Postdoc positions in big-data materials science available!

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People currently working in our group:

Juliane Mörsel
Markus Scheidgen
Archana Manoharan
Christian Vorwerk
Claudia Draxl
Benedikt Maurer
Helen Jurscha
Hannah Kleine
Maria Troppenz
Le Fang
Olga Turkina
Pasquale Pavone
Santiago Rigamonti
Benedikt Hoock
Sven Lubeck
Julian Graupner
Tim Bechtel
Jungho Shin
Konstantin Lion
Martin Kuban
Sebastian Tillack
Axel Hübner
Victoria Coors
Daniel Speckhard
Mao Yang
Cecilia Vona
Maximilian Schebeck
Simon Gabaj
Peter Weber
Ignacio Gonzalez
  • Juliane Mörsel
  • Markus Scheidgen
  • Archana Manoharan
  • Christian Vorwerk
  • Claudia Draxl
  • Benedikt Maurer
  • Helen Jurscha
  • Hannah Kleine
  • Maria Troppenz
  • Le Fang
  • Olga Turkina
  • Pasquale Pavone
  • Santiago Rigamonti
  • Benedikt Hoock
  • Sven Lubeck
  • Julian Graupner
  • Tim Bechtel
  • Jungho Shin
  • Konstantin Lion
  • Martin Kuban
  • Sebastian Tillack
  • Axel Hübner
  • Victoria Coors
  • Daniel Speckhard
  • Mao Yang
  • Cecilia Vona
  • Maximilian Schebeck
  • Simon Gabaj
  • Peter Weber
  • Ignacio Gonzalez

Job opportunities

Master and bachelor topics

Electronic screening in transition metal dichalcogenides and their interfaces

Transition metal dichalcogenides (TMDCs), which are layered 2D materials, are being investigated for a wide variety of applications. They present disparate conductive and optical properties, and many support interesting phases such as topologically protected bands, charge density wave ordering, superconductivity, or exciton condensates. Moreover, the material possibilities offered by the TMDCs greatly expand when we also consider monolayers or bilayers of these systems, or build heterostructures from layers of different TMDCs. Calculating the properties of the 2D TMDCs requires the ability to accurately treat electronic interactions and the coupling of electrons to collective excitations, such as phonons and plasmons. Since these systems are intrinsically 2-dimensional, electronic screening is weakened and electron-electron interactions can be significant, indicating that many-body effects beyond standard density functional theory need to be accounted for to accurately describe their electronic structures. We propose two avenues for improving the accounting of electronic interactions in the TMDCs and their interfaces:

1. Implement and apply the GW+cumulant formalism in order to study electron-plasmon interactions within 2D TMDCs.

2. Interface systems can require large supercells, making standard GW calculations extremely expensive. We will apply a new approximate technique for hybrid GW calculations in order to improve our ability to accurately characterize the electronic structure and material behavior of these technologically important systems.

Spectroscopic determination of hydrogen ordering in pressurized metal hydrides

The search for room temperature superconductors has focused almost exclusively on unconventional superconductors, those not explained by the theory of Bardeen-Cooper-Schrieffer (BCS). However, the record Tc value of 203 K was demonstrated in hydrogen-sulfide, a BCS superconductor, pressurized to 155 GPa. This greatly reinvigorated work on hydride materials as superconducting candidates and many related materials have since been proposed. To solidify our understanding of hydride superconductivity and to identify lower pressure candidate materials we need a clear verification of the high-pressure crystal structures. We will use X-ray spectroscopic techniques to reveal the complete crystallographic structure of pressurized metal hydrides. This will require studying structural compression of hydride materials under high pressure and performing spectroscopic calculations on candidate structures. For both tasks, it will be essential to accurately account for zero-point fluctuations of hydrogen.

Polaron formation in SrTiO3

Recent theoretical developments demonstrate the ability to perform detailed calculations of the coupled electronic and phonon composition of polarons in insulators and semiconductors. It is highly desirable to implement and make use of this new methodology and apply it to several important problems. In particular, SrTiO3 is a “standard problem” material for which the presence and nature of polarons has been vigorously debated. Studying SrTiO3 is particularly appealing because superconductivity has been discovered at the interface of LaAlO3/SrTiO3 and CaCuO2/SrTiO3 heterostructures. Within doped SrTiO3, which itself becomes superconducting with just one extra electron per 104 Ti ions, an enhanced effective carrier mass is often attributed to coupling to optical modes and the formation of large polarons. It would be particularly interesting to quantify the polaron size and formation energy as a function of doping density.

Non-equilibrium spectroscopy for charge dynamics in energy materials

While we have learned a lot about materials from studying their ground-state properties, important processes such as photo-absorption and charge transfer involve the dynamics of an excited state. Exceptional advancements in optical high-harmonic generation and the recent construction of x-ray free electron lasers have opened this new frontier of ultrafast science on the experimental side. Rigorous computational work remains limited, though. This project will implement femtosecond computational pump-probe spectroscopy using both time-dependent density functional theory and many-body perturbation theory methods. Applications may include metal-ligand and interfacial charge transfer processes in solar cell materials, polaron and charge-density wave formation in transition metal dichalcogenides, and electronic screening dynamics.

