EVENTS/GRANTS

WISE PhD project accepted

WISE, the Wallenberg Initiative Material Science for Sustainability, launched during summer the first call for Academic PhD-student and postdoctoral research projects. The call was intended to attract research projects addressing sustainability challenges for materials science.

After the evaluation, 90 projects have been granted, and among these the project “Strain engineering of High-Tc thin films for clean energy”, applied by Floriana Lombardi, got funding. To read more on the call and on the granted projects you can read here and here!

New VR grant

Floriana Lombardi received a new Swedish Research Council (VR) project grant, for the period 2023-2026, with the project “Revealing strongly entangled quantum matter in High-Tc Superconductor nanodevices”. The complete list of researchers, receiving grants at Chalmers (5 in the MC2 Department!), is available here.

TOPOLOGICAL INSULATORS

Nanometric Moiré Stripes on the Surface of Bi₂Se₃ TI

Read our work on topological insulator Bi₂Se₃ nanobelt, done in collaboration with Matteo Salvato from Università di Roma “Tor Vergata” and with the group of Donats Erts at University of Latvia. The paper has been recently published on ACS Nano (link).

Mismatch between adjacent atomic layers in low-dimensional materials, generating moiré patterns, has recently emerged as a suitable method to tune electronic properties by inducing strong electron correlations and generating novel phenomena. Beyond graphene, van der Waals structures such as three-dimensional (3D) topological insulators (TIs) appear as ideal candidates for the study of these phenomena due to the weak coupling between layers.

In this work we have discovered and investigated the origin of 1D moiré stripes on the surface of Bi₂Se₃ TI thin films and nanobelts, using scanning tunneling microscopy and high-resolution transmission electron microscopy. The 1D stripes are characterized by a spatial modulation of the local density of states, which is strongly enhanced compared to the bulk system. Density functional theory calculations confirm the experimental findings.

The strongly enhanced density of surface states in the TI 1D moiré superstructure can be instrumental in promoting strong correlations in the topological surface states, which can be responsible for surface magnetism and topological superconductivity.

HIGH Tc SUPERCONDUCTORS

Determining the upper magnetic critical field in YBCO films

In the quest to increase the critical temperature of superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field H_c,2 is a fundamental measure of that strength. Only in recent years, its magnitude and doping dependence has been measured in cuprate superconductors, but only when they are in single crystal form. It is therefore very interesting to measure its behavior in thin films, where the ground state can be modified through strain and intertwining and competition between different orders can be enhanced.

For the abovementioned reasons, in the work, written by our postdoc Eric Wahlberg and published on Superconductor Science and Technology (link), we have reported on measurements of the doping dependence of the upper critical field H_c,2 in 50 nm thick YBa₂Cu₃O_7−δ films. The films are untwinned and are characterized by a small in-plane compressive strain. We find that the H_c,2 shows a strong decrease in the underdoped region of the phase diagram, in agreement with what has been measured in relaxed single crystals. The origin of the decrease of H_c,2 in the underdoped regime is discussed within a scenario where charge density wave order competes with superconductivity.

This work demonstrates the potential of using thin films for studying the phase diagram of high-Tc materials under strain, and opens up the possibility to investigate the interplay between charge density wave order and superconductivity tuned by strain.