Small Doping Research Articles

The effects of Eu3+ doping on the epitaxial growth and photovoltaic properties of BiFeO3 thin films

1 Eu 3+ doping induces a phase transition from tetragonal to rhombohedral for BiFeO 3 film; 2 A decrease in coercivity and a sharp change in band gap upon Eu 3+ doping have been observed; 3 Eu 3+ can be used to tailor the photovoltaic effect of ITO/Eu x Bi 1- x FeO 3 / Ca 0.96 Ce 0.04 MnO 3 ; Nowadays, photovoltaic effect has been widely studied in various ferroelectric materials due to its applications as optoelectronic devices. In this work, with BiFeO 3 (BFO) films as the photovoltaic materials, we report the effects of Eu 3+ doping content on the phase structure, ferroelectric and optical properties of BFO films grown on Ca 0.96 Ce 0.04 MnO 3 /YAlO 3 (001) substrate. We found that a small doping level of 0.05 could induce a phase change of BFO from tetragonal to rhombohedral, due to the shrinking of the lattice upon Eu 3+ doping and the breaking of surface terrace structure induced by Ca 0.96 Ce 0.04 MnO 3 layer. This results in a sharp band gap reduction from 3.30 eV to 2.60 eV, and a decrease in the coercivity of ferroelectric polarization switching. Based on these findings, we investigate the photovoltaic effects of ITO/Eu x Bi 1- x FeO 3 /Ca 0.96 Ce 0.04 MnO 3 vertical capacitors. It is found that the short-circuit current density ( J sc ) decreases with increasing Eu 3+ doping, whereas the open-circuit voltage ( V oc ) first increases to a level of 0.1 V and then decreases with further Eu 3+ doping. This could be explained by the combined effect of Schottky-junction and depolarization field on the photovoltaic process. Our research suggests that a moderate Eu 3+ doping is helpful for enhancing the photovoltaic effect of BFO thin film devices. .

Stripes and spin-density waves in the doped two-dimensional Hubbard model: Ground state phase diagram

We determine the spin and charge orders in the ground state of the doped two-dimensional (2D) Hubbard model in its simplest form, namely with only nearest-neighbor hopping and on-site repulsion. At half-filling, the ground state is known to be an antiferromagnetic Mott insulator. Doping Mott insulators is believed to be relevant to the superconductivity observed in cuprates. A variety of candidates have been proposed for the ground state of the doped 2D Hubbard model. A recent work employing a combination of several state-of-the-art numerical many-body methods established the stripe order as the ground state near $1/8$ doping at strong interactions. In this paper, we apply one of these methods, the cutting-edge constrained-path auxiliary field quantum Monte Carlo (AFQMC) method with self-consistently optimized gauge constraints, to systematically study the model as a function of doping and interaction strength. With careful finite size scaling based on large-scale computations, we map out the ground state phase diagram in terms of its spin and charge order. We find that modulated antiferromagnetic order persists from near half-filling to about $1/5$ doping. At lower interaction strengths or larger doping, these ordered states are best described as spin-density waves, with essentially delocalized holes and modest oscillations in charge correlations. When the charge correlations are stronger (large interaction or small doping), they are best described as stripe states, with the holes more localized near the node in the antiferromagnetic spin order. In both cases, we find that the wavelength in the charge correlations is consistent with so-called filled stripes.

Stability of the nonconstant stationary solution to the Poisson–Nernst–Planck–Navier–Stokes equations

We study the three-dimensional Cauchy problem of the Poisson–Nernst–Planck–Navier–Stokes equations. We first show that the corresponding stationary system has a unique semi-trivial solution under a general doping profile. Under initial small perturbations around such the semi-trivial stationary solution and small doping profile, we obtain the unique global-in-time solution to the non-stationary system. Moreover, we prove the asymptotic convergence of the solution toward the semi-trivial stationary solution as time tends to infinity.

