Sharp Metal-insulator Transition Research Articles

Double peak behavior of resistivity curves in Cd doped LaMnO 3 perovskite systems

The effect of Cd doping on transport, magnetotransport, and magnetic properties has been investigated in the perovskite La 1− x Cd x MnO 3 (0⩽ x⩽0.5) systems. The ρ( T) curves exhibit a sharp metal insulator transition ( T p1), which is close to paramagnetic to ferromagnetic transition ( T c) obtained from M− T curves for all samples. In addition, ρ( T) curves for Cd doped samples exhibit another broad transition ( T P2) below T p1. This transition becomes more prominent and the transition temperature ( T p2) shifts to lower temperature with increasing Cd content. Such double peak behavior in the ρ( T) curve is attributed to the phase separation between the ferromagnetic metallic phases and the ferromagnetic insulating phases induced by the electronic inhomogeneity in the samples.

Beta' '-(BEDT-TTF)4[(H3O)Cr(C2O4)3]CH2Cl2: effect of included solvent on the structure and properties of a conducting molecular charge-transfer salt.

The new charge-transfer salt β″-(BEDT-TTF)[(HO)Cr(CO )]S, where BEDT-TTF = bis-(ethylenedithio)tetrathiafulvalene and S = CHCl has a broad metal-insulator transition at approximately 150 K. This differs from similar salts in the BEDT-TTF tris(oxalato) metalate series, notably the superconducting materials with S = PhCN and the salt with S = CHN, which has a sharp metal-insulator transition.

Perovskite heterostructures grown by MOCVD

3d-metal perovskites are promising thin film materials with a great variety of electrical and magnetic properties. We have deposited the epitaxial heterostructures including different combinations of CMR manganites (La 1-x Pr x ) 0.7 Ca 0.3 MnO 3 , (x=0-1), metallic nickelates RNiO 3 (R = Pr, Nd, Sm) with sharp metal-insulator transition and antiferromagnetic insulators RFeO 3 (R = Nd, Eu). The heterostructures were characterised by XRD, SEM, EDX, HREM, RBS, electric and magnetic measurements. Particular attention is paid to the lattice strain in the layers and the structure of their interfaces. The prototype electronic devices based on the heterostructures are discussed.

Transport properties of La0.8Ca0.2MnO3 epitaxial films prepared by rf magnetron sputtering using soft targets

Epitaxial La0.8Ca0.2MnO3 films have been prepared by the rf magnetron sputtering method using so-called ”soft” (or powder) targets instead of solid-state ones. The as-grown films have a highly oriented perovskite-like structure, a sharp metal–insulator transition in zero magnetic field at a temperature (Tp) near 220 K, and a temperature coefficient of resistance (TCR) of 900%. We found that a reduction in the film thickness leads to a shift of Tp in the low-temperature range and a significant suppression in the TCR value. The obtained films also exhibited an anisotropic temperature behavior of the magnetoresistance in a magnetic field of 1.5 T.

Synthesis and physical properties of (DIETS) 2[Au(CN) 4]: A new θ-salt with a unique donor⋯anion network

A new θ-type salt, (DIETS) 2 [Au(CN) 4 ] (DIETS = diiodo(ethylenedithio)dithiadiselenafulvalene) has been prepared. A characteristic I…N intermolecular contact (d I … N = 3.018(7) A) exists in the crystal and forms a unique supramolecular donor…anion network. Although the calculated Fermi surface is a closed two-dimensional one, a sharp metal-insulator transition occurs at 226 K. The transition temperature is lowered under high pressure, which is in contrast to the pressure effect in the conventional θ-salts of BEDT-TTF (ET = bis (ethylenedithio)tetrathiafulvalene).

Growth of epitaxial and polycrystalline thin films of the electron doped system La1−xCexMnO3 through pulsed laser deposition

The polycrystalline La1−xCexMnO3 manganites do not exist in single phase in bulk under the preparation conditions so far employed, but their polycrystalline and epitaxial films deposited by the pulsed laser deposition (PLD) technique form readily in single phase. The cerium oxide (CeO2) remains partially unreacted when the bulk sample is prepared through the solid state reaction route. The resistivity of the bulk La0.7Ce0.3MnO3 sample shows a broad metal insulator transition (MIT) clearly resolved into two peaks, suggesting the presence of a second (impurity) phase, which is identified as unreacted CeO2 by the intensity analysis of the x-ray diffraction (XRD) data. However, when prepared as thin films by PLD, La0.7Ce0.3MnO3 forms in single phase, as corroborated by the uniqueness and sharpness of the MIT peak and also by the XRD patterns of the polycrystalline films. We also performed a detailed study of the epitaxial films by a high-resolution XRD system with a four-circle goniometer and did not find any impurity phase. The magnetization data shows a very sharp transition followed by a sharp MIT in resistivity at the same temperature in the epitaxial thin film. These results suggest that PLD can be used as a useful technique to synthesize unconventional compounds, which do not form easily in bulk.

