Nonmetallic Regime Research Articles

Probing half-metallicity in Mn2CoSi/Si(100) thin film structures using electrical transport measurements towards spintronic applications

The structural, magnetic, and electronic transport properties of Mn₂CoSi (MCS) thin film have been studied to explore the possibility of half-metallicity of MCS Heusler alloy (HA) in thin film form. Grazing incidence X-ray diffraction (GIXRD) data indicated the presence of the rhombohedral crystal structure with a space group of R 3‾ (148). Spectrum fitting of X-ray reflectivity (XRR) suggests the deposited film has smooth surface with uniform density. Magnetic analysis reveals the ferrimagnetic nature of the film with a transition temperature well above the room temperature. Electric transport study of MCS thin film indicates the non-metallic behavior (< 250 K) and metallic behavior (> 250 K) in different temperature regimes. The persistence of half-metallicity across the entire temperature range is supported by the presence of T7/2 terms in the resistivity data due to two-magnon scattering. Arrhenius equation fitting of the electrical resistivity data in the non-metallic regime results the activation energy of 4.98 meV. At room temperature, the electrical resistivity is 1.372 mΩ-cm which is consistent with the values reported previously for other well-known half-metallic HAs. The observed results of HA in thin film form seems encouraging to us which could find its applications as a magnetic electrode for future spintronics.

Observation of dopant-profile independent electron transport in sub-monolayer TiOx stacked ZnO thin films grown by atomic layer deposition

Dopant-profile independent electron transport has been observed through a combined study of temperature dependent electrical resistivity and magnetoresistance measurements on a series of Ti incorporated ZnO thin films with varying degree of static-disorder. These films were grown by atomic layer deposition through in-situ vertical stacking of multiple sub-monolayers of TiOx in ZnO. Upon decreasing ZnO spacer layer thickness, electron transport smoothly evolved from a good metallic to an incipient non-metallic regime due to the intricate interplay of screening of spatial potential fluctuations and strength of static-disorder in the films. Temperature dependent phase-coherence length as extracted from the magnetotransport measurement revealed insignificant role of inter sub-monolayer scattering as an additional channel for electron dephasing, indicating that films were homogeneously disordered three-dimensional electronic systems irrespective of their dopant-profiles. Results of this study are worthy enough for both fundamental physics perspective and efficient applications of multi-stacked ZnO/TiOx structures in the emerging field of transparent oxide electronics.

On the bad metallicity and phase diagrams of Fe1+δX (X =Te, Se, S, solid solutions): an electrical resistivity study

Based on a systematic analysis of the thermal evolution of the resistivities of Fe-based chalcogenides Fe1+δTe1-xXx (X = Se, S), it is inferred that their often observed nonmetallic resistivities are related to a presence of two resistive channels: one is a high- temperature thermally-activated process while the other is a low-temperature log-in-T process. On lowering temperature, there are often two metal-to-nonmetall crossover events: one from the high-T thermally-activated nonmetallic regime into a metal-like phase and the other from the log-in-T regime into a second metal-like phase. Based on these events, together with the magnetic and superconducting transitions, a phase diagram is constructed for each series. We discuss the origin of both processes as well as the associated crossover events. We also discuss how these resistive processes are being influenced by pressure, intercalation, disorder, doping, or sample condition and, in turn, how these modifications are shaping the associated phase diagrams.

