The rise of the electronic age sparked a quest for increasingly faster and smaller switches, since this element is ubiquitous and foundational in any electronic circuit to regulate the flow of current. Mott insulators are promising candidates to meet this need as they undergo extremely fast resistive switching under electric field initiated by an avalanche phenomena. However, the nature of the final switched state is still under debate. The spatially resolved micro‐X‐ray Diffraction imaging and micro‐Raman experiments carried out on the prototypal Mott insulator (V 0.95 Cr 0.05 ) 2 O 3 show that the resistive switching is associated with the creation of a conducting filamentary path consisting in an isosymmetric compressed phase without any chemical or symmetry change. This strongly suggests that the avalanche initiated resistive switching mechanism is inherited from the bandwidth‐controlled Mott‐Hubbard transition just like the laser induced insulator to metal transition recently studied in the same system. This discovery may hence ease the development of a new branch of electronics called Mottronics.

The rise of the electronic age sparked a quest for increasingly faster and smaller switches, since this element is ubiquitous and foundational in any electronic circuit to regulate the flow of current. Mott insulators are promising candidates to meet this need as they undergo extremely fast resistive switching under electric field initiated by an avalanche phenomena. However, the nature of the final switched state is still under debate. The spatially resolved micro-X-ray Diffraction imaging and micro-Raman experiments carried out on the prototypal Mott insulator (V0.95Cr0.05)(2)O-3 show that the resistive switching is associated with the creation of a conducting filamentary path consisting in an isosymmetric compressed phase without any chemical or symmetry change. This strongly suggests that the avalanche initiated resistive switching mechanism is inherited from the bandwidth-controlled Mott-Hubbard transition just like the laser induced insulator to metal transition recently studied in the same system. This discovery may hence ease the development of a new branch of electronics called Mottronics.

This paper aims to review the physical properties and crystal chemistry of the family of correlated quantum materials AM4Q8 (A = Ga, Ge; M = V, Nb, Ta, Mo; Q = S, Se). These compounds exhibit a lacunar spinel structure with tetrahedral transition metal clusters which favor a correlated state. But compared to most other inorganic Mott insulators, the AM4Q8 compounds show very small Mott–Hubbard gaps (0.1–0.3 eV). These small values originate from the weak electronic repulsion occurring on the scale of the tetrahedral clusters and from the effect of spin–orbit coupling. As a consequence, AM4Q8 quantum materials show a great variety of ground states and astonishing electronic properties depending on cluster filling, compression, or distortion. For example, they exhibit a multiferroic behavior related to an orbital ordering on the clusters and a variety of magnetic ordering like skyrmions. Under external pressure they undergo a bandwidth-controlled insulator to metal transition (IMT) that leads to a superconducting state at low temperature. When chemically doped, the AM4Q8 compounds undergo a filling-controlled insulator to metal transition with appearance of a half ferromagnetic metallic behavior or colossal negative magnetoresistance. The AM4Q8 compounds are also sensitive to the electric field and exhibit a striking resistive switching above a threshold electric field of a few kV/cm which is related to the breakdown of the Mott insulating state at the nanoscale. This phenomenon unlocks new functionalities that may be used to build up a new type of resistive random-access memory (RRAM) or an artificial neuron. All these examples show the potential of this family of quantum materials whose exploration has only just begun.

Metal chalcogenide semiconductors are being widely investigated for applications in solar energy conversion, such as photovoltaics and visible light photocatalysis. Herein, an initial assessment of potentialities of new lamellar chalcogenides named CIGSn is provided, while comparing them with that of the well-known CIGS chalcopyrite. The main difference between CIGS and CIGSn compounds concerns their electronic properties and more precisely the nature of charge carriers. Cu0.32In1.74Ga0.84S4 (CIGS4) is an n-type semiconductor, unlike the chalcopyrite CuIn0.7Ga0.3S2 (CIGS) that is a p-type semiconductor. More noticeable, Cu1.44In2.77Ga0.76S6 (CIGS6) and in a lesser extent Cu0.65In1.75Ga1.4S5 (CIGS5), exhibit an ambipolar character with a slight predominance of electron transport. The Fermi levels of all lamellar CIGSn compounds are similar (-4.5 eV) and higher that of the chalcopyrite CIGS (-5.1 eV). In addition, the charge carrier densities of CIGSn compounds (1014 - 1017 cm-3) are significantly lower than that of CIGS (1020 cm-3), which is consistent with their higher resistivity. Photoluminescence measurements and OCP decays suggest much more in-gap defect states in the lamellar compounds. These results suggest that CIGSn compounds would not be suitable for photovoltaic applications. Nevertheless, their energy bands show an interesting positioning, with respect to redox potentials involved in water splitting and CO2 reduction. In addition, ambipolarity could enhance the efficiency of catalytic reactions, because a type of minority charge carriers does not limit the charge transport.

