Ferromagnetic Resonance in Ga1?xMnxAs

We report on ferromagnetic resonance measurements of Ga_1–x Mn _x As thin films with Mn contents 0.022 x 0.051. For x 0.036 and the external magnetic field normal to the thin film, we observe several resonances, which we identify as spin wave resonances. The non-quadratic mode spacing can be consistently explained by a linear gradient in the magnetic properties of the films. From the measurements, the exchange constant A can be deduced for different Mn contents x.     

Heat conduction in Ba1?xKxBiO3

We have investigated heat conduction of single crystal Ba_1–xK_xBiO₃ in the temperature range of 2–300 K and in a magnetic field of up to 6 Tesla. Temperature dependence of thermal conductivity(T) reveals the participation of both electrons and phonons with their relative contributions that depend critically on the potassium doping concentration. Crystals underdoped with potassium (samples with higherT _c) exhibit a strong suppression of and a glass-like temperature dependence. In contrast, those with a higher potassium content (lowerT _c) show an increase as temperature decreases with a peak near 23 K. Field dependence of(H) is also very sensitive to the level of potassium doping. Crystals exhibiting a large phonon contribution show an initial drop in(H) at low fields followed by a minimum and then a slow rise to saturation as the field increases. The initial drop is due to the additional phonon scattering by magnetic vortices as the sample enters a mixed state. The high field behavior of(H), arising from a continuous break-up of Cooper pairs, exhibits scaling which suggests the presence of an unconventional superconducting gap structure in this material.     

GaxIn1 ? xBiyAszSb1 ? y ? z/InSb and InBiyAszSb1 ? y ? z/InSb heterostructures grown in a temperature gradient

Heterophase equilibria in the Ga _x In_{1 − x} Bi _y As _z Sb_{1 − y − z} -InSb and InBi _y As _z Sb_{1 − y − z} -InSb systems have been analyzed within the simple solution approximation. Ga _x In_{1 − x} Bi _y As _z Sb_{1 − y − z} /InSb and InBi _y As _z Sb_{1 − y − z} /InSb heterostructures have been grown in a temperature gradient, and their composition and structural perfection have been assessed.     

Magnetoresistance of Domain Walls in?Superconductor/Ferromagnet Hybrid Systems

The anisotropic magnetoresistance of single domain walls in S/F hybrid devices was studied with a view of examining the superconducting proximity effect at a single domain wall. By changing the geometry of a Py/Nb nanowire, the character of the domain wall that is nucleated can be controlled. The pinning of both vortex and transverse domain walls at narrow notches in the wires was investigated. In addition, the effect of the interfacial transparency between a superconductor and ferromagnet is also briefly discussed, showing a reduction of the superconducting proximity effect with poorer transparency.     

Normal-state Conduction Mechanisms in GdBa2Cu3?xRuxO7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3−x Ru _x O_7−δ , Gd-123, with x=0.0–0.3 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD), scanning electron microscope (SEM), electron dispersive X-ray (EDX) and electrical-resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of both phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3−x Ru _x O_7−δ phase up to x=0.05 was observed from XRD and electrical-resistivity measurement, respectively. This enhancement was confirmed with the calculated relative volume fraction. For x>0.05, suppression of both phase formation and T _c was obtained and the superconductivity was completely destroyed around x=0.3. The normal-state electrical resistivity was analyzed by the two- and three-dimensional variable range hopping (2D-VRH and 3D-VRH) and Coulomb gap CG. The results showed that the dominant mechanism was CG for x≤0.075, while was 3D-VRH for x≥0.15.     

Polaron Absorption and the Insulator to Metal Transition in Nd2?xCexCuO4?y Cuprates

The optical conductivity of Nd_2–x Ce_xCuO_4–y single crystals with x ranging from 0 to 0.122 has been measured at differenttemperatures. The polaron band observed at 1100 cm–1 in Nd₂CuO_4–y shifts to lower frequencies when the Ce concentration xapproaches 0.125, namely, the critical value for the insulator to metal transition. Thisunexpected behavior indicates a progressive delocalization of polarons in theCu–O plane for increasing x and may explain the anomalous Drudebehavior reported for the carriers in the metallic phase of superconducting cuprates.     

