Doping Range Research Articles

Effects of Ba doping on the structural and electronic properties of La[formula omitted]BaxCuO4

The structural and electronic properties of La2−xBaxCuO4 were investigated as a function of Ba-concentration 0 ≤ x ≤ 1.2, within the virtual crystal approximation (VCA) by means of first-principles total-energy calculations. We found Ba doping induces a quasi-rigid displacement of Cu dx2−y2 and dz2 bands toward higher energies. This effect generates important changes in the Fermi surface topology, which manage to imitate the ARPES momentum distribution map (MDM) measurements and tight-binding calculations recently reported for doped La-based cuprates. The calculations exhibit the significant increase in the contribution of Cu dz2 states at the Fermi level for greater Ba doping content to 0.3, which has effects on orbital hybridisation with Cu dx2−y2 states and therefore in the suppression of the superconducting state. For this Ba doping range, no ferromagnetism was found.

Effects of Ordering in Fe-xAl Alloys

The evolution of the structurally different phases and of the microstructural state of Fe-xAl alloys is studied in neutron diffraction experiments performed in the high resolution and continuous temperature scanning modes. It is found that the D03 phase in a weakly nonequilibrium state of alloys occurring in the doping range from x ≈ 23 to x ≈ 31 at % has the form of nanoclusters (L ≈ 100–800 A) dispersed within a host material (matrix) such as the disordered (A2) or partially ordered (B2) phase. The B2 → D03 → B2 → A2 phase transitions are accompanied by a decrease or increase in the lattice parameters of these phases in the course of ordering or disordering of the atomic structure, respectively. In this situation, the lattice parameters of both the matrix and clusters change simultaneously and a high degree of coherence of the crystal lattices of the matrix and clusters is observed.

Precision silicon doping with acceptors by temperature gradient zone melting

We present the results of optimization of the temperature gradient zone melting technique, also known as the thermomigration (ThM) technique, aimed on improvement of the quality of p-layers and formation of p-n junctions with sharper boundaries compared to those obtained by conventional thermal diffusion technique. In addition, ThM allows an expansion of the range of doping of silicon substrates with an acceptor impurity, usually limited by the solidus value. The ternary Al-Ga-Si and Al-Sn-Si melts were used as ligatures. The dependences of the migration rate of the liquid zones on temperature and composition for the Al-Ga and Al-Sn solvent metal are presented. The possibility of changing the acceptor concentration from 2·1019 cm−3 to 6·1019 cm−3 is shown. A threshold temperature of 1400 K was experimentally found for the ThM process with stable migration of triple liquid zones in a crystal. X-ray diffractional rocking curves and projection topography confirmed high structural quality of the Si(Al) p-layer with a thickness of 25 µm, obtained by ThM with stable moving liquid zones. The experimentally measured strain Δd/d=2.3×10−5 for the p-layer and the silicon substrate was used in the substitution model to estimate the Al concentration of 1×1019 cm−3. X-ray topographic images of the layers did not reveal both growth S-defects and misfit dislocations, confirming the high structural perfection of the layers and phase boundaries.

Inter-layer modified -wave superconductivity in an effectively doped spin-1 ladder

We construct a four-leg spin-1/2 t–J type model to simulate a doped two-leg spin-1 antiferromagnetic Heisenberg ladder. Employing renormalized mean-field theory with simple Gutzwiller factors, we obtain three degenerate superconducting states with different pairing symmetry. Through improving the Gutzwiller factors, we find that the state C with inter-layer modified -wave pairing has the lowest energy in a large doping range. Besides, we use the density matrix renormalization group method to solve the model. The negative binding energy reveals the pairing tendency, and the pair–pair correlation functions exhibit a slowly decaying behavior on certain types of bonds. From the pair correlations, we confirm the inter-layer modified -wave superconducting state as the ground state of the model.

