Localized Polarons Research Articles

Magnetic and Electronic Inhomogeneity in Sm1−xEuxB6

While SmB6 attracts attention as a possible topological Kondo insulator, EuB6 is known to host magnetic polarons that give rise to large magnetoresistive effects above its ferromagnetic order transition. Here, we investigate single crystals of Sm1−xEuxB6 by magnetic and magnetotransport measurements to explore a possible interplay of these two intriguing phenomena, with a focus on the Eu-rich substitutions. Sm0.01Eu0.99B6 exhibits generally similar behavior as EuB6. Interestingly, Sm0.05Eu0.95B6 combines a global antiferromagnetic order with local polaron formation. A pronounced hysteresis is found in the magnetoresistance of Sm0.1Eu0.9B6 at low temperature (T= 1.9 K) and applied magnetic fields between 2.3 and 3.6 T. The latter is in agreement with a phenomenological model that predicts the stabilization of ferromagnetic polarons with an increasing magnetic field within materials with a global antiferromagnetic order.

Atomic-Scale Direct Observation on Cation Antisite Intermixing and Identification of Charge Conduction Mechanism in p-Type CuBi2O4 for Photocathodes

Cu-based p-type semiconducting oxides have been investigated for their potential as water-reduction photocathodes in order to enhance energy conversion efficiency in photoelectrochemical cells. Among these oxides, CuBi2O4 has recently gained significant attention as a promising candidate for converting light energy into electrochemical reactions because of its good light absorption property with an adequate bandgap. Despite the importance of identifying the major defect structure to comprehend the electronic conduction behavior, however, no direct experimental analysis has been suggested yet. While the formation of Cu cation vacancies may be discussed as a possible explanation of the p-type conduction behavior, direct evidence regarding the atomic-scale structural and chemical characteristics is still required.In this research, a combination of advanced characterization techniques is employed to figure out a representative defect structure affecting the optoelectronic properties in CuBi2O4. Cs-corrected scanning transmission electron microscopy (STEM) analysis is conducted to identify a noteworthy occurrence of antisite point defects. Atomic-level energy-dispersive X-ray spectroscopy (EDS) analysis is also employed simultaneously, to clearly show the intermixing of BiCu-CuBi antisite cations as a crucial type of point defect in CuBi2O4. Additionally, a combined result of optical and electrical measurement and density functional theory (DFT) calculations demonstrates the presence of local Cu 3d polaron states in the bandgap. A new type of charge conduction mechanism is created by the hopping of hole-polarons between Cu sites, but it is seriously hindered by the BiCu cations occupying Cu sites. A higher degree of intermixing leads to a greater activation energy and decreased electronic conductivity due to the hindrance of hole-polaron hopping, as evidenced in the Arrhenius plot.Overall, this research provides valuable insights into the role of antisite defects in CuBi2O4 and their impact on its potential as a photocathode material. The results highlight the significance of the combination of atomic-scale structure analysis and theoretical calculations to identify the possible existence of defects and gain a comprehensive understanding of the electric characteristics in complex oxides for (photo)electrocatalysis. By understanding how antisite defects affect electronic conductivity, various strategies can be developed by controlling the possible defects to optimize not only CuBi2O4 but also other complex oxides for energy-harvesting applications. Figure 1

