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).
Shedding light on the strange metal regime of cuprate superconductors
In the work just published in Communications Physics (link) we have presented the result of our collaboration with Politecnico di Milano, Brandenburg University of Technology and La Sapienza University in Rome.
Here we show a theoretical proposal, which investigates the consequences of the charge density fluctuations – we have discovered two years ago by Resonant Inelastic X-ray Scattering – on the electron and transport properties of cuprate high critical temperature superconductors. The finding is that these charge density fluctuations are likely the long-sought microscopic mechanism underlying the peculiarities of the metallic state of cuprates. This might represent a decisive step toward the understanding of this fascinating but still very mysterious class of materials.
Additional information about the paper can be found at the following link.
Size matters for transport in topological insulator nanoribbons
Read our recent work on topological insulator Bi2Se3 nanoribbons.
We have grown Bi2Se3 nanoribbons by catalyst-free Physical Vapor Deposition, and employed them to fabricate high quality Josephson junctions. In these devices we have observed a pronounced size effect in the transport properties: a strong reduction of the Josephson critical current density Jc occurs by reducing the width of the junction, which in our case corresponds to the width of the nanoribbon.
Since the topological surface states extend over the entire circumference of the nanoribbon, the superconducting transport associated to these states is carried by modes on both the top and bottom surfaces of the nanoribbon. The Jc reduction as a function of the nanoribbons width shows that only the modes traveling on the top surface contribute to the Josephson transport. The reduction qualitatively agrees with the calculation of the top surface modes by using geometrical considerations.
This finding, recently published on Journal of Applied Physics (link), is of a great relevance for topological quantum circuitry schemes, since it indicates that the Josephson current is mainly carried by the topological surface states. The work has been done in collaboration with the University of Latvia.
Discovering the secrets of a one atomic layer thick superconductor
The journal Nature Communications just published the results of a Resonant Inelastic X-ray Scattering (RIXS) experiment (link), led by the MIT professor Riccardo Comin, and conducted in collaboration with the Politecnico group of Prof. Ghiringhelli and with the QMand researcher Riccardo Arpaia.
The experiment aimed at discovering the origin of superconductivity in iron selenide (FeSe), which is characterized by a low critical temperature (Tc = 8 K) in the bulk, while becoming a high-critical temperature superconductor (Tc = 65 K) when it is shrank to dimensions of only one atomic layer.
RIXS spectra of bulk samples have been compared with those of monolayer samples. The result of the experiment, supported by quantum Monte Carlo calculations, points toward the crucial role played by spin excitations in enhancing the critical temperature of this quantum material. This discovery might help shedding light on the origin of the anomalously high Tc in other superconductor compounds, as the cuprates.
A cover story about this work, issued by the MIT Materials Research Laboratory, can be found at the following link.