Spin Gap Effects on Bulk R1+x ba2−x Cu3O7−δ (R=Eu or Nd and 0≤x≤0.4)

Spin gap effects on the underdoping states of the bulk system of R_1+x Ba_2?x Cu₃O_7?δ (R = Eu or Nd and 0 ≤ x ≤ 0.4) were investigated through transport property measurements. The underdoping states were achieved by, alternatively substituting R³⁺ for Ba²⁺ ions in the system rather than adjusting the oxygen deficiency. The excess R³⁺ ions were to occupy the Ba sites of the crystalline lattice as revealed from Rietveld analysis for powder X-ray diffraction. The underdoped materials were observed to first undergo spin pairing transition in the temperature range well above T _c, and come across with superconducting transition at T _c. The increasing feature observed for spin gap temperature and the decreasing one for T _c, as the concentration of holes decreases, are in qualitatively good agreement with theoretical predictions from the mean-field RVB model.     

Ab Initio Investigations of Structural,Elastic, Mechanical,Electronic, Magnetic,and Optical Properties of Half-Heusler Compounds RhCrZ (Z = Si,Ge)

The first principle study of half-Heusler compounds RhCrZ (Z = Si, Ge) is performed in the framework of density functional theory (DFT). The compounds are found to have small band gap in the minority spin channel (spin-down). While the majority spin channel (spin-up) is metallic. Therefore, both compounds are half-metallic and 100 % spin polarized at Fermi level. Several properties including structural, mechanical, elastic, electronic, magnetic, and optical are computed using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2k simulation package. Equilibrium lattice constants for both compounds are found to be in the range 5.5–6.0 Å. Elastic properties indicate the ductile nature of the compounds. The total magnetic moments for these compounds are approximately equal to 1μ _B, i.e., M_Tot ≈ 1μ _B. Hence, the compounds are weak ferromagnetic materials. We have calculated the complex dielectric function. Many optical properties including reflectivity, refractive index, conductivity, and absorption coefficients are obtained form dielectric function. Imaginary part of the dielectric functions shows that compounds are optically metallic and become transparent above 17 and 13 eV, respectively. It is also observed that compounds are more active in the infrared region.     

Effect of Ga doping on micro/structural, electrical and optical properties of pulsed laser deposited ZnO thin films

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al₂O₃ substrates using the 1, 3 and 5 at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E₂^High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B₁ modes (B₁^Low, 2B₁^Low and B₁^High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300 K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.     

Neutron scattering study in the spin-1/2 ladder system: Sr14Cu24O41

Inelastic neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system Sr₁₄Cu₂₄O₄₁, which has both simple chains and two-leg ladders of copper ions. We have observed that both the chain and the ladder exhibit a spin gap, which originate from a dimerized state.     

Multifunctional Gadolinium‐Doped Mesoporous TiO2 Nanobeads: Photoluminescence,Enhanced Spin Relaxation,and Reactive Oxygen Species Photogeneration,Beneficial for Cancer Diagnosis and Treatment

Materials with controllable multifunctional abilities for optical imaging (OI) and magnetic resonant imaging (MRI) that also can be used in photodynamic therapy are very interesting for future applications. Mesoporous TiO₂ sub‐micrometer particles are doped with gadolinium to improve photoluminescence functionality and spin relaxation for MRI, with the added benefit of enhanced generation of reactive oxygen species (ROS). The Gd‐doped TiO₂ exhibits red emission at 637 nm that is beneficial for OI and significantly improves MRI relaxation times, with a beneficial decrease in spin–lattice and spin–spin relaxation times. Density functional theory calculations show that Gd³⁺ ions introduce impurity energy levels inside the bandgap of anatase TiO₂, and also create dipoles that are beneficial for charge separation and decreased electron–hole recombination in the doped lattice. The Gd‐doped TiO₂ nanobeads (NBs) show enhanced ability for ROS monitored via ^?OH radical photogeneration, in comparison with undoped TiO₂ nanobeads and TiO₂ P25, for Gd‐doping up to 10%. Cellular internalization and biocompatibility of TiO₂@x Gd NBs are tested in vitro on MG‐63 human osteosarcoma cells, showing full biocompatibility. After photoactivation of the particles, anticancer trace by means of ROS photogeneration is observed just after 3 min irradiation.     

