Metal-insulator Transition Temperature Research Articles

Effects of Thermal Energy on the Formation of Lattice Strain in VO2 Thin Films Grown on TiO2(001).

Film thickness is a well-known experimental parameter for controlling lattice strain in oxide films. However, due to environmental and resource conservation considerations, films need to be as thin as possible, increasing the need to find alternative factors for strain management. Herein, we present the importance of thermal energy as a factor for the formation of lattice strain in the oxide films, specifically focusing on the effects of laser fluence during pulsed laser deposition (PLD) on the in-plane lattice strain of vanadium dioxide (VO2) thin films grown on titanium dioxide (TiO2) (001). VO2 thin films were deposited using a KrF excimer laser (λ = 248 nm) at laser fluences ranging from 0.88 to 1.70 J/cm2. The film thickness ranged from 10-15 nm, below the critical thickness. Films grown at higher laser fluences exhibited smooth surfaces and completely strained in-plane lattices. In contrast, films grown at lower laser fluences displayed numerous small islands and relaxed in-plane lattice strain. The metal-insulator transition (MIT) temperature was lower for films grown at higher laser fluencies compared to those grown at lower laser fluences. It was also revealed that Ti-V interdiffusion occurs, forming a solid solution (V1-xTixO2) near the interface. These observations suggest that the thermal energy of the particles, influenced by laser fluence, is a critical factor in the formation of lattice strain in metal oxide films and also that laser fluence in PLD is an effective experimental parameter for strain management in oxide films. Our findings enhance the understanding of lattice strain formation in metal oxides and offer insights for establishing effective methods for controlling lattice strain in metal oxide films.

Modeling of the metal–insulator transition temperature in alio-valently doped VO2 through symbolic regression

The correlated semiconductor vanadium dioxide (VO2) exhibits an insulator–metal transition (IMT) near room temperature, which is of interest in various device applications. Precise IMT temperature control is crucial to determine the use cases across technologies such as thermochromic windows, actuators for robots or neuronal oscillators. Doping the cation or anion sites can modulate the IMT by several tens of degrees and control hysteresis. However, modeling the effects of control parameters (e.g., doping concentration, type of dopants) is challenging due to complex experimental procedures and limited data, hindering the use of traditional data-driven machine learning approaches. Symbolic regression (SR) can bridge this gap by identifying nonlinear expressions connecting key input parameters to target properties, even with small data sets. In this work, we develop SR models to capture the IMT trends in VO2 influenced by different dopant parameters. Using experimental data from the literature, our study reveals a dual nature of the IMT temperature with varying tungsten (W) doping concentrations. The symbolic model captures data trends and accounts for experimental variability, providing a complementary approach to first-principles calculations. Our feature-driven analysis across a broader class of dopants informs selectivity and provides qualitative insights into tuning phase transition properties valuable for neuromorphic computing and thermochromic windows.

Electrical transport properties of topographically demorphed films of La1-xBaxMnO3 grown on (00l)-oriented LaAlO3 substrate via spin-coating method

Herein, the crystal structure, valence, and electrical transport properties of La1-xBaxMnO3 films were examined by preparing La1-xBaxMnO3 films with varying Ba content (x = 0.22, 0.24, 0.26, 0.28) on LaAlO3 substrates using the sol-gel spin coating technique. The La1-xBaxMnO3 films prepared in the study exhibit topographically demorphed and non-dense characteristics in the microstructure, which is mainly caused by the Volmer-Weber growth mode of the films and the volatilization of organic matter during the sol-gel sintering process for films preparation. The increase of Ba doping leads to MnO6 deformation, decrease in grain boundary scattering, increase in Mn4+ ion content, and enhancement of both the double exchange mechanism and the Jahn-Teller effect. The four-probe results showed that the peak resistivity of the films decreased significantly with the increase in Ba2+ doping, and the metal-insulator transition temperature (Tp) shifted to higher temperatures. Peak temperature coefficient of resistivity (TCR) corresponding temperature (Tk) increased from 283.05 K (x = 0.22) to 309.31 K (x = 0.28), which was consistent with the trend of Tp; the peak value of TCR decreased with the increase of Ba doping. When x = 0.24, the Tk of the La1-xBaxMnO3 films reach room temperature (294.25 K) with TCR = 9.01 % K−1. In conclusion, the electrical properties of La1-xBaxMnO3 can be optimized by varying the amount of Ba doping to obtain La1-xBaxMnO3 films with room-temperature Tk and higher TCR values, providing reasonable data support and theoretical explanation for the Ba doping mechanism.

