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
Dilution refrigerator with base temperature 20 mK. 3He refrigerator with base T=300 mK. Dipsticks for liquid helium (T=4.2 K) and liquid nitrogen (T=77 K). Two different superconducting coils for magnetic fields 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).
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.