A conjectured explanation for room-temperature superconductivity in narrow channels in oxidized polypropylene

Two groups of scientists have observed conductivity at least five orders of magnitude higher than that of copper at room temperature in narrow channels perpendicular to surfaces of films in oxidized polypropylene. For pulsed currents, this conductivity starts at a minimum value of applied current, and is destroyed at a current of about 30–60 times this value. Because of the existence of an upper critical current and of the observation that electronic thermal conductivity is negligible in the channels, it is thought that the channels are superconducting. A study is made of the hypothesis that these results are due to enhanced pairing, as first suggested by Parmenter, when the drift velocity of current carriers becomes close to the velocity of sound or, in work by Hone and by the present author, to an appropriate phase velocity of optical phonons. Such enhancements can be expected to be larger in quasi-one-dimensional systems.     

Dielectric properties of AlN films prepared by laser-induced chemical vapour deposition

The dielectric properties and electrical conductivity of AlN films deposited by laser-induced chemical vapour deposition (LCVD) are studied for a range of growth conditions. The static dielectric constant is 8.0 ± 0.2 over the frequency range 10²−10⁷ Hz and breakdown electric fields better than 10⁶ V cm⁻¹ are found for all films grown at temperatures above 130°C. The resistivity of the films grown under optimum conditions (substrate temperature above 170°C, NH₃/TMA flow rate ratio greater than 300 and a deposition pressure of 1–2 Torr) is about 10¹⁴ Ω cm and two conduction mechanisms can be identified. At low fields, F < 5 × 10⁵ V cm⁻¹ and conductivity is ohmic with a temperature dependence showing a thermal activation energy of 50–100 meV, compatible with the presumed shallow donor-like states. At high fields, F > 1 × 10⁶ V cm⁻¹, a Poole-Frenkel (field-induced emission) process dominates, with electrons activated from traps at about 0.7–1.2 eV below the conduction band edge. A trap in this depth region is well-known in AlN. At fields between 4 and 7 × 10⁵ V cm⁻¹ both conduction paths contribute significantly. The degradation of properties under non-ideal growth conditions of low temperature or low precursor V/III ratio is described.     

Positive exchange bias in epitaxial permalloy/MgO integrated with Si (1 0 0)

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni_82.5Fe_17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (1 0 0).     

Electrical transport and magnetoresistance in La2/3Sr1/3MnO3 polycrystalline composite thin films

The manganite La_2/3Sr_1/3MnO₃ (LSMO)/CuO and LSMO/Al₂O₃ polycrystalline composite thin films are deposited on Si (111) substrates in a magnetron sputtering system, using the tandem deposition method. The electrical transport and magnetoresistance properties of the films have been systematically investigated. By considering two parallel conduction channels at the grain boundary, we obtain a general expression for the temperature dependency of the resistance that agrees with the experimental data measured in LSMO polycrystalline composite thin films at whole temperature region of 300 K-10 K under the condition of zero magnetic field. Also, the resistance vs. temperature curve under an external magnetic field can be obtained by only varying one parameter in the model. It provides an effective way to obtain the high-temperature grain boundary magnetoresistance in the polycrystalline manganite.     

Room temperature ferromagnetism and hopping transport in amorphous CrN thin films

Magnetic and electronic properties of stoichiometric amorphous CrN thin films grown on MgO (001) substrates by radio-frequency nitrogen-plasma-assisted molecular beam epitaxy have been investigated. The magnetic property of the amorphous CrN thin films shows a ferromagnetic behavior even at room temperature, and can be interpreted by the percolation theory of magnetic polaron where we consider Cr³⁺ defects as magnetic impurities which lead to the formation of bound magnetic polarons. The obtained results of electrical conductivity are explained by the variable-range-hopping theory of the Mott and Davis model.     

