**Êtes-vous un étudiant de l'EPFL à la recherche d'un projet de semestre?**

Travaillez avec nous sur des projets en science des données et en visualisation, et déployez votre projet sous forme d'application sur Graph Search.

Publication# Strain Gradients and Magnetoresistance in Microstructured 3D Topological Semi-metals

Résumé

Three-dimensional topological semimetals have emerged as strong candidates to probe new fundamental physical phenomena that could be exploited to develop next generation electronics. However, many aspects of their electronic properties remain unclear. This thesis addresses mainly two of them: (i) the interplay between their lattice deformations and charge carrier dynamics and (ii) the possible origin of their magnetoresistive behaviour. This is done by investigating the transport properties of microstructures fabricated with the focused ion beam. The results are presented in three main parts: In the first part, I describe the development of an apparatus for the controlled application of strain gradients at cryogenic temperatures. This is achieved by mechanically bending 3D crystalline cantilevers. A simple sample design aimed at transport characterization under strain is adopted. The bending apparatus is also fitted to a rotator probe for the simultaneous application of strain gradients and the rotation with respect to the applied magnetic field. Strain gradients exceeding 1.3 % um^-1 are achieved at a surface strain value of approx. 0.65 %. Moreover, the results establish that the quantum transport characteristics of the cantilevers are unaltered by the large strain gradients, demonstrating that no plastic deformation is induced. In the second part, I apply the newly developed technique to explore the effects of strain gradients on the Dirac semimetal Cd3As2. This material hosts novel electron orbits, called Weyl orbits, that combine bulk chiral states coming from Landau quantization with Fermi arc states that are located on the surface. Theoretical models that treat strain gradients in the framework of pseudo-electromagnetic fields predict that the periodicity of the Weyl orbits is sensitive to strain gradients. Despite features of the experimental data compatible with some of the expected effects of pseudo-fields, the results reveal a more complicated experimental scenario with important aspects that cannot be easily captured by the available simplified models. In the final part, I investigate the transport properties of the Dirac nodal arc semimetal PtSn4. This material is characterized by its extremely large magnetoresistance at low temperatures. There has been an intense debate on whether such behaviour is connected to its peculiar electronic structure that contains a short Dirac node line in momentum space associated to graphene-like surface states. By fabricating the first microstructures of this material, I study the transport behaviour for currents applied along all main crystallographic directions. While for some directions the usual extremely large magnetoresistance is observed, for one particular direction the magnetoresistance is suppressed. An inspection of the complex Fermi surface shows that this suppression is associated to the existence of open orbits in one of the branches. The presence of the open orbits is very sensitive to the magnetic field orientation. These results suggest a semi-classical origin for the characteristic large magnetoresistance observed in this material.

Source officielle

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Concepts associés (33)

Publications associées (69)

MOOCs associés (32)

Déformation plastique

La théorie de la plasticité traite des déformations irréversibles indépendantes du temps, elle est basée sur des mécanismes physiques intervenant dans les métaux et alliages mettant en jeu des mouvements de dislocations (un réarrangement de la position relative des atomes, ou plus généralement des éléments constitutifs du matériau) dans un réseau cristallin sans influence de phénomènes visqueux ni présence de décohésion endommageant la matière. Une des caractéristiques de la plasticité est qu’elle n’apparaît qu’une fois un seuil de charge atteint.

Finite strain theory

In continuum mechanics, the finite strain theory—also called large strain theory, or large deformation theory—deals with deformations in which strains and/or rotations are large enough to invalidate assumptions inherent in infinitesimal strain theory. In this case, the undeformed and deformed configurations of the continuum are significantly different, requiring a clear distinction between them. This is commonly the case with elastomers, plastically-deforming materials and other fluids and biological soft tissue.

Deformation (engineering)

In engineering, deformation refers to the change in size or shape of an object. Displacements are the absolute change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain is the relative internal change in shape of an infinitesimally small cube of material and can be expressed as a non-dimensional change in length or angle of distortion of the cube. Strains are related to the forces acting on the cube, which are known as stress, by a stress-strain curve.

Plasma Physics: Introduction

Learn the basics of plasma, one of the fundamental states of matter, and the different types of models used to describe it, including fluid and kinetic.

Plasma Physics: Introduction

Learn the basics of plasma, one of the fundamental states of matter, and the different types of models used to describe it, including fluid and kinetic.

Plasma Physics: Applications

Learn about plasma applications from nuclear fusion powering the sun, to making integrated circuits, to generating electricity.

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

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 character ...

Pedro Miguel Nunes Pereira de Almeida Reis, Dong Yan, Bastien Freddy Gustave Aymon

We develop a reduced-order model for thin plates made of hard magnetorheological elastomers (hard-MREs), which are composed of hard-magnetic particles embedded in a polymeric matrix. First, we propose a new magnetic potential, as an alternative to an exist ...

Dragan Damjanovic, Fangping Zhuo

We report an intrinsic strain engineering, akin to thin filmlike approaches, via irreversible hightemperature plastic deformation of a tetragonal ferroelectric single-crystal BaTiO3. Dislocations wellaligned along the [001] axis and associated strain field ...