Critical Doping Level Research Articles

Appearance of Fröhlich-like phonon mode and defect dynamics in La3+-doped ceria

The underlying physics which explains the role of cationic and anionic defect dynamics in determining the catalytic activity and ionic conductivity of aliovalent ion-doped ceria is complex and not yet fully understood. To address this issue, we have investigated the structural evolution of LaxCe1-xO2–δ, where x = 0.1, 0.2, 0.3, 0.4, and 0.5. Rietveld refinements of the X-ray diffraction patterns reveal that the crystal structure belongs to a disordered fluorite phase for all samples. We find a systematic increase in the lattice parameter along with a gradual decrease in oxygen site occupancy with the increase in La3+ doping concentration. In Raman spectra of doped compounds, other than well-known crystalline F2g and defect related modes, we observe the presence of an additional Raman mode. The Raman line-shape analysis indicates the non-resonant Fröhlich character of this mode. Temperature dependent Raman measurements demonstrate unique characteristics of this Raman peak. In addition, from the change in relative intensity ratio of the Raman modes related to different types of defect states, we show that vacancies cannot be considered as isolated defects beyond a critical doping level. We propose that the evolution of Raman intensities of defect related modes and Fröhlich mode with the doping level can be used as a marker to determine the role of electron-phonon coupling and anion vacancies in the catalytic activity of doped ceria. Furthermore, by studying the photocatalytic measurements for La3+ doped ceria compounds, we demonstrate that the anion vacancies do not always play a direct role in controlling functional properties.

Critical boron-doping levels for generation of dislocations in synthetic diamond

Defects induced by boron doping in diamond layers were studied by transmission electron microscopy. The existence of a critical boron doping level above which defects are generated is reported. This level is found to be dependent on the CH4/H2 molar ratios and on growth directions. The critical boron concentration lied in the 6.5–17.0 × 1020at/cm3 range in the ⟨111⟩ direction and at 3.2 × 1021 at/cm3 for the ⟨001⟩ one. Strain related effects induced by the doping are shown not to be responsible. From the location of dislocations and their Burger vectors, a model is proposed, together with their generation mechanism.

Effect of Al doping on structural, optical and electrical properties of SnO2 thin films synthesized by pulsed laser deposition

Aluminium-doped (Al = 0–5 wt.%) SnO2 thin films with low-electrical resistivity and high optical transparency have been successfully synthesized by pulsed laser deposition technique at 500 °C. Structural, optical and electrical properties of the as-deposited and post-annealed thin films were investigated. X-ray diffraction patterns suggest that the films transform from crystalline to amorphous state with increasing aluminium content. The root mean square (Rq) surface roughness parameter, determined by atomic force microscopy decreases upon annealing of the as-deposited film. While resistivity of the film is the lowest (9.49 × 10−4 Ω-cm) at a critical doping level of 1 wt.% Al, optical transparency is the highest (nearly 90%) in the as-deposited condition. Temperature dependence of the electrical resistivity suggests that the Mott’s variable range hopping process is the dominant carrier transport mechanism in the lower temperature range (40–135 K) for all the films whereas, thermally activated band conduction mechanism seems to account for conduction in the higher temperature region (200–300 K).

Probing high‐energy ion‐implanted silicon by micro‐Raman spectroscopy

The effect of ion implantation (4 MeV12C2+, 5 MeV16O2+, and 8 MeV28Si2+) on [110] silicon wafers in channeling and random orientation is investigated by micro‐Raman spectroscopy. The profiles were measured using Scanning Electron Microscope (SEM) showing that the ions were penetrating deeper inside the wafer in the channeling case creating a 1–2 µm wide strongly modified region and agreeing with the d‐nuclear reaction analysis measurements. Micro‐Raman spectroscopy was employed for the assessment of the lattice damage, probing the side surface of the cleaved wafers at submicron step. The phonon modifications show strong lattice distortions in zones parallel to the front surface of the wafers and at depths, which agree with the results of the characterization techniques. In these strongly damaged zones, there is a substantial reduction in the phonon intensity, a small shift in wavenumber position, and a large increase in the phonon width. On the basis of a modification of the phonon confinement model that takes under consideration the laser beam profile, the reduction in intensity of scattered light, and the nanocrystallite size distribution from the simulation of the lattice displacements, the main characteristics of the Raman spectra could be reproduced for the random C and O implantations. The results indicate that at a critical doping level, the induced defects and lattice distortions relax by breaking the silicon single crystal into nanocrystallites, thus creating the observed zones of strongly distorted lattice. Copyright © 2014 John Wiley & Sons, Ltd.