Proton Incorporation and Electrical Leakage in BaZrO3 and BaCeO3

Molecular hydrogen (H2) is a vital next-generation energy carrier, due to its high energy density, universal abundance, and ability to produce electricity in a fuel cell without greenhouse gas emissions. Critical to the adoption of hydrogen energy is the ability to produce H2 cleanly via the electrolysis of water. Solid oxide electrolyzer cells (SOECs) are attractive technologies to achieve this goal, as they can use excess thermal energy or other renewable energy sources to convert water into H2. However, the fundamental physics that determines the performance of SOECs—including sources of performance loss and degradation—is relatively poorly understood.Here, we describe first-principles calculations based on hybrid density functional theory (DFT) aimed at studying efficiency loss in proton-conducting SOECs based on the oxide electrolytes BaZrO3 (BZO) and BaCeO3 (BCO). These materials are acceptor-doped with a trivalent element such as yttrium to introduce protons; however, such doping can also increase electrical leakage in devices, which limits their Faradaic efficiency. Our calculations describe the properties of localized polarons in BZO and BCO: hole polarons form favorably in both materials, while electron polarons are only stabile in BCO and in Ce-containing alloys. Under typical operating conditions, large concentrations of hole polarons may be present; therefore, to limit the risk of p-type electrical leakage, it is advantageous to avoid extreme oxygen-rich conditions and high dopant concentrations [1].High ionic conductivity also requires large proton concentrations. We examine avenues to achieve this requirement by calculating the formation energy of protons and their concentrations under various conditions. Proton concentrations will be highest under wet conditions, with oxygen vacancies becoming more dominant under drier conditions. Encouragingly, higher protonation levels correlate with lower concentrations of hole polarons, implying lower electrical leakage with higher hydration levels. In addition to traditional protonation pathways, we also explore a novel hydrogenation mechanism via hydroxyl incorporation, which does not require acceptor doping. However, this process is unfavorable except under extremely wet conditions [2].These results provide crucial insights for understanding the complex behavior of these materials in situ. Determining how to promote ionic conductivity at the expense of electrical leakage is critical to optimizing their performance in SOECs and related devices.This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.[1] A. J. E. Rowberg, M. Li, T. Ogitsu, J. B. Varley, Phys. Rev. Mater. 7, 015402 (2023).[2] A. J. E. Rowberg, M. Li, T. Ogitsu, J. B. Varley, Mater. Adv. 4, 6233-6243 (2023).

Hole Localization Induces Anionic Redox in Li-Ion Cathodes

Anionic redox is defined as the depopulation of electronic states consisting of unhybridized or minimally hybridized O 2p orbitals. In recent years, anionic redox has been investigated in great detail due its potential to increase the high-voltage capacity of Na-ion and Li-ion intercalation positive electrodes (cathodes) for batteries. However, anionic redox almost always manifests alongside a gamut of undesirable effects, from > 1 V of hysteresis, to voltage and capacity fade over hundreds of cycles. These effects have hindered the widespread utilization and commercialization of anionic redox-active cathodes. Understanding the origin and role of anionic redox is vital if we are to leverage it in high-voltage cathodes and mitigate its negative effects.Anionic redox occurs across many bulk structural motifs - from Na and Li-layered oxides to cation-disordered rocksalt-type (DRX) structures. While it was initially thought to only occur in systems with alkali metal-excess environments (such as the Li-O-Li type local configurations), it has hence been observed even in conventional stoichiometric cathodes at high states-of-charge. The crystal structure origins of anionic redox in terms of metal-oxygen decoordination and metal migrations have been well explored in the literature, a similar robust origin theory stemming from electronic structure arguments is lacking.To address this, we note that there are two manifestations of anionic redox observed in the literature - the kind that involves O-O dimerization (observed in several systems such as DRX compounds, Li2RuO3, Li- and Mn-rich NMCs, etc.) and the kind that involves localized polarons without dimers (observed in Na2Mn3O7 and Na0.6Li0.2Mn0.8O2). In attempting to reconcile both observed types of behaviour, we observe that the chief difference stems from the stabilization mechanism for adding holes to the ligand orbitals. In both cases, holes are added to ligand or ligand-dominated orbitals, with minimal to no hybridization with the adjacent transition metals. In the former kind of anionic redox, the ligand holes are stabilized by forming a sigma-like O-O bond, while in the latter, it is a weak pi-type hybridization that stabilizes the addition of ligand holes. We hypothesize that the localization of the ligand holes governs the manifestation of anionic redox in a system.In this work, we investigate this relationship between localized ligand holes and anionic redox by tuning the cation ordering in Li4FeSbO6 (LFSO). This honeycomb-ordered Li-excess material is the perfect model system to use for this study, as the electrochemical behaviour of this system features a single high-voltage plateau at 4.2 V vs. Li+/Li which is charge compensated by a novel Fe(III)/(V) redox couple. The Fe redox couple involves an electronic transition from a 3d5 majority ground state to a negative charge transfer 3d5 L 2 ground state, involving highly hybridized and localized ligand holes on Fe-O orbitals. Despite the presence of hole density on O orbitals and the presence of Li-O-Li local environments, the system cannot be classified as being anion redox active due to the contribution of the metal states to the charge compensation.However, by systematically tuning the extent of cation disorder in LFSO, we are able to convert highly metal-hybridized holes into unhybridized ligand holes, inducing anionic redox. We use a combination of soft X-ray spectroscopy, synchrotron diffraction, neutron diffraction, charge-transfer atomic multiplet calculations and density functional theory to unravel the mechanistic relationship between cation disorder, ligand hole localization and anionic redox, giving us a rational design rule for synthesizing materials with low hysteresis at high potentials. Our findings reveal the crucial role of cation disorder in promoting anionic redox.

