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

Detection of relativistic fermions in topological semimetals with magnetostriction

leader: Prof. T. Cichorek

Description

One of the major themes of condensed-matter physics has been the discovery and classification of miscellaneous phases of matter. Historically, it was believed that characterization of states through the principle of spontaneous symmetry breaking and local order parameters can give a universal description of all kinds of states. The discovery and understanding of quantum Hall states opened up a new 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 and topological phase transitions. Particularly significant effort has been devoted to searching and characterizing topological semimetal phases in the past few years. Topological semimetals are characterized by bulk band crossings in their electronic structures, which are expected to give rise to gapless electronic excitations and topological features that underlie exotic physical properties. The most famous examples are Dirac and Weyl semimetals, in which the corresponding low-energy fermionic excitations are direct analogues of relativistic particles in quantum field theory.

The unique topological nature of topological semimetals promises many novel properties, such as protection from back-scattering, monopoles, and Fermi arcs on the surface. Moreover, research interest in materials with linearly dispersing bands is fueled by their technological potential for exploiting the relativistic nature of Dirac and Weyl fermions in high-speed electronics.

A research project Detection of relativistic fermions in topological semimetals with magnetostriction addresses a fundamental problem related to experimental investigations of Weyl and Dirac quasiparticles. First-principles calculations and angle-resolved photoemission spectroscopy measurements can point towards new materials with nontrivial band topology. However, other experimental signatures of relativistic fermions are often subtle and indirect, since in these materials conventional, massive charge carriers also exist. Hence, new experimental methods for determining the relativistic character of the quasiparticles are highly desirable to set the stage for investigations of their relevance for electronic applications.

Our proposal draws attention to magnetostriction, which in a nonmagnetic semimetal results from the interaction between the electronic and elastic degrees of freedom in a crystal, and thus is determined by the change in charge-carrier density in an intense magnetic field. Furthermore, for a multiband material with a multivalley structure, such as semimetals or degenerate semiconductors, this direction-dependent thermodynamic quantity is greatly enhanced due to band overlap and electron redistribution between the bands upon switching on the magnetic field.

Employing a theory of magnetostriction for topological semimetals developed by our collaborators, we have recently demonstrated that measuring the field-induced length change in the quantum limit allows one to clearly distinguish between relativistic and conventional electrons, owing to fundamentally different contributions from linearly crossing and trivial parabolic bands when relativistic fermions are confined to the zeroth Landau level [T. Cichorek et al.].

The main research task is to study relativistic quasiparticles in topological semimetals using magnetostriction as an experimental probe. We propose comprehensive investigations of the angle-dependent field-induced length change of selected representative TSMs with bulk band crossings sufficiently close to the Fermi energy, giving rise to robust gapless electronic excitations.

Our second objective, which constitutes the main experimental challenge of the project, is to explore the effect of uniaxial stress on magnetostriction. Because this thermodynamic quantity is sensitive to the position of the Fermi level, we plan to study magnetostrictive effects when the enclosed nodes are tuned under uniaxial tension to the Fermi level, thereby searching for new physics.

In a broader context, the observation of large and strongly anisotropic length changes under magnetic fields may be relevant for future Weyltronic devices, since strained thin films could be realized using magnetostrictive stress.

Main tasks

• Detection of relativistic fermions in topological semimetals

  Using magnetostriction as an experimental probe

• Studies of magnetic-field-induced length changes depending on the direction of the applied field

• Influence of uniaxial stress on magnetostriction

  Magnetostrictive effects when Weyl (Dirac) nodes are tuned to the 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)

Links

BIP – special recruitment announcement
PRELUDIUM BIS 3 NCN-2021/43/O/ST3/03000 (2022)