Homogeneous Coexistence in CaFe1−x Co x AsF and Phase Segregation in LaFeAsO1−x F x Studied via NMR

In iron-based pnictides, one of the interesting topics is homogeneous coexistence or phase segregation at the boundary between antiferromagnetic (AF) and superconducting (SC) phases. We addressed this problem on a microscopic level by means of ⁷⁵As NMR measurements in LaFeAsO_1?x F _x (x=0.026) (La1111), and CaFe_1?x Co _x AsF (x=0.06) (Ca1111) having an intermediate electronic phase diagram between Ba(Fe_1?x Co _x )₂As₂ and the La1111 series. NMR spectra for 6 % Co-doped Ca1111 samples were very similar to those for the undoped samples even below T _c , suggesting homogeneous coexistence of the AF and SC states. For 2.6 % F-doped La1111 samples, AF and paramagnetic (PM) domains coexist at ambient pressure, and AF and SC domains coexist at 3.0 GPa. The coexistence of domains is explained by considering a SC dome separated from the AF phase in the phase diagram. The homogeneous coexistence support S _±-wave superconductivity, whereas separation of the AF and SC phases gives credence to S ₊₊-wave superconductivity.     

Structural, Raman Spectroscopy, and Magnetic Ordering in New Delafossite-Type Oxide CuCr1−x Ti x O2 (0≤x≤0.1)

We investigated in this paper a new mixture +3/+2 B cations delafossites by the doping at Cr³⁺ sites in CuCrO₂ by nonmagnetic cation Ti²⁺. Ceramics of CuCr_1?x Ti _x O₂ (0≤x≤0.1) were prepared via solid state synthesis techniques in a controlled atmosphere of O₂. The compound crystallized into the 3R delafossite phase with hexagonal structure. The slightly different size in B cations induced a slightly variation in unit cell parameters. We modulate the spin chirality by the effect of spin dilution for Ti²⁺ on the structural and magnetic properties of delafossite CuCrO₂ having a S=3/2 antiferromagnetic triangular lattice (ATL).     

High-Pressure Studies on Superconductivity in LaFeAsO1−x F x and SmFeAsO1−x F x

The pressure dependence on the superconducting transition temperature (T _c ) was investigated for the iron-based superconductors LaFeAsO_1−x F _x and SmFeAsO_1−x F _x . The T _c ’s increase largely for LaFeAsO_1−x F _x with a small increase of pressure, while a sharp decrease of T _c was observed for SmFeAsO_1−x F _x . The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO_1−x F _x system.     

Photoinduced Superconductivity in La2?x Sr x CuO4

Dependences of the superconducting transition temperature (T _c) and chemical potential shift (????) on the hole concentration (n _h ) for La_2?x Sr _x CuO₄ cuprate is obtained taking into account of canonical two-band BCS model containing Fermi surfaces of p and d holes. The shift of chemical potential (??) leads to the curve T _c(n _h ) with a maximum. The dependences of T _c(n _h ) for our system compared with available experimental results. Downward shift (????) of the electron chemical potential (??) with the hole concentration (n _h ) have been found. Self-consistent equations for superconducting order parameters ( $\bar{\Delta}_{p}$ and $\bar{\Delta}_{d}$ ) for both p and d holes are derived using Green??s function and equation of motion method. The temperature dependences of superconducting gaps and specific heat based on this model are also calculated. The enhancement of T _c due to doping is observed.     

Zr1?x Yb x WMoO8?x/2 (x?=?0, 0.04) ceramics fabricated by in situ synthesis from trigonal polymorph: preparation, sintering process, and negative thermal expansion properties

In this study, negative thermal expansion (NTE) Zr_1−x Yb _x WMoO_8−x/2 (x = 0, 0.04) ceramics were fabricated by in situ synthesis from trigonal polymorphous precursors for the first time. Phase transition was studied by means of powder X-ray diffraction. Study on the sintering process of Zr_1−x Yb _x WMoO_8−x/2 (x = 0, 0.04) ceramics was performed by calcining a series of precursor pellets at 950, 980, and 1000 °C for different times, varying from 1 min to 1 h. The results indicate that the sintering process can be mainly divided into three stages: phase transition from trigonal precursors to cubic Zr_1−x Yb _x WMoO_8−x/2 (c-Zr_1−x Yb _x WMoO_8−x/2, 0–5 min), densification of c-Zr_1−x Yb _x WMoO_8−x/2 (6–30 min), and the final sintering stage with little densification (>30 min). Densification reaches almost the maximum in the duration of ~30 min at assigned temperature. Temperature has little influence on densification of ZrWMoO₈, but improves that of Zr_0.96Yb_0.04WMoO_7.98 evidently. In addition, densification of ZrWMoO₈ can be promoted markedly by introduction of Yb³⁺.     