Enhancement in structural and optical properties of Cd doped hybrid organic-inorganic halide perovskite CH3NH3Pb1-xCdxI3 photo-absorber

We report the solution-based synthesis and material characterization of the Cd substituted hybrid organic-inorganic lead halide perovskite CH3NH3Pb1-xCdxI3 (x = 0.0, 0.03, 0.05, 0.08). Structure and morphology of the samples were investigated using XRD, FESEM and HRTEM techniques. XRD results confirmed a tetragonal perovskite structure (I4/mcm) through the doping range. Optical absorption, band gap and bonding character were studied by UV–visible, FTIR and photoluminescence spectroscopy. Oxidation states, binding energy and chemical composition were determined by XPS and EDAX. A low value of Urbach energy (~14 meV) indicated high sample quality with low defects and trap states. The undoped samples had a direct energy gap of 1.56 eV, which reduced to 1.50 eV upon Cd doping. Most significantly, the absorption coefficient of CH3NH3Pb1-xCdxI3 remained high (~103) above 750 nm in the spectrally important infrared range as compared to CH3NH3PbI3, which approached zero. The enhanced structural and optical properties upon Cd doping make these materials promising photo absorber candidates compared to undoped CH3NH3PbI3.

High-performance room-temperature TiO2-functionalized GaN nanowire gas sensors

Hybrid gas sensors based on TiO2 functionalized gallium nitride nanowires have been prepared by nanofabrication and comprehensively studied for high-responsivity applications. The devices exhibited a high responsivity (25%) to 500 ppm NO2 assisted with ultraviolet illumination at room temperature. The thickness and doping concentration of TiO2 were engineered to improve the transducer function. The result indicated that an excellent n-type response can be stably obtained for a doping range from 1 × 1017 cm−3 to 1 × 1019 cm−3. The TiO2 thickness and doping concentration can be further fine-tuned to achieve optimal performance. In addition, a comprehensive device simulation was carried out to understand the device operation and gain insight for optimizing the device performance.

(Invited) Electrical Properties of (100) β-Ga2O3 Schottky Diodes with Four Different Metals

Due to its large bandgap (~4.6-4.8 eV), wide range of n-type doping, and single-crystal wafer availability, β-Ga2O3 holds promise for high-efficiency power devices. R&D of β-Ga2O3 has been conducted in Japan for more than a decade; however, funded research programs on this semiconductor material began in the U.S. only five years ago, highlighting its early stage of development. Device development of a semiconductor technology requires the ability to produce suitable ohmic and rectifying (Schottky) contacts. Whereas ohmic and Schottky contacts have been demonstrated on n-type β-Ga2O3, certain characteristics of contacts to β-Ga2O3 differ from contacts to other wide bandgap semiconductors. For example, we found that morphology was problem with many ohmic contact metals, and it was concluded that metal work function is not a dominant predictive factor for forming an ohmic contact to β-Ga2O3 [1]. Furthermore, thermal instability of the standard ohmic contact metallization (Ti/Au) is a concern for operation of devices at elevated temperatures. Schottky contacts to β-Ga2O3 have shown dependence on the orientation, growth method, and choice of metal. For example, on (-201) β-Ga2O3 Schottky barrier heights calculated from I-V measurements were typically between 0.9 and 1.3 eV and displayed little dependence on the metal work function [2]. Our preliminary measurements of selected metals on (100) β-Ga2O3 substrates yielded lower ideality factors and a correlation between barrier height and metal workfunction. The results for Schottky contacts on the (100) surface show some similarities to those previously reported on the (010) surface. In this presentation specific examples of ohmic and Schottky contact metallizations to β-Ga2O3 will provide a platform for more detailed discussion. [1] Y. Yao, R.F. Davis, and L.M. Porter, J. Electron. Mater. 46(4), 2053-2060 (2016). [2] Y. Yao, R. Gangireddy, J. Kim, K. Das, R.F. Davis, and L.M. Porter, J. Vac. Sci. Technol. B 35(3), 03D113.1-7 (2017).

