Sharp Metal-insulator Transition Research Articles

Electronic band structure of Ti2O3 thin films studied by angle-resolved photoemission spectroscopy

Ti2O3 exhibits a unique metal-insulator transition (MIT) at approximately 450 K over a wide temperature range of ~ 150 K. This broad MIT accompanied by lattice deformation differs from the sharp MITs observed in most other transition-metal oxides. A long-standing issue is determining the role of electron-electron correlation in the electronic structure and MIT of Ti2O3. However, the lack of information about the band structure of Ti2O3 has hindered investigating the origin of its unusual physical properties. Here, we report the electronic band structure of insulating Ti2O3 films with slight hole doping by angle-resolved photoemission spectroscopy (ARPES). ARPES showed clear band dispersion on the surface of single-crystalline epitaxial films. The experimentally obtained band structures were compared with band-structure calculation results based on density functional theory (DFT) with generalized gradient approximation + U correction. The obtained band structures are in good agreement with the DFT calculations at U = 2.2 eV, suggesting that electron-electron correlation plays an important role in the electronic structure of Ti2O3. Furthermore, the detailed analyses with varying U suggest that the origin of the characteristic MIT in Ti2O3 is a semimetal-semimetal or semimetal-semiconductor transition caused by changes in the Fermi surface due to lattice deformation.

Effect of chemical doping on memristive behavior of VO2 microcrystals

Strongly correlated oxides, such as vanadium dioxide that undergoes a sharp metal-insulator transition when triggered by different stimuli, are of high relevance for novel electronic devices. In this work, we show the variation in threshold voltage of memristor behavior with systematic doping of tungsten (W) in VO2 crystals grown by the vapor transport method. Chemical doping effects on metal insulator transition are further correlated with Raman spectroscopy studies and differential scanning calorimetry studies. Furthermore, bi-polar threshold switching of VO2 memristor behavior is demonstrated in VO2 microcrystals with different contents of W. Threshold voltage for electrical triggering in W doped VO2 is reduced to about 0.547 V from 2.27 V of undoped VO2.

Metal–Insulator Transitions in Stable V2O3 Thin Films: Atomic Layer Deposition and Postdeposition Annealing Studies

New, stable V2O3 thin films are prepared using VO(acac)2 and ozone (O3) by atomic layer deposition (ALD) and a post‐treatment process on a c‐Al2O3 substrate. The obtained V2O3 thin films are crystalline, and have single‐phase and rhombohedral structure. The present ALD process yields a thickness of 48 nm by 1000 ALD cycles at a temperature of 200 °C; it portrays uniformity on planar sapphire substrates. The V2O3 films (48 nm) exhibit a sharp metal–insulator transition (MIT) at a temperature of ≈165 K with five orders of magnitude change in electrical resistivity.

Probing thermal properties of vanadium dioxide thin films by time-domain thermoreflectance without metal film* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61825102, 51872038, and 52021001) and the “111” Project, China (Grant No. B18011).

Vanadium dioxide (VO2) is a strongly correlated material, and it has become known due to its sharp metal-insulator transition (MIT) near room temperature. Understanding the thermal properties and their change across MIT of VO2 thin film is important for the applications of this material in various devices. Here, the changes in thermal conductivity of epitaxial and polycrystalline VO2 thin film across MIT are probed by the time-domain thermoreflectance (TDTR) method. The measurements are performed in a direct way devoid of deposition of any metal thermoreflectance layer on the VO2 film to attenuate the impact from extra thermal interfaces. It is demonstrated that the method is feasible for the VO2 films with thickness values larger than 100 nm and beyond the phase transition region. The observed reasonable thermal conductivity change rates across MIT of VO2 thin films with different crystal qualities are found to be correlated with the electrical conductivity change rate, which is different from the reported behavior of single crystal VO2 nanowires. The recovery of the relationship between thermal conductivity and electrical conductivity in VO2 film may be attributed to the increasing elastic electron scattering weight, caused by the defects in the film. This work demonstrates the possibility and limitation of investigating the thermal properties of VO2 thin films by the TDTR method without depositing any metal thermoreflectance layer.