2D transition metal chalcogenides have been examined as versatile platforms for exotic quantum phenomena, optoelectronic and photocatalytic applications. La2O2S2, a layered oxysulfide built of [La2O2](2+) slabs and 2D arrays of [S-2](2-) dimers, was recently found to be a promising precursor to fabricate such 2D materials. Redox reactions with external zerovalent metals cleaved its S-S bonds, triggering intercalation of those metal guests. This process serves as a novel approach to construct 2D metal sulfides between rigid [La2O2](2+) slabs, but so far demonstrated only for Cu (+) cations. We herein report that the same intercalation process takes place also when Ni and Fe were used as reagents. While XRD indicated that the reactions with Ni and Fe converted La2O2S2 into the sulfur-deficient La2O2S1.5-x (0 <= x <= 0.38) phase, our TEM analyses evidenced diffusion of those metals in between [La2O2](2+) slabs at the local scale. This finding suggested the formation of 2D nickel and iron sulfides intergrown with [La2O2](2+) slabs, either as the unprecedented La-O-M-S (M = Ni, Fe) phase, or a biphasic heterostructure. In addition, our computational structure prediction also supported stability of such intergrowth [La2O2][MxS2-y] structures, encouraging future attempts to isolate those elusive 2D materials.

Nowadays, materials science is moving towards the understanding and control of materials in nonequilibrium states by making use of perturbative techniques to investigate their dynamical responses. From this perspective, the use of ultrashort light pulses seems to be a relevant approach as it can selectively address different degrees of freedom in solid-state systems and more particularly electrons. Such a method can help to decipher the physical phenomena arising from electronic correlations and complements a more conventional methodology where the phase diagrams of materials are investigated at thermodynamical equilibrium. Here, we combine femtosecond optical spectroscopy and a high-pressure setup to monitor the ultrafast out-of-equilibrium photo response of a V_{2}O_{3} thin film across the pressure driven insulator-to-metal transition. The experimental results demonstrate the possibility to use the spectroscopy of coherent phonons as a thermodynamical phase marker in V_{2}O_{3} thin films. In addition, the frequency behavior of the ultrafast coherent phonon mode (A_{1g} character) seems to reflect the manifestation of a strong coupling between the lattice and electronic degrees of freedom near first-order transition lines with a pronounced drop in frequency around the critical pressure.

Ultrafast photoexcitation can generate internal compressive stress in Mott insulators that lead to strain waves from free surfaces. These photoinduced elastic waves can trigger phase transitions in materials. However, a comprehensive physical picture of the phase transformation dynamics that includes acoustic-scale propagation has not yet been developed. Here we demonstrate that such a strain-wave mechanism drives the ultrafast insulator-to-metal phase transition in granular thin films of the Mott material V2O3. Our time-resolved optical reflectivity and X-ray diffraction measurements reveal that an inverse ferroelastic shear occurs before the insulator-to-metal transition, which propagates in the wake of a compressive strain wave. These dynamics are governed by the domain size and film thickness, respectively. Our results clarify the morphological conditions for the ultrafast phase transition that is favoured in granular thin films and hindered in single crystals. The resulting physical picture sheds light on the ultrafast phase transitions in quantum materials and future devices based on Mott insulators.

