Title: Modeling the surface structural of two-dimensional materials by Crystal Truncation Rods analysis

Abstract: In this work, processes for modeling the surface electronic density of two-dimensional materials with lamellar stacking using Crystal Truncation Rods are discussed. This method allow for checking intercalation and diffucion profiles in systems such as topological insulators, synthesized graphene bilayers and exfoliated minerals. Examples of these three cases will be discussed with simplified models.

Title: Search for new electride high temperature superconductors

Abstract: In this century a new class of materials have been shown to present superconductivity. Under pressure, many new electride superconductors have been discovered. The present work discusses two new materials proposed as new high temperature superconductors, one under pressure, Li5C, the other, Mo2N at ambient pressure. Using density functional theory (DFT) and particle swarm search method (PSO), we discovered the electride superconductor, Li5C, stable for pressures from 50 to 210 GPa. Li5C has a significant electron-phonon coupling (EPC) with superconducting critical temperature Tc = 48.3 K at pressure 210 GPa. We also discuss superconductivity in the 2-D electrene Mo2N. Using DFT and Eliasberg approach, we show how biaxial strain affects superconductivity in Mo2N. Results indicate that Mo2N presents strong (EPC) with large anisotropy in the superconducting energy gap. This material shows superconductivity and calculations point to Tc = 24.7 K.

Title: Probing charge density waves using dark field X-ray microscopy

Abstract: Charge density waves (CDW) are a collective phenomenon where, below a specific transition temperature, there is a modulation of conduction electrons accompanied by a distortion in the crystal lattice. Experimental signatures of CDWs can be observed even in macroscopic measurements such as thermal and electrical transport and X-ray diffraction, where forbidden reflections appear in the diffraction pattern below the transition temperature. We employ the dark-field X-ray microscopy technique to explore and characterize the mesoscopic and macroscopic properties of CDW in the intermetallic compound Sr0.4Ca0.6Rh4Sn13.

Title: Spin-State Ordering and Intermediate States in a Mixed-Valence Cobalt Oxyborate with Spin Crossover 

Abstract: Spin-state ordering – a periodic pattern of ions with different spin-state configurations along a crystal lattice – is a rare phenomenon, and its possible interrelation with other electronic degrees of freedom remains little explored. Here we perform a structural investigation of the mixed-valence Co homometallic ludwigite Co₂²⁺Co³⁺O₂BO₃. A superstructure consistent with a long-range Co³⁺ spin-state ordering is observed between T₄=580 K and T₃=510 K (see Fig. 1). Intermediate states with mesoscopic correlations are detected below T₃ down to T₁=480 K with a change of dimensionality at T₂=495 K. The spin-state correlations are connected to the charge sector as revealed by the abrupt changes in the electrical resistance at T₁ and T₂. The evolution of the structural parameters below T₁ indicate that the spin crossover is ignited by a moderate degree of thermally-induced Co²⁺/Co³⁺ charge disorder. Charge and spin-state degrees of freedom can be interrelated in mixed-valence spin-crossover materials, leading to sharp transitions involving intermediate spin-state/charge correlated states at the mesoscale.

TBA

Title: The polymorphous nature of halide perovskites

Abstract: Many halide perovskites exhibit a cubic crystal structure (Pm-3m) at elevated temperatures but transition to lower symmetry structures such as orthorhombic or tetragonal at lower temperatures. Recent theoretical findings have cast doubts upon the cubic structure, as it may exhibit negative phonon modes, unusual band gap trends, and incomplete alignment with PDF measurements. In this context, we propose the concept of polymorphous structures, suggesting that the cubic structure emerges as a result of temporal and spatial averaging of lower symmetry structures. To substantiate this idea, we will employ ab initio and molecular dynamics simulations.

Title: Unveiling emergent phenomena in Bi2Se3 and V5S8 single crystals through structure-property relationships

Abstract: This talk will highlight three recent works currently submitted for publication, conducted by our research group and collaborators on the title binary compounds. With a large band gap and a single Dirac cone responsible for the topological surface states, Bi2Se3 is widely regarded as a prototypical 3D topological insulator. We have used the self-flux method to obtain large, high-quality Bi2Se3 single crystals in the entire concentration range available for such on the binary phase diagram. By combining basic structural characterization with resistivity, Hall effect and Shubnikov-de Haas (SdH) quantum oscillations, different types of lattice defects are identified and their detailed role on the bulk transport are investigated. Previous open questions on Bi2Se3, such as the different scattering times in transport and quantum oscillations, and the presence of additional low mobility bands, are addressed. For V5S8, we have grown high-quality single crystals by chemical vapour transport (CVT) and report the observation of an unexpected phase transition at high magnetic fields between the spin-flop and spin-flip transitions in this d-electron antiferromagnetic quantum material. High-precision magnetic, thermal and electrical transport measurements enable us to track the transitions up to fields as high as 35 T and at temperatures down to the millikelvin range, revealing three distinct magnetic quantum phase transitions. We present a model that finds agreement with our observation of a triad of spin transitions involving two sublattices with frustrated inter- and intra-sublattice spin couplings. Additionally, our heat capacity studies on V5S8 have revealed an unexpected and highly unusual (for a bulk material) quadratic dependence of the specific heat at low temperatures. The regime is independent of applied magnetic field and therefore not magnetic in origin, but rather related to the system’s particular layered structure, as part of the broader chalcogenide family VxS8 (x=4,5,6,8). We present a model wherein the anomalous heat capacity is described with an unconventional acoustic phonon spectrum, which is linear in wavevector in the c direction, but quadratic in the a-b plane, indicating a form of geometrical elastic criticality.

