Entire Doping Range Research Articles

Transport properties of highly dense proton-conducting BaSn1−xInxO3−δ ceramics

Doped barium stannate belongs to the perovskite-structured complex oxides with proton transport capability. Such objects are of high interest in terms of their application in various protonic ceramic electrochemical cells. In the present work, In-doped barium stannate (BaSn1−xInxO3−δ, 0 ≤ x ≤ 0.4) materials were prepared and thoroughly characterized by structural, microstructural, and electrochemical techniques. The single-phase and high-dense ceramic materials were prepared by the conventional solid-state synthesis route over the entire doping range investigated. The chemical composition of these materials was found to be in close agreement with the nominal values, indicating no evaporation of Ba-, Sn-, or In-containing phases during sintering. The high-temperature electrochemical analysis showed that the ionic conductivity of BaSn1−xInxO3−δ gradually increases with increasing the In-content without reaching a concentration maximum, even for high dopant concentrations. The relatively high activation energy values indicate that this ionic conductivity is determined by oxygen-ionic transport, which, however, becomes predominantly protonic at reduced temperatures. The determination of bulk and grain boundary transport showed that both were improved with the In-doping. The possible reasons for these observations were discussed in detail in this work. Therefore, the obtained data are important for discovering the transport properties of stannates and their potential applications.

Spin-orbital Jahn-Teller bipolarons

Polarons and spin-orbit (SO) coupling are distinct quantum effects that play a critical role in charge transport and spin-orbitronics. Polarons originate from strong electron-phonon interaction and are ubiquitous in polarizable materials featuring electron localization, in particular 3d transition metal oxides (TMOs). On the other hand, the relativistic coupling between the spin and orbital angular momentum is notable in lattices with heavy atoms and develops in 5d TMOs, where electrons are spatially delocalized. Here we combine ab initio calculations and magnetic measurements to show that these two seemingly mutually exclusive interactions are entangled in the electron-doped SO-coupled Mott insulator Ba2Na1−xCaxOsO6 (0 < x < 1), unveiling the formation of spin-orbital bipolarons. Polaron charge trapping, favoured by the Jahn-Teller lattice activity, converts the Os 5d1 spin-orbital Jeff = 3/2 levels, characteristic of the parent compound Ba2NaOsO6 (BNOO), into a bipolaron 5d2 Jeff = 2 manifold, leading to the coexistence of different J-effective states in a single-phase material. The gradual increase of bipolarons with increasing doping creates robust in-gap states that prevents the transition to a metal phase even at ultrahigh doping, thus preserving the Mott gap across the entire doping range from d1 BNOO to d2 Ba2CaOsO6 (BCOO).

Synthesis and phase transformations of hexagonal, orthorhombic, and cubic ScxLu1-xFeO3 (0 ≤ x ≤ 1) nanocrystals

Hexagonal, orthorhombic, and cubic ScxLu1-xFeO3 (0 ≤ x ≤ 1) nanocrystals were obtained through solution combustion synthesis with subsequent heat treatment. The structure and phase transformations of the nanocrystals were comprehensively characterized using EDX, PXRD, and 57Fe MS methods. It was observed that an increase in the Sc content within the ScxLu1-xFeO3 structure led to a phase transition from the orthorhombic phase to the hexagonal phase, and further to the bixbyite-type cubic phase. It was determined that the hexagonal phase was present throughout the entire doping range. Hexagonal substitutional solid solutions were stabilized with the content of scandium cations in range of 50–80 mol.%. The substitution limits of scandium for lutetium were determined for each phase. Based on the obtained results, the possibility of controlled synthesis of ScxLu1-xFeO3 nanoparticles with desired composition and crystal structure was successfully demonstrated, that open up the way to the design of novel advanced functional materials.