Confirmation of Superconductivity in Sr2YRu1?uCuuO6

Evidence is reported that Sr₂YRu_1–u Cu _u O₆ superconducts with a transition temperature of at least T _c 30 K for u = 0.1, confirming the reports by Wu et al. that this material superconducts despite having no cuprate planes. (Such planes are widely believed to be necessary for high-temperature superconductivity.) Coexisting with the superconductivity, Sr₂YRu_1–u Cu _u O₆ exhibits magnetic order due to antiferromagnetic Ru and Cu.     

Influence of Co Doping on the Zn Distribution in?Nd1.05Ba1.95Cu3?xZnxOz

The structural effects of Zn doping and Zn, Co co-doping in Nd_1.05Ba_1.95Cu₃O _z have been investigated by using neutron diffraction. The Zn atoms are found to occupy exclusively the planar sites in Nd_1.05Ba_1.95Cu_3−x Zn _x O _z samples with x≤0.4. An orthorhombic-to-tetragonal transition behavior was observed in the Nd_1.05Ba_1.95Cu_3−x Zn _x O _z (x=0–0.6) system, while a tetragonal-to-orthorhombic transition was observed in the Nd_1.05Ba_1.95Cu_2.8−x Zn _x Co_0.2O _z (x=0–0.6) system as the Zn doping content x increases. A comparative analysis of the structural behavior of Nd_1.05Ba_1.95Cu_3−x Zn _x O _z and Nd_1.05Ba_1.95Cu_2.8−x Zn _x Co_0.2O _z (x=0, 0.2, 0.4, 0.6) samples showed that the Co doping can induce an increase in Zn occupancy at the Cu(1) sites as the Zn doping concentration increases above x=0.4, implying that the distribution of Zn ions between the chain and planar sites can be tuned by the Co substitution for the chain Cu sites.     

Relation of Structure and Superconductivity in Self-compensated Y1?xCaxBa2Cu3?xAlxOz System

The co-doped compounds of Y_1−x Ca _x Ba₂Cu_3−x Al _x O _z , with x from 0.1 to 0.4, were synthesized through a solid-state reaction method. Structural and superconducting properties have been investigated by X-ray diffraction, Rietveld refinement, and DC magnetization measurement. The lattice constant a decreases while b increases with the addition of x. The difference between a and b diminishes gradually. Careful study of the crystalline structure shows that the critical temperature (T _c ) changes monotonically with some local structural parameters, such as the difference between Ba and Cu(2) atoms’ Z coordinates, the bond length of Cu(2)–O(4), and the bond angle of Cu(2)–O(2)–Cu(2), which are all closely related to the interaction between the perovskite block and the rock salt block in the unit cell. The results indicated that the influence of the crystalline structure on superconductivity is important and independent of the carrier concentration.     

Solid ? liquid phase transformations in hypereutectic P/M Al-Si-Fe-X alloys

In this study, the metastable and stable solid liquid phase transformations of the hypereutectic alloys Al-20Si-5Fe-2Ni (wt%) and Al-17Si-5Fe-3.5Cu-1.1Mg-0.6Zr in as-atomised powder and in as-hot-forged material have been investigated. Differential Scanning Calorimetry (DSC) measurements at high temperatures have been performed. The resultant products have been thoroughly analysed using Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) microanalysis. During solidification large Si, (Al₄FeSi₂), FeNiAl₉, Al₇Cu₂Fe and Q(Cu₂Mg₈Si₆Al₅) are formed. A cooling rate of 5 °C/min and 1 °C/min is too high for the formation of the equilibrium phases (Al₅FeSi) and Al₇Cu₂Fe. The understanding of the sequence of transformations is useful in order to define appropriate processing parameters for these alloys produced by powder metallurgy. The temperature at which the first liquid phase appears during heating at 5 °C/min is 559 °C for the Al-Si-Fe-Ni powder and 506 °C for the Al-Si-Fe-Cu-Mg-Zr powder.     