Co-sprayed manganese doped zinc sulfide films

Manganese doped zinc sulfide thin films were deposited on glass substrates using co-spray method. Different characterization techniques such as XRD, FESEM, AFM, UV–vis and PL have been used in order to investigate the structural, morphological and optical properties of these films. X-ray diffraction (XRD) analysis indicated that pure and Mn-doped films showed only hexagonal polycrystalline wurtzite structure with [001] vertical preferential direction. SEM images revealed that manganese inclusion has a significant effect on the morphological structure; films have nano-sized grains with a size of approximately 100nm. All films have high transmittance of about 82% in the visible region. Manganese inclusion has been shown to decrease overall optical gap from 3.69 to 3.63eV. The photoluminescence intensity was found to reach a maximum and decline at higher manganese content, assisting to the well-known concentration quenching effect. The critical doping level of Mn ions has been determined to be 2%.

Polarization Sensitive Surface Band Structure of DopedBaTiO3(001)

We present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO(3)(001) single crystal. The n-type doping of the BaTiO(3) is controlled by in situ vacuum and oxygen annealing, providing experimental evidence of a surface paraelectric-ferroelectric transition below a critical doping level. Real space imaging of photoemission threshold, core level and valence band spectra show contrast due to domain polarization. Reciprocal space imaging of the electronic structure using linearly polarized light provides unambiguous evidence for the presence of both in- and out-of-plane polarization with two- and fourfold symmetry, respectively. The results agree well with first principles calculations.

Disappearance of nodal gap across the insulator–superconductor transition in a copper-oxide superconductor

The parent compound of the copper-oxide high-temperature superconductors is a Mott insulator. Superconductivity is realized by doping an appropriate amount of charge carriers. How a Mott insulator transforms into a superconductor is crucial in understanding the unusual physical properties of high-temperature superconductors and the superconductivity mechanism. Here we report high-resolution angle-resolved photoemission measurement on heavily underdoped Bi₂Sr₂-xLaxCuO(₆+δ) system. The electronic structure of the lightly doped samples exhibit a number of characteristics: existence of an energy gap along the nodal direction, d-wave-like anisotropic energy gap along the underlying Fermi surface, and coexistence of a coherence peak and a broad hump in the photoemission spectra. Our results reveal a clear insulator-superconductor transition at a critical doping level of ~0.10 where the nodal energy gap approaches zero, the three-dimensional antiferromagnetic order disappears, and superconductivity starts to emerge. These observations clearly signal a close connection between the nodal gap, antiferromagnetism and superconductivity.

Correlations between mechanical and photoluminescence properties in Eu doped sodium bismuth titanate

Nanoindentation technique is utilized to examine mechanical property variation in Eu doped Na0.5Bi0.5TiO3 (NBT). Doping levels of Eu in NBT is systematically varied. Dilute doping results in a linear reduction in both modulus and hardness. At higher concentrations, a recovery of the mechanical properties (to undoped NBT values) is observed. These experimental trends mirror variations in the optical emission intensities with Eu concentration. Observed trends are rationalized on the basis of a model, which hypothesizes phase segregation beyond a critical Eu doping level. Such segregation leads to the formation of pure NBT, nano-Eu saturated NBT, and nano-mixed Eu oxides in the microstructure. Pure NBT is optically inactive, while saturated Eu:NBT is a much better emitter when compared to europium oxide. Hence beyond the critical concentration, luminescence signal comes primarily from the saturated Eu:NBT phase. The model presented is supported by nanoindentation, and spectroscopic results.

