Intermediate Doping Research Articles

Model for the electron distribution in modulation-doped heterostructures with high density of surface states

We study heterostructures, consisting of parallel layers of GaAs and ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ containing $n$-doped sheets and, on both sides, GaAs cap layers with a high density of surface states. Assuming thermal equilibrium between the electrons in the surface states, the donor-induced states, and the eigenstates near the $\text{GaAs}/{\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ interface (IF), we obtain three different doping regimes. At low doping only the surfaces are charged with electrons, not the IF. At intermediate doping also a two-dimensional electron system (2DES) near the IF occurs. At high doping the electron densities of the surface layers and of the 2DES saturate, and a parallel channel occurs at the highly doped sheet, which accommodates the electrons resulting from further doping. Besides the results of a self-consistent Hartree-type calculation we present a quasiclassical approach, which gives very good results for the electron distribution over the sample in all three doping regimes. In the heavy-doping saturation regime also a simple electrostatic estimate is applicable.

Substitutional disorder and charge localization in manganites

In the manganites RE1 − xAExMnO3 (RE and AE being rare-earth and alkaline-earth elements, respectively) the random distribution ofRE3 + andAE2 + induces random, but correlated, shifts of site energies of charge carriers in the Mn sites. Weconsider a realistic model of this diagonal disorder, in addition to the double-exchangehopping disorder, and investigate the metal–insulator transition as a function oftemperature, across the paramagnetic–ferromagnetic line, and as a function of doping x. Contrary to previous results, we find that values of parameters, estimated fromthe electronic structure of the manganites, are not incompatible with thepossibility of a disorder-induced metal to insulator transition accompanyingthe ferromagnetic to paramagnetic transition at intermediate doping (x ∼ 0.2–0.4). These findings indicate clearly that substitutional disorder has to be considered as animportant effect when addressing the colossal magnetoresistance properties of manganites.

Preparation and Characterization of Mn-Doped Titanates with a Bicrystalline Framework: Correlation of the Crystallite Size with the Synergistic Effect on the Photocatalytic Activity

Mn2+ ion was doped into the sol−gel anatase TiO2 in the concentration range of 0.02−0.1%. Powder X-ray diffraction results revealed that the phase transformation from anatase to rutile proceeds at a slower rate up to an intermediate dopant concentration of 0.06% and accelerates for a higher dopant concentration of 0.1%. In comparison with the previous studies, the phase transition temperature was reduced by 100 °C for the intermediate dopant concentration. The photocatalytic activity of Mn2+ (0.06%)−TiO2 for the degradation of an oxo-fused polycyclic aromatic dye, Indanthrane BR Violet, was almost equal to that of Degussa P-25 under UV light, but under solar light its efficiency was nearly 4 times higher. This enhanced activity can be attributed to the (i) synergistic effect in the bicrystalline framework of anatase and rutile, (ii) small crystallite size, and (iii) high intimate contact between both the phases. The correlation of the crystallite size with the synergistic effect on the photocatalytic acti...

Isotope effect in the Hubbard model with local phonons

The isotope effect (IE) in the two-dimensional Hubbard model with Holstein phonons is studied using the dynamical cluster approximation with quantum Monte Carlo. At small electron-phonon (EP) coupling the IE is negligible. For larger EP coupling there is a large and positive IE on the superconducting temperature that decreases with increasing doping. A significant IE also appears in the low-energy density of states, kinetic energy, and charge excitation spectrum. A negligible IE is found in the pseudogap and antiferromagnetic (AF) properties at small doping, whereas the AF susceptibility at intermediate doping increases with decreasing phonon frequency ${\ensuremath{\omega}}_{0}$. This IE stems from increased polaronic effects with decreasing ${\ensuremath{\omega}}_{0}$. A larger IE at smaller doping occurs due to stronger polaronic effects determined by the interplay of the EP interaction with stronger AF correlations. The IE of the Hubbard-Holstein model exhibits many similarities with the IE measured in cuprate superconductors.

