Institut Néel

Accurate temperature determination at the nanoscale is crucial for understanding and optimizing the photothermal properties of plasmonic nanoparticles (NPs). Here, we introduce confocal dynamic light scattering (confocal DLS) as a novel method for in-situ measurement of the temperature of individual colloidal NPs. By coupling tightly focused heating-probe configuration with single-particle DLS, we quantify NPs heating at their surface from their Brownian motion, exploiting the temperature dependence of the solvent viscosity. Experiments on Au NPs (20-50 nm) reveal excellent agreement with the hot Brownian motion theory, enabling reliable temperature increases determination up to ∼ 150 K, without any adjustable parameters. Importantly, the method extends to Au@SiO2 core-shell nano-heaters, where a transition from the high-density to the low-density hot Brownian motion regime is experimentally observed for the first time, demonstrating that the density ratio between the NPs and the solvent governs the motion of the heated NPs. Confocal DLS thus provides unprecedented access to the photothermal response of nanoscale heaters at the single-particle level, including mesoporous core-shell architectures, paving the way for applications in nanoscale catalysis, thermally responsive nanomaterials and controlled drug release.

CoV___O___ (CVO) stands out among spinel vanadates for its ultra-short V-V distances, placing it at the brink of itinerant electron behaviour-an ideal playground for strain engineering. In this work, we exploit this sensitivity by growing high-quality epitaxial CVO thin films on SrTiO___ (001) and MgO (001), inducing compressive and tensile strain, respectively. Using pulsed laser deposition under ultra-low oxygen pressure, we achieve high crystalline quality and straincontrolled tetragonal distortions: c > a under compression (STO) and c < a under tension (MgO). Resonant elastic X-ray scattering confirms a normal spinel structure, with cobalt occupying tetrahedral sites and vanadium octahedral ones. Both strain types reduce charge transport, driving the system into a highly resistive state. Magnetic measurements reveal strain-driven anisotropy switching: STO films transition from out-of-plane to in-plane easy axis below 90 K, while MgO films flip from in-plane to out-of-plane below 45 K. These results highlight CVO's exceptional responsiveness to lattice strain, unlocking a path to finely tunable electronic and magnetic properties. With its strong spin-lattice coupling and potential in spin Hall magnetoresistance, strained CVO emerges as a compelling platform for next-generation lowpower spintronic devices.

Abstract In the current study, the structural (X-ray diffraction XRD), magnetic, critical behavior and magnetocaloric properties of the polycrystalline Pr 0.55 Sr 0.45 Mn 1−x Cr x O 3 Manganites with (x = 0.0, 0.05, 0.1 and 0.15) samples were examined. All our samples have been elaborated by means of the solid-state reaction from stoichiometric powder mixtures of binary oxides at high temperatures. The compounds crystallize in the orthorhombic structure with Pnma space group, according to Rietveld refinement of the XRD pattern. All of the samples exhibit a second-order FM to PM phase transition, according to temperature and field-dependent magnetization measurements; however, the Curie temperature T C value decreases from 300 K to 275 K as the Cr (% x) content increases from 0.00 to 0.15. Using Maxwell thermodynamic relations, the magnetocaloric effect MCE in terms of maximum entropy change \(\:{-\varDelta\:S}_{M}^{max}\), and relative cooling power RCP has been calculated using isothermal magnetization data around T C . In a magnetic field shift of 5 T, the highest values of the magnetic entropy change \(\:{-\varDelta\:S}_{M}^{max}\) have been determined to be 3.8 J/kg.K, 3.63 J/kg.K, 3.87 J/kg.K, and 2.55 J/kg.K for x = 0.0, x = 0.05, x = 0.1, and x = 0.15, respectively. For x = 0.0, x = 0.05, x = 0.1, and x = 0.15 at 5 T, the highest value of the relative cooling power RCP is found to be 247 J/kg, 254.1 J/kg, 205.1 J/kg, and 201 J/kg, respectively. The RCP value of 254.1 J/kg (5% of chromium) is equivalent to 58% of the RCP value of gadolinium metal. Technically, the developed material is highly promising for magnetic refrigeration because of these significant values.

The dielectric response of amorphous organic solids at the timescale of atomic motion has broad implications for their photophysics, governing spectroscopic properties and excited-state relaxation, among other phenomena. We present an original approach to the calculation of the vibrational contribution to the frequency-dependent dielectric permittivity. Our methodology, combining molecular dynamics simulations, density functional theory calculations, and linear response theory, is applied to a set of amorphous carbazole-based small-molecule matrices, widely used as host materials in organic light-emitting diodes. Our analysis reveals the crucial role of hindered molecular rotations (librations) in the solid-state matrices of dipolar compounds. These modes manifest in the very-low frequency ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mo>&lt;</a:mo> <a:mn>100</a:mn> <a:mspace width="0.28em"/> <a:msup> <a:mrow> <a:mi>cm</a:mi> </a:mrow> <a:mrow> <a:mo>−</a:mo> <a:mn>1</a:mn> </a:mrow> </a:msup> </a:mrow> </a:math> ) region of the dielectric function, accounting for up to one-third of the total vibrational contribution to the static dielectric constant in a dipolar compound like 3,3'-di(carbazol-9-yl)-5-cyano-1,1'-biphenyl. This low-frequency contribution is found to be crucial for determining the variations in the static dielectric constant across various materials.