Local Structure and Valence State of Mn in Ga1?xMnxN Epilayers

Mn has been incorporated in epilayers of the large-gap semiconductor GaN grown by molecular beam epitaxy using a nitrogen plasma cell. Detailed extended X-ray absorption fine structure (EXAFS) studies of a Ga_0.98Mn_0.02N epilayer confirm that the Mn atoms substitute the Ga atoms, with an increase by 2.7% of the distance to the nearest nitrogen atoms. Near-edge spectroscopy results tend to indicate that the valence state of Mn is slightly higher than 3+, while EXAFS analysis suggests an electron transfer to the N neighbors.     

Anisotropic Superconductivity and Vortex Dynamics in?Magnetically Coupled F/S and F/S/F Hybrids

Magnetically coupled superconductor?Cferromagnet hybrids offer advanced routes for nanoscale control of superconductivity. Magnetotransport characteristics and scanning tunneling microscopy images of vortex structures in superconductor?Cferromagnet hybrids reveal rich superconducting phase diagrams. Focusing on a particular combination of a ferromagnet with a well-ordered periodic magnetic domain structure with alternating out-of-plane component of magnetization, and a small coherence length superconductor, we find directed nucleation of superconductivity above the domain wall boundaries. We show that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels. In order to explore the Abrikosov flux line ordering in F/S hybrids, we use a combination of scanning tunneling microscopy and Ginzburg?CLandau simulations. The magnetic stripe domain structure induces periodic local magnetic induction in the superconductor, creating a series of pinning?Canti-pinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes nonlinear due to a change in both the number of vortices and the confining potential. In F/S/F hybrids we demonstrate the evolution of the anisotropic conductivity in the superconductor that is magnetically coupled with two adjacent ferromagnetic layers. Stripe magnetic domain structures in both F-layers are aligned under each other, resulting in a directional superconducting order parameter in the superconducting layer. The conductance anisotropy strongly depends on the period of the magnetic domains and the strength of the local magnetization. The anisotropic conductivity of up to three orders of magnitude can be achieved with a spatial critical temperature modulation of 5% of T _c. Induced anisotropic properties in the F/S and F/S/F hybrids have a potential for future application in switching and nonvolatile memory elements operating at low temperatures.     

Optical Study of Plasmon–LO Phonon Modes in Ga1?xMnxAs

Ga_1–xMn _x As layers with Mn fractions 0 x 2.8 % grown on GaAs(001) substrates by low-temperature molecular beam epitaxy were investigated using micro-Raman spectroscopy and far-infrared (FIR) reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of coupled modes of longitudinal optical phonons and hole plasmons. A full line-shape analysis of the spectra was performed within a Lindhard–Mermin model for the dielectric function, including intraband and interband hole transitions. Annealing at temperatures between 250 and 500°C results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the number of Mn atoms on Ga lattice sites is deduced from high-resolution X-ray diffraction. After annealing at 450°C the Raman lines of elemental As are observed, which are due to the precipitation of As on the sample surface.     

Critical and Gaussian Conductivity Fluctuations in Granular Ho1?xPrxBa2Cu3O7?δ Superconductor

Fluctuations in the electrical conductivity of polycrystalline Ho_1−x Pr _x Ba₂Cu₃O_7−δ superconductors were investigated with Ho contents from x=0.01 up to 0.10. Samples were prepared by the standard solid-state reaction technique. The method of analysis is based on the determination of the quantity c_s^-1=-\fracddTlnDs\chi_{\sigma}^{-1}=-\frac{d}{dT}\ln\Delta\sigma, where Δσ=σ−σ _R is the fluctuation conductivity. The results show that the resistive transition proceeds in two stages as seen by the temperature derivative of the resistivity near T _C . In the normal phase, Gaussian and critical fluctuation conductivity regimes were identified. The pairing transition splitting, associated with Pr doping and related to the occurrence of a phase separation, as observed in studies with other rare earth elements, was not clearly observed. On approaching the zero-resistance state, our results show a power-law behavior that corresponds to a phase transition from a paracoherent to a coherent state of the granular array. This behavior was not affected by Pr doping.     

