Atomistic simulation of sodium–gadolinium molybdate of stoichiometric (Na<Subscript>1/2</Subscript>Gd<Subscript>1/2</Subscript>MoO<Subscript>4</Subscript>) and cation-deficient (Na<Subscript>2/7</Subscript>Gd<Subscript>4/7</Subscript>MoO<Subscript>4</Subscript>) compositions

Crystals of sodium–gadolinium molybdates of two compositions: stoichiometric (Na_1/2Gd_1/2MoO₄) and cation-deficient (Na_2/7Gd_4/7MoO₄) composition in which 1/7 of the corresponding cation positions are not occupied are simulated by the method of interatomic potentials. For cation-deficient crystals, two kinds of cation position distribution are considered: the statistical distribution of sodium, gadolinium, and unoccupied cation positions in the I4₁/a structure and their partial ordering in the I space group. As a result of the simulation, structural characteristics of sodium–gadolinium molybdates agreeing well with the known experimental data are obtained. In addition, a number of important elastic and thermodynamic properties of these compounds are predicted. The results obtained in the partial-occupancy approximation and by constructing a 7 × 2 × 2 supercell are compared. The local structure of sodium–gadolinium molybdates are analyzed in detail. The influence of the deviation from the stoichiometry as well as cation ordering on the properties of these crystals is discussed.     

Surface of underdoped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-</Subscript><Subscript>δ</Subscript> as revealed by STM/STS

We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa₂Cu₃O_{7- δ} at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance.     

Synthesis and characterization of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>·LiMn<Subscript>0.33</Subscript>Fe<Subscript>0.67</Subscript>PO<Subscript>4</Subscript>/C cathode materials

A new polyanionic cathode material, Li₃V₂(PO₄)₃·LiMn_0.33Fe_0.67PO₄/C for lithium-ion batteries, was synthesized using a sol-gel method and with N,N-dimethyl formamide as a dispersion agent. The analysis of electron transmission spectroscopy and X-ray diffraction revealed that the composite contained two phases. The material has high crystallinity with a grain size of 20–50 nm. The valence states of Mn, V, and Fe in the composite were analyzed by X-ray photoelectron spectroscopy. The electrochemical kinetics in Li₃V₂(PO₄)₃ is effectively enhanced by the incorporation of LiMnPO₄ and LiFePO₄, via structure modification and reduced Li diffusion length. The Li₃V₂(PO₄)₃·LiMn_0.33Fe_0.67PO₄/C materials displayed high rate capacity and steady cycle performance with discharge capacity remained 148 mAh g^?1 after 50 cycles at the rate of 0.2C. In particular, the composite exhibited excellent reversible capacities, with the values of 157, 134, 120, 102, and 94 mAh g^?1 at charge/discharge 0.2, 0.5, 1, 2, and 5C rates, respectively.     

Charge dynamics in quasi-one dimensional β-Sr<Subscript>1/6</Subscript>V<Subscript>2</Subscript>O<Subscript>5</Subscript>

Polarized infrared reflectivity was measured as a function of temperature on a quasi-one dimensional β-Sr_0.17V₂O₅ single crystal. Along the conduction direction, the optical conductivity exhibits a weak electronic background in the far infrared and a large mid infrared band. Sum rule analysis shows that both far infrared and midinfrared bands arise from 3d vanadium electrons. As temperature is decreased, the mid infrared band slightly narrows and redshifts, but does not exhibit drastic change. In contrast, the spectral weight grows at low frequencies and peaks appear in the phonon energy range. In the transverse direction, β-Sr_0.17V₂O₅ shows an insulator-like behaviour at all temperatures. The optical response is compared with the iso-electronic and iso-structural compound β-Na_0.33V₂O₅. Although spectra are very similar in both compounds at room temperature, their temperature dependence differs. However, some general trends indicate that charge carriers are of the same nature in both compounds. The results suggest that electron- phonon coupling is relevant to explain the optical properties of β-Sr_0.17V₂O₅. The optical response along the conducting direction is assigned to an adiabatic small polaron absorption, in intermediate or strong electron-phonon coupling regime. In this framework, the optical conductivity is in qualitative agreement with recent Dynamical Mean Field Theory results over a wide temperature range.     

Influence of Zn/Co ratio on dielectric behavior of Na<Subscript>2</Subscript>Zn<Subscript>1 ‐ x</Subscript>Co<Subscript>x</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript> glasses

A series of zinc phosphate glass doped with cobalt Na₂Zn_(1???x)Co_xP₂O₇ (x = 0, 1, 2 and 5 mol%) was synthesized. These glasses were characterized by both infrared and large broadband dielectric spectroscopy. Infrared spectra indicate the increase of Zn/Co ratio creates defect in phosphate network due to the depolymeration of phosphate anions. The dc conductivity increases and activation energy decreases with the amount of cobalt ions in the glass network. The impedance measurements reveal that the total conductivity follows Jonscher’s power law. The dielectric constant and dielectric loss increased with the temperature and decreased with the frequency whatever the cobalt proportion.     