Diamond Light Source , B18 under the Energy Materials Block Allocation Group (BAG)

The study of quantum vortex dynamics in He II offers great potential for advancing quantum-fluid models. Bose-Einstein condensates, neutron stars, and even superconductors exhibit quantum vortices, whose interactions are crucial for dissipation in these systems. These vortices have quantized velocity circulation around their cores, which, in He II, are of atomic size. They have been observed indirectly, through methods such as second sound attenuation or electron bubble imprints on photosensitive materials. Over the past twenty years, decorating cryogenic flows with particles has become a powerful approach to studying these vortices. However, recent particle visualization experiments often face challenges with stability, initial conditions, stationarity, and reproducibility. Moreover, most dynamical analyses are performed in 2D, even though many flows are inherently 3D. We constructed a rotating cryostat with optical ports on an elongated square cupola to enable 2D and 3D Lagrangian and Eulerian studies of rotating He II flow. Using this setup, individual quantum vortices have been tracked with micron-sized particles, as demonstrated by Peretti etal. [Sci. Adv. 9, eadh2899 (2023)]. The cryostat and associated equipment-laser, cameras, sensors, and electronics-float on a 50 μm air cushion, allowing for precise control of the experiment's physical parameters. The performance during rotation is discussed, along with details on particle injection.

Mixed ionic electronic transfer (MIET) reactions, such as the oxygen reduction reaction (ORR) at oxide surfaces, are of paramount importance to manifold technologically highly relevant processes and fundamental understanding must be developed to improve performance and tailor highly efficient electrodes and catalysts. Understanding such complex multi-step reactions, requires the study of kinetic processes, underlying thermodynamic properties, i.e. ionic and electronic defect concentrations and electrostatic surface effects. However conventional techniques struggle to uncover the complete picture within the same sample/measurement. Here, we overcome this limitation by introducing bias-triggered conductivity relaxation (BCR) as a novel tool to investigate MIET reactions on oxides. It is based on alternating out-of-plane coulometric titration/polarization and in-plane electrical conductivity relaxation measurements, providing simultaneous electronic, ionic and extraordinarily rich surface kinetics information. This innovative combination of electrical and chemical driving forces synergizes information depth, with enhanced time resolution, versatility and speed, yet it lifts the weaknesses of the individual approaches, while remaining cost-effective and surprisingly simple. Furthermore, BCR allows to disentangle overpotential induced electrostatic modifications of the surface kinetics in a unique manner. We showcase the advantages of BCR in this work by studying the ORR in model (La,Sr)FeO$_{3-δ}$ thin film electrodes and reporting on their thermodynamic and kinetic properties.

The structural X-ray diffraction (XRD), magnetic, critical behavior, and magnetocaloric characteristics of polycrystalline Pr0.55Sr0.45Mn1-x Ni x O3 manganites with 0%, 5%, 10% and 15% of Ni doping were investigated in the current work. A solid-state reaction using the stoichiometric powder mixtures of binary oxides at high temperatures was employed for the synthesis of all the samples. The Rietveld refinement of the XRD patterns indicate that the compounds crystallize in an orthorhombic structure with the Pnma space group. Temperature- and field-dependent magnetization experiments show a second-order ferromagnetic (FM) to paramagnetic (PM) phase transition in all the samples. However, when the Ni (%, x) content increases from 0.00 to 0.15, the Curie temperature (T C) decreases from 300 K to 180 K. For all the compounds, Arrott plots reveal a second-order phase transition. Using various methods, data from static magnetization measurements at the FM-PM phase transition is used to analyze the critical parameters. Using the isothermal magnetization data around T C, the magnetocaloric effect (MCE) in terms of maximum entropy change (-ΔS max M) and relative cooling power (RCP) are determined using Maxwell thermodynamic relations. In a µ 0 H = 5 T magnetic field shift, the highest values of magnetic entropy changes (-ΔS max M) are calculated to be 3.8 J kg-1 K-1, 1.71 J kg-1 K-1, 1.62 J kg-1 K-1, and 1.29 J kg-1 K-1 for 0%, 5%, 10% and 15% of Ni, respectively. The maximum relative cooling power (RCP) values for x = 0.0, x = 0.05, x = 0.1, and x = 0.15 at µ 0 H = 5 T are 247 J kg-1, 216 J kg-1, 211 J kg-1, and 215.94 J kg-1, respectively. The RCP of compound (0% of nickel) = 254.1 J kg-1 is 56% of the RCP value of Gd gadolinium metal. These important features make these new materials extremely promising for magnetic refrigeration from a technical standpoint.

Many problems in physical chemistry involve systems that are coupled to an environment, such as a molecule interacting with an adjacent surface, possibly resulting in meta-stable molecular states where electron density is transferred to the surface. Such systems can be described by non-Hermitian quantum mechanics (NHQM), where the Hamiltonian includes a complex potential. Within NHQM, one can also formulate the Hartree–Fock (HF) and Kohn–Sham (KS) methods and, as in the conventional theory, an effective independent-particle picture is employed. The crucial observation of the present study is that even for systems that are not coupled to an environment, in the non-Hermitian HF or KS equation, a single electron is interacting with the remaining electrons, which act as an environment, allowing for the exchange of current density between the one-electron and the remaining (N − 1)-electron system. The corresponding self-consistent states represent a new uncharted space of solutions to the HF and KS equations. We show that the additional solutions can have a physical interpretation and, therefore, extend the range of problems HF and KS can be applied to. If open-system HF and KS calculations are performed, the new class of solutions is always encountered, but this has also not been noted previously.