Tomczak Group

Computational Many-Body Physics

welcome

We study electronic, optical, and thermoelectric properties of correlated materials, such as transition-metal oxides, intermetallics, rare-earth compounds, and iron pnictides and chalcogenides, using realistic simulations. Owing to strong Coulomb interactions, electrons in these materials are in a collective state that is extremely sensitive to external perturbations, resulting in rich phase-diagrams and a propensity for large response functions. This high sensitivity is a harbinger for many technological applications, such as optoelectronic switches, sensors, memory storage, and thermoelectrics. In our work, we aim to accelerate the fundamental and microscopic understanding of correlated materials and make accurate theoretical predictions for their properties and applicability in functional devices a reality.

We focus on developing and applying state-of-the-art realistic many-body methods to unravel the microscopic origins of unresolved physical phenomena in strongly correlated materials.

Motivation

We are interested in identifying the microscopic origins of unresolved phenomena in strongly correlated materials. We study transition metal oxides (e.g., cuprates), pnictides, intermetallics, and rare-earth compounds (particularly Kondo insulators) using realistic many-body methods that we are also developing, such as DFT+dynamical mean-field theory (DMFT), GW, GW+DMFT, and ab initio dynamical vertex approximation (AbinitioDΓA). Our simulations—motivated by and validated against experiments—allow to test previously hypothesized scenarios or develop new theories for the targeted phenomenon, and to make quantitative predictions for future experiments. Indeed, having identified the control parameter (e.g., an interatomic hybridization, the Hund's rule coupling) of a studied phenomenon, we can propose how to best manipulate properties in experiment (e.g., by applying pressure). Also, minimal many-body models (e.g., of Hubbard or Anderson type) containing all relevant degrees of freedom can be designed and investigated further (see 'Many-body models' below). 

Transport and spectroscopic properties

A credible theory must describe a wide range of complementary experimental observables. To better link theory with experiment, we aim to make physical observables, particularly transport and spectroscopic quantities, describable by and interpretable from realistic many-body simulations. Having been validated against experiments, our simulations are able to elucidate microscopic insights inaccessible to experiments. Furthermore, response functions may motivate exploiting a material's functionality in novel technologies, e.g., thermoelectrics, transparent conductors, intelligent window coatings, pigments, Mott transistors, or molecular spintronic current switches.

➜ The Linear Response Transport Centre (LinReTraCe).➜ Realistic Simulations of correlations and disorder in materials (RECORD).

Ultra-thin films and heterostructures

In ultra-thin films and heterostructures, charge transfers, quantum confinement, lower symmetries and lifted degeneracies lead to properties that are radically different from the behaviour of the individual materials in their bulk. We study the rich physics unleashed by geometric and chemical engineering, e.g., strong non-local spin, charge and orbital fluctuations; competing long-range orders; structural relaxations. A particular focus is on optimizing thin-film properties, e.g., thermoelectric powerfactors or superconducting transition temperatures, through the right choice of substrate on which to grow the sample.
➜ Surface effects, non-local fluctuations, and spectral coherence in ultra-thin oxide films  
➜ Tuning of electronic structures and functionalities in ultra-thin oxide films by substrate-strain

Many-body models

Once the control parameter (e.g., crystal-field splittings, orbital hybridizations, Hund's rule coupling) of a phenomenon is identified, one can build a minimal model that specifically contains said parameter. Properties of this many-body model can then be investigated in much greater depth than what is currently possible in the ab initio setting (due to high computational cost). In particular, phase-space can be scanned to characterise neighbouring regimes, providing insight (e.g., novel phases in proximity to the original coordinates) that can motivate additional ab initio simulations (e.g., how can a regime observed in a many-body model be realised in an actual material through external perturbation). Many-body models also allow one to study key signatures of a phenomenon without the many complications (e.g., defects) in actual materials.
 Local moment screening across and above the Mott transition
Insights into Kondo insulators from the periodic Anderson model
Non-local fluctuations—from 3D to 2D 

Keywords: Strongly correlated materials, thermoelectricity, transport and spectroscopic properties, Kondo insulators, electronic structure, many-body, dynamical mean-field theory, GW , GW + DMFT, dynamical vertex approximation

publication highlights

LinReTraCe: The Linear Response Transport Centre


github.com/LinReTraCe / SciPost Physics Codebases 16 (2023)

The "Linear Response Transport Centre" (LinReTraCe), a package for the simulation of transport properties of solids. LinReTraCe captures quantum (in)coherence effects beyond semi-classical Boltzmann techniques, while incurring similar numerical costs. The enabling algorithmic innovation is a semi-analytical evaluation of Kubo formulae for resistivities and the coefficients of Hall, Seebeck and Nernst. We detail the program's architecture, its interface and usage with electronic-structure packages such as WIEN2k, VASP, and Wannier90, as well as versatile tight-binding settings.

Resistivity saturation in
Kondo insulators


Commun. Phys. 4, 226 (2021)

Resistivities of heavy-fermion insulators such as Ce3Bi4Pt3 typically saturate below a characteristic temperature T*. We establish a new mechanism for this phenomenon: At low temperature, in addition to the charge gap, the scattering rate turns into a relevant energy scale, invalidating the semiclassical Boltzmann picture. Finite lifetimes of intrinsic carriers drive residual conduction, impose the existence of a crossover T*, and control the quantum regime emerging below it.

Zoology of spin & orbital fluctuations in ultrathin oxide films


PRB (2021) 104:024307

Our many-body calculations reveal a cornucopia of nonlocal fluctuations in an ultra-thin oxide film associated with ferromagnetic, (in)commensurate antiferromagnetic, and stripe and checkerboard orbital ordering instabilities. By understanding how the film geometry controls the crystal-field splitting, we elucidate the ensuing phases through the lens of the orbital degrees of freedom.

Thermoelectricity in correlated narrow-gap semiconductors


J. Phys.: Condens. Matter (2018) 30:183001

We review many-body effects, their microscopic origin, and their impact on thermoelectricity in correlated narrow-gap semiconductors. This class of  materials (e.g., FeSi and FeSb2) display an unusual temperature dependence in many experimental observables. We present an overview of our current understanding of these phenomena and new results, discuss the relation to heavy-fermion Kondo insulators (e.g., Ce3Bi4Pt3), and propose a general classification of paramagnetic insulators.