Quantum Materials and nanoDevices group at Chalmers University
Quantum materials bring together a variety of problems at the border between physics, materials science and engineering. The properties of these systems are uniquely defined by quantum mechanical effects which persist at high temperatures and macroscopic length scales. Some examples are unconventional superconductors, topological insulators, Weyl semimetals.
Superconductors are quantum materials which allows to transport a zero resistance electrical current while being perfect diamagnet (i.e. they expel magnetic field from the interior).
3D Topological insulators
Topological insulators form a new class of quantum matter with an insulating bulk and metallic Dirac surface states protected by topology.
Nanodevices are fabricated in the cleanroom of our department at Chalmers using state of art tools. Quantum materials in form of nanobelts, heterostructures and very thin films are nanopatterned to study basic physics effects and to realize a variety of quantum limited sensors. Examples are SQUIDs, single photon detectors and charge pump. Dimensions down to 10 nm are achieved.
The transport in quantum materials is investigated in our measurement lab via: electric resistivity as a function of temperature and magnetic field; RF and microwave measurements; Magnetic field/flux sensing; Hall measurements and high voltage gating effects.
Low temperature – High magnetic field
Cryogen free dilution refrigerator with base temperature of 18 mK and persistent magnetic field up to 12 T. 3He dipstick with base temperature of 300 mK and magnetic fields up to 30 mT. Dipstick for characterization in liquid helium (T=4.2 K) and liquid nitrogen (T=77 K) with magnetic fields up to 10 mT. Quantum Design Physical Property Measurement System with base temperature of 1.9 K and magnetic field up to 14 T.
The crystal structure of the quantum materials is investigated via non-resonant X-ray diffraction. Their electronic structure, which includes a very broad class of intrinsic excitations (driven by charge, spin, lattice, orbitals), is probed via synchrotron-based X-ray spectroscopies, which include resonant X-ray scattering, performed in several European facilities (ESRF, DLS, BESSY II).
A new way to control quantum materials
Read our last paper on Science
The article, ‘Restored strange metal phase through suppression of charge density waves in underdoped YBa2Cu3O7–δ’, is based on a research led by our group, in collaboration with researchers from Politecnico di Milano, University La Sapienza, Brandenburg University of Technology and the European Synchrotron facility (ESRF).
The presented research focuses on understanding and controlling the enigmatic state called ‘strange metal’, appearing in high temperature superconductors at temperatures above the superconducting transition.
The main result of the paper is new evidence of an intimate connection between the strange metal state and a “directional” local charge modulation in the conducting electrons called charge density waves (CDW). More specifically, the strange metal state is suppressed by the appearance of these charge modulations, providing valuable insights into the possible mechanism behind this enigmatic state.
The experiment also shows that CDW can be controlled by applying strain to the material, leading to a novel technique of using strain to turn the strange metal state on or off. This is the first step towards a systematic study of ultra quantum matter in the lab, where strain control can be used to manipulate this new class of quantum materials.
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.
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 Hc,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 Hc,2 in 50 nm thick YBa2Cu3O7−δ films. The films are untwinned and are characterized by a small in-plane compressive strain. We find that the Hc,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 Hc,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.
Quantum Device Physics Laboratory
Department of Microtechnology and Nanoscience
Chalmers University of technology
Kemivägen 9, 412 58 Göteborg