Effect of Y doping on transport properties of La0.8Sr0.2MnO3 polycrystalline ceramics

In this study, La0.8-xYxSr0.2MnO3 (LYSMO) polycrystalline ceramics were prepared by means of sol-gel technique using methanol as solvent. X-ray diffraction (XRD) showed all samples to possess standard perovskite structure. Scanning electron microscopy (SEM) revealed samples with high compactness and grain size from 27.80 to 29.73 μm. Resistivity–temperature tests indicated sharp metal-insulator transition behavior of all samples accompanied by rapid transformation from ferromagnetism to paramagnetism (FM-PM). As Y3+ doping amounts rose, radius of A-site ions decreased, metal-insulator transition temperature (Tp) of polycrystalline samples shifted to lower temperatures, and resistivity increased. Temperature coefficient of resistance (TCR) and magnetoresistance (MR) were affected by introduction of Y3+. At x = 0.06, peak TCR and peak MR reached 4.85% K−1 and 52.34%, respectively. Using double exchange (DE) interaction mechanism, electric transport performances of as-prepared ceramics were explained. These findings look promising for future applications of LYSMO materials in magnetic devices and infrared detectors.

Van der Waals Heteroepitaxial VO2/Mica Films with Extremely Low Optical Trigger Threshold and Large THz Field Modulation Depth

AbstractMaterials with low optical trigger threshold and large THz field modulation depth are highly desirable for THz switches and modulators. VO2, a classical strongly correlated oxide undergoing a sharp metal–insulator transition (MIT), is advantageous for its large THz field modulation depth by virtue of MIT. However, for optically triggered VO2‐based THz switches and modulators, high pump laser intensity is normally required to trigger the MIT, adding considerable costs to THz systems. Reducing optical trigger threshold of VO2 is of substantial interest in the THz science. Here, van der Waals (vdW) heteroepitaxial VO2/mica films are reported with extremely low optical trigger threshold and large THz field modulation depth. Such VO2 films can be used to fabricate low cost but high performance THz switches and modulators. The vdW‐bonded VO2 film–substrate interface allows for negligible disturbance to the VO2 lattice by the mica substrate and reduces thermal conduction to the substrate, boosting the MIT and thus, reducing the pump threshold.

Competing phases in epitaxial vanadium dioxide at nanoscale

Phase competition in correlated oxides offers tantalizing opportunities as many intriguing physical phenomena occur near the phase transitions. Owing to a sharp metal-insulator transition (MIT) near room temperature, the correlated vanadium dioxide (VO2) exhibits a strong competition between insulating and metallic phases, which is important for practical applications. However, the phase boundary undergoes a strong modification when strain is involved, yielding complex phase transitions. Here, we report the emergence of nanoscale M2 phase domains in VO2 epitaxial films under anisotropic strain relaxation. The competing phases of the films are imaged by multilength-scale probes, detecting the structural and electrical properties in individual local domains. Competing evolution of the M1 and M2 phases indicates the critical role of lattice-strain on both the stability of the M2 Mott phase and the energetics of the MIT in VO2 films. This study demonstrates how strain engineering can be utilized to design phase states, which allow deliberate control of MIT behavior at the nanoscale in epitaxial VO2 films.

Improved temperature coefficient of resistance in La1-xCaxMnO3:Ag0.2 (0.25 ≤ x≤ 0.33) ceramics prepared by sol–gel method

A series of La1−xCaxMnO3:Ag0.2 (0.25 ≤ x ≤ 0.33) polycrystalline ceramic materials were prepared by sol–gel method using methanol as solvent. X-ray diffraction analysis showed that all samples consisted of pure perovskite structurewith no second phase. The grain size and morphology of the samples were studied by scanning electron microscopy. The results showed the samples with good crystallinity, large grain size, and high density. X-ray photoelectron spectroscopy measurement results shows that Ag exists in manganese oxides in two forms (Ag and Ag+). The resistivity temperature curves showed that all samples exhibited a sharp metal–insulator transition. At x = 0.28, the temperature coefficient of resistance was 78.8%·K−1. The magnetization-temperature curves also showed a sharp ferromagnetism–paramagnetism transition, and the Curie temperature increased gradually. These ceramics act as promising candidates for application in infrared and bolometric detectors.

