Tuning Of Metal-insulator Transition Research Articles

Flat band induced metal-insulator transitions for weak magnetic flux and spin-orbit disorder

We consider manifolds of tunable all-bands-flat (ABF) lattices in dimensions $d=1,2$, parameterized by a manifold angle parameter $\ensuremath{\theta}$. We study localization properties of eigenstates in the presence of weak magnetic flux disorder and weak spin-orbit disorder. We demonstrate that weakly disordered ABF lattices are described by effective scale-free models, where the disorder strength is scaled out. For weak magnetic flux disorder, we observe subexponential localization at flatband energies in $d=1$, which differs from the usual Anderson localization. We also find diverging localization length at flatband energies for weak flux values in $d=2$; however, the character of the eigenstates at these energies is less clear. For weak spin-orbit coupling disorder in $d=2$ we identify a tunable metal-insulator transition with mobility edges. We also consider the case of mixed spin-orbit and diagonal disorder and obtain the metal-insulator transition driven by the manifold parameter $\ensuremath{\theta}$.

Interfacial eg orbital reconstruction: Modulation of Metal–Insulator transitions of ultrathin NdNiO3 films by two-dimensional electronic gas

Tunable metal–insulator transitions (MIT) of ultrathin NdNiO3 (NNO) film grown on the LaAlO3/SrTiO3 two-dimensional electronic gas structures have been investigated by interfacial eg orbital reconstruction. Spectroscopy studies suggest that the polar catastrophe of the two-dimensional electronic gas results in Ni2+ to Ni3+ charge transfer of the NNO owing to the band-alignment condition. The orbital-hybridization of Ni 3d and O 2p can directly increase in-plane eg orbital occupancy, and the physical mechanism and modulation of the MIT can be clearly elucidated. This study not only provides new insight into the relationship between the electronic properties and orbital occupation, but it will also promote future application of epitaxial NNO thin films.

Tunable metal-insulator transition of V2O3 thin films strained by controlled inclusion of crystallographic defect

Current understanding of the metal-insulator transition (MIT) of V2O3 based on the electronic band structures states that the electrical properties of V2O3 are extremely sensitive to the crystallographic structures. In the present report, we present the systematic tuning of the MIT characteristics through the control of crystallographic structure of the polycrystalline V2O3 thin film. Both crystallographic textures and the residual strains due to the presence of controlled amount of stacking faults in the films could be adjusted with the proper control of deposition conditions and the corresponding changes in the electrical properties have be tracked to establish the tunability of MIT temperature as well as the transition rates. Observed change was understood with the consideration of electronic band structures, through which the effect of atomic distances to the electron band gap was demonstrated. Additionally, it was found that the lattice strain of a strongly textured V2O3 influences both room temperature resistivity and MIT temperature.

Metal–insulator transition tuned by oxygen vacancy migration across TiO2/VO2 interface

Oxygen defects are essential building blocks for designing functional oxides with remarkable properties, ranging from electrical and ionic conductivity to magnetism and ferroelectricity. Oxygen defects, despite being spatially localized, can profoundly alter global properties such as the crystal symmetry and electronic structure, thereby enabling emergent phenomena. In this work, we achieved tunable metal–insulator transitions (MIT) in oxide heterostructures by inducing interfacial oxygen vacancy migration. We chose the non-stoichiometric VO2-δ as a model system due to its near room temperature MIT temperature. We found that depositing a TiO2 capping layer on an epitaxial VO2 thin film can effectively reduce the resistance of the insulating phase in VO2, yielding a significantly reduced ROFF/RON ratio. We systematically studied the TiO2/VO2 heterostructures by structural and transport measurements, X-ray photoelectron spectroscopy, and ab initio calculations and found that oxygen vacancy migration from TiO2 to VO2 is responsible for the suppression of the MIT. Our findings underscore the importance of the interfacial oxygen vacancy migration and redistribution in controlling the electronic structure and emergent functionality of the heterostructure, thereby providing a new approach to designing oxide heterostructures for novel ionotronics and neuromorphic-computing devices.