Optimized TCR and MR properties of La0.7-xSmxCa0.3MnO3 ceramics by Sm3+ doping

La0.7Ca0.3MnO3 (LCMO), with its high temperature coefficient of resistance (TCR) and large magnetoresistance (MR) effect, has emerged as a leading material in the field of new multi-functional ceramics. Enhancing the magnetoresistive effect of LCMO in low magnetic field and expanding its application range is one of the main research goals of this kind of materials. In this work, La0.7-xSmxCa0.3MnO3 (LSCMO) (0≤x≤0.09) ceramic targets with different doping amounts were successfully prepared by sol-gel method, and the influence mechanism of Sm3+ doping on the magnetoresistive effect and electrical transport performance of LSCMO was systematically analyzed. X-ray diffraction (XRD) results showed that the lattice parameters (a,b,c,V) of LSCMO decreased slightly, indicating that the doping amount of Sm3+ at the A-site increased. Scanning electron microscopy (SEM) showed that the grains were closely connected without obvious pores, the grain size decreased slightly. The ρ-T curve shows an obvious metal-insulator (M-I) transition, with the increase of Sm3+, the increase of resistivity, metal-insulator transition temperature (TMI) and ferromagnetic-paramagnetic (FM-PM) transition temperature (TC) move towards low temperature. When x=0.015, the peak value of TCR reaches 41.94%·K-1, and when x= 0.06, the peak value of MR reaches 80.25%, both TCR and MR are improved. The results show that: Sm3+ doping can effectively enhance the electromagnetic transport performance of LSCMO. This material has a very wide application prospect in the fields of magnetic storage, and infrared detection.

Structural, magnetic, and electrical properties of LaMnO3 doped with Na by microwave-assisted synthesis method

Abstract The electric and magnetic transport properties of La1-xNaxMnO3 (x = 0.15, 0.2, 0.25, 0.3) are studied using the X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and four-probe resistivity measurement method. The samples are synthesized by the method of microwave heating. The obtained samples show phase changes around x = 0.25. The magnetization measurement shows the curie temperature (Tc) increases as the level of doping increases and saturates around x = 0.25. The resistivity measurement dramatically decreases at a metal-insulator transition temperature close to Tc. These findings are discussed along with the structural features of different Na concentrations.

The visible magneto-optical response of RE1-xAxMnO3manganites: relationship with the charge component of the material.

Magnetic circular dichroism (MCD) spectroscopy for manganite films of various compositions and morphologies has been studied in the range of 1.2-3.7 eV. The primary focus was on the temperature behavior of the MCD spectra, as well as the magnetization and resistivity of the films. The data obtained were analyzed in comparison with magneto-optical spectroscopy of the Kerr rotation (KR) on both single crystal and thin film of manganites. It has been established that the MCD response at 2.3 eV is typical for manganites transitioning into a conducting state. Consequently, it reflects a change in the band structure of the material. This response is also observed in the KR spectrum of manganites in the range 2.3-2.6 eV below the metal-insulator transition temperature. These findings complement the understanding of the electronic structure of manganites in general. Moreover, they also provide a basis for the search for new functional materials.