Dielectric response of SrTiO<Subscript>3</Subscript>–SrMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> solid solutions in the terahertz–infrared range

The reflection and transmission spectra of ceramic samples of SrTiO₃–SrMg_1/3Nb_2/3O₃ solid solutions have been measured in the frequency range of 5–5000 cm^–1 and in the temperature range of 5–370 K. Based on these spectra, the spectra of the real ε'(ν) and imaginary ε''(ν) parts of the complex permittivity ε*(ν) have been simulated by the method of dispersion analysis. It has been found that the temperature evolution of the dielectric constant is entirely determined by the behavior of the soft mode.     

Crossing of the πd<Subscript>5/2</Subscript> and πg<Subscript>7/2</Subscript> orbitals in the <Subscript>51</Subscript>Sb isotopes

Self-consistent calculations using the D1S Gogny force have been performed in order to study the mechanism involved in the crossing of the πd _5/2 and πg _7/2 orbitals in the Sb isotopes. This inversion is well predicted by the HFB + blocking calculations with spherical symmetry performed for the odd-A Sb isotopes. In addition, several HFB and HF calculations have been performed for even-even nuclei of the five neighbouring isotopic chains (Z = 46 to 54, from the proton dripline to N = 82). The results obtained for the binding energies of the two proton orbitals indicate that the radii of the systems play an important role in the crossing, even though some particular πν interactions also give a contribution. The spin-orbit interaction, which is known to be concentrated mainly at the nuclear surface, is proposed to be the main responsible of the crossing.     

Magnetic and magnetoresistive properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sr<Subscript>2</Subscript>FeMoO<Subscript>6–δ</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoheterostructures

A method has been developed for fabricating nanoporous matrices based on anodic aluminum oxide for the deposition of ferromagnetic nanoparticles in them. The modes of deposition of strontium ferromolybdate thin films prepared by the ion-plasma method have been worked out, and the magnetic and magnetoresistive properties, structure, and composition of the films have been investigated. It has been revealed that the microstructure and properties of the strontium ferromolybdate films deposited by ionplasma sputtering depend on the deposition rate and the temperature of the substrate. Based on the measurement of the electrical resistivity of nanoheterostructures in a magnetic field, it has been found that the magnetoresistance reaches 14% at T = 15 K and B = 8 T, which is due to the manifestation of tunneling magnetoresistance.     

Internal friction and magnetoelectric response in two-layer composites Tb<Subscript>0.12</Subscript>Dy<Subscript>0.2</Subscript>Fe<Subscript>0.68</Subscript>–PbZr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>O<Subscript>3</Subscript>

The inverse magnetoelectric effect and internal friction in two-layer composites based on ferromagnetic Tb_0.12Dy_0.2Fe_0.68 and piezoelectric PbZr_0.53Ti_0.47O₃ are studied in an ac electrical field in the frequency range of 52–213 kHz at temperatures of 293 to 400 K. A correlation is found between the internal friction and the efficiency of the inverse magnetoelectric transformation at resonant frequencies.     

μSR study of Eu<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> and EuMn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

A comparative μSR study of ceramic samples of the EuMn₂O₅ and Eu_0.8Ce_0.2Mn₂O₅ multiferroics is performed in the temperature range from 15 to 300 K. It is found that the Ce doping of the EuMn₂O₅ sample slightly reduces the temperature of the magnetic phase transition from T _N = 45 K for the EuMn₂O₅ sample to T _N = 42.5 K for the Eu_0.8Ce_0.2Mn₂O₅ sample. Below the temperature T _N for both samples, there are two types of localization of a thermalized muon with different temperature dependences of the precession frequency of the magnetic moment of the muon in an internal magnetic field. The higher frequency in both samples refers to the initial antiferromagnetic matrix. The behavior of this frequency in Eu_0.8Ce_0.2Mn₂O₅ follows the Curie–Weiss law with the exponent β = 0.29 ± 0.02, which differs from the value β = 0.39 standard for 3D Heisenberg magnetics and is observed in EuMn₂O₅, because of the strong frustration of the doped sample. The temperature-independent low frequency is due to the presence of Mn³⁺–Mn⁴⁺ ferromagnetic pairs located along the b axis of the antiferromagnetic matrix and in the regions of phase separation, which contain such ion pairs and e _g electrons recharging them. In both samples, polarization losses are the same (about 20%) and are associated with the formation of Mn⁴⁺–Mn⁴⁺ + Mu complexes near Mn³⁺–Mn⁴⁺ ferromagnetic pairs. In the temperature interval from 25 to 45 K, the separation of the Eu_0.8Ce_0.2Mn₂O₅ structure into two fractions where the relaxation rates of polarization of muons differ by an order of magnitude is revealed. This effect is due to a change in the state of regions of phase separation (1D superlattices) at the indicated temperatures. Such effect in EuMn₂O₅ is significantly weaker.     

