Half-metallicity in Rh-doped TiO<Subscript>2</Subscript> from ab initio calculations

First-principles calculations based on density functional theory (DFT) are performed to study the electronic structures and magnetic properties of Rh-doped TiO₂ crystals. The hybridization between Rh-4d and O-2p results in Rh becoming ferromagnetic with a magnetic moment of about 1.0 μ _B per supercell. The Rh-doped TiO₂ system exhibits half-metallic ferromagnetism based both DFT and DFT + U. The strong ferromagnetic couplings between local magnetic moments can be attributed to both the p-d hybridization and double-exchange mechanisms, as well as superexchange interaction. These results suggest an alternative approach to achieve promising dilute magnetic semiconductors by doping non-magnetic transition metals in a TiO₂ host.     

The structural,electronic and optical properties of CuGa (Se<Subscript>x</Subscript>S<Subscript>1-x</Subscript>)<Subscript>2</Subscript> compounds from first-principle calculations

The structural, electronic and optical properties of theCuGa (Se _x S_1-x )₂alloy system have been performed systematic within generalized gradient approximation(GGA) of Perdew-Burke-Ernzerhof (PBE) implemented in the Cambridge serial total energypackage (CASTEP) code. We calculate the lattice parameters and axial ratio, which agreewith the experimental values quite well. The anion position parameters uare also predicted using the model of Abrahams and Bernstein and the results seem to betrustworthy as compared to the experimental and theoretical values. The total and partdensity of states are discussed which follow the common rule of the conventionalsemiconductors. The static dielectric tenser and refractive index are summarized comparedwith available experimental and theoretical values. Also the spectra of the dielectricfunctions, refractive index, reflectance, absorption coefficient and real parts ofphotoconductivity are discussed in details.     

Magnetic behaviour of doped dimer compounds Sr<Subscript>3</Subscript>Cr<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>8</Subscript> (M = V,Mn)

Wilson chains, based on a logarithmic discretization of a continuous spectrum, are widely used to model an electronic (or bosonic) bath for Kondo spins and other quantum impurities within the numerical renormalization group method and other numerical approaches. In this short note we point out that Wilson chains can not serve as thermal reservoirs as their temperature changes by a number of order ΔE when a finite amount of energy ΔE is added. This proves that for a large class of non-equilibrium problems they cannot be used to predict the long-time behavior.     

The effect of Zr content on electrical properties of Ba(Ti<Subscript>1-x</Subscript>Zr<Subscript>x</Subscript>)O<Subscript>3</Subscript> ceramics

The effect of Zr content on the crystal structure and electrical properties of barium zirconate titanate (Ba(Zr,Ti)O₃) was studied by X-ray diffraction and dielectric, ferroelectric and impedance spectroscopy. An increase of Zr content into BaTiO₃ leads to a reduction in its c-parameter and an increase in its a-parameter, resulting in a change from tetragonal to cubic symmetry of the BaTiO₃ unit cell. The Curie temperatures are lowered and the relative permittivity values are decreased with increasing Zr content. The presence of BaZrO₃ secondary phases has the affect of decreasing tanδ. A higher applied electric field is required during the polarization process because of the effect of domain-wall pinning caused by oxygen vacancies. Impedance spectroscopy studies of Ba(Ti_0.95Zr_0.05)O₃ ceramics show a decrease in the bulk resistance with increasing temperature, indicating a typical negative temperature coefficient of resistance. PACS 74.62.Bf; 74.62.Yb; 77.22.Ch; 77.80.Bh; 77.84.Dy     

On the interpretation of the phase diagram of La<Subscript>2−x</Subscript>M<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

