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Cu nanocrystals (NCs) were produced by multiple high-energy ion implantations into 5-μm-thick amorphous silica (SiO2) at liquid-nitrogen temperature. The Cu-rich SiO2 films were subsequently annealed to reduce irradiation-induced damage and promote NC formation. The NC size distribution and structure were studied utilizing a combination of Rutherford backscattering spectroscopy, x-ray diffraction, cross-sectional transmission electron microscopy, and extended x-ray-absorption fine-structure (EXAFS) spectroscopy. We present results derived from all four techniques, focussing on EXAFS measurements to study the local atomic structure surrounding Cu atoms, and comparing NC samples with bulk standards. Using a unique sample preparation method, we drastically improve the signal-to-noise ratio to extract high-quality EXAFS data to enable the determination of a non-Gaussian bond length distribution via the third-order cumulant. We quantify subtle concentration- and annealing-temperature-dependent changes in the Cu NC short-range order and relate such changes to NC size. Relative to a bulk Cu standard, enhanced structural disorder is observed in addition to both a suppressed coordination number and bond length contraction. Deviations from bulklike structure increase as the NC size decreases. Samples of low Cu concentration and/or low annealing temperature contain a significant fraction of Cu oxides, as either oxidized NCs or Cu bonding to O in the SiO2 matrix. EXAFS and x-ray-absorption near-edge structure analyses demonstrate Cu in an oxidized form exhibits an oxidation state and local coordination similar to crystalline Cu2O albeit in a disordered form. [ABSTRACT FROM AUTHOR]

The ac losses in three disk-shaped YBa[sub 2]Cu[sub 3]O[sub 7] (YBCO) films, which were deposited on SrTiO[sub 3] by the pulsed-laser-deposition technique and had thicknesses, d, of 0.2, 1.0, and 3.0 μm, were measured in perpendicular applied ac magnetic fields to ∼0.14 T at 10 Hz in liquid nitrogen. The losses at low fields were found to be a strong function of the film thickness. The measured losses were compared with the theoretically calculated losses. The ac losses calculated using a field-independent critical-current density, the Bean model [J. R. Clem and A. Sanchez, Phys. Rev. B 50, 9355 (1994)], agreed very well with the 0.2-μm-thick film, while the calculated losses agreed well with the measured ones when a field-dependent critical-current density, the Kim model [D. V. Shantsev, Y. M. Galperin, and T. H. Johansen, Phys. Rev. B 61, 9699 (2000)], was used for the films of thickness 1.0 and 3.0 μm. However, a surprising discrepancy was found in the values of B[sub c] and B[sub 0] for thinner YBCO films depending on whether they were determined by ac or dc measurements. B[sub c] is defined as B[sub c]=μ[sub 0]J[sub c](0)d/2, B[sub 0] is the characteristic field in the Kim-model critical-current density J[sub cK](B[sub a])=J[sub c](0)/(1+B[sub a]/B[sub 0]), and J[sub c](0) is the critical-current density at applied magnetic field B[sub a]=0. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ac losses at 20 and 30 Hz were measured for two disk-shaped YBa[sub 2]Cu[sub 3]O[sub 7] films in perpendicular peak applied magnetic fields up to ∼0.2 T in liquid nitrogen. One of the films had a significantly higher critical-current density than the other as determined from the loss measurements. Also, it exhibited a more-uniform flux penetration around the circumference of the disk than the other as observed by magneto-optical images of these films in perpendicular dc fields. The results from this film were compared with theoretical predictions of ac losses for disks of thin superconducting films in perpendicular magnetic fields using the Bean model [J. R. Clem and A. Sanchez, Phys. Rev. B 55, 9355 (1994)], and the Kim critical-current model [D. V. Shantsev et al., Phys. Rev. B 61, 9699 (2000)]. The asymptotic Bean model predictions for low and high fields were in reasonably good agreement with the data. The numerical calculation of the losses following Shantsev et al. was found to give extremely good agreement with the loss data throughout the entire field range of the measurement when the Kim model for the critical-current density was used. [ABSTRACT FROM AUTHOR]