Electronic and optical properties of boron-doped nanocrystalline diamond films

We report on the electronic and optical properties of boron-doped nanocrystalline diamond (NCD) thin films grown on quartz substrates by ${\text{CH}}_{4}/{\text{H}}_{2}$ plasma chemical vapor deposition. Diamond thin films with a thickness below 350 nm and with boron concentration ranging from ${10}^{17}$ to ${10}^{21}\text{ }{\text{cm}}^{\ensuremath{-}3}$ have been investigated. UV Raman spectroscopy and atomic force microscopy have been used to assess the quality and morphology of the diamond films. Hall-effect measurements confirmed the expected $p$-type conductivity. At room temperature, the conductivity varies from $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }{\ensuremath{\Omega}}^{\ensuremath{-}1}\text{ }{\text{cm}}^{\ensuremath{-}1}$ for a nonintentionally doped film up to $76\text{ }{\ensuremath{\Omega}}^{\ensuremath{-}1}\text{ }{\text{cm}}^{\ensuremath{-}1}$ for a heavily $B$-doped film. Increasing the doping level results in a higher carrier concentration while the mobility decreases from 1.8 down to $0.2\text{ }{\text{cm}}^{2}\text{ }{\text{V}}^{\ensuremath{-}1}\text{ }{\text{s}}^{\ensuremath{-}1}$. For NCD films with low boron concentration, the conductivity strongly depends on temperature. However, the conductivity and the carrier concentration are no longer temperature dependent for films with the highest boron doping and the NCD films exhibit metallic properties. Highly doped films show superconducting properties with critical temperatures up to 2 K. The critical boron concentration for the metal-insulator transition is in the range from $2\ifmmode\times\else\texttimes\fi{}{10}^{20}$ up to $3\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }{\text{cm}}^{\ensuremath{-}3}$. We discuss different transport mechanisms to explain the influence of the grain boundaries and boron doping on the electronic properties of NCD films. Valence-band transport dominates at low boron concentration and high temperatures, whereas hopping between boron acceptors is the dominant transport mechanism for boron-doping concentration close to the Mott transition. Grain boundaries strongly reduce the mobility for low and very high doping levels. However, at intermediate doping levels where hopping transport is important, grain boundaries have a less pronounced effect on the mobility. The influence of boron and the effect of grain boundaries on the optoelectronic properties of the NCD films are examined using spectrally resolved photocurrent measurements and photothermal deflection spectroscopy. Major differences occur in the low energy range, between 0.5 and 1.0 eV, where both boron impurities and the $s{p}^{2}$ carbon phase in the grain boundaries govern the optical absorption.

Vanishing conductivity of quantum solitons in polyacetylene

Quantum solitons or polarons are supposed to play a crucial role in the electric conductivity of polyacetylene, in the intermediate doping regime. We present an exact fully quantized calculation of the quantum soliton conductivity in polyacetylene and show that it vanishes exactly. This is obtained by applying a general method of soliton quantization, based on order–disorder duality, to a Z(2)-symmetric complex extension of the TLM dimerization effective field theory. We show that, in this theory, polyacetylene solitons are sine-Gordon solitons in the phase of the complex field.

Local structural changes in paramagnetic and charge-ordered phases ofSm0.2Pr0.3Sr0.5MnO3: an EXAFS study

Sm0.5−xPrxSr0.5MnO3 exhibits a varietyof ground states as x is varied from 0 to 0.5. At an intermediate doping ofx = 0.3 a charge-ordered CE-type antiferromagnetic insulating (AFI) ground state is seen. Thetransition to this ground state is from a paramagnetic-insulating (PMI) phase through aferromagnetic-metallic phase (FMM). Local structures in PMI and AFI phases of thex = 0.3 sample have been investigated using Pr K-edge and Sm K-edge extended x-rayabsorption fine structure (EXAFS). It can be seen that the tilting and rotation of theMnO6 octahedraabout the b-axis are responsible for the charge-ordered CE-type antiferromagnetic ground state at lowtemperatures. In addition a shift in the position of the rare-earth ion along thec-axis has to be considered to account for observed distribution of bond distances aroundthe rare-earth ion.

