Electronic structure of Ti-doped Sr<Subscript>4</Subscript>Sc<Subscript>2</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>O<Subscript>6</Subscript> as a possible parent phase for the new FeAs superconductors

First principle FLAPW-GGA calculations have been performed with the purpose to understand the effect of Ti-doping on the electronic properties for the newly discovered tetragonal iron arsenide-oxide Sr₄Sc₂Fe₂As₂O₆ (abbreviated as FeAs42226) as the possible parent phase for the new FeAs superconductors. Our results show that the insertion of Ti into Sc sublattice of this five-component iron arsenide-oxide phase leads to the resolute change of electronic structure of FeAs42226. Namely, the insulating oxygen-containing [Sr₄Sc₂O₆] blocks in Ti-doped FeAs42226 became conducting, and this differs essentially from the known picture for all others FeAs superconductors where the conducting [Fe₂As₂] blocks are alternated with insulating blocks. Moreover in sharp contrast with FeAs-based superconductors with Fe 3d bands near the Fermi level, for Ti-doped FeAs42226 in this region the Ti 3d states are dominated, whereas the Fe 3d states are suppressed.     

Band structure of new layered arsenides SrRu<Subscript>2</Subscript>As<Subscript>2</Subscript> and BaRu<Subscript>2</Subscript>As<Subscript>2</Subscript>

This paper reports on the results of the ab initio FLAPW-GGA band structure calculations for two new layered phases SrRu₂As₂ and BaRu₂As₂, which are isostructural and isoelectronic to the known tetragonal (Ca,Sr,Ba)Fe₂As₂ basis phases of the FeAs superconductor family. The energy bands, densities of states, topology of the Fermi surface, low-temperature electron specific heats, and molar Pauli paramagnetic susceptibilities of SrRu₂As₂ and BaRu₂As₂ are determined for the first time and discussed in comparison with those for BaFe₂As₂ and BaRh₂As₂.     

Superconductivity in Ti-doped iron-arsenide compound Sr<Subscript>4</Subscript>Cr<Subscript>0.8</Subscript>Ti<Subscript>1.2</Subscript>O<Subscript>6</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

Superconductivity was achieved in Ti-doped iron-arsenide compound Sr₄Cr_0.8Ti_1.2O₆Fe₂As₂ (abbreviated as Cr-FeAs-42622). The X-ray diffraction measurement shows that this material has a layered structure with the space group of P4/nmm, and with the lattice constants a = b = 3.9003 Å and c = 15.8376 Å. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2 K, which may be induced by the nonuniform distribution of the Cr/Ti content in the FeAs-42622 phase.     

Optical studies of phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>Ti(O<Subscript>2</Subscript>)F<Subscript>5</Subscript> crystal

Polarization-optical study of twinning and measurements of the Raman spectra and birefringence in oxyfluoride (NH₄)₃Ti(O₂)F₅ were carried out over the temperature range 90–350 K. Phase transitions were detected at temperatures T ₀₁ = 266 K (second-order transition) and T ₀₂ = 225 K (first order). It is assumed that the crystal symmetry is changed as follows: Fm3m ? I4/mmm ? I4/m. Anomalies of the spectral parameters are established in the frequency range of internal vibrations of ammonium ions and Ti(O₂)F₅ complexes. An analysis of the results shows that the transition at T ₀₁ is likely due to small shifts of the tetrahedral groups from their position on the triad axis and that the transition at T ₀₂ is due to fluorine-oxygen ordering of Ti(O₂)F₅ complexes.     

Temperature evolution of the dielectric response function of Pb(Fe<Subscript>0.95</Subscript>Sc<Subscript>0.05</Subscript>)<Subscript>2/3</Subscript>W<Subscript>1/3</Subscript>O<Subscript>3</Subscript> relaxor ceramics in a wide frequency range

Electrodynamic properties of Pb(Fe_0.95Sc_0.05)_2/3W_1/3O₃ solid solution belonging to A(B'B'')O₃ perovskite structural family have been investigated by broadband dielectric spectroscopy in a wave-number range of (4 × 10^–9–4 × 10³) cm^–1 and a temperature range of 100–600 K. The influence of low-frequency relaxations on the vibrational spectrum is determined within the four-parameter factorized dispersion model. Anomalies in the behavior of the dielectric response function are found near the temperature-diffuse maximum of permittivity.     