Cd[SUB1 - x] Mn[SUBx]Te/CdTe superlattices were grown by molecular beam epitaxy with a CdTe buffer layer on GaAs(001) substrate. Photoreflectance spectra were performed on Cd[SUB1- x]Mn[SUBx]Te/CdTe superlattices with high compositions of x = 0.4, 0.8 at room temperature and liquid nitrogen temperature. The exciton transitions related to the heavy (H) and light (L) holes of 11H, 11L, 22H, and 33H are observed. After taking into account the strain-induced and quantum confinement effects, the theoretical calculations are in good agreement with the photoreflectance measurement results except x = 0.8 of 33H. Photoluminescence measurements were also performed at room temperature and low temperature in order to compare with our photoreflectance results. Our results show that the photoreflectance spectroscopy technique is a powerful probe for the study of quantized state structures in superlattices systems. [ABSTRACT FROM AUTHOR]

Structural characteristics of single-crystal wurtzite AlN epilayers (grown on sapphire substrates) bombarded with 300 keV [sub 197]Au[sup +] ions at room and liquid-nitrogen temperatures (RT and LN[sub 2]) are studied by a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy. Results reveal extremely strong dynamic annealing of ion-beam-generated defects in AlN. Lattice amorphization is not observed even for very large doses of keV heavy ions at LN[sub 2]. An increase in irradiation temperature from LN[sub 2] to RT has a relatively small effect on the production of stable structural damage in AlN. In contrast to the case of AlxGa[sub 1-x]N with x ≤ 0.6, neither damage saturation in the crystal bulk (below the random level) nor preferential surface disordering is revealed for AlN. Results also show that structural lattice disorder produced in A1N by high-dose keV heavy-ion bombardment is stable to rapid thermal annealing at temperatures as high as 1000 °C. [ABSTRACT FROM AUTHOR]

The carrier transport properties across Ge nanocrystalline films (hereafter referred to as nanofilms) deposited by the cluster beam evaporation technique have been thoroughly studied. A thin nanofilm deposited at liquid nitrogen substrate temperature (Ge-LNT) exhibits the Coulomb blockade characteristic at low temperatures, while that deposited at room temperature (Ge-RT) does not show any evidence of single electron tunneling. This is explained by the difference in the nature of the overall electron transport due to a structural difference between the two types of nanofilms. The electron transport between two adjacent nanocrystals in both types of nanofilms was also studied in detail. It was found that, under relatively low electric fields, a Ge-LNT nanofilm exhibited T[sup -½] conductivity dependence, which could be explained by the variable range hopping model of Efros and Shklovskii [A. L. Efros and B. I. Shklovskii, J. Phys. C 8, L49 (1975)], while in the Ge-RT nanofilms, the conductivity-temperature dependence was explained by the variable range hopping mechanism as proposed by Mott and Greaves [N. F. Mott and E. A. Davis, Electronic Process in Non-Crystalline Materials (Clarendon, Oxford, 1979); G. N. Greaves, J. Non-Crystalline Solids 11, 427 (1973)] in the given temperature range. [ABSTRACT FROM AUTHOR]

The influence of structural disorder on the magnetic, transport, and optical properties of Ni[sub 0.50]Al[sub 0.50] alloy films has been investigated. A significantly disordered state was obtained by using vapor-quenching deposition onto substrates cooled by liquid nitrogen. The loss of translational invariance in the disordered state leads to a prominent increase of the magnetic moment of alloy below 50 K. This growth can be explained by the appearance of antistructure Ni atoms (or their clusters) in the disordered state of alloy and their ferromagnetic coupling below 50 K. The temperature dependences of resistivity for both ordered and disordered states of the Ni[sub 0.50]Al[sub 0.50] alloy films exhibit the resistivity minimum at 17-18 K, which has a nonmagnetic nature and can be related mainly to the quantum corrections to the electron-electron interactions in the presence of weak localization. It was shown that the resistivity of the ordered Ni[sub 0.50]Al[sub 0.50] alloy films in 50-300 K temperature range originates mainly from the electron-phonon scattering, while in the disordered state the contribution from the electron-phonon-vibrating impurity scattering becomes dominant. The structural disordering also leads to a noticeable change in the optical properties of alloy, especially in the infrared region. The observed temperature and structural dependences of the resistivity as well as the optical properties in the intraband region confirm the thesis on partial localization of the electronic states near the Fermi level. [ABSTRACT FROM AUTHOR]

