Quantum Dynamics of Atom-Molecule BECs in a Double-Well Potential

We investigate the dynamics of two-component Bose-Josephson junction composed of atom-molecule BECs. Within the semiclassical approximation, the multi-degree of freedom of this system permits chaotic dynamics, which does not occur in single-component Bose-Josephson junctions. By investigating the level statistics of the energy spectra using the exact diagonalization method, we evaluate whether the dynamics of the system is periodic or non-periodic within the semiclassical approximation. Additionally, we compare the semiclassical and full-quantum dynamics.     

NMR Study on a Pnictide Superconductor LaFeAsO1−x H x in a H-Overdoped Regime: Revival of Antiferromagnetic Fluctuations

An electron-doped high-T _c pnictide LaFeAsO_1?x H _x is recently known for having double superconducting (SC) domes in the phase diagram. We have performed nuclear magnetic resonance (NMR) measurements in an H-overdoped regime to investigate the second SC dome on a microscopic level. We unexpectedly discovered that the linewidths of both ⁷⁵As and ¹H spectra broaden at low temperatures for the 58 and 62.5 % H-doped samples, indicating that the antiferromagnetic (AF) ordering returns in the heavily H-doped regime. The finding itself is rare because an excess carrier doping tends to cause Fermi-liquid behavior in strongly correlated electron systems. We demonstrate that the new AF ordering originates from the nesting between electron pockets.     

Deterministic Model of Homophase and Heterophase Fluctuations in a Liquid–Vapor System

Within the framework of the assembly-type catastrophe model, a nonlinear dynamic equation (DE) homogeneous in the parameter t with an aftereffect is constructed, in which t characterizes the deviation of the reduced density of a thin surface layer on the liquid-vapor interface from the mean density of the vapor-liquid system. This equation is used to treat a second-order nonlinear DE with a variable damping coefficient for a vapor-liquid system excited by periodic impacts (acts of evaporation and condensation of molecules). This DE is integrated over a finite time interval to find a two-dimensional mapping whose numerical solution describes the chaotic dynamics of the density in time, including homophase and heterophase fluctuations. For this system, the bifurcation diagrams are constructed and the Lyapunov exponents are found.     

Fe–Ga–As precipitates and their magnetic domain structures in high-dose iron implanted GaAs

The integration of ferromagnetism with semiconductors to fabricate ferromagnetic semiconductors has been believed to be a potential way to make new spintronic devices. We have synthesized ferromagnetic Fe₃Ga_2−x As _x nanoparticles in the surface of GaAs by employing ion implantation and rapid thermal annealing processes. Transmission electron microscopy revealed that these nanoparticles exist only in the top surface of the GaAs samples, with sizes from several to hundreds of nanometers. They have two orientation relationships to the GaAs matrix: [1 − 210]_p//[011]_m, (10 − 10)_p//(−42 − 2)_m and (0002)_p//(11 − 1)_m; and [1 − 210]_p//[011]_m, (−1010)_p//(42 − 2)_m and (0002)_p//(−11 − 1)_m. The magnetic structures of the precipitates were studied by magnetic force microscopy. Results indicate that most of the ferromagnetic nanoparticles have single magnetic domains with their magnetic poles randomly orientated.     

Coarsening kinetics of metastable nanoprecipitates in a Fe–Ni–Al alloy

Microstructure evolution of a Fe–Ni–Al alloy has been examined during annealing at temperatures between about 700 and 800 °C. This material is brittle in the cast state but shows good strength with ductility after a stabilising anneal at 1100 °C when it has a duplex microstructure of B2 dendrites with fcc interdendritic phase. The 700–800 °C ageing leads to the formation of metastable bcc precipitates within the dendrites with less change within the interdendritic regions. The long-term coarsening of these precipitates is controlled by diffusion within the B2 phase. The composition of the B2 phase changes with annealing temperature, which is believed to modify the diffusion rate and, correspondingly, the rate of particle coarsening. The present coarsening study serves to define annealing conditions for preparation of optimum microstructure before material testing, as well as define upper temperature limits for possible long-term application, where stable microstructures are required.     

Coarsening of Co-rich precipitates in a Cu–Co–Fe ternary alloy

The effect of coherency on coarsening of fcc Co–Fe precipitates in a Cu–1.47 wt.%Co–0.56 wt.%Fe (Co : Fe = 7:3 in atomic ratio) alloy aged at 873–973 K has been studied by measuring both the precipitate size by transmission electron microscopy and the solute concentration in the Cu matrix by electrical resistivity measurements. The precipitate phase consists of 7 parts of Co and 3 parts of Fe in atomic ratio, irrespective of the precipitate size. The precipitates smaller than about 8 nm in radius are coherent with the Cu-matrix. When the average precipitate radius is over 18 nm, all the precipitates become semi-coherent. The coarsening rates are not affected by the coherency of the precipitates. The precipitate/matrix interface energy γ has been derived, independently of the diffusivities of solute atoms using a coarsening model developed by Kuehmann and Voorhees for ternary systems. The precipitates are coherent or semi-coherent with the matrix, the experimentally obtained value of γ is 0.2 J/m². This value lies between the reported values of γ  = 0.15 J/m² for Co precipitates and γ = 0.25 J/m² for γ-Fe precipitates.     