Preparation and Properties of Stoichiometric Vanadium Oxides

Stoichiometric films of the V2O3–V2O5system, including the Magneli phases V n O2n– 1(3 ≤n≤ 9), were prepared by thermal oxidation of metallic vanadium between 720 and 950 K in quartz tubes evacuated to a residual pressure below 10–3Pa. The oxidation was performed in the presence of powder mixtures of two vanadium oxides with adjacent stability fields so as to ensure an oxygen partial pressure corresponding to the stoichiometry of the required vanadium oxide. The oxide films thus prepared showed a sharp metal–insulator transition, with a conductivity change comparable to that in single crystals.

Low-temperature MFM studies of CMR manganites

We have performed low-temperature Magnetic Force Microscopy (MFM) studies of colossal magneto-resistive (CMR) (La 0.67Ca 0.33)MnO 3 films prepared by physical vapor codeposition (PVCD). The samples exhibit sharp metal-insulator transitions at T c =265 K, and 120 nm grain structures. MFM studies performed down to T = 77 K showed irregularly-shaped magnetic features at temperatures below T c . Characteristic magnetic domain sizeswere 1–2 μm, which exceeds the grain size for these films, implying coherence across grain boundaries in this system.

分子性伝導体（ＤＭｅ‐ＤＣＮＱＩ）２Ｃｕの物性と低温構造

The mixed-valence Cu salt of DMe-DCNQI provides unique physical properties due to the interaction between the one-dimensional (1D) organic pπ band and the d-orbital of Cu ion. This system is metallic down to very low temperature at ambient pressure in spite of the 1D column structure, whereas the system exhibits a sharp metal-insulator (M-I) transition accompanied by a threefold superstructure under pressure. Reentrant behavior (M-I-M transition) is observed in the boundary pressure region. The selective dueteration of (DMe-DCNQI) 2Cu has enabled to control the chemical pressure. Application of this technique produces the Petf-T phase diagram of this system under ambient pressure. The effect of (chemical) pressure on these transitions in the system is generally understood from the change in charge transfer amount from metal ions to DCNQI molecules by low temperature crystal structure analysis and a doping method.

B-Site Doped Lanthanum Strontium Manganites by the DAAS Technique

AbstractMagnetic perovskites of the general form La0.7Sr0.3Mn1−yFeyO3 (y = 0, 0.05, 0.10, and 0.15) have successfully been synthesized using deposition by aqueous acetate solution (DAAS). Crystalline, iron-doped, lanthanum strontium manganite (Fe-doped LSMO) powders are obtained by preparing an aqueous solution of metal acetate precursors in the proper stoichiometry, drying the solution to generate a glassy gel, consolidating the gel, and then firing the gel for short periods of time (<2hrs). This novel technique has the potential for depositing large area thin films with high throughput and low cost. Powder samples of La0.7Sr0.3Mn1−yFeyO3 prepared by DAAS and annealed for 100 minutes at 1200°C are of high purity, are single phase, and exhibit excellent electrical and magnetic characteristics. Powders annealed at 1200°C or greater exhibit sharp metalinsulator transitions. Increasing the iron dopant concentration in these powders from 0% to 15% decreases the metal-insulator transition temperature of these samples from ∼360K to about 140K. The resistivity of these powders also increases with increasing substitution of the lattice B-site with iron, as does the unit cell volume of the lattice. Preparation of an iron doped LSMO powder that exhibits a maximum magnetoresistance at 305K was successfully carried out via a careful selection of iron content and anneal temperature. This compound, La0.7Sr0.3Mn0.93Fe0.07O3, exhibits a magnetoresistance of 40% at 305K and an applied field of 5 Tesla.

Field-Restored Highly Conducting State in .LAMBDA.-BETS2FeCl4 and .LAMBDA.-BETS2FeBr0.6Cl3.4 under Pressure.

Magnetoresistance measurement of λ-BETS2FeCl4 and λ-BETS2FeBr0. 6Cl3. 4 were performed under pressure. λ-BETS2FeBr0. 6Cl3. 4 showed a sharp metal-insulator (MI) transition at 13. 7K under ambient pressure, which is 5. 2K higher than the MI transition temperature (TMI) of λ-BETS2FeCl4. Under magnetic field, TMI of λ-BETS2FeBr0. 6Cl3. 4 was almost constant up to 12 T. But at high pressure, λ-BETS2FeBr0. 6Cl3. 4 showed the field-restored highly conducting state (FRHCS). Magnetic susceptibility measurement of λ-BETS2FeBr0. 6Cl3. 4 showed no clear indication of antiferromagnetic transition at least down to 0. 1 T for the magnetic field parallel to the c axis.