Pressure-induced superconductivity and structural transitions in Ba(Fe<Subscript>0.9</Subscript>Ru<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

Electrical transport and structural characterizations of isoelectronically substituted Ba(Fe_0.9Ru_0.1)₂As₂ have been performed as a function of pressure up to ～ 30 GPa and temperature down to ～ 10 K using designer diamond anvil cell. Similar to undoped members of the AFe₂As₂ (A = Ca, Sr, Ba) family, Ba(Fe_0.9Ru_0.1)₂As₂ shows anomalous a-lattice parameter expansion with increasing pressure and a concurrent ThCr₂Si₂ type isostructural (I4/mmm) phase transition from tetragonal (T) phase to a collapsed tetragonal (cT) phase occurring between 12 and 17 GPa where the a is maximum. Above 17 GPa, the material remains in the cT phase up to 30 GPa at 200 K. The resistance measurements show evidence of pressure-induced zero resistance that may be indicative of high-temperature superconductivity for pressures above 3.9 GPa. The onset of the resistive transition temperature decreases gradually with increasing pressure before completely disappearing for pressures above ～ 10.6 GPa near the T-cT transition. We have determined the crystal structure of the high-T _c phase of Ru-doped BaFe₂As₂ to remain as tetragonal (I4/mmm) by analyzing the X-ray diffraction pattern obtained at 10 K and 9.7 ± 0.7 GPa, as opposed to inferring the structural transition from electrical resistance measurement, as in a previous report [S.K. Kim, M.S. Torikachvili, E. Colombier, A. Thaler, S.L. Bud’ko, P.C. Canfield, Phys. Rev. B 84, 134525 (2011)].     

Magnetic structure and phase diagram of Sr<Subscript>5</Subscript>Rh<Subscript>4</Subscript>O<Subscript>12</Subscript> frustrated Ising magnet

The magnetic structure of Sr₅Rh₄O₁₂ is based on Ising chains of rhodium ions with a variable valence, Rh³⁺-Rh⁴⁺. The ordering in the chains is assumed to be ferromagnetic. It has been shown that the magnetic structure and phase diagram of Sr₅Rh₄O₁₂ are well described in a model taking into account weak antiferromagnetic interactions between the nearest and next-nearest neighbors on the triangular lattice of ferromagnetic Ising chains. The ground state at low temperatures is the two-sublattice stripe phase; this phase in the magnetic field is transformed to the ferrimagnetic phase and, then, to the ferromagnetic phase. Small plateaus can be observed in the region of the transition from the ferrimagnetic phase to the ferromagnetic one.     

Molecular structure of tris(acetylacetonato)scandium Sc(C<Subscript>5</Subscript>H<Subscript>7</Subscript>O<Subscript>2</Subscript>)<Subscript>3</Subscript>

Molecular structure of tris(acetylacetonato)scandium, Sc(C₅H₇O₂)₃, is investigated by gas-phase electron diffractometry. The main structural parameters of the molecule are evaluated. The average intemuclear distances and angles correspond to C₃ symmetry. The chief structural motif is trigonal antiprisms of six oxygen, carbon, and hydrogen atoms with a scandium atom at the center. It is found that r_g[Sc-O) = 204.1(8) pm and r_g(C−O) = 124.7(4) pm Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 4, pp. 633–639, July–August, 1998.     

Dielectric dispersion and magnetic properties of Ba-modified Pb(Fe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>)O<Subscript>3</Subscript>

The polycrystalline samples of Ba-modified Pb(Fe_1/2Nb_1/2)O₃ (i.e., (Pb_1-xBa_x)(Fe_1/2Nb_1/2)O₃ PBFN, with x=0,0.05,0.07) were synthesized by a mechanosynthesis (i.e., high-energy ball milling) route followed by a mixed oxide method. Structural analysis provides the information on formation of single-phase orthorhombic structure on substitution of a small amount (x=0.07) of Ba at the Pb-site of Pb(Fe_0.50Nb_0.50)O₃ (PFN). The ferroelectric–paraelectric phase transition in PFN was observed at 383 K, which decreases on increasing Ba-concentration in PBFN. Detailed studies of dielectric properties of PBFW show the following: (i) diffuse phase transition, (ii) low loss tangent, (iii) low activation energy, and (iv) low frequency dielectric dispersion. An anomaly in the ac conductivity was found very close to phase transition temperature. The activation energy is found to decrease from 0.19 to 0.01 eV on increasing Ba-concentration to 7% (x=0.07). Temperature field-dependent magnetization measurements of all the samples showed antiferromagnetic transition at ∼15 K (for x=0.07). PBFN sample showed a slight increase in the coercivity (i.e., from 400 Oe (PFN) to 500 Oe (PBFN, for x=0.07) at 2 K. PACS 61.10.Nz; 68.37.Hk; 75.50.Ss; 75.60.Ej; 77.22.Ch; 77.22.Gm     

