Metal-insulator Transition Temperature Research Articles

Giant nonvolatile resistive switching in a Mott oxide and ferroelectric hybrid

Controlling the electronic properties of oxides that feature a metal-insulator transition (MIT) is a key requirement for developing a new class of electronics often referred to as "Mottronics." A simple, controllable method to switch the MIT properties in real time is needed for practical applications. Here we report a giant, nonvolatile resistive switching (ΔR/R > 1,000%) and strong modulation of the MIT temperature (ΔTc > 30 K) in a voltage-actuated V2O3/PMN-PT [Pb(Mg,Nb)O3-PbTiO3] heterostructure. This resistive switching is an order of magnitude larger than ever encountered in any other similar systems. The control of the V2O3 electronic properties is achieved using the transfer of switchable ferroelastic strain from the PMN-PT substrate into the epitaxially grown V2O3 film. Strain can reversibly promote/hinder the structural phase transition in the V2O3, thus advancing/suppressing the associated MIT. The giant resistive switching and strong Tc modulation could enable practical implementations of voltage-controlled Mott devices and provide a platform for exploring fundamental electronic properties of V2O3.

Broadband switching of mid-infrared atmospheric windows by VO2-based thermal emitter.

Atmospheric windows play an important role in the field of infrared detection and radiative cooling. In this paper, the development of VO2-based metamaterial emitter brings broadband thermal-switching light to mid-infrared atmospheric windows. At room temperature, the emitter radiates light in both 3-5μm and 8-14μm atmospheric windows. At high temperature, the radiation peaks move out of the atmospheric windows and result a strong radiation at 5-8μm. The underlying mechanism relies on the relationship between VO2 metal-insulator transition (MIT) and resonant absorption modes coupling. Corresponding thermal imaging experiment exhibits two distinct phenomena. One is the observation of unchanged thermal radiation around MIT temperature. The other phenomenon regards the concealment of the emitter from Al background at specific temperatures. These two phenomena show potential application in infrared anti-detection.

Metal-insulator transition and novel magnetoresistance effects in amorphous carbon films

Amorphous carbon films were successfully prepared by radio frequency magnetron sputtering at different deposition temperatures. We observed metal-insulator transition in the amorphous carbon films, and found that the metal-insulator transition temperature moved to low temperature with the increase of the deposition temperature. A local sandwich structure formed by sp2 and sp3 hybridized carbon domains was thought to exist in the amorphous carbon films, which is responsible for the negative giant magnetoresistance of the film. Ferromagnetism and antiferromagnetism coexist in the localized sandwich structure, and they are tuned by temperature and external magnetic field, resulting in a small bulge and signal reversal of the magnetoresistance of the film. The proportion of sp2 and sp3 hybridized carbon in the carbon films can be adjusted by temperature. Different bonding configurations of sp2 and sp3 hybridized carbon result in two kinds of unpaired electrons, thus exhibiting two ferromagnetic phases.

Electronic structure of the α-(BEDT-TTF)2I3 surface by photoelectron spectroscopy

We report anomalies observed in photoelectron spectroscopy measurements performed on α-(BEDT-TTF)2I3 crystals. In particular, above its metal-insulator transition temperature (T = 135 K), we observe the lack of a sharp Fermi edge in contradiction with the metallic transport properties exhibited by this quasi-bidimensional organic material. We interpret these unusual results as a signature of a one-dimensional electronic behavior confirmed by DFT band structure calculations. Using photoelectron spectroscopy we probe a Luttinger liquid with a large correlation parameter (α > 1) that we interpret to be caused by the chain-like electronic structure of α-(BEDT-TTF)2I3 surface doped by iodine defects. These new surface effects are inaccessible by bulk sensitive measurements of electronic transport techniques.

Substrate-induced strain effect on magnetic and electrical transport properties of La0·67Ca0·33MnO3 thin films with varying orientation

This paper carries out a detailed study on magnetic and electrical transport properties of La0·67Ca0·33MnO3 (LCMO) thin films deposited on SrTiO3 substrates with different orientations, which provide varying degrees of in-plane tensile strain. As confirmed by the structure and morphology analyses, the homogeneous LCMO thin films with columnar grains are epitaxially grown along the orientations of the substrates, and the suffered tensile strain in the film is increasing from (100) LCMO to (111) LCMO. The obtained data suggest that the magnetic and electrical transport properties of the thin films are sensitive to the substrate-induced tensile strain. The resistivity, temperature coefficient of resistivity (TCR) and magnetoresistance (MR) of the thin films were in ascending order of (1 0 0) < (1 1 0) < (1 1 1), while the ferromagnetic-paramagnetic transition temperature (TC), magnetization and metal-insulator transition temperature (TIM) were in descending order of (1 0 0) > (1 1 0) > (1 1 1).

