Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - SUPER-2D (Many-body physics and superconductivity in 2D materials)

Teaser

Layered materials could play a major role for solving the grand societal challenges like energy supply and human health. The goal of this project is related to a rational design and spectroscopic characterization approach. Layered materials are synthesized in-situ and...

Summary

Layered materials could play a major role for solving the grand societal challenges like energy supply and human health. The goal of this project is related to a rational design and spectroscopic characterization approach. Layered materials are synthesized in-situ and characterized using a novel combination of electrical transport, photoelectron and optical spectroscopy. This approach addresses the intense research efforts in trying to engineer, probe and unravel many-body physics and the superconducting coupling mechanism in layered solids. The materials under investigation are based on the families of graphene, dichalcogenides and iron based superconductors. Chemical functionalization using dopants and strain allows for an unprecedented control over their physical properties. The proposed material systems provide a new arena to explore diverse condensed matter phenomena such as electron correlation, electron-phonon coupling and superconductivity. The groundbreaking issues that this proposal addresses are as follows: (1) development of a unique setup where electrical transport, angle-resolved photoemission (ARPES) and optical spectroscopy is measured in-situ on the same sample, (2) large-area deterministic layer-by-layer growth by chemical vapour deposition (CVD) and molecular beam epitaxy, (3) the effects of mechanical strain and hence large pseudomagnetic fields on the electronic band structure will be investigated using ARPES, (4) the effects of alkali metal doping on the superconducting transition temperature and the spectral function will be investigated using transport, ARPES and optical spectroscopies shining light onto the superconducting pairing mechanisms in different classes of materials.

Work performed

\"In the present reporting period we have performed instrumental development and carried out work pertaining to the goals specified in the work packages. Regarding instrumental development, we have produced the prototype of the sample holder and transfer system which allows combined transport and UHV spectroscopy experiments.
In particular we have published ten papers in peer reviewed journals (among them are 4 highlight papers in high-impact journals such as Nano Letters, 2D Materials, Advanced Electronic Materials and Scientific Reports). Our work was also recognized by other bodies, e.g. the ELETTRA synchrotron has chosen one work carried out by the ERC grant as a “top story” and a Phys. Rev. B paper first-authored by an ERC-funded Master student was selected as a “Editoral Suggestion” (only 5% of PRB papers are selected).

Below the implementations and achievements are listed in detail:
- We have successfully synthesized a new possible 2D superconductor based on lithium doped black phosphorous. This work is related to alkali metal functionalization WP1 (Task 5) and band structure measurements WP4 (Task 1). These works have been published in 2D Materials and Phys. Rev. B. The Phys. Rev. B paper was selected by the Editors as an “Editor suggestion” (awarded only to about 5% of all papers published in Phys. Rev. B). The main author of this work, Niels Ehlen was financed during his Master course studies by the ERC grant and is now moving on to a PhD.
- We have performed UHV Raman spectroscopy at lowest temperatures of alkali metal doped monolayer graphene. This work is related to WP1 (all tasks) and WP4 (Task 5 - pair breaking peaks). A little more work is required to align the low-wavenumber filter for measurements inside UHV. Then we shall be able to detect superconductivity by optical methods.
- We have found proof for a substantial environmental effect in barium doped graphene on Germanium. This work has been published in the renowned journal 2D Materials. The work was carried out at the ELETTRA synchrotron (Trieste, Italy) and was selected by the ELETTRA beamtime committee as a “top story” of 2016. This work pertains the growth, intercalation and alkali earth doping described in WP1 (Tasks 1,4,5,6) and the ARPES measurements of the band structure and analysis of electron-phonon coupling WP4 (Tasks 1 and 2).
- We have found evidence for nematic phases in FeSe which are important for superconductivity in these compounds. This work relates to WP1 (Tasks 1 and 2) and WP4 (Tasks 3).
- Atomically precise Ge-graphene interphases. This work is a prerequisite to obtained graphene on insulating substrates. We found that by intercalation of Ge under graphene we can obtain quasi-free-standing graphene on an insulator without transfer. This work is related to WP1 (Tasks 1-3) and WP4 (Task 2).
- Controlled thermodynamics for achieving highest possible doping levels in graphene. In this work we have studied the thermodynamics of alkali metal intercalation in between graphene and a substrate. We have found that using controlled thermodynamics, we can achieve maximum doping levels. (related to WP4, Task4).
- We have succeeded to observe a spin-polarized Dirac cone in graphene on Cobalt. This work is related to WP1 (Tasks 1-3) and WP4 (Task 1). This work is published in the renowned Journal “Nano Lett.”
- We have discovered a new one-dimensional metal ; lithium doped graphene nanoribbons. This work is just accepted in the renowned journal \"\"Adv. Electr. Materials\"\"\"

Final results

We can only speculate but we could imagine that the transport equipment in ultra-high vacuum (UHV) and the graphene based devices we have made can have an impact in the field of sensors. For example, if we are able to detect minute quantities of harmful gases in an UHV environment using a graphene based field effect transistor, it can be useful for the automotive industries or home safety. In particular, we will could enhance the chemical sensitivity towards certain gases by using not graphene but nitrogen or boron doped graphene. Also, we have built first transistors of graphene nanoribbons. We believe that in the inevitable post-silicon era, graphene nanoribbons could play a major role.

Website & more info

More info: http://www.ph2.uni-koeln.de/684.html.