Substantial Improvement of Operating Stability by Strengthening Metal‐Halogen Bonds in Halide Perovskites

AbstractSolution‐processed lead halide perovskites (LHPs) hold great promise for low‐cost high‐performance solar cells and light‐emitting devices, but they also suffer from a serious operating instability problem due to the ionic migration and lattice decomposition driven by strong electric fields. Here, considerably suppressed ionic migration and enhanced lattice stability in LHPs with partial substitution of Pb with 3d transition metal (TM: Mn and Ni) are reported. It is experimentally shown that the energy barrier for ionic migration in CsPbBr3 can be increased fourfold by Mn and Ni substitution, even with a small doping level (<4%). However, post‐TM Zn and non‐TM Bi incorporations are less efficient in suppressing ionic migration. The theoretical results reveal that Ni and Mn ions with partially filled 3d orbitals can passivate the active lone‐pair electron of surrounding Pb‐Br octahedrons via a coordination effect and reduce the Pb 6s‐Br 4p antibonding states, resulting in long‐range lattice stabilization and suppressed ionic migration. The Ni incorporation strategy in mixed‐halogen CsPbBr1.5I1.5 is further demonstrated, for which the field‐driven halogen segregation is significantly mitigated and the associated emission color variation is reduced sixfold. This study paves the way for improving the operating stability in LHP‐based optoelectronic and electronic devices.

Self-Doping and the Mott-Kondo Scenario for Infinite-Layer Nickelate Superconductors

We give a brief review of the Mott-Kondo scenario and its consequence in the recently-discovered infinite-layer nickelate superconductors. We argue that the parent state is a self-doped Mott insulator and propose an effective t- J-K model to account for its low-energy properties. At small doping, the model describes a low carrier density Kondo system with incoherent Kondo scattering at finite temperatures, in good agreement with experimental observation of the logarithmic temperature dependence of electric resistivity. Upon increasing Sr doping, the model predicts a breakdown of the Kondo effect, which provides a potential explanation of the non-Fermi liquid behavior of the electric resistivity with a power law scaling over a wide range of the temperature. Unconventional superconductivity is shown to undergo a transition from nodeless (d+is)-wave to nodal d-wave near the critical doping due to competition of the Kondo and Heisenberg superexchange interactions. The presence of different pairing symmetry may be supported by recent tunneling measurements.

Stripes versus superconductivity in the doped Hubbard model on the honeycomb lattice

We study the ground state of the doped Hubbard model on the honeycomb lattice in the small doping and strongly interacting region. The nature of the ground state by doping holes into the anti-ferromagnetic Mott insulating states on the honeycomb lattice remains a long-standing unsolved issue, even though tremendous efforts have been spent to investigate this challenging problem. An accurate determination of the ground state in this system is crucial to understand the interplay between the topology of Fermi surface and strong correlation effect. In this work, we employ two complementary, state-of-the-art, many-body computational methods -- constrained path (CP) auxiliary-field quantum Monte Carlo (AFQMC) with self-consistent constraint and density matrix renormalization group (DMRG) methods. Systematic and detailed cross-validations are performed between these two methods for narrow systems where DMRG can produce reliable results. AFQMC are then utilized to study wider systems to investigate the thermodynamic limit properties. The ground state is found to be a half-filled stripe state in the small doping and strongly interacting region. The pairing correlation shows $d$-wave symmetry locally, but decays exponentially with the distance between two pairs.

Halide-containing organic persistent luminescent materials for environmental sensing applications.

Great progress has been made in the development of various organic persistent luminescent (OPL) materials in the past few years, and increasing attention has been paid to their interesting applications in environmental sensing due to their long emission lifetimes and high sensitivity. Especially, the introduction of different halogen elements facilitates highly efficient OPL emission with distinct lifetimes and colours. In this review, we summarize the current status of the halide-containing OPL materials for environmental sensing applications. To begin with, the photophysical processes and luminescence mechanisms of OPL materials are expounded in detail to better understand the relationship among molecular structures, OPL properties, and sensing applications. Then, representative halide-containing material systems, such as small molecules, polymers, and doping systems, are summarized with their interesting applications in sensing temperature, oxygen, H2O, UV light and organic solvents. In addition, several challenges and future research opportunities in this field are discussed. This review aims to provide some reasonable guidance on the material design of OPL sensors and their practical applications, and tries to provide a new perspective on the application direction of organic optoelectronics.