Low-temperature transport properties of Tl-doped Bi2Te3single crystals

While the bulk, stoichiometric Bi${}_{2}$Te${}_{3}$ single crystals often exhibit $p$-type metallic electrical conduction due to the Bi${}_{\mathrm{Te}}$-type antisite defects, doping by thallium (Bi${}_{2\ensuremath{-}x}$Tl${}_{x}$Te${}_{3}$, $x=0\ensuremath{-}0.30$) progressively changes the electrical conduction of single crystals from $p$ type (0 \ensuremath{\le} $x$ \ensuremath{\le} 0.08) to $n$ type (0.12 \ensuremath{\le} $x$ \ensuremath{\le} 0.30). This is observed via measurements of both the Seebeck coefficient and the Hall effect performed in the crystallographic (0001) plane in the temperature range of 2--300 K. Since any kind of substitution of Tl on the Bi or Te sublattices would result in an enhancement of the density of holes rather than its decrease, and because simple incorporation of Tl at interstitial sites or in the van der Waals gap is unlikely as it would increase the lattice parameters which is not observed in experiments, incorporation of Tl likely proceeds via the formation of TlBiTe${}_{2}$ fragments coexisting with the quintuple layer structure of Bi${}_{2}$Te${}_{3}$. At low levels of Tl, 0 \ensuremath{\le} $x$ \ensuremath{\le} 0.05, the temperature-dependent in-plane ($I\ensuremath{\perp}c$) electrical resistivity maintains its metallic character as the density of holes decreases. Heavier Tl content with 0.08 \ensuremath{\le} $x$ \ensuremath{\le} 0.12 drives the electrical resistivity into a prominent nonmetallic regime displaying characteristic metal-insulator transitions upon cooling to below \ensuremath{\sim}100 K. At the highest concentrations of Tl, 0.20 \ensuremath{\le} $x$ \ensuremath{\le} 0.30, the samples revert back into the metallic state with low resistivity. Thermal conductivity measurements of Bi${}_{2}$Te${}_{3}$ single crystals containing Tl, as examined by the Debye-Callaway phonon conductivity model, reveal a generally stronger point-defect scattering of phonons with the increasing Tl content. The systematic evolution of transport properties suggests that the Fermi level of Bi${}_{2}$Te${}_{3,}$ which initially lies in the valence band (for $x$ = 0), gradually shifts toward the top of the valence band (for 0.01 \ensuremath{\le} $x$ \ensuremath{\le} 0.05), then moves into the band gap (for 0.08 \ensuremath{\le} $x$ \ensuremath{\le} 0.12), and eventually intersects the conduction band (for 0.20 \ensuremath{\le} $x$ \ensuremath{\le} 0.30).

Optical conductivity of a metal-insulator transition for the Anderson-Hubbard model in three dimensions away from half filling

We have completed a numerical investigation of the Anderson-Hubbard model for three-dimensional simple cubic lattices using a real-space self-consistent Hartree-Fock decoupling approximation for the Hubbard interaction. In this formulation we treat the spatial disorder exactly, and therefore we account for effects arising from localization physics. We have examined the model for electronic densities well away 1/2 filling, thereby avoiding the physics of a Mott insulator. Several recent studies have made clear that the combined effects of electronic interactions and spatial disorder can give rise to a suppression of the electronic density of states, and a subsequent metal-insulator transition can occur. We augment such studies by calculating the ac conductivity for such systems. Our numerical results show that weak interactions enhance the density of states at the Fermi level and the low-frequency conductivity, there are no local magnetic moments, and the ac conductivity is Drude-like. However, with a large enough disorder strength and larger interactions the density of states at the Fermi level and the low-frequency conductivity are both suppressed, the conductivity becomes non-Drude-like, and these phenomena are accompanied by the presence of local magnetic moments. The low-frequency conductivity changes from a sigma-sigma_dc omega^{1/2} behaviour in the metallic phase, to a sigma omega^2 behaviour in the nonmetallic regime. Our numerical results show that the formation of magnetic moments is essential to the suppression of the density of states at the Fermi level, and therefore essential to the metal-insulator transition.

Magnetotransport in the amorphous carbon films near the metal–insulator transition

The low temperature electrical conductivity behavior of the amorphous carbon films in the non-metallic regime of the metal–insulator transition is studied in magnetic fields up to 7 T. The films are prepared from the pyrolysis of succinic anhydride in the temperature range 700–980 ∘C. A positive magnetoresistance (MR) is observed in films that are in the insulating regime and follows the variable range hopping (VRH) mechanism of conduction. In the VRH regime, at low temperatures B 2 dependence on the magnetoresistance is observed. The conductivity of the films in the critical regime follows the power law behavior. The MR is positive both at low and high field limits. At high fields the magnetoconductance is proportional to B 1 / 2 and is negligible at high temperatures.