A spin-valve-system in ferromagnetic semiconductor Sr2FeMoO6/SrMoO4

In this study, we designed a novel magnetic semiconductor Sr2FeMoO6/SrMoO4 with a sharp metal-insulator transition based on the spin-glass freezing and antiferromagnetic transition behavior, which has been confirmed by various magnetic and transport measurements. To investigate the charge transport mechanisms of the sample, the electrical resistivities have been measured over a wide range of temperatures from 400 K down to liquid helium temperatures under different magnetic fields. The typical semiconductor behavior of the sample is observed. The relatively high conduction behavior is blocked below 80 K, and thereafter into the insulator state. The Vogel-Fulcher and the Mott variable-range-hopping (VRH) law are also used to study its conduction features and the relationship between magnetic and electrical properties. The spin-glass like component around the Sr2FeMoO6 crystal is initially observed by TEM and this provides a robust evidence able to verify the results from magnetic measurements. The novel features of the metal-insulator transition shielding external magnetic effects in highly disordered Sr2FeMoO6 double perovskite is dominantly controlled by a new spin-valve-system present in the spin-glass layers. Our insights into the role of the B-site disorder, spin-glass layer around the soft ferromagnetic grains, grain boundaries, and non-magnetic second phase effects demonstrate how the static and dynamic properties of magnetic materials might be tuned by designing the composition with a high B-site cation disordered matrix ferromagnetic material.

Electronic properties across metal-insulator transition in β -pyrochlore-type CsW2O6 epitaxial films

In CsW2O6, which undergoes a metal-insulator transition (MIT) at 213 K, the emergence of exotic properties associated with rattling motion of Cs is expected owing to its characteristic \beta-pyrochlore-type structure. However, a hurdle for crystal growth hampers elucidation of detailed properties and mechanisms of the MIT. Here we report on the epitaxial growth of \beta-pyrochlore-type CsW2O6 films and their electronic properties across the MIT. Using pulsed-laser deposi-tion technique, we grew single-crystalline CsW2O6 films exhibiting remarkably lower resistivity compared with a poly-crystalline bulk and sharp MIT around 200 K. Negative magnetoresistance and positive Hall coefficient were found, which became pronounced below 200 K. The valence-band and core-levels photoemission spectra indicated the drastic changes across the MIT. In the valence band photoemission spectrum, the finite density of states was observed at the Fermi level in the metallic phase. In contrast, an energy gap appeared in the insulating phase. The split of W 4f core-level spectrum suggested the charge disproportionation of W5+ and W6+ in the insulating phase. The change of spectral shape in the Cs 4d core levels reflected the rattling motion of Cs+ cations. These results strongly suggest that CsW2O6 is a novel material, in which MIT is driven by the charge disproportionation associated with the rattling motion.

Investigating Metal⁻Insulator Transition and Structural Phase Transformation in the (010)-VO₂/(001)-YSZ Epitaxial Thin Films.

The VO2 thin films with sharp metal–insulator transition (MIT) were epitaxially grown on (001)-oriented Yttria-stabilized zirconia substrates (YSZ) using radio-frequency (RF) magnetron sputtering techniques. The MIT and structural phase transition (SPT) were comprehensively investigated under in situ temperature conditions. The amplitude of MIT is in the order of magnitude of 104, and critical temperature is 342 K during the heating cycle. It is interesting that both electron concentration and mobility are changed by two orders of magnitude across the MIT. This research is distinctively different from previous studies, which found that the electron concentration solely contributes to the amplitude of the MIT, although the electron mobility does not. Analysis of the SPT showed that the (010)-VO2/(001)-YSZ epitaxial thin film presents a special multi-domain structure, which is probably due to the symmetry matching and lattice mismatch between the VO2 and YSZ substrate. The VO2 film experiences the SPT from the M1 phase at low temperature to a rutile phase at a high temperature. Moreover, the SPT occurs at the same critical temperature as that of the MIT. This work may shed light on a new MIT behavior and may potentially pave the way for preparing high-quality VO2 thin films on cost-effective YSZ substrates for photoelectronic applications.

Enhanced Metal-Insulator Transition Performance in Scalable Vanadium Dioxide Thin Films Prepared Using a Moisture-Assisted Chemical Solution Approach.

Vanadium dioxide (VO2) is a strong-correlated metal-oxide with a sharp metal-insulator transition (MIT) for a range of applications. However, synthesizing epitaxial VO2 films with desired properties has been a challenge because of the difficulty in controlling the oxygen stoichiometry of VO x, where x can be in the range of 1 < x < 2.5 and V has multiple valence states. Herein, a unique moisture-assisted chemical solution approach has been developed to successfully manipulate the oxygen stoichiometry, to significantly broaden the growth window, and to significantly enhance the MIT performance of VO2 films. The obvious broadening of the growth window of stoichiometric VO2 thin films, from 4 to 36 °C, is ascribed to a self-adjusted process for oxygen partial pressure at different temperatures by introducing moisture. A resistance change as large as 4 orders of magnitude has been achieved in VO2 thin films with a sharp transition width of less than 1 °C. The much enhanced MIT properties can be attributed to the higher and more uniform oxygen stoichiometry. This technique is not only scientifically interesting but also technologically important for fabricating wafer-scaled VO2 films with uniform properties for practical device applications.