Quantum Interference Effects in p-Si1−x Ge x Quantum Wells

Quantum interference effects, such as weak localization and electron-electron interaction (EEI), have been investigated in magnetic fields up to 11 T for hole gases in a set of Si_1?x Ge _x quantum wells with 0.13<x<0.95. The temperature dependence of the hole phase relaxation time has been extracted from the magneto-resistance between 35 mK and 10 K. The spin-orbit effects that can be described within the Rashba model were observed in low magnetic fields. A quadratic negative magneto-resistance was observed in strong magnetic fields, due to the EEI effect. The hole-phonon scattering time was determined from hole overheating in a strong magnetic field.     

Nanocrystalline microstructures in Fe93 − x Zr7B x alloys

The crystallization behaviour of amorphous Fe_{93 – x} Zr₇B _x (x = 3, 6, 12 at.%) alloys, the microstructures of the primary crystallization products of stable and metastable phases and the subsequent transformations, have been studied using a combination of differential scanning calorimetry, differential thermal analysis, X-ray diffraction and transmission electron microscopy, including microdiffraction. It has been found that, for x = 3 and 6 at.%, the sole product of primary crystallization is the bcc -Fe phase and the average grain sizes of the crystalline phase were 14 nm and 12 nm for the two alloys, respectively. However, when x = 12 at.%, primary crystallization results in more than one crystalline phase, and a metastable phase with the cubic Fe₁₂Si₂ZrB structure is the major crystallization product after the primary crystallization reaction, accompanied by the -Fe phase. The average grain size of this metastable phase was 35 nm for the alloy heated to 883 K at 20 K/min. Isothermal heat treatments at 873 K and 973 K confirm that after being heated for 240 h, this metastable phase transforms into equilibrium phases: bcc -Fe, hcp ZrB₂ and probably hcp Fe₂Zr. The apparent activation energies for the primary crystallization reaction during continuous heating for these three alloys are 4.4 ± 0.2 eV, 3.5 ± 0.2 eV and 6.9 ± 0.3 eV, respectively.     

Flux Dynamics in NdO1−x F x FeAs Bulk Sample

We present data of multiharmonic magneto-dynamic experiments. In particular, we performed ac magnetic susceptibility experiments on layered pnictide-oxide quaternary compound NdOFeAs doped with fluorine. The experiments allow one to measure the critical temperature and probe the flux dynamic behavior using the third harmonic component of the ac susceptibility of an NdF_0.16FeAsO_0.84 bulk sample as a function of temperature and frequency of the applied ac magnetic fields. Measured signals are connected with the nonlinear superconducting flux dynamic behavior and are characterized by a “flux critical states” sustaining a superconducting critical current. In this framework the irreversibility line that describes the stable superconducting state has been extracted from the onset of the third harmonic signal vs. frequency. Finally we present also the analysis of the flux dynamic dimensionality in the investigated sample.     

Grain growth in (Ca1−x,Mg x )Zr4(PO4)6 ceramics

Grain growth in (Ca_1–x ,Mg _x )Zr₄(PO₄)₆ (CMZP) ceramics in the final stage of sintering has been investigated. The grain growth in CMZP ceramics obeys the isothermal grain-growth kinetics with time exponent,n, lying between 1.8 and 2.4 which depends on magnesium content, indicative of a change in grain-growth rate. The time exponent for the grain growth of CMZP can be taken as 2.0 which implies that a normal grain growth develops in the CMZP ceramics. The apparent activation energy for grain growth demonstrates a maximum atx = 0.0 and a minimum atx = 0.1, with 103.2 and 39.4 kcal mol^–1, respectively, indicating that a small amount of magnesium promotes grain-boundary migration. The critical grain size for initiating microcracks in the CMZP increases with increasing magnesium and reaches 9–12 m whenx = 0.4.     

Crystallization kinetics in glassy Ge20Se80−x In x alloys

Crystallization kinetics is studied in glassy Ge₂₀Se_80–xIn_x (0 x 20) using isothermal annealing at temperatures between the glass transition and melting. D.c. conductivity is taken as a parameter to estimate the extent of crystallization (). The activation energy of crystallization (E) and the order parameter (n) are calculated by fitting the values of in the Avrami equations of isothermal crystallization. The results indicate that E is highly composition-dependent, which is explained in terms of the stable phases in the Ge-Se-In system.     