Strange metal electrodynamics across the phase diagram of Bi2−xPbxSr2−yLayCuO6+δ cuprates

Unlocking the mystery of the strange metal state has become the focal point of high T$_{c}$ research, not because of its importance for superconductivity, but because it appears to represent a truly novel phase of matter dubbed `quantum supreme matter'. Detected originally through high magnetic field, transport experiments, signatures of this phase have now been uncovered with a variety of probes. Our high resolution optical data of the low T$_{c}$ cuprate superconductor, Bi$_{2-x}$Pb$_{x}$Sr$_{2-y}$La$_{y}$CuO$_{6+\delta}$ allows us to probe this phase over a large energy and temperature window. We demonstrate that the optical signatures of the strange metal phase persist throughout the phase diagram. The strange metal signatures in the optical conductivity are two-fold, (i): a low energy Drude response with Drude width on the order of temperature and (ii): a high energy conformal tail with doping dependent power-law exponent. While the Drude weight evolves monotonously throughout the entire doping range studied, the spectral weight contained in the high energy conformal tail appears to be doping and temperature independent. Our analysis further shows that the temperature dependence of the optical conductivity is completely determined by the Drude parameters. Our results indicate that there is no critical doping level inside the superconducting dome where the carrier density starts to change drastically and that the previously observed 'return to normalcy' is a consequence of the increasing importance of the Drude component relative to the conformal tail with doping. Importantly, both the doping and temperature dependence of the resistivity are largely determined by the Drude width.

Pervasive beyond Room-Temperature Ferromagnetism in a Doped van der Waals Magnet.

The existence of long-range magnetic order in low-dimensional magnetic systems, such as the quasi-two-dimensional van der Waals (vdW) magnets, has attracted intensive studies of new physical phenomena. The vdW Fe_{N}GeTe_{2} (N=3, 4, 5; FGT) family is exceptional, owing to its vast tunability of magnetic properties. In particular, a ferromagnetic ordering temperature (T_{C}) above room temperature at N=5 (F5GT) is observed. Here, our study shows that, by nickel (Ni) substitution of iron in F5GT, a record high T_{C}=478(6) K is achieved. Importantly, pervasive, beyond room-temperature ferromagnetism exists in almost the entire doping range of the phase diagram of Ni-F5GT. We argue that this striking observation in Ni-F5GT can be possibly due to several contributing factors, including increased 3D magnetic couplings due to the structural alterations.

Sintering characteristics, crystal structure and dielectric properties of cobalt-tungsten doped molybdate-based ceramics at microwave frequency

In this study, a sequence of Ce2[Zr1‐x(Co1/2W1/2)x]3(MoO4)9 (CZ1‐x(CW)xM) (x = 0.02–0.10) ceramics with excellent microwave dielectric properties were obtained by the traditional solid-phase method. The crystal structure, dielectric properties, and chemical bond characters of the ceramics were characterized and analyzed. X-ray diffraction and Rietveld refinement analysis show that CZ1‐x(CW)xM could form a single-phase of the triangular crystal system in the entire doping range. The microstructure of the ceramic samples was obtained by scanning electron microscopy. The sintering temperature was reduced and the gain of the sample was refined as the increase of doping ion content. Furthermore, the intrinsic factors affecting the properties of CZ1‐x(CW)xM were analyzed by employing P-V-L theory and through in-depth infrared analysis. When x was 0.04 and the sintering temperature was 750 °C, the best dielectric properties of the samples were achieved, including εr = 9.95, Q·f = 80,803 GHz (at 9.99 GHz), and τf = − 9.10 ppm/°C.

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.

Low-temperature specific heat of doped SrTiO3 : Doping dependence of the effective mass and Kadowaki-Woods scaling violation

We report wide-doping-range ($8 \times 10^{17}$ to $4 \times 10^{20}$ cm$^{-3}$ Hall electron density) low temperature specific heat measurements on single crystal SrTiO$_3$:Nb, correlated with electronic transport data and tight-binding modeling. Lattice dynamic contributions to specific heat are shown to be well understood, albeit with unusual sensitivity to doping, likely related to the behavior of soft modes. Electronic contributions to specific heat provide effective masses that increase substantially, from $1.8$ to $4.8 m_e$, across the two SrTiO$_3$ Lifshitz transitions. It is shown that this behavior can be quantitatively reconciled with quantum oscillation data and calculated band structure, establishing a remarkably doping-independent mass enhancement factor of $2.0$. Most importantly, with the doping-dependent $T^2$ resistivity prefactor and Sommerfeld coefficient known, Kadowaki-Woods scaling has been tested over the entire doping range probed. Despite classic Fermi liquid behavior in electronic specific heat, standard Kadowaki-Woods scaling is dramatically violated, highlighting the need for new theoretical descriptions of $T^2$ resistivity in SrTiO$_3$.