Accumulation of structural disorder in Si bombarded at -196 °C with 0.5 MeV [sup 209]Bi[sub 1] and 1 MeV [sup 209]Bi[sub 2] ions (the so-called molecular effect) is studied by Rutherford backscattering/channeling spectrometry. Results show that the damage buildup is sigmodal even for such heavy-ion bombardment at liquid nitrogen temperature. This strongly suggests that, for the implant conditions of this study, the buildup of lattice damage cannot be considered as an accumulation of completely disordered regions. Instead, damage-dose curves are well described by a cascade-overlap model modified to take into account a catastrophic collapse of incompletely disordered regions into an amorphous phase after damage reaches some critical level. Results also show that Bi[sub 2] ions produce more lattice damage than Bi[sub 1] ions implanted to the same dose. The ratio of lattice disorder produced by Bi[sub 2] and Bi[sub 1] ions is 1.7 near the surface, decreases with depth, and finally becomes close to unity in the bulk defect peak region. Parameters of collision cascades obtained using ballistic calculations are in good agreement with experimental data. The molecular effect is attributed to a spatial overlap of (relatively dense) collision subcascades, which gives rise to (i) nonlinear energy spike processes and/or (ii) an increase in the defect clustering efficiency with an effective increase in the density of ion-beam-generated defects. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The conditions for microwave power generation at low temperatures under optical phonon emission are analyzed by Monte Carlo simulations of both small- and large-signal responses in bulk zinc blende and wurtzite GaN. As a result of the high optical phonon energy and the strong interaction of electrons with optical phonons in GaN a general improvement on the transit-time resonance and a considerable increase in the maximum generation frequency and power can be achieved in comparison to the widely studied III-V materials such as GaAs and InP. A dynamic negative differential mobility caused by transit-time resonance occurs in a wide frequency range of about 0.05-3 THz and persists in the THz frequency range up to the liquid nitrogen temperature with doping levels up to about 5x10[sup 16] cm[sup -3]. The efficiency of the amplification and generation is found to depend nonmonotonously on static and microwave electric field amplitudes, generation frequency, and doping level so that for each generation frequency there exists an optimal range of parameter values. Under optimal conditions a generation efficiency of about 1% to 2% can be achieved in the 0.5-1.5 THz frequency range. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoemission from GaAs cathodes with negative electron affinity (NEA) is applied for producing electron beams with very low longitudinal and transverse velocity spread. GaAs transmission-mode cathodes were activated with Cs and either O[sub 2] or NF[sub 3] in an extremely high vacuum setup (base pressure below 10[sup -12] mbar). Quantum efficiencies of 20%-25% (at 670 nm) and long dark lifetimes (about 1000 h) could be achieved for both types of activation in a reproducible way. Using a method based on the adiabatic transverse expansion of an electron beam in a spatially decreasing magnetic field, the mean transverse energy (MTE) of the photoemitted electrons was measured systematically, recording longitudinal energy distribution curves. Both the MTE and the longitudinal energy spread strongly depend on the value of NEA and the position of the extracted distribution relative to the bulk conduction band minimum. Electrons with energies above the conduction band minimum are thermalized with the lattice temperature of the cathode, while electrons with energies below this level show a non-Maxwellian distribution with enhanced transverse energies. Thus, when extracting all electrons in a current limited emission mode, the MTE increases with the absolute value of NEA and reaches values up to approx. 100 meV. By cutting off the low energy electrons with an external potential barrier, the longitudinal as well as transverse energy spread of the extracted electron ensemble are reduced. The MTE could be reduced down to about 28 meV at room temperature and to about 14 meV at liquid nitrogen temperature. The behavior of the MTE was found equivalent for (Cs, O) and for (Cs, F) activation layers on the same cathode. Conclusions about energy loss and scattering in the emission of photoelectrons from NEA GaAs cathodes are discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the electron mobility as a function of the carrier concentration mu(n) in a silicon doped GaAs-AlAs short period superlattice at the temperature of liquid nitrogen. Tuning of the metastable electron density; Influence of charge correlations on the mobility.

Provides information on a study which investigated the effect of coating thickness on the thermal stresses in tungsten-coated optical fibers. Theory and experiment; Results and conclusion.

Interband recombination in bulk indium-rich InGaN is studied via both spontaneous and stimulated emissions. Based on the low-temperature luminescence and absorption data, the magnitude of the edge tails in conduction and valence bands is determined, and the non-thermal energy distribution of excess holes localized in the fluctuating band potential is revealed. We show that the combination of carrier localization effects and Auger-determined interband rates fully accounts for the experimentally observed stimulated emission thresholds and gain values (∼20–30 kW/cm2 and >100 cm−1, respectively) at low temperatures (T < 100 K). It is suggested that exploiting structural disorder to keep injected holes below the mobility edge, thus suppressing defect-related recombination, is a prerequisite for high-temperature infrared lasing from degenerate InGaN with relatively temperature-stable threshold intensities of some 100 kW/cm2. [ABSTRACT FROM AUTHOR]