Superconductivity in the two-dimensional nonbenzenoid biphenylene sheet with Dirac cone

During the past decade, two-dimensional materials have attracted much attention in superconductivity due to their feasible physical properties and easy chemical modifications. Herein, we use a recently literature reported novel biphenylene sheet (BP sheet) for investigating superconductivity-related physical properties. The electronic states of BP sheet that appeared near the Fermi level are composed of p z orbital of carbon due to sp2 hybridization. Also, an anisotropic Dirac cone is formed just above the Fermi level by crossing two bands comprised of different carbon atoms. One of the two bands is quasi-flat thus leading to a peak of electronic density of states above the Fermi level. In addition, the rotational-vibration phonon mode of the six-membered carbon ring is strongly coupled with electrons. The electron-phonon coupling induces the superconductivity of 6.2 K in BP sheet. Furthermore, both small uniaxial strains and electronic doping can take the Dirac cone and high electronic density of state close to the Fermi level and further raise the superconducting critical temperature to 27.4 K and 21.5 K, respectively. The obtained result suggests that BP sheet with Dirac fermions and superconductivity can be a potential material for the development of future superconducting devices.

Ground-state phase diagram of the t-t′-J model

We report results of large-scale ground-state density matrix renormalization group (DMRG) calculations on t-[Formula: see text]-J cylinders with circumferences 6 and 8. We determine a rough phase diagram that appears to approximate the two-dimensional (2D) system. While for many properties, positive and negative [Formula: see text] values ([Formula: see text]) appear to correspond to electron- and hole-doped cuprate systems, respectively, the behavior of superconductivity itself shows an inconsistency between the model and the materials. The [Formula: see text] (hole-doped) region shows antiferromagnetism limited to very low doping, stripes more generally, and the familiar Fermi surface of the hole-doped cuprates. However, we find [Formula: see text] strongly suppresses superconductivity. The [Formula: see text] (electron-doped) region shows the expected circular Fermi pocket of holes around the [Formula: see text] point and a broad low-doped region of coexisting antiferromagnetism and d-wave pairing with a triplet p component at wavevector [Formula: see text] induced by the antiferromagnetism and d-wave pairing. The pairing for the electron low-doped system with [Formula: see text] is strong and unambiguous in the DMRG simulations. At larger doping another broad region with stripes in addition to weaker d-wave pairing and striped p-wave pairing appears. In a small doping region near [Formula: see text] for [Formula: see text], we find an unconventional type of stripe involving unpaired holes located predominantly on chains spaced three lattice spacings apart. The undoped two-leg ladder regions in between mimic the short-ranged spin correlations seen in two-leg Heisenberg ladders.

Modeling and simulation of the lateral photovoltage scanning method

The goal of this paper is to model and simulate the lateral photovoltage scanning (LPS) method which detects inhomogeneities in semiconductor crystals. Our drift-diffusion model is based on the semiconductor device equations combined with an implicit boundary condition which models a volt meter. To validate our 2D and 3D finite volume simulations, we use theory developed by Tauc [1] to derive three analytical predictions which our simulation results corroborate, even for anisotropic 2D and 3D meshes. Our code runs about two orders of magnitudes faster than earlier implementations based on commercial software [2]. It also performs well for small doping concentrations which previously could not be simulated at all due to numerical instabilities. We present a convergence study which shows that the LPS voltage converges quadratically. Finally, our simulations provide experimentalists with reference laser powers for which meaningful voltages can still be measured. For higher laser power the screening effect does not allow this anymore.

Electron–phonon coupling in lightly [formula omitted]-doped 1T monolayers of PdSTe and PdSeTe: A rigid band approximation approach

In this paper, we present a preliminary study to determine the electron–phonon (el–ph) coupling in Janus 1T monolayered PdSTe and PdSeTe within the rigid band approximation (RBA). Under the assumption that both electronic band structures and phonon dispersions remain intact, or suffer slight changes at most, the Fermi energy is manually shifted towards the conduction bands. Without spin–orbit coupling (SOC), these structures are predicted to be semiconductors with electronic bandgaps of 0.33 eV for PdSTe and 0.34 eV for PdSeTe and with an insignificant electron–phonon coupling. We find that minimal small Fermi energy shifts of the order of 35 meV and 65 meV applied to 1T PdSTe and PdSeTe respectively, are sufficient to produce non-zero values of λ in both systems. These shifts could physically correspond to small electron doping of the materials, not exceeding 4%. Our calculations give that λ=1.77 and λ=0.5 for PdSTe and PdSeTe respectively. The corresponding lowest superconducting temperatures (Tc) register 0.28 K for PdSeTe and 28.19 K for PdSTe. We also demonstrate that the longitudinal optical and acoustical phonon modes may play a key role in the process of superconductivity of the doped materials. The above temperature of PdSTe is larger than those for 2D borophene, stanene, phosophorene and arsenene indicating that, in principle if the right dopant is found, lightly n-doped PdSTe could be promising in the area of 2D superconductors.

