

Quasi two-dimensional (2D) layered transition metal chalcogenides are remarkable for their tunable electronic and optical properties including semiconducting, metallic, superconducting (SC) and charge density wave (CDW) states. Common wisdom is that electronic correlations are weak in these compounds and that the SC mechanism is of the conventional BCS electron-phonon type. In fact, the discovery of high-temperature SC in the FeSe system has led to propose an alternative scenario of a non-Fermi liquid state and of unconventional superconductivity driven by electronic excitations, such as magnons.

In the present PhD project, we shall address this issue by investigating the electronic and transport properties of layered transition metal sulfides where charge fluctuations arise from the mixed-valence properties of early transition metal ions, such as V. Following previous proposals by Little and Ginzburg, we shall investigate the possibility that these fluctuations are enhanced by the poor screening of the charge carriers, a characteristic property of low-density 2D metals. In this research project, we shall focus on the following two systems:

1. MxVS2 compounds characterized by VS2 layers intercalated with a transition metal M. This crystal structure enables us to tune the electron doping and the bandwidth by using the intercalant concentration, x, and pressure as control parameters, respectively.

2. Layered ternary transition metal sulfides belonging to the Ruddlesden-Popper series, An+1MnS3n, where A is a rare-earth or alkali-earth ion, M a transition metal ion of the IV column (Ti, Zr or Hf) and n a integer number. In these compounds, doping is provided by heterovalent substitutions at the A site, while the effective dimensionality and thus the strength of the electronic correlations and the exciton binding energy can be varied as a function of n.

Main objective of the PhD project will be to uncover experimentally the signature of charge fluctuations in the system by using advanced spectroscopic techniques, such as EELS and angular photoemission spectroscopy (ARPES). This activity will be supported by ab initio calculations using density functional theory or many-body Green-function methods that take into account electronic correlations (coll. M. Helgren, B. Lenz and M. Casula, IMPMC).

A systematic study by means of specific heat, magnetic, magnetotransport and thermopower measurements at low temperature and under high pressure shall provide further indications as to the role of charge fluctuations on the thermodynamic and transport properties.
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Quasi two-dimensional (2D) layered transition metal chalcogenides are remarkable for their tunable electronic and optical properties including semiconducting, metallic, superconducting (SC) and charge density wave (CDW) states. Common wisdom is that electronic correlations are weak in these compounds and that the SC mechanism is of the conventional BCS electron-phonon type. In fact, the discovery of high-temperature SC in the FeSe system has led to propose an alternative scenario of a non-Fermi liquid state and of unconventional superconductivity driven by electronic excitations, such as magnons.

In the present PhD project, we shall address this issue by investigating the electronic and transport properties of layered transition metal sulfides where charge fluctuations arise from the mixed-valence properties of early transition metal ions, such as V. Following previous proposals by Little and Ginzburg, we shall investigate the possibility that these fluctuations are enhanced by the poor screening of the charge carriers, a characteristic property of low-density 2D metals. In this research project, we shall focus on the following two systems:

1. MxVS2 compounds characterized by VS2 layers intercalated with a transition metal M. This crystal structure enables us to tune the electron doping and the bandwidth by using the intercalant concentration, x, and pressure as control parameters, respectively.

2. Layered ternary transition metal sulfides belonging to the Ruddlesden-Popper series, An+1MnS3n, where A is a rare-earth or alkali-earth ion, M a transition metal ion of the IV column (Ti, Zr or Hf) and n a integer number. In these compounds, doping is provided by heterovalent substitutions at the A site, while the effective dimensionality and thus the strength of the electronic correlations and the exciton binding energy can be varied as a function of n.

Main objective of the PhD project will be to uncover experimentally the signature of charge fluctuations in the system by using advanced spectroscopic techniques, such as EELS and angular photoemission spectroscopy (ARPES). This activity will be supported by ab initio calculations using density functional theory or many-body Green-function methods that take into account electronic correlations (coll. M. Helgren, B. Lenz and M. Casula, IMPMC).

A systematic study by means of specific heat, magnetic, magnetotransport and thermopower measurements at low temperature and under high pressure shall provide further indications as to the role of charge fluctuations on the thermodynamic and transport properties.
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Début de la thèse : 01/10/2026

Public funding alone (i.e. government, region, European, international organization research grant)

Concours pour un contrat doctoral

Sorbonne Université SIS (Sciences, Ingénierie, Santé)

397 Physique et Chimie des Matériaux

Strong background in materials physics, condensed matter physics or solid state chemistry. Experimental skills in the laboratory, data acquisition and analysis. Autonomy, good communication skills and ability to work in a team.
Strong background in materials physics, condensed matter physics or solid state chemistry. Experimental skills in the laboratory, data acquisition and analysis. Autonomy, good communication skills and ability to work in a team.