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Personne# Xiangwei Huang

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Sonde ionique focalisée

La sonde ionique focalisée, plus connue sous le nom du sigle anglais FIB (Focused ion beam), est un instrument scientifique qui ressemble au microscope électronique à balayage (MEB). Mais là où le M

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Jonas De Jesus Diaz Gomez, Chunyu Guo, Xiangwei Huang, Philip Johannes Walter Moll, Matthias Carsten Putzke, Yi-Chiang Sun

The concept of quasi-symmetry-a perturbatively small deviation from exact symmetry-is introduced and leads to topological materials with strong resilience to perturbations. The crystal symmetry of a material dictates the type of topological band structure it may host, and therefore, symmetry is the guiding principle to find topological materials. Here we introduce an alternative guiding principle, which we call 'quasi-symmetry'. This is the situation where a Hamiltonian has exact symmetry at a lower order that is broken by higher-order perturbation terms. This enforces finite but parametrically small gaps at some low-symmetry points in momentum space. Untethered from the restraints of symmetry, quasi-symmetries eliminate the need for fine tuning as they enforce that sources of large Berry curvature occur at arbitrary chemical potentials. We demonstrate that quasi-symmetry in the semi-metal CoSi stabilizes gaps below 2 meV over a large near-degenerate plane that can be measured in the quantum oscillation spectrum. The application of in-plane strain breaks the crystal symmetry and gaps the degenerate point, observable by new magnetic breakdown orbits. The quasi-symmetry, however, does not depend on spatial symmetries and hence transmission remains fully coherent. These results demonstrate a class of topological materials with increased resilience to perturbations such as strain-induced crystalline symmetry breaking, which may lead to robust topological applications as well as unexpected topology beyond the usual space group classifications.

Jonas De Jesus Diaz Gomez, Amelia Emily-Kay Estry, Xiangwei Huang, Philip Johannes Walter Moll, Matthias Carsten Putzke

We present an experimental set-up for the controlled application of strain gradients by mechanical piezoactuation on 3D crystalline microcantilevers that were fabricated by focused ion beam machining. A simple sample design tailored for transport characterization under strain at cryogenic temperatures is proposed. The topological semi-metal Cd3As2 serves as a test bed for the method, and we report extreme strain gradients of up to 1.3% mu m(-1) at a surface strain value of approximate to 0.65% at 4 K. Interestingly, the unchanged quantum transport of the cantilever suggests that the bending cycle does not induce defects via plastic deformation. This approach is a first step towards realizing transport phenomena based on structural gradients, such as artificial gauge fields in topological materials.

Topological semimetals are frequently discussed as materials platforms for future electronics that exploit the remarkable properties of their quasiparticles. These ideas are mostly based on dispersion relations that mimic relativistic particles, such as Weyl Fermions, and the nontrivial quantum phase evolution in a Berry curvature landscape. These ideas are fascinating, yet so-far entirely theoretical. This thesis is concerned with experimentally investigating these prospects using Focused Ion Beam prototyping. Three science cases are pursued in its course:The first project involves searching for the chiral zero sound in microstructuredWeyl semimetals. The chiral zero sound mode is a collective excitation of the Fermi surface, that is predicted to lead to giant magnetic quantum oscillations in the thermal conductivity. We developed a technique based on the idea of the 3w method and successfully measured the thermal conductivity of a TaAs microbar at cryogenic temperatures. We found that the third harmonic voltage of the material within the magnetic field showed large quantum oscillations without the magnetoresistance background as in the regular Shubnikov-de Haas (SdH) effect. We demonstrated this effect comprehensively in topological semimetal CoSi, due to its simpler oscillation frequencies. Third harmonic voltage is related to the derivative of the resistance due to the nonlinear current-voltage relation of a semimetal under self-heating. The enhanced quantum oscillations appear because the derivative of the resistance reacts more sensitive to Landau quantization than the resistance. This alternative way of measuring quantum oscillations may improve the sensitivity with which we probe topological semimetals.The aim of the second experiment is the search for nonlocal voltages in microstructured Cd3As2 devices. Theory predicts non-local voltages to appear due to a quantum process combining Fermi arc states with bulk chiral Landau levels, known as the Weyl orbit. We demonstrate that due to the field-induced giant conductivity anisotropy in semimetals, the current path is strongly altered, forming two long-ranged current beams propagating along the magnetic field. This phenomenon sometimes referred to as "current jetting", is a general property of high mobility semimetals. It can also cause nonlocal signals, leaving it impossible to unequivocally identify the nonlocal voltages from topological effects. Finite element simulations of the current jetting effect in Cd3As2 microstructures quantitatively capture the current path in the devices. This experiment presents a significant step towards understanding the microscopic current patterns in 3D Dirac semimetal microdevices.In the third experiment, we report a thorough study on the transport properties of the recently discovered Kagome superconductor CsV3Sb5, which is considered an ideal platform to study the interplay between topology and correlations. By designing a unique membrane-based low-strain microstructure, we are able to detect the intrinsic properties of this material. This allows us to settle the ongoing debate about the existence of 3D ellipsoidal Fermi surfaces and to uncover the unexpected absence of magnetoresistance at moderately high temperatures, which violates Kohler scaling. Our results indicate that the electrical properties of CsV3Sb5 have mixed dimensionality, and the 3D nature has to be considered in the physical description of this material.