High-Temperature Heat Capacity of Zn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>–Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript> Solid Solutions

Differential scanning calorimetry has been used to study the influence of temperature on the heat capacity of synthesized vanadates Zn₂V₂O₇, (Cu_0.56Zn_1.44)V₂O₇, and (Cu_1.0Zn_1.0)V₂O₇. It is found that dependences C_p = f(T) have extremes. The thermodynamic properties of Zn₂V₂O₇ have been determined.     

Microwave characterization of Pb<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> multiferroics at X-band

The microwave characteristics of Pb_1?x Ca _x Fe_0.5Nb_0.5O₃ multiferroics (x = 0.0, 0.4, 0.45, 0.5, 0.55, 0.6), have been investigated as a function of frequency and substitution. The results depict ?13.99 dB reflection loss at 11.65 GHz in composition x = 0.6. Microwave absorption is enhanced with substitution of Ca²⁺ ions and undoped composition 0.0 behaves as electromagnetic shield. The model governing microwave absorption is discussed and different compositions for electromagnetic applications have been suggested.     

Cluster spin glass and superparamagnetism in RuSr<Subscript>2</Subscript>Eu<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10-</Subscript><Subscript>δ</Subscript>

We investigate in detail the dc magnetization and nonlinear ac susceptibility behavior of the superconducting ferromagnet RuSr₂Eu_1.5Ce_0.5Cu₂O_{10- δ} (Ru1222) to develop a comprehensive understanding of the spin glass and superparamagnetism in this material. The structural properties of the system result in the formation of magnetic (ferromagnetic) clusters of different sizes, shapes and properties. The magnetic clustering of the system leads to observation of various features in dc magnetization and ac susceptibility consistent with superparamagnetism and cluster spin glass states, which can coexist or stand alone, depending on the temperature range considered. Experimental results of magnetic measurements in combination with their analysis have enabled us to explain and distinguish these phenomena, as well as to propose a temperature dependent scenario of the system behavior.     

Intermediate phases in [111]- and [001]-oriented PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>–29PbTiO<Subscript>3</Subscript> single crystals

Phase transformations in [111]- and [001]-oriented PbMg_1/3Nb_2/3O₃–29PbTiO₃ single crystals have been studied using dielectric and optical measurements before and after applying an electric field. It is shown that the subsequence of phase transitions rhombohedral (R)—tetragonal (T)—cubic (C) phases is observed in nonpolarized samples of both orientations as temperature increases. In the [111]-oriented crystal, an additional intermediate monoclinic phase (it is possible, M _a ) is induced after preliminary polarization at room temperature and the R–M _a –T–C phase transitions are observed on heating. In the [001]-oriented crystal, after its polarization, the monoclinic phase forms instead of the rhombohedral phase even at room temperature and the M _a –T–C transitions occur on heating. The results are discussed from the point of view of the existence polar nanoregions with different local symmetries in a glasslike matrix.     

Electrical switching in the MoO<Subscript>3</Subscript>–WO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> and MoO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

High field electrical switching on blown films of MoO₃(60%)–P₂O₅(40%), MoO₃(50%)–WO₃(10%)–P₂O₅(40%), and MoO₃(45%)–WO₃(15%)–P₂O₅(40%) having different thicknesses was studied and compared. Switching was observed using two terminal samples. S-type current–voltage characteristic (current-controlled negative resistance—CCNR) with memory was observed in molybdenum–phosphate glasses, but N-type characteristic (voltage-controlled negative resistance—VCNR) with threshold in tungsten–molybdenum–phosphate glasses was observed. The important observation was that with the addition of WO₃ to binary MoO₃–P₂O5 led to a change of I–V characteristic from CCNR with memory to VCNR with threshold. The measurements of density and molar volume showed linear relation between MoO₃ content and density which decreased with the increase of MoO₃ content. The samples’ thickness had no significant effect on threshold voltage. The attained results also indicated that the electrode material had no effect on switching property of devices. The switching behavior of the devices did not show any dependence on the polarity of the applied voltage. In terms of the effect of heat on the switching behavior of molybdenum–phosphate glasses, it was found that threshold voltage decreases with increasing of temperature. Finally, the switching phenomenon was explained by thermal (formation of crystalline filaments) and electronic models.     