Reversible modulations of insulator–metal transition in an epitaxial VO2 film through thermal crystallization and femtosecond laser-induced-amorphization of capping Ge2Sb2Te5 layer

We demonstrate reversible modulation of an insulator–metal transition (IMT) of a VO2 film grown on an Al2O3 (001) substrate through crystallization and re-amorphization of a chalcogenide germanium–antimony–telluride (Ge2Sb2Te5: GST) capping layer. After succeeding in the negative shift of IMT temperature (Tr) of the VO2 film through the crystallization of the GST layer accompanied by volume reduction, we performed re-amorphization of the crystalline GST by femtosecond laser irradiation. Under the optimized conditions of laser irradiation considering the penetration depth, re-amorphization of the GST layer was fully achieved, resulting in the shift-back of Tr toward a high-temperature side. Such a reversal of IMT through the crystallization and re-amorphization of the capping GST layer was demonstrated over two cycles. It was suggested that capping GST effectively induces interfacial strain modifications in the VO2 film underneath. Although the shifts in the IMT are still small, reversible modulation of IMT shown here will be beneficial for applications of VO2 films with controllable IMT.

The magnificent manifestation of Nd0.7Sr0.3MnO3 ceramics: A comprehensive exploration of different wet chemical routes via sol-gel and thermal treatment methods

The mixed-valence manganite has sparked immense interest owing to its intriguing colossal magnetoresistance (CMR) effect. However, a research gap exists in the current literature regarding Nd-based manganites with intermediate bandwidth. This paper addresses this gap by investigating Nd0.7Sr0.3MnO3 (NSMO) ceramics prepared through two different synthesis approaches: the newly developed thermal treatment method (TT) and the well-established sol-gel method (SG). The XRD analysis revealed a single high-purity NSMO phase with insignificant alteration in Mn-O bond length and Mn-O-Mn bond angle for both samples. Nevertheless, SG exhibited larger particle size (209.3 nm) and better crystallinity in comparison to TT (178.6 nm). These factors significantly affect the magnetic transition width while preserving the Curie temperature, TC ≈ 246 K. Regarding electrical behaviour, TT showed a robust suppression of the metal-insulator transition temperature, TMI from 224 K (SG) to 182 K. This difference can be attributed to the existence of magnetic disordered layer at the grain boundaries. Consequently, this resulted in an increased scattering process and elevated resistivity from 8 Ω (SG) to 22 Ω (TT). Various theoretical models were employed to elucidate the transport mechanism at different temperature ranges. Interestingly, both samples displayed distinct magneto-transport behaviour despite negligible differences in their structural parameters. The occurrence of magnetoresistance peak in SG suggests a pronounced intrinsic CMR effect, while its absence in TT implies a dominant influence of extrinsic CMR. These findings highlight that the physical properties of the manganite are not only governed by their intragrain properties (TC) but are also influenced by intergrain contributions (TMI) when various synthesis routes were utilised.

Investigation on characteristic of vanadium trioxide insulation mixed with metal powder for rare-earth barium copper oxide coils

This study examined the turn-to-turn contact resistance (R ct) between rare-earth barium copper oxide (REBCO) tapes and layers of vanadium trioxide (V2O3) and V2O3 mixed with metal powder mixture. V2O3 in single crystal structure was electrically characterised to exhibit resistivity with negative temperature dependence, allowing the turn-to-turn insulation to self-regulate the current bypass between REBCO tapes. To facilitate effective quench protection of V2O3-insulated REBCO magnets above the metal-insulator transition temperature (T rt), R ct must be further reduced to a level similar to those of non- and metal as insulated (NI and MI) REBCO magnets. Thus, we explored the mixing of conductive metal powders such as molybdenum (Mo) with V2O3 paste and investigated the transition properties of R ct. The resistance versus temperature characteristics, microscopic morphologies of the V2O3 layers, and thermal conductivity (k v) were appropriately assessed to determine the effects of mixing the metal powder with V2O3. The R ct of virgin V2O3 exhibited variations of 107–105 μΩ cm2 under 77–293 K. As the mixing concentration of the metal powder was increased, the reduction magnitude on R ct increased for > T rt (approximately 150 K). Furthermore, the transition slope became gentler for a wider temperature range of < T rt. For metal powder concentrations exceeding 50 wt%, R ct decreased by approximately 2 orders of magnitude (∼103 μΩ cm2) for > 150 K compared with that for virgin V2O3 paste. Moreover, compared to that of pure V2O3, k v demonstrated a remarkable increase of approximately 352% at 91 K for Mo powder mixed at a concentration of 60 wt%. The improved electrical and thermal properties of the V2O3 insulation layer owing to the mixing of metal powders can help REBCO magnets operate in an insulated state under normal conditions and effectively convert to a non-insulated state under quenching.

