Sharp Metal-insulator Transition Research Articles

High magnetic entropy change in La0.70Ca0.21Ag0.09MnO3 compound

In this work, the structural, electrical, magnetic and magneto-caloric properties of the La0.70Ca0.21Ag0.09MnO3 compound were investigated. The compound was prepared by the sol gel method and sintered at 1000 °C for 24 h. Powder X-ray diffraction pattern (XRD) analyzes reveals a single phase orthorhombic structure with the Pnma space group as shown by the Rietveld analysis. The grainy morphology was confirmed with a grain size variation of 0.5–1.5 μm by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. High resolution transmission electron microscopy (HR-TEM) results strongly support an orthorhombic structure with the parameters calculated from the XRD data. Ferromagnetic-paramagnetic phase transitions occur at ∼263 K (Curie temperature, TC) and ∼265 K (metal-insulator, TMI). The sample undergoes a sharp metal–insulator transition at TMI following the sharp paramagnetic–ferromagnetic transition TC. From the isothermal magnetization measurements (M−H), the maximum magnetic entropy change |ΔSM| was calculated to be ∼4.8 J/kgK for an applied field change of 1 T, which is greater than that of pure Gd.

Electronic and magnetic phenomena at the interface of LaAlO3 and Ru doped SrTiO3

We have investigated the effect of Ru doping the SrTiO3 (STO) side of the LaAlO3/STO (LAO/STO) interface. The metallic behavior at the interface is remarkably robust to defects and disorder. Despite spin moment contribution from Ru ions, we see no evidence of magnetic ordering at the Ti L3,2 edge in either doped or undoped interfaces using X-ray magnetic circular dichroism. Magnetotransport measurements also do not show any evidence of magnetic scattering beyond that observed in undoped LAO/STO interfaces. Insertion of more than 7 unit cells of Ru doped STO at the interface suppresses metallic conductivity with a surprisingly sharp metal insulator transition. A similar metal-insulator transition is observed when a homoepitaxial STO film is grown on the single crystal substrate before LAO deposition. Together, our results indicate that ferromagnetism is not intrinsic to the interface, magnetic Ru dopants are not significant sources of scattering, and that cation vacancy formation alone cannot explain the insulating behavior observed in thick homoepitaxial LAO/STO/STO bilayers.

Transport properties of ultra-thin VO2 films on (001) TiO2 grown by reactive molecular-beam epitaxy

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproducible metal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinner films with sharp MITs are discussed, including the Volmer-Weber type growth mode due to the high energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2 substrate, which we quantify using low angle annular dark field scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy. We find that controlling island coalescence on the (001) surface and minimization of cation interdiffusion by using a low growth temperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2 films with abrupt MIT behavior.

Tunable metal-insulator transition in Nd1−xYxNiO3 (x = 0.3, 0.4) perovskites thin film at near room temperature

Metal-insulator transition (MIT) occurs due to the charge disproportionation and lattice distortions in rare-earth nickelates. Existing studies revealed that the MIT behavior of rare-earth nickelates is fairly sensitive to external stress/pressure, suggesting a viable route for MIT strain engineering. Unlike applying extrinsic strain, the MIT can also be modulated by through rare-earth cation mixing, which can be viewed as intrinsic quantum stress. We choose Nd1−XYXNiO3 (x = 0.3, 0.4) perovskites thin films as a prototype system to exhibit the tunable sharp MIT at near room temperature. By adjusting Y concentration, the transition temperature of the thin films can be changed within the range of 340–360 K. X-ray diffraction, X-ray absorption fine structure (XAFS), and in situ infrared spectroscopy are employed to probe the structural and optical property variation affected by composition and temperature. The infrared transmission intensity decreases with temperature across the MIT, indicating a pronounced thermochromic effect. Meanwhile, the XAFS result exhibits that the crystal atomistic structure changes accompanying with the Y atoms incorporation and MIT phase transition. The heavily doped Y atoms result in the pre-edge peak descent and Ni-O bond elongation, suggesting an enhanced charge disproportionation effect and the weakening of hybridization between Ni-3d and O-2p orbits.