The Structural,Anisotropic Magnetization,and Spectroscopic Study of Delafossite CuCr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> Systems

This work describes the chemical doping effect on the magnetic properties of p-type transparent semiconductor with delafossitetype oxides CuCr _0.95 M _0.05 O ₂ (M = Ni, Cr, Ga, V, Fe, Mn, and Zn). The lattice parameters were found to vary according to Vegard’s law. Reflection broadening is observed, that is ascribed to local lattice distortion due to the ionic radius difference between Cr ³⁺ and M dopants Raman spectra at a room temperature of CuCr _0.95 M _0.05 O ₂ displayed three intense modes which are identified as σ(A _1g) at 691 cm ^?1, σ(E _g) at 444 cm ^?1 and σ(A _g) at 207 cm ^?1 that matched with other delafossite structures. Magnetic susceptibility measurements show that the dominant interactions are antiferromagnetic (AFM) but that doping induces significant changes. A large ferromagnetic component arises in the Fe, V and Mn-substituted samples below ≈120 K. The Ni, Zn and Ga-substituted phases retain antiferromagnetic ordering at T _N≈ 24 K. The main origin of the stabilization of the AFM state is probably the randomness induced by doping. The coupling between the local spins at the Cr sites and doped metal transition may enhance spin fluctuations at the Cr sites, which break the residual magnetic degeneracy as fluctuation-induced symmetry breaking in a highly magnetic degenerate groundstate manifold of some frustrated systems.     

Spin Gap Effects on Bulk R1+xba2?x Cu3O7?δ (R=Eu or Nd and 0≤x≤0.4)

Spin gap effects on the underdoping states of the bulk system of R_1+x Ba_2–x Cu₃O_7– (R = Eu or Nd and 0 x 0.4) were investigated through transport property measurements. The underdoping states were achieved by, alternatively substituting R³⁺ for Ba²⁺ ions in the system rather than adjusting the oxygen deficiency. The excess R³⁺ ions were to occupy the Ba sites of the crystalline lattice as revealed from Rietveld analysis for powder X-ray diffraction. The underdoped materials were observed to first undergo spin pairing transition in the temperature range well above T _c, and come across with superconducting transition at T _c. The increasing feature observed for spin gap temperature and the decreasing one for T _c, as the concentration of holes decreases, are in qualitatively good agreement with theoretical predictions from the mean-field RVB model.     

Ab-initio spin polarized electronic structure calculations for Ti<Subscript>x</Subscript>Ga<Subscript>n</Subscript>As<Subscript>m</Subscript> photovoltaic materials

A half-metallic isolated band in the band-gap of GaAs and GaP semiconductors has been found for Ti and Sc transition metal impurities and proposed as highly-efficient photovoltaic materials. In this paper, we have investigated by first principle calculations, the spin polarized and non-polarized dispersion band structures and lattice constants of Ga₃As₄Ti and Ga₄As₃Ti alloy semiconductor compounds. We have carried out a comparative study of these compounds in order to identify the basic features of the isolated intermediate band formation in the semiconductor band-gap. We use an ab-initio fully self-consistent density functional theory method in the local density approximation (LDA), with norm-conserving, non-local pseudopotentials for core electrons. To assess the results, we first determined the electronic properties of GaAs and compared them with the experimental results. We find that spin wave functions of the polarized Ga_nAs_mTi compounds noticeably modify the nature and properties of the intermediate band that have already shown in the corresponding paramagnetic compounds.     