Effect of magnetic fields on the abnormal electroresistance behavior in epitaxial thin films of La0.8Ca0.2MnO3

We report an unusual electroresistance (ER) behavior induced by a current and its response to magnetic fields in La_0.8Ca_0.2MnO₃ epitaxial thin films. These thin films were fabricated on SrTiO₃ (1 0 0) substrate using pulsed laser deposition (PLD) technique. It is found that the electric resistivity in these films is significantly enhanced by applying a dc current over a threshold value. Simultaneously, an abnormal electroresistance behavior appears in the temperature range from 10 to 300 K. The enhanced resistance turns out to be very sensitive to a weak current. Even a very small dc current can remarkably depress the high resistance, showing an unusual colossal ER effect. The ER reaches ∼1175% at temperatures lower than ∼50 K, and ∼705% at 300 K for a current changing from 0.72 to 10.5 μA. The influence of magnetic fields on the transport was also studied. The I–V curves can be strongly influenced by a low magnetic field even at room temperature. The deduced magnetoresistance (MR) reaches 120% at 300 K upon applying a magnetic field of 0.25 T. An interesting phenomenon is that the observed MR is current dependent.     

Conditions for and limitations of high-power handling capabilities of planar YBa2Cu3O7-x filters

The conditions for the design of compact high-power HTS filters are deduced from basic considerations, and related to the microwave properties of the HTS films and the substrates. Potential mechanisms limiting the linear microwave response of HTS films are analyzed systematically. Magnetic and thermal field breakdowns are found to confine the power handling capability to comparable field levels. While magnetic limitation is related to flux penetration at the lower critical field B_c1, thermally induced quenches occur dependent on the presence of defects and on the thermal conductivity of the substrate. The detrimental role of film inhomogeneities for the linear microwave response is confirmed experimentally for microwave fields at 19 GHz and for dc fields at 87 GHz.     

Micromagnetics of Ni-nanowires filled in nanochannels of porous silicon

An array of ferromagnetic nanowires embedded in silicon is fabricated using an anodizing and electroplating process. During the former one oriented porous silicon channels are fabricated whereas by the latter process Ni is incorporated into these channels. The electrodeposited Ni-wires have a great pore-length to pore-diameter (aspect) ratio up to 1000 : 1 (typical diameters between 10 and 60 nm, pore-length from 10 to 30 μm). Due to this property the samples exhibit a magnetic perpendicular anisotropy. The micromagnetic properties of Ni (e.g. Bloch-wall thickness) are responsible for the peculiar magnetic behaviour of this ferromagnet/silicon nanocomposite. Structural as well as magnetic investigations like scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), and SQUID-magnetometry display this nanocomposite as a bimodal ferromagnetic structure consisting of perpendicularly oriented Ni-wires and of single domain Ni-granules embedded in the silicon based matrix. The hysteresis curve outlines a two-fold magnetic switching, one at low fields (∼0.05 T) and the second at high fields (> 3 T). The micromagnetic magnetization reversal is treated in terms of an analytic formulation.     

Controllable Self‐Assembled Microstructures of La0.7Ca0.3MnO3:NiO Nanocomposite Thin Films and Their Tunable Functional Properties

Two‐phase self‐assembled nanocomposite films have attracted increasing interest in recent years because of their potential applications in novel technological devices. However, tuning the physical properties by modulating the microstructure of self‐assembled nanocomposite films is still a challenge. In this study, epitaxial La_0.7Ca_0.3MnO₃:NiO nanocomposite films are synthesized by pulsed laser deposition. In the composite films with a NiO ratio of 50%, microstructures with nanomultilayer, nanogranular, and nanocolumnar characteristics are successfully obtained by using different growth modes. The metal–insulator transition and magnetic transition can be separately modulated by tuning the microstructures. By precisely modulating the microstructure, a significantly enhanced low‐field magnetoresistance (>80% at a magnetic field of 1 T) with an unusual plateau in the temperature interval from 10 to 110 K is realized in these films, which is expected to be applicable in field‐sensor devices that can be operated in a wide temperature range.     

FePt nanocluster films for high-density magnetic recording

High anisotropy L1(0) ordered FePt thin films are considered to have high potential for use as high areal density recording media, beyond 1 Tera bit/in2. In this paper, we review recent results on the synthesis and magnetic properties of L1(0) FePt nanocomposite films. Several fabrication methods have been developed to produce high-anisotropy FePt films: epitaxial and non-epitaxial growth of (001)-oriented FePt:X (X = Au, Ag, Cu, C, etc.) composite films that might be used for perpendicular media; monodispersed FePt nanocluster-assembled films grown with a gas-aggregation technique and having uniform cluster size and narrow size distribution; self-assembled FePt particles prepared with chemical synthesis by reduction/decomposition techniques, etc. The magnetic properties are controllable through variations in the nanocluster properties and nanostructure. FePt and related films show promise for development as heat-assisted magnetic recording media at extremely high areal densities. The self-assembled FePt arrays show potential for approaching the ultimate goal of single-grain-per-bit patterned media.     