Evidence of Electron–Phonon Interaction in Al-Substituted Mg1?xAlxB2

Phonon density of states in Al substituted Mg_1–x Al _x B₂ has been studied by means of inelastic neutron scattering experiments with pulsed neutron time-of-flight technique. Phonon dynamics have a significant role for realizing superconductivity in the recent discovered superconductor MgB₂. We present a systematic investigation on generalized phonon density of states (PDOS) up to E = 120 meV on Al content of x = 0.1, 0.25, and 0.5. We have clearly observed a large effect on the PDOS by Al substitution; a large energy shift occurs around at 70 meV and 89 meV. We have also observed a visible increase of PDOS at a particular energy range below T _c. Those phonon states are related with E _2g mode to be expected to have a strong coupling with the electron states at the –A region.     

Suppression of Superconductivity Due to Enhanced Co Doping in?Cu0.5Tl0.5Ba2Ca2Cu3?yCoyO10?δ Superconductors

Fluctuation Induced Conductivity (FIC) of Co-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Co _y O_10−δ (y=0, 0.05, 0.1, 0.5) Superconductors have been studied as to find the possible cause of T _c suppression in these compounds. We used Aslamazov–Larkin (AL) and Lawrence–Donaich (LD) equations to analyze the FIC of our samples. In this regard, it is observed that the data seems to fit well with 3D (three-dimensional) AL equations. In addition, crossover temperature from 2D to 3D has been found. The width of the temperature window for 3D conductivity is continuously decreasing with enhanced Co concentration. The shrinking of 3D temperature region with increased Co doping can be correlated with the suppression of superconductivity. It is observed that the coherence length and th interplanar coupling strength have also been decreased with increased Co doping, consistent with the magnetic pair-breaking effect.     

Thermodynamic evaluation and potentiometric study of Cd1?xMnxTe and Cd1?xZnxTe dissolution in acid and alkaline solutions

We have performed thermodynamic calculations and have constructed Pourbaix diagrams representing redox processes in the MnTe-H₂O, ZnTe-H₂O, Cd_1−x Mn _x Te-H₂O, and Cd_1−x Zn _x Te-H₂O systems. Analysis of the Pourbaix diagrams and potentiometric data attests to different etching behaviors of Cd_1−x Mn _x Te and Cd_1−x Zn _x Te solid solutions in alkaline and acid solutions. Original Russian Text ? S.G. Dremlyuzhenko, A.G. Voloshchuk, Z.I. Zakharuk, I.N. Yurijchuk, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 1, pp. 26–34.     

Fluctuation Conductivity and Phase Separation in Polycrystalline Y1?xCexBa2Cu3O7?δ Superconductors

Fluctuations on the electrical conductivity of polycrystalline Y_1−x Ce _x Ba₂Cu₃O_7−δ superconductors were investigated with Ce contents from x=0.01 up to 0.10. The results showed that the resistive transition proceeded in two stages, intragranular and intergranular, as indicated by the temperature derivative of the resistivity and verified by critical exponent analysis. Also, from the critical exponent analysis, we have observed a splitting of the pairing transition in all investigated samples. Our results strongly suggest that this pairing transition splitting is related to the occurrence of a phase separation associated with Ce doping. In the regime of near-zero resistance state, the temperature dependence of our data is consistent with a power-law behavior which is indicative of a phase transition from a paracoherent to a coherent state of the granular array.     

Finite-Wave-Vector Phonon Coupling to Degenerate Electronic States in La2?xSrxCuO4

We consider finite-k electron–phonon coupling appropriate for a superconductor with a very short coherence length and local intersite pairs. From group theoretical analysis we find that k 0 phonons can couple to degenerate electronic states in La_{2– x} Sr _x CuO₄ and a Jahn-Teller-like distortion is found to be operative for E _g- and E _u-symmetry O states. Experimentally observed anomalies in inelastic neutron scattering, ESR, and EXAFS are found to be consistent with such an interaction of the ₁ symmetry -point mode with the planar O P _x and P _y states. The proposed interaction is found to naturally facilitate symmetry breaking associated with pairing and/or stripe formation.