Band structure of a new layered La<Subscript>3</Subscript>Ni<Subscript>4</Subscript>P<Subscript>4</Subscript>O<Subscript>2</Subscript> superconductor

The ab initio FLAPW-GGA calculations of the band structure of a new layered low-temperature (T _C ～ 2.2 K) La₃Ni₄P₄O₂ superconductor are presented. The energy bands, distributions of the densities of electron states, charge states of the atomic layers, low-temperature electron specific heat, and molar Pauli paramagnetic susceptibility for La₃Ni₄P₄O₂ have been determined. They are discussed compared to the existing experimental data.     

Observation of multiple superconducting gaps in the infrared reflectivity spectra of Ba(Fe<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe_0.9Co_0.1)₂As₂ are reported. The reflectivity is found to be close to unity at frequencies ω lower than 2Δ/h (2Δ is the superconducting gap and h is Planck’s constant). This is evidence for the s ^+/− or s ^+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe_0.9Co_0.1)₂As₂ manifest opening of several superconducting gaps at temperatures lower than critical T _c .     

Phase stability and elasticity of Sc<Subscript>2</Subscript>O<Subscript>3</Subscript> at high pressure

An investigation into the high-pressure behavior of Sc₂O₃ was conducted using first-principle calculations based on density functional theory within the generalized gradient approximation. Nine candidate phases were considered and simulated, C-, B-, A-RES, Rh₂O₃(II), Gd₂S₃, U₂S₃, Al₂Er₃, corundum and PPv respectively. Our results demonstrate phase transitions of Sc₂O₃ from C- to B-RES phase at 15 GPa, then to Gd₂S₃ phase at 18 GPa. Elastic constants of Sc₂O₃ present softening from about 270 GPa to 337 GPa, associated with a semiconductor-to-metal crossover. The Gd₂S₃-type Sc₂O₃ is both mechanically and dynamical stable at least up to 302 GPa supported by the mechanically stable criteria and the phonon spectrum.     

Band structure of new layered superconductors BaRh<Subscript>2</Subscript>P<Subscript>2</Subscript> and BaIr<Subscript>2</Subscript>P<Subscript>2</Subscript>

The results of ab initio FLAPW-GGA computations of the band structure of two new layered low-temperature superconductors BaRh₂P₂ and BaIr₂P₂ (with a ThCr₂Si₂ tetragonal structure) are presented. As distinct from the family of the isostructural FeAs superconductors, they feature the complete replacement of the magnetic (Fe) metal by the nonmagnetic 4d (Rh) and 5d (Ir) metals. For BaRh₂P₂ and BaIr₂P₂, the energy bands, the distributions of the densities of electronic states, the Fermi surface topology, and the coefficients of the low-temperature electron specific heat and the molar Pauli paramagnetic susceptibility have been determined. An increase in T _C in the BaRh₂P₂ (1 K) → BaIr₂P₂ (2.1 K) transition can assumingly be attributed to the features of their phonon subsystem.     

Sub-unit cell layer-by-layer growth of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>, MgO,and Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript> thin films

The use of oxide materials in oxide electronics requires their controlled epitaxial growth. Recently, it was shown that Reflection High Energy Electron Diffraction (RHEED) allows the growth of oxide thin films to be monitored, even at high oxygen pressures. Here, we report the sub-unit cell molecular or block layer growth of the oxide materials Sr₂RuO₄, MgO, and magnetite using Pulsed Laser Deposition (PLD) from stoichiometric targets. Whereas a single RHEED intensity oscillation is found to correspond to the growth of a single unit cell for perovskites such as SrTiO₃ or doped LaMnO₃, in materials where the unit cell is composed of several molecular layers or blocks with identical stoichiometry, sub-unit cell molecular or block layer growth is established, resulting in several RHEED intensity oscillations during the growth of a single unit cell. PACS 61.14.Hg; 74.76.Db; 75.70.-i; 81.15.Fg     

Giant positive magnetoresistance in heterostructure (La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>) coated with YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> composites

