Metal-insulator transition in Nd1-xEuxNiO 3 compounds

Polycrystalline Nd1-xEuxNiO3 (0≤x≤0.5) compounds were synthesized in order to investigate the character of the metal-insulator (MI) phase transition in this series. Samples were prepared through the sol-gel route and subjected to heat treatments at ∼1000 °C under oxygen pressures as high as 80bar. X-ray diffraction (XRD) and neutron powder diffraction (NPD), electrical resistivity ρ(T), and magnetization M(T) measurements were performed on these compounds. The NPD and XRD results indicated that the samples crystallize in an orthorhombic distorted perovskite structure, space group Pbnm. The analysis of the structural parameters revealed a sudden and small expansion of ∼0.2% of the unit cell volume when electronic localization occurs. This expansion was attributed to a small increase of ∼0.003 of the average Ni-O distance and a simultaneous decrease of ∼-0.5° of the Ni-O-Ni superexchange angle. The ρ(T) measurements revealed a MI transition occurring at temperatures ranging from TMI∼193 to 336K for samples with x ≤ 0 and 0.50, respectively. These measurements also show a large thermal hysteresis in NdNiO3 during heating and cooling processes, suggesting a first-order character of the phase transition at TMI. The width of this thermal hysteresis was found to decrease appreciably for the sample Nd 0.7Eu0.3NiO3. The results indicate that cation disorder associated with increasing substitution of Nd by Eu is responsible for changing the first-order character of the transition in NdNiO3. © 2006 IOP Publishing Ltd.

Metal-insulator transition in Nd1-x Eu-x NiO3 probed by specific heat and anelastic measurements

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Magnetic field induced lattice effects in a quasi-two-dimensional organic conductor close to the Mott metal-insulator transition

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Metal-insulator transition in perovskite oxides: Tunneling experiments

In this paper we have investigated the composition-driven metal-insulator (MI) transitions in two ABO3 classes of perovskite oxides (LaNixCo1-xO3 and NaxTayW1-yO3) in the composition range close to the critical region by using the tunneling technique. Two types of junctions (point-contact and planar) have been used for the investigation covering the temperature range 0.4 KMI transition is approached. However, there is a fairly strong thermal-smearing effect near the zero-bias region for ?V?<10kBT/e. $G(V)� has been found to follow a power law of the type G(V)=G0(1+{?V?/V*}n) with V*=const and with n=0.5 for samples in the weak-localization region. However, as the critical region of the MI transition is approached G0?0 and n?1. We also find that for samples lying in the weak-localization region ?=eV* has a well-defined dependence on ?0, the zero-temperature conductivity. The observed behavior can be explained either as a manifestation of depletion of density of states at the Fermi level as the MI transition is approached or as a manifestation of strong inelastic scattering in the junction region.

Non-metallic cold-cathode electron emission from composite metal-insulator microstructures

A detailed investigation has been undertaken into a field-induced electron emission (FIEE) mechanism that occurs at microscopically localised `sites' on uncoated, dielectric-coated and composite-coated metallic cathodes. An optical imaging technique has been used to observe and characterize the spatial and temporal behaviour of the populations of emission sites on these cathodes under various experimental conditions, e.g. pulsed-fields, gas environment etc. This study has shown that, for applied fields of 20MVm^-1, thin dielectric (750AA) and composite metal-insulator (MI) overlayers result in a dramatic increase in the total number of emission sites (typically 30cm^-2), and hence emission current. The emission process has been further investigated by a complementary electron spectroscopy technique which has revealed that the localised emission sites on these cathodes display field-dependent spectral shifts and half-widths, i.e. indicative of a `non-metallic' emission mechanism. Details are also given of a comprehensive investigation into the effects of the residual gas environment on the FIEE process from uncoated Cu-cathodes. This latter study has revealed that the well-known Gas Conditioning process can be performed with a wide range of gas species (e.g. O_2, N_2 etc), and furthermore, the degree of conditioning is influenced by both a `Voltage' and `Temperature' effect. These experimental findings have been shown to be particularly important to the technology of high-voltage vacuum-insulation and cold-cathode electron sources. The FIEE mechanism has been interpreted in terms of a hot-electron process that is associated with `electroformed' conducting channels in MI, MIM and MIMI surface microstructures.