Role of annealing temperature on the sol-gel synthesis of VO2 nanowires with in situ characterization of their metal-insulator transition.

Among the techniques to create VO2 nanostructures, the sol–gel method is the most facile and benefits from simple, manipulable synthetic parameters. Here, by utilizing various TEM techniques, we report the sequential morphological evolution of VO2 nanostructures in a sol–gel film spin-coated on a customized TEM grid, which underwent oxygen reduction as the annealing temperature increased. In situ TEM dark-field imaging and Raman spectroscopy allowed us to confirm the sharp phase transition behavior of an individual nanowire by illustrating the effect of electrode-clamping-induced tensile stress on the nucleation of the R phase from the M1 phase. The electrical transport properties of a single-nanowire device fabricated on a customized TEM grid showed excellent control of the stoichiometry and crystallinity of the wire. These results offer critical information for preparing tailored VO2 nanostructures with advanced transition properties by the sol–gel method to enable the fabrication of scalable flexible devices.

Effects of A-site cationic radius and cationic disorder on the electromagnetic properties of La0.7Ca0.3MnO3 ceramic with added Sr, Pb, and Ba

A series of polycrystalline ceramic samples of La0.7Ca0.25X0.05MnO3 (X = Sr, Pb, and Ba) were prepared using sol-gel method with methanol as the solvent. Resistivity-temperature curves show sharp metal-insulator transitions; furthermore, magnetization-temperature curves reveal a transition from ferromagnetism to paramagnetism. After doping with Sr, Pb, and Ba, both average ionic radius of A site and tolerance factor increase. Double-exchange also increases, which supports the metallicity of polycrystalline ceramic. However, larger lattice distortion is also introduced via substitution of larger ions at A site, and this tends to localize carriers. There is competing relationship between tolerance factor enhancement and lattice distortion; in combination, they determine electronic and magnetic properties of polycrystalline ceramics. As a result, after doping with Sr, Pb, and Ba, resistance temperature coefficient decreases, and metal-insulator transition temperature (TIM) increases. Meanwhile, ferromagnetic-paramagnetic transition temperature shows the same tendency as TIM.

Unveiling the control of quenched disorder in rare earth nickelates

The role of quenched disorder, a key control to obtain novel phases and functionalities, has not yet been determined in the complex phase diagram of $\mathrm{RNi}{\mathrm{O}}_{3}$ (R = rare-earth ion) perovskites. Here we present such a study by investigating $(\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{E}{\mathrm{u}}_{0.5})\mathrm{Ni}{\mathrm{O}}_{3}$ (LENO) having large R-site cation disorder. We show that in the presence of quenched disorder, (i) the resistivity drops by a few orders of magnitude across the metal-insulator transition (MIT) but the MIT shows only a subtle decrease, (ii) compressive films are completely metallic while largely tensile films are completely insulating sans a MIT, (iii) orthorhombic distortion promotes sharp MIT, and (iv) a Fermi liquid behavior even as high temperature resistivity exceeds the Mott-Ioffe-Regel limit with a bad metallic state. The low-energy terahertz conductivity dynamics obey Drude and Drude-Smith models for compressive and tensile films, respectively. All these features of disordered LENO, which are not typical of prototype ordered $\mathrm{NdNi}{\mathrm{O}}_{3}$, reveal an extraordinary sensitivity to slight structural perturbations. This study depicts the ease with which a variety of electronic phases can be tuned in disordered nickelates and emphasize the need to incorporate quenched disorder as a key control in the phase diagram of nickelates.

Sharpened VO2 Phase Transition via Controlled Release of Epitaxial Strain.

Phase transitions in correlated materials can be manipulated at the nanoscale to yield emergent functional properties, promising new paradigms for nanoelectronics and nanophotonics. Vanadium dioxide (VO2), an archetypal correlated material, exhibits a metal-insulator transition (MIT) above room temperature. At the thicknesses required for heterostructure applications, such as an optical modulator discussed here, the strain state of VO2 largely determines the MIT dynamics critical to the device performance. We develop an approach to control the MIT dynamics in epitaxial VO2 films by employing an intermediate template layer with large lattice mismatch to relieve the interfacial lattice constraints, contrary to conventional thin film epitaxy that favors lattice match between the substrate and the growing film. A combination of phase-field simulation, in situ real-time nanoscale imaging, and electrical measurements reveals robust undisturbed MIT dynamics even at preexisting structural domain boundaries and significantly sharpened MIT in the templated VO2 films. Utilizing the sharp MIT, we demonstrate a fast, electrically switchable optical waveguide. This study offers unconventional design principles for heteroepitaxial correlated materials, as well as novel insight into their nanoscale phase transitions.