Title: Thermal transport and structural transitions in Barium Bismuthate

Abstract: Perovskite-type complex oxides are a family of compounds that have attracted growing interest because of the variety of tunable physical properties making them attractive for technological applications in different areas [1]. At LQMEC, we are interested in investigating how structural properties and transitions impact the evolution of heat transport in some selected perovskites from bulk to thin films.
In this talk, I will present and discuss our recent investigations of thermal transport 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 the attempt to explain the electronic states of this compound [4]. In our thermal conductivity experiments, we found and unexpected ~T2 power law at low temperatures, reminiscent of a glass-like behavior, and possible interpretations will be discussed in terms of the temperature -dependent structural phase diagram.
Finally, I will discuss our perspective as users on experiments we expect to carry out at the Synchrotron light source Sirius, in BBO and in other compounds, to bring further evidence to the interpretation of our (electrical, thermal, thermoelectric) transport experiments.

REFERENCES
1. AS. Bhalla et al. Mat. Res. Innov. 4.1, 3, (2000)
2. Henriques et al. arXiv:2308.13681
3. RL. Bouwmeester, et al. Rev. in Phys., 6, 100056. (2021)
4. B. G. Jang et al, Physical Review Letters 130, 136401 (2023)

Title: Resonant X-ray Scattering Studies of Charge Density Wave Correlations in the Cuprates

Abstract: Resonant X-ray Scattering (RXS) is a photon scattering technique that can be used for the study of electron states in quantum materials. In the last 15 years we have witnessed the development of ever more advanced RXS synchrotron beamlines working in the soP x-ray regime (energies below 2 keV) and the many important scientific findings enabled by this technique. The study of charge density waves (CDWs) in cuprate high-temperature superconductors over the last 12 years has evolved in tandem with the technologica improvements of RXS instruments, from the early stages of detecting CDW correlations with energy-integrated RXS (EI-RXS) to the study of dynamic CDW correlations with the newest resonant inelastic x-ray scattering (RIXS) instruments that can resolve the energy and polarization of the scattered photons. I will present an overview of various RXS studies of the CDW in the cuprates and the resulting insights about the superconductivity, pseudogap state and strange metal behavior in those systems, as well as the form of the effective interaction between electrons.

Title: Search for the Kitaev quantum spin liquid state in honeycomb iridates at high pressures

Abstract: Compounds with 5d transition metal ions have attracted recent attention due to the prediction and observation of novel forms of topological magnetic and electronic states. Among these, particular attention has been given to the Kitaev quantum spin liquid state that is expected to occur in honeycomb iridates, and which is a potential candidate for topologically protected quantum computing. However, the presence of a spin liquid in these materials rely on the delicate balance between Kitaev and Heisenberg exchange interactions; such balance is very sensitive to structural deviations away from a perfect honeycomb motif. In this talk I will present our efforts in the study of honeycomb iridates using several x-ray techniques at high pressure to control and probe their structure, as well as electronic and magnetic ground states. Particular focus will be given to our recent results on Cu2IrO3 and Na2IrO3, highlighting their distinct and complex phase diagram.

Title: Polarised Soft X-ray Imaging of Quantum Materials

Abstract: In order to explore the novel functionalities of quantum materials, a comprehensive understanding of their microstructure is essential. Macroscopic probes provide an average assessment of the material’s properties and offer indirect insights into the pivotal roles played by magnetic and microstructural inhomogeneities. The array of polarized soft X-ray spectroscopic techniques available at the I06 beamline of Diamond Light Source (UK) can be seamlessly integrated with Photoemission Electron Microscopy (PEEM) or coherent diffraction imaging, enabling high-resolution imaging of fundamental phenomena governing the properties of functional and quantum materials.
In this talk, I will provide an overview of the key scientific areas addressed by the beamline, with a particular emphasis on the in-situ manipulation of quantum materials. This manipulation encompasses the control of domains and domain walls in antiferromagnets, magnetic domain imaging of novel topological spin textures, and the investigation of light-induced phenomena in strongly correlated materials.