Variation of the metal-insulator phase transition temperature in VO2: An overview of some possible implementation methods

The great interest in VO2, which has stimulated a large number of studies and publications in recent decades, is caused by the reversible metal-insulator phase transition (MIT) that occurs at T = 68 °C and is accompanied by the transformation of a low-temperature dielectric (semiconductor) monoclinic phase into a high-temperature metallic phase with a rutile structure. Despite the ongoing discussion about the physical mechanism of this transition, the concomitant rapid change in the electrical and optical characteristics of the material by several orders of magnitude already finds numerous applications in optics, optoelectronics and sensors. At the same time, it became obvious that both the number and performance of the applications of VO2 would greatly increase, if it were possible to decrease the temperature of the phase transition without deterioration of other properties. This issue has become the subject of numerous studies. Mechanical stress and oxygen vacancies in the VO2 lattice, the concentration of free charge carriers, tuned by impurity doping or implantation, have been investigated and discussed as the main factors affecting the transition temperature. In this review, we intend to summarize and analyze the literature data on these ways, primarily those which are most efficient in influencing the transition temperature while maintaining a significant change in the modulation characteristics.

Effect of Zn2+ doping on the structure and metal–insulator transition of La2CuO4 ceramics

The structure and electrical properties of layered perovskites La2Cu1-xZnxO4 have been investigated. The resistivity of these samples has been measured in the temperature range of 90–300 K. The Zn2+doping induces a shift of the metal–insulator transition temperature towards the high-temperature region. The d9 electronic configuration of Cu2+ in La2CuO4 leads to an uneven distribution of electron clouds, inducing axial stretching of the surrounding oxygen octahedra and resulting in Jahn-Teller distortion. After Zn2+ doping, the charge compensation induces a partial transformation of Cu2+ to Cu+. As a result, the positions occupied by Cu2+ are replaced by Zn2+ and Cu+ with perfectly symmetric electron clouds in the 3d10 outer electronic configuration, thus suppressing the Jahn-Teller effect. This shifts the phonon scattering, which is responsible for the emergence of the metal’s conductive properties, to higher temperatures.

Optimized TCR and MR of La0.67Ca0.33-xSrxMnO3: Ag0.15 ceramics by Sr doping

Herein, in order to successfully create high-density La0.67Ca0.33-xSrxMnO3: Ag0.15 (LCSMO: Ag) ceramics (0 ≤ x ≤ 0.05), this research combines sol-gel and solid-phase processes. LCSMO: Ag ceramics electrical transport and magnetic properties were thoroughly investigated. XRD verified that the high-purity orthorhombic perovskite structure (space group Pnma) was produced. As Sr doping rises, the grain size grows, the single electron bandwidth increases, the metal-insulator transition temperature (TMI) continues to climb, and temperature coefficient of resistance (TCR) and magnetoresistance (MR) steadily drop. When x = 0.04, the TMI reaches 302.2 K. At this time, the TCR peak (TCRpeak) reaches 24.0 %·K−1 and the MRpeak reaches 49.2 %. These results demonstrate that Sr may effectively raise the TMI of LCMO: Ag ceramics to room temperature while maintaining high TCR and MR Values, which benefits the material's potential applications in devices such as uncooled infrared detectors.