Enhancement of temperature coefficient of resistance (TCR) and Magneto-resistance (MR) in La1–x Ca x MnO3:Ag0.2 polycrystalline composites

La1−x Ca x MnO3 (x ranges from 0.28 to 0.34) ceramics with Ag addition were synthesized by sol–gel method using methanol as solvent. The EDS elemental mapping shows that Ag element was detected dominantly on the sample surface. The sample of x = 0.28 has almost no pore and bigger grain size, which indicated that it has better crystallization and high density. Temperature dependence of resistivity shows that the samples for all Ca content exhibit sharp metal–insulator transition, and the corresponding metal–insulator temperature shift toward higher temperature with increasing Ca content. The temperature coefficient of resistance value for x = 0.28 reaches its the highest value, 71.8%·K−1. This temperature coefficient of resistance value is even higher than the previously reported for LCMO films and single crystals, and it shows a very promising application for the infrared and bolometric detectors. The high magnetoresitance for x = 0.28 reaches up to 69.3% in magnetic field of 1 T near room temperature. It was concluded that it improved the properties of LCMO:Ag composites are attributed to its improved crystallization by Ag addition, homogeneity and Mn4+/Mn3+ ratio. Different theoretical models are employed to analyze the resistance behaviors in different temperature regions, which give good agreements with experimental results.

Investigation of the metal-insulator transition in NdNiO3 films by site-selective X-ray absorption spectroscopy.

In this work, multifunctional oxide NdNiO3 (NNO) thin films grown on a SrTiO3 (STO) substrate using pulsed-laser deposition are studied. Temperature dependent resistivity measurements revealed that NNO/STO samples exhibit a sharp thickness dependent metal-insulator transition (MIT) over a range of 150-200 K. It is known that the electronic properties of correlated oxides are extremely complex and sensitive to changes in orbital occupancy. To evaluate the changes in the electronic and/or crystallographic structure responsible for the MIT, a site-selective (O, Ni and Nd) X-ray absorption near edge structure (XANES) analysis is performed above and below the transition temperature. Analysis of XANES spectra suggests that: (i) in NNO films nominally trivalent Ni ions exhibit multiple valency (bond disproportionation), (ii) intermetallic hybridization plays an important role, (iii) the presence of strong O 2p-O 2p hole correlation at low temperature results in the opening of the p-p gap and (iv) the valency of Nd ions matches well with that of Nd3+. For NNO films exhibiting a sharp MIT, Ni 3d electron localization and concurrent existence of Ni 3d8 and Ni 3d8L[combining low line]2 states are responsible for the observed transition. At temperatures below the MIT the O 2p-O 2p hole correlation is strong enough to split the O 2p band stabilizing insulating phase. Temperature and thickness dependent differences observed in the site-selective XANES data are discussed in terms of possible mechanisms for the MIT (negative charge-transfer type).

Metal-insulator transition in epitaxial NdNiO3 thin film: A structural, electrical and optical study

NdNiO3 thin film has been prepared by pulsed laser deposition on LaAlO3 (001) single crystalline substrate. Temperature-dependent resistivity measurement shows a sharp metal-insulator transition in such thin film. The phase transition temperature can be tuned from 90K to 121K by changing the thickness of thin film. The structure evolution during phase transition is studied by Raman spectroscopy. Optical conductivity reveals that the variation carrier density in the process of phase transition. The results of structural, electrical and optical studies provide useful insights to understand the mechanism of metal-insulator transition of NdNiO3 thin film.

Van der Waal Epitaxy of Flexible and Transparent VO2 Film on Muscovite

Vanadium dioxide (VO2) is a compelling candidate for next-generation electronics beyond conventional silicon-based devices due to the exhibition of a sharp metal–insulator transition. In this study, in order to realize functional VO2 film for flexible electronics, the growth of VO2 film directly on a transparent and flexible muscovite via van der Waals epitaxy is established. The heteroepitaxy and structural transition of VO2 films on muscovite are examined by a combination of high-resolution X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The unique metal–insulator transition of VO2 is further revealed with a change in electrical resistance over 103 and a more than 50% variation of optical transmittance. Furthermore, due to the nature of muscovite, the VO2/muscovite heterostructure possesses superior flexibility and optical transparence. The approach developed in this study paves an intriguing way to fabricate functional VO2 film for the applications in flexible electronics.