Electronic Structure Evolution across the Peierls Metal-Insulator Transition in a Correlated Ferromagnet

Transition metal compounds often undergo spin-charge-orbital ordering due to strong electron-electron correlations. In contrast, low-dimensional materials can exhibit a Peierls transition arising from low-energy electron-phonon-coupling-induced structural instabilities. We study the electronic structure of the tunnel framework compound K2Cr8O16, which exhibits a temperature-dependent (T-dependent) paramagnetic-to-ferromagnetic- metal transition at T-C = 180 K and transforms into a ferromagnetic insulator below T-MI = 95 K. We observe clear T-dependent dynamic valence (charge) fluctuations from above T-C to T-MI, which effectively get pinned to an average nominal valence of Cr+3.75 (Cr4+:Cr3+ states in a 3:1 ratio) in the ferromagnetic-insulating phase. High-resolution laser photoemission shows a T-dependent BCS-type energy gap, with 2G(0) similar to 3.5(k(B)T(MI)) similar to 35 meV. First-principles band-structure calculations, using the experimentally estimated on-site Coulomb energy of U similar to 4 eV, establish the necessity of strong correlations and finite structural distortions for driving the metal-insulator transition. In spite of the strong correlations, the nonintegral occupancy (2.25 d-electrons/Cr) and the half-metallic ferromagnetism in the t(2g) up-spin band favor a low-energy Peierls metal-insulator transition.

Direct observation of the structural component of the metal-insulator phase transition and growth habits of epitaxially grown VO2 nanowires.

We have grown epitaxially orientation-controlled monoclinic VO2 nanowires without employing catalysts by a vapor-phase transport process. Electron microscopy results reveal that single crystalline VO2 nanowires having a [100] growth direction grow laterally on the basal c plane and out of the basal r and a planes of sapphire, exhibiting triangular and rectangular cross sections, respectively. In addition, we have directly observed the structural phase transition of single crystalline VO2 nanowires between the monoclinic and tetragonal phases which exhibit insulating and metallic properties, respectively, and clearly analyzed their corresponding relationships using in situ transmission electron microscopy.

Predicting a new phase (T′′) of two-dimensional transition metal di-chalcogenides and strain-controlled topological phase transition

Single layered transition metal dichalcogenides have attracted tremendous research interest due to their structural phase diversities. By using a global optimization approach, we have discovered a new phase of transition metal dichalcogenides (labelled as T′′), which is confirmed to be energetically, dynamically and kinetically stable by our first-principles calculations. The new T′′ MoS2 phase exhibits an intrinsic quantum spin Hall (QSH) effect with a nontrivial gap as large as 0.42 eV, suggesting that a two-dimensional (2D) topological insulator can be achieved at room temperature. Most interestingly, there is a topological phase transition simply driven by a small tensile strain of up to 2%. Furthermore, all the known MX2 (M = Mo or W; X = S, Se or Te) monolayers in the new T′′ phase unambiguously display similar band topologies and strain controlled topological phase transitions. Our findings greatly enrich the 2D families of transition metal dichalcogenides and offer a feasible way to control the electronic states of 2D topological insulators for the fabrication of high-speed spintronics devices.

Metal-Insulator Transition in the Hubbard Model on a Triangular Lattice

We discuss the results of an extensive mean-field investigation of the half-filled Hubbard model on a triangular lattice at zero temperature. At intermediate U we find a first-order metal-insulator transition from an incommensurate spiral magnetic metal to a semiconducting state with a commensurate linear spin density wave ordering stabilized by the competition between the kinetic energy and the frustrated nature of the magnetic interaction. At large U the ground state is that of a classical triangular antiferromagnet within our approximation. In the incommensurate spiral metallic phase the Fermi surface has parts in which the wave function renormalization Z is extremely small. The evolution of the Fermi surface and the broadening of the quasi-particle band along with the variation of the plasma frequency and a charge stiffness constant with U/t are discussed.