High-temperature interface superconductivity in bilayer copper oxide films by pulsed laser deposition

In a seminal work, Gozar et al. reported on the high-temperature interface superconductivity in bilayers of insulating La2CuO4 and metallic La2−xSrxCuO4 (x=0.45). An interesting question to address is how general and robust this interface superconductivity is. In the past, the cuprate bilayers were grown in a unique atomic-layer molecular beam epitaxy system, with a Sr doping range of x≤0.47, and the atomically flat interface was thought to be indispensable. Here, we have fabricated bilayers of La2CuO4 and La2−xSrxCuO4 by pulsed laser deposition. We have tried to extend the nominal doping range of Sr from the previous maximum of 0.47 to the present 1.70 (the nominal Sr content in the targets). X-ray diffraction result indicates that our La2−xSrxCuO4 films with x≤0.60 have very high crystalline quality; but the film crystalline structure degrades gradually with further increasing x, and finally the structure is fully lost when x reaches 1.40 and higher. Although the film quality scatters dramatically, our experiments show that there exists superconductivity for bilayers in nearly the entire over-doped Sr range, except for a non-superconducting region at x∼0.80. These observations demonstrate that the interface superconductivity in copper oxides is very general and robust.

Resonant Inelastic Soft X-ray Scattering Study of Co-Doped Maghemite Nanoparticles.

Cobalt ferrite nanoparticles have been attracting considerable interest in the recent years because of the large number of potential applications, including magnetic storage, magnetic fluid hyperthermia and as contrast agents for magnetic resonance imaging. Physical properties of this class of materials depend critically on a number of parameters, including crystallinity, stoichiometry and cation distribution. In this work we have performed a Resonant Inelastic soft X-ray Scattering (RIXS) study on a series of 5 nm cobalt-doped maghemite nanoparticles to obtain direct quantitative information on cation distribution as a function of cobalt doping. We found that the distribution of divalent cobalt is stable in the investigated doping range and slightly different from that of bulk, stoichiometric cobalt ferrite. These results confirm that cobalt doping can be used to finely tune the magnetic properties of nanostructured ferrites without modifying their structural integrity.

Electron Paramagnetic Resonance in HoxLu1−xB12 Dodecaborides

The high-frequency (60 GHz) electron paramagnetic resonance is studied in HoxLu1−xB12 single crystals at low temperatures (2–80 K) in an applied magnetic field up to 7 T within a wide doping range, 0.01 ≤ x ≤ 1. These compounds are characterized by the presence of a low-temperature cage glass phase with random displacements of magnetic holmium ions from the centrosymmetric position in B12 boron cuboctahedra. For the samples with x ≥ 0.1, it is demonstrated that the electron paramagnetic resonance in the form of a single broad line with a g-factor of about 5 appears in the cage glass phase at T 0.3, the pronounced broadening of the resonance line and a steep decrease in the g-factor related to antiferromagnetic correlations is observed on cooling at T < 30 K. The performed simulation of the electron paramagnetic resonance spectra suggests the dominant role of the exchange interaction effects and positional disorder in determining the characteristic features of spin dynamics in HoxLu1−xB12.

Spectroscopic signatures of many-body correlations in magic-angle twisted bilayer graphene.

The discovery of superconducting and insulating states in magic-angle twisted bilayer graphene (MATBG)1,2 has ignited considerable interest in understanding the nature of electronic interactions in this chemically pristine material. The transport properties of MATBG as a function of doping are similar to those of high-transition-temperature copper oxides and other unconventional superconductors1-3, which suggests that MATBG may be a highly interacting system. However, to our knowledge, there is no direct experimental evidence of strong many-body correlations in MATBG. Here we present high-resolution spectroscopic measurements, obtained using a scanning tunnelling microscope, that provide such evidence as a function of carrier density. MATBG displays unusual spectroscopic characteristics that can be attributed to electron-electron interactions over a wide range of doping levels, including those at which superconductivity emerges in this system. We show that our measurements cannot be explained with a mean-field approach for modelling electron-electron interactions in MATBG. The breakdown of a mean-field approach when applied to other correlated superconductors, such as copper oxides, has long inspired the study of the highly correlated Hubbard model3. We show that a phenomenological extended-Hubbard-model cluster calculation, which is motivated by the nearly localized nature of the relevant electronic states of MATBG, produces spectroscopic features that are similar to those that we observed experimentally. Our findings demonstrate the critical role of many-body correlations in understanding the properties of MATBG.

