New picture of high-temperature superconductivity

The case for high-temperature superconductivity originating in SrO or BaO planes, or in interstitial regions, is made, including (i) four successfully predicted superconductors; (ii) evidence that the superconductivity of the major cuprates is associated with holes in these layers; (iii) data showing that Pr on one side of a cuprate-plane kills the superconductivity, but Pr on the other side does not; and (iv) evidence that doped Sr₂YRuO₆ has an onset of superconductivity at ∼45 K despite having no cuprate-planes.     

Evidence for a two-band behavior of MgB2 from point-contact and tunneling spectroscopy

The break-junction tunneling has been systematically investigated in MgB₂. Two types of the break-junction contacts have been exploited on the same samples, which demonstrated tunnel contact like (SIS) and point contact like (SnS) behavior. Both of them have shown the existence of the two distinct energy gaps. We have also observed peculiarities on the I(V)- characteristics related to Leggett's collective mode assisted tunneling.     

On the possibility of a phonon mediated transport anomaly in MgB2 under compression

Results of electrical resistance measurements on MgB₂ at ambient temperature up to 25 GPa are presented. An abrupt reduction of nearly 30% in resistance around 18 GPa is observed. Band structure calculations in the presence of a frozen-in distortion of the E_2g phonon mode reveal that one of the closed Fermi sheets corresponding to the σ-band opens along the Γ-A direction at this pressure. It is suggested that the anomaly observed in the resistance is due to this phonon mediated electronic topological transition (ETT).     

Surface modification of biphasic calcium phosphate bioceramic powders

Biphasic calcium phosphate (BCP)/poly l-lactide (PLLA) biocomposite is proven to be a promising bone graft material or scaffold for bone tissue engineering. To improve the interfacial compatibility of BCP bioceramic with biopolymer-PLLA, BCP powders were surface-modified in different condition to graft polymer groups onto the surface of the BCP powders. l-lactide and l-lactic acid (LA) oligomer were used to modify the BCP surface with stannous octanoate (Sn(Oct)₂) and stannous chloride (SnCl₂) as catalyst, respectively. Results show that the surface modification effect is obvious and the amount of grafted organic group is above 6.5 wt.%. Sn(Oct)₂ and SnCl₂ are the optimal catalysts for the surface grafting reaction of l-lactide and l-LA oligomer, respectively. The surface grafting slightly increase the particle size of BCP powders and reduce the tendency for their agglomeration.     

Compressibility crossover and quantum opening of a gap for two-dimensional disordered clusters with Coulomb repulsion

Using Hartree-Fock orbitals with residual Coulomb repulsion, we study spinless fermions in a two-dimensional random potential. When we increase the system size L at fixed particle density, the size dependence of the average inverse compressibility exhibits a smooth crossover from a 1/L ² towards a 1/L decay when the Coulomb energy to Fermi energy ratio increases from 0 to 3. In contrast, the distribution of the first energy excitation displays a sharp Poisson-Wigner-like transition at . Received 13 March 2000     

Spin-deposited nanocrystalline lithium ferrite thin films: Fabrication and characterization

Thin films of lithium ferrite (with general composition Li_0.5Fe_2.5O₄) were fabricated at low temperatures (up to 650 °C) by citrate-route using spin-deposition technique. Deposited films consisted of nanometer-sized grains as evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. XRD patterns for annealed films showed broad peaks exhibiting a spinel phase. Size of nanocrystallites is estimated to be 3-7 nm using Scherrer's equation. Average grain size ∼8.5 nm is observed from TEM images of films annealed at 650 °C. Scanning electron micrographs show the formation of spherical aggregates of around 130 nm in diameter. The AFM analysis clearly evidenced the development of nanograins even at low (∼500 °C) annealing temperatures. Significant decrease in complex dielectric permittivity (∈′ − j∈″) with frequency is observed in the low frequency (100 Hz-1 MHz) as well as in X-band microwave frequency (8-12 GHz) region. ∈′ is found to be in the range of 15.7-33.9 in low frequency region, whereas in X-band microwave frequency region, it is found to lie between 3.9 and 4.9. Similarly, ∈″ is found to be 0.16-5.9 in the low frequency region, and 0.002-0.024 in the X-band microwave frequency region. Room temperature dc resistivity of these films is estimated to lie in the range of 10⁶-10⁸ Ω cm. These results strongly suggest that citrate-route processed nanocrystalline lithium ferrite thin films are promising candidates for monolithic microwave integrated circuits (MMICs).     