(La_0.7Sr_0.3MnO₃) _x /(YBa₂Cu₃O₇) _y composites were prepared by mixing La_0.7Sr_0.3MnO₃ powders and the sol–gel-derived YBa₂Cu₃O₇ matrix, followed by high-temperature calcinations. Their structural, magnetic properties and magnetoresistance effect have been investigated systematically. A giant positive magnetoresistance (PMR) at low magnetic field is observed at low temperatures. In the case of (La_0.7Sr_0.3MnO₃)₁/(YBa₂Cu₃O₇)₉ composite, the PMR achieves 260% under a magnetic field of 5800 Oe. However, the PMR value sharply decreases with increasing temperature and no magnetoresistance effects are found above metal-insulator transition temperature. The enhancement of spin-dependent scattering at the grain boundaries should be responsible for the observed PMR. In addition, the temperature dependence of resistance under magnetic field could be explained by the competition between diamagnetism and paramagnetism in YBCO phase. At low temperature, the diamagnetism is predominant over paramagnetism and the interface scattering between LSMO grains is enhanced correspondingly. As a result, the low-temperature resistance increases and large PMR appears.     

Acoustic emission and thermal expansion of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> and Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> crystals

The temperature dependence of the elongation per unit length for Pb(Mg_1/3Nb_2/3)O₃ crystals unannealed after growth and mechanical treatment is investigated in the course of thermocycling. It is revealed that this dependence deviates from linear behavior at temperatures below 350°C. The observed deviation is characteristic of relaxors, is very small in the first cycle, increases with increasing number n of thermocycles, and reaches saturation at n≥3. In the first cycle, a narrow maximum of the acoustic emission activity is observed in the vicinity of 350°C. In the course of thermocycling, the intensity of this maximum decreases and becomes zero at n>3. For (1?x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ crystals, the dependence of the temperature of this acoustic emission maximum on x exhibits a minimum. It is assumed that the phenomena observed are associated with the phase strain hardening due to local phase transitions occurring in compositionally ordered and polar nanoregions.     

Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).     

The possible mechanisms of phase transitions in RMn<Subscript>2</Subscript>O<Subscript>5</Subscript> oxide family

The magnetic, magnetoelectric, and magnetoelastic properties of ErMn₂O₅ single crystals have been studied at low temperatures and strong magnetic fields (up to 250 kOe) and compared to the analogous results obtained previously for YMn₂O₅. Based on these data, the possible mechanisms of various spontaneous and magnetic-field-induced phase transitions in these compounds are considered within the framework of the theory of representations of the Pbam-D _2h ⁹ space group. It is shown that a biquadratic exchange plays an important role in the formation (and mutual transformation) of magnetic structures revealed by neutron diffraction in the RMn₂O₅ oxide family.     

Structural changes during phase transitions and the critical and noncritical order parameters in the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>Nb(O<Subscript>2</Subscript>)<Subscript>2</Subscript>F<Subscript>4</Subscript> crystal

The structures of two phases of the (NH₄)₃Nb(O₂)₂F₄ crystal, namely, the parent cubic phase and the most distorted low-temperature phase, have been determined from data of an X-ray diffraction experiment performed for a powder sample. The profile and structural parameters have been refined according to the procedure implemented in the DDM program. The results obtained have been discussed with invoking the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic displacements and allows the interpretation of the obtained experimental data. It has been found that the most probable sequence of structural transformations occurring in the crystal can be schematically represented in the following form:

Electronic structure of prototype AFe<Subscript>2</Subscript>As<Subscript>2</Subscript> and ReOFeAs high-temperature superconductors: A comparison

We have performed ab initio LDA calculations of the electronic structure of newly discovered prototype high-temperature superconductors AFe₂As₂ (A = Ba, Sr) and compared it with the previously calculated electronic spectra of ReOFeAs (Re = La, Ce, Pr, Nd, Sm). In all cases, we obtain almost identical densities of states in a rather wide energy interval (up to 1 eV) around the Fermi level. Energy dispersions are also very similar and almost two dimensional in this energy interval, leading to the same basic (minimal) model of the electronic spectra, determined mainly by Fe d orbitals of the FeAs layers. The other constituents, such as A ions or rare-earth Re (or oxygen states) are more or less irrelevant for superconductivity. LDA Fermi surfaces for AFe₂As₂ are also very similar to that of ReOFeAs. This makes the more simple AFe₂As₂ a generic system to study the high-temperature superconductivity in FeAs-layered compounds. The text was submitted by the authors in English.