Nernst effect of stripe ordering La<Subscript>1.8−x</Subscript>Eu<Subscript>0.2</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

We investigate the transport properties of La_1.8−xEu_0.2Sr_xCuO₄ (x = 0.04, 0.08, 0.125, 0.15, 0.2) with a special focus on the Nernst effect in the normal state. Various anomalous features are present in the data. For x = 0.125 and 0.15 a kink-like anomaly is present in the vicinity of the onset of charge stripe order in the LTT phase, suggestive of enhanced positive quasiparticle Nernst response in the stripe ordered phase. At higher temperature, all doping levels except x = 0.2 exhibit a further kink anomaly in the LTO phase which cannot unambiguously be related to stripe order. Moreover, a direct comparison between the Nernst coefficients of stripe ordering La_1.8−xEu_0.2Sr_xCuO₄ and superconducting La_2−xSr_xCuO₄ at the doping levels x = 0.125 and x = 0.15 reveals only weak differences. Our findings make high demands on any scenario interpreting the Nernst response in hole-doped cuprates.     

Large electrostrictive strain in lead-free Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–BaTiO<Subscript>3</Subscript>–KNbO<Subscript>3</Subscript> ceramics

In the present work, (1−x)(0.935Bi_0.5Na_0.5TiO₃–0.065BaTiO₃)–xKNbO₃ (BNT–BT–KN, BNT–BT–100xKN) ceramics with x ranging from 0 to 0.1 were prepared by the conventional ceramic fabrication process. A large electrostrictive coefficient of ∼10⁻² m⁴ C⁻² is obtained with the composition x ranging from 0.02 to 0.1, which is close to the well-known electrostrictive material Pb(Mg_1/3Nb_2/3)O₃. Under an electric field of 4 kV/mm, the electrostrictive strain can reach as high as 0.08%. Besides, the electric field induced strain behavior indicates a temperature independent behavior within the temperature range of 20 to 150°C. The large electrostrictive strain is suggested to be ascribed to the formation of non-polar (NP) phase developed by the KNbO₃ substitution, and the high electrostrictive coefficient of BNT–BT–KN ceramics makes them great candidates to be applied in the new solid-state actuators.     

Galvanomagnetic study of the quantum-well valence band of germanium in the Ge<Subscript>1−x</Subscript>Si<Subscript>x</Subscript>/Ge/Ge<Subscript>1−x</Subscript>Si<Subscript>x</Subscript> potential well

The structure of the quantum-well valence band in a Ge(111) two-dimensional layer is calculated by the self-consistent method. It is shown that the effective mass characterizing the motion of holes along the germanium layer is almost one order of magnitude smaller than the mass for the motion of heavy holes along the [111] direction in a bulk material (this mass is responsible for the formation of quantum-well levels). This creates a unique situation in which a large number of subbands appear to be populated at moderate values of the layer thickness d _w and the hole concentration p _s . The depopulation of two or more upper subbands in a 38-nm-thick germanium layer at a hole concentration p _s = 5 × 10¹⁵ m^?2 is revealed from the results of measuring the magnetoresistance in a strong magnetic field aligned parallel to the germanium layers. The destruction of the quantum Hall state at a filling factor ν = 1 indicates that the two lower subbands merge together in a self-formed potential profile of the double quantum well. It is demonstrated that, in a quasi-two-dimensional hole gas, the latter effect should be sensitive to the layer strain.     

Investigation of quasicrystalline Al<Subscript>62.5</Subscript>Cu<Subscript>25</Subscript>Fe<Subscript>12.5</Subscript> and crystalline β-Al<Subscript>50</Subscript>Cu<Subscript>33</Subscript>Fe<Subscript>17</Subscript> alloys by X-ray photoelectron spectroscopy

The electronic spectra of the valence band and core levels of the surface of polygrain alloys with the icosahedral structure and the β-(CsCl)-type solid solution of Al₅₀Cu₃₃Fe₁₇ were investigated by X-ray photoelectron spectroscopy (XPS). The obtained XPS spectra of the Al_62.5Cu₂₅Fe_12.5 alloy, in comparison with those of the crystalline Al₅₀Cu₃₃Fe₁₇ alloy demonstrate narrowing and a decrease in asymmetry of the Fe2p core level and a decrease in the electron state density N(E _F ) near the Fermi level, features expected for the poorly conducting icosahedral phase. The XPS data are compared with the estimates of N(E _F ) based on the low-temperature specific heat measurements.