Checkerboard patterns and magnetic resonance peak in cuprate superconductors

We investigate a kind of spin-Peierls transition (SP) in high T_c superconductivity. It is found the antiferromagnetic exchange integral of SP corresponds to the magnetic resonance peak. The kind of spin-Peierls transition applied to cuprate superconductors is that without dimerization of lattice ions and with dimerization of localized hole h_Cu attached to the ion. Absence of the magnetic resonance peak in La-Sr-Cu-O results from the dimerized state of localized hole, h_Cu below T_c into tetramerized phase above T_c in SP transition without dimerization of copper-ion. The checkerboard patterns with four unit cell period originate from the SP of electronic part without ion-dimerization and from charge occupation probability of oxygen-atom around Cu.     

Heat treatment induced intermetallic phase transition of arc-sprayed coating prepared by the wires combination of aluminum-cathode and steel-anode

A method to prepare intermetallic composite coatings employing the cost-efficient electric arc spraying twin wires assistant with suitable heat treatment was developed. In this study, a Fe-Al composite coating was produced by spraying twin wires, i.e. a carbon steel wire as the anode and an aluminum wire as the cathode. The inter-deposited Fe-Al coating was transformed in-situ to Fe-Al intermetallic composite coating after a post annealing treatment. The effect of annealing treatment conditions on phase composition, microstructure and mechanical properties of the coating was investigated by using XRD, SEM, EDS and OM as well as microhardness tester. The results show that the desirable intermetallic phases such as Fe₂Al₅, FeAl and Fe₃Al are obtained under the annealing condition. The main oxide in the coating is FeO which can partially transform to Fe₃O₄ up to the annealing condition.     

Low-energy signatures of charge and spin fluctuations in Raman and optical spectra of the cuprates

We calculate the optical and Raman response within a phenomenological model of fermion quasiparticles coupled to nearly critical collective modes. We find that, whereas critical scaling properties might be masked in optical spectra due to charge conservation, distinct critical signatures of charge and spin fluctuations can be detected in Raman spectra exploiting specific symmetry properties. We compare our results with recent experiments on the cuprates.     

Energy exchange in heterogeneous cumulative mediums induced by nonstationary quantum flux

It is known that energy flux from nonstationary source is able to modulate the energy exchange in the heterogeneous cumulative medium in the quantum optics approximation. This paper presents the results of further investigation of energy exchange problem in the heterogeneous medium with cumulative features. Equation for dynamics of dispersion term is derived and relative contribution of dissipative and dispersion term is estimated. The possibility of the expansion of nonstationary solution of energy exchange equation into basis of corresponding stationary solutions (dissipative structures) is shown. The physical interpretation of obtained results in terms of the dissipative structure variations is given.     

Enhanced corrosion resistance of Zr coating on biomedical TiNi alloy prepared by plasma immersion ion implantation and deposition

Zirconium film was prepared on TiNi alloy by plasma immersion ion implantation and deposition (PIIID) technique to enhance its corrosion resistance and prolong its working lifetime. The atomic force microscopy (AFM) results show that the film was relatively smooth with the root mean square roughness being 9.166 nm. The X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results indicate that the implant element of Zr was oxidation partialness. The potentiodynamic polarization measurements in the Hank's solution at 37 °C show that the corrosion resistance of the alloy was improved by the Zr coating film and the atomic absorption spectrometry (AAS) tests also indicate that Ni ion concentration released from the substrate in the Hank's solution after the polarization test was reduced greatly, in comparison to the unmodified TiNi alloy sample.     

The effect of thermal annealing and laser irradiation on the microstructure of vanadium oxide nanotubes

The microstructure of vanadium oxide nanotubes (VONTs) have been characterized using FTIR spectroscopy and Raman spectroscopy. The temperature effects on the VONTs were studies by changing the laser irradiation power and thermal annealing temperature in air. Raman spectroscopy studies showed that the VONTs could be decomposed even at low laser power irradiation. Also, together with scanning electron microscopy, it was found that thermal annealing in air could lead to the collapse of the tubular structure and convert the nanotubes into V₂O₅ nanoparticle. It was found that the thermal stability of VONTs was relatively low and the tubular morphology was destroyed at temperatures higher than 300 °C. The spectroscopic analyses showed that the Raman signature of the VONTs could be established for probing tubular structure.     

