Phase Separations in Highly Correlated Materials

Abstract

The X-ray absorption near edge structure spectroscopy is a unique powerful local and fast experimental method to study complex systems since it probes the nanoscale structure around selected atoms giving evidence for different local and instantaneous phases present in multiscale highly correlated granular systems. Transition metals and rare earth oxides like manganites, cuprates or pnictides superconductors show a rich variety of different competing structural, electronic and magnetic phases, which spatially coexist forming complex lattice textures. Many recent experimental data have pointed out the presence of arrested phase separation and the interplay of different phases occurring from nano-to micrometer-scale. This scenario opens the possibility to manipulate the mesoscopic phases to get new material functionalities. Therefore there is increasing need to develop methods to probe morphology and phase distribution at multiple length scales. Actually, combining X-ray imaging at high spatial resolution with µ-XANES spectroscopy both mesoscale, nanoscale and atomic structural changes can be identified. The µ-XANES spectroscopy technique is rapidly growing to investigate adaptive matter, high temperature superconductors, complex materials showing arrested phase separation at the mesoscale. IntroductionIt is well known that phase separation occurs where two or more phases with comparable free energies coexist giving two or more macroscopic phases each one with a large coherence length, however there is growing scientific interest for the case of arrested phase separations in transition metal oxides where two or more phases form complex textures extending in the mesoscale between the atomic scale and the macroscopic world, made of small domains with a size ranging from nanoscale to micron-scale. The mesoscale phase separation leads to strong anomalies or dramatic changes of system properties and material functionality. Clear anomalies are observed in the behaviour of different observable quantities such as the resistivity and the optical transmission [1,2]. The origin of the mesoscale phase separation phenomenon responsible for a variety of unusual phenomena is due to the appearance of voxels of competing thermodynamic phases inside a host phase [3][4][5][6]. Transition metals and rare earth oxides, as for example high temperature superconductors, as cuprates or pnictides present multiple electronic states with different orbital symmetries at the Fermi energy and competing anisotropic Coulomb, magnetic and electron-lattice short range interactions together with long range Coulomb and elastic interaction, giving complex lattice architectures and a rich variety of different coexisting electronic and magnetic phases. Their interplay plays a key role in colossal magnetoresistance materials [7,8], ferroelectrics [9,10] and in high temperature superconductors (HTS) showing arrested phase separation at mesoscopic or nanoscopic length scales [11][12][13][14][15].

Metastability Phenomena in VO2 Thin Films

“…This sample shows a clear metastability of the final resistivity value in the insulating/metallic phase after hysteresis temperature cycles. The emerging nanoscale inhomogeneity in the vanadium dioxide at the metal-to-insulator transition is well known and makes this system similar to other complex high correlated quantum materials [8,9,[16][17][18][19][20] where the phase separation landscape depends on misfit strain [16,17]. We show that by reducing the thickness below 40 nm in micron-sized ribbons, we can control the complex nano-phase separation at MIT of VO 2 .…”

Nanoscale Phase Separation and Lattice Complexity in VO2: The Metal–Insulator Transition Investigated by XANES via Auger Electron Yield at the Vanadium L23-Edge and Resonant Photoemission

“…XANES can be particularly useful for probing and mapping the changes in the electronic structure of an MIT transition, providing a correlation between macroscopic observables and microscopic models associated with coexisting metastable configurations, which are tuned by the thickness-dependent misfit strain and stress distributions induced by the mismatch between thin film and substrate [26][27][28]. Since, in the VO 2 electronic MIT-driven transition, a relatively large change in the electronic configuration between the metallic and insulating phases is expected, an accurate measurement of the difference among V 2p X-ray absorption spectra will probe changes of the local 3d orbital occupancy.…”

Interplay among work function, electronic structure and stoichiometry in nanostructured VOx films

“…VO2 and V2O3 exhibit sharp, fast and reversible metal insulator transitions (MIT) triggered by temperature 19 and characterized by a nanoscale phase separation. [20][21][22][23] Synthetizing nano-sized samples with a tunable electronic structure is therefore relevant for many applications.…”