Visualizing Ultrafast Photogenerated Electron and Hole Separation in Facet-Engineered Bismuth Vanadate Crystals.

Photogenerated charge separation is pivotal for effecting efficient photocatalytic reactions. Understanding this process with spatiotemporal resolution is vital for devising highly efficient photocatalysts. Here, we employed pump-probe transient reflection microscopy to directly observe the temporal and spatial evolution of photogenerated electrons and holes on the surface of facet-engineered bismuth vanadate (BiVO4) crystals. The findings suggest that the anisotropic built-in field of BiVO4 crystals propels the separation of photogenerated electrons and holes toward different facets through a two-step process across varying time scales. Photogenerated electrons and holes undergo ultrafast separation within ∼6 ps, with electrons transforming into localized small polarons toward the {010} facets of truncated BiVO4 octahedral crystals. However, the photogenerated holes prolong their separation up to ∼2000 ps in a drift-diffusion manner before ultimately accumulating on the {120} facets. This work provides a comprehensive visualization of spatiotemporal charge separation at the nano/microscale on semiconductor photocatalysts, which is beneficial for understanding the photocatalysis mechanism.

Exploring photoconduction mechanisms in Lead‐Free Cs3Sb2I9 single crystal thin films

The future commercial advancement of lead-halide perovskites (LHPs) faces obstacles due to the existence of harmful lead and the inadequate stability associated with these materials. To tackle this challenge, we have prepared a Cs3Sb2I9 perovskite single-crystalline thin film (Cs3Sb2I9 device) using a technique based on the restricted evaporation of solvents in space, aiming for effective photodetection. The photodetector in the current study shows a responsivity (R) of around 111 mA/W and a detectivity (D∗) of approximately 3.7 × 1012 Jones. While these performance metrics are comparable to other photodetectors, there is a need for additional optimization to match the capabilities of commercial silicon and germanium-based counterparts. The examination of ultrafast transient absorption provides insights into the essential photophysics inherent in Cs3Sb2I9. The synergy between the deformation potential and the Fröhlich effect plays a crucial role in shaping electronic dynamics. This synergy leads to the self-trapping of charge carriers, resulting in the creation of localized polarons within the Cs3Sb2I9 lattice within just some picoseconds. The restriction of carrier mobility within Cs3Sb2I9 arises from the self-capturing and confinement of small polarons (SPs). Furthermore, it was noted that SPs in a localized state could undergo absorption into an elevated state (photon energy ⁓ 1.60 eV). This process effectively facilitates the charge carriers' mobilization to a more dispersed eigenstate. Our research provides essential comprehension into the light-induced processes of lead-free halide perovskites (LFHPs), offering valuable insights for their prospective use in optoelectronics as promising future semiconducting agents.

