Perovskite-type complex oxides are a family of compounds that have attracted growing interest because of the variety of tunable physical properties, which makes them attractive for technological applications in different areas [1]. Thermal transport can reveal important information about phonons, which can play an important role in electronic collective transport properties. At LQMEC, we have implemented advanced experimental platforms for the investigation of thermal transport in a range of quantum materials under multiple extreme conditions.
In this talk, I will present and discuss our recent investigations of thermal diffusivity in a representative of this class of oxides: the Barium Bismuthate BaBiO3 (BBO) [2]. BBO exhibits an insulating ground state with a still debated origin and a superconducting state upon hole-doping, besides being predicted to host a topological insulating (TI) state upon electron doping [3]. A complex relation between electronic and lattice degrees of freedom has  been called into question in an attempt to explain the electronic states of this compound [4]. Our thermal conductivity experiments found a remarkably low value at room temperature (~1W/mK), a plateau at intermediate temperatures, and an unexpected ~T2 power law at low temperatures (T < 5K), reminiscent of a glass-like behavior [2].
I will finally present our current experimental efforts in pushing our investigation of thermal conductivity towards thin films limit by employing the 3-Omega technique, aiming at investigating how structural distortion induced by strain can tune the heat transport.

REFERENCES
1.    AS. Bhalla et al. Mat. Res. Innov. 4.1, 3, (2000).
2.    Henriques et al. New Physics: Sae Mulli, 73 (12), 1183 (2023).
3.    RL. Bouwmeester, et al. Rev. in Phys., 6, 100056. (2021).
4.    B. G. Jang et al, Physical Review Letters 130, 136401 (2023)
 
We acknowledge the support of FAPESP (Grants n. 2018/19420-3, 2018/08845-3, 2022/01742-0, 2021/00625-7, 2022/03262-5), UGPN-2020 and CNPq (402919/2021-1).

The interplay between charge-density waves (CDW) and other ordered states in intermetallic materials remains an intriguing question in condensed matter physics. For example, despite the many low-dimensional examples, the role of CDW and its relationship with the superconducting state for three-dimensional compounds has yet to be completely unraveled. In this talk, we present experimental X-ray diffraction and electrical resistivity studies on some single-crystalline intermetallic compounds to explore the charge-ordered phase in these materials.

Light-matter interaction at the Nanoscale is a captivating field that delves into the intricate interplay between photons and nanoscale materials. At this scale, matter exhibits unique properties, and light behaves in ways that diverge from macroscale observations. This dynamic interaction underpins applications like nanophotonics, optoelectronics, quantum technologies, and beyond, driving advances in sensing, imaging, and communication. Nanoscale materials, such as semiconductor quantum dots and 2D materials, possess dimensions comparable to or smaller than the wavelength of light. Consequently, they can manipulate light at a fundamental level, leading to phenomena like plasmonics, excitonics, and quantum confinement effects. Plasmonics, for instance, involves the collective oscillation of free electrons in metallic nanostructures when stimulated by light, enabling unprecedented control over light localization and enhancement at the Nanoscale. In this talk, we will present the results of the interaction of semiconductor quantum dots and two-dimensional semiconductor materials with electromagnetic fields confined in metallic nanostructures, the effects that can be observed in this, and its possible applications.

In this talk, we will explore the results of spin and orbital Hall effects in two-dimensional (2D) topological materials. We have conducted high-throughput calculations to determine the spin (SHC) and orbital (OHC) Hall conductivities across hundreds of 2D materials, aiming to understand the relationship between these Hall effects and topological properties. Initially, we will discuss the OHC plateau observed within the insulating gap of 2D transition metal dichalcogenides (TMDs). These TMDs are characterized as higher-order topological insulators (HOTIs) using a Z4 topological invariant. Employing general symmetry principles, we will establish a linkage between these phenomena, highlighting potential applications in spin-orbitronics. A broader perspective will be provided by examining the SHC and OHC data compiled from an extensive 2D materials database.