Electron-Conduction Mechanism and Specific Heat Above Transition Temperature in LaFeAsO and BaFe2As2 Superconductors

The ionization energy theory is used to calculate the evolution of the resistivity and specific heat curves with respect to different doping elements in the recently discovered superconducting pnictide materials. Electron-conduction mechanism in the pnictides above the structural transition temperature is explained unambiguously, which is also consistent with other strongly correlated materials, such as cuprates, manganites, titanates and magnetic semiconductors.     

Pearlite growth rate in Fe–C and Fe–Mn–C steels

The kinetic theory for the growth of pearlite in binary and ternary steels is implemented to ensure local equilibrium at the transformation front with austenite, while accounting for both boundary and volume diffusion of solutes. Good agreement is on the whole observed with published experimental data, although the reported growth rate at the lowest of temperatures is much smaller than predicted. To investigate this, experiments were conducted to replicate the published data. It is found that the cooperation between cementite and ferrite breaks down at these temperatures, and surface relief experiments are reported to verify that the resulting transformation product is not bainite.     

Phase Equilibria in the Cd–Ga–As–Te System

Inorganic Materials - An isothermal subsolidus x–y–z phase diagram, a block diagram of invariant phase equilibria, and a model for the p–T projection of the...     

Hot ductility of a Fe–Ni–Co alloy in cast and wrought conditions

The hot ductility of Fe–29Ni–17Co alloy was studied in both cast and wrought conditions by hot tensile tests over temperature range of 900–1250 °C and at strain rates of 0.001–1 s⁻¹. Over the studied temperature range, the wrought alloy represented higher elongation and reduction in area as compared to the cast alloy. Dynamic recrystallization was found responsible for the higher hot ductility of the wrought alloy and the improvement of hot ductility of the cast alloy at high temperatures. At temperature range of 1000–1150 °C the wrought alloy exhibited a hot ductility drop while a similar trough was not observed in case of the cast alloy. It was also found that at temperatures of 1150–1250 °C the best hot ductility is achieved in both cases of cast and wrought alloy. The experimental data of flow stress were constitutively analyzed and the apparent activation energy of deformation was estimated to be 344 kJ/mol.     

Effect of homogenization on recrystallization in a twin-roll cast Al–Fe–Si alloy

The effect of homogenization treatment on the recrystallization process in a twin-roll cast AlFeSi alloy was investigated by means of calorimetry, microstructural analysis, electrical conductivity, and hardness measurements and cupping tests. The response to annealing of cold-rolled AlFeSi sheet processed with a homogenization treatment at the cast gauge is a typical two-stage, recovery and recrystallization process, while that processed without homogenization softens without recovery. The rather limited precipitation capacity in the former allows recrystallization to occur largely discontinously, favoring the annealing texture. The nucleation rate and the volume fraction of the discontinously recrystallized grains are largely reduced in the sheet processed without homogenization, owing to extensive dynamic precipitation. This reduces the strength of the annealing texture components and gives a more or less random crystallographic texture after annealing. With a relatively finer-grain structure and a nearly random crystallographic texture, AlFeSi sheet processed to soft temper at 1 mm without a homogenization treatment is an attractive foil stock material.     

Synthesis of a decagonal phase in the Al–Cu–Fe–Cr system by mechanical alloying

Quasicrystalline Al–Cu–Fe–Cr alloys have been prepared by mechanical activation. The morphology of powder particles has been investigated after thermomechanical processing under various conditions. We have identified the sequence of phase transformations in the quaternary alloys in the stability region of quasicrystalline phases and optimized conditions for obtaining a maximum fraction of a decagonal state in powder materials.     

Pairing Fluctuations, the BCS–BEC Crossover and Strong Disorder in Superconductors

The mean field theory due to Bardeen, Cooper, and Schrieffer (BCS) provides the conceptual foundation of our understanding of superconductivity, but many examples over the last few decades have forced condensed matter physicists to extend the BCS framework. In particular, the extension to strong coupling, the BCS to Bose–Einstein condensation (BEC) crossover, requires the treatment of amplitude and phase fluctuations above the mean field state. Similarly, the presence of disorder can lead to strong inhomogeneity in the pairing amplitude, enhance phase fluctuations, and suppress the transition temperature. Finally, magnetic scattering quickly leads to a gapless superconducting state and then the loss of order. All of these involve physics beyond the BCS scenario. We employ a real space method that reduces to inhomogeneous mean field theory in the ground state, but fully retains the amplitude and phase fluctuations of the pairing field at finite temperature. This paper reviews some of our work in the weak to strong coupling (BCS–BEC) crossover, the disorder driven superconductor-insulator transition, and the role of magnetic impurities.     