Electrical resistivity and magnetoresistance of Sr3Fe2+xMo1–xO9–3x/2 (x = 0.45, 0.60, and 1.00) prepared by the solid-state reaction

In this paper, we focus on understanding the behavior of the temperature dependence of electrical resistivity (ρ) and magnetoresistance (MR) in Sr3Fe2+xMo1–xO9–3x/2 perovskites with x = 0.45, 0.60, and 1.00. Three polycrystalline materials were synthesized using conventional solid-state reaction techniques. Rietveld analysis of room-temperature X-ray diffraction data reveals that all three compositions crystallize in tetragonal symmetry; where x = 0.45 and 0.60 undergo an I4/mcm space group, while x = 1.00 adopts a P4/mmm space group. Electrical resistivity measurements carried out at magnetic fields (0 and 1 T) reveal the presence of a transition from the ferromagnetic phase to the paramagnetic phase in the vicinity of the TMI. The metal-insulator transition temperature (TMI) decreases with increasing Fe content and rises slightly with changing magnetic field from 0 to 1 T. The magnetoresistance (MR) of the SrFeO3–δ sample (x = 1.00) has a large value of about 63.6 %, indicating that it can be exploited for certain devices. A variety of theoretical models are used to study the behavior of electrical resistivity in various temperature ranges, showing excellent agreement with experimental results.

Nitrogen Doping in VO2 Thin Films on Synthetic Mica Substrates Through Mist Chemical Vapor Deposition: Lowering the Metal–Insulator Transition Temperature Toward Smart Windows

AbstractIn this study, nitrogen (N) is doped into VO2 thin films through mist chemical vapor deposition (CVD), and the effect of the doping on metal–insulator transition (MIT) temperatures is investigated. The N‐doped VO2 thin films are grown on an SnO2 buffer layer. The N‐doped VO2 lattice spacing tends to expand as the growth temperature decreased, which indicates that the incorporation of N into the lattice is derived from the Ethylenediamine. Secondary ion mass spectrometry (SIMS) is conducted to investigate the relationship between the decrease in the transition temperature and N concentration. The results reveal that the sample grown at 425 °C contains approximately 2 × 1020 cm−3 of N. Thus, efficient nitrogen doping can be achieved through mist CVD. The temperature‐resistance characteristics of VO2 thin films are measured to investigate their electrical properties and MIT temperatures. The results reveal that for undoped samples, the transition temperature slightly decreases with the decrease in the growth temperature. Furthermore, the sample grown at 425 °C exhibits a considerable change in resistance because of MIT at approximately 29.5 °C. These results prove the potential of using mist CVD N‐doped thin films for smart window applications to address future energy problems.

Tuning the metal-insulator transition temperature by the controlled generation of oxygen vacancies on La0.7Ca0.3MnO3-x epitaxial thin films

This report presents a study on the effect of the controlled generation of oxygen vacancies on the electronic transport properties of La0.7Ca0.3MnO3-x epitaxial thin films. Oxygen defects cause significant changes in the crystalline structure of mixed-valence manganites and lower the Mn valence, resulting in a shift of the metal-insulator transition to lower temperatures due to modifications in the angles and lengths of Mn-O bonds. Experiments using Polarized Extended X-Ray Absorption Fine Structure Spectroscopy have provided strong evidence that oxygen vacancies are mainly formed in the basal plane of the MnO6 octahedra. The accurate control of oxygen vacancies generation opens possibilities for managing and enhancing the magneto-electronic properties of epitaxial manganite thin films with potential in industrial applications.