Silicene, a 2D hexagonal structure of silicon, has shown great promise for next-generation advanced electronic devices due to recent progress. However, one major obstacle to its application and integration into devices is its strong tendency of rapid oxidation in an ambient environment. Enhancing the chemical inertness by means of chemical modification without comprising its unique structure and properties is vital. Here, we report the fabrication of hydrogenated silicene grown on polycrystalline Ag films by means of plasma-enhanced chemical-vapor deposition (PECVD) with proper hydrogen dilution. Our Raman spectroscopy and x-ray diffraction results show the existence of typical monolayer and few-layer silicene. The x-ray photoelectron spectroscopy demonstrates its oxidation resistance after a few minutes of air exposure in a class 100 clean room. We propose that PECVD silicene can be an alternative to molecular-beam-epitaxy silicene for potential low-cost and large-area silicene applications. [ABSTRACT FROM AUTHOR]

Carbon-doped Ge2Sb2Te5 (CGST) is a potential candidate in phase change random access memory (PCRAM) with superb thermal stability and ultrahigh cycle endurance. Direct observation of the microstructure evolution of CGST is desirable to uncover the phase transformation mechanism on the relationship of nucleation/crystalline behaviors of the crystalline phase at elevated temperatures and the pristine amorphous phase at room temperature. Here, we investigate the structural evolution of CGST using combined in situ techniques. Our in situ x-ray diffraction and ellipsometry results demonstrate that CGST exhibits a much higher phase transition temperature than undoped one. Temperature-dependent in situ transmission electron microscopy observations further reveal that carbon doping plays a critical role in tailoring the properties of GST by tuning the stochasticity of nucleation/crystallization, stabilizing amorphous and crystalline GST via isolating and refining the grain size at room temperature and elevated temperature. Our work provides detailed information for understanding the microscopic origin of crystallization kinetics of carbon-doped phase change materials toward high-performance PCRAM. [ABSTRACT FROM AUTHOR]

As the main merits, self-stabilization and no magnetic resistance make the high-temperature superconducting (HTS) magnetic levitation technology a crucial area for high-speed magnetic levitation development. To guarantee a stable operation of superconducting magnetic levitation systems, the dynamic characteristics of superconducting bulk materials occupy a significant place. However, in the previous research, there is still a lack of a simulation method that can describe 6 degree of freedom (DOF) motion of the superconductor. In this paper, an electromagnetic–thermal–dynamic coupling calculation model was established first. Then, the damping characteristics of 5-DOF superconducting levitation were experimentally tested, and the response analysis of the superconductor under 1–20 Hz excitation was carried out to explore the coupled motion relationship between the various degrees of freedom of the superconductor. In addition to the above, the operating conditions and primary resonance intervals that should be avoided by the HTS maglev system were identified. Additionally, a numerical simulation was conducted to investigate the dynamic response of the HTS maglev system under impact loads. All in all, this study explored the temperature rise conditions of superconducting bulk materials under excitation force through magnetic-thermal-force multi-physics coupling research. This 6-DOF model can provide a comprehensive simulation method for superconducting maglev systems in superconductor's motion behavior, attitude, and thermal state monitoring. [ABSTRACT FROM AUTHOR]

High-entropy alloys possess significant potential for material applications in demanding structural components subjected to extreme conditions. Through experimental investigations, we have validated that the CoCrFeNiAl0.1Ti0.05 high-entropy alloy, which was fabricated using a cold rolling and annealing process, exhibits an excellent balance of strength and ductility at 77 K. Specifically, it demonstrates a yield strength of 575 MPa, a tensile strength of 1145 MPa, and a ductility of 55%. This enhanced performance can be attributed to the altered deformation mechanism at low temperatures, leading to the multiplications of stacking faults and twins. Additionally, the introduction of small quantities of aluminum and titanium results in the formation of precipitated phases within the alloy matrix. The synergistic influence of these two factors contributes to the augmentation of cryogenic plasticity in the alloy. [ABSTRACT FROM AUTHOR]

Direct bandgap group IV materials could provide intimate integration of lasers, amplifiers, and compact modulators within complementary metal–oxide–semiconductor for smaller, active silicon photonics. Dilute germanium carbides (GeC) with ∼1 at. % C offer a direct bandgap and strong optical emission, but energetic carbon sources such as plasmas and e-beam evaporation produce defective materials. In this work, we used CBr4 as a low-damage source of carbon in molecular beam epitaxy of tin-free GeC, with smooth surfaces and narrow x-ray diffraction peaks. Raman spectroscopy showed substitutional incorporation of C and no detectable sp2 bonding from amorphous or graphitic carbon, even without surfactants. Photoluminescence shows strong emission compared with Ge. [ABSTRACT FROM AUTHOR]

