NCN PRELUDIUM BIS 3: 2021/43/O/ST3/03000

Detection of relativistic fermions in topological semimetals with magne-tostriction

leader: Prof. T. Cichorek

Description

One of the major themes of condensed-matter physics has been the discovery and classification ofmiscellaneous phases of matter. Historically, it was believed that characterization of states throughthe principle of spontaneous symmetry breaking and local order parameters can give a universal de-scription of all kinds of states. The discovery and understanding of quantum Hall states open up anew era of condensed-matter physics: topological quantum states of matter.

In the last 40 years, we have witnessed the emergence of many types of topological states andtopological phase transitions. Particularly significant effort has been devoted to searching and charac-terizing topological semimetal phases in the past few years. Topological semimetals are characterizedby bulk band crossings in their electronic structures, which are expected to give rise to gapless elec-tronic excitations and topological features that underlie exotic physical properties. The most famousexamples are Dirac and Weyl semimetals, in which the corresponding low-energy fermionic excita-tions are direct analogues of relativistic particles in quantum field theory.

Unique topological nature of topological semimetals promise many novel properties, such as pro-tection from back-scattering, monopoles, and Fermi arcs on the surface. Moreover, the research inter-est in materials with linearly dispersing bands is fuelled by their technological potential for exploitingthe relativistic nature of the Dirac and Weyl fermions in high-speed electronics.

A research project Detection of relativistic fermions in topological semimetals with magne-tostriction addresses a fundamental problem related to experimental investigations of Weyl and Diracquasiparticles: First-principles calculations and angle-resolved photoemission spectroscopy measure-ments can point towards new materials with nontrivial band topology. However, other experimentalsignatures of relativistic fermions are often subtle and indirect, since in these materials conventional,massive charge carriers exist as well. Hence, new experimental methods for determining the rela-tivistic character of the quasiparticles are highly desirable to set the stage for investigations of theirrelevance for electronic applications.

Our proposal draws attention to the magnetostriction that in a nonmagnetic semimetal resultsfrom the interaction between the electron and elastic degrees of freedom in a crystal, and thus it isdetermined by the change of the charge-carrier density in an intense magnetic field. Furthermore, fora multiband material with multivalley structure, such as semimetals or degenerate semiconductors,this directional dependent thermodynamic quantity is greatly enhanced due to a band overlap andan electron redistribution between the bands at the switching-on of magnetic field. Employing atheory of the magnetostriction for topological semimetals developed by our collaborators, we haverecently demonstrated that measuring the field-induced length change in the quantum limit, one canclearly distinguish between the relativistic and conventional electrons owing to cardinally differentcontributions from the linearly crossing and trivial parabolic bands when relativistic fermions areconfined at the zeroth Landau level [T. Cichorek et al. arXiv:2106.06062].

The main research task is intended to study relativistic quasiparticles in topological semimetalsusing the magnetostriction as an experimental probe. We propose comprehensive investigations ofthe angle-dependent field-induced length change of selected representative TSMs with bulk bandcrossings sufficiently close to the Fermi energy, and hence giving rise to robust gapless electronicexcitations. Our second intention, which constitutes the main experimental challenge of the project,is to explore effect of uniaxial stress on the magnetostriction. Because this thermodynamic quantity issensitive to the position of the Fermi level, we plan to study magnetostrictive effects when the enclosednodes will be tuned under uniaxial tension to the Fermi level, and thus to search for new physics. Ina wider context, the observation of large and strongly anisotropic length changes under magneticfields can be relevant for future Weyltronic devices, since strained thin-films might be realized usinga magnetostrictive stress.

Main tasks

• detection of relativistic fermions in topological semimetals

  using magnetostriction as an experimental probe

• studies of the change in length induced by the magnetic field depending on the direction of the applied field

• influence of uniaxial stress on magnetostriction

  magnetostrictive effects when Weyl (Dirac) nodes are tuned to Fermi level

Financing

• PhD scholarship (2022-2026)

  • PLN 5,000 / month - first two years
  • PLN 6,000 / month - next two years

• Foreign stay (Max Planck Institute for Chemical Physics of Solids in Dresden)

  • PLN 9,000 / month - 6 months (financed by NAWA)

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