Master and PhD project topics

Thanks to strong light-matter interactions in cavity-coupled systems, new important vacuum phenomena are emerging in condensed matter physics. In analogy with exciton- and phonon-polaritons that are now routinely observed in semiconductors an exciting perspective is to tune or even enhance the properties of a superconductor (SC) at equilibrium by dressing it with vacuum photons. Strong light-matter interactions offer unprecedented control opportunities of SC states since they can be shaped by modifying the spatial geometry and spectrum of the cavity electromagnetic modes. Beyond static properties such as enhancing the SC transition temperature, of particular interest is the possibility to directly couple the SC Higgs mode, a superconducting analogue of the Higgs boson well-known from high-energy physics, to a THz cavity mode. In this setting a new hybrid light-matter excitation coined the Higgs polariton is formed. To date none of the effects described above has been observed.

2D SC transition metal dichalcogenides (TMDs) like NbSe2, and NbS2 are extremely attractive platforms to demonstrate these effects. Their 2D nature make them particularly suitable for integrating into deep sub-wavelength THz cavities based on split-ring resonators which have been used successfully to produce strong light-matter polariton state in semiconductor heterostructures. During this internship, the fabrication of TMD-based van der Waals heterostructure will be carried out and integrated into THz split resonant cavities. An originality of the project will be to go beyond static properties like Tc and explore the effect of strong light matter coupling on the SC excitation spectrum and search for spectroscopic fingerprints of the Higgs polariton via Raman and THz spectroscopic techniques. The work will be performed in close collaboration with THz cavity experts at the LSI lab of Ecole Polytechnique.

References :

1. Alloca et al. Phys. Rev. B 99, 020504 (2021)
2. Garcia Vidal et al. Science 373, eabd0336 (2021)
3. Grasset et al. npj Quantum Materials 7, 4 (2022)
4. K. Katsumi et al. Phys. Rev. Lett. 120, 117001  (2018)

Contact: Yann Gallais and Alain Sacuto

Transition metal dichalcogenides (TMDs) have recently attracted significant interest because they allow the exploration of novel quantum phenomena down to the 2D limit. Of particular interest for the present project are metallic TMD like NbSe₂ which displays various quantum phases like Superconductivity (SC) and charge density wave (CDW) states. In addition, the possibility of creating Van der Waals heterostructures (VdW) by vertically stacking 2D materials provide a fertile playground to engineer novel properties and devices. Of particular interest are the Moiré patterns due to the lattice mismatch and crystalline misalignment between vertically stacked layers. Indeed, quantum interference effects between sheets of the 2D TMD with a twist angle allows an unprecedented control of the effective electron kinetic energy scale, driving the system to an interaction dominated regime and drastically enhancing anisotropies, thus providing a pathway to engineer SC properties at the 2D scale.

During the internship, the student will initiate the fabrication of TMD-based VdW heterostructures displaying SC properties using exfoliation techniques. Samples of NbSe₂ will be fabricated and characterized as a function of thickness and twist angle. The obtained samples will be first measured by transport measurements to assess their presence of SC and its critical temperature. Going beyond traditional transport measurements, an originality of the project will be the use of low temperature spectroscopic techniques with micron-size spatial resolution like Raman scattering to probe the SC state.

 

References :

1. M. Kennes et al. Nature Physics, 17, 155 (2021)
2. Grasset et al. Physical Review Letters 122, 127001 (2019)
3. Yoshikawa et al. Nature Physics (2021)

 

Contact: Yann Gallais

The control of correlated electronic phases has emerged as a central challenge in the research on quantum materials. In these materials many intertwined electronic orders compete or cooperate, making them an attractive playground to discover or engineer novel quantum phases. Of particular interest in these quantum materials is the possibility of continuously tuning one of their phases towards a quantum critical point (QCP) using a non-thermal tuning parameter. A QCP is defined as the T=0K end point of a continuous phase transition. Interestingly, high-temperature superconductivity is often found to develop around a QCP. This is the case in heavy fermions and iron-based superconductors where the maximum critical temperature T_c lies nearby the end point of a magnetic, nematic or density wave phase. However, the nature of the QCP and the coupling between the nearby electronic order and superconductivity remain poorly understood because of a lack of experimental techniques capable of tuning electronic orders in a selective and continuous way. A promising tool to achieve this goal is tunable uni-axial strain because it can couple selectively to some electronic orders, like nematicity and charge density wave. Anisotropic strain provides a novel way to continuously tuned the materials towards various QCPs, and possibly engineer novel phases hitherto inaccessible by other means.

In this internship we propose to implement a novel uni-axial strain optical set-up recently developed in the SQUAP team of MPQ to tune charge density-wave and nematic orders across QCP. The resulting strain tuned electronic orders will be monitored by Raman spectroscopy and transport measurements. Candidate materials of interests are transition metal dichalcogenides (TMDs) displaying both charge density wave and superconductivity, and also chalcogenides like FeSe with both nematic and superconducting orders.

References :

1. J.C. Philippe et al. Phys. Rev. Lett. 129, 187002 (2022)
2. Straquadine et al. Phys. Rev. X 12, 021046 (2022)

Contact: Yann Gallais