Metal-like Behavior Research Articles

Anomalous Hall Effect in Ge1-x-yPbxMnyTe Composite System

The purpose of this study was to investigate the magnetotransport properties of the Ge0.743Pb0.183Mn0.074Te mixed crystal. The results of magnetization measurements indicated that the compound is a spin-glass-like diluted magnetic semiconductor with critical temperature TSG = 97.5 K. Nanoclusters in the sample are observed. Both, matrix and clusters are magnetically active. Resistivity as a function of temperature has a minimum at 30 K. Below the minimum a variable-range hopping is observed, while above the minimum a metallic-like behavior occurs. The crystal has high hole concentration, p = 6.6× 10 cm−3, temperature-independent. Magnetoresistance amplitude changes from 0.78 to 1.18% with increase of temperature. In the magnetotransport measurements we observed the anomalous Hall e ect with hysteresis loops. Calculated anomalous Hall e ect coe cient, RS = 2.0× 10 m/C, is temperature independent. The analysis indicates the extrinsic skew scattering mechanism to be the main physical mechanism responsible for anomalous Hall e ect in Ge0.743Pb0.183Mn0.074Te alloy.

Optical Properties of Triangular Molybdenum Disulfide Nanoflakes.

The results from calculations of optical and electronic properties of triangular MoS2 nanoflakes with edge lengths ranging from 1.6 to 10.4 nm are presented. The optical spectra were calculated using the time-dependent extension of the density-functional tight-binding method (TD-DFTB). The size effect in the optical absorption spectra is clearly visible. With decreasing length of the nanoflakes edges, the long-wavelength absorption in the range of visible light is shifted toward short-wavelength absorption, confirming a quantum-confinement-like behavior of these flakes. In contrast, the edges of the nanoflakes exhibit a distinct metallic-like behavior. The relation of the absorption properties to the observed photoluminescence of MoS2 nanoflakes is discussed in a qualitative manner.

Do dielectric nanostructures turn metallic in high-electric dc fields?

Three-dimensional dielectric nanostructures have been analyzed using field ion microscopy (FIM) to study the electric dc field penetration inside these structures. The field is proved to be screened within a few nanometers as theoretically calculated taking into account the high-field impact ionization process. Moreover, the strong dc field of the order of 0.1 V/Å at the surface inside a dielectric nanostructure modifies its band structure leading to a strong band gap shrinkage and thus to a strong metal-like optical absorption near the surface. This metal-like behavior was theoretically predicted using first-principle calculations and experimentally proved using laser-assisted atom probe tomography (APT). This work opens up interesting perspectives for the study of the performance of all field-effect nanodevices, such as nanotransistor or super capacitor, and for the understanding of the physical mechanisms of field evaporation of dielectric nanotips in APT.

Crystal Structure and Physical Properties of Ternary Phases around the Composition Cu5Sn2Se7 with Tetrahedral Coordination of Atoms

A new monoclinic selenide Cu5Sn2Se7 was synthesized, and its crystal and electronic structure as well as thermoelectric properties were studied. The crystal structure of Cu5Sn2Se7 was determined by electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data: space group C2, a = 12.6509(3) A, b = 5.6642(2) A, c = 8.9319(4) A, β = 98125(4)°, Z = 2; T = 295 K. Thermal analysis and high-temperature synchrotron X-ray diffraction indicated the decomposition of Cu5Sn2Se7 at 800 K with formation of the tetragonal high-temperature phase Cu4.90(4)Sn2.10(4)Se7: space group I42m, a = 5.74738(1) A, c = 11.45583(3) A; T = 873 K. Both crystal structures are superstructures to the sphalerite type with tetrahedral coordination of the atoms. In agreement with chemical bonding analysis and band structure calculations, Cu5Sn2Se7 exhibits metal-like electronic transport behavior.

Low temperature electronic transport in sputter deposited a-IGZO films

We report on the electrical properties of a-IGZO thin films prepared by reactive sputtering. Without oxygen injection, dc resistivity measured at room temperature is ρ300K = 1.22 × 10−3 Ωm. The lowest resistivity ρ300K = 4.86 × 10−5 Ωm is obtained at a certain oxygen supply into the deposition process. Hall effect measurements of these films reveal a metallic-like behavior from mobility and carrier concentration vs. temperature in the range 15–300 K whereas films deposited without oxygen or for the highest oxygen flows behave as semiconductors. These enhanced electrical properties are connected to the oxygen vacancies and the local coordination structure around the In3+ cations.