Generic strange-metal behaviour of overdoped cuprates

We present an analysis of the temperature dependence of the zero-field in-plane resistivity ρab(T) of overdoped Tl2Ba2CuO6+δ (Tl2201). Taking our cue from earlier resistivity and angle-dependent magnetoresistance studies of Tl2201, as well as high-field measurements on La2−xSrxCuO4 (LSCO), we delineate ρab(T) into two T-dependent components below 200 K, one linear in temperature, the other quadratic. As found in LSCO, the T-linear component α1(0) is finite for all superconducting samples, its magnitude scaling with the transition temperature Tc. By contrast, the T2 coefficient α2(0) is essentially doping independent. Such an extended regime (in doping) of T-linear resistivity at low T is at odds with conventional quantum critical scenarios involving the collapse of an ordered phase, possibly associated with the normal state pseudogap, to 0 K at a critical doping level. Its confirmation in Tl2201, whose electronic state (as revealed by quantum oscillation experiments) is highly homogeneous over hundreds of unit cells, appears to rule out phase separation or electronic inhomogeneity as the origin of this extended critical behaviour.

Ferromagnetism in ABC-stacked trilayer graphene

In this article we study the ferromagnetic behavior of ABC-stacked trilayer graphene. This is done using a nearest-neighbor tight-binding model, in the presence of long-range Coulomb interactions. For a given electron-electron interaction g and doping level n, we determine whether the total energy is minimized for a paramagnetic or ferromagnetic configuration of our variational parameters. The g versus n phase diagram is first calculated for the unscreened case. We then include the effects of screening using a simplified expression for the fermion bubble diagram. We show that ferromagnetism in ABC-trilayer graphene is more robust than in monolayer, in bilayer, and in ABA-trilayer graphene. Although the screening reduces the ferromagnetic regime in ABC-trilayer graphene, the critical doping level remains one order of magnitude larger than in unscreened bilayer graphene.

Effect of doping on polarization profiles and switching in semiconducting ferroelectric thin films

This paper proposes a theory to describe the polarization and switching behavior of ferroelectrics that are also wide-gap semiconductors. The salient feature of our theory is that it does not make any a priori assumption about either the space charge distribution or the polarization profile. The theory is used to study a metal-ferroelectric-metal capacitor configuration, where the ferroelectric is n-type doped. The main result of our work is a phase diagram as a function of doping level and thickness that shows different phases, namely, films with polarization profiles that resemble that of undoped classical ferroelectrics, paraelectric, and a new head-to-tail domain structure. We have identified a critical doping level, which depends on the energy barrier in the Landau energy and the built-in potential, which is decided by the electronic structures of both the film and the electrodes. When the doping level is below this critical value, the behavior of the films is almost classical. We see a depleted region, which extends through the film when the film thickness is very small, but is confined to two boundary layers near the electrodes for large film thickness. When the doping level is higher than the critical value, the behavior is classical for only very thin films. Thicker films at this doping level are forced into a tail-to-tail configuration with three depletion layers, lose their ferroelectricity, and may thus be described as nonlinear dielectric or paraelectric. For films which are doped below the critical level, we show that the field required for switching starts out at the classical coercive field for very thin films, but gradually decreases.

Analysis of critical doping level of sprayed SnO2:F films

Fluorine doped tin oxide films were deposited using a simplified spray technique. The concentration of the host precursor (SnCl2.2H2O) in the starting solution is varied between 0·2 and 1·0M. The critical doping level of fluorine for which the sheet resistance Rsh acquires a minimum value is observed for each host precursor concentration (HPC). It is found that for a 0·2M solution, Rsh is minimum (42·59 Ω sq−1) when the fluorine doping level is 40 at-%, and the critical doping level gradually increases as the HPC increases. This variation in the critical doping level is correlated with the increase in the number of oxygen sites in the SnO2 lattice caused by the increase in the HPC. The transmittance in the visible range is 80% when the HPC is 0·2M, and the transmittance decreases as the HPC increases.