Ab initio study of electromagnetic modes in two-dimensional semiconductors: Application to doped phosphorene

Starting from the rigorous quantum-field-theory formalism, we derive a formula for the screened conductivity designed to study the coupling of light with elementary electron excitations and the ensuing electromagnetic modes in two-dimensional (2D) semiconductors. The latter physical quantity consists of three fully separable parts, namely, intraband, interband, and ladder conductivities, and is calculated beyond the random phase approximation as well as from first principles. By using this methodology, we study the optical absorption spectra in 2D black phosphorous, so-called phosphorene, as a function of the concentration of electrons injected into the conduction band. The mechanisms of phosphorene exciton quenching versus doping are studied in detail. It is demonstrated that already small doping levels ($n\ensuremath{\sim}{10}^{12}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$) lead to a radical drop in the exciton binding energy, i.e., from 600 meV to 128 meV. The screened conductivity is applied to study the collective electromagnetic modes in doped phosphorene. It is shown that the phosphorene transversal exciton hybridizes with free photons to form an exciton-polariton. This phenomenon is experimentally observed only for the case of confined electromagnetic microcavity modes. Finally, we demonstrate that the energy and intensity of anisotropic 2D plasmon-polaritons can be tuned by varying the concentration of injected electrons.

High Temperature Superconductivity in a Lightly Doped Quantum Spin Liquid.

We have performed density-matrix renormalization group studies of a square lattice t-J model with small hole doping, δ≪1, on long four and six-leg cylinders. We include frustration in the form of a second-neighbor exchange coupling, J_{2}=J_{1}/2, such that the undoped (δ=0) "parent" state is a quantum spin liquid. In contrast to the relatively short range superconducting (SC) correlations that have been observed in recent studies of the six-leg cylinder in the absence of frustration, we find power-law SC correlations with a Luttinger exponent, K_{SC}≈1, consistent with a strongly diverging SC susceptibility, χ∼T^{-(2-K_{SC})} as the temperature T→0. The spin-spin correlations-as in the undoped state-fall exponentially suggesting that the SC "pairing" correlations evolve smoothly from the insulating parent state.

Tunable nano-distribution of Pt on TiO2 nanotubes by atomic compression control for high-efficient oxygen reduction reaction

Achieving cost-competitive catalysts with low Pt utilization and improving the durability caused by the corrosion of supports in the catalysts must be solved for the high activity in the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Here, we show an innovative technique to synthesize unique nanotube supports for the ORR catalysts based on the combination of experimental and theoretical studies. The method precisely controls the atomic morphology of TiO2 nanotubes by a small amount of atomic substitution, maximizing their efficiency as catalyst supports. The spontaneous change in the size and dispersibility of the Pt nanoparticles appears by only small lattice contraction on the metal-doped TiO2 (M-TiO2) nanotubes. To study this phenomenon, various dopants such as V, Nb, and Cr were added to the M-TiO2 nanotubes. The compression arising out of each metal-support interaction resulted in the diverse shape of the nanoparticles on similar supports, which is revealed based on the X-ray absorption fine structure (XAFS) and the density-functional-theory (DFT) calculations. Based on a comprehensive understanding of inter-and intracrystal interactions in the small substitution doping process, we can control the size and dispersibility of the Pt nanoparticles, catalytic activity, and durability of catalysts for ORR.

Competition of spatially inhomogeneous phases in systems with nesting-driven spin-density wave state

In this work we study zero-temperature phases of the anisotropic Hubbard model on a three-dimensional cubic lattice in a weak coupling regime. It is known that, at half-filling, the ground state of this model is antiferromagnetic (commensurate spin-density wave). For non-zero doping, various types of spatially inhomogeneous phases, such as phase-separated states and the state with domain walls ("soliton lattice"), can emerge. Using the mean-field theory, we evaluate the free energies of these phases to determine which of them could become the true ground state in the limit of small doping. Our study demonstrates that the free energies of all discussed states are very close to each other. Smallness of these energy differences suggests that, for a real material, numerous factors, unaccounted by the model, may arbitrary shift the relative stability of the competing phases. This further implies that purely theoretical prediction of the true ground state in a particular many-fermion system is unreliable.