Transparent conducting oxide nanotubes

Thin film or porous membranes made of hollow, transparent, conducting oxide (TCO) nanotubes, with high chemical stability, functionalized surfaces and large surface areas, can provide an excellent platform for a wide variety of nanostructured photovoltaic, photodetector, photoelectrochemical and photocatalytic devices. While large-bandgap oxide semiconductors offer transparency for incident light (below their nominal bandgap), their low carrier concentration and poor conductivity makes them unsuitable for charge conduction. Moreover, materials with high conductivity have nominally low bandgaps and hence poor light transmittance. Here, we demonstrate thin films and membranes made from TiO2 nanotubes heavily-doped with shallow Niobium (Nb) donors (up to 10%, without phase segregation), using a modified electrochemical anodization process, to fabricate transparent conducting hollow nanotubes. Temperature dependent current–voltage characteristics revealed that TiO2 TCO nanotubes, doped with 10% Nb, show metal-like behavior with resistivity decreasing from 6.5 × 10−4 Ωcm at T = 300 K (compared to 6.5 × 10−1 Ωcm for nominally undoped nanotubes) to 2.2 × 10−4 Ωcm at T = 20 K. Optical properties, studied by reflectance measurements, showed light transmittance up to 90%, within wavelength range 400 nm–1000 nm. Nb doping also improves the field emission properties of TCO nanotubes demonstrating an order of magnitude increase in field-emitter current, compared to undoped samples.

Relation between Thermally Induced Structural Distortions and Electronic Properties of the Layered Misfit Chalcogenide (LaS)1.196VS2

Interplay between electronic and structural properties of the misfit layered chalcogenide (LaS)1.196VS2 crystals was investigated by transport, optical measurements, angle-resolved photoemission (ARPES), Raman spectroscopy, and X-ray diffraction. Although no clear anomaly is found in temperature-dependent transport measurements, a large spectral weight transfer, around 1 eV, is observed by both optical and photoemission spectroscopies. ARPES reveals a nearly filled band with negative curvature, close enough from the Fermi level at room temperature to produce metallic-like behavior as observed in optical conductivity spectra. At low temperature, the band structure is strongly modified, yielding to an insulating state with an optical gap of 120 meV. Both Raman spectroscopy and accurate (3 + 1)D analysis of X-ray diffraction data show that, although a phase transition does not occur, structural distortions increase as temperature is decreased, and vanadium clusterization is enhanced. We found that the change...

Electronic properties and Compton scattering studies of monoclinic tungsten dioxide

We present the first ever Compton profile measurement of WO2 using a 20 Ci 137Cs γ-ray source. The experimental data have been used to test different approximations of density functional theory in linear combination of atomic orbitals (LCAO) scheme. It is found that theoretical Compton profile deduced using generalized gradient approximation (GGA) gives a better agreement than local density approximation and second order GGA. The computed energy bands, density of states and Mulliken׳s populations (MP) data confirm a metal-like behavior of WO2. The electronic properties calculated using LCAO approach are also compared with those obtained using full potential linearized augmented plane wave method. The nature of bonding in WO2 is also compared with isoelectronic WX2 (X=S, Se) compounds in terms of equal-valence-electron-density profiles and MP data, which suggest an increase in ionic character in the order WSe2→WS2→WO2.

Correlation between magnetic and electrical properties of Co0.6Sn0.4Fe2O4 nanoparticles

The Co0.6Sn0.4Fe2O4 nanoparticles with 20-nm size were prepared by chemical coprecipitation method. The material exhibits wasp-waist type M(H) loop at lower temperatures (below 360 K) that has been attributed to the simultaneous existence of double-exchange and superexchange interactions in these nanoparticles. It has been investigated that at 360 K and above, the double-exchange interactions (Fe3+–O2−–Fe2+ and Co2+–O2−–Fe3+) are stronger than the superexchange (Fe3+–O2−–Fe3+) and vice versa. The temperature-dependent ac conductivity reveals the semiconductor–metal transition around 360 K. It is suggested that around 360 K, the metal-like behavior of nanoparticles is due to the dominant ferromagnetic order in the system, while below this temperature, the charge carriers are thermally activated and the material becomes antiferromagnetic in nature.

Electrical and thermal transport properties of intermetallic RCoGe2 (R = Ce and La) compounds

To investigate the electronic structure of the intermetallic compound CeCoGe2, we performed electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements in a temperature range of 10–300 K. For comparison, the non-magnetic counterpart LaCoGe2 is also studied. It is found that CeCoGe2 exhibits a broad maximum in the S(T) near 75 K, at which the sudden drop in the ρ(T) is observed. Temperature-dependent electrical resistivity and the Seebeck coefficient of CeCoGe2 can be described well by a two-band model, which reveals the signature of Kondo scattering in CeCoGe2. On the other hand, a typical metallic-like behavior is seen in the non-magnetic LaCoGe2 from the ρ(T) and S(T) studies. Analysis of the thermal conductivity indicates that the electronic contribution dominates thermal transport above 100 K in both CeCoGe2 and LaCoGe2. In addition, it is found that the variation in low-temperature lattice thermal conductivity of CeCoGe2 as compared to that of LaCoGe2 is most likely due to the phonon-point-defect scattering.