Spinodally decomposed nanostructures in a TiO2–VO2 crystal

We have prepared a single crystal of Ti0.4V 0.6O2 which forms a solid solution crystallizing in the rutile structure at high temperatures and undergoes phase separation due to the spinodal decomposition when cooled to room temperature. The spinodally decomposed crystal consists of a self-assembled, mega stack of alternate Ti- and V-rich layers with an approximate period of 33 nm along the c axis. The unidirectional microstructure causes a large anisotropy in resistivity and a small one in thermal conductivity. A sharp metal–insulator transition as well as a structural transition to a monoclinic structure is observed in the thin V-rich layers.

Strain induced modulation of the correlated transport in epitaxial Sm0.5Nd0.5NiO3 thin films

We report a study of the effect of epitaxial strain on the correlated transport properties of 2–40 nm Sm0.5Nd0.5NiO3 (SNNO) films grown on different substrates. The metal–insulator transition (MIT) temperature TMI of the SNNO films increases with increasing tensile strain. While films on (0 0 1) LaAlO3 and (1 1 0) NdGaO3 substrates exhibit a sharp MIT and thermal hysteresis in the cooling–heating cycle, signaling a first-order transition, films on (0 0 1) SrTiO3 show a broad, second-order MIT. Hall effect measurements reveal hole-type charge carriers and thermally activated temperature dependence of the carrier density below TMI. The corresponding activation energy is ∼80 meV for films on LaAlO3 and NdGaO3, and is suppressed to 25 meV for films on SrTiO3. The carrier mobility in the metallic state and variable range hopping (VRH) transport at a low temperature point significantly enhanced electron localization in SNNO on STO, which we believe is not simply driven by extrinsic effects such as oxygen vacancies, but rather is an intrinsic characteristic for films subject to tensile strain due to the elongated Ni–O bond and hence enhanced dynamic Jahn–Teller distortion. In ultrathin films above the electrical dead layer thickness (2–3 nm), we observe a more than 100 K increase of TMI for films on LaAlO3, which has been correlated with a crossover from 3D to 2D transport as revealed from VRH. We attribute the distinct transport characteristics to strain induced modulation of various energy scales associated with the Ni–O–Ni bond angle and Ni–O bond length, which collectively determine the delocalization bandwidth of the system.

Controlling the sharpness of metal-insulator transition in epitaxial (La1−xPrx)0.67Ca0.33MnO3 (0 ≤ x ≤ 0.35) films

We report that epitaxial strain and chemical doping can be used cooperatively to tune the sharpness of metal-insulator transition (MIT) in epitaxial (La1−xPrx)0.67Ca0.33MnO3 (LPCMO) films. Compared to multiple MITs in anisotropically strained LPCMO/(LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7(001)C (LSAT) films with a phase-separated ground state, the lattice-matched LPCMO/NdGaO3(110)Or (NGO) films show a sharp MIT near the Curie temperature (TC), with a ferromagnetic-metallic ground state. The sharpness of MIT, as evaluated by the temperature coefficient of resistance (TCR), can be two times larger in LPCMO/NGO films than in LPCMO/LSAT films. Moreover, for LPCMO/NGO films, TCR greatly relies on the Pr doping level x, where a maximum TCR value of 88.17% K−1 can be obtained at x = 0.25, but shows less dependence on the film thicknesses. These results suggest that the combination of epitaxial strain and chemical doping could be employed to control not only the ground state of the manganite films, but the sharpness of MIT at various TC, providing the feasibility to design manganite-based infrared devices in a broad temperature range.

바나듐 산화물의 금속-절연체 전이현상 기반 센서 연구

Abstract Here, we review the various methods for the preparation of vanadium dioxide (VO 2 ) films and nanowires, and their potential appli-cations to the sensors such as gas sensor, strain sensor, and temperature sensor. VO 2 is an interesting material on account of its easilyaccessible and sharp Mott metal-insulator transition (MIT) at ~68°C in the bulk. The MIT is also triggered by the electric field, stress,magnetic field etc. This paper involves exceptionally sensitive hydrogen sensors based on the catalytic process between hydrogen mol-ecules and Pd nanoparticles on the VO 2 surface, and fast responsive sensors based on the self-heating effects which leads to the phasechanges of the VO 2 . These features will be seen in this paper and can enable strategies for the integration of a VO 2 material in advancedand complex functional units such as logic gates, memory, FETs for micro/nano-systems as well as the sensors.Keywords: Sensors, Oxide semiconductors, VO 2 , Metal-insulator transition, Films, Nanowires