Kondo effects in variable-valence manganese-substituted thulium selenide

The MnXTm1‒XSe (0 ≤ Х ≤ 0.2) solid solutions have been first synthesized and their structural, magnetic, and transport properties have been studied in the temperature range of 80–1000 K and magnetic fields of up to 12 kOe. The surface morphology of the samples has been examined and the chemical analysis has been carried out. It is shown that the valence change with the increasing substitution concentration is accompanied by a change in the lattice parameter and a decrease in the magnetic moment of the samples. The Kondo temperatures caused by the manganese and thulium subsystem have been found in the low- and room-temperature regions. The temperature of localization of small-radius polarons has been determined. A drastic decrease in the relaxation time in the range of the manganese ion percolation through the lattice in the MnXTm1‒XSe system has been established. The change of the current carrier type upon variation in the temperature and substitution concentration was determined from the Seebeck coefficient. A high-temperature extremum of thermopower was revealed, which is explained within the framework of the Anderson model.

Reduction of (100)-Faceted CeO2 for Effective Pt Loading.

Although the function and stability of catalysts are known to significantly depend on their dispersion state and support interactions, the mechanism of catalyst loading has not yet been elucidated. To address this gap in knowledge, this study elucidates the mechanism of Pt loading based on a detailed investigation of the interaction between Pt species and localized polarons (Ce3+) associated with oxygen vacancies on CeO2(100) facets. Furthermore, an effective Pt loading method was proposed for achieving high catalytic activity while maintaining the stability. Enhanced dispersibility and stability of Pt were achieved by controlling the ionic interactions between dissolved Pt species and CeO2 surface charges via pH adjustment and reduction pretreatment of the CeO2 support surface. This process resulted in strong interactions between Pt and the CeO2 support. Consequently, the oxygen-carrier performance was improved for CH4 chemical looping reforming reactions. This simple interaction-based loading process enhanced the catalytic performance, allowing the efficient use of noble metals with high performance and small loading amounts.

Electron Localization and Mobility in Monolayer Fullerene Networks.

The novel 2D quasi-hexagonal phase of covalently bonded fullerene molecules (qHP C60), the so-called graphullerene, has displayed far superior electron mobilities, if compared to the parent van der Waals three-dimensional crystal (vdW C60). Herein, we present a comparative study of the electronic properties of vdW and qHP C60 using state-of-the-art electronic-structure calculations and a full quantum-mechanical treatment of electron transfer. We show that both materials entail polaronic localization of electrons with similar binding energies (≈0.1 eV) and, therefore, they share the same charge transport via polaron hopping. In fact, we quantitatively reproduce the sizable increment of the electron mobility measured for qHP C60 and identify its origin in the increased electronic coupling between C60 units.

Investigation of spectroscopic and electrical properties of doped poly(pyrrole-3-carboxylic acid)

The spectroscopic and electrical properties of poly(pyrrole-3-carboxylic acid) doped with p-TSA- (p-toluenesulfonate) and AQS- (anthraquinone sulfonate) were investigated. The variation in electrical conductivity as a function of temperature shows that the systems have semiconductor-like electrical characteristics. The investigated polymers exhibit 3D conductivity and less than 0.6 eV energy gaps. The IR and Raman spectra show that the charge carriers are polarons and bipolarons. Doping the poly(pyrrole-3-carboxylic acid) increases the number of charge carriers. Electron paramagnetic resonance has shown that localized polarons and bipolarons are formed within these polymers.