Magnetism, charge ordering, and metal-insulator transition in the lanthanum manganites

We examine the magnetic and structural properties of the lanthanum manganite-based double-exchange magnets exhibiting colossal magnetoresistance. A model Hamiltonian containing the double-exchange, superexchange, and the Hubbard terms, with parameters obtained from density–functional calculations (Ref. 1), is studied within a mean-field approximation both at temperature T=0 and T>0 and with the effects of the magnetic field included. The phase diagrams we obtain with magnetic and charge-ordered phases enable us to examine the competition between the double- and superexchange terms as functions of doping and temperature. Our theoretical study provides a qualitative understanding of the phase diagram observed in the experiments. © 1997 American Institute of Physics.

Photoelectron spectromicroscopy study of metal-insulator transition in NaxWO3

We have investigated the validity of percolation model, which is quite often invoked to explain the metal-insulator transition in sodium tungsten bronzes, NaxWO(3) by photoelectron spectromicroscopy. The spatially resolved direct spectromicroscopic probing on both the insulating and metallic phases of high quality single crystals of NaxWO(3) reveals the absence of any microscopic inhomogeneities embedded in the system within the experimental limit. Neither any metallic domains in the insulating host nor any insulating domains in the metallic host have been found to support the validity of percolation model to explain the metal-insulator transition in NaxWO(3). The possible origin of insulating phase in NaxWO(3) is due to the Anderson localization of all the states near E-F. The localization occurs because of the strong disorder arising from random distribution of Na+ ions in the WO3 lattice.

Metal-insulator transition characteristics of vanadium dioxide thin films synthesized by ultrasonic nebulized spray pyrolysis of an aqueous combustion mixture

We report the synthesis of high quality vanadium dioxide (VO2) thin films by a novel spray pyrolysis technique, namely ultrasonic nebulized spray pyrolysis of aqueous combustion mixture (UNSPACM). This simple and cost effective two step process involves synthesis of a V2O5 film on an LaAlO3 substrate followed by a controlled reduction to form single phase VO2. The formation of M1 phase (p21/c) is confirmed by Raman spectroscopic studies. A thermally activated metal-insulator transition (MIT) was observed at 61 degrees C, where the resistivity changes by four orders of magnitude. Activation energies for the low conduction phase and the high conduction phase were obtained from temperature variable resistance measurements. The infrared spectra also show a dramatic change in reflectance from 13% to over 90% in the wavelength range of 7-15 mu m. This indicates the suitability of the films for optical switching applications at infrared frequencies.

Tuning the metal-insulator transition in NdNiO3 heterostructures via Fermi surface instability and spin fluctuations

We employed in situ pulsed laser deposition (PLD) and angle-resolved photoemission spectroscopy (ARPES) to investigate the mechanism of the metal-insulator transition (MIT) in NdNiO3 (NNO) thin films, grown on NdGaO3(110) and LaAlO3(100) substrates. In the metallic phase, we observe three-dimensional hole and electron Fermi surface (FS) pockets formed from strongly renormalized bands with well-defined quasiparticles. Upon cooling across the MIT in NNO/NGO sample, the quasiparticles lose coherence via a spectral weight transfer from near the Fermi level to localized states forming at higher binding energies. In the case of NNO/LAO, the bands are apparently shifted upward with an additional holelike pocket forming at the corner of the Brillouin zone. We find that the renormalization effects are strongly anisotropic and are stronger in NNO/NGO than NNO/LAO. Our study reveals that substrate-induced strain tunes the crystal field splitting, which changes the FS properties, nesting conditions, and spin-fluctuation strength, and thereby controls the MIT via the formation of an electronic order parameter with QAF similar to (1/4,1/4,1/4 +/- delta).

Incommensurate spin-density-wave and metal-insulator transition in the one-dimensional periodic Anderson model

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Unoccupied electronic states in NiS2-xSex across the metal-insulator transition

We investigate the evolution of the electronic structure across the insulator-metal transition in NiS2-xSex with changing composition, but in the absence of any structural or magnetic changes. A comparison of the inverse photoemission spectra with band-structure calculations establishes the importance of correlation effects in these systems. Systematic changes in the spectral distribution establish the persistence of the upper Hubbard band well into the metallic regime, with the insulator-to-metal transition being driven by a transfer of spectral weight from the Hubbard band to states close to the Fermi energy.