Electrical response in atomic layer deposited Al:ZnO with varying stack thickness

We report on the effects of stacking of the macrocycles in atomic layer deposited (ALD) Al:ZnO thin films on the structural and electrical properties. There is a large change in the resistivity ranging from as high as 1.19 × 10−3 Ω cm for 760 growth cycles film down to as low as 7.9 × 10−4 Ω cm for the 4000 cycles. The electrical transport demonstrates a transition from a semiconductor behavior at 760 cycles to a metallic behavior in the 4000 cycle, due to an increase in electron scattering as well as increase in the carrier concentration. However, interestingly the carrier concentration sharply increases with increasing macrocycles containing Al and Zn, exhibiting a nearly metal-like behavior in thicker films. We anticipate that the change in Zn-vacancy, Vzn, formation energy is related to the increase in surface area of the ALD precursor deposition plane. The increase in Vzn density allows for more adsorption of Al-precursor into the doped monolayer, showing interesting electrical properties.

Density functional theory study of Rh(n)S(0,±) and Rh(n+1)(0,±) (n = 1-9).

A basin-hopping search strategy has been used to determine likely candidates for low-energy Rh(n)S(0,±) (n = 1-9) cluster structures. Cluster structures were optimized at the density functional level of theory using the PBE and PBE0 functionals. Ionization energies, electron detachment energies, HOMO-LUMO gap energies, UV-visible spectra, low-lying fragmentation channels and energies, cluster structures, spin multiplicities, and vibrational spectra are predicted for Rh(n)S(0,±) and Rh(n+1)(0,±) (n = 1-9). Donation from valence sulfur atomic p orbitals to valence rhodium atomic d orbitals and back-donation to valence sulfur atomic d orbitals leads to electron density delocalization and metal-like behavior for rhodium sulfide clusters.

Electrical conduction mechanisms of Au/NiO/heavily doped p-type Si memory devices

The fabrication of memory devices based on the Au/NiO/heavily doped p-type Si (p+-Si) structures and their current-voltage characteristics were reported. The Au/NiO/p+-Si devices show hysteresis behavior. The fitting data of the temperature-dependent off-state current–voltage curves demonstrated that the carrier transport mechanism of the Au/NiO/p+-Si device was attributed to the space change limited conduction. However, the difference between the temperature-dependent on-state currents in the forward-bias and reverse-bias regions was found. The different electrical conduction mechanisms (hopping conduction and Ohmic conduction with metal-like behaviors) were discussed. This phenomenon is related to the different interfacial characteristics between Au/NiO and NiO/p+-Si.

Ferromagnetic ordering and metallic-like conductivity in sputtered SnNx films

Polycrystalline SnNx films were fabricated by reactive sputtering with various substrate temperatures. The film has a hexagonal tin-rich structure and decomposition temperature is above 500°C. The band-gap enlarged to 3.10eV due to the complex defects and the conductivity of the films shows metallic-like behavior, which has the opposite variation trend with ferromagnetism as the substrate temperature changes. The origin of ferromagnetism may be from the intrinsic point defects, which result in a net moment by unpaired spin electrons. The feeble coercivity with no obvious magnetocrystalline anisotropy suggests that the magnetism in our samples is not a bulk effect, and it distributes unevenly in the grain boundaries or interfaces. Furthermore, thermal decomposition also has significant influence on the magnetic and transport properties.

The effects of Si doping on the thermoelectric and magnetic properties of Ca0.98Bi0.02Mn1−xSixO3−δ with x=0.00, 0.02 and 0.03

• The Si doping element has remarkable effects on the thermoelectric and magnetic properties of the system. • Ca 0.98 Bi 0.02 Mn 0.98 Si 0.02 O 3− δ exhibits the highest dimensionless figure of merit of 0.045 at 540 K. • The Neel temperature occurred at around 120 K for all the samples. Ca 0.98 Bi 0.02 Mn 1− x Si x O 3− δ ( x = 0.00, 0.02 and 0.03) samples are prepared by solid state reaction. All the samples are single phase with an orthorhombic structure. A metal-like behavior is observed for x = 0.00, and it switches to nonmetal-like behavior for x = 0.03. Co-substituting bismuth for calcium and silicon for manganese could modify the electronic states and hence the transport properties. The thermopower of all the samples is negative, indicating that the predominant carriers are electrons over the entire temperature range. The electrical resistivity, thermopower increase and thermal conductivity decreases with increasing Si content. Among the samples, Ca 0.98 Bi 0.02 Mn 0.98 Si 0.02 O 3− δ exhibits the highest dimensionless figure of merit (0.045), which is about 84% higher than that of the undoped at 540 K. Magnetic moments of doped samples decrease as compared to undoped sample. The Neel temperature occurred at around 120 K for all the samples.