The difference between the phase diagrams of La_2?x(Ba,Sr)_xCuO₄ and Nd_2?xCe_xCuO₄ is discussed. It is proposed that the discrepancy of x-values corresponding to the transition from antiferromagnetic dielectric state to conducting one (respectively, x≈0.06 in La_2?x(Ba,Sr)_xCuO₄ and x≈0.125 in Nd_2?xCe_xCuO₄) results from non-homogeneous doping of La_2?x(Ba,Sr)_xCuO₄ over the range 0.06<x<0.125, when localized holes are added to each second CuO₂ layer. Therefore the actual phase diagram of the “holedoped” superconductor La_2?x(Ba,Sr)_xCuO₄ coincides with one for the “electron-doped” superconductor Nd_2?xCe_xCuO₄. It is shown that all features observed in La_2?x(Ba,Sr)_xCuO₄ around x=0.125 are clarified on this basis.     

High-pressure phases of MgSiN<Subscript>2</Subscript> from first-principles calculations

The relationship between structure and pressure for MgSiN₂ has been simulated using a density functional approximation within the local density approximation. The low-pressure (LP-) phase of MgSiN₂ with an orthorhombic structure transforms to a high-pressure (HP-) modification with a CsICl₂-type structure at a pressure of about 16.5 GPa. HP-MgSiN₂ , in which both Mg and Si are octahedrally coordinated by N, has a bulk modulus of about 238 GPa, much higher than that of the LP-modification (about 182 GPa) with tetrahedrally coordinated metal atoms. HP-MgSiN₂ is a wide-gap semiconductor with an indirect energy gap of about 4.3 eV, similar to that of LP-MgSiN₂. The direct gap at is about 5.8 eV. PACS 71.15.Mb; 61.50.Ks; 61.50.Ah     

Characterizing the insulator adjacent to the superconductor in Bi<Subscript>2</Subscript>Sr<Subscript>2-x</Subscript>La<Subscript>x</Subscript>CuO<Subscript>6+δ</Subscript> (x = 0.3)

The time-averaged and low-frequency noise transport properties were investigated in the vicinity of the superconductor-insulator transition for a Bi₂Sr_2-x La _x CuO_6+δ (x = 0.3) thin film. The results are consistent with a superconductor (metal) embedded in a strong insulator, the latter showing two-dimensional variable range hopping properties. The weak insulator behavior - if any - is attributed to the metallic inclusions only.     

On the magnetism of Ln<Subscript>2/3</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> (Ln = lanthanide)

The magnetic and thermodynamic properties of the complete Ln_2/3Cu₃Ti₄O₁₂ series were investigated. Here Ln stands for the lanthanides La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. All the samples investigated crystallize in the space group Im[`3]Im\bar{3} with lattice constants that follow the lanthanide contraction. The lattice constant of the Ce compound reveals the presence of Ce⁴⁺ leading to the composition Ce_1/2Cu₃Ti₄O₁₂. From magnetic susceptibility and electron-spin resonance experiments it can be concluded that the copper ions always carry a spin S = 1/2 and order antiferromagnetically close to 25 K. The Curie-Weiss temperatures can approximately be calculated assuming a two-sublattice model corresponding to the copper and lanthanide ions, respectively. It seems that the magnetic moments of the heavy rare earths are weakly coupled to the copper spins, while for the light lanthanides no such coupling was found. The 4f moments remain paramagnetic down to the lowest temperatures, with the exception of the Tm compound, which indicates enhanced Van-Vleck magnetism due to a non-magnetic singlet ground state of the crystal-field split 4f manifold. From specific-heat measurements we accurately determined the antiferromagnetic ordering temperature and obtained information on the crystal-field states of the rare-earth ions.     