Mott insulator to metal transition in filling-controlled SmMnAsO1−x

Transport and magnetic properties have been systematically investigated for SmMnAsO_{1-x} with controlled electron-doping. As the electron band-filling is increased with the increase of oxygen deficiency (x), the resistivity monotonically decreases and the transition from Mott-insulator to metal occurs between x=0.17 and 0.2. Seebeck coefficient at room temperature abruptly changes around the critical doping level from a large value (\sim -300 \mu V/K) to a small one (\sim -50 \mu V/K) both with negative sign. In the metallic compounds with x=0.2 and x=0.3, Mn spins order antiferromagnetically around 30 K, and the Hall coefficient with the negative sign shows a reduction in magnitude upon the magnetic transition, indicating the change in the multiple Fermi surfaces. Gigantic positive magnetoresistance effect is observed in a wide range of temperature, reaching up to 60 % at 2 K for the x=0.3 compound. The effect is attributed to the field-induced change of the complex Fermi surfaces in this multi-orbital correlated electron system.

Valence and origin of metal–insulator transition in Mn doped SrRuO 3 studied by electrical transport, X-ray photoelectron spectroscopy and LSDA+ U calculation

We have studied the valence and electronic properties of Mn doped SrRuO 3 using electrical transport measurement, X-ray photoelectron spectroscopy (XPS) and local (spin) density approximation plus Coulomb interaction strength calculation (LSDA+ U). The resistivity data revealed that the system undergoes transition from metal to insulator at the critical Mn doping level, x∼0.2, which is accompanied by the structural transition from orthorhombic to tetragonal crystal symmetry. Besides, the significant reduction of the spectral weight at the coherent zone (0.8 eV) of the valence band is observed for x>0.2. The core XPS spectra suggest that both the transition elements exist in the mixed ionic pair, Ru +4/Ru +5↔Mn +3/Mn +4. The detail analysis of the results suggests that the Coulomb correlation effect in conjugation with localization of the charge carriers predominate over the mixed ionic pair effect and responsible for the metal–insulator transition in the series.

Magnetic fluctuations and superconductivity inLaFeAsO1−xFxunder pressure as seen viaA75sNMR

The relationship between antiferromagnetic (AF) fluctuation and superconductivity was investigated in the La1111 series, ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ ($x=0$, 0.05, 0.08, 0.10, and 0.14) by examining nuclear relaxation rates $(1/{T}_{1})$ at both ambient pressure and 3.0 GPa. The results show that the critical doping level at which low-frequency AF fluctuation vanishes is around the optimally doped regime $(x\ensuremath{\sim}0.10)$. Although the AF fluctuation is enhanced by applying pressure in the underdoped regime $(0.05\ensuremath{\le}xl0.10)$, the increase in critical transition temperature $({T}_{c})$ is small, whereas ${T}_{c}$ remarkably increases in the overdoped regime $(x=0.14)$, implying that the AF fluctuation is less important to the high-${T}_{c}$ mechanism than the density of states at the electron pocket. The $x$ dependence of ${T}_{c}$ at 3.0 GPa is similar to that of $R1111$ ($R=\text{Ce}$, Pr, Nd, etc.) with ${T}_{c}\ensuremath{\ge}40\text{ }\text{K}$ at ambient pressure. The relationship between ${T}_{c}$ and the pnictogen height or lattice constant indicates that pressure application is equivalent to full rare-earth substitution. This equivalence suggests that high ${T}_{c}$ above 40 K is realized when the AF fluctuation is absent.