Characterization of Mo Doped TiO2 and its Enhanced Photo Catalytic Activity Under Visible Light

Transition metal doping into the TiO2 lattice can expand the response of these metal oxide nano particles to the visible region. In view of this, Mo6+ ion is doped into the TiO2 lattice in order to understand the mechanism of its photo response. The prepared photocatalysts were characterized by X-ray diffraction, UV–Visible absorption spectroscopy, UV–Visible diffuse reflectance spectroscopy, SEM, EDX, FTIR and BET specific surface area techniques. The characterization results have confirmed the incorporation of metal ions into TiO2 lattice. XRD analysis shows no change in crystal structure except a slight variation in crystallite size and elongation along the c-axis with increase in the concentration of the dopant. Diffuse reflectance measurements showed a shift in the band edge position to longer wavelengths and an extension of the absorption to the visible region. The photo degradation efficiencies of these catalysts were studied with Tebuconazole pesticide as model pollutant. Under UV light, undoped catalyst showed higher activity than doped catalyst. But in the case of visible light irradiation Mo doped TiO2 with intermediate dopant concentration of 0.06 atom % had the highest photocatalytic reactivity. This may be due to the narrowing of band gap so that it could effectively absorb the light of longer wavelength. The degradation path way was followed by UV–Visible spectroscopy.

Simultaneous lasing of interband and intersubband transitions in InAs∕AlSb quantum cascade laser structures

We show that both interband and intersubband transitions in n-InAs∕AlSb quantum cascade laser structures can have sufficient gain to enable simultaneous lasing at the two transitions. High electric fields generate holes, which are necessary for interband lasing in InAs. The doping concentration in the injection parts is shown to be critical for controlling the electric fields in the device, enabling the selection of lasing at the intersubband transition alone (high doping), at the interband transition alone (low doping), and simultaneous lasing at both transitions (intermediate doping).

Bipairing and the stripe phase in four-leg Hubbard ladders

Density matrix renormalization group (DMRG) calculations on four-leg $t\text{\ensuremath{-}}J$ and Hubbard ladders have found a phase exhibiting ``stripes'' at intermediate doping. Such behavior can be viewed as generalized Friedel oscillations, with wavelength equal to the inverse hole density, induced by the open boundary conditions. So far, this phase has not been understood using the conventional weak-coupling bosonization approach. Based on studies of a general bosonization proof, finite size spectrum, an improved analysis of weak-coupling renormalization group equations, and the decoupled two-leg ladder limit, we here find new types of phases of four-leg ladders, which exhibit stripes. They also inevitably exhibit ``bipairing,'' meaning that there is a gap to add one or two electrons (but not four) and that both single electron and electron pair correlation functions decay exponentially, while correlation functions of charge-4 operators exhibit a power-law decay. Whether or not bipairing occurs in the stripe phase found in DMRG is an important open question.

Novel Electrical Properties of Nanoscale Thin Films of a Semiconducting Polymer: Quantitative Current-Sensing AFM Analysis

Thin films (20-150 nm thickness) of poly(o-anthranilic acid) with various doping levels were prepared on silicon substrates with deposited indium tin oxide, and their topography and current-voltage (I-V) characteristics were quantitatively investigated using current-sensing atomic force microscopy with a platinum-coated tip. The films were found to have a surface morphology like that of orange peel, with a periodic modulation and a surface roughness. The films exhibited nonuniform current flows and I-V characteristics that depended on the doping level as well as on the film thickness. Films with a high doping level were found to exhibit Zener diode switching behavior, whereas the films with a very low doping level (or that were dedoped) exhibited no current flow at all, and so are insulators. Interestingly, self-doped films (which are at an intermediate doping level) were found to have a novel electrical bistability, i.e., a switching characteristic like that of Schottky diodes, and increasingly insulating characteristics as the film thickness was increased. The films with thickness < or =62 nm, which exhibited this novel and stable electrical bistability, can potentially be used in the fabrication of high-density, stable, high-performance digital nonvolatile memory devices based only on transistors. The measured current images and I-V characteristics indicate that the electrical switching and bistability of the films are governed by local filament formation and charge traps.

Fermi surface and pseudogap of electron-doped cuprate superconductors: Numerical study of thet−t′−Jmodel

Spectral functions are evaluated numerically within the $t\text{\ensuremath{-}}{t}^{\ensuremath{'}}\text{\ensuremath{-}}J$ model as relevant for electron-doped cuprates. The Fermi surface develops from a pocketlike into a large one with doping. The corresponding pseudogap in the nodal direction is well resolved at low doping and vanishes at intermediate doping. Its presence is strongly temperature dependent and thus connected to longer-range antiferromagnetic correlations. Features including a double effective band and optical conductivity are consistent with experiments on electron-doped cuprates without invoking the closing of the Mott-Hubbard gap.