Magnetization reversal and ferrimagnetism in Pr1−x Nd x MnO3

Detailed magnetic properties of Pr_{1? x} Nd _x MnO₃ (x = 0.3, 0.5 and 0.7) have been reported. All the samples crystallize in orthorhombic perovskite structure with Pnma space group. Magnetization measurements under field cooled (FC) protocal reveal magnetization reversal at low temperatures and low magnetic field. This indicates clear evidence of two magnetic sublattices aligned opposite to each other. There is a well-defined maximum around 48 K in the x = 0.7 sample (i.e. Pr_0.3Nd_0.7MnO₃) in the χ′ value which is identified as paramagnetic to ferrimagnetic transition. The peak value shifts to higher temperature with decrease of x and width of the maximum broadened. It is also observable that with decrease of Nd, both the value of χ′ and χ″ decrease. An attempt is made to explain the magnetization reversal within the framework of available models.     

Magnetic Moments in SmCo5 and SmCo5?x Cu x Films

SmCo₅ is an emerging perpendicular magnetic material for super-high density magnetic recording, due to its large magnetic anisotropy energy. In this paper, the magnetic moments of SmCo_5?x Cu _x have been studied using first principles calculation based on density-functional theory (DFT). Calculations are performed using the pseudopotential plane wave DFT code Vienna ab initio simulation package (VASP) with the projector augmented wave (PAW) method. The local density approximation LDA+U method is used for the calculation of the exchange correlation energy of Sm. The calculation results show that the average Co magnetic moment of SmCo_5?x Cu _x decreases with the increase of Cu doping concentrations, and the influence of the Cu doping on the spin state of Co is greater than that of Sm. The magnetic anisotropy energy of SmCo₅ is analyzed. The electronic density of states and the differential in spin densities of atoms show that the spatial distribution of 4f electron and the 4f?C3d coupling are the controlling factors of the magnetic anisotropy energy of SmCo₅.     

Point-Contact Andreev-Reflection Spectroscopy in the Fe-based Superconductor LaFeAsO1−x F x

Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-based superconductor LaFeAsO_1−x F _x in order to investigate the energy gaps. The Andreev spectra show clear and reproducible features that could be related to two nodeless gaps. Their values, as determined by fitting the conductance curves measured in junctions with local T _c =27.3–28.6 K within the two-band BTK model, are Δ ₁=2.75–3.8 meV for the small gap and Δ ₂=7.9–10.2 meV for the larger one, respectively. The absence of zero-bias conductance peaks and considerations regarding the nonperfectly directional current injection in PCAR experiments and the polycrystalline nature of the samples rule out the possibility of nodal gaps in the superconductor. Moreover, the conductance curves show pseudogap-like features coexisting, at low temperature, with superconductivity and disappearing close to the Néel temperature of the parent compound, T _N≈140 K.     

Synthesis, Crystal Growth, and Acoustophotovoltaic Effect of TlIn1 – x Pr x Se2 and TlIn1 – x Pr x Te2

Procedures were developed for growing perfect TlIn_{1 – x} Pr _x Se₂ and TlIn_{1 – x} Pr _x Te₂ (0 x 0.08) single crystals. The growth charges were synthesized by a process involving partial cooling of a tilted, rotating tube. The crystals grown by the floating zone technique were found to readily cleave in two, mutually perpendicular mirror planes to give rectangular parallelepipeds. The crystals were shown to exhibit a sizeable acoustophotovoltaic effect: a combined action of radiation and sound waves produced an emf between the electrodes or a short-circuit current.     

Magnetic clusters in Cu1 ? x In1 ? x Fe2x Se2 solid solutions

We have synthesized Cu_{1 ? x} In_{1 ? x} Fe_2x Se₂ solid solutions (CuInSe₂-FeSe join). The ??-FeSe end-member of the solid-solution system is an antiferromagnet and a superconductor with a superconducting transition temperature of 8 K. The magnetic properties of the solid solutions were studied at temperatures from 5 to 300 K in magnetic fields H = 3.98 and 3184 kA/m. By fitting experimental ??(H) magnetization data with Langevin and Curie equations, we determined the magnetic moment, size, and concentration of the clusters and the concentration of noninteracting Fe²⁺ ions. The magnetic moment of the Fe²⁺ ions in the clusters is ??0.8??_B at 2x = 0.06 and 0.08, with a tendency to decrease with increasing iron content. This points to antiferromagnetic exchange interaction, which becomes stronger with increasing cluster size. The number of clusters in the materials increases with iron content up to a critical level 2x _c < 2x = 0.06. In the range 2x = 0.06 to 0.10, the cluster size increases until a limiting composition determined by the boundary of the solid-solution range is reached.     

