Fermi Energy Tuning in Topological Materials – FETTOM

Initially, we have proposed different approaches to tune the Fermi energy of the different systems of interest including chemical doping, ionic liquid and solid gating, reduction of dimensionality, and pressure (uniaxial and hydrostatic). We have chosen to concentrate our effort particularly on the pressure studies, which is a domain of expertise of partners I and III, under extreme condition. The reduction of the dimentionnality was used only to optimized the measurement of quantum oscillations under pressure but no specific study on the modification of the physical properties with the thickness has been done.

The sample were synthesized either in PHELIQS Grenoble, or in Hong Kong. The measurements at low temperature and under pressure were done both in PHELIQS Grenoble and in Hong Kong. We performed specific very high magnetic field experiment in LNCMI Grenoble to confirm or infirm different hypothesis on the fermiology on the different systems we have studied, this was a real added value in the project with respect to the competitors.

The family of kagome metals AV3Sb5 (A: K, Rb, Cs) presents an intriguing interplay of non-trivial electronic band topology, superconductivity, electronic correlations leading to charge density wave (CDW) instability, and magnetic frustration due to its unique crystalline structure. ​​

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The entire consortium of the project conducted a detailed study of the Fermi surface of CsV3Sb5, a member of the kagome family. In this material, the Fermi surface is reconstructed due to the CDW instability. ​​

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To better understand the material, it is crucial to know the Fermi surface of the “pristine” metal, before the electronic instability sets in. This has been achieved in the present study through a probe of the Fermi surface of CsV3Sb5 as a function of hydrostatic pressures up to 30 kbars and high magnetic fields up to 30 T. The first outcome is the discovery of a large Fermi surface, which is a signature of the pristine state and absent in the reconstructed Fermi surface at ambient pressure. ​​

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It is the first time that such a strong effect of the CDW instability on the Fermi surface is directly demonstrated by quantum oscillations. Another important result is that the effective mass of the charge carriers is noticeably enhanced near the critical pressure where the CDW instability is suppressed by pressure. Both findings emphasize the influence of the charge density wave on the electronic structure, together with the sensitivity of the electronic correlations driving this electronic instability to a modest pressure.

​​Zhang W, Poon TF, Tsang CW, Wang W, Liu X, Xie .J, Lam ST, Wang S, Lai KT, Pourret A, Seyfarth G, Knebel G, Yu WC and Goh SK.

Large Fermi surface in pristine kagome metal CsV3Sb5 and enhanced quasiparticle effective masses

Proceedings of the National Academy of Sciences of the United States of America (2024)

The future of the collaboration:

The ANR Fettom had a strong impact on the collaboration with the team of Swee Goh in Hong Kong. We plan to continue our fruitful collaboration on a large variety topological interesting systems with joint experiment in LNCMI Grenoble.

The future of the project:

After investigating the tuning of the Fermi level with pressure in the present project, we plan in the next years to change the Fermi level of these systems with peculiar electronic topology using chemical or ionic gating doping. The idea is to emphasize the topological properties (chiral electrons, Weyl electrons, surface states...) of these materials and detect their influence on the Ferminology.

We plan to explore other Europium or Gadolinium based materials presenting exotic magnetic topological phases like the skyrmion lattice phase with two objectives. The first one is to understand the role of the lattice symmetry (comparing non-centrosymmetric versus centrosymmetric systems) on the type/size of the skyrmions, on the robustness and temperature range of the skyrmion lattice phase. The second one is to understand the dynamic of the skyrmions under the influence of either a thermal gradient or an electrical field.

This joint project aims to investigate the electronic states of selected topological quantum solids via a systematic tuning of the Fermi energy. Topological quantum materials host Dirac points in the electronic band structure. The electronic properties are sensitively dependent on the precise location of the Fermi energy, which can be controlled by hydrostatic pressure, uniaxial pressure, electric field and dimensionality. Unconventional behavior related to massless chiral fermions occurs only if the Fermi energy is close to these Dirac points. This project seeks to achieve such systematic tuning of the Fermi energy by employing multiple tuning parameters, and to probe the underlying electronic structure via high magnetic field straddling the quantum limit. This offers the prospect of extracting fundamental physics and the functionalization of these exciting materials. The results of this project will be of interest to the community of condensed matter physics and device engineering.