Intercalation/deintercalation reactions enable introduction/removal of intercalants without destructive structure transformation of host lattices. They are among the most versatile ways to design metastable phases attainable in mild synthesis conditions. Recently, topochemical deintercalation of oxygen anions has opened up an avenue to access new compounds with unusual transition metal oxidation states and interesting properties. So far, the scope of such anion deintercalation was mainly restricted to oxides and their oxyhalide derivatives. However, lately, we presented a proof-of-concept study on the sulfur deintercalation reaction driven by anionic redox in La2O2S2. Here, we extend this work and present the design of new members of a family of slightly colored non-centrosymmetric metastable oxysulfides. Our work shows that the reduction with an alkali metal of sulfur dimers in Ln(2)O(2)S(2) (Ln = Pr, Nd) precursors leads to the topochemical deinsertion of half of the sulfur atoms of each S-2 pair, producing two new metastable oA-Ln(2)O(2)S (Ln = Pr, Nd) phases. The non-centrosymmetric compounds oA-Ln(2)O(2)S (Ln = La, Pr, Nd) evidence second and third harmonic generation effects, suggesting the potential applicability of the topochemical route for design of nonlinear optical materials.

Ultrafast physics opens new avenues for directing materials to different functional macroscopic states on non-thermal dynamical pathways. In any phase transition involving volume and/or ferroelastic deformation, an often overlooked mechanism emerges whereby photoinduced elastic waves drive the transition. However, a comprehensive physical picture of transformation dynamics which includes acoustic scale propagation remained elusive. Here we show that such a strain wave mechanism drives the ultrafast insulator-to-metal phase transition (IMT) in the V2O3 Mott material. We discuss the underlying physics based on time-resolved optical reflectivity and X-ray diffraction probing granular thin films. We evidence the role of strain wave mechanisms in ultrafast changes either with or without symmetry breaking. We reveal inverse ferroelastic shear occurring before the IMT propagating in the wake of compressive strain wave. These dynamics are shown to be governed by the domain size and the film thickness, respectively. A fluence threshold is evidenced for the onset of IMT at macroscopic scale, as well as phase separation at intermediate fluence and complete transformation at saturating fluence. We clarify the morphological conditions for the ultrafast IMT that is favoured in granular thin films, and hindered in single crystals. The resulting physical picture shed new light on ultrafast phase transitions in quantum materials and future devices based ond Mott insulators.

Memories based on the insulator-to-metal transition in correlated insulators are promising to overcome the limitations of alternative nonvolatile memory technologies. However, associated performances have been demonstrated so far only on narrow-gap compounds, such as (V0.95Cr0.05)2O3, exhibiting a tight memory window. In the present study, V-substituted Cr2O3 compounds (Cr1-xVx)2O3 have been synthesized and widely investigated in thin films, single crystals, and polycrystalline powders, for the whole range of chemical composition (0 < x < 1). Physicochemical, structural, and optical properties of the annealed magnetron-sputtered thin films are in very good agreement with those of polycrystalline powders. Indeed, all compounds exhibit the same crystalline structure with a cell parameter evolution consistent with a solid solution over the whole range of x values, as demonstrated by X-ray diffraction and Raman scattering. Moreover, the optical band gap of V-substituted Cr2O3 compounds decreases from 3 eV for Cr2O3 to 0 eV for V2O3. In the same way, resistivity is decreased by almost 5 orders of magnitude as the V content x is varying from 0 to 1, similarly in thin films and single crystals. Finally, a reversible resistive switching has been observed for thin films of three selected V contents (x = 0.30, 0.70, and 0.95). Resistive switching performed on MIM devices based on a 50 nm thick (Cr0.30V0.70)2O3 thin film shows a high endurance of 1000 resistive switching cycles and a memory window ROFF/RON higher by 3 orders of magnitude, as compared to (Cr0.05V0.95)2O3. This comprehensive study demonstrates that a large range of memory windows can be reached by tuning the band gap while varying the V content in the (Cr1-xVx)2O3 solid solution. It thus confirms the potential of correlated insulators for memory applications.

Controlling material properties with light pulses represents one of the next great challenge in material science. To achieve this goal, one needs to impact the material on the relevant time scale for controlling electronic and phononic properties. Targeting this challenge becomes possible thanks to the emerging field of photo-induced phase transition when a femtosecond light pulse interacts with the system. Along this line, we demonstrate the possibility to perform femtosecond optical spectroscopy under control thermodynamical environment. This new experimental setup includes a very precise pressure control between 1 and 6 kbar. These experimental results clearly demonstrate an effect of hydrostatic pressure onto the out-of-equilibrium photo-response of V_2O_3 . The main feature is a blue shift of the Brillouin frequency that correlates with an increasing speed of sound.