The response of plane-parallel 2 mm thick samples of 47.3Ni-52.7Ti alloy was studied in two series of planar impact tests at temperatures between 300 and 473 K and between 473 and 318 K (heating to 473 K followed by cooling). In two additional series, the samples of 0.4–4 mm thickness were tested at 300 and 338 K (after preheating up to 473 K). In all the tests, the samples were loaded by 1 mm thick copper impactors having velocities equal to 314 ± 2 m/s. The velocity of the rear sample surface was continuously monitored by a laser Doppler velocimeter. It was shown that substantial, by an order of magnitude, variation of Hugoniot elastic limit σ H E L and compressive strength Y of the nitinol with temperature are caused by the martensite–austenite transformation and its reversal. The variation of the dynamic tensile (spall) strength σ s p of the nitinol along the heating–cooling path was found similar to that of σ H E L although the difference between σ s p values of austenite and martensite, ∼20%, is much more modest than in the case of σ H E L . The test series performed at constant temperatures with samples of different thicknesses allows one to conclude that the plastic deformation in shocked austenite is presumably realized by dislocation motion and multiplication controlled by phonon viscosity. In the shocked martensite, the plastic deformation mechanism at a stress lower than ∼0.3 GPa is likely a thermally activated combination of deformation twinning and slip of kinking dislocations. [ABSTRACT FROM AUTHOR]

The study of insulation degradation processes is crucial for the reliable operation of power equipment and electronic devices. The phase field method has been widely used in recent years to simulate the degradation process of insulation materials. However, the effect of space charge was ignored. In this paper, a novel phase field method based on the bipolar carrier transfer model is suggested. This model can simulate degradation under different temperatures and DC or combined AC/DC voltage. The cases under AC and combined AC/DC voltage at different temperatures are simulated by COMSOL. Methods to ensure model convergence are proposed. The results show that the field strength distribution under a combined AC/DC field is more uniform, especially for negative polarity. For the same voltage form, temperature and breakdown time show an exponential relationship. The results are consistent with those of previous experimental studies, proving the usability of the model. In addition, the processes of initiation, growth, and breakdown stage of the dielectric degradation process, as well as the stagnation period, are explored theoretically. The stagnation time at low temperatures under combined AC/negative DC voltage is long. The research in this paper is useful for insulation optimization design, condition assessment, and longevity prediction. [ABSTRACT FROM AUTHOR]

This study presents an approach to achieve a triaxial magnetic alignment through a linear drive type of modulated rotating magnetic field by using a permanent magnet array. This magnet array generates a modulated rotating magnetic field (MRF) without moving the sample and can produce an MRF of ∼ 0.8 T. By utilizing this magnet array, DyBa2Cu3Oy (Dy123, y ∼ 7) grains were biaxially aligned. After cutting the as-aligned sample that was magnetically aligned, we observed a location dependence of the orientation degree. We achieved the higher in-plane orientation degree of ∼ 8.5° and the c-axis orientation degree of ∼ 6.0°, respectively. The 3D simulation analysis played a key role in identifying the region where the uniformity of the MRF is within the magnet array. Compared to the epitaxial growth techniques, the use of the magnet array offers a cost-effective and versatile solution for achieving triaxial magnetic alignment. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the temperature dependence of current-voltage (I-V) and capacitance voltage characteristics of the electron-irradiated Yb/p-InP metal-insulator-semiconductor (MIS) tunnel diode. Replacement of MIS tunnel diodes in the continuous flow liquid-nitrogen cryostat, after irradiation; Measurement of the forward I-V characteristics of the irradiated MIS tunnel diodes; Results of deep-level transient spectroscopy.

Describes high-Curie temperature superconducting wires using a combination of bismuth high-temperature phase and powder-in-tube technique. Fabrication of bundle conductors composed of the aforementioned thin tape-type wires; Large current conductor application; Electromagnetic properties of the superconducting wire.

Presents a study of photomodulated transmission and reflectivity at room and liquid-nitrogen temperatures of a series of indium gallium arsenide/gallium arsenide superlattices. Comparison of the features of miniband dispersion; Experimental details; Results of miniband dispersion.