Effects of competing orders and quantum phase fluctuations on the low-energy excitations and pseudogap phenomena of cuprate superconductors

We investigate the low-energy quasiparticle excitation spectra of cuprate superconductors by incorporating both superconductivity (SC) and competing orders (CO) in the bare Green’s function and quantum phase fluctuations in the proper self-energy. Our approach provides consistent explanations for various empirical observations, including the excess subgap quasiparticle density of states, “dichotomy” in the momentum-dependent quasiparticle coherence and the temperature-dependent gap evolution, and the presence (absence) of the low-energy pseudogap in hole- (electron-) type cuprates depending on the relative scale of the CO and SC energy gaps.     

Effect of an absorbent overlay on the residual stress field induced by laser shock processing on aluminum samples

Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field, which increases fatigue crack initiation life and reduces fatigue crack growth rate. Specimens of 6061-T6 aluminum alloy are used in this investigation. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switch Nd:YAG laser, operating at 10 Hz. The pulses are focused to a diameter of 1.5 mm onto aluminum samples. Density of 2500 pulses/cm² with infrared (1064 nm) radiation was used. The effect of an absorbent overlay on the residual stress field using this LSP setup and this energy level is evaluated. Residual stress distribution as a function of depth is assessed by the hole drilling method. It is observed that the overlay makes the compressive residual stress profile move to the surface. This effect is explained on the basis of the vaporization of the coat layer suppressing thermal effects on the metallic substrate. The effect of coating the specimen surface before LSP treatment may have advantages on improving wear and contact fatigue properties of this aluminum alloy.     

First-principles calculations on the pressure induced zircon-type to scheelite-type phase transition of CaCrO4

The zircon-type and scheelite-type CaCrO₄ are investigated by first-principles calculations based on density-functional theory. The calculated zircon-type lattice parameters and the oxygen positions are in agreement with the experimental results and those of scheelite-type structure are studied for the first time in this work. The theoretical phase transition pressure of CaCrO₄ from zircon phase to scheelite phase is about 5.8 GPa, which is consistent with the experimental observation. From the density of states and the electronic band structures, CaCrO₄ is an insulator with a direct band gap (2.16 eV) for zircon-type structure and an indirect band gap (1.98 eV) for scheelite-type structure. The bulk moduli of the two phases are evaluated from the Murnaghan equation fit to the total energies as a function of the unit cell volume.     

Surface modification by oil jet peening in Al alloys, AA6063-T6 and AA6061-T4: Residual stress and hardness

The life of structural members that experience cyclic loading is improved by the introduction of surface compressive residual stresses. A high-pressure oil jet is used for the introduction of surface compressive residual stresses in aluminum alloys, AA6063-T6 and AA6061-T4. The peening machine designed and developed in the laboratory is capable of generating high pressures using hydraulic oil. The magnitude of residual stress developed depends upon the stand-off distance and yield strength of the material. A hardened layer up to a depth of about 350 μm was developed in the materials investigated. The residual stresses and surface hardening induced are comparable to that produced by other peening processes. An analytical model is proposed to predict the impact pressure.     

Duality in 2 + 1D quantum elasticity: superconductivity and quantum nematic order

