Periodic Reporting for period 5 - QUANTMATT (Dynamics and transport of quantum matter --- exploring the interplay of topology, interactions and localization)
Berichtszeitraum: 2020-09-01 bis 2021-03-31
A quantum technology is reliant on a fundamental understanding of quantum matter. In quantum matter the quantum nature of the matter constituents is of paramount interest. Quantum particles, such as the electron, have a wave-like character and therefore have an associated phase. Particles moving from A to B simultaneously take all possible paths connecting the two points, with their different phase factors interfering in the process. This makes quantum particles fundamentally different from their classical counterparts, those that we encounter in daily life and know always just take one path at a time in getting between places. In a newly discovered family of materials, which we refer to as topological quantum matter, the phase factor of the electrons winds in a topologically nontrivial way---in some sense it is tied into a knot. This results in some unique electronic properties: topological insulator are bulk insulators with a stable metallic surface shell; topological semimetals (or Weyl semimetals) have unusual response to electromagnetic fields, and topological superconductors host exotic particles, called Majorana fermions, that may potentially be used in quantum computing. In another class of material, interactions between different particles results in strong correlations and entanglement. Entanglement is a unique quantum phenomena in which the state of one particle can instantaneously affect the state of another distant particle; it is also the resource needed for performing quantum computations.
The major objective of this project was to develop a fundamental understanding of the inner working of such quantum materials, and to find ways of uncovering their deep quantum nature in an experimental setup. This includes developing new computational ways of simulating quantum materials, and designing and modeling devices that allow for experimental verification of their theoretical description. Studies like this build the fundamental foundation on which the rapidly emergent quantum technology will rest.
The work on Weyl semimetals first discussed the possibility of realizing what one can call a fractional Weyl semimetal, and showed that it would respond to electromagnetic fields as if the constituents of the material had a fraction of the elementary charge of an electron. We then analyzed how the resistance of a Weyl semimetal in a magnetic field can have an extremely strong dependence on the direction in which the magnetic field is pointing and discussed how this could be important for analyzing conductance measurements. We have also discussed how to construct even more exotic material, so called nodal-line Weyl semimetals, from heterostructures of Weyl semimetals, and demonstrated how certain types of Weyl material may have interesting thermal properties. An further important outcome of the project was the link between phenomenology from high-energy physics and topological quantum matter, especially the understanding of so-called chiral anomalies. This is a phenomena that involves chiral particles, namely particles with a definite handedness, either left- or right-handed. The anomaly is the result that this handedness can be changed by quantum fluctuations, resulting in right-handed particles becoming left-handed, and vice versa. We have been able to show how this can be best observed in real materials and further how this can be used in spintronics materials to manipulate magnetic degrees of freedom.
Our work on the many-body localized insulator, which is an insulator of many interacting correlated electrons, revealed how one can obtain information about the properties of the system by looking at certain one-particle properties. This allows for a clear way of obtaining information in simulations, but may also be useful for experimentally exploring these materials. An important insight into the nature of the many-body localization transition requires understanding of the structure of entanglement in such states. To capture this structure in simulations turns out to be quite difficult, but we have devised a new algorithm that can do this in an accurate albeit approximate way. This allowed new insights into the nature of the many-body localization transition.
These and other related results that resulted from this project where published in at least 47 peer-reviewed articles, and were presented in over twenty lectures in conference and workshops and colloquia around the world. One popular science article in Swedish was also published.