EW METAL/SUPERCONDUCTOR-INSULATOR TRANSITIONS AND THEIR EFFECTS ON HIGH-TCSUPERCONDUCTIVITY INUNDERDOPED AND OPTIMALLY DOPED CUPRATES

Abstract.Anew approach to the metal/superconductor-insulator transition in doped cuprates by studying the polaron formation and localization of doped charge carriers (holes) in them and the possibility of transforming a metallic or superconducting system into an insulatorwas developed. Amore suitable criterion for such a phase transition by comparing the bandwidth (or Fermi energy) of large polarons with their binding energies in the cuprateswas derived. The possibility of the metal/superconductor-insulator transition and phase separation in doped cuprates resulting in the formation of competing metallic/superconducting and insulating phases in underdoped, optimally doped and even in overdoped high-Tccuprateswas predicted. Then the possible detrimental and beneficial effects of the different disorders (e.g. polaron formation and charge-density-wave transition) and the coexisting insulating and superconducting phases on the critical temperature 𝑇𝑐of the superconducting transition of underdoped and optimally doped cuprateswas examined. The actual superconducting transition temperature 𝑇𝑐in these materials using the theory of Bose-liquid superconductivity, and not theBardeen-Cooper-Schrieffer-like theory of Fermi-liquid superconductivity, which is incapable of predicting the relevant value of 𝑇𝑐in high-𝑇𝑐cuprateswas determined. We find thatthe suppressing of the polaronic and charge-density-waveeffects in optimally doped cuprates results in the enhancement of 𝑇𝑐, while some lattice defects (e.g., anion vacancies) in the cuprates may strongly affect, on 𝑇𝑐and enhance high-𝑇𝑐superconductivity in them.

Coherent Transient Localization Mechanism of Interchain Exciton Transport in Regioregular P3HT: A Quantum-Dynamical Study.

Transient localization has been proposed as a transport mechanism in organic materials, for both charge carriers and excitons. Here, we characterize a quantum coherent transient localization mechanism using full quantum simulations of an H-aggregated model system representative of regioregular poly(3-hexylthiophene) (rrP3HT). A Frenkel-Holstein Hamiltonian parametrized from first principles is considered, including local high-frequency modes and anharmonic, site-correlated interchain modes. Quantum-dynamical calculations are carried out using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for a 13-site system with 195 vibrational modes, under periodic boundary conditions. It is shown that temporary localization of exciton polarons alternates with resonant transfer driven by interchain modes. While the transport process is mainly determined by exciton-polarons at the low-energy band edge, persistent coupling with the excitonic manifold is observed, giving rise to a nonadiabatic excitonic flux. This elementary transport mechanism remains preserved for limited static disorder and gives way to Anderson localization when the static disorder becomes dominant.

The Role of Water Molecules on Polaron Behavior at Rutile (110) Surface: A Constrained Density Functional Theory Study.

Understanding the behavior of a polaron in contact with water is of significant importance for many photocatalytic applications. We investigated the influence of water on the localization and transport properties of polarons at the rutile (110) surface by constrained density functional theory. An excess electron at a dry surface favors the formation of a small polaron at the subsurface Ti site, with a preferred transport direction along the [001] axis. As the surface is covered by water, the preferred spatial localization of the polarons is moved from the subsurface to the surface. When the water coverage exceeds half a monolayer, the preferred direction of polaron hopping is changed to the [110] direction toward the surface. This characteristic behavior is related to the Ti3d-orbital occupations and crystal field splitting induced by different distorted structures under water coverage. Our work describes the reduced sites that might eventually play a role in photocatalysis for rutile (110) surfaces in a water environment.

Thermal Disorder-Induced Strain and Carrier Localization Activate Reverse Halide Segregation.

The reversal of halide ions is studied under various conditions. However, the underlying mechanism of heat-induced reversal remains unclear. This work finds that dynamic disorder-induced localization of self-trapped polarons and thermal disorder-induced strain (TDIS) can be co-acting drivers of reverse segregation. Localization of polarons results in an order of magnitude decrease in excess carrier density (polaron population), causing a reduced impact of the light-induced strain (LIS - responsible for segregation) on the perovskite framework. Meanwhile, exposing the lattice to TDIS exceeding the LIS can eliminate the photoexcitation-induced strain gradient, as thermal fluctuations of the lattice can mask the LIS strain. Under continuous 0.1Wcm⁻2 illumination (upon segregation), the strain disorder is estimated to be 0.14%, while at 80°C under dark conditions, the strain is 0.23%. However, in situ heating of the segregated film to 80°C under continuous illumination (upon reversal) increases the total strain disorder to 0.25%, where TDIS is likely to have a dominant contribution. Therefore, the contribution of entropy to the system's free energy is likely to dominate, respectively. Various temperature-dependent in situ measurements and simulations further support the results. These findings highlight the importance of strain homogenization for designing stable perovskites under real-world operating conditions.