Organic mixed ion-electron conductors are renowned for their unique ability to efficiently conduct both electrons and ions, finding applications across various fields such as electronics and bioelectronics. In this colloquium, I will delve into the fundamentals of ionic-electronic transport in organic mixed conductors and showcase their applications in electrochemical transistors, biosensors, and low-voltage artificial synapse memory systems, which emulate the function of neurons (also known as neuromorphic devices).

Graphene is a crystalline 2D material and considered the thinnest possible membrane. Because of its high chemical stability, combined with its physical properties, graphene is promising in a variety of applications. Particularly, liquid/graphene interfaces have been exploited for bio-applications on cellular flow sensing, liquid sensing, DNA sequencing, and transparent windows in liquid cells. In such devices, understanding the interaction between water and graphene is crucial for building up novel and smart bio-interfaces. Additionally, the study of reactivity and structure of water at the graphene interface has also generated intriguing questions and controversial results. For instance, several experimental works demonstrate that the charge transfer process that happens between graphene and water molecules is highly dependent on the underlying substrate. Thus, it would be highly desirable to elucidate the above discussion by probing the electrical response of a suspended graphene membrane in contact with water without the presence of any substrate. We also believe that a precise understanding of the electrochemical behavior of water/graphene interface would be fundamental for developing novel and superior electrical, mechanical and optical devices. In this work, we develop a microfluidic platform that integrates suspended graphene membrane windows (with electrical contacts) with buried fluid channels to probe the electrical response of a graphene membrane in contact with water [1,2,3]. The platform design provides a direct probing of the electrical response of the air/graphene/liquid interface without the presence of any underlying substrate.

 I will present a detailed study of the water-induced electromechanical response in suspended graphene atop a microfluidic channel. The graphene membrane resistivity rapidly decreases ~ 25% upon water injection into the channel, defining a sensitive “channel wetting” device – a wetristor. The physical mechanism of the wetristor operation is investigated using two graphene membrane geometries, either uncovered or covered by an inert and rigid lid (h-BN multilayer or PMMA film). The wetristor effect, namely the water-induced resistivity collapse, occurs in uncovered devices only. AFM and Raman spectroscopy indicate substantial morphology changes of graphene membranes in such devices, while covered membranes suffer no changes, upon channel water filling. Our results suggest an electromechanical nature for the wetristor effect, where the resistivity reduction is caused by un-wrinkling of the graphene membrane through channel filling, with an eventual direct doping caused by water being of much smaller magnitude, if any. The wetristor device should find useful sensing applications in general micro- and nano-fluidics and provides novel insights on the interface interactions of 2D materials with liquids.

Acknowledgments: The authors acknowledge the support of FAPEMIG (Rede 2D), CNPQ/MCTI, and INCT de Nanocarbono.

[1] Ferrai et al., Graphene nanoencapsulation action at an air/lipid interface Journal of Materials Science 57, 6223 (2022).

[2] Ferrari et al., Apparent Softening of Wet Graphene Membranes on a Microfluidic Platform ACS Nano 12, 5, 4312 (2018)

[3] Meireles, Leonel et al., Graphene Electromechanical Water Sensor: The Wetristor. Advanced Electronic Materials, 6, 1901167, 2020.

Since the breakdown of Dennard scaling approximately 15 years ago, the clock frequency of processors has remained stalled at a few GHz. Although all-optical transistors that can switch at THz speed could bring a leap in performance, this promise was not fulfilled during decades of research due to low optical nonlinearities and bulky components. Now the foundations of a new generation of devices are investigated that harnesses the so-called strong light-matter interaction regime with novel materials and integrated photonic structures that could enable compact, ultrafast all-optical logic circuits with attojoule switching energy [1,2].

In this talk, the experimental progress towards this goal will be presented, including a cascading setup where a spontaneous polariton condensate is created in one cavity (Seed) and fed into another cavity (Transistor) to induce polariton condensation [3]. Additionally, rapid polariton condensation dynamics on a sub-picosecond timescale will be presented, and important transistor metrics such as signal amplification (up to a factor of 60) and on/off extinction ratio (up to 9:1) will be determined.