Tunable metal–insulator transition in LaTiO3/CaVO3 superlattices: A theoretical study* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11804168 and 51872145), the Natural Science Foundation of Jiangsu Province, China (Grant Nos. BK20180736 and BK20190726), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 18KJB140009), and the Science Foundation from Nanjing University of

As one of intriguing physical results of electronic reconstruction, the metal–insulator transition plays an important role in exploring new electronic devices. In this study, the density functional theory is employed to investigate the metal–insulator transition in (LaTiO3) m /(CaVO3) n superlattices. Herein, three kinds of physical avenues, i.e., stacking orientation, epitaxial strain, and thickness periods, are used to tune the metal–insulator transition. Our calculations find that the [001]- and [110]-oriented (LaTiO3)1/(CaVO3)1 superlattices on SrTiO3 substrate are insulating, while [111]-oriented case is metallic. Such metallic behavior in [111] orientation can also be modulated by epitaxial strain. Besides the structural orientation and strain effect, the highly probable metal–insulator transition is presented in (LaTiO3) m /(CaVO3) n superlattices with increasing thickness. In addition, several interesting physical phenomena have also been revealed, such as selective charge transfer, charge ordering, and orbital ordering.

Tunable low-field magnetoresistance properties in (La0.7Ca0.3MnO3)1−x:(CeO2)x vertically aligned nanocomposite thin films

Vertically aligned nanocomposite (VAN) (La0.7Ca0.3MnO3)1−x:(CeO2)x thin films have been deposited on SrTiO3 (001) substrates by pulsed laser deposition. Enhanced low-field magnetoresistance properties and tunable metal-insulator transition temperature (TMI) have been demonstrated via modulating the composition of (La0.7Ca0.3MnO3)1−x:(CeO2)x (x = 0, 0.05, 0.1, and 0.2). By increasing the atomic percentage of the CeO2 phase to 20%, a maximum magnetoresistance value of 51.8% can be achieved and the TMI value can be tuned from 113 K to 210 K. The enhanced magnetoresistance properties are attributed to the disordered grain boundary and tunneling structure generated by the insulating CeO2 phase. The change in the TMI value is attributed to the strain state in the La0.7Ca0.3MnO3 phase. Furthermore, high ferromagnetic anisotropy and enhanced magnetization have been demonstrated in the VAN system. This work demonstrates the power of multifunctionalities and property tuning in VAN thin films.

Electrochemical Tuning of Metal Insulator Transition and Nonvolatile Resistive Switching in Superconducting Films.

Modulation of carrier concentration in strongly correlated oxides offers the unique opportunity to induce different phases in the same material, which dramatically change their physical properties, providing novel concepts in oxide electronic devices with engineered functionalities. This work reports on the electric manipulation of the superconducting to insulator phase transition in YBa2Cu3O7−δ thin films by electrochemical oxygen doping. Both normal state resistance and the superconducting critical temperature can be reversibly manipulated in confined active volumes of the film by gate-tunable oxygen diffusion. Vertical and lateral oxygen mobility may be finely modulated, at the micro- and nano-scale, by tuning the applied bias voltage and operating temperature thus providing the basis for the design of homogeneous and flexible transistor-like devices with loss-less superconducting drain–source channels. We analyze the experimental results in light of a theoretical model, which incorporates thermally activated and electrically driven volume oxygen diffusion.

Tunable metal-insulator transition, Rashba effect and Weyl Fermions in a relativistic charge-ordered ferroelectric oxide

Controllable metal–insulator transitions (MIT), Rashba–Dresselhaus (RD) spin splitting, and Weyl semimetals are promising schemes for realizing processing devices. Complex oxides are a desirable materials platform for such devices, as they host delicate and tunable charge, spin, orbital, and lattice degrees of freedoms. Here, using first-principles calculations and symmetry analysis, we identify an electric-field tunable MIT, RD effect, and Weyl semimetal in a known, charge-ordered, and polar relativistic oxide Ag2BiO3 at room temperature. Remarkably, a centrosymmetric BiO6 octahedral-breathing distortion induces a sizable spontaneous ferroelectric polarization through Bi3+/Bi5+ charge disproportionation, which stabilizes simultaneously the insulating phase. The continuous attenuation of the Bi3+/Bi5+ disproportionation obtained by applying an external electric field reduces the band gap and RD spin splitting and drives the phase transition from a ferroelectric RD insulator to a paraelectric Dirac semimetal, through a topological Weyl semimetal intermediate state. These findings suggest that Ag2BiO3 is a promising material for spin-orbitonic applications.

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.

Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces by charge-transfer-induced modulation doping.