Correlation between Dynamics of Polaronic Photocarriers and Photoelectrochemical Performance in Mo-Doped Bismuth Vanadate.

Bismuth vanadate (BiVO4) has received intense research interest due to its outstanding performance for solar water splitting, and doping it with molybdenum (Mo) ions can effectively boost photoelectrochemical performance. In this material, highly localized polarons play a key role in the photoconversion process. Herein, we uncovered the influence of Mo dopants on the dynamics of polaronic transient species using transient absorption spectroscopy. We find that the preexisting electron small polarons stemming from the thermal ionization of dopants provide additional centers to capture itinerant holes, which significantly decrease the hole lifetime. However, the introduction of dopants increases the lifetime of self-trapped excitons that arise from the binding of electron polarons and holes. The dependence of the photoelectrochemical performance of BiVO4 photoelectrodes on doping levels can be well explained by combining the dopant effects on the lifetimes of delocalized and self-trapped transient species. Our findings provide guidance for rational optimization of dopant concentration to maximize the PEC efficiency.

Understanding Effects of Alkyl Side-Chain Density on Polaron Formation Via Electrochemical Doping in Thiophene Polymers.

Polarons exist when charges are injected into organic semiconductors due to their strong coupling with the lattice phonons, significantly affecting electronic charge-transport properties. Understanding the formation and (de)localization of polarons is therefore critical for further developing organic semiconductors as a future electronics platform. However, there are very few studies reported in this area. In particular, there is no direct in situ monitoring of polaron formation and identification of its dependence on molecular structure and impact on electrical properties, limiting further advancement in organic electronics. Herein, how a minor modification of side-chain density in thiophene-based conjugated polymers affects the polaron formation via electrochemical doping, changing the polymers' electrical response to the surrounding dielectric environment for gas sensing, is demonstrated. It is found that the reduction in side-chain density results in a multistep polaron formation, leading to an initial formation of localized polarons in thiophene units without side chains. Reduced side-chain density also allows the formation of a high density of polarons with fewer polymer structural changes. More numerous but more localized polarons generate a stronger analyte response but without the selectivity between polar and non-polar solvents, which is different from the more delocalized polarons that show clear selectivity. The results provide important molecular understanding and design rules for the polaron formation and its impact on electrical properties.

Effects of Acceptors on the Charge Photogeneration Dynamics of PM6-Based Solar Cells

In this work, we investigated the effects of different acceptors (IT−4F and PC71BM) on the charge dynamics in PM6-based solar cells. The correlation between different acceptors and the performance of organic solar cells was studied by atomic force microscope, steady-state absorption spectrum, transient absorption spectrum, and electrical measurements. Optical absorption exhibited that IT−4F has strong absorption in the near-infrared region for the active layer. Transient absorption measurements showed that different acceptors (IT−4F and PC71BM) had a significant influence on the behaviors of PM6 excitons and charge dynamics. That is, the exciton dissociation rate and delocalized polaron transport in the PM6:IT−4F active layer were significantly faster than that in the PM6:PC71BM active layer. The lifetime of localized polaron in the PM6:PC71BM active layer was longer than that in the PM6:IT−4F active layer. Conversely, the lifetime of delocalized polaron in the PM6:IT−4F active layer was longer than that in the PM6:PC71BM active layer. Electrical measurement analysis indicated that lower bimolecular recombination, higher charge transport, and charge collection ability were shown in the PM6:IT−4F device compared with the PM6:PC71BM device. Therefore, PM6:IT−4F solar cells achieved a higher power conversion efficiency (12.82%) than PM6:PC71BM solar cells (8.78%).