Interplay of local structure, charge, and spin in bilayered manganese perovskites

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. Here, we show results of high-energy resolution x-ray absorption and emission spectroscopies on a ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ family of bilayered manganites in a broad doping range $(0.5\ensuremath{\le}x\ensuremath{\le}1)$. We established a relation between local Mn charge and Mn-O distances as a function of doping. Based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.

Enhanced superconducting transition temperatures in the rocksalt-type superconductors In1−xSnxTe (x≤0.5)

We investigate superconductivity in ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}\mathrm{Te}$ ($x\ensuremath{\le}0.5$) synthesized at high pressures of up to 2 GPa and observe an enhancement of the superconducting transition temperature ${T}_{\mathrm{c}}$ for increasing tin concentration $x$. These compounds have not been accessible in rocksalt structure via conventional ambient pressure synthesis. While the lattice constant smoothly increases with $x$, ${T}_{\mathrm{c}}$ saturates around $x=0.4$. Electronic structure calculations indicate that the ${T}_{\mathrm{c}}$ modulation is brought on by the change of the density of states in the vicinity of the Fermi energy $[N({E}_{\mathrm{F}})]$. However, differences between the calculated $N({E}_{\mathrm{F}})$ and the observed electronic specific-heat coefficient indicate that the phonon dispersion plays an important role in the system and that the mechanism of superconductivity may not be the same in the entire doping range.

Doping and compensation in Al-rich AlGaN grown on single crystal AlN and sapphire by MOCVD

In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the “knee behavior” in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.

Interplay of long-range and short-range Coulomb interactions in an Anderson-Mott insulator

In this paper, we tackle the complexity of coexisting disorder and Coulomb electron-electron interactions (CEEIs) in solids by addressing a strongly disordered system with intricate CEEIs and a screening that changes both with charge carrier doping level Q and temperature T . We report on an experimental comparative study of the T dependencies of the electrical conductivity σ and magnetic susceptibility χ of polyaniline pellets doped with dodecylbenzenesulfonic acid over a wide range. Thismaterial is special within the class of doped polyaniline by exhibiting in the electronic transport a crossover between a low-T variable range hopping (VRH) and a high-T nearest-neighbor hopping (NNH) well below room temperature. Moreover, there is evidence of a soft Coulomb gap C in the disorder band, which implies the existence of a long-range CEEI. Simultaneously, there is an onsite CEEI manifested as a Hubbard gap U and originating in the electronic structure of doped polyaniline, which consists of localized electron states with dynamically varying occupancy. Therefore, our samples represent an Anderson-Mott insulator in which long-range and short-range CEEIs coexist. The main result of the study is the presence of a crossover between low- and high-T regimes not only in σ(T ) but also in χ(T ), the crossover temperature T ∗ being essentially the same for both observables over the entire doping range. The relatively large electron localization length along the polymer chains results in U being small, between 12 and 20 meV for the high and low Q, respectively. Therefore, the thermal energy at T ∗ is sufficiently large to lead to an effective closing of the Hubbard gap and the consequent appearance of NNH in the electronic transport within the disorder band. C is considerably larger than U, decreasing from 190 to 30 meV as Q increases, and plays the role of an activation energy in the NNH.

Electronic structure of Pr1−xCaxMnO3

The electronic structure of Pr$_{1-x}$Ca$_x$MnO$_3$ has been investigated using a combination of first-principles calculations, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption. The full range of compositions, $x=0,1/2,1$, and a variety of magnetic orders have been covered. Jahn-Teller as well as Zener polaron orders are considered. The free parameters of the local hybrid density functionals used in this study has been determined by comparison with measured XPS spectra. A model Hamiltonian, valid for the entire doping range, has been extracted. A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided. The comparison of theoretical calculations and different experimental sprectra provide a detailed and consistent picture of the electronic structure. The large variations of measured optical absorption spectra are traced back to the coexistence of magnetic orders respectively to the occupation of local orbitals. A consistent treatment of the Coulomb interaction indicate a partial cancellation of Coulomb parameters and support the dominance of the electron-phonon coupling.