Microscopic Scenario for x = 1/8 Anomaly in La2 − x Sr x CuO4

We adopt a t ₁-t ₂-t ₃-J-G model for explanation of x = 1/8 anomaly in La_{2 ? x} Sr _x CuO₄ family compound. The calculated charge susceptibility shows a maximum near Q = (π, π) at intermediate temperatures and near (π, π/2) as temperature approaches zero, in agreement with neutron scattering experiments. Coulomb repulsion G between the first neighbors turns out to be the source of Charge Density Waves (CDW) in narrow band t ^eff ₁, t ^eff ₂, t ^eff ₃ < G. For physically realistic hopping values we obtain the CDW amplitude e _Q = x. The in-phase domain structure as a candidate for “stripe” picture is proposed.     

Conduction in nanostructured La1?x Sr x FeO3 (0 ≤ x ≤ 1)

We investigated electrical properties of nanostructured La_1?x Sr _x FeO₃ (0 ?? x ?? 1) from 300 K?C400 K. The nanostructured La_1?x Sr _x FeO₃ (0 ?? x ?? 1) was synthesized by citrate gel method requiring no pH control. X-ray diffraction pattern showed that single phase LaFeO₃ with an orthorhombic structure was formed. The structure changed into rhombohedral for x = 0.5 and it became cubic for x = 1.0. For x ?? 0.5, our material showed non-linear current-voltage characteristics and for x > 0.5 it showed linear current-voltage characteristics. Poole Frenkel type conduction mechanism was found to be operative in LaFeO₃ from 300 K?C400 K. The experimental values of field-lowering coefficient were by 2.56?C6.41 times higher than the predicted value and were attributed to the presence of localized fields. The increase in conductance with Sr content was due to formation of Fe⁴⁺ ions in addition to Fe³⁺ with the increase in Sr content. Impedance spectroscopy and ac conductivity analysis of La_1?x Sr _x FeO₃ (0 ?? x ?? 1) was also carried out in the temperature range from 300 K?C400 K and frequency was varied from 20 Hz - 2 MHz. The ac conduction followed the correlated barrier hopping model in La_0.9Sr_0.1FeO₃.     

Structural, transport, magnetic, and dielectric properties of La1−x Te x MnO3 (x = 0.10 and 0.15)

In this study, we have investigated the structure, temperature-dependent resistivity, magnetization, and dielectric properties of La_1?x Te _x MnO_3±δ (x = 0.10 and 0.15). X-ray diffraction analysis confirms the rhombohedral crystal symmetry with space group R $ \overline{3} $ c. For both the samples, the temperature dependence of magnetization plots show paramagnetic-to-ferromagnetic phase transition. The Curie temperature (T _c) and magnitude of magnetization increase with the Te concentration. Field-dependent magnetization produces the asymmetric hysteresis loop that has been attributed to the magneto crystalline anisotropy induced by lattice distortion and the rare earth spin coupling at room temperature. Temperature-dependent resistivity plots exhibit metal–insulator transition (MIT) and charge-ordering state. These plots have been fitted using variable range hopping model, and the density of states [N(E_F)] has been estimated. Magnetoresistance is measured as a function of temperature in the field of 1T, 5T, and 8T. The dielectric constant shows an anomaly near MIT. The dielectric constant exhibits a peaking behavior with the applied frequency and the temperature dependence of dielectric constant attains colossal values at high temperatures.     

1/8 Anomaly in the Excess-Oxygen-Doped La2−x A x CuO4+δ (A = Nd, Bi, Pr)

We have investigated in detail dependence on p (the hole concentration per Cu) of the transport properties and T _c in the excess-oxygen-doped La_2–x A _x CuO₄₊ (A=Nd, Bi, Pr), where the phase separation of the excess oxygen is suppressed. The so-called 1/8 anomaly has been found in La_1.8Nd_0.2CuO₄₊. The 1/8 anomaly has become marked through the 1% substitution of Zn for Cu. The SR measurements have revealed that the magnetic correlation develops at low temperatures below about 40 K in both Zn-free and 1% Zn-substituted samples with p=1/8 and that a long-range magnetic order is formed below 1 K in the 1% Zn-substituted sample with p=1/8. These results support the stripe-pinning model. Neither La_2–x Bi _x CuO₄₊ nor La_2–x Pr _x CuO₄₊ has exhibited the clear 1/8 anomaly.