Anomalous Metal–Insulator Transitions in Reduced Molybdenum Oxides,A4Mo18O32(A=Ca, Y, Gd–Yb) with Mon(n=2, 4, 6) Cluster Chains

Single crystals ofA4Mo18O32(A=Ca, Y, Gd–Yb) large enough for transport measurements were grown by high-temperature reactions or molten-salt electrolysis and their electrical and magnetic properties were investigated. The crystal structure of these phases indicates the presence of three distinctly different Mo metal atom clusters (Mo2, Mo4, and Mo6), which extend in complex chains along the monoclinicbaxis. The temperature-dependent resitivity displays a sharp metal–insulator transition in the range 70–120 K for the Y and all the rare earth members. The metal–insulator transition temperature (TIM) was observed to increase with decreasing size of the rare earth ions. No anomalies were observed in their magnetic susceptibilities at temperatures in the vicinity of metal–insulator transition. The corresponding Ca phase, however, is semiconducting. An attempt has been made to correlate the observed physical properties with the structural arrangement of the Moncluster chains.

Reflectivity, transmission, and photoinduced infrared spectra ofNdNiO3

We report temperature-dependent infrared spectra of ${\mathrm{NdNiO}}_{3}$ prepared by synthesis under high oxygen pressure. Its room-temperature reflectivity is characteristic of a metal oxide with a Drude tail asymptotically extending to higher frequencies. The sharp metal-insulator transition is detected at about 200 K as phonons become unscreened. While their activity is in the frequency range for a distorted perovskite, the strength of involving the Nd ion against the ${\mathrm{NiO}}_{6}$ octahedra at $183{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is remarkable. This and all phonon groups show strong antiresonances near the longitudinal optical frequencies and these are unusually close to absorptions seen in transmission measurements. We interpret this as evidence of strong electron-phonon interactions with carrier localization in a temperature-dependent regime. Low-temperature photoinduced spectra, excited with argon laser lines, support this picture by showing weak absorptions at about those frequencies as well as a relatively very strong photoinduced band at $810{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and one very broad and weak peak at $\ensuremath{\sim}2300{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and tail up to $4000{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}.$ They are assigned to electronic transitions involving localized defects. Our measurements suggest that the metal-insulator phase transition in ${\mathrm{NdNiO}}_{3}$ is triggered by electron localization in a polaronic environment. We conclude that a suitable order parameter for perovskites with a metal-insulator phase transition may be identified in the coupling between those electrons and local spins

Magnetoresisnve La-Sr-Mn-O Powders and Films by Daas Technique

ABSTRACTMagnetic perovskites of the general form La1-xSrxMnO3 have successfully been synthesized using deposition by aqueous acetate solution (DAAS). This novel technique, which has the potential for depositing large area min films with high throughput and low cost, involves the preparation of an aqueous metal acetate precursor solution, drying the solution to generate a glassy gel, consolidating the gel, and then firing it for short periods of time (<2hrs) to produce crystalline lanthanum strontium manganate (LSMO). The DAAS method has been used to prepare both powders and thin films of LSMO. Powder samples of La0.83Sr0.17MnO3 annealed for 100 minutes at 1200°C were high purity, single phase, and exhibited excellent electrical and magnetic characteristics. Thin films of La0.7Sr0.17MnO3 deposited onto both sapphire and strontium titanate substrates and annealed at 900°C were also found to be crystalline, and substrate choice was found to influence thin film crystal structure. These films exhibited sharp metal-insulator transitions, and in the case of LSMO films on strontium titanate, magnetoresistance was observed at the unusually high temperature of ∼360K.

Electric-field-induced Mott metal-insulator transition.

The Mott metal-insulator transition induced by an external electric field is discussed based on an analysis of a double-layered two-dimensional lattice model. A sharp transition between a metallic phase with the antiferromagnetic order and a Mott insulating phase is found to be driven by the voltage difference between the two layers. A sharp metal-insulator transition is transformed to a smooth metal-insulator transition by a change in the coupling between sites This model is applicable to a Mott transition in a two-dimensional array of very small quantum dots whose shape is asymmetric in the direction perpendicular to the substrate. \textcopyright{} 1996 The American Physical Society.