Textured metastable VO2 (B) thin films on SrTiO3 substrates with significantly enhanced conductivity

Textured metastable VO2 (B) thin films with a layered structure were grown on SrTiO3 (001) by pulsed laser deposition. The X-ray diffraction and transmission electron microscopy results indicate that VO2 (B) films exhibit c-axis out-of-plane, while the films have 4 possible in-plane matching relations. In addition, a small amount of VO2 (M) phase can co-grow in the VO2 (B) phase when the film thickness exceeds a threshold. The thick VO2 films on STO exhibit a sharp metal-insulator transition with an increase of electrical conductivity in two orders of magnitude. This study may provide an alternative approach to enhance the performance of insulating VO2 (B) based batteries with increased electrical conductivity by incorporating VO2 (M) phase in the VO2 (B) phase layered network.

Controlling the sharpness of room-temperature metal-insulator transition in epitaxial Sm0.5Nd0.5NiO3 films

Sm0.5Nd0.5NiO3 (SNNO) films with metal-insulator transition (MIT) at room-temperature (∼300 K) have been grown on NdGaO3 (001) substrates by pulsed laser deposition. By modifying the parameters of oxygen pressure, substrate temperature, and film thickness, the role of oxygen vacancies and strain relaxation on the MIT of SNNO films was systematically analyzed. The strain status of the films was carefully characterized by means of high resolution x-ray diffraction. The results revealed that for the fully strained films (≤20 nm) an increment of deposition oxygen pressure (and/or temperature) would decrease the content of oxygen vacancies and Ni2+ in the films, leading to a sharp MIT. In contrast, the strain relaxation occurs in the thicker films (>20 nm) despite an optimized oxygen pressure (temperature) was adapted for the deposition, which results in an inferior transport property and surface morphology. Specifically, a broadening MIT and a doublet TMI was observed in the partially strained films, where one TMI kept a stable value around 300 K in analogues to that of fully strained film, and another one increased with the increment of the film thickness, reaching a highest value of 330 K. This might be induced by the coexistence of a fully strained part and a strain-relaxed portion in the thicker films that observed on high resolution X-ray reciprocal space mappings.

Voltage-Triggered Ultrafast Phase Transition in Vanadium Dioxide Switches

Electrically driven metal-insulator transition in vanadium dioxide (VO2) is of interest in emerging memory devices, neural computation, and high speed electronics. We report on the fabrication of out-of-plane VO2 metal-insulator-metal (MIM) structures and reproducible high-speed switching measurements in these two-terminal devices. We have observed a clear correlation between electrically-driven ON/OFF current ratio and thermally-induced resistance change during metal-insulator transition. It is also found that sharp metal-insulator transition could be triggered by external voltage pulses within 2 ns at room temperature and the achieved ON/OFF ratio is greater than two orders of magnitude with good endurance.

Properties in vanadium dioxide thin film synthesized from V2O5 annealed in H2/Ar ambience

Owning to its sharp metal-insulator transition at ～340 K, VO2 is becoming an attractive candidate for the electrical and optical material. Here we report on the fabrication and characterization of VO2 thin film obtained from the V2O5 thin film annealed in Ar/H2 ambience. V2O5 thin film is fabricated by using the pulsed laser deposition system on the R-sapphire substrate under several different conditions by varying the substrate temperature and the pressure of the growth atmosphere to optimize the growth condition. Then we carry out the annealing treatment on the V2O5 thin film in different annealing conditions. The VO2 thin films are characterized using X-ray diffraction, X-ray photoelectron spectroscopy and R-T measurement. When annealed in a temperature range of 500–525 ℃ for 3 h in H2(5%)/Ar ambience, the V2O5 thin film can be converted into the mixed-structures of metastable monoclinic structure (B) and the monoclinic rutile structure (M) which is responsible for the phase-change property. And under the same conditions, when the annealing time reaches 4.5 h, the pure VO2(B) is obtained. Further we anneal the VO2(B) in pure Ar ambience and tentatively realize the resistivity reduced by nearly four orders with the temperature increasing from 25 ℃ to 105 ℃. The transition temperature is nearly 350 K. And the transition between VO2 (B) and VO2 (M) is realized.