Synthesis and characterization of mesoporous and hollow-mesoporous M<Subscript>x</Subscript>Fe<Subscript>3-x</Subscript>O<Subscript>4</Subscript> (M=Mg,Mn, Fe,Co, Ni,Cu, Zn) microspheres for microwave-triggered controllable drug delivery

Spinel ferrites can be used in magnetic targeting and microwave heating and can therefore be used for targeted and controllable drug delivery. We used the cetyltrimethylammonium bromide-assisted solvothermal method to synthesize a series of spinel ferrites (M_xFe_3-xO₄, M=Mg, Mn, Fe, Co, Ni, Cu, Zn) with a mesoporous or hollow-mesoporous structure suitable for direct drug loading and the particle diameters ranging from 200 to 350 nm. We investigated the effects of M²⁺ cation on the morphology and properties of these products by analyzing their transmission electron microscopy images, mesoporous properties, magnetic properties, and microwave responses. We chose hollow-mesoporous M_xFe_3-xO₄ (M=Fe, Co, Zn) nanoparticles, which had better overall properties, for the drug VP16 (etoposide) loading and microwave-controlled release. The Co_xFe_3-xO₄ and Fe₃O₄ particles trapped 61.5 and 64.8%, respectively, of the VP16, which were higher than that (60.4%) of Zn_xFe_3-xO₄. Controllable drug release by these simple magnetic nanocarriers can be achieved by microwave irradiation, and VP16-loaded Co_xFe_3-xO₄ released the most VP16 molecules (more than 50% after 1 h and 69.1% after 6 h) under microwave irradiation. Our results confirm the favorable drug loading and microwave-controlled delivery by these ferrites, and lay a theoretical foundation to promote clinical application of the targeted controllable drug delivery system.

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Graphical abstract In the present study, we prepared mesoporous or hollow-mesoporous spinel ferrites (M_xFe_3-xO₄, M=Mg, Mn, Fe, Co, Ni, Cu, Zn) by CTAB-assisted solvothermal method and solved the problem of Cu and Ni impurities in Cu_xFe_3-xO₄ and Ni_xFe_3-xO₄ products by means of magnetic separation and additional redox reactions, respectively. We investigated the effects of the M²⁺ cation on the morphology, mesoporous properties, magnetic properties, and microwave responses of these ferrites. Then, the drug loading and microwave-controlled drug release of hollow-mesoporous M_xFe_3-xO₄ (M?=?Fe, Co, Zn) nanoparticles with better overall properties were also studied. Co_xFe_3-xO₄ has the best overall performances for microwave-controlled drug release.

     

Growth of polycrystalline LaNi<Subscript>1-x</Subscript>Co<Subscript>x</Subscript>O<Subscript>3</Subscript> (<Emphasis Type="BoldItalic">x</Emphasis> =0.3,0.5) thin films on Si(100) by pulsed laser deposition

Polycrystalline LaNi_1-xCo_xO₃ (x=0.5,0.3) thin films have been deposited on polished Si(100) substrates by pulsed laser deposition. The films are grown at 650 °C in ambient oxygen pressure of 0.4 mbar with an incident laser fluence of 1.5 J/cm² delivered by a KrF excimer laser. The lattice parameters of the as-grown films are slightly larger (0.05–0.4%) than those of the powders used to prepare the targets. The films exhibit weak texturing along the (012) direction. The low-temperature magnetic properties of the films, i.e. the coercive force, the remanence and the saturation magnetization, are enhanced compared to the powders. Furthermore, the x=0.3 film exhibits a low, almost temperature-independent resistivity above 200 K [(300 K)30 cm] and thus we propose it as a potential candidate material for electrode applications, e.g. in ferroelectric devices. PACS 68.55.-a; 73.61.-r; 81.15.Fg     

Magnetic ordering in Dy<Subscript>1-x</Subscript>Ca<Subscript>x</Subscript>BaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript> for x = 0.0 and 0.1