Structural and electrical transport properties of (La0.7-Y )Ca0.3MnO3 manganites

Abstract The effect of yttrium (Y) substitution at La-site on structural, microstructural and electrical transport properties of (La0.7-xYx)Ca0.3MnO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) were investigated. Samples were prepared by conventional solid state reaction method and structural properties of the samples were obtained using the Rietveld refinement of X-ray diffraction data. X-ray diffraction patterns show orthorhombic distorted perovskite structures. Resistivity measurement results of the LaYCaMnO3 compounds show that metal–insulator transition temperature, TMI shifts to lower temperature while the resistivity increases by increasing Y3+ substitution. In the metallic region (T TMI), resistivity data were fitted to model of small polaron hopping conduction mechanism and the activation energy, Ea increased as Y substitutions increases.

Metal-insulator transition of monoclinic VO2 thin film without Peierls distortion

VO2 thin film with a metal-insulator transition (MIT) temperature of about 37 °C was fabricated on the 300 °C indium-tin oxide substrates, in which the evident lattice distortion was seen because the 2θ value of the as-deposited film was even smaller than that of the standard VO2(R) (110) plane. Transmission electron microscopy demonstrated that the crystal structure of the as-deposited film was VO2(M) rather than VO2(R). In-situ x-ray diffraction at varied temperatures revealed that the crystal structure transition was suppressed with the temperature increasing, indicating that MIT was only driven by the electron-electron correlations. The experimental result was further verified by the density functional theory calculations, proving the presence of the monoclinic metal phase for the first time.

Structure and electromagnetic properties of La0.7Ca0.3-xKxMnO3 polycrystalline ceramics

In this study, La0.7Ca0.3-xKxMnO3 (LCKMO) polycrystalline ceramics were synthesized by sol–gel method. Their structures, surface morphologies, and electromagnetic transmission characteristics were studied and discussed. X-ray diffraction results showed that all prepared specimens highly crystalized as single phase with orthorhombic perovskite structure and Pnma space group. Characterization by scanning electron microscopy revealed that grain size increased with the increase in the K+ content. Electrical resistivity as function of temperature (ρ-T) was investigated through standard four-probe method. With the increase in the K+ content, metal–insulator transition temperature (Tp) and the Curie temperature (TC) shifted to higher temperatures, while the resistivity (ρ) reduced. Careful analysis of data indicated that peak temperature coefficient of resistance (TCR) and magnetoresistance (MR) were affected by K+ substitution. At x = 0.02, peak TCR and MR values reached 6.03%·K−1 and 30.26%, respectively. Characteristics of resultant ceramics were explained based on double exchange interaction. Overall, LCKMO materials act as promising candidates for broad range of applications in magnetic devices and infrared detectors at room temperature.

Controlled tungsten doping of vanadium dioxide grown through alternating-target pulsed laser deposition

Thin films of vanadium dioxide were grown with various fractions of tungsten doping controlled during growth through alternating-target pulsed laser deposition. A range of V1−xWxO2 films were grown on (0001) sapphire substrates with x values on intervals from x = [0, 0.025]. The metal-insulator transition temperature was measured through the change in infrared transmission, and the change in transition temperature as a function of the dopant-fraction x was found to be −18.89 °C per atomic percent tungsten, consistent with studies using various other forms of growth.

Study of Microstructural, Electrical and Dielectric Properties of La0.9Pb0.1MnO3 and La0.8Y0.1Pb0.1MnO3 Ceramics