Terahertz beam reconfigurable phase gradient metasurface of VO2 based on different metal–insulator transition temperatures

Terahertz beam reconfigurable phase gradient metasurface of VO2 based on different metal–insulator transition temperatures

Nanoelectrical monitoring the nonvolatile behavior of VO2 under multi-field stimulate by conductive atomic force microscopy

The ability to control the metal–insulator phase transition is crucial for future neuromorphic and memristive technologies. The key goal of this implementation is to understand and control the nanoscale mechanisms that support these two fundamental switching modalities (volatile and non-volatile states). Here, by adjusting the temperature (60 °C) close to the metal–insulator transition temperature, external application of low voltage (1 V) and laser excitation (18.6 mw · cm−2) can complete the metal–insulator transition of vanadium dioxide (VO2). The nonvolatile behavior of the VO2 devices was nanoelectrically monitored using conductive atomic force microscopy (C-AFM). Using this technique, we were able to reversibly transform the local switching response from volatile to nonvolatile, providing a unique approach for nanoelectrical monitoring in phase transition materials.

Insulator Metal Transition-Based Selector in Crossbar Memory Arrays

This article investigates resistive random access memory (ReRAM) crossbar memory arrays, which is a notable development in non-volatile memory technology. We highlight ReRAM’s competitive edge over NAND, NOR Flash, and phase-change memory (PCM), particularly in terms of endurance, speed, and energy efficiency. This paper focuses on the architecture of crossbar arrays, where memristive devices are positioned at intersecting metal wires. We emphasize the unique resistive switching mechanisms of memristors and the challenges of sneak path currents and delve into the roles and configurations of selectors, particularly focusing on the one-selector one-resistor (1S1R) architecture with an insulator–metal transition (IMT) based selector. We use SPICE simulations based on defined models to examine a 3 × 3 1S1R ReRAM array with vanadium dioxide selectors and titanium dioxide film memristors, assessing the impact of ambient temperature and critical IMT temperatures on array performance. We highlight the operational regions of low resistive state (LRS) and high resistive state (HRS), providing insights into the electrical behavior of these components under various conditions. Lastly, we demonstrate the impact of selector presence on sneak path currents. This research contributes to the overall understanding of ReRAM crossbar arrays integrated with IMT material-based selectors.

Growth and phase transition properties of Ta-doped VO2 single crystal microtubes

Ta-doped VO2 single crystal microtubes with rectangular cross-sections were grown by a thermal oxidation process from V–Ta alloy foils in air. Characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy revealed their morphology as hollow, rod-like single crystals. Phase transition examinations used differential scanning calorimetry demonstrated that Ta doping can effectively decrease the metal-insulator transition temperatures of VO2 by approximately 9.8 °C/at. % Ta. The phase transition behavior and growth mechanism of Ta-doped VO2 single crystal microtubes are also elucidated.

Effects of oxygen vacancies and interfacial strain on the metal-insulator transition of VO2 nanobeams.

The role of oxygen vacancies and interfacial strain on the metal-insulator transition (MIT) behavior of high-quality VO2 nanobeams (NBs) synthesized on SiO2/Si substrates employing V2O5 as a precursor has been investigated in this research. Selective oxygen vacancies have been generated by argon plasma irradiation. The MIT is progressively suppressed as the duration of plasma processing increases; in addition, the temperature of MIT (TMIT) drops by up to 95 K relative to the pristine VO2 NBs. Incorporating oxygen vacancies into VO2 may increase its electron concentration, which might shift the Fermi levels upward, strengthen the electronic orbital overlap of the V-V chains, and further stabilize the metallic phase at lower temperatures, based on first-principles calculations. Furthermore, in order to evaluate the influence of substrate-induced strain in our situation, the MIT in two distinct types of VO2 NB samples is examined without metal contacts by using the distinctive light scattering characteristics of the metal (M) and insulator (I) phases (i.e., M/I domains) by optical microscopy. It is found that the domain structures in the "clamped" NBs persisted up to ∼453 K, while the "released" NBs (transferred to a new substrate) did not exhibit any domain structures and turned into an entirely M phase with a dark contrast above ∼348 K. When combined with first-principles calculations, the electronic orbital occupancy in the rutile phase contributes to explaining the interfacial strain-induced modulation of MIT. The current findings shed light on how interfacial strain and oxygen vacancies impact MIT behavior. It also suggests several types of control strategies for MIT in VO2 NBs, which are essential for a broader spectrum of VO2 NB applications.