Two functionals approach in DFT for the prediction of thermoelectric properties of Fe2ScX (X = P, As, Sb) full-Heusler compounds

In the quest for new thermoelectric materials with high power factors, full-Heusler compounds having flat band are found to be promising candidates. In this direction, Fe2ScX (X = P,As,Sb) compounds are investigated using mBJ for the band gap and SCAN to describe the electronic bands and phonon properties for thermoelectric applications. The band gaps obtained from mBJ are 0.81 eV, 0.69 eV and 0.60 eV for Fe2ScX compounds. The phonon dispersion, phonon density of states (DOS) and partial DOS are calculated. The phonon contributions to specific heat are obtained as a function of temperature under harmonic approximation. The electronic band structutre calculated from mBJ and SCAN functionals are qualitatively compared. The effective mass values are calculated at the band extrema from SCAN functional. The thermoelectric parameters are calculated for both hole and electron dopings under semiclassical theory. We use a simple, but reasonable method to estimate the phonon relaxation time (). Using the specific heat, estimated and slopes (phase velocity) of acoustic branches in the linear region, lattice thermal conductivity () at 300 K is calculated for three compounds. The obtained values of with constant are 18.2, 13.6 and 10.3 Wm−1 K−1, respectively. Finally, the temperature dependent figure of merit ZT values are calculated for optimal carrier concentrations in the doping range considered, to evaluate the materials for thermoelectric application. The ZT values for n-type Fe2ScX, in 900–1200 K, are 0.34–0.43, 0.40–0.48 and 0.45–0.52, respectively. While, the p-type Fe2ScX have ZT values of 0.25–0.34, 0.20–0.28 and 0.18–0.26, respectively in the same temperature range. The ZT values suggest that, Fe2ScX compounds can be promising materials in high temperature power generation application on successful synthesis and further reduction by methods like nanostructuring.

Correlation between vacancy defects and magnetic properties of the GdMn1-xZnxO3 multiferroic ceramics studied by positron annihilation

The GdMn1-xZnxO3 multiferroic ceramics were synthesized using the solid state reaction method, and the effects of Zn doping on the microstructure and magnetic properties of these ceramics were investigated. All of the samples formed single phase structure, and no structural transition occurred in the entire doping range. The SEM images showed that Zn doping prompted grain growth, and the high doping samples (x = 0.15 and 0.20) exhibited denser microstructure and lager, more well-connected grains. The Positron annihilation results revealed that the vacancy concentration first increased and then decreased with the increase in the Zn content; Zn doping does not change the defect species. Zn doping could obviously modulate the long-range order of Gd moments, transition temperature, and magnetization of the GdMn1-xZnxO3. The interplay between the microstructure and properties suggests that vacancy-type defects concentration is one of the most important factors that can tailor the magnetization of GdMn1-xZnxO3 ceramics.

Crystal structure and magnetic properties of disordered alloy ErGa3−xMnx**Project supported by the National Natural Science Foundation of China (Grant No. 11274110).

ErGa3−xMnx disordered alloy is successfully prepared by the vacuum arc melting technology, and the crystal structure and magnetic properties are investigated by using the x-ray diffraction and magnetic measurements. The Rietveld structural analysis indicates that the ErGa3−xMnx crystallizes into a cubic structure with space group of in Mn doping range of x = 0–0.1. However, the disordered alloy with structural formula of (GaII, Mn)0.4 as the second phase is separated from cubic phase for the samples with x = 0.2 and 0.3, which is induced by substituting the (GaII, Mn)–(GaII, Mn) pair at 2e crystal position for the rare earth Er at 1a site. The lattice parameters tend to increase with Mn content increasing due to the size effect at Ga (1.30 Å) site by substituting Mn (1.40 Å) for Ga. The paramagnetic characteristic is observed by doping Mn into ErGa3 at room temperature. With Mn content increasing from x = 0 to 0.1, the magnetic susceptibility χ tends to increase. This phenomenon can be due to the increase of effective potential induced by doping Mn into ErGa3. However, the magnetic susceptibility χ continues to decrease with the increase of Mn content in a range of , which is due to the phase separation from the cubic Er(Ga, Mn)3 to the hexagonal Er0.8Ga2(Ga, Mn)0.4.

