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 new article, ‘Restored strange metal phase through suppression of charge density waves in underdoped YBa2Cu3O7–δ’, now available in the leading scientific journal Science. The research has been 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.
Read more about our paper here!
High-Mobility Ambipolar Magnetotransport in TI Nanoribbons
Read our work on topological insulator Bi2Se3 nanoribbons, 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 Physical Review Applied (link).
Nanoribbons of topological insulators (TIs) have been suggested for a variety of applications exploiting the properties of the topologically protected surface Dirac states. In these proposals it is crucial to achieve a high tunability of the Fermi energy, through the Dirac point, while preserving a high mobility of the involved carriers. Tunable transport in TI nanoribbons has been achieved by chemical doping of the materials so to reduce the bulk carriers’ concentration, however at the expense of the mobility of the surface Dirac electrons, which is substantially reduced.
In this work we have studied bare Bi2Se3 nanoribbons transferred on a variety of oxide substrates and demonstrate that the use of a large relative permittivity SrTiO3 substrate enables the Fermi energy to be tuned through the Dirac point and an ambipolar field effect to be obtained. Through magnetotransport and Hall conductance measurements, performed on single Bi2Se3 nanoribbons, we have demonstrated that electron and hole carriers are exclusively high-mobility Dirac electrons, without any bulk contribution. The use of SrTiO3 allows therefore an easy field effect gating in TI nanostructures providing an ideal platform to take advantage of the properties of topological surface states.
Tuning the oxygen content of YBCO nanowire with electromigration
Precise control of the doping in cuprates is fundamental for studying the phase diagram of these material, and is an important tool for technological applications. We recently published two articles on tuning the doping of YBCO nanowire ex-situ using electromigration (EM).
In our first work (link), we show that an AC biasing scheme allows to fine tune the oxygen in our nanowires. The phase diagram of YBCO was reproduce using a single nanowire and successive steps of EM. We investigate the homogeneity of the nanowires after EM using Kelvin Probe Atomic Force Microscopy, which shows uniform distribution of oxygen in the nanowires.
In our second work (link) we use EM to tune the superconducting properties of nanowires-based SQUIDs, exploring the effect on different wire geometries. Here, we found that EM can be used to suppress the critical current of the SQUID and enhance the voltage modulation depth by factors as high as 8.