Thesis presented December 19, 2023

Abstract:
In this thesis, we have studied the electrical and thermal transport properties of the non-centrosymmetric compound EuPtSi, at low temperatures and under magnetic field.
The first part of the results focus on the low-field (H < 5 T) magnetic phases that are observed in EuPtSi below the ordering temperature T = 4.1 K for the three main directions H ∥ [110], H ∥ [111] and H ∥ [100]. From the angular dependence of the resistivity under magnetic field, we confirmed the presence of anomalous phases: for the direction H ∥ [111] the so-called skyrmion lattice A-phase, and for the H ∥ [100] direction the skyrmion-related A’ and B phases. By studying these phases with various transport probes, we have established the magnetic phase diagram with precision. We have also evidenced the presence of additional anomalies at low-temperature related to the skyrmion phases. Our temperature-dependent results for the H ∥ [111] and H ∥ [100] evidence the metastable behavior of the A, A’ and B phases under field cooling. From the peculiar metastable state for the H ∥ [100] direction, further field sweeps show that both the A’ and the B phases are created simultaneously through the same mechanism. From transport measurements as a function of temperature, we also show the existence of a non-Fermi liquid behavior in the conical ordered state.
The second part of our results focus on the Fermi surface of EuPtSi, studied with quantum oscillations in the thermoelectric power for high fields (H > 6 T). The temperature dependence of the quantum oscillations recovers the effective mass of the carriers for each Fermi surface branch, with results comparable to the literature. From the field-dependence of the quantum oscillations, we have recovered the Dingle temperature and mean-free path of the carriers using a new analysis framework for the thermoelectric power.

Keywords:
Topological materials, EuPtSi, Skyrmion, Quantum oscillations, Transport

On-line thesis. ​