Epitaxial growth and metal-insulator transition of vanadium oxide thin films with controllable phases

Vanadium oxide thin films with well controlled phases such as rhombohedra V2O3 and monoclinic VO2 were synthesized on Al2O3 (0001) substrates by optimizing the processing parameters of a polymer assisted deposition technique. X-ray diffraction and high-resolution transmission electron microscopy studies revealed that both V2O3 and VO2 films can be well controlled with good epitaxial quality. The temperature dependency of electrical resistivity demonstrated sharp metal-insulator transitions (MITs) for V2O3 and VO2 films. The crystallinity and the strains in the films are believed to play critical roles in determining the MIT properties.

Strain effect caused by substrates on phase separation and transport properties in Pr0.7(Ca0.8Sr0.2)0.3MnO3 thin films

Tensile and compressive strains were introduced by epitaxially growing Pr(Ca0.8Sr0.2)0.3MnO3 thin films on different substrates with different lattice constants. The large tensile strain in films on SrTiO3 enhances the stability of the long-range charge-orbital (CO) phase and a 5 T magnetic field cannot melt the CO phase. However, the CO phase in the film on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) is less robust due to a smaller tensile strain and can be partially melt by a 5T field. On the other hand, compressive strained films on LaAlO3 (LAO) show different behavior. Sharp metal-insulator transition with pronounced hysteresis was observed, indicating that the long-range CO is suppressed, whereas the short-rang ferromagnetic (FM)/CO is favored. Upon magnetic field application, resistance reduces remarkably, whereas the insulator-metal (IM) transition and hysteresis becomes broader and insignificant.

Work Function of Vanadium Dioxide Thin Films Across the Metal-Insulator Transition and the Role of Surface Nonstoichiometry

Vanadium dioxide (VO(2)) undergoes a sharp metal-insulator transition (MIT) in the vicinity of room temperature and there is great interest in exploiting this effect in novel electronic and photonic devices. We have measured the work function of vanadium dioxide thin films across the phase transition using variable temperature Kelvin force microscopy (KFM). The work function is estimated to be ∼5.15 eV in the insulating phase and increases by ∼0.15 eV across the MIT. We further show that the work function change upon the phase transition is highly sensitive to near-surface stoichiometry studied by X-ray photoelectron spectroscopy. This change in work function is distinct from bulk resistance-versus temperature trends commonly used to evaluate synthesis protocols for such vanadium oxide films and optimize stoichiometry. The results are pertinent to understanding fundamental electronic properties of vanadium oxide as well as charge injection phenomena in solid-state devices incorporating complex oxides containing multivalence cations.

Oxide Electronics Utilizing Ultrafast Metal-Insulator Transitions

Although phase transitions have long been a centerpiece of condensed matter materials science studies, a number of recent efforts focus on potentially exploiting the resulting functional property changes in novel electronics and photonics as well as understanding emergent phenomena. This is quite timely, given a grand challenge in twenty-first-century physical sciences is related to enabling continued advances in information processing and storage beyond conventional CMOS scaling. In this brief review, we discuss synthesis of strongly correlated oxides, mechanisms of metal-insulator transitions, and exploratory electron devices that are being studied. Particular emphasis is placed on vanadium dioxide, which undergoes a sharp metal-insulator transition near room temperature at ultrafast timescales. The article begins with an introduction to metal-insulator transition in oxides, followed by a brief discussion on the mechanisms leading to the phase transition. The role of materials synthesis in influencing functional properties is discussed briefly. Recent efforts on realizing novel devices such as field effect switches, optical detectors, nonlinear circuit components, and solid-state sensors are reviewed. The article concludes with a brief discussion on future research directions that may be worth consideration.

Analysis of “on” and “off” times for thermally driven VO 2 metal-insulator transition nanoscale switching devices

Vanadium oxide undergoes a sharp metal-insulator transition in the vicinity of room temperature and there is considerable interest in exploring novel device applications that utilize this phase transition. Using experimentally determined values of the thermal conductivity across the metal-insulator transition in VO 2 thin films, we estimate the switching characteristics of two-terminal VO 2 devices. The minimum switching time for both heating (“on” state) and cooling (“off” state) processes is explored using a simple resistance-capacitance thermal circuit model. The estimated minimum switching time is on the order of ∼1 ns for 20 nm VO 2 films which is comparable to experimentally observed switching times. Optimal operating temperatures to maximize switching times are estimated. Methods to further enhance the switching kinetics by device thickness, carrier doping/strain, interfacial thermal resistance and input thermal energy are discussed. The simulations are compared with a 3-D model for VO 2 devices on Si substrates utilizing COMSOL. The results are of potential relevance to the emerging field of correlated oxide electronics with fast phase transitions.