The present work studies the microstructural and electrical properties of La0.9Pb0.1MnO3 and La0.8Y0.1Pb0.1MnO3 ceramics synthesized by solid-state route method. Microstructure and elemental analysis of both samples were carried out by field emission scanning electron microscope (FESEM) and energy dispersive spectroscopy (EDS) method, respectively. Phase analysis by X-ray diffraction (XRD) indicated formation of single phase distorted structure. The XRD data were further analyzed by Rietveld refinement technique. Raman analysis reveals that Y atom substitutes La site into the LPMO with shifting of phonon modes. The temperature variation of resistivity of undoped and Y-doped La0.9Pb0.1MnO3 samples have been investigated. The electrical resistivity as a function of temperature showed that all samples undergo an metal-insulator (M-I) transition having a peak at transition temperature TMI. Y-doping increases the resistivity and the metal-insulator transition temperature (TMI) shifts to lower temperature. The temperature-dependent resistivity for temperatures less than metal-insulator transition is explained in terms the quadratic temperature dependence and for T &gt; TMI, thermally activated conduction (TAC) is appropriate. Variation of frequency dispersion in permittivity and loss pattern due to La-site substitution in LPMO was observed in the dielectric response curve.

Strain and property tuning of the 3D framed epitaxial nanocomposite thin films via interlayer thickness variation

This work demonstrates the growth of three-dimensional (3D) ZnO-framed La0.7Sr0.3MnO3 (LSMO)-ZnO heteroepitaxial thin films in a vertically aligned nanocomposite (VAN) form on SrTiO3 (STO) substrates. Such 3D framed structures are formed by interlayering pure ZnO layers in LSMO-ZnO VAN thin films to thus achieve a ZnO-framed nanocomposite structure. Tailoring the thickness of the ZnO interlayer enables fine-tuning of the overall strain state of the two phases and thus leads to the tuning of the physical properties, such as the metal-insulator transition temperature, and magnetotransport properties. The optimum thickness of the ZnO interlayer is determined to be ∼2 nm to obtain a maximum magnetoresistance of 31% by a combined strain tuning and magnetoresistance tunneling effect. This work demonstrates effective strain tuning using the 3D framed design and provides a comprehensive perspective on the strain- and property-tuning using 3D nanocomposite frameworks.

Control of metal-insulator transition temperature in VO2 thin films grown on RuO2/TiO2 templates by strain modification

Ruthenium dioxide (RuO2) is an ideal buffer layer for vanadium dioxide (VO2) heterostructures due to its high electrical conductivity and matching crystal structure with metallic VO2. VO2 thin films were deposited on single crystal TiO2 (001) substrates with RuO2 buffer layers via pulsed laser deposition. The metal-insulator transition temperature (TMIT) in VO2 films can be controlled by the epitaxial strain between the VO2 film and RuO2 buffer layer by adjusting the buffer layer thickness (10 - 50 nm). We observed a decrease in the TMIT of VO2 films from 59 °C to 24 °C as the RuO2 thickness decreased from 50 nm to 10 nm. Additionally, we show that the RuO2 buffer layer can sustain an intermediate strain state in VO2 films up to 100 nm in thickness with a subsequently lower TMIT (30 °C). The 10 nm thick RuO2 buffer layer can reduce the TMIT in VO2 films by providing a pathway to relieve the strain through grain boundaries.

Size and crystallinity control of dispersed VO2 particles for modulation of metal–insulator transition temperature and hysteresis

Growth mechanism of VO2 particles with size dependent crystallinity: a solid-state dewetting and pyrolysis synergistic effect. Crystallinity, strain and defects optimize and modulate the MIT behavior of VO2 particles.

Monovalent doping effects on the structural, magnetic and magnetotransport properties of La0.833R0.167MnO3 (R = Li+, Na+, Ag+, K+)

This work elaborates the effects of monovalent doping on the structural, magnetic and magnetotransport properties of La0.833R0.167MnO3, where R = Li+, Na+, Ag+ and K+. Rietveld refinement of XRD profiles reveal that the compounds crystallize in rhombohedral structure, with the rhombohedral distortion angle (α) exhibiting an inverse relation with A-site ionic radius (˂ rA˃). A detailed investigation on room temperature Fourier transform infrared spectra (FTIR) of samples comprehends the evolution of Mn-O bond vibrations across the metal-insulator transition temperature (Tp). Surface morphological studies done by field emission scanning electron microscopy (FESEM) confirm the presence of well grown grains in the samples. The magnetization studies on La0.833Li0.167MnO3 indicate that the sample exhibits a magnetically frustrated spin glass state with irreversible metamagnetic transition at high magnetic fields. On the other hand, Na, Ag and K doped samples exhibit a soft ferromagnetic nature, with faster saturation at lower fields. Magnetotransport studies indicate that Li doped sample possess a charge ordered state close to its spin glass transition temperature, Tg = 113 K. Double resistivity peaks exhibited by La0.833Na0.167MnO3, La0.833Ag0.167MnO3 and La0.833K0.167MnO3 are discussed based on magnetic phase separation inherent in these samples. The transport and magnetic studies of the present samples contradict the widely accepted notion that Curie temperature (Tc) and Tp in manganites closely follow ˂rA˃ values. Fitting the resistivity data of samples to various theoretical models reveal the nature of conduction mechanisms realized in different temperature regimes. High resistivity of La0.833Li0.167MnO3 signals that the charge carriers are in a highly localized state. This is attributed to its low ˂ rA˃ and the consequent structural distortions.