Theoretical insights into the hydroxyl-promoted H2 releasing reaction after H2O splitting on Pu-oxide surfaces

Within DFT+U-D3 scheme, we systematically investigate the reaction of H2O on Pu-oxide surfaces to reveal the effects of *H/*OH produced by water splitting on subsequent H2 releasing and surface oxidation. We find that both *H and *OH can promote H2 releasing, especially when PuO2 reduced to α-Pu2O3, but the micro mechanisms of *H/*OH driven H2 releasing process on PuO2(111) are different from α-Pu2O3(111) surface. On PuO2(111) surface, the adsorbed H2O and *H/*OH show a dynamic coupling with the localized electron/hole polarons induced by *H/*OH. Due to the significant heterogeneity of surface structure and Pu-coordination of α-Pu2O3(111), *H and *OH produce non-localized and localized electron polarons respectively, and notably facilitate the first-round exothermic H2 releasing process. After the first-round reaction, the residual surface O prefers to migrate into the inner layer to oxidize the α-Pu2O3 matrix, then the surface is recovered with *OH and ready for further H2 releasing reaction. Thus, we predict the eventual cyclic reaction on α-Pu2O3(111) surface, H2O + Pu2O3→*OH H2(g) + Pu2O3+, which can continually emit H2. In sum, our study provides new insights into the effects of surface structure and polarons on H2 releasing reaction, which we believe is of value to the Pu corrosion science.

Hopping conduction of localized polarons with scaling behaviour in ceramic composite (YCrO3)1 -x - (CoFe1.6Cr0.4O4)x

The percolative (YCrO3)1 -x – (CoFe1.6Cr0.4O4)x (x = 0.3, 0.5, 0.7, 1.0), ceramic composite belongs to a family of materials that exhibit colossal dielectric permittivity. Our fundamental interests lie in the investigation of the conduction mechanism of the composite from both microscopic and macroscopic perspectives. Here, all three ceramic composites have been chosen for a comparative study containing CFCO above fc, near fc, and below fc. The electrical conductivity of the composite follows Mott’s variable range-hopping model in the temperature range of 300 – 500 K, indicating domination of this model in conduction of localized polarons in the electrical behaviour. Parameters, including the dc conductivity, hopping, most probable hopping range, the activation energies involved in the said model, the density of localized states at the Fermi level, and relaxation were obtained and analyzed. Scaling behaviors of conductivity and imaginary part of complex impedance were investigated, showing a temperature-independent distribution of relaxation times. By looking at the impedance data at a variety of temperatures, semiconducting behaviour is demonstrated, which has been modelled by an equivalent circuit model that incorporates grain and grain boundary responses. The ferroelectric nature of the samples resembled lossy characteristics due to the presence of the CFCO phase. The correlation between conductivity and colossal apparent permittivity helps in better understand the conduction mechanism in ceramic composites comprising semiconducting and insulating phases.

Microstructure-dependent magnetic anisotropy in nanorod vanadium oxide room-temperature ferromagnetic thin films using reactive sputtering

Vanadium oxides (VOx) with a strong correlated electronic effect have been widely used in the photoelectric field due to their metal–insulator transition characteristics. More attention should be paid to the magnetic properties of VOx thin films when developing spintronic devices. In this study, we prepare a nanorod VOx thin film on a sapphire Al2O3(0001) substrate by reactive sputtering. The results indicate that the prepared film exhibits room-temperature ferromagnetic semiconductor behaviors and is a mixed phase VOx film. The ferromagnetic source of the prepared film is well explained by the local electronic polaron model. In addition, magnetic anisotropy seems to depend on the orientation of nanorods of the thin film, which aids in the design of functional spintronic devices by controlling the microstructure to change the direction of the magnetic moment. Such findings will pave the way for the use of VOx films in spintronics.