These findings indicate the potential for developing integrated, ultrafast all-optical transistors that are scalable, allowing for more complex all-optical logic circuits.

This work was funded by EU H2020 EIC Pathfinder Open project “PoLLoC” (grant agreement no. 899141) and EU H2020 MSCA-ITN project “AppQInfo” (grant agreement no. 956071).

References

[1] Anton V. Zasedatelev, Anton V. Baranikov, Denis Sannikov, Darius Urbonas, Fabio Scafirimuto, Vladislav Yu. Shishkov, Evgeny S. Andrianov, Yurii E. Lozovik, Ullrich Scherf, Thilo Stöferle, Rainer F. Mahrt, Pavlos G. Lagoudakis, “A room-temperature organic polariton transistor,” Nat. Photonics 13, 378–383 (2019).

[2] Anton V. Zasedatelev, Anton V. Baranikov, Darius Urbonas, Fabio Scafirimuto, Ullrich Scherf, Thilo Stöferle, Rainer F. Mahrt, Pavlos G. Lagoudakis, “Single-photon nonlinearity at room temperature,” Nature 597, 493–497 (2021).

[3] P. Tassan, D. Urbonas, B. Chmielak, J. Bolten, T. Wahlbrink, M. C. Lemme, M. Forster, U.Scherf, R.F. Mahrt, T. Stöferle, “Integrated ultrafast all-optical polariton transistors,” arXiv:2404.01868v1, (2024).

Scanning probe microscopy allows a better understanding of phenomena at the nanoscale. The advent of CO-functionalized tips allowed for something extraordinary: atomically resolved images on planar molecules and the elucidation of chemical structures.  
The combination of high-resolution scanning tunneling microscopy and atomic force microscopy has allowed different textbook chemistry concepts (e.g. bond-order, aromaticity and oxidation states) to be probed in unforeseen ways. Moreover, it is possible to characterize batches of synthesized products one-molecule-at-a-time and create a library of observed chemical structures in ways that are prohibitive to standard analytical tools. Here, I will show how high-resolution AFM measurements are performed. Then, we will peruse through different aspects of on-surface reactions and its wonders.

   The symmetries of crystals play an important role in the properties of their phonons. When the mirror symmetries are broken, the lattice ions can display circular motion with finite angular momentum. These modes, known as chiral phonons, have recently been demonstrated in both rotating and propagating lattice motions. Usually, phonons are insensitive to magnetic fields. On the contrary, chiral phonons carry magnetic moment and directly couple to magnetic fields.
    In this talk, I will present a review of the recent progress on the study of chiral phonons using terahertz time-domain spectroscopy. Our contributions to this exciting new field will be highlighted [1-3]. In particular, I will show that the phonon magnetic moment is largely enhanced in topological materials. Furthermore, unpublished results will be discussed to provide future perspectives.
    Our research was supported by FAPESP Grants No. 2015/16191-5 and 2018/06142-5, 2021/12470-8, 2023/04245-0, and CNPq Grants No. 307737/2020-9 and 409245/2022-4.

[1] A. Baydin et al., Physical Review Letters 128, 075901 (2022).
[2] F. G. G. Hernandez et al., Science Advances 9, eadj4074 (2023).
[3] N. M. Kawahala et al., Coatings 13, 1855 (2023).

Transition metal dichalcogenides (TMDs) are one of the most explored classes of two-dimensional materials. The experimental routes for synthesis and device construction in these materials are well established allowing future applications. Materials with topological phases of matter present the possibility of electronic transport with long coherence length, due to the protection of their surface states by time-reversal symmetry. Such systems allow for the development of low-power electronics and new device functionalities. We show that energetically favorable defects in TMDs, Hg doping, and chalcogen vacancies, introduce topological states in their semiconductor gap. The transition from trivial to non-trivial occurs at a critical concentration of defects and is robust against disorder.