Thickness effects on the epitaxial strain states and phase transformations in (001)-VO2/TiO2 thin films

The thickness-dependent epitaxial strains and phase transformations of (001)-VO2/TiO2 thin films are investigated systematically in a wide thickness range (from 9 to 150 nm). Under a thickness of 18 nm, the tensile in-plane strain is maintained, owing to the good lattice and the symmetry matching between the VO2 thin film and the TiO2 substrate, but the compressive out-of-plane epitaxial strain is gradually relaxed. The epitaxial strains co-stabilize the rutile phase (R phase) in this thickness range. Beyond a thickness of 18 nm, the out-of-plane lattice c exhibits a sudden elongation and reaches the bulk level of 2.8528 Å at a thickness of 20 nm, which indicates a structural phase transition (SPT). A further increase of the film thickness results in another new phase (tetragonal-like or T-like) with lattice distortion, which maintains the tetragonal symmetry in the thickness range of 20 to 55 nm. From a thickness of 60 nm, the monoclinic phase (M1 phase) appears, which indicates another SPT from T-like to the monoclinic M1 phase. This SPT is more favorable energetically, owing to the assistance of the strain relaxation in the thicker films. Additionally, the metal-insulator transition temperature positively increases as a function of the out-of-plane strain. This result is consistent with the fact that the tensile strain along the cR axis (V-V atom chain) is conducive for the stabilized insulating phase. This work highlights strain engineering as a crucial avenue for manipulating the phase transformations and properties in the correlated electron system.

Single-axis control of manganite films by helium doping via He-co-sputtering

We have studied the effect of the out-of-plane lattice on tensile strained (001) La0.7Ca0.3MnO3 thin films. The films were deposited on SrTiO3 substrates through magnetron sputtering technique under different Ar/O2/He gas flow ratios, varying the out-of-plane lattice from 3.823 Å to 3.845Å, which corresponds to an increase in the metal-insulator transition temperature. These changes are reversible after high-temperature anneal due to a massive helium release from the films occurring at temperatures around 540°C. The dependence of the transition temperature on lattice distortion is in good agreement with the prediction proposed by Millis et al. [J. Appl. Phys. 83, 1588 (1998)]. Considering the Jahn-Teller distortion enhanced by the in-plane biaxial strain in the films, we attribute the elevated transition temperature to the distortion relaxation due to He doping in the tensile strained films. The effective He doping by magnetron sputtering technique provides a simple strategy for manipulating functionality of oxide films.

Comparative Study on Structural, Electrical Transport and Magnetic Properties of Cr-Doped in Charge-Ordered Pr0.75Na0.25Mn1-Xcrxo3 and Nd0.75Na0.25Mn1-Ycryo3 Manganites

Cr doping in charge-ordered Pr0.75Na0.25Mn1-xCrxO3 and Nd0.75Na0.25Mn1-yCryO3 have been synthesized using conventional solid-state method to investigate its effect on structural, electrical transport and magnetic properties. X-ray diffraction (XRD) analysis for both compounds showed that the samples were crystallized in an orthorhombic structure with Pnma group. The unit cell volume value decrease as the Cr-doped increased indicating the possibility of Mn3+ ion was replaced by Cr3+ due to the different of ionic radius. The temperature dependence of electrical resistivity showed an insulating behavior down to the lower temperature the both parent compound (x = 0 and y = 0). Successive substitution of Cr at Mn-site in Pr0.75Na0.25Mn1-xCrxO3 manganites induced the metal-insulator (MI) transition temperature around TMI~120 K and TMI~122 K for x = 0.02 and x = 0.04 samples respectively suggestively due to the enhancement of double-exchange (DE) mechanism as a result of suppress the CO state. Analysis of resistivity data of dlnρ/dT-1 vs. T in Nd0.75Na0.25Mn1-yCryO3 manganite, showed a peak around 210 K and 160 K for y = 0 and 0.02 samples respectively while no peak was observed for y = 0.05 sample indicate the charge-ordered (CO) weakened. AC susceptibility, χ’ measurements in Pr0.75Na0.25Mn1-xCrxO3 exhibits paramagnetic to ferromagnetic-like with curie temperature, TC increases from 132 K for x = 0.02 to 141 K for x = 0.04 with Cr content indicate the suppression of CO state meanwhile in Nd0.75Na0.25Mn1-yCryO3 showed paramagnetic to anti-ferromagnetic transition as Neel temperature TN increases from 115 K for y = 0.02 to 125 K for y = 0.05.