Mott insulators are a class of strongly correlated materials with emergent properties important for modern electronics applications, such as artificial neural networks. Under an electric field, these compounds undergo a resistive switching that may be used to build up artificial neurons. However, the mechanism of this resistive switching is still under debate and may depend on the Mott material involved. Some works suggest an electronic avalanche phenomenon, while others propose an electrothermal scenario. As electric pulses produce both Joule heating and hot carriers, disentangling their respective roles requires the use of another external stimulus. Here, an ultrashort light pulse is used to tune the number of photogenerated carriers and the energy provided to the system. In these pump-pump-probe experiments, a crystal of the Mott insulator GaTa4Se8 is simultaneously excited by electric and laser pulses while an electric probe monitors its conductivity. The study shows that the resistive switching is affected by the number of generated photocarriers rather than by the accumulation of energy deposited by the femtosecond laser. It supports therefore a mechanism driven by generation of hot carriers. Finally, our work opens the possibility to build up an artificial electro-optical "Mott" neuron tuned by a femtosecond laser pulse.

Time-resolved studies with temporal resolution that separate molecular level dynamics from macroscopic changes, allow clear distinction between the time scales of the different degrees of freedom involved. Cooperative molecular switching in the solid state is exemplified by spin crossover phenomenon in crystals of transition metal complexes. Here we show the existence of a delay between the crystalline volume increase, and the cooperative macroscopic switching of molecular state. Using 100 ps X-ray diffraction, we track the molecular spin state and the structure of the lattice during the photoinduced low spin to high spin transition in microcrystals of [Fe III (3-MeO-SalEen) 2 ]PF 6 . Model simulations explain the phenomenon with thermally activated kinetics governed by local energy barriers separating the molecular states. Such behaviour is different from that encountered in materials with no local energy barriers, where phase transformation can occur simultaneously with propagation of strain. Broadly, this motivates an optimised material design, scalable with size and intrinsic energetics.

The resistive switching observed under electric pulses in Mott materials has a high potential for micro and nanoelectronics. Here we report on the study of the resistive switching observed at the surface of single crystals of the canonical Mott semiconductor GaMo 4 S 8 . The study is made using a multiprobe setup with 4 nanopositionable tips under the supervision of a high resolution scanning electron microscop. We find a resistivity of 38 Ω.cm by four-point probe measurements, in agreement with the literature. The volatile insulator to metal transition is studied with a two probes configuration for interelectrode distances varying between 4 and 200 microns. Finite element simulations are performed to determine the spatial distribution of the electric field prior to the transition. Our results are in agreement with i) an intrinsic voltage threshold of 60 mV independent of the interelectrode distance ii) a maximum electric field close to the electrodes and iii) a threshold electric field of 0.2 kV/cm.

Controlling material properties with light pulses represents one of the next great challenge in material science. To achieve this goal, one needs to impact the material on the relevant time scale for controlling electronic and phononic properties. Targeting this challenge becomes possible thanks to the emerging field of photo-induced phase transition when a femtosecond light pulse interacts with the system. Along this line, we demonstrate the possibility to perform femtosecond optical spectroscopy under control thermodynamical environment. This new experimental setup includes a very precise pressure control between 1 and 6 kbar. These experimental results clearly demonstrate an effect of hydrostatic pressure onto the out-of-equilibrium photo-response of $$\hbox {V}_{2} \hbox {O}_{3}$$ . The main feature is a blue shift of the Brillouin frequency that correlates with an increasing speed of sound.

The common approach to modify the thermoelectric activity of oxides is based on the concept of selective metal substitution. Herein, we demonstrate an alternative approach based on the formation of multiphase composites, at which the individual components have distinctions in the electric and thermal conductivities. The proof-of-concept includes the formation of multiphase composites between well-defined thermoelectric Co-based oxides: Ni, Fe co-substituted perovskite, LaCo0.8Ni0.1Fe0.1O3 (LCO), and misfit layered Ca3Co4O9. The interfacial chemical and electrical properties of composites are probed with the means of SEM, PEEM/XAS, and XPS tools, as well as the magnetic susceptibility measurements. The thermoelectric power of the multiphase composites is evaluated by the dimensionless figure of merit, ZT, calculated from the independently measured electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (λ). It has been demonstrated that the magnitude's electric and thermal conductivities depend more significantly on the composite interfaces than the Seebeck coefficient values. As a result, the highest thermoelectric activity is observed at the composite richer on the perovskite (i.e., ZT = 0.34 at 298 K).