Flat band carrier confinement in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress but improving sample quality is essential for separating the delicate correlated electron physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist angle variation which has been studied elsewhere. Here, by using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, we demonstrate that flat bands in twisted bilayer graphene can amplify small doping inhomogeneity that surprisingly leads to carrier confinement, which in graphene could previously only be realized in the presence of a strong magnetic field.

Anderson localization effects on the doped Hubbard model

We derive the disorder vs. doping phase diagram of the doped Hubbard model via Dynamical Mean Field Theory combined with Typical Medium Theory, which allows the description of both Mott (correlation driven) and Anderson (disorder driven) metal-insulator transitions. We observe a transition from a metal to an Anderson-Mott insulator for increasing disorder strength at all interactions. In the weak correlation regime and rather small doping, the Anderson-Mott insulator displays properties which are alike to the ones found at half-filling. In particular, this phase is characterized by the presence of empty sites. If we further increase either the doping or the correlation however, an Anderson-Mott phase of different kind arises for sharply weaker disorder strength. This phase occupies the largest part of the phase diagram in the strong correlation regime, and is characterized by the absence of the empty sites.

Possible Spin-Density Wave on Fermi Arc of Edge State in Single-Component Molecular Conductors [Pt(dmdt)2] and [Ni(dmdt)2

We construct three-orbital tight-binding models describing single-component molecular conductors [Pt(dmdt)$_2$] and [Ni(dmdt)$_2$] using first-principles calculations. We show that [Ni(dmdt)$_2$] is a Dirac nodal line system with highly one-dimensional edge states at the (001) edge, similar to [Pt(dmdt)$_2$], as demonstrated in prior studies. To investigate possible edge magnetism, we calculate longitudinal and transverse spin susceptibilities using real-space-dependent random-phase approximation (RPA) in three-orbital Hubbard models in the presence of spin--orbit coupling. We find that the edge spin-density wave (SDW) is induced by the Coulomb repulsion and incommensurate nestings of the Fermi arcs. We also find that the magnetic structure of the edge SDW can be changed via extremely small carrier doping, which is controllable in molecular conductors.

Low-Energy Optical Conductivity of TaP: Comparison of Theory and Experiment

The ab-plane optical conductivity of the Weyl semimetal TaP is calculated from the band structure and compared to the experimental data. The overall agreement between theory and experiment is found to be best when the Fermi level is slightly (20 to 60 meV) shifted upwards in the calculations. This confirms a small unintentional doping of TaP, reported earlier, and allows a natural explanation of the strong low-energy (50 meV) peak seen in the experimental ab-plane optical conductivity: this peak originates from transitions between the almost parallel non-degenerate electronic bands split by spin-orbit coupling. The temperature evolution of the peak can be reasonably well reproduce by calculations using an analog of the Mott formula.

Structural investigations of ZnO nanostructure

Undoped and Mn-doped ZnO samples with different percentages of Mn content (1, 5 and 10 at%) were synthesized by a dip coating sol–gel method. We have studied the structural, chemical and optical properties of the samples by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-visible spectroscopy. The XRD spectra show that all the samples are hexagonal wurtzite structures. We note that doping favors c-axis orientation along (002) planes. Up to 5 at% of Mn doping level, the c- axis lattice parameter shifts towards higher values with the increase of manganese content in the films. The expansion of the lattice constant of ZnO–Mn indicates that Mn is really doped into the ZnO. The SEM investigations of all samples revealed that the crystallites are of nanometer size. The sur- face quality of the ZnO–Mn film increases with Mn doping but no significant change of the grain size is observed from SEM images. The transmittance spectra show that the trans parency of all the samples is greater than 85 %. We note, also, that a small doping (1 %) lowered the refractive index while the thickness of the layers and the gap increase. However, on raising the proportion of Mn beyond 5 %, practically the same values of index and gap as pure ZnO are found.