Description of the thesis topic

Discovery of topological matter has triggered an intense research work motivated by the emergence of promising features among which the existence of helical ballistic edge states. For 2D systems or 3D higher order topological insulators (TI), they form one dimensional conducting channels on the edges with two time-reversed spin-momentum-locked states which do not backscatter. A given projection of the spin is locked to the propagation direction, theoretically preventing any backscattering. The topological protection manifests itself in the supercurrent which appears when the TI is connected to superconducting electrodes. Our project aims at investigating the poorly understood dynamics of helical edge states, determining their relaxation mechanisms and the robustness of their topological protection in superconducting junctions as well as in isolated TI.
We have proved that finite-frequency supercurrent response to a magnetic flux as well as the supercurrent noise at equilibrium contain clear signature of the relaxation mechanism in the GHz regime for non-topological junctions. For topological edge states, the expected millisecond lifetime calls for experiments below the MHz to detect the current fluctuations.
We propose for the internship to use our newly developed ultrasensitive TMR-based (Tunnel Magnetoresistance) magnetic field sensor to detect the supercurrent carried by the helical edge states as well as its fluctuations at equilibrium, which are predicted to contain signatures of the topological protection. Using nanolithography, the student will fabricate and deposit the junctions made from TI material (WTe2, BiBr) on the TMR sensor fabricated in a CEA lab. The TMR response is then measured at very low temperature in a dilution refrigerator (mK) with low noise electronic technics.

Work Context

The research will be conducted within the Mesoscopic Physics group at the Laboratoire de Physique des Solides (LPS), using the lab's micro/nanofabrication facilities and the group's low-temperature and high-frequency equipment.

LPS is a joint research unit (UMR 8502) of Université Paris-Saclay and CNRS, affiliated with the CNRS Institute of Physics and the 28th section of the Conseil National des Universités. It is part of the Friedel-Jacquinot Federation, which coordinates physics research on the Moulon plateau in Orsay (IdF). The lab hosts about 100 researchers and faculty, supported by 60 engineers, technicians, and staff. Each year, many undergraduate and graduate students, PhD candidates, postdocs, and visiting scientists join its activities.

Research at LPS covers a broad range of condensed matter physics, structured around three main axes:

Novel electronic states of matter (correlated systems, unconventional superconductivity, magnetism, metal-insulator transitions, etc.)

Reduced-dimensionality phenomena (nanoscience and size-dependent fundamental properties)

Soft matter and physics–biology interfaces (complex systems, living tissues, liquid crystals, foams, polymers, granular matter).

The PhD will take place in the Mesoscopic Physics team of LPS (CNRS–UMR 8502). The position falls under the protection of scientific and technical potential (PPST) regulations and therefore requires authorization from the French Ministry of Higher Education and Research (MESR).