Effect of Cu insertion on structural, local electronic/atomic structure and photocatalyst properties of TiO2, ZnO and Ni(OH)2 nanostructures: XANES-EXAFS study

We report detailed investigations on the synthesis, structural, morphology, electronic/atomic structure and photocatalyst properties of Cu doped TiO2, ZnO and Ni(OH)2 nanostructures. All of the samples were synthesized by using the chemical precipitation method. Samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and photocatalyst measurements. XRD studies revealed single phase nature of the samples and omitted the presence of trivial metallic or binary oxide phases. TiO2 set of samples have shown nanorod kind of morphology, however TEM images of ZnO and Ni(OH)2 set of samples depicted the spherical morphology of particles. XANES spectra at the Cu K-edge and Cu L-edge, along with the atomic multiplet calculations, revealed the predominance of Cu2+ ions in all of the samples, within the entire doping range. Ti L-edge and Ti K-edge XANES confirmed the existence of Ti4+ ions in the pure and Cu doped TiO2 samples with anatase local structure. Zn L-edge XANES results confirmed the divalent character of Zn ions in the pure and Cu doped ZnO, which is further validated by the Zn K-edge XANES. Ni L-edge and Ni K-edge XANES conveyed the +2 valence state of Ni ions in the pure and Cu doped Ni (OH)2 samples. EXAFS analysis at the Cu K-edge nullifies the formation of Cu metallic clusters and other trivial phases, suggesting random distribution of Cu atoms in the oxide materials. Though, local atomic arrangement of Cu ions is disparate in the different oxide compounds. As an application of the pure and Cu doped TiO2, ZnO and Ni(OH)2 nanostructures, towards the degradation of water pollutant dyes, we demonstrate that all of the samples can serve as effective photocatalyst materials towards the degradation of methyl orange aqueous pollutant dye under the UV-light irradiation.

Evolution of structure, magnetic and transport properties of

The present paper seeks to investigate effect of Mn doping in superconducting FeSe. It is found that over the entire doping range in MnxSe (), Mn does not substitute Fe in the superconducting tetragonal phase. Instead two impurity phases, NiAs type hexagonal phase and NaCl type cubic phase, grow with increasing Mn content. Initially, hexagonal phase has a higher content than the cubic phase but beyond x = 0.5, the cubic phase grows rapidly and for , the sample is monophasic with cubic NaCl type structure. The superconducting tetragonal phase content steadily decreases with increasing Mn concentration and completely disappears beyond x = 0.5. The premise that Mn never replaces Fe in the superconducting phase is further strengthened by observation of a sharp drop in AC susceptibility akin to superconducting transition at the Tc of FeSe up to x = 0.5. EXAFS studies at the Fe K edge also show that the Fe has a four coordinated tetragonal local structure in all compositions below x = 0.5, similar to that in FeSe and it gradually changes to a six coordinated one as is expected for a NaCl type cubic phase for .

Fermi surface evolution of Na-doped PbTe studied through density functional theory calculations and Shubnikov–de Haas measurements

We present a combined experimental and theoretical study of the evolution of the low-temperature Fermi surface of lead telluride, PbTe, when holes are introduced through sodium substitution on the lead site. Our Shubnikov-de-Haas measurements for samples with carrier concentrations up to $9.4\times10^{19}$cm$^{-3}$ (0.62 Na atomic $\%$) show the qualitative features of the Fermi surface evolution (topology and effective mass) predicted by our density functional (DFT) calculations within the generalized gradient approximation (GGA): we obtain perfect ellipsoidal L-pockets at low and intermediate carrier concentrations, evolution away from ideal ellipsoidicity for the highest doping studied, and cyclotron effective masses increasing monotonically with doping level, implying deviations from perfect parabolicity throughout the whole band. Our measurements show, however, that standard DFT calculations underestimate the energy difference between the L-point and $\Sigma$-line valence band maxima, since our data are consistent with occupation of a single Fermi surface pocket over the entire doping range studied, whereas the calculations predict an occupation of the $\Sigma$-pockets at higher doping. Our results for low and intermediate compositions are consistent with a non-parabolic Kane-model dispersion, in which the L-pockets are ellipsoids of fixed anisotropy throughout the band, but the effective masses depend strongly on Fermi energy.