Relationship between Spin Gap and Crystal Structure in La-214 Cuprate Superconductor

To investigate the intrinsic nature of spin correlations in La-214 high-T _c cuprate, we performed neutron scattering measurement on La_1.94−x Sr _x Ce_0.06CuO₄ (LSCCO) samples for two different hole concentrations. Since the Ce-doping reduces the hole concentration without changing the bulk lattice distortion, the corrugation of CuO₂ planes in the Sr and Ce co-doped LSCCO system is more relaxed compared with that in La_2−x Sr _x CuO₄ (LSCO) at equivalent hole concentration. In the optimally doped x=0.18 sample, clear spin gap structure was observed, as is the case of the optimally doped LSCO. Therefore, the spin gap structure is insensitive to the lattice distortion. In the underdoped x=0.14 sample, the local spin susceptibility (χ″(ω)) above 2 meV increases with increasing the energy transfer (ω) and reaches an intensity maximum at ∼7 meV, exhibiting a gap-like structure. The observation of gap-like structure in the present underdoped sample is quite different from the energy-independent χ″(ω) reported for underdoped LSCO. The possible origin of difference of spin excitations is discussed from the viewpoint of two-component picture for spin fluctuations.     

Yellow and warm white light emitting Zn doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript> for near UV excitable phosphor converted WLED

Optical properties of Zn doped Y₂O₃ microsheets prepared by sol–gel combustion method have been investigated and their application in phosphor converted white LED has been examined. The formation of single phase, well crystalline cubic Y₂O₃ is confirmed from powder XRD results. Effective substitution of Zn in Y₂O₃ crystal lattice is inferred from shifting of diffraction peaks. SEM images have showed that undoped as well as Zn doped Y₂O₃ formed as microsheets. Doping of Zn enhanced the growth of the sheets and its length increased from 1.5 to 19 µm. Development of structural disorder in Y₂O₃ crystal structure after Zn doping and confirmation of the conserved cubic structure of Zn doped Y₂O₃ without any secondary phase have been revealed from micro-Raman spectra. The optical band gap of Y₂O₃ has been altered after Zn doping and it is found to be decreased from 5.6 to 5.22 eV as increasing Zn concentration. Both undoped and Zn doped Y₂O₃ showed a broad visible emission from blue to green region due to various defects and impurities present in it. Broad PL excitation spectrum inferred the possibility to attain the visible emission under the excitation of light with wide range of wavelength from near UV to blue region. Excitation of pure Y₂O₃ under near UV (375 nm) LED chip lead to the emission of yellow light whereas Zn doped Y₂O₃ emitted warm white light with color coordinate of (0.42, 0.35), colour rendering index of 77.6 and correlated color temperature (CCT) of 2840 K. Hence, Zn doped Y₂O₃ discussed in the present work can be a better replacement for various rare earth doped phosphors in the application of phosphor converted WLED (pc-WLED).     

First principle investigation of AlAs and AlP compounds and ordered AlAs1−xPx alloys

We have investigated the structural and electronic properties of AlAs and AlP compounds and of ordered AlAs_1−xP_x alloys using the full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method based on density functional theory. The total energies and structural quantities of those compounds have been calculated for different approximations of exchange–correlation energy. The electronic quantities have been found to be in good agreement with the corresponding measured ones when the compounds were defined by the lattice constants of Perdew–Wang-generalized gradient approximation (PW-GGA) scheme. The PW-GGA approach was also applied on ordered AlAs_1−xP_x alloys to study the effect of composition on lattice constant, band gap, and refractive index of AlAs_1−xP_x ternary alloys. The calculated lattice constants scale linearly with composition (Vegard’s law). The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. In addition to (FP-LAPW + lo) method, the composition dependence of the refractive index was studied by Reedy and Nazeer model. The thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram.     

Spectroscopic investigations on hybrid nanocomposites: CdS : Mn nanocrystals in a conjugated polymer

We have used electron paramagnetic resonance (EPR) spectroscopy for investigating the properties of spins, such as those carried by polarons which carry both spin and charge in poly (meta/para phenylene) PMPP: CdS doped Mn based nanocomposites. To identify the nature of paramagnetic species in PMPP matrix, we have studied the effect of different physical parameters. It was found that we are in presence of trapped polarons and localized spins which concentration has been estimated. Moreover, spin–spin and spin–lattice relaxation rates have been calculated. Then, we discussed the results of optical and EPR study on the hybrid nanocomposite (CdS nanostructures, doped with manganese (II) ions, incorporated in PMPP conjugated polymer matrix). The optical spectra of these nanocomposites were compared to the existing models of energy levels in quantum dots. Moreover, by the use of electronic paramagnetic resonance, conclusions about the location and the symmetry of Mn²⁺ ions have been drawn. The nanocomposite energy gap is in the 3.2–3.3 eV range. The size of the nanoparticle is about 3.3 nm and Mn²⁺ ions are located at or near the nanoparticle surface.     