Dependence of the a-Si:H defect density of states on the magnetic field profile of an electron cyclotron resonance microwave plasma

The magnetic field profile of an electron cyclotron resonance microwave plasma was systematically altered to determine subsequent effects on a-Si:H film quality. The mobility gap deep density N_D deposition rate and light-to-dark conductivity were determined for the a-Si:H films. By variation of the magnetic field profile N_D could be altered by more than an order of magnitude, from 1 × 10¹⁶ to 1 × 10¹⁷ cm⁻³ at 0.7 mTorr and 1 × 10¹⁶ to 5 × 10¹⁷ cm⁻³ at 5 mTorr as determined by junction capacitance techniques. Two deposition regimes were found to occur for the conditions of this study. Highly divergent magnetic fields resulted in poor quality a-Si:H, while for magnetic field profiles defining a more highly confined plasma, the a-Si:H was of device quality and relatively independent of the magnetic field configuration. The data is interpreted as a consequence of silane depletion for highly divergent magnetic field profiles.     

Development of transparent and conductive ZnO films by spray pyrolysis

Indium-doped zinc oxide (IZO) films were deposited on Corning 7059 substrates by the spray pyrolysis technique. To achieve higher electrical conductivity both the zinc acetate concentration and indium concentration in the solution were varied. The films were characterized for their structural and electrical properties. Film stability in H₂ plasma was also checked for possible use in amorphous and microcrystalline silicon related fields. It was observed that the films can be sustained in a hydrogen plasma, and hence IZO films of high conductivity can be used for the development of amorphous and microcrystalline silicon solar cells.     

Oscillations in longitudinal magnetoresistance in single-crystal films of bismuth

Theoretically, it has been shown by Erukhimov and Tavger that, when the magnetic and electric fields are in the same direction and parallel to the film surface, a change in the film thickness or the magnetic field leads to oscillations in conductivity for experimentally attainable conditions. We report small amplitude oscillations in the longitudinal magnetoresistance of single-crystal bismuth films in weak magnetic fields at liquid nitrogen temperature, due to the influence of the magnetic field on size quantization.     

The effect of static electric and magnetic fields on the optical properties of amorphous hydrogenated silicon films produced by r.f. glow discharge

The aim of the present work was to study the optical properties of amorphous hydrogenated silicon films produced by capacitive and inductive r.f. glow discharge in a 3%SiH₄-Ar gas mixture. The effect of the application of static electric and magnetic fields during the film formation on the photoconductivity, photoactivation energy, recombination mechanisms and optical gap was thoroughly investigated. Films prepared in a capacitively or inductively coupled discharge show bias-dependent photoconductivities, which reach about 10^-4 Ω^-1 cm^-1 for an inductive discharge with a negative bias and about 10^-5 Ω^-1 cm^-1 for a capacitive discharge with a positive bias. The optical gap is of the order of 1.55 eV for capacitive films and is bias dependent for inductive films (1.45–1.85 eV). A superimposed magnetic field (of about 1 kG) increases the photoconductivity by one order of magnitude for both deposition methods. The optical gap is field dependent for inductive films (1.6–1.8 eV) and is about 1.6 eV for capacitive films. The main recombination mechanism at a moderate photon flux (less than 10¹⁴ cm^-2 s^-1) is monomolecular for all deposition conditions. The photoactivation energy lies between 0.1 and 0.2 eV for capacitive films and is about 0.1 eV for inductive films. It was also found that, by using suitable crossed electric and magnetic fields, it was possible to control the density and nature of the defect states in the films. These are correlated with the type of hydrogenated silicon species and with the amount of hydrogen incorporated into the films, which markedly influence the film properties.     

Annealing behavior of magnetic anisotropy in CoNbZr films

The effect of magnetic and nonmagnetic annealing on the magnetic anisotropy in CoNbZr films, formed by a DC opposing-targets sputtering method, was investigated. It was revealed that the origin of the magnetic anisotropy is the directional ordering of the magnetic atoms. The anisotropy fields and the direction of the easy axis obtained when the films are annealed in zero magnetic field are almost the same as those for the magnetic field parallel to the easy axis of the as-deposited films. When the films are annealed in a magnetic field perpendicular to the easy axis, the anisotropy field induced in parallel with the magnetic field, H_k(t), is well represented by the following formula: ln {1-H_k(t )/H_k(∞)∝-√Tt, where H_k(∞) is the thermal equilibrium value of the anisotropy field and D is the diffusion constant. The activation energy of the as-deposited film is 0.86 eV. Annealing the film increases the activation energy which is 2.1 eV when the film is annealed at a temperature of 450°C for 2 h     

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.     

