“Transition metal halides are a new class of van der Waals materials that we have identified as an ideal platform for many-body engineering. We are very enthusiastic about this class of materials because we think that it is one of the best and cleanest examples of a Mott insulator that currently exists”, says Erik van Loon, associate senior lecturer at NanoLund and the division of mathematical physics.
The transition metal halides are Niobium combined with Fluorine, Chlorine, Bromine, or Iodine – Nb3(F,Cl,Br,I)8.
“It turns out that by changing the halide element from I via Br, and Cl to F, the electronic properties of the material change dramatically, eventually turning the material into what is called a Mott insulator: a system where the repulsive interaction between electrons is so strong that they can no longer move, similar to cars stuck in a traffic jam. This phase is of great interest to physicists, but it has been hard to find materials that provide a clean realization of the phase.”
A system where the repulsive interaction between electrons is so strong that they can no longer move, similar to cars stuck in a traffic jam
A surprise to the team was that the material should not be understood in terms of a monolayer but in terms of bilayers.
“It turns out that there is a very strong coupling between pairs of layers, and then weak coupling to the next pair. This was a surprise because two-dimensional materials usually have very weak interlayer coupling between all layers – so-called van der Waals bonds”, says Erik van Loon.
A more stable platform for studying quantum correlations
This class of materials, Erik van Loon states, provides a highly tunable platform for studying quantum correlations: the strength of the correlations is controlled by the choice of the halide element.
“Furthermore, the correlations are much more robust, for example, stable up to high temperature, than other platforms for two-dimensional correlations, such as twisted bilayer graphene or star-of-david 1T-TaS2. The reason is that the correlations take place within Nb trimers and are well-separated spatially and energetically from the rest of the material.”
Superconducting currents
“The reason for the great interest in this class of materials is that previous experiments have shown a field-free Josephson diode effect, which means that superconducting current flows preferentially in one direction. This kind of diode effect could be used as a building block for computing using superconducting currents. However, so far, it is not really understood why this Josephson diode effect occurs, so there are still open questions for theory and experiment.”
Figuring out the properties of these materials was teamwork:
“A combination of synchrotron-based angle-resolved photoelectron spectroscopy and theoretical calculations demonstrates that the correlations in transition metal halide compounds can be tuned by chemical composition, thickness, and doping, thereby opening the door to designing materials that host a wide range of strongly interacting quantum states.”
The paper is an experiment-theory collaboration, with ARPES (angle-resolved photoelectron spectroscopy) experiments performed in Aarhus and theory done in Nijmegen, Örebro, and Lund.