Effect of the substrate temperature on the structural, optical and electrical properties of spray-deposited CdS:B films

Boron doped CdS films have been deposited by spray pyrolysis method onto glass substrate temperature in the range of 350–450 °C. And the effect of substrate temperature (T _s) on the structural, electrical and optical properties of the films were studied. The structural properties of boron doped CdS films have been investigated by (XRD) X-ray diffraction techniques. The X-ray diffraction spectra showed that boron doped CdS films are polycrystalline and have a hexagonal (wurtzite) structure. By using SEM analysis, the surface morphology of the films was observed as an effect of the variation of substrate temperature. The substrate temperature is directly related with the shift detected in the band gap values derived from optical of parameters and the direct band gap values were found to be in the region of 2.08–2.44 eV. The electrical studies showed that the film deposited at the substrate temperature 400 °C had high carrier concentration and Hall mobility and minimum resistivity. This resistivity value decreased with increase in temperature up to 400 °C indicating the semiconducting nature of B- doped CdS films. The lattice parameter, grain size, microstrain and dislocation densities were calculated and correlated with the substrate temperature (T _s ).     

Diverse Defects in Alkali Niobate Thin Films: Understanding at Atomic Scales and Their Implications on Properties

Defects in ferroelectric materials have many implications on the material properties which, in most cases, are detrimental. However, engineering these defects can also create opportunities for property enhancement as well as for tailoring novel functionalities. To purposely manipulate these defects, a thorough knowledge of their spatial atomic arrangement, as well as elastic and electrostatic interactions with the surrounding lattice, is highly crucial. In this work, analytical scanning transmission electron microscopy (STEM) is used to reveal a diverse range of multidimensional crystalline defects (point, line, planar, and secondary phase) in (K,Na)NbO₃ (KNN) ferroelectric thin films. The atomic-scale analyses of the defect-lattice interactions suggest strong elastic and electrostatic couplings which vary among the individual defects and correspondingly affect the electric polarization. In particular, the observed polarization orientations are correlated with lattice relaxations as well as strain gradients and can strongly impact the properties of the ferroelectric films. The knowledge and understanding obtained in this study open a new avenue for the improvement of properties as well as the discovery of defect-based functionalities in alkali niobate thin films.     

Ferromagnetism from Acceptors of Native Point Defects in (Zn, Mn)O Doped Systems

In this paper, we represent a theoretical study of the (Zn, Mn)O doped system with native point defects of ZnO as oxygen interstitials (O_i) and zinc vacancies (V_Zn). Under these defects, it has been shown that the ground state can be converted from spin glass to ferromagnetic phase, by performing ab initio density functional theory calculations relying on the Korringa?CKohn?CRostoker coherent potential approximation (KKR-CPA) method employing the local density approximation (LDA) with the parameterization by Morruzi, Janak, and Williams. The stability of magnetism in the (Zn, Mn)O doped system with different native point defects has been discussed. We find that acceptor-like defects (that is, O_i, O_Zn, and V_Zn) may cause the enhancement of the ferromagnetic characteristics in ZnMnO with increasing T _c . Based on the theoretical results, we suggest that the native point defects have a key role with respects to the FM properties. Using the mean field approximation, the Curie temperature in our studied model is estimated.     

Evolution of Magnetic Properties and Spin Kinetics of Cuprates with Doping

We have shown that properties of lightly doped quasi-layered cuprates can be described on the basis of topological excitation known as skyrmions both thermally excited and induced by quasi-localized electronic holes. We have calculated the average skyrmion radius r ₀ and nuclear spin relaxation rate 1/T ₁ as a function of temperature and hole concentration. The results are in qualitative agreement with experiments.     