Fourfold oscillations and anomalous magnetoresistance irreversibility in the nonmetallic regime ofPr1.85Ce0.15CuO4

Using magnetoresistance measurements as a function of applied magnetic field and its direction of application, we present sharp angular-dependent magnetoresistance oscillations for the electron-doped cuprates in their low-temperature non-metallic regime. The presence of irreversibility in the magnetoresistance measurements and the related strong anisotropy of the field dependence for different in-plane magnetic field orientations indicate that magnetic domains play an important role for the determination of electronic properties. These domains are likely related to the stripe phase reported previously in hole-doped cuprates.

Electronic transport of bilayer manganite La 1.2Sr 1.8Mn 2O 7

We report magnetoresistivity measurements on a La 1.2Sr 1.8Mn 2O 7 single crystal. The charge transport in the ferromagnetic phase is dominated by two-magnon scattering in the high T range and by weak localization effects at lower T, in the non-metallic regime. In the paramagnetic phase, ρ ab ( T) obeys the adiabatic small polaron hopping mechanism, while ρ c ( T) follows an Arrhenius behavior with the same activation energy.

Low-temperature electrical transport in bilayer manganiteLa1.2Sr1.8Mn2O7

The temperature $T$ and magnetic field $H$ dependence of anisotropic in-plane $\rho_{ab}$ and out-of-plane $\rho_{c}$ resistivities have been investigated in single crystals of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$. Below the Curie transition temperature $T_c=$ 125 K, $\rho_{ab}$ and $\rho_{c}$ display almost the same temperature dependence with an up-turn around 50 K. In the metallic regime (50 K $\leq T \leq$ 110 K), both $\rho_{ab}(T)$ and $\rho_{c}(T)$ follow a $T^{9/2}$ dependence, consistent with the two-magnon scattering. We found that the value of the proportionality coefficient $B_{ab}^{fit}$ and the ratio of the exchange interaction $J_{ab}/J_c$ obtained by fitting the data are in excellent agreement with the calculated $B_{ab}$ based on the two-magnon model and $J_{ab}/J_c$ deduced from neutron scattering, respectively. This provides further support for this scattering mechanism. At even lower $T$, in the non-metallic regime ($T<$ 50 K), {\it both} the in-plane $\sigma_{ab}$ and out-of-plane $\sigma_{c}$ conductivities obey a $T^{1/2}$ dependence, consistent with weak localization effects. Hence, this demonstrates the three-dimensional metallic nature of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ at $T<T_c$.

Saturation of the phase coherence length at low temperatures in Pr 1.95Ce 0.05CuO 4

Using fits of two-dimensional weak-localization theory to the magnetoresistance in the non-metallic regime, we extract the temperature dependence of the phase coherence length of non-superconducting Pr 2−χCe χCuO 4 with x= 0.05. We find that this characteristic length saturates at low temperatures, correlating with the deviations from ln( T) behavior observed in the resistivity.

The thermopower of Nd1+xBa2−xCu3Oy in a comparative study of effects of Ba-site doping versus chain-Cu-site doping

We report the results of measurements of resistivity up to 300 K and of thermoelectric power up to 400 K on ceramic samples of Nd1+xBa2−xCu3Oy with 0 ≤ x ≤ 0.65. The samples were fully oxygenated; they were characterized by x-ray diffraction and iodometric titration. The results are compared with data from literature reporting on effects of substitutions on the Ba site in 1–2–3 compounds and with our earlier experiments on substitution of Co for chain Cu. The focus is on the metal-nonmetal transition and on the nonmetallic regime in all these systems. Tc and the temperature dependence of S close to the metal-nonmetal transition and in the nonmetallic state seem to be determined by a single parameter, irrespective of the nature of the dopant. This result and its implications on the electronic structure of Rba2Cu3Oy (R = Y, or lanthanide ion) will be discussed.