Geometric confinement effects on the metal-insulator transition temperature and stress relaxation in VO2 thin films grown on silicon

VO2 undergoes a sharp metal-insulator transition at ∼67 °C with several orders of change in conductivity and optical transmittance. Understanding and control of the properties of vanadium oxide layers grown on technologically relevant substrates such as Si (100) single crystals is therefore of great interest. In this work, we show tunability of metal-insulator transition temperature as well as recoverable stress in VO2 thin films grown on Si substrate by introducing nanoscale atomic layer deposited HfO2 interfacial layers with no degradation in the resistance ratio. For a confined VO2 film, the metal-insulator transition temperature is suppressed by ∼16 °C and the recoverable stress is 150 MPa, compared to 400 MPa for a bare film. These observations are further correlated with in situ variable temperature measurement of stress changes occurring during the phase transition. Structural and microstructural studies on the various samples have been carried out by x ray diffraction and cross-sectional transmission electron microscopy. The strategy of tuning the metal-insulator transition characteristics by nanoscale interfacial dielectrics is of broader relevance in design of programmable materials and integration into solid state devices for electronics.

Structurally-driven metal–insulator transition in Ca 2Ru 1− xCr xO 4 (0≤ x

Correlation between structure and transport properties are investigated in high-quality single-crystals of Ca 2Ru 1− x Cr x O 4 with 0< x<0.14 using single crystal X-ray diffraction and by electronic studies. The parent compound was known to exhibit an intriguing first-order structurally driven metal–insulator (MI) transition at 357 K. Upon chromium doping on the ruthenium site, the metal–insulator transition temperature ( T MI) was drastically reduced, and is related to the competition between structural changes that occur upon Cr doping and with decreasing temperature. A strong suppression of structural distortions with increasing Cr substitution was identified. No clear T MI can be observed when x>13.5% and the system behaves as an insulator. Such a large, sharp metal–insulator transition and tuneable transition temperature may have potential applications in electronic devices.

Dielectric and carrier transport properties of vanadium dioxide thin films across the phase transition utilizing gated capacitor devices

Vanadium dioxide $({\text{VO}}_{2})$ is a strongly correlated oxide that undergoes a sharp metal-insulator transition (MIT) in the vicinity of room temperature. Fundamental knowledge of the semiconducting properties of thin film ${\text{VO}}_{2}$ is needed to advance our understanding of the microscopic transition mechanisms that are presently being actively explored and also for novel electron devices that could utilize the phase transition. In this report, the temperature dependence of the dielectric constant and carrier conduction in ${\text{VO}}_{2}$ thin films are investigated, from quantitative capacitance-voltage analyses of a multilayer capacitor with ${\text{HfO}}_{2}/{\text{VO}}_{2}/{\text{HfO}}_{2}/n\text{-Si}$-substrate stack structure. The finite conductance of the ${\text{VO}}_{2}$ in the capacitor structure is taken into account in the impedance transformations to obtain material properties as a function of temperature. The dielectric constant of ${\text{VO}}_{2}$ increases from a value of $\ensuremath{\sim}36$ at room temperature to a value exceeding $6\ifmmode\times\else\texttimes\fi{}{10}^{4}$ at $100\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The carrier type of ${\text{VO}}_{2}$ thin film is electronic, determined by the polarity of the capacitance-voltage spectra. The electron carrier concentration of the ${\text{VO}}_{2}$ thin films shows about four orders of magnitude increase from room temperature to the temperature near phase transition. The approach of deriving insights into carrier conduction from capacitance measurement analyses can be applied to other materials beyond ${\text{VO}}_{2}$, wherein sensitive temperature-dependence or low carrier mobility makes Hall measurements challenging. Furthermore, the stack structure consisting of ${\text{HfO}}_{2}/{\text{VO}}_{2}/{\text{HfO}}_{2}/n\text{-Si}$ shows a strong temperature dependence of capacitance due to the MIT in the sandwiched ${\text{VO}}_{2}$ layer. This suggests the potential for strongly correlated oxides that undergo a hysteretic MIT in thermally tunable capacitors that could be of interest for solid-state devices.