Nothing is perfect the quantum mechanics of defects
CECAM-ETHZ, Zurich, Switzerland
Recent progress along the aforementioned challenges has been staggering in first principles defect science. However, due to the widespread interest in defects, new developments have been disconnected with little crosstalk between the various disciplines and communities. In our experience, defects are typically discussed at topical conferences on specific materials or material classes. It is our aim to change this state of affairs with our proposed workshop.
By pooling expertise in a single event we expect to provide a unique opportunity for assessing the current state of the field. We plan to bring together a representative selection of distinguished researchers in defect science. In oral presentations, the invited speakers will address the different aspects of the four grand challenges. Ample discussion time is reserved after each presentation to reflect on the immediate challenge and its ramifications. In addition, we will organize a round table discussion, in which the broader picture can be discussed that might lead to roadmaps for future developments. We plan to structure the proposed workshop along the four main lines of challenges we have identified. Specifically we will address:
1) Computational efficiency: In experiment, the properties of defects are generally measured for dilute defect concentrations (i.e. a very small number of defects in an otherwise perfect material). Present day computers facilitate calculations for structures that encompass of the order of a few thousand atoms. This is still too small to capture the dilute limit. The workshop will discuss strategies that could address this scale problem. Potential avenues are i) accelerating the existing techniques, e.g. with localized orbitals or with “order N methods”, ii) correcting the spurious (elastic, electrostatic, quantum-mechanical) interactions induced by too small system sizes.
2) Adequacy: The predominant approach to describe the quantum mechanical interaction between electrons in materials has been DFT in the local or semilocal-density approximations (LDA, GGA). While LDA and GGAs facilitate calculations for structural models of up to a few thousand atoms, their intrinsic deficiencies (e.g. self-interaction errors, absence of long-range van der Waals effects and the band-gap problem) severely limit their predictive power. Alternatives to LDA and GGA exist. However, they come at the price of a higher computational expense and therefore smaller tractable structure models. In our workshop we plan to address these alternative techniques, namely i) hybrid functionals, ii) quantum Monte Carlo or iii) many-body perturbation theory in the GW approximation. A critical review of their performance seems timely since more applications for defects appear in the literature.
3) Complexity: As defect science progresses, more and more complex defects are being investigated. The semiconductor industry is exploring if the conductivity of materials for photovoltaic-, electronic-, and optoelectronic-devices can be more efficiently engineered with dopant complexes instead of single impurity atoms. Rare earth compounds have assumed enormous technological importance, while actinides are of interest to the nuclear industry. In the emergent field of quantum computing defects like the NV-center in diamond have been proposed as solid-state quantum bits. Magnetic impurities can make semiconductors magnetic. Nanostructures might exhibit novel types of defects, to which our current concepts derived from bulk structures do not apply. These examples illustrate that complexity can derive from structural variety, chemical diversity or intricate quantum effects. In addition, factors like finite temperatures and background gases introduce environmental effects in experiments that are typically not taken into account in first principles calculations. In our workshop we will contrast “hot” applications of complex defect physics and chemistry with methodological development that tackles the aforementioned forms of complexity.
4) Sophistication: To bridge the gap between the idealized environment of first principles computer simulations and actual experimental setups, the portfolio of first principles techniques needs to be extended. This forces us to going beyond standard zero temperature ground state methods like DFT and to introduce complexity into the theoretical framework. Different aspects need to be distinguished:
i) Experiments are always performed at finite temperature, whereas first principles calculations are typically at zero temperature. At the workshop we will review how vibrations and entropy are currently being introduced into quantum-mechanical calculations.
ii) Spectroscopic measurements involve the interaction between a probe and the defect, giving rise to an excitation of the system. The proper account of quantum-mechanical excitations is crucial to simulate experimental spectra, e.g. in photoluminescence and absorption spectroscopies.
iii) Another widely used technique to characterize defects is electron paramagnetic resonance (EPR). It requires the interactions between electronic and nuclear spins, which is typically not included in standard DFT calculations.
Patrick Rinke (Aalto University, Helsinki) - Organiser
Fabien Bruneval (Université Paris-Saclay / CEA) - Organiser
Alfredo Pasquarello (EPFL) - Organiser