Understanding of metal-insulator transition in VO2 based on experimental and theoretical investigations of magnetic features

The metal-insulator transition temperature Tc in VO2 is experimentally shown to be almost the same as a magnetic transition temperature Tm characterized by an abrupt decrease in susceptibility, suggesting the evidence of the same underlying origin for both transitions. The measurement of susceptibility shows that it weakly increases on cooling for temperature range of T > Tm, sharply decreases near Tm and then unusually increases on further cooling. A theoretical approach for such unusual observations in susceptibility near Tm or below is performed by modeling electrons from each two adjacent V4+ ions distributed along V-chains as a two-electron system, which indicates that the spin exchange between electrons could cause a level splitting into a singlet (S = 0) level of lower energy and a triplet (S = 1) level of higher energy. The observed abrupt decrease in susceptibility near Tm is explained to be due to that the sample enters the singlet state in which two electrons from adjacent V4+ ions are paired into dimers in spin antiparallel. By considering paramagnetic contribution of unpaired electrons created by the thermal activation from singlet to triplet levels, an expression for susceptibility is proposed to quantitatively explain the unusual temperature-dependent susceptibility observed at low temperatures. Based on the approach to magnetic features, the observed metal-insulator transition is explained to be due to a transition from high-temperature Pauli paramagnetic metallic state of V4+ions to low-temperature dimerized state of strong electronic localization.

Infrared-light switching in highly oriented VO2 films on ZnO-buffered glasses with controlled phase transition temperatures

Switching ability for infrared-light (IR) of commercial glass is of great interest for realizing practical smart windows which enable energy savings. In this article, we report on highly oriented growth of vanadium dioxide (VO2) films on ZnO-buffered glass substrates by means of biased reactive sputtering technique and their high switching ability for IR against temperature based on the insulator-metal transition (IMT). In addition, lower shift of transition temperature down to 55 °C was realized without deterioration of IMT properties in which resistance change nearly three orders of magnitude was kept. Shortening of in-plane V-V atoms distance through the effect of in-plane compressive stress responsible for energetic ion irradiation to biased substrate was supposed to contribute the achievement of controlled IMT temperatures. Large switching of around 50% for transmittance of IR with wavelength of 2.5 µm was realized under controlled phase transition temperatures. We emphasized that high biasing power enabled the shift of transition temperature for IR to nearly room temperature (～38 °C) by combination of the shortening of V-V distance and the oxygen defect formation, while the IMT reveals gradual change at biasing power higher than certain threshold value.

Bandwidth controlled metal‐insulator transition in Au–VO2 nanocomposite thin films

AbstractRecognizing and controlling the metal‐insulator transition (MIT) in VO2 transition‐metal oxides is interesting for the future electronic devices. However, the effect of the electron correlation for the structure‐coupled MIT in VO2 is as yet an open question. In this study, we present for the first time direct spectroscopic evidence for the charge‐transfer assistance bandwidth controlled MIT (BC‐MIT) in Au–VO2 nanocomposite thin films (NCTFs). A significantly enhancement of the MIT temperature (about 350 K) is realized in Au–VO2 films with Au volume ratio of 1.1 mol%. However, by further increasing Au ratios, the MIT temperature in Au–VO2 NCTFs is downward shifted by ~16 K and forward shifted 6 K. The V L‐edge and O K‐edge have been investigated. The basic electronic parameters such as the covalency (W) have been tuned. The relationship between bandwidth and the MIT temperature has been clearly elucidated a linear relationship. The experimental results demonstrate that MIT in VO2 is BC‐MIT which improved our understanding of the electron correlation effect in VO2 systems.