Two-dimensional electron gases (2DEGs) formed at the interface of insulating complex oxides promise the development of all-oxide electronic devices. These 2DEGs involve many-body interactions that give rise to a variety of physical phenomena such as superconductivity, magnetism, tunable metal-insulator transitions and phase separation. Increasing the mobility of the 2DEG, however, remains a major challenge. Here, we show that the electron mobility is enhanced by more than two orders of magnitude by inserting a single-unit-cell insulating layer of polar La(1-x)Sr(x)MnO3 (x = 0, 1/8, and 1/3) at the interface between disordered LaAlO3 and crystalline SrTiO3 produced at room temperature. Resonant X-ray spectroscopy and transmission electron microscopy show that the manganite layer undergoes unambiguous electronic reconstruction, leading to modulation doping of such atomically engineered complex oxide heterointerfaces. At low temperatures, the modulation-doped 2DEG exhibits Shubnikov-de Haas oscillations and fingerprints of the quantum Hall effect, demonstrating unprecedented high mobility and low electron density.

Pulsed laser deposition of La1−xSrxMnO3: thin-film properties and spintronic applications

Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.

Controlling interfacial states in amorphous/crystalline LaAlO3/SrTiO3 heterostructures by electric fields

The tunable metal-insulator transition in crystalline LaAlO3/SrTiO3 heterostructures constitutes a central element in the range of remarkable interface properties that has made this oxide system subject to extensive research. Recently, metallic interfaces have also been realized when depositing amorphous LaAlO3 films on SrTiO3. Here, we present a non-volatile and reversible tuning of the interface conductivity by more than 3 orders of magnitude at room temperature by applying an electric field to such amorphous/crystalline heterostructures with amorphous LaAlO3 film thicknesses of ∼2 nm. We show that the tunability is strongly temperature dependent, and demonstrate a simple protocol for enhancing the tunability.

Tunable metal–insulator transition in double-layer graphene heterostructures

We report a double-layer electronic system made of two closely-spaced but electrically isolated graphene monolayers sandwiched in boron nitride. For large carrier densities in one of the layers, the adjacent layer no longer exhibits a minimum metallic conductivity at the neutrality point, and its resistivity diverges at low temperatures. This divergence can be suppressed by magnetic field or by reducing the carrier density in the adjacent layer. We believe that the observed localization is intrinsic for neutral graphene with generic disorder if metallic electron-hole puddles are screened out.

Tunable metal–insulator transitions in bilayer graphene by thermal annealing

Tunable and highly reproducible metal–insulator transitions have been observed in bilayer graphene upon thermal annealing at 400 K under high vacuum conditions. Before annealing, the sample is metallic in the whole temperature regime of study. Upon annealing, the conductivity changes from metallic to that of an insulator and the transition temperature is a function of annealing time. The pristine metallic state can be reinstated by exposing to air thereby inducing changes in the electronic properties by adsorbing water vapor, which makes graphene a technologically promising material for sensor applications.

Spin Polarized Tunneling at the Metal-Insulator Transition

We have determined the spin polarized density of states (DOS) of 3-d amorphous Gd x Si 1-x in the quantum critical regime of a tunable Metal-Insulator Transition (MIT). Using a spin polarized BCS DOS we fit the data and extract the spin polarization P.

Publisher’s Note: Hall effect at a tunable metal-insulator transition [Phys. Rev. B67, 121102 (2003)

Publisher’s Note: Hall effect at a tunable metal-insulator transition [Phys. Rev. B<b>67</b>, 121102 (2003)

Hall effect at a tunable metal-insulator transition

Using a rotating magnetic field, the Hall effect in three-dimensional amorphous ${\mathrm{Gd}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}$ has been measured in the critical regime of the metal-insulator transition for a constant total magnetic field. The Hall coefficient ${R}_{0}$ is negative, indicating electronlike conductivity, with a magnitude that increases with decreasing conductivity. ${R}_{0}$ diverges at the metal-insulator transition, and displays critical behavior with exponent $\ensuremath{-}1$ $[{R}_{0}\ensuremath{\sim}(H\ensuremath{-}{H}_{C}{)}^{\ensuremath{-}1}].$ This dependence is interpreted as a linear decrease in the density of mobile carriers $n\ensuremath{\sim}{R}_{0}^{\ensuremath{-}1}\ensuremath{\sim}H\ensuremath{-}{H}_{C},$ indicative of the dominant influence of interaction effects.

Sm x Nd 1−x NiO 3 thin-film solid solutions with tunable metal–insulator transition synthesized by alternate-target pulsed-laser deposition

Thin-film solid solutions of SmxNd1−xNiO3 were synthesized on NdGaO3 substrates by pulsed-laser deposition using alternating NdNiO3 and SmNiO3 targets. The films were characterized by x-ray diffraction and variable-temperature four-probe conductivity measurements. The films grow in the {100}pseudocubic direction. There is a nearly linear increase of the metal–insulator transition from 199 K for x=0 to 378 K for x=1, with the composition corresponding to x=0.6 displaying a transition near room temperature.