Strongly inhomogeneous phases and non-Fermi-liquid behavior in randomly depleted Kondo lattices

We investigate the low-temperature behavior of Kondo lattices upon random depletion of the local $f$-moments, by using strong-coupling arguments and solving SU($N$) saddle-point equations on large lattices. For a large range of intermediate doping levels, between the coherent Fermi liquid of the dense lattice and the single-impurity Fermi liquid of the dilute limit, we find strongly inhomogeneous states that exhibit distinct non-Fermi liquid characteristics. In particular, the interplay of dopant disorder and strong interactions leads to rare weakly screened moments which dominate the bulk susceptibility. Our results are relevant to compounds like Ce_{x}La_{1-x}CoIn_5 and Ce_{x}La_{1-x}Pb_3

Phase diagram of La5/8−yNdyCa3/8MnO3 manganites

We report a detailed study of the electric transport and magnetic properties of theLa5/8−yNdyCa3/8MnO3 manganite system.Substitution of La3+ by smaller Nd3+ ions reduces the mean ionic radius of the A-site ion. We have studiedsamples in the entire range between La-rich and Nd-rich compounds(0.1<y<0.625). Results of dc magnetization and resistivity show that doping destabilizes theferromagnetic (FM) character of the pure La compound and triggers the formation of aphase-separated state at intermediate doping. We have also found evidence of a dynamicalbehaviour within the phase-separated state. A phase diagram is constructed, summarizingthe effect of chemical substitution on the system.

Orbital degree of freedom in single-layeredLa1−xSr1+xMnO4: Doping- and temperature-dependent rearrangement of orbital states

Oxygen $1s$ near-edge x-ray absorption spectroscopy and x-ray diffraction investigations on ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.5)$ single crystals evidence a doping- and temperature-dependent redistribution of ${e}_{g}$ electrons between ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$, ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$, and ${d}_{3{x}^{2}\ensuremath{-}{r}^{2}}∕{d}_{3{y}^{2}\ensuremath{-}{r}^{2}}$ orbitals: For undoped $\mathrm{La}\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{4}$ the ${e}_{g}$ electrons predominantly reside on ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals, nevertheless a small amount of charge carriers is found on in-plane ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals. These in-plane electrons are gradually transferred to the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ states upon cooling. Sr doping, however, does not only provide holes to the system but also induces a continuous transfer of electrons from ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ to ${d}_{3{x}^{2}\ensuremath{-}{r}^{2}}∕{d}_{3{y}^{2}\ensuremath{-}{r}^{2}}$ orbitals and, finally long-range charge-exchange-type orbital ordering develops for ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}\mathrm{Mn}{\mathrm{O}}_{4}$ below $T\ensuremath{\leqslant}225\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The data indicate a short-range orbital ordering for the intermediate doping range.

Structural studies of Sr- and Mg-doped LaGaO 3

A series of La 1− x Sr x Ga 1− y Mg y O 3− δ (LSGM) solid solution were prepared by solid state synthesis and characterized by X-ray phase analysis at room temperature. A systematic change of crystal structure with the increase in A- and B-site dopant contents was observed. At a total dopant concentration of 25 at%, the crystal structure was found to be orthorhombic whereas at a total dopant concentration of 35 at% keeping at least one component at 20 at%, the structure was observed to be cubic. Mixed structures of orthorhombic and rhombohedral were also found at intermediate dopant concentrations. A structural phase diagram is proposed based on the findings and co-related with the oxygen ionic conductivity.