Thickness-Dependent Superconductivity in a Layered Electride on Silicon

Layered materials exhibit a plethora of fascinating properties. The challenge is to make the materials into epitaxial films, preferably integrated with mature technological platforms to facilitate their potential applications. Progress in this direction can establish the film thickness as a valuable parameter to control various phenomena, superconductivity in particular. Here, a synthetic route to epitaxial films of SrAlSi, a layered superconducting electride, on silicon is designed. A set of films ranging in thickness is synthesized employing a silicene-based template. Their structure and superconductivity are explored by a combination of techniques. Two regimes of T_C dependence on the film thickness are identified, the coherence length being the crossover parameter. The results can be extended to syntheses of other honeycomb-lattice ternary compounds on Si or Ge exhibiting superconducting, magnetic, and other properties.     

Magnetotransport Anomaly in Room‐Temperature Ferrimagnetic NiCo2O4 Thin Films

The inverse spinel ferrimagnetic NiCo₂O₄ presents a unique model system for studying the competing effects of crystalline fields, magnetic exchange, and various types of chemical and lattice disorder on the electronic and magnetic states. Here, magnetotransport anomalies in high‐quality epitaxial NiCo₂O₄ thin films resulting from the complex energy landscape are reported. A strong out‐of‐plane magnetic anisotropy, linear magnetoresistance, and robust anomalous Hall effect above 300 K are observed in 5–30 unit cell NiCo₂O₄ films. The anomalous Hall resistance exhibits a nonmonotonic temperature dependence that peaks around room temperature, and reverses its sign at low temperature in films thinner than 20 unit cells. The scaling relation between the anomalous Hall conductivity and longitudinal conductivity reveals the intricate interplay between the spin‐dependent impurity scattering, band intrinsic Berry phase effect, and electron correlation. This study provides important insights into the functional design of NiCo₂O₄ for developing spinel‐based spintronic applications.     

Theoretical Investigations of Properties of New Half-Heusler Compounds NiFeZ (Z = Si,Ge)

The half-Heusler alloys NiFeZ (Z = Si, Ge) are investigated theoretically in the general framework of Density Functional Theory (DFT). We have calculated structural, mechanical, elastic, electronic, magnetic, and optical properties using WIEN2k simulation package. To incorporate the electronic exchange-correlation energy we have used the GGA+U approximation. Both the compounds are found structurally stable, however, elastic properties show that NiFeSi is ductile while NiFeGe is brittle. Electronically, the compounds are found to be half-metals with small band gaps in the spin-down channels whereas spin-up channels are conducting. The total magnetic moments for both the compounds are ≈ 2μ _B. Optical properties including dielectric function, refractive index, extinction coefficient, reflectivity, conductivity, and absorption coefficient are also calculated and analyzed.     

Grating-coupled surface plasmon enhanced short-circuit current in organic thin-film photovoltaic cells

In this study, we demonstrate the fabrication of grating-coupled surface plasmon resonance (SPR) enhanced organic thin-film photovoltaic cells and their improved photocurrent properties. The cell consists of a grating substrate/silver/P3HT:PCBM/PEDOT:PSS structure. Blu-ray disk recordable substrates are used as the diffraction grating substrates on which silver films are deposited by vacuum evaporation. P3HT:PCBM films are spin-coated on silver/grating substrates. Low conductivity PEDOT:PSS/PDADMAC layer-by-layer ultrathin films deposited on P3HT:PCBM films act as the hole transport layer, whereas high conductivity PEDOT:PSS films deposited by spin-coating act as the anode. SPR excitations are observed in the fabricated cells upon irradiation with white light. Up to a 2-fold increase in the short-circuit photocurrent is observed when the surface plasmon (SP) is excited on the silver gratings as compared to that without SP excitation. The finite-difference time-domain simulation indicates that the electric field in the P3HT:PCBM layer can be increased using the grating-coupled SP technique.