The crystal and magnetic structures of Dy_1-xCa_xBaCo₂O_5.5 for x = 0.0 and 0.1 have been studied by neutron powder diffraction and the crystal structures of both compounds were found to be best described in space group Pmmm with a a_p × 2a_p × 2a_p unit cells where a_p is the lattice parameter of the cubic perovskite unit cell. The a- and b-axes were found to decrease and increase abruptly between 315 and 350 K as the temperature increases and the unit cell volumes exhibit signs of excess thermal expansion in the temperature range from 260 to 315 K. Dy_0.9Ca_0.1BaCo₂O_5.5 orders antiferromagnetically for T ≤ 305 K into a G-type magnetic structure with a 2a_p × 2a_p × 2a_p magnetic unit cell. DyBaCo₂O_5.5 exhibits two magnetically ordered phases and a G-type magnetic structure was observed at the investigated temperatures 260 and 290 K. A 2a_p × 2a_p × 4a_p magnetic unit cell was needed for indexing of the magnetic reflections observed for T ≤ 230 K. The low temperature magnetic structure of DyBaCo₂O_5.5 is different from the observed magnetic structures of TbBaCo₂O_5.5 and HoBaCo₂O_5.5 despite the proximity of Tb, Dy and Ho in the periodic table. It is a relatively complex antiferromagnetic structure with both pyramidally and octahedrally coordinated Co ions in the intermediate spin state. It contains both ferro- and antiferromagnetic interactions and the magnetic moments are canted in the a, b-plane. The canting angles between the magnetic moments and the b-axis are 6.6 and 50.0° at 20 K for the pyramidally and octahedrally coordinated Co ions, respectively. The high and low temperature magnetic phases were found to coexist at 230 K.     

Synthesis and Luminescent Properties of M<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>: Eu (M=Sr,Ba) Nanophosphors

A solution combustion route for the synthesis of Eu³⁺-activated M₂V₂O₇ (M = Sr, Ba) and their luminescent properties have been investigated. Structure and luminescent characteristics of Sr₂V₂O₇:Eu³⁺ and Ba₂V₂O₇:Eu³⁺ nanophosphors have been studied by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, fluorescence spectrometry and Fourier transform infra-red spectroscopy. The incorporation of Eu³⁺ activator in these nanoparticles has been checked by luminescence characteristics. These nanoparticles have displayed red color under a UV source which is due to characteristics transition of Eu³⁺ from ⁵D₀ → ⁷F₂ at 613 nm in both Sr₂V₂O₇:Eu³⁺ and Ba₂V₂O₇:Eu³⁺ nanophosphors. In addition, the optimal Eu³⁺ - doped contents of Sr_2(1-x)Eu_2xV₂O₇ and Ba_2(1-x)Eu_2xV₂O₇ nanophosphors for both were 4 mol%.     

Special electronic structures of inverse spinels LiMVO<Subscript>4</Subscript>(M = Ni and Cu): a first-principles study

We use the Ulam method to study spectral properties of the Perron-Frobenius operators of dynamical maps in a chaotic regime. For maps with absorption we show numerically that the spectrum is characterized by the fractal Weyl law recently established for nonunitary operators describing poles of quantum chaotic scattering with the Weyl exponent ν = d-1, where d is the fractal dimension of corresponding strange set of trajectories nonescaping in future times. In contrast, for dissipative maps we numerically find the Weyl exponent ν = d/2 where d is the fractal dimension of strange attractor. The Weyl exponent can be also expressed via the relation ν = d₀/2 where d₀ is the fractal dimension of the invariant sets. We also discuss the properties of eigenvalues and eigenvectors of such operators characterized by the fractal Weyl law.     

Lattice dynamics of Eu<Subscript>1-x</Subscript>Y<Subscript>x</Subscript>MnO<Subscript>3</Subscript> (0 ≤ x ≤ 0.5)