Electron localization and metal–nonmetal transition in fluid KxKCl1−x: An electron spin resonance study of the magnetic properties with in situ variation of x (10−4⩽x⩽10−1)

We have measured the electron spin resonance (ESR) spectra of saltrich KxKCl1−x melts at temperatures near 800 °C and over a broad composition range, 10−4≤x≤10−1, approaching the metal–nonmetal (M–NM) transition in these solutions. Emphasis has been given to a precise in situ variation of composition which has been achieved for the first time in a high temperature ESR experiment by Coulometric titration. The spectra are characterized by a motional linewidth narrowing with a Lorentzian line shape. The ESR characteristics as determined from the Lorentz fits of the spectra exhibit the following features as a function of x: In the nonmetallic regime the g factor (g=1.9938±0.0002) and the peak-to-peak halfwidth (ΔBPP=0.2 mT) stay constant up to x∼0.05. Above this composition a clear increase of ΔBPP is observed indicating the NM–M transition. The spin susceptibility ϰs has been determined from the imaginary part of the Lorentz fits and has been calibrated against a sapphire signal measured simultaneously with the KxKCl1−x liquid samples. The spin susceptibility strongly deviates from simple Curie behavior even at low x which gives evidence for spin paired states in the nonmetallic solutions. Above x∼0.05 ϰs rises steeply with x. The overall variation of ϰs with x is interpreted by two limiting models: a thermodynamic defect model focusing on the particle character of localized paramagnetic and diamagnetic states for x≤0.05, and a simple band model describing the strong increase of ϰs approaching the NM–M transition. Both approaches yield a satisfactory description of the observed variation of ϰs. Further support of this interpretation is found in the spin dynamics which are qualitatively discussed.

Fully oxygenated RBa2Cu3−xCoxOy, (R=Y, Eu, Pr, and 0≤x≤1)— from high-temperature superconductors to high-resistivity nonmetals

Reported here are the results of measurements of the resistivity (ρ) up to 300 K and of the thermoelectric power (TEP) up to 400 K of ceramic samples of the title materials. We determined also their room-temperature lattice parameters and oxygen content as functions of Co concentrations. The metal–nonmetal transition in YBa2Cu3−xCoxOy and in EuBa2Cu3−xCoxOy is marked by the onset of the deviation of the maximum absolute TEP from a value calculated from a simple narrow-band formula. The results indicate that the effective valency of Pr in PrBa2Cu3−xCoxOy varies from ∼3.5 at x=0 to ∼3 for x=0.5. For x=0 this material is close to the metal–nonmetal transition. In the nonmetallic regime the electrical transport is by activated hopping. In certain ranges of Co content the results are consistent with two-band hopping conductivity with two branches, a low-temperature and a high-temperature branch of variable-range-hopping (VRH) conductivity. In the VRH regime there is a remarkable correlation between the two resistivity parameters, ρ0 and T0 in Mott’s equation ρ=ρ0 exp(T0/T)1/4. The resistivity of these Co-rich compounds is very high: thus, e.g., ρ(80 K)≳107 Ω cm in PrBa2Cu2.2Co0.8O7.15. These materials may find applications as insulating barriers in thin-film high-temperature superconducting devices.

A study of chemically prepared small metal clusters in the nonmetallic regime

Small gold clusters [mean diameter (d)≲ 1.4 nm], unlike larger clusters, show a higher Au(4f) binding energy relative to the bulk value and the presence of a conductance gap in tunnelling measurements, just as the molecular cluster compound, Au55(PPh3)12Cl6; small platinum clusters show similar nonmetallic features.

NEUTRON SCATTERING MESOSCOPIC AND MICROSCOPIC STRUCTURE DETERMINATION OF BINARY LIQUID MIXTURES UNDERGOING NON-METAL TO METAL TRANSITION (APPLICATION TO METAL-AMMONIA AND METAL MOLTEN SALTS)

NEUTRON SCATTERING MESOSCOPIC AND MICROSCOPIC STRUCTURE DETERMINATION OF BINARY LIQUID MIXTURES UNDERGOING NON-METAL TO METAL TRANSITION (APPLICATION TO METAL-AMMONIA AND METAL MOLTEN SALTS)

Soliton lattice in highly doped trans-polyacetylene.

