Focus of my research:

In order to develop the next "next-generation" device or technology we need to first explore and understand the limitations of the existing ones. The key requirement of any technological advancement is a comprehensive knowledge about basic properties of materials.

The expression "quantum materials" comprises a huge variety of materials which host emerging physical properties rooted in basic quantum effects. A huge choice of interesting physics is covered by the notion "Quantum Matter". For example, materials have been identified in which their macroscopic physics is governed by exceptional single particle conditions (e.g. Dirac- or Weyl physics in semi-metals) or collective excitation of multiple strongly correlated particles (e.g. nematicity, unconventional superconductivity and magnetism, or spin- and charge-density order).

In order to contribute to a better understanding of emergent physical phenomenae we try to disturb exciting materials by systematic changes of certain conditions that may alter their equilibrium ground states. Extreme conditions available in our laboraties, e.g., the world's strongest magnetic fields and lowest temperatures, high pressure, and reduced dimensions can act as external non-invasive tuning parameters. In my research I am using a combination of these external parameters on a broad range of quantum materials.

I am particularly interested in the mesoscopic behavior of those materials as their dimensions approach the mesoscale regime, i.e. the micro- and nano meter range. New insights into the physics of any material of interest can be revealed as the dimensions are shrunk down to its typical intrinsic length scales. For example, we are investigating how collective ground states, such as superconductivity or magnetic order, behave if the dimensions of their host are reduced to below the relevant coherence lengths. With our approach we can tune the specimen dimensions by at least three orders of magnitude, that is through a range of 0.1-100 micrometers.

Here at the Dresden High Magnetic Field Laboratory we aim to investigate, understand, and control strong correlation effects in topical quantum materials. Our tools are, for example non-destructively generated pulsed magnetic fields up to 100 Tesla, low temperatures down to below 100 Milli Kelvin, and high pressures reaching beyond 1 Mega Bar. Together with the Institute of Ion Beam Physics and Materials Research (IBC) the Helmholtz-Zentrum Dresden-Rossendorf offers an impressive amount of expertise and experimental opportunities for my research. In addition I have the great opportunity to collaborate with the Max Planck Institute for Chemical Physics of Solids (MPI CPFS) in Dresden.

One of the specialties at IBC and MPI CPfS is the fabrication of 3D nano- and micro-structures from quantum materials by the application of focused-ion-beam (FIB) and electron-beam lithography methods. At the MPI CPfS we have access to one of the world's leading facilities in the Department for "The Physics of Quantum Materials" headed by Prof. Andrew Mackenzie. The availability of Dual Beam FIB-Scanning Electron Microscope (SEM) systems as well as cleanroom facilities for micro- and nano-fabrication processes enables us to develop innovative experiments for our research.

A powerful tool to scrutinize the underlying physics that is responsible for any electronic ground state in metallic materials is magnetotransport, that is electrical transport under the influence of an applied external magnetic field.

Below, topics are listed and materials I have been working on:

Unconventional Superconductors, Iron Pnictides, Quasi 1D Superconductors, Cuprates Superconductors, 4f/5f Heavy Fermion, Frustrated Quantum Magnets, Dirac and Weyl Semi-Metals, Quantum Magnets, Kitaev Honeycomb Lattices, Skyrmion systems, Anti- and Ferromagnets, Actinide metals, Topological Matter, ...

Main methods:

We are accessing physical properties of exotic single-crystalline materials by the application of focused ion beams. This approach enables investigations of electrical transport on sub-micron sized devices cut from single crystals of almost any material and along any desired orientation.

Micro-fabrication of devices by the application of focused ion beams and UV-light lithography

• • micron scale devices

  • feature sizes of the order of 10 nm

Electrical and thermolelectric transport, magnetic torque experiments under extreme conditions:

• • high current densities (sub-micron squared cross sections)

  • low temperatures (T = [0.01 - 1000] K)

  • high magnetic field (B < 100 T)

  • high pressures (p < 500 kbar)

Toni Helm, PhD