Observation of two types of charge-density-wave orders in superconducting La2-xSrxCuO4

The discovery of charge- and spin-density-wave (CDW/SDW) orders in superconducting cuprates has altered our perspective on the nature of high-temperature superconductivity (SC). However, it has proven difficult to fully elucidate the relationship between the density wave orders and SC. Here, using resonant soft X-ray scattering, we study the archetypal cuprate La2-xSrxCuO4 (LSCO) over a broad doping range. We reveal the existence of two types of CDW orders in LSCO, namely CDW stripe order and CDW short-range order (SRO). While the CDW-SRO is suppressed by SC, it is partially transformed into the CDW stripe order with developing SDW stripe order near the superconducting Tc. These findings indicate that the stripe orders and SC are inhomogeneously distributed in the superconducting CuO2 planes of LSCO. This further suggests a new perspective on the putative pair-density-wave order that coexists with SC, SDW, and CDW orders.

Role of coupling divalent cations on the physical properties of SmFeO3 prepared by citrate auto-combustion technique

Controlling the magnetic properties of a material has great importance for variety of potential applications. X-ray diffraction and high-resolution transmission electron micrographs (HRTEM) analysis are utilized to study the effect of Sr2+ substitution on the physical properties of the investigated samples. The estimated crystallite sizes are found in the range of 14.315 and 22.129 nm. The X-ray diffraction patterns show that Sm1 − xSrxFeO3 remains as an orthorhombic structure of space group Pbnm within the whole doping range. The improvement of the magnetic and structural properties of the investigated multiferroic is achieved due to the changes in the tolerance factor as well as crystallite size caused by strontium substitution. The magnetic data show that either the antiferromagnetic (AFM) or ferromagnetic (FM) interactions can be dominant depending on the Sr2+ ion concentration. However, doping by Sr2+ reduces energy loss by more than 83%. On the other hand, the anisotropy constant and coercivity are decreased by more than 75% and 60%, respectively. Finally, one can recommend the use of such orthoferrite in electronic industry and different microwave components.

Bi(Mg0.5Ti0.5)O3-doped NaNbO3 ferroelectric ceramics: Linear regulation of Curie temperature and ultra-high thermally stable dielectric response

Sodium niobate (NaNbO3, NN)-based ferroelectric ceramics have been extensively studied owing to their antiferroelectric (AFE) nature. However, the dielectric properties of NN-based ceramics, which would be beneficial to capacitor devices, gain less research interest. In this study, (1−x)NaNbO3-xBi(Mg0.5Ti0.5)O3 (NN-xBMT, x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) ferroelectric ceramics were manufactured by a traditional solid state method. Structure and dielectric/ferroelectric properties were studied in detail by X-ray diffraction, dielectric spectrum as well as polarization-electric field hysteresis loops. All ceramic samples show an AFE P phase in the doping range. As x increases, the grain size of studied compositions gradually decreases from 5.36 μm in x = 0 to 2.05 μm in x = 0.05. In addition, the incorporation of BMT plays a regulatory role to the Curie temperature TC of ceramic samples, which decreases almost linearly from 375 °C x = 0 to 216 °C x = 0.05 at a rate of 33 °C/1 at. %. However, the degree of diffusion for each composition does not change significantly. More importantly, ultra-high thermally stable dielectric responses in a wide temperature range are obtained in NN-xBMT. Especially in x = 0.03, its permittivity is around 500 at room temperature and shows a variation less than 4.4% from 30 to 150 °C. Our results may have some guiding significance for the preparation of NN-based ceramics with specific TC, and are significantly important for capacitor applications requiring a wide temperature range stability.

