■ The electrodynamics of iron-arsenide superconductors: Consequences of the multiband physics on electronic correlations and competing orders

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Description of the PhD project

Superconductivity is a phenomenon where the electrical resistivity vanishes below a certain temperature. It was discovered in the early 20th century and for several decades was observed only in metallic alloys close to liquid helium temperatures. The key point of superconductivity is the formation of pairs of electrons that condensate into a single macroscopic quantum state. In the mid 80s, superconductivity was observed in copper oxides breaking the liquid nitrogen barrier. Another dimension to the problem was added in 2008 with the discovery of superconductivity in several classes of iron based materials. In addition to the surprising magnetic nature of these compounds, iron-based superconductors are also multiband superconductors. Indeed, the d-nature and Hund’s coupling of Fe orbitals create up to five bands at the Fermi level all of which become superconducting. If the reason why metallic alloys are superconducting is well known, the explanation of high temperature superconductivity remains elusive, in particular the nature of the pairing mechanism.
The multiband character of Fe-based superconductors and the strong role played by magnetism puts these compounds at the center of a dynamic and rapidly changing world-wide scientific activities. These materials show magnetic, structural, and electronic phase transitions which compete and cooperate among them. The optical conductivity is a particularly powerful tool in the study of these properties as it directly probes the charge redistribution across the excitation spectra of charge particles. Hence, it allows to probe the nature of charge carriers that form the superfluid; the origin of gaps at the Fermi surface due to magnetic ordering; the effects of interband scattering on the superconducting pairing energy; and new exotic electronic states.
The main goals of the present proposal are (a) to understand the microscopic nature of the formation of Cooper pairs; (b) to study its interactions with magnetism and lattice effects and to establish the cases where these orders are competing and when they cooperate; (c) to analyze the role the presence of multiple superconducting bands at the Fermi level into the normal state and superconducting properties; (d) to determine the condition where exotic states are observed. This last point is particularly interesting as it was predicted, and recently measured by us, that for specific band configurations, topologically protected Dirac fermions - excitations typical of 2D materials such as graphene - appear and dominate the optical response.
This work is essentially in experimental physics but a strong interaction with the theory group of LPEM is expected. The leader of this theory group is a co-advisor of this work. We will be particularly interested in systems such as CaFeAsF as well as more conventional AFe2As2, where A is an alkaly metal. These materials are yet very poorly studied and are the subject of many theoretical predictions.

Keywords

Superconductivity; iron-pnictides; optical conductivity; superconducting mechanism, Dirac states, electronic correlations, quantum materials

Research unit

UMR7636 Physics & Materials

Description of the research Unit/subunit

LPEM is a research unit overseen by ESPCI, the CNRS and Sorbonne Université. Its activities are divided into three main areas: Quantum Materials; Nanomaterials; and Instrumentation. The most important subjects in quantum materials cover superconductivity; bi-dimensional electron gazes; dimensionality and size effects; ferroelectrics and multiferroics; and transport at the quantum limit. Recently, the laboratory started a theory group on strongly correlated systems. For this project, the candidate will have access to spectrometers covering the energy range from 1 meV (10 cm-1; 1 mm) to 5 eV (40 000 cm-1; 250 nm), liquid helium cryostats, and a 10 T split coil magnet with optical access.

Name of the supervisor
Ricardo Lobo (ricardo.lobo (arobase) espci.fr)

Name of the co-supervisor
Luca De’Medici (luca.demedici (arobase) espci.fr)

3i Aspects of the proposal

The present project is an effort to understand the fundamental nature of complex quantum many-body physics that dominate high temperature superconductors, in particular iron-based materials. This project fosters a strong interaction between experimental results and advanced theoretical modelling of quantum materials. It also requires a close collaboration with solid state chemists to the fabrication of the materials to be studied. This project is embedded in a long standing collaboration between LPEM and the Chinese Academy of Sciences in Beijing, China, and a more recent one with the Karlsruhe Institute of Technology in Germany. The PhD candidate will strongly benefit from this environment and international collaboration. Even though the core of this proposal is research in a fundamental are, the successful candidate will be able to interact with LPEM scientists working in topics utilizing High-Tc superconductivity in societal applications. One of the research teams in LPEM works with liquid nitrongen cooled superconducting power transmission lines for electricity. In addition, the co-advisor of the present thesis proposal have a patent deposited for self-cooling superconducting cables.

Expected Profile of the candidate

The candidate is expected to be a physicist, with a background in condensed-matter physics and motivated to carry out careful experiments. An strong interest in and basic knowledge of the quantum theory of solids is required. The desired candidate should be an outstanding experimentalist willing to strongly interact with the theory group of LPEM.

Important dates

Call for applications : from July 16th to September 17th 2018
Eligibility check results : Late September
3i Committee evaluation results : Late October
Interviews from the shortlisted candidates with the Selection Committee : Mid-December (week of December 10th)
Final results : Late December





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