A systematic and detailed study of Raman and infrared active lattice excitations in the orthorhombic multiferroic manganite Eu_1−xY_xMnO₃ (0 ≤ x ≤ 0.5) was carried out at room temperature. For the infrared active phonon modes the eigenfrequencies, damping constants and oscillator strengths were analyzed by Fourier-Transform Infrared Spectroscopy in the far infrared frequency range. For the Raman active phonons the same analysis for eigenfrequencies and damping constants was carried out using Raman spectroscopy in the range from 200 cm⁻¹ to 700 cm⁻¹. Y doping leads to mode-dependent phonon frequency shifts up to 8%. These are interpreted in terms of the interplay between the decrease of the reduced ion masses and the axis-dependent change of bond lengths. The latter leads to a bond softening along the a-axis and a strengthening along the c-axis, for which the highest phonon frequency increase is observed. The application of both Raman and Infrared Spectroscopy gives us sensitivity not only to symmetry properties via the selection rules but also to the involvement of different ion types within the unit cell. It is clearly shown that the disorder induced effects are of minor impact on the lattice properties and solely detected on the rare earth sites. The MnO₆ octahedra remain unaffected and show the same behavior as in the stochiometric RMnO₃ making Eu_1−xY_xMnO₃ an excellent model system for a quasi-continuous fine-tuning of the lattice parameters relevant for the appearance of multiferroicity.     

Structural and thermoelectric properties in (Sb<Subscript>1-x</Subscript>Bi<Subscript>x</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> thin films

We have investigated the structural and thermoelectric properties of (Sb_1-xBi_x)₂Te₃ thin films on CdTe(111)B. Analysis of X-ray diffraction patterns (–2 scans and rocking curves) of the films shows that they are of high quality and that they are well aligned with their (00.1) axis normal to the substrates. Measurements of the temperature-dependent thermoelectric power, resistivity, and Hall coefficient of the films were performed with respect to the binary composition, x. For the samples in the range 0.2<x<0.3, the room-temperature thermopower values were in the range 159–184 V/K, the room-temperature carrier concentrations were 3.93–5.13×10¹⁹ cm^-3, and the room-temperature mobilities were 24.6–64.0 cm²V^-1s^-1. PACS 72.20.Pa; 72.80.Jc; 73.6l.Le     

Influence of different precursor surface layers on Cu(In<Subscript>1-x</Subscript>Ga<Subscript>x</Subscript>)Se<Subscript>2</Subscript> thin film solar cells

The properties of Cu(In_1-xGa_x)Se₂ (CIGS) thin films obtained by selenization of the precursors with different surface layers have been studied, and photovoltaic devices based on the absorbers were measured and analyzed. The devices constructed by the absorbers obtained by selenization of the precursors with CuGa-rich surface layers are improved, compared with those with In-rich surface layers. Through XRD, SEM, SIMS, illuminated J–V, QE and Raman spectra measurements, it was found that the increased Ga contents within the surface region of films and the graded Ga distribution can be realized in the selenized thin films fabricated by the precursors with the CuGa-rich surface layer. Consequently, the performances of the photovoltaic devices based on these thin films are further improved. PACS 61.72.Ss; 87.64.Jt; 68.60.Bs; 81.15.Cd; 84.60.Jt     

Enhanced luminescence in ZnGa<Subscript>2</Subscript>O<Subscript>4-x</Subscript>S<Subscript>x</Subscript>:Mn<Superscript>2+</Superscript> thin-film phosphors grown on MgO(100) substrate

Mn-doped ZnGa₂O_4-xS_x thin-film phosphors have been grown using a pulsed laser deposition technique at varying growth conditions. Structural characterization was carried out on a series of ZnGa₂O_4-xS_x:Mn²⁺ films grown on MgO(100) substrates using Zn-rich ceramic targets. Oxygen pressure was fixed at 100 mTorr and substrate temperatures were varied from 500 to 700 °C. The results of X-ray-diffraction patterns showed that the lattice constants of the ZnGa₂O_3.95S_0.05:Mn²⁺ thin films decrease with the substitution of sulfur for the oxygen in ZnGa₂O₄. Measurements of photoluminescence (PL) properties of ZnGa₂O_4-xS_x:Mn²⁺ thin films have indicated that MgO(100) is one of the most promising substrates for the growth of high-quality ZnGa₂O_4-xS_x:Mn²⁺ thin films. In particular, the incorporation of sulfur into the ZnGa₂O₄ lattice could induce a remarkable increase of PL. The highest green-emission intensity was observed with ZnGa₂O_3.95S_0.05:Mn²⁺ films, whose brightness was increased by a factor of 3.5 in comparison with that of ZnGa₂O₄:Mn²⁺ films. This phosphor may be promising for application to flat-panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