First-principles study of electronic properties of B and C doped CuNNi3

We have performed ab initio electronic structure calculations for and () alloys using the Green's-function-based Korringa–Kohn–Rostoker method, formulated within atomic sphere approximation. The disorder in sub-lattice is treated within single-site coherent potential approximation. The spin-polarized calculations predict CuNNi3 and its alloys and to be non-magnetic for the entire doping range. For ordered phase, the density of states (DOS) at Fermi energy is mainly composed of Ni 3d states with small contributions from N 2p and Cu 3d states as well. Additionally, we observe a high DOS peak at which arises from Ni and Cu 3d states. For both and alloys, the DOS at Fermi energy decreases monotonically with concentration. This is in sharp contrast to what is expected for a hole doped system with high DOS peak below Fermi energy. The decrease in DOS at Fermi energy is more prominent in alloys. Hence the doping is found to be more deteriorating to superconductivity than . Moreover, the doping induced effects are not entirely rigid-band like, rather a moderate redistribution of electronic states is observed.

Surface plasmon polaritons with plasma frequencies above 1 eV in Sn‐doped In2 O3

AbstractSn‐doped In2 O3 films with free‐electron concentration varied up to 1.710cm are prepared by molecular beam epitaxy. In this way, a metallic Drude‐type dielectric function with a negative real part extending beyond = 1050 nm is created. Despite essentially polycrystalline structure of the layers, the plasmonic damping is found not to exceed 70 meV in the entire doping range making excitation of low‐loss surface plasmon polaritons at frequencies fully covering the telecommunication band feasible. A monotonically increasing discrepancy between the carrier concentration obtained from Hall‐effect and the concentration extracted from fitting the optical spectra hints at a change of the band‐structure related parameters of In2 O3 with increasing Sn‐doping.

Composition dependence of structural and optical properties in epitaxial Sr(Sn1−xTix)O3 films

Epitaxial Sr(Sn1−xTix)O3 (SSTO, x = 0–1) thin films were grown on MgO substrates by a pulsed laser deposition technique. The effects of composition on the structural and optical properties of SSTO films were investigated. X-ray diffraction studies show that the lattice parameter decreases from 4.041 to 3.919 Å gradually with increasing Ti content from 0 to 1 in SSTO films. Optical spectra analysis reveals that the band gap energy Eg decreases continuously from 4.44 to 3.78 eV over the entire doping range, which is explained by the decreasing degree of octahedral tilting distortion and thus the increasing tolerance factor caused by the increasing small-Ti-ion doping concentration.

Competing charge, spin, and superconducting orders in underdopedYBa2Cu3Oy

To explore the doping dependence of the recently discovered charge density wave (CDW) order in YBa2Cu3Oy, we present a bulk-sensitive high-energy x-ray study for several oxygen concentrations, including strongly underdoped YBa2Cu3O6.44. Combined with previous data around the so-called 1/8 doping, we show that bulk CDW order exists at least for hole concentrations (p) in the CuO2 planes of 0.078 <~ p <~ 0.132. This implies that CDW order exists in close vicinity to the quantum critical point for spin density wave (SDW) order. In contrast to the pseudogap temperature T*, the onset temperature of CDW order decreases with underdoping to T_CDW ~ 90K in YBa2Cu3O6.44. Together with a weakened order parameter this suggests a competition between CDW and SDW orders. In addition, the CDW order in YBa2Cu3O6.44 shows the same type of competition with superconductivity as a function of temperature and magnetic field as samples closer to p = 1/8. At low p the CDW incommensurability continues the previously reported linear increasing trend with underdoping. In the entire doping range the in-plane correlation length of the CDW order in b-axis direction depends only very weakly on the hole concentration, and appears independent of the type and correlation length of the oxygen-chain order. The onset temperature of the CDW order is remarkably close to a temperature T^\dagger that marks the maximum of 1/(T_1T) in planar 63^Cu NQR/NMR experiments, potentially indicating a response of the spin dynamics to the formation of the CDW. Our discussion of these findings includes a detailed comparison to the charge stripe order in La2-xBaxCuO4.