Strain glass in Fe-doped Ti–Ni

We report the existence of strain glass in Ti 50Ni 50− x Fe x , beyond a critical Fe doping level x > x c(5 < x c < 6). The strain glass state is confirmed by the appearance of a frequency-dependent anomaly in the AC mechanical modulus/loss at a freezing temperature T 0 and by the breaking of ergodicity shown in a zero-field-cooling/field-cooling experiment. Based on the experimental results, a phase diagram is established in which both the normal martensitic transformations (for x < x c) and the strain glass transition (for x > x c) are indicated. The new phase diagram allows for explanations of two long-standing puzzles in Ti 50Ni 50− x Fe x and Ti–Ni alloys: (i) the origin of nano-domains present prior to the martensitic transformation (for x < x c) and (ii) the negative temperature dependence of electrical resistivity in abnormal non-transforming compositions (for x > x c).

Strain glass in doped Ti 50(Ni 50−xD x) (D = Co, Cr, Mn) alloys: Implication for the generality of strain glass in defect-containing ferroelastic systems

Strain glass is a new glassy state discovered recently in Ni-rich Ti–Ni ferroelastic alloys. However, it remains unclear whether or not strain glass can be found in a wide range of ferroelastic systems. Here, we investigated the transition behavior of three different defect-doped systems Ti 50(Ni 50− x D x ) (D = Co, Cr, Mn) and found a striking similarity in their transition behavior as a function of defect concentration x. In all these systems, there exists a critical doping level x c at which the transition behavior shows an interesting crossover. At x < x c, these materials undergo a martensitic transition and the transition temperature decreases with increasing x. However, at x > x c, the martensitic transition is suppressed and a strain glass transition occurs. These results imply that strain glass may be quite general in defect-containing ferroelastic systems. An analysis with a modified Landau-type free energy landscape suggests that although point defects always favor the formation of strain glass, to actually form a strain glass they need to destabilize the martensite of the system.

Analysis of critical doping level of sprayed antimony doped tin oxide films

Antimony doped tin oxide films are fabricated using a simplified and inexpensive spray pyrolysis technique. The variation in the sheet resistance, as a function of Sb/Sn ratio in the spraying solution, is studied. The sheet resistance decreases with the increase in doping level, attains a minimum at a particular doping level and increases with further doping. This critical value of the doping ratio (Sb/Sncritical), at which the transition takes place, is found to be 2.5 at.%, when the concentration of the precursor solution is 0.8 M and 4.5 at.% when the concentration is 0.4 M. The reason for this variation is analyzed with the support of the Sb/Sncritical values, obtained by various researchers. This Sb/Sncritical is found to depend mainly on the carrier concentration of the films.

Influence of Y3+ doping on the high-temperature transport mechanism and thermoelectric response of misfit-layered Ca3Co4O9

The high-temperature transport and thermoelectric characteristics of Ca(3-x) Y (x) Co(4)O(9) (x=0-0.75) series were studied up to 1000 K. The results reveal that the substitution of Y(3+) for Ca(2+) not only increases resistivity but also gradually alters the transport mechanism. The localization of carriers narrows bandwidth, which induces the evolution of the system from metal to variable-range hopping semiconductor and then to thermally activated semiconductor. The increasing thermopower with doping originates from the reduction of carrier concentration along with enhanced electronic correlations. Thermoelectric figure of merit ZT of Ca(3)Co(4)O(9) system is improved by Y doping. However, the optimal thermoelectric performance is found to only exist at the critical doping level where doping-induced metal-semiconductor transition occurs. This result suggests an intrinsic correlation between transport mechanism and thermoelectric response in this system.

Evolution of the Fermi Surface of the Electron-Doped High-Temperature SuperconductorNd2−xCexCuO4Revealed by Shubnikov–de Haas Oscillations

We report on the direct probing of the Fermi surface in the bulk of the electron-doped superconductor Nd(2-x)Ce(x)CuO(4) at different doping levels by means of magnetoresistance quantum oscillations. Our data reveal a sharp qualitative change in the Fermi surface topology, due to translational symmetry breaking in the electronic system which occurs at a critical doping level significantly exceeding the optimal doping. This result implies that the (pi/a, pi/a) ordering, known to exist at low doping levels, survives up to the overdoped superconducting regime.