Study of the Structural,Electronic, Magnetic,and Optical Properties of Mn<Subscript>2</Subscript>ZrGa Full-Heusler Alloy: First-Principles Calculations

In this work, the structural, electronic, magnetic, and optical properties of Mn₂ZrGa full-Heusler alloy were investigated by using density functional theory (DFT) calculations. It is found that the spin-up states have a metallic character, but the spin-down bands have a pseudo-gap at the Fermi level. The total spin magnetic moment of Mn₂ZrGa (per formula unit) is 3.00 µ _B at an equilibrium lattice parameter of 6.15 Å. The calculations show that Mn₂ZrGa is a ferrimagnetic with 81% spin polarization at equilibrium lattice parameter. The effect of lattice parameter distortion on the magnetic properties and spin polarization is also studied. It is found that the total magnetic moment preserves its value for a lattice parameter range of 5.96–6.30 Å. The decreasing of the lattice parameter leads to improvement of spin polarization. The real and imaginary parts of dielectric function and hence the optical properties including energy absorption spectrum, reflectivity, and optical conductivity are also calculated. The value of plasma frequency for spin-up and down electrons is located at 1.78 and 0.74 eV, respectively.     

Structural study, photoluminescence, and photocatalytic activity of semiconducting BaZrO3:Bi nanocrystals

Wide band gap nanocrystalline bismuth doped barium zirconate is synthesized by a facile hydrothermal method at 100 °C. The obtained cubic perovskites are characterized by powder X-ray diffraction (XRD), UV-VIS diffuse reflectance spectroscopy, photoluminescence (PL) spectroscopy, and photocatalytic activity. The estimated band gap in the 2.4-4.9 eV range, depending on Bi concentration, suggests nanocrystalline BaZrO₃:Bi as a useful visible-light activated photocatalyst under excitation wavelengths <800 nm. Displacement of main XRD pattern peaks suggest that bismuth ion mostly substitutes into Zr⁴⁺ sites within the BaZrO₃ host lattice. It is found that BaZrO₃:Bi decomposes methylene blue (MB) under both UV and visible light irradiation. The photocatalyst efficiency depends strongly on Bi content and induced defects.     

Influence of substitutional doping on the electronic properties of carbon nanotubes with Stone Wales defects: density functional calculations

Abstract

In this work, we implemented density function theory to investigate the structural and the electronic properties of nitrogen doped single walled carbon nanotube under different orientations of Stone Wales defect. We have found that, the doped defected structures are more stable than the non-doped defected structures. Furthermore, doping defected carbon nanotubes with a nitrogen atom has significantly narrowed the band gap and slightly shifted the Fermi level toward the conduction band. Moreover, nitrogen substitution creates new band levels just above the Fermi level which exemplifies an n-type doping. However, the induced band gap is indirect band gap compared to direct band gap as in pristine carbon nanotubes. Furthermore, the electronic and structural properties of nitrogen doped carbon nanotube with Stone Wales defects is crucially affected by the dopant site as well as the orientations of Stone Wales defects.     

Recent advances in β-FeSe1−x and related superconductors

Abstract

It has been more than four years since the discovery of β-FeSe_1?x superconductors. Through the efforts of many outstanding research groups, unprecedented advances in the field have been achieved. High-quality single crystals of β-FeSe_1?x and related compounds have been prepared by various techniques, allowing us to explore in detail the physical properties of this class of materials. Detailed characterizations of the structure and properties of these crystals have helped us to understand the origin of superconductivity in β-FeSe_1?x. The occurrence of superconductivity is associated with the low-temperature structure distortion, which is accompanied by several anomalies. Recent measurements on quasiparticle and acoustic phonon dynamics with respect to the orbital modification in β-FeSe_1?x suggest the opening of an energy gap below 130–140 K, accompanied by a coincident transfer of optical spectral weight in the visible range and alterations in transport properties. These observations provide convincing evidence that the modification of the electronic structure occurs prior to the lattice distortion. They further suggest that the high-temperature gap and the lattice symmetry breaking are driven by short-range orbital and/or charge orders.