Superfluidity and superconductivity are traditionally understood in terms of an adiabatic continuation from the Bose-gas limit. Here we demonstrate that at least in a 2 + 1D Bose system, superfluidity can arise in a strict quantum field-theoretic setting. Taking the theory of quantum elasticity (describing phonons) as a literal quantum field theory with a bosonic statistic, superfluidity and superconductivity (in the EM charged case) emerge automatically when the shear rigidity of the elastic state is destroyed by the proliferation of topological defects (quantum dislocations). Off-diagonal long range order in terms of the field operators of the constituent particles is not required. This is one of the outcomes of the broader pursuit presented in this paper. In essence, it amounts to the generalization of the well known theory of crystal melting in two dimensions by Nelson et al. [Phys. Rev. B 19 (1979) 2457; Phys. Rev. B 19 (1979) 1855], to the dynamical theory of bosonic states exhibiting quantum liquid-crystalline orders in 2 + 1 dimensions. We strongly rest on the field-theoretic formalism developed by Kleinert [Gauge fields in Condensed Matter, vol. II: Stresses and Defects, Differential Geometry, Crystal Defects, World Scientific, Singapore, 1989] for classical melting in 3D. Within this framework, the disordered states correspond to Bose condensates of the topological excitations, coupled to gauge fields describing the capacity of the elastic medium to propagate stresses. Our focus is primarily on the nematic states, corresponding with condensates of dislocations, under the topological condition that disclinations remain massive. The dislocations carry Burgers vectors as topological charges. Conventional nematic order, i.e., the breaking of space-rotations, corresponds in this field-theoretic duality framework with an ordering of the Burgers vectors. However, we also demonstrate that the Burgers vectors can quantum disorder despite the massive character of the disclinations. We identify the physical nature of the ‘Coulomb nematic’ suggested by Lammert et al. [Phys. Rev. Lett. 70 (1993) 1650; Phys. Rev. E 52 (1995) 1778] on gauge-theoretical grounds. The 2 + 1D quantum liquid crystals differ in fundamental regards from their 3D classical counterparts due to the presence of a dynamical constraint. This constraint is the glide principle, well known from metallurgy, which states that dislocations can only propagate in the direction of their Burgers vector. In the present framework this principle plays a central role. This constraint is necessary to decouple compression rigidity from the dislocation condensate. The shear rigidity is not protected, and as a result the shear modes acquire a Higgs mass in the dual condensate. This is the way the dictum that translational symmetry breaking goes hand in hand with shear rigidity emerges in the field theory. However, because of the glide principle compression stays massless, and the fluids are characterized by an isolated massless compression mode and are therefore superfluids. Glide also causes the shear Higgs mass to vanish at orientations perpendicular to the director in the ordered nematic, and the resulting state can be viewed as a quantum smectic of a novel kind. Our most spectacular result is a new hydrodynamical way of understanding the conventional electromagnetic Meissner state (superconducting state). Generalizing to the electromagnetically charged elastic medium (‘Wigner Crystal’) we find that the Higgs mass of the shear gauge fields, becoming finite in the nematic quantum fluids, automatically causes a Higgs mass in the electromagnetic sector by a novel mechanism.     

Growth and thermal stability of epitaxial BiFeO3 thin films

We have investigated the oxygen pressure and the temperature dependence on BiFeO₃ thin films deposited on SrTiO₃ substrates by pulsed laser deposition. Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction measurements indicate that high-quality epitaxial thin films are obtained for and T=650 °C. Outside of this pressure-temperature window, parasitic peaks attributed to β-Bi₂O₃ appear. We find an increase of the out-of-plane lattice parameter with oxygen pressure that we ascribe to Bi-deficiency due to its high volatility at low pressure. Ex-situ anneals have been performed and results show that as-grown single-phase BiFeO₃ thin films degrade after annealing, whereas as-grown BiFeO₃ containing impurity phases evolve toward a single-phase structure. These experiments demonstrate that parasitic phases can stabilize compounds which are usually unstable in air at elevated temperatures.     

Assignment of quantum number for plasmon energies in carbon layer systems

Electron-emission distribution curves of carbon layer surfaces excited by primary electrons of energies in the 118-534 eV range have been measured. The first four peaks in the plasmon spectrum are observed. It is concluded that the oscillator energies are presented to explain the assignment of the quantum number (n = 0,1,2,3) for internal plasmons in carbon layer systems. The preliminary assignment is in good agreement with the experimental results. It is also shown that the existence of limit between internal and surface plasmons. It is pointed out that the plasmon energy does not depend on both the external electrostatic voltage and the sample temperature. Moreover, the quantum number was adopted to the names of internal plasmons in the observed spectra.     

Nuclear polarization effects in spectra of multicharged ions

We evaluate the nuclear-polarization energy shifts for one-electron ²⁰⁸₈₂Pb and ²³⁸₉₂U ions. In contrast to previous calculations, the effect of nuclear polarizability is found to be essentially of minor significance for high-precision measurements of the Lamb shift in heavy multicharged ions. The calculations are carried out employing different numerical methods. We also provide a comparison of our numerical results with analytical estimates.