New BETS Conductors with Magnetic Anions (BETS = bis(ethylenedithio)tetraselenafulvalene)

The preparation, crystal structures, and electric and magnetic properties of (BETS)2MX4 molecular conductors (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Mn2+, Fe3+, Co2+, Ni2+, Cu2+; X = Cl, Br, CN) are reported. Resistivity measurements down to 2 K reveal the coexistence of the BETS π conduction electrons and the localized magnetic moments of the anions in the κ-(BETS)2FeCl4 and κ-(BETS)4(CoCl4)(C2H3Cl3) salts. Another FeCl4 phase, λ-(BETS)2FeCl4, undergoes a sharp metal-insulator (MI) transition around 8 K. At the same temperature, a magnetic transition of the FeCl4-anions takes place cooperatively. A superconducting transition is observed at 4.6 K in λ-(BETS)2(FeCl4)0.5(GaCl4)0.5, where half of the anion sites are occupied by magnetic ions. The crystals prepared from 1,1,2-trichloroethane solutions with the NiCl42- and MnCl42- anions exhibit the behavior of a semimetal down to ≈100 K. The (BETS)4(Cu2Cl6) salt remains metallic down to 4.2 K. ESR studies show that the Fe3+ ions in κ- and λ-(BETS...

Niobium induced metal-superconductor to metal-insulator transition in the YBCO system

In this work a novel high-T/sub c/ compound of the form YBa/sub 2/Cu/sub 3(1-x)/Nb/sub x/O/sub 7/ as been synthesized and investigated for a broad range of Nb contents O<x<0.9. The compound displays a sharp metal-insulator (M-I) transition at x=0.21 associated with an increase in resistivity in the range x<0.21 of approx. One order of magnitude. The transition temperature T/sub c/ in contrast, remains fairly constant at approx. 90 K. It was shown that Nb added to YBCO results in a formation of dielectric inclusions YBa/sub 2/NbO/sub 6/, Y/sub 2/BaCuO/sub 5/, and CuO, which dilute the superconducting grains. For the case x>0.21 corresponding to a no-superconducting state, the conduction mechanism is adequately described through the variable-range-hopping (VRH) model. Comparison of experimental data with theory provides an estimation of both, carrier localization length and density of states at the Fermi energy.<<ETX>>

Angular and magnetic field dependencies of the magnetoresistance of λ-(BETS) 2FeCl 4 in the field-restored high conductivity state

The very sharp metal-insulator transition, which occurs at 8 K at ambient pressure in λ-(BETS) 2FeCl 4 is suppressed when applying magnetic fields >10T. This unusual phenomenon may be connected to ferromagnetic ordering of the Fe 3+ moments, which could drive some “evaporation” of the previously condensed conduction electrons, into a field-restored highly conducting state (FRHCS) in which oscillations of the magnetoresistance are reported.

New organic superconductor, λ-(BETS)2GaCl4 and metal-insulator transition of BETS conductor with magnetic anions (BETS=bis(ethlenedithio)tetraselenafulvalene)

Abstract Crystal structures and electronic properties of BETS compounds with tetrahedral anions, MX4 (BETS=bis(ethylenedithio)tetraselenafulvalene; MGa, Fe, In; XCl, Br) were examined. The salts can be classified into two types (κ, λ). All the compounds with so-called κ-type molecular arrangements retained metallic behavior down to 4 K. λ-(BETS)2GaCl4 demonstrated a a superconducting transition around 8 K. Whereas λ-(BETS)2FeCl4 with the isomorphic λ-type structure exhibited a sharp metal-insulator transition at 8 K. Low-temperature X-ray studies showed that the structural difference between λ-GaCl4 and λ-FeCl4 salts is very small. The structure analysis of κ-(BETS)2FeCl4 at 18 K indicated that the FeCl4 anion has approximately C3v symmetry. The ESR measurements revealed the high-spin state of Fe3+ in λ-FeCl4 salt and and low-spin state in κ-FeCl4 salt. ESR intensities of κ- and λ-FeCl4 salts suggested antiferromagnetic interaction between Fe3+ ions.

Metal - insulator transition in CuIr 2S 4 : Comparison with CuIr 2Se 4

CuIr 2S 4 exhibits a sharp metal-insulator transition around 230 K. The metal-insulator transition of CuIr 2S 4 is ascribed to the structural phase transition from the cubic high-temperature metallic phase to the tetragonal low-temperature insulating phase. There is no localized magnetic moment in both of metallic and insulating phases. On the other hand, CuIr 2Se 4 stays metallic down to 0.5 K without any anomaly. The magnetic susceptibility of CuIr 2Se 4 shows a weak positive temperature dependence of dχ/dT>0 with rather large diamagnetism of core electrons, which might indicate a characteristic of strongly-correlated electrons.