Dynamic exchange effect induced multi-state magnetic phase diagram in manganese oxide Pr1-xCaxMnO3

The crystal structure and magnetic properties of Pr1-xCaxMnO3 (PCMO) were investigated systematically. It is shown that the symmetry of the structure is improved with the increase of Ca2+ ions. At the same time, the distortion of the MnO6 octahedron is reduced due to the weakening of the Jahn-Teller (J-T) effect in the system. Magnetic characterization shows that the original ferromagnetic (FM) at the low doping range (x = 0.2, 0.3) begins to appear antiferromagnetic (AFM) at the half doping range (x = 0.4, 0.5). Such magnetic transition is derived from the dynamic competition between the double exchange (DE) effect and the super exchange (SE) effect. These two exchange effects are significantly affected by structural distortions and charge-ordered (CO) phases. The coexistence and transition of multiple magnetic states embody complex internal competition in PCMO. Analysis of its intrinsic mechanism is helpful for designing spintronic devices.

Emergence of long-range magnetic order stabilized by magnetic impurities in pnictides

We examine the temperature dependence of the magnetic ordering in the frustrated Heisenberg $J_1-J_2$ model in presence of two different kind of dopants: vacancies or magnetic impurities. We demonstrate that, irrespective to their magnetic ratio, the introduction of impurities quenches the order by disorder selection mechanism associated with an Ising-like phase transition at low temperatures and gives way to a $90^\circ$ (anticollinear) order . The presence of dopants triggers a non trivial competition between entropically selected states (collinear) and energetically favoured ones (anticollinear) in dependence of both dilution and temperature. While in case of magnetic impurity, the interesting magnetic phases are observed for full range of temperature and doping, in case of nonmagnetic impurities every magnetic order is destroyed at all temperatures above $12\%$ dilution. At fixed low temperature and tuning the doping we show a first order phase transition leading to the re-entrance of the Ising-like order with percolation of islands of $90^\circ$ order. At fixed doping and varying the temperature we observe a transition from the anticollinear to the collinear phase assisted by a new emerging magnetic phase in the presence of magnetic impurities, whilst in case of vacancies this transition is characterised by a coexistent region of both. Furthermore, tuning the magnetic moment of the impurities, a complete collapse of the Ising-like order is attained. This is in agreement with observations of Ir dopant atoms in superconducting Ba(Fe$_{1-x}$Ir$_x$)$_2$As$_2$ with $x<0.047$.

Physical and chemical transformations of zirconium doped ceria nanoparticles in the presence of phosphate: Increasing realism in environmental fate and behaviour experiments

During their lifecycle, many engineered nanoparticles (ENPs) undergo significant transformations that may modify their toxicity, behaviour, and fate in the environment. Therefore, understanding the possible environmentally relevant transformations that ENPs may undergo as a result of their surroundings is becoming increasingly important. This work considers industrially produced ceria (CeO2) and focuses on a particle library consisting of seven zirconium-doped variants (Ce1-xZrxO2) where the Zr doping range is x = 0–1. The study assesses their potential transformation in the presence of environmentally relevant concentrations of phosphate. These ENPs have an important role in the operation of automotive catalysts and therefore may end up in the environment where transformations can take place. Samples were exposed to pH adjusted (c. 5.5) solutions made up of either 1 mM or 5 mM each of KH2PO4, citric acid and ascorbic acid and the transformed particles were characterised by means of DLS – size and zeta potential, UV/VIS, TEM, FT-IR, EDX and XRD. Exposure to the phosphate solutions resulted in chemical and physical changes in all ceria-containing samples to cerium phosphate (with the monazite structure). The transformations were dependent on time, ceria concentration in the particles (Ce:Zr ratio) and phosphate to ceria ratio. The presence of Zr within the doped samples did not inhibit these transformations, yet the pure end member ZrO2 ENPs showed no conversion to phosphate. The quite dramatic changes in size, structure and composition observed raise important questions regarding the relevant form of the materials to investigate in ecotoxicity tests, and for regulations based on one or more dimensions in the nanoscale.