Semimétal

Résumé

redresse=1.75|vignette| Diagramme représentant la distribution des électrons dans les bandes de différents types de matériaux à l'équilibre. De gauche à droite : métal ; semimétal ; semiconducteur (dopé p, intrinsèque, dopé n) ; isolant. L'énergie est représentée par l'axe vertical, tandis que l'épaisseur horizontale des bandes représente la densité d'états.La densité électronique par niveau d'énergie suit la statistique de Fermi-Dirac et est représentée par un dégradé de noir. Le niveau de Fermi E des métaux et des semimétaux se trouve dans au moins l'une des bandes, tandis qu'il se trouve loin de toute bande dans les isolants, et suffisamment proche d'une bande dans les semiconducteurs pour peupler la bande de conduction ou la bande de valence d'électrons ou de trous. Un semimétal est un matériau dont les états d'énergie inférieurs de la bande de conduction recouvrent très faiblement les états d'énergie supérieurs de la bande de valence. Il s'agit de l'un des types de substanc

Source officielle

https://fr.wikipedia.org/wiki/Semim%C3%A9tal

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Three-dimensional topological semi-metals carry quasiparticle states that mimic massless relativistic Dirac fermions, elusive particles that have never been observed in nature. As they appear in the solid body, they are not bound to the usual symmetries of space-time and thus new types of fermionic excitations that explicitly violate Lorentz-invariance have been proposed, the so-called type-II Dirac fermions. We investigate the electronic spectrum of the transition-metal dichalcogenide PtSe2 by means of quantum oscillation measurements in fields up to 65 T. The observed Fermi surfaces agree well with the expectations from band structure calculations, that recently predicted a type-II Dirac node to occur in this material. A hole- and an electron-like Fermi surface dominate the semi-metal at the Fermi level. The quasiparticle mass is significantly enhanced over the bare band mass value, likely by phonon renormalization. Our work is consistent with the existence of type-II Dirac nodes in PtSe2, yet the Dirac node is too far below the Fermi level to support free Dirac-fermion excitations.

2018

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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.

EPFL2022

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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.

NATURE PORTFOLIO2022

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EPFL2022