Designing Dirac materials for the direct detection of dark photons with directional sensitivity

Since the 1980s, efforts toward direct detection of dark matter (DM) have focused on weakly interacting massive particles (WIMPs) within the anticipated mass range of 1 GeV – 1 TeV. Given the null results of these experiments, the focus of dark matter hunters is now shifting to the light DM mass range of 1 keV – 10 GeV, where new detection concepts rely on observing electronic excitations in quantum materials. Various materials have been proposed as targets for direct detection of DM by electronic excitation including superconductors, narrow-gap organic molecules, graphene and Dirac semimetals. The objective of this project is to rigorously quantify and optimize the sensitivity reach of Dirac materials for the detection of dark photons. Specifically, we will investigate the suitability of several Dirac materials as DM targets including 2D materials. The focus will be to optimize key material quantities directly relevant to DM detection such as the value and sharpness of the electronic gap, the Fermi velocity and its anisotropy in the vicinity of the gap, thermal noise, optical permittivity and absorption cross section.

Highly precise electronic band structures of metals

Density-functional theory is the state of the art for computing materials properties without any parameters. Typical DFT functionals that describe the quantum-mechanical contributions to the electron-electron interaction are particularly well-suited for metals, such that DFT results are often interpreted as "real" band structures. Having high-resolution photoemission experiment in hand, we can assess the shortcomings of doing so. We propose corrections obtained by many-body perturbation theory to go beyond.

Within this thesis, our GW code (D. Nabok, et al., Phys. Rev. B. 94, 035418 (2016).) will be used and slightly modified to explore excitations in metallic systems.

Electron transport in wide-gap oxides

Wide-gap oxides, such as Ga2O3, are fascinating electronic materials that are widely studied at the HU and its partner organizations within the Leibniz Science Campus GraFOx. Due to their rather low thermal conductivity, they can be considered also as candidates for thermoelectricity. A key quantity for such applications is the electrical conductivity that shall be investigated from first principles. Depending on the level of involvement, this topic can be carried out either as bachelor or master thesis.

Maximally localized Wannier functions considering spin-orbit coupling

Maximally localized Wannier functions (MLWFs) are a well-established tool in solid-state calculations. Due to their localized nature they are superior to the equivalent Bloch representation in terms of chemical interpretation. They provide inexpensive access to both single-particle eigenvalues and eigenfunctions at any point in reciprocal space in terms of the so-called Wannier interpolation scheme. A recent implementation in the exciting code, developed in the group, has been successfully applied to various systems. A variety of topical advanced materials are strongly affected by spin-orbit coupling, that impact, e.g. the band gap of their bandgaps. For reliably capturing them, extensions to the Wannierization scheme is required. This thesis will enable highly-precise calculations of materials like hybrid perovskites or transition-metal dichalcogendies that are studied by the group and its experimental partners. 

Photon-energy dependence and final-state effects in photoemission spectroscopy

Based on the photoelectric effects, angle-resolved photoemission spectroscopy (ARPES) provides information on the electronic structure of a system by measuring the kinetic energy of the electron which is emitted upon absorption of a photon. The photoemission intensity thereby typically exhibits a strong dependence on the photon energy and the polarization of light which is critical to achieve a realistic comparison between theory and experiment but most often neglected in calculations. Green function methods, such as the GW approximation, provide an ideal framework for a systematic and accurate theoretical description of ARPES experiments. Within this master project, the student will develop a theoretical and computational approach to predict the intensity of ARPES measurements on the energy and polarization of the probing photon. This will be accomplished by state-of-the-art methodology and it will provide numerous opportunities for collaboration with experimental groups.

Reference data of electronic-structure theory

There exist many ab initio methods and computer packages to calculate ground-state properties, the electronic structure or various excitation spectra of materials. The exciting code, developed in the group, is able to provide results of highest precision. It is used to generate materials-science data that are valuable benchmarks for the international community.

Recent publications along these lines:
Kurt Lejaeghere et al., Reproducibility in density-functional theory calculations of solids, Science 351, aad3000 (2016); DOI: 10.1126/science.aad3000
A. Gulans, A. Kozhevnikov, and C. Draxl, Microhartree precision in density functional theory calculations, Phys. Rev. B 97, 161105(R) (2018).

More topics to be explored .... just contact us for more information

  • Database of core-level spectra (cooperation with Brookhaven National Lab and others)
  • Assessing methodology for the electronic structure of correlated superconductors
  • Implementing symmetry in the electronic-structure code exciting
  • Elastic properties of emerging materials
  • Role of phonons on the properties of thermoelectric clathrate materials
  • Implementation of new density functionals