After a forced pause of a couple of years, it is finally time to meet at conferences, taking part in lectures and tutorials and discussing research ideas and results with each other.

The first chance has arrived from the third Nordic Nanolab User Meeting (NNUM), which took place at Chalmers on May 5-6.

The Nordic Nanolab Network is a collaborative network formed by the national research infrastructures for micro- and nanofabrication in the Nordic countries.

QManD took part to the meeting thanks to the posters presented by the master students Andrea D’Alessio and Núria Alcalde Herraiz, and respectively entitled “Transport measurements in YBCO nanowires to study nematic order” (left panel) and “Topological Insulator junctions fabrication to probe zero energy bound states with circuit-QED” (right panel).

To read more on the meeting you can read here!

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!

Precise control of the doping in cuprate high-Tc superconductors is fundamental for studying the phase diagram of these materials, and can have a crucial role for technological applications. We recently developed a novel procedure, in order to tune the doping of YBCO nanostructures by using an ex-situ electromigration (EM). On this topic, we have recently published two papers.

In the first work, published on Physical Review Applied and featured as editors’ suggestion (link), we have shown that an AC biasing scheme allows to fine tune the oxygen content in our nanowires. A phase diagram, in good agreement with that of bulk YBCO, can be built using a single nanowire and successive steps of EM. Kelvin Probe Atomic Force Microscopy, used to investigate homogeneity of the nanowires after EM, confirmed the uniform distribution of oxygen atoms within the nanowires.

In the second work, published on Superconductor Science and Technology (link), we have used EM to tune the superconducting properties of nanowires-based SQUIDs. Here, EM is instrumental to suppress the critical current of the SQUID, therefore enhancing the voltage modulation depth by factors as high as 8. This result shows that EM can be used to significantly improve the nanoSQUIDs performances. Moreover, this technique can be extended to a large range of devices: indeed, it is particularly well suited for constriction type weak links and grooved Dayem bridges, where EM is expected to occur locally.

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.

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.

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.

Kiryl, one of our former guests, will join our group at Chalmers from the 5th of January until next September.

Kiryl is PhD student at the Institute of Physical Chemistry, University of Latvia, working in the field of micro- and nano electronics. During his stay with us at Chalmers, he will be engaged in the fabrication of quantum dots based on topological insulator (TI) Bi₂Se₃ nanoribbons, using electron-beam lithography and reactive ion etching.

The long-term goal of this research is to use these patterned TI nanoribbons in novel high frequency devices. In particular, the aim is to create a topologically protected single-electron charge pump that can be used as a metrological quantum current standard or, in other words, to lay the technological foundations for a TI-based device that can realize the SI Ampere.

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 J_c 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 J_c 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.

Thilo Bauch (on the left) and Riccardo Arpaia (on the right) received new Swedish Research Council (VR) project grants for the period 2021-2024. Thilo has been awarded for the project “Quantum Fluctuations and Quantum Entanglement in High Critical Temperature Superconductors”, Riccardo for the project “Resonant Inelastic X-ray Scattering to study changes in the HTS phase diagram induced by strain and confinement”.

Additional information on all the 43 granted researchers at Chalmers and on Riccardo’s research project can be found at the following link.

Matteo, one of the guest researcher collaborating with the QManD group, will join us back in Chalmers from the 3rd of November until the 19th of December.

Matteo is a researcher in the field of condensed matter physics at the University of Roma2 “Tor Vergata”. During his stay with us at Chalmers, he will perform transport measurements on Bi thin films, which were previously deposited by thermal evaporation at University of Roma2. Moreover, he will use vapor solid deposition in the cleanroom here at Chalmers to fabricate Bi and Bi2Se3 topological insulator nanowires and thin films.