Post-doctoral positions
We have proposals for Master level internships
Ultrafast light-control of electronic orders in van der Waals materials
Context:
Left panel: schematics of ultrafast pump-probe Raman spectroscopy. Right panel: Laser pulses can switch or push the phase of a matter (in blue) to new metastable phases (orange) in ultrafast timescales.
In this internship, which is expected to lead to a PhD thesis, the student will use laser-based techniques to induce phase transitions in the least dissipative and fastest ways. This will be achieved by using a novel ultrafast Raman scattering set-up (left panel in figure) capable of “probing” how the electronic ground state is modified by a “pump” pulse with sub-picosecond time resolution. During the internship the technique will be applied to 2D vdW transition metal dichalcogenides (TMD) like TaS2 and NbSe2 displaying CDW order coexisting with either chiral or superconducting order. Specifically, the student will investigate whether light can efficiently switch the balance between these orders, or even induces completely novel orders that do not exist in equilibrium (right panel in figure). The ultimate goal will be to establish light as a selective control parameter of quantum phases.
Contact: Yann Gallais (yann.gallais@u-paris.fr) and Niloufar Nilforoushan (niloufar.nilforoushan@u-paris.fr)
Funding: the internship and the following PhD thesis are funded by ANR project SUPER2DTMD
Shining light on superconducting 2D transition metal dichalcogenides
Context:
(top) Superconducting electron Cooper pairs on a single layer of the 2D TMD NbSe2. (bottom) Twisted layers yielding a Moire pattern that can tune drastically the 2D material’s properties.
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 [Xi16]. The possibility of fabricating these 2D crystals into vertical “van der Waals” (VdW) heterostructures make them ideal candidate for the integration into cavities to enhance light-matter interaction and achieve cavity control of quantum phases. In addition, the formation of Moiré patterns due to the lattice mismatch and crystalline misalignment between vertically stacked layers is another unique aspect of the VdW layered structures, offering opportunities for quantum engineering of material properties. 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 [Kennes21].
During the internship, the student will participate in the first steps of this ambitious project. He/she will study TMD-based VdW heterostructures displaying SC properties using exfoliation techniques. He/she will work in close collabration with our partners experts in TMD fabrication (LPS, U. Paris Saclay) and THz cavities (LSI, Polytechnique). Going beyond traditional transport measurements, an originality of the project will be the use of low temperataure spectroscopic techniques with micron-size spatial resolution like Raman scattering to probe the SC state [Grasset2018,Grasset2019]. In the longer term these optical techniques will be implemented in out-of-equilibrium pump-probe schemes and in equilibrium on cavity-integrated samples.
[Grasset18]. Grasset, R., T. Cea, Y. Gallais, M. Cazayous, A. Sacuto, L. Cario, L. Benfatto, and M.-A. Méasson. Higgs- Mode Radiance and Charge-Density-Wave Order in 2H−NbSe2. Phys. Rev. B 97, 094502 (2018).
[Grasset19] Grasset, R., Y. Gallais, A. Sacuto, M. Cazayous, E. Coronado, et M.-A. Méasson. « Pressure-Induced Collapse of the Charge Density Wave and Higgs Mode Visibility in 2HTaS2 « . Physical Review Letters 122, no 12 (2019): 127001.
[Kennes21] D. M. Kennes et al. « Moiré heterostructures as a condensed matter quantum simulator » Nature Physics 17, 155-163 (2021)
[Xi16] Xi, X., Z. Wang, H. Berger, L. Forró, J. Shan, et K. F. Mak. Ising Pairing in Superconducting NbSe2 Atomic Layers », Nat. Phys. 12, 2, 13943 (2016)
Contact: Yann Gallais (yann.gallais@u-paris.fr)
We have a post-doctoral position opening !
Probing and manipulating the superconducting state of NbSe2 in the 2D limit
2D TMD embedded in a THz split ring cavity (left) The resulting coupled light-SC matter state is probed by Raman and THz spectroscopy. The SC Higgs mode is a collective oscillation of the SC condensate (right) that can couple to cavity photon to form cavity-Higgs polaritons.
Context:. The dynamics of superconductors (SC) driven out-of-equilibrium is governed by their collective mode spectrum, and in particular the SC amplitude mode which is an analog of the Higgs mode in high-energy physics [1]. In many materials the coexistence of SC with other competing phases and/or the presence of competing SC pairing states leads to a rich spectrum of collective excitations. Examples include materials like the transition metal dichalcogenide (TMD) NbSe2 where SC Higgs and charge-density-wave (CDW) amplitude modes displays coupled dynamics [2]. These SC collective modes not only give fingerprints of the nature of the ground state [3], but also a path to dynamically drive or even control SC order.
TMDs like NbSe2 are among the few SCs that can be isolated in the few-layer limit while retaining excellent crystalline quality and sizable Tc > 4K. This allows their integration into THz cavities to achieve both strong THz light fields for collective mode driving, and strong-light matter coupling in equilibrium. 2D TMDs also offer an ideal playground to explore novel exotic SC states due to their quasi-2D character and strong spin-orbit coupling [4,5].
Goals: The present project, funded by a collaborative ANR grant between several labs in the Paris region and Strasbourg (MPQ, INSP, LSI and ISIS), aims at exploring the collective mode spectrum of the exotic SC state of 2D TMDs using complementary Raman and THz spectroscopies. We will also investigate SC 2D TMDs in the unchartered regimes of periodically driven out-of-equilibrium and strong light-matter coupling when embedded in THz cavities [6]
Tasks: The post-doc will be in charge of TMD sample fabrication and their integration into van der Waals heterostructures and cavities using state of the art 2D material transfer techniques at MPQ lab. He/she will then probe the SC state using low temperature Raman scattering and pump-probe set-ups available at MPQ labs. He/she will also be involved in complementary THz spectroscopy measurements with our partner at LSI, Ecole Polytechnique.
Profile of the candidate: the candidate is expected to have a strong expertise in 2D sample fabrication and transfer. A background in optical spectroscopy is a plus but not mandatory.
Duration of the contract: 24 months
Salary: between 3000 and 3500 euros gross per month depending on experience.
Location: MPQ lab, Université Paris Cité, 75013 Paris
Contact: Yann Gallais yann.gallais@u-paris.fr
References
[1] R. Shimano and N. Tsuji, Ann. Rev. Cond. Matt. 11, 103124, 2020
[2] Grasset, R., T. Cea, Y. Gallais, A. Sacuto, L. Cario, L. Benfatto, and M.-A. Méasson. Phys. Rev. B 97, 094502 (2018).
[3] Grasset, R. K. Katsumi, P. Massat, H.-H. Wen, Y. Gallais, and R. Shimano, npj Quantum Materials 7, 4 (2022)
[4] Xi, X., Z. Wang, H. Berger, L. Forró, J. Shan, et K. F. Mak. Nat. Phys. 12, 2, 13943 (2016)
[5] David Möckli, Maxim Khodas, Phys. Rev. B 98, 144518 (2018).
[6] Raines, Zachary M., Andrew A. Allocca, M. Hafezi, et V. M. Galitski. Physical Review Research 2, n(2020): 013143
