Theory of spectroscopy, dynamics and numerical methods for complex materials

Our research would not be possible without the funding of basic research. We therefor thank gratefully the different German and European funding agencies enabling fundamental research. We are part of the following research projects:

Radioactive elements have been of significant scientific and technological interest ever since their discovery. They undergo change in a variety of ways, Electron Capture (EC) decay being one such pathway. New theoretical calculation techniques and measurement improvements for this process would deliver wide-ranging benefits, including: greater knowledge of the effects of EC decay at the DNA level in cancer therapy; a better understanding of the early history of the solar system; updated and improved radionuclide standards data.

This project will improve novel detection techniques and theoretical models to understand better the behaviour of radioactive elements undergoing EC decay. X-ray emissions from selected radionuclides will be measured more precisely, using recently developed Metallic Magnetic Calorimeters (MMCs) and Microwave Coupled Resonators (MCRs); the high-precision experimental data to be obtained will help validate modelling and calculations. The potential for MMC and MCR detector systems will be demonstrated, with project results delivering improved theoretical methods and standards data for users in nuclear physics.

For more information please visit the site of MetroMMC

Neutrino Mass Determination by Electron Capture in Holmium-163 – ECHo

For many years, the kinematic determination of the electron anti-neutrino mass has been associated to the measurement of the tritium beta spectrum. This approach led to the most stringent upper limit on the electron anti-neutrino mass of about 2 eV/c² at 95% C.L.. At the same time the most stringent upper limit on the electron neutrino mass is presently 225 eV/c² at 95% C.L. obtained by the analysis of the ¹⁶³Ho X-ray spectrum. The Electron Capture ¹⁶³Ho experiment, ECHo, is designed to investigate the electron neutrino mass in the sub-eV/c² range by means of the analysis of the calorimetrically measured energy spectrum following the electron capture process of 163Ho. In the ECHo experiment, arrays of low temperature metallic magnetic calorimeters with high energy resolution and fast response time, having the high purity 163Ho source embedded in the absorber, will be used to calorimetrically measure the EC spectrum in a low background environment. Thanks to the results obtained by the working groups of the DFG Research Unit ECHo within the first funding period, the possibility to improve the limit on the electron neutrino mass at the level of the limit on the anti- neutrino mass seems today in reach. Not only methods for the production of large activity of high purity 163Ho sources, the development of high energy resolution large MMC arrays with multiplex readout and the identification and suppression of background sources which allow for the precise measurement of the ¹⁶³Ho spectrum, but also a more precise determination of the expected spectral shape have been obtained during the last three years. Moreover, the independent determination of the QEC-value by Penning-Trap Mass Spectrometry and by the analysis of the calorimetrically measured spectrum showed consistent results pointing out the reliability of the 163Ho spectrum measurement technique.

For more information please visit the site of FOR 2202: ECHo

Dynamical Mean-Field Approach with Predictive Power for Strongly Correlated Materials

During the last few years conventional band-structure calculations in the local density approximation (LDA) have been merged with a modern many-body approach, the dynamical mean-field theory (DMFT), into a novel computational method referred to as LDA+DMFT. While this framework has proved to be a breakthrough for the realistic modeling of the electronic, magnetic, and structural properties of materials such as transition metals and their oxides, it still needs to be considerably advanced to be able to treat increasingly complex systems. This requires, for example, an improvement of the interface between the band structure and many-body constituents of the approach, the refinement and integration of efficient quantum impurity solvers, the realistic computation of free energies and forces, and the development of schemes to treat non-local correlations.

The goal of the Research Unit FOR 1346 is to develop the dynamical mean-field approach into a comprehensive computational scheme for the investigation of materials with strong electronic correlations. To this end it brings together experts in electronic band structure theory, materials science, many-body approaches, quantum impurity solvers, and numerical optimization techniques. The Research Unit FOR 1346 coordinates the development of electronic structure calculations on the basis of dynamical mean-field approaches within the German speaking part of Europe and is the first concerted research activity in this field worldwide. The ultimate goal of the Research Unit is to create a new standard of computational electronic structure scheme which is suitable to compute, and even predict, the properties of complex, correlated materials.

The Research Unit FOR 1346 is funded by the Deutsche For­schungs­ge­mein­schaft (DFG) since 2010. During its first funding period (August 2010 - July 2013) the Research Unit FOR 1346 made great progress in the combination of the main density-functional codes with the state-of-the-art DMFT solvers. The aim of the second funding period (August 2013 - July 2016) is to bring the dynamical mean-field approach to the full first-principles level, which is free from adjustable parameters.

For more information please visit the site of FOR 1346