The geometrical and electronic properties of highly doped trans-polyacetylene are studied by use of a total Hamiltonian consisting of the Su-Schrieffer-Heeger Hamiltonian, describing the electron hopping along the polymer chain, an extended Hubbard term due to the intrachain electron-electron interaction, and an external Coulomb potential arising from interchain electrostatic interactions and interactions with counterions. The three-dimensional structure of the system including polymer chains and dopant ions is that obtained from x-ray crystallography studies of sodium-doped trans-polyacetylene. At all doping levels (y\ensuremath{\le}12 at. %), and for all chain lengths (100N224) included in this study, the ground-state configuration is a soliton lattice, commensurate with the periodicity of the external potential. A self-consistent treatment of the soliton charge distribution entering the external potential is essential in order to calculate the electronic properties of the system correctly. For a regular distribution of the counterions, the evolution of the single-particle gap in the electronic spectrum exhibits a sharp decrease in the gap at doping levels between y\ensuremath{\sim}8 and 10 at. %. Increasing the doping level further does not lower the gap significantly. This result indicates the possibility of having a purely electronic phase transition within the soliton-lattice configuration, in agreement with experimental data. The size of the energy gap at high doping levels is, however, in the nonmetallic regime. When disorder in the positions of the counterions is introduced, the electronic states at the soliton and conduction-band edges tail off and close the gap around the Fermi level above \ensuremath{\sim}10 at.% doping. This phenomenon occurs already at very weak (intrinsic) disorder for which the counterions are displaced by maximum one-half of a carbon-carbon bond length.

Concentration-Fluctuation Model of a Doped Semiconductor in the Nonmetallic Regime. III. Coulomb Gap

Using the unrestricted Hartree-Fock scheme in the framework of pseudocluster calculation developed by us earlier, the Coulomb gap is investigated for compensated doped semiconductors in deep nonmetallic regime. For given degree of compensation, we obtain a power law behavior of the Coulomb gap Δc ∝ Pν, where P is the donor concentration and ν ≃ 1/3. Above the Coulomb gap the excitation spectrum exhibits a sharp threshold. Relation between the present quantum mechanical treatment and the classical treatment is discussed.

Concentration-fluctuation model of a doped semiconductor in the nonmetallic regime. II. Excitation spectrum

With the use of the unrestricted Hartree-Fock scheme developed by us earlier, the excitation spectra are calculated for doped semiconductors in the nonmetallic regime. The excitation spectrum suggests the possible self-compensation effect proposed by Bhatt and Rice. The spectral density so derived explains the dependence of the photoconducting current on the impurity concentration.

Concentration-fluctuation model of a doped semiconductor in the nonmetallic regime: Pseudocluster investigation

We have performed an improved unrestricted Hartree-Fock pseudocluster calculation with a correlated two-electron wave function for the ${D}^{\ensuremath{-}}$ state to investigate the microscopic structure of impurity bands in doped semiconductors. Though the impurity density of states and the estimated specific heat support the Mott-Hubbard-Anderson model, in the nonmetallic regime the impurity states are cluster states and a doped semiconductor can be described as statistically distributed clusters of various sizes. The distribution function of the cluster states agrees with the recent conclusion obtained from analyzing the optical, magnetic, dielectric, and transport data. A new picture of thermally activated hopping is provided which is relevant to the observed non-Ohmic conductivity and large characteristic electronic length.

Localization in a three-dimensional metal

The transition from a positive to a negative temperature coefficient of resistivity (TCR) has been studied in the artificially layered metallic system Nb-Cu. We find the TCR to undergo a rather sudden change in sign from positive to negative as the layer thickness is decreased below approximately 10 \AA{}. The temperature dependence of the conductivity in the nonmetallic regime can be explained by localization-type theories, and implies that both localization and Coulomb effects are important in this purely metallic three-dimensional system.