Analysis of Time-Resolved Donor–Acceptor Photoluminescence of N-Doped ZnO

The wide-gap semiconductor ZnO has a band gap of ’3:3 eV at room temperature and is of current interest due to its potential optoelectronic applications. Since undoped ZnO is known to be grown under fairly high residual n-type, the trial of the p-type doping generally tends to result in an observation of the photoluminescence (PL) of donor–acceptor pair (DAP) bands. The optical properties of the DAP emissions have not been fully understood. In a previous report, we described the growth and the steady-state PL properties of ZnO:N epitaxial layers grown by repeated temperature modulation (RTM) epitaxy. Evidence of p-type conductivity for RTM-grown ZnO:N has been accumulated from a variety of experimental results, such as the Hall-effect and electroluminescence measurements in a p–n juction structure. For the analysis of PL peak energy and excitation-induced peak shift on p-type ZnO, we took the effect of donor wavefunction overlapping and the fact of impurity distribution inhomogeneity into account by adopting the fluctuation theory developed by Kuskovsky et al. Using this theory, we were able to consistently explain the excitation-intensity dependences of the PL properties of ZnO:N grown with three different techniques and conditions. In this short communication, we report on the analysis of the time-resolved PL (TRPL) spectra taken for the RTMgrown sample at an intermediate doping level to experimentally detemine the transition probability of the DAP (being W0) by using all the other material parameters that have been deduced in our previous work. It should be noted that the recombination dynamics of the DAPs has been studied for ZnO:N within the low-doping regime, where the approach of Thomas, Hopfield, and Augustyniak (THA) is still valid. The doped epilayer was grown by the RTM epitaxy on a lattice-matched ScAlMgO4 (SCAM) substrate. 7) The detailed methods of fabrication have been given elsewhere. Secondary-ion mass spectroscopy (SIMS) yielded a total nitrogen concentration [N] of 6 10 cm . Our ZnO:N sample dominantly showed the DAP band (D being a shallow donor) with zero-phonon energies of 3:235 eV, a donor activation energy of ED 1⁄4 60meV, and an acceptor activation energy of EA 1⁄4 171meV at 5K. These have been already determined from the energy of the bound exciton line (unchanged from that for undoped samples) and the excitation-intensity dependence of the PL. In addition, the energy of EA has turned out to be almost independent of the nitrogen concentration. For the time-resolved experiments, pulsed excitation was provided by the frequency-doubled beam of a picosecond mode-locked Ti:sapphire laser (360 nm, 2 ps). Here, we focus our attention to experimental total-lightdecay data (a spectrally integrated PL decay curve), because one can fit to the data with a smaller number of running parameters than the case of spectrally resolved decay curves, i.e., EA, ED, and ðNA NDÞ (acceptor and donor concentrations). Irrespective of the model adopted, in both cases of the theories of THA and of the fluctuation, the intensity IðtÞ of light emitted at time t is

Nonresonant Raman response of inhomogeneous structures in the electron-dopedt‐t′Hubbard model

We calculate the non-resonant Raman response, the single particle spectra and the charge-spin configuration for the electron doped $t-t'$ Hubbard model using unrestricted Hartree-Fock calculations. We discuss the similarities and differences in the response of homogeneous versus inhomogeneous structures. Metallic antiferromagnetism dominates in a large region of the $U-n$ phase diagram but at high values of the on-site interaction and for intermediate doping values, inhomogeneous configurations are found with lower energy. This result is in contrast with the case of hole doped cuprates where inhomogeneities are found already at very low doping. The inhomogeneities found are in-phase stripes compatible with inelastic neutron scattering experiments. They give an incoherent background in the Raman response. The $B_{2g}$ signal can show a quasiparticle-like component even when no Fermi surface is found in the nodal direction.

High-temperature magnetization in Y 1−xCa xMnO 3

We have studied the T dependence of the susceptibility, χ( T), in the paramagnetic phase of the distorted perovskite series Y 1− x Ca x MnO 3. We have observed different regimes depending on the level of doping. For x∼0 and 1 we found a localized behavior for the e g electrons and a delocalized one for intermediate doping (0.2< x⩽0.8). For the delocalized regime we present a mean-field description for χ( T) that includes the interaction between localized and itinerant e g electrons. We compare our results with the less distorted series La 1− x Ca x MnO 3.

Jahn-Teller distortions and phase separation in doped manganites

A "minimal model" of the Kondo-lattice type is used to describe a competition between the localization and metallicity in doped manganites and related magnetic oxides with Jahn-Teller ions. It is shown that the number of itinerant charge carriers can be significantly lower than that implied by the doping level x. A strong tendency to the phase separation is demonstrated for a wide range of intermediate doping concentrations vanishing at low and high doping. The phase diagram of the model in the x-T plane is constructed. At low temperatures, the system is in a state with a long-range magnetic order: antiferromagnetic (AF), ferromagnetic (FM), or AF-FM phase separated (PS) state. At high temperatures, there can exist two types of the paramagnetic (PM) state with zero and nonzero density of the itinerant electrons. In the intermediate temperature range, the phase diagram includes different kinds of the PS states: AF-FM, FM-PM, and PM with different content of itinerant electrons. The applied magnetic field changes the phase diagram favoring the FM ordering. It is shown that the variation of temperature or magnetic field can induce the metal-insulator transition in a certain range of doping levels.