Determination of thicknesses of oxide films grown on titanium under argon irradiation by spectroscopic ellipsometry

In this article we present a study of the oxidation of pure titanium bulk samples under argon ion irradiation at 500°C under rarefied air. In particular we follow the dependence of the oxide thickness as a function of the energy of argon ions. The novelty of this study consists in the range of ion energy explored, from 1 to 9MeV. Until very recently it was commonly accepted that metal surfaces were transparent to ion beams in this low energy range (few MeV), and no surface modifications were expected. In a previous paper by the authors of this work, the formation of shallow craters in the surface of titanium was reported as a result of argon ion bombardment with energies of 2, 4 and 9MeV under the same environmental conditions. We show here that around 3MeV the oxide growth is unexpectedly enhanced. We think that an interplay of electronic excitations and nuclear ballistic collisions could possibly explain this enhanced oxide growth. We have used spectroscopic visible ultraviolet ellipsometry and XPS to determine the thickness of the oxide layers and characterize their optical properties. From the optical properties of the oxides we observed that for ion energies below 3–4MeV the oxides show a dielectric-like behavior, whereas for ion energies above 3–4MeV the oxides show a metal-like behavior. These findings indicate also that ion bombardment in this energy range may change substantially the oxygen-to-titanium ratio in the oxide films grown under irradiation leading to the formation of titanium sub-oxides.

Electronic structure of Al- and Ga-doped ZnO films studied by hard X-ray photoelectron spectroscopy

Al- and Ga-doped sputtered ZnO films (AZO, GZO) are semiconducting and metallic, respectively, despite the same electronic valence structure of the dopants. Using hard X-ray photoelectron spectroscopy we observe that both dopants induce a band in the electronic structure near the Fermi level, accompanied by a narrowing of the Zn 3d/O 2p gap in the valence band and, in the case of GZO, a substantial shift in the Zn 3d. Ga occupies substitutional sites, whereas Al dopants are in both substitutional and interstitial sites. The latter could induce O and Zn defects, which act as acceptors explaining the semiconducting character of AZO and the lack of variation in the optical gap. By contrast, mainly substitutional doping is consistent with the metallic-like behavior of GZO.

Revealing the metal-like behavior of iodine: an iodide-catalysed radical oxidative alkenylation

In this work, we have described an alternative alkenylation approach to illustrate the metal-like behaviour of iodine in cross-coupling reactions. Alkenylation could proceed through iodide catalysed radical initiation, radical addition and iodine promoted alkenyl functionality recovery. Catalytic HI elimination similar to the β-hydride elimination of transition metals was realized for the radical alkenylation of sulfonyl hydrazides. Operando IR and cyclic voltammetry experiments were carried out to confirm the crucial role of iodine in the radical alkenylation process.

Highly conductive SiC ceramics containing Ti2CN

Abstract Highly conductive SiC ceramics were fabricated by sintering β-SiC and TiN powder mixture in N 2 atmosphere. SiC ceramics exhibited decreased electrical resistivity ( ρ ) with increasing TiN content. X-ray diffraction data indicated that the specimens consisted of β-SiC grains without a detectible secondary phase for low TiN content (≤2 vol%) but contained a Ti 2 CN phase as the TiN content increased. The temperature-dependent resistivity ρ ( T ) of specimens revealed semiconductor-like behavior for TiN content up to 10 vol% and metal-like behavior above 20 vol%. For the specimen with TiN content of 15 vol%, ρ ( T ) remained almost constant (2.06 ± 0.01 × 10 −3 Ω cm) in the 4–300 K range. The resistivity of metal-like specimens were as low as 3.5 × 10 −4 Ω cm for TiN content of 20 vol%. For semiconductor-like specimens, ρ ( T ) was primarily affected by N donors in the β-SiC grains. Metal-like specimens were primarily affected by metallic Ti 2 CN clusters.

Structural heterogeneity induced plasticity in bulk metallic glasses: From well-relaxed fragile glass to metal-like behavior

To reveal the structural origin responsible for the sharp change of the fracture mode on the as-cast and thermally-relaxed status, we use nanomechanical testing to measure the stresses for the onset of plasticity of a metallic glass and develop a stochastic statistical model, which can be used to characterize structural heterogeneity (defect density and strength) inside the metallic glass. Our experiments and calculations found that, with increasing the structural relaxation, the defect density drops by two orders of magnitude. Correspondingly, the fracture of metallic glasses changes from a significantly plastic (metal-like) mode to an extremely brittle (fragile glass) one.