NMR study of the electric field gradient in the paramagnetic phase of M<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> (M = Co,Ni) compounds

The NMR spectra and the decay of a spin echo signal from ⁵¹V nuclei in Kagome-staircase Co₃V₂O₈ (CVO) and Ni₃V₂O₈ (NVO) single crystals are measured in the temperature range 30–300 K and a magnetic field H ₀ = 20 kOe. The orientation dependences of the ⁵¹V NMR line shape are used to determine the electric field gradient (EFG) parameters, namely, quadrupole frequency ν _Q and asymmetry parameter η. These parameters for NVO and CVO are ν _Q = 180(10) kHz, η = 0.5(1) and ν _Q = 130(10) kHz, η = 0.6(1), respectively. A comparison of the results of calculating EFG tensors with a point charge model and the NMR data indicates that the crystallographically equivalent vanadium atoms in the Ni₃V₂O₈ and Co₃V₂O₈ compounds differ in the EFG axis orientation. M₃V₂O₈ crystals are found to have vanadium positions (V1, V2) with different orientations of the z axis, which specifies the direction of the principal value of EFG (V _zz ): these orientations lie in the bc plane and make an angle of either +51(5)° (V1) or −51(5)° (V2) with axis c. In the temperature range 30–300 K, the EFG tensor components and the local symmetry of the charge surrounding of the vanadium positions in NVO and CVO oxides are found to change insignificantly.     

Effect of Ba-substitution on the structure and properties of Pb<Subscript>0.8</Subscript>Ba<Subscript>0.2</Subscript>[(In<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>)<Subscript>1-x</Subscript>Ti<Subscript>x</Subscript>]O<Subscript>3</Subscript> ceramics

Ferroelectric ceramics with formula Pb_0.8Ba_0.2[(In_1/2Nb_1/2)_1-xTi_x]O₃ (PBINT) (x=0.0,0.1,0.2,0.3,0.4 and 0.5) were prepared via a two-step solid state reaction method. It was found that ceramics with compositions in the range of x=0.0∼0.3 showed a pseudo-cubic structure, whereas the ceramic with x=0.5 displayed a tetragonal structure. All compositions showed significant frequency dispersion in their dielectric properties. The remanent polarization P_r as well as the coercive field E_c, measured at room temperature, increases with the Ti content. The experimental results obtained in this system are summarized into a phase diagram, with the morphotropic phase boundary (MPB) located at x=0.4. Compared with the Pb[(In_1/2Nb_1/2)_1-xTi_x]O₃ solid solution system, incorporating Ba in the A-site leads to a significant decrease in the dielectric maximum temperature T_max, a suppression of the dielectric relaxation parameter γ, and a shift of the MPB composition to a higher Ti content. PACS 77.84.Dy; 77.80.Bh; 77.22.Ch     

Investigations of dielectric enhancement in (Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>)<Subscript>1-x</Subscript>(TiO<Subscript>2</Subscript>)<Subscript>x</Subscript> ceramics prepared by laser-sintering technique

The maximum dielectric permittivity of Ti-doped Ta₂O₅ ceramics may reach 450 by a laser-sintering technique. The aim of this study is to investigate the mechanisms of the dielectric enhancement based on the unique structural and morphological properties of the laser-sintered ceramics. The reason for the dielectric enhancement is due to the crystal structure distortion in the high-temperature phase, the oriented grain growth taking place in a direction deviating from [001] in the laser-sintered ceramics. The concurrent nature of quenching effects, a sharp temperature gradient and mass transfer in liquid phase originated from laser high energy irradiation with strict directivity leads to the structural and morphological properties. PACS 81.40.Tv; 61.80.Ba; 77.22.-d; 77.22.Ch