Fe-rich border and activation energy of phase decomposition in a Fe–Cr alloy

Concentration of Cr in the Fe-rich α-phase, x, resulted from a phase decomposition caused by an isothermal annealing at T = 415 and 450 °C of a non-irradiated (NR) Fe–Cr14 EFDA sample and that of a He-ions irradiated one (IR) annealed at 415 °C was determined with Mossbauer spectroscopy. The x-value in the latter was by ∼3 at% higher than the one in the NR-counterpart. The activation energy for the phase decomposition in the NR-sample was 122 kJ mol⁻¹. In the IR-sample its value was by 12 kJ mol⁻¹ lower. Avrami exponents for the NR-samples were close to 0.5, and that the IR-sample had a value of about 1.     

Landau–Zener spin transitions in Fe2+–Fe2+ quantum dots controlling dislocation mobility in NaCl:Fe crystals

Magnetic field induces transition in the distorted Fe²⁺–Fe²⁺ pairs (quantum dots) from the initial bonding singlet state to the high spin antibonding state providing decay of the pairs for two separated Fe²⁺ ions. Dislocations moving under internal stresses easily overcome separated Fe²⁺ ions in comparison with Fe²⁺–Fe²⁺ pairs lying close to the glide plane. Non-monotonous field dependence of dislocation displacements under internal stresses governed by short (100 μs) impulse of high magnetic fields up to 31 T was revealed in NaCl:Fe crystals. This non-typical dependence is the fingerprint of the Landau–Zener non-adiabatic spin transition between singlet and high spin states in quantum dots distorted by mechanical stresses of moving dislocations.     

SbSI nanocrystal formation in As–Sb–S–I glass under laser beam

As–Sb–S–I glasses are obtained by co-melting of As₂S₃ and SbSI in a broad compositional interval. Their structure and composition are confirmed by the studies of scanning electron microscopy, energy dispersive X-ray spectroscopy, and micro-Raman scattering. Laser-induced crystallization of SbSI crystallites from the glass matrix is observed in the course of the micro-Raman measurement as a result of local laser beam heating.     

Interfacial magnetic coupling between Fe nanoparticles in Fe–Ag granular alloys

The role of the interface in mediating interparticle magnetic interactions has been analysed in Fe50Ag50 and Fe55Ag45 granular thin films deposited by the pulsed laser deposition technique (PLD). These samples are composed of crystalline bcc Fe (2–4 nm) nanoparticles and fcc Ag (10–12 nm) nanoparticles, separated by an amorphous Fe50Ag50 interface, occupying around 20% of the sample volume, as determined by x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and high resolution transmission electron microscopy (HRTEM). Interfacial magnetic coupling between Fe nanoparticles is studied by dc magnetization and x-ray magnetic circular dichroism (XMCD) measurements at the Fe K and Ag L2,3 edges. This paper reveals that these thin films present two magnetic transitions, at low and high temperatures, which are strongly related to the magnetic state of the amorphous interface, which acts as a barrier for interparticle magnetic coupling.     

Positron annihilation study on deformation-induced Au precipitation in Fe–Au and Fe–Au–B–N alloys

We have investigated the potential self-healing of deformation-induced defects by Au precipitation during isothermal aging at 550 °C in Fe–Au and Fe–Au–B–N alloys using positron annihilation spectroscopy. Two different samples with 0 and 24 % pre-strain were used to study the influence of dislocations on the Au precipitation. Dislocations introduced prior to the aging process play an essential role in the formation of Au precipitates. The Coincidence Doppler broadening (CDB) technique shows that Au precipitation in the matrix occurs in the pre-strained samples only. TEM observations confirm the heterogeneous nature of the Au precipitation which occurs exclusively on dislocations and grain boundaries. The evolution of S and W parameters derived from the CDB indicates a three-stage precipitation process for the pre-strained samples. Both the hardness tests and the positron annihilation spectroscopy indicate that the addition of boron and nitrogen to the Fe–Au alloy causes a deceleration of the Au precipitation in the pre-strained samples, but does not alter the defect-induced mechanism of the Au precipitation. The defect-induced Au precipitation provides a promising site-specific autonomous repair mechanism to extend the lifetime of Fe-based alloys for high temperature structural applications.     

Preparation of high purity glasses in the Ga–Ge–As–Se system

Small additions of Ga to Ge–As–Se glasses are known to enhance rare earth ion solubility in Ge–As–Se chalcogenide glasses designed for active optical applications. The effect of variants and conditions for producing samples of an exemplar Ge₁₆As₁₇Se₆₄Ga₃ (atomic%) glass on optical transmission, and the content of limiting impurities, is investigated. To prepare the high-purity glass samples, chemical distillation for purification of the Ge–As–Se base-glass is used. Next, a new vapor phase transport approach of metallic Ga transfer in a GaI₃ flow is developed to purify and add the batch of metallic gallium into the silica-glass reactor for the Ge–As–Se–Ga glass synthesis. A thermodynamic equilibrium based vapor phase transport model is discussed. In the best examples of these glasses, the content of residual impurities is: hydrogen – 0.15 ppm, oxygen – <1 ppm, and transition metals – less than 0.1 ppm.