This presentation was first delivered at Photonics West 2020 on 4 February 2020 and has been included as part of this Digital Forum to enable scholarly dialogue. Please use the original citation when citing: Proceedings Volume 11284, Smart Photonic and Optoelectronic Integrated Circuits XXII; 112840W (2020) https://doi.org/10.1117/12.2545127
Searching for natural materials exhibiting larger electron-electron interactions constitutes a traditional approach to high temperature superconductivity research. Very recently we pointed out that the newly developed field of electromagnetic metamaterials deals with the somewhat related task of dielectric response engineering on a sub-100 nm scale. Considerable enhancement of the electron-electron interaction may be expected in such metamaterial scenarios as in epsilon near zero (ENZ) and hyperbolic metamaterials. In both cases dielectric function may become small and negative in substantial portions of the relevant four-momentum space, leading to enhancement of the electron pairing interaction. This approach has been verified in experiments with aluminium-based metamaterials. Metamaterial superconductor with Tc = 3.9 K have been fabricated, that is three times that of pure aluminium (Tc = 1.2 K), which opens up new possibilities to considerably improve Tc of other simple superconductors. A theoretical model based on the Maxwell-Garnett approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation. We report the observations of this excitation in Al-Al2O3 core-shell metamaterials using inelastic neutron scattering. This result provides support for this novel mechanism of superconductivity in metamaterials.
Metamaterial approach is capable of drastically increasing the critical temperature, Tc, of composite metal-dielectric superconductors. Tripling of Tc was observed in bulk Al-Al2O3 core-shell metamaterials. A theoretical model based on the Maxwell-Garnett approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation. We report the first observation of this excitation in Al-Al2O3 core-shell metamaterials using inelastic neutron scattering. This result provides support for this novel mechanism of superconductivity in metamaterials and explains the 50 year old mystery of enhanced Tc in granular aluminium films.
Conventional metals with high carrier concentrations have served to date as the materials of choice for plasmonic and metamaterial devices. However, typical metals are not well suited for near IR (NIR) plasmonic applications because their associated plasma frequencies correspond to the visible and ultraviolet regions of the spectrum. On the other hand, materials with lower plasma frequencies such as conducting oxides like ZnO and VO2 are capable of more efficiently coupling the electromagnetic radiation for optical metamaterial and plasmonic applications in the NIR. Furthermore, unlike metals, the electrical transport properties of conductive oxides can be modulated intrinsically by doping or extrinsically by applying heat, light or an electrical bias, thus allowing tuning of their electro-optical behavior. At the Naval Research Laboratory (NRL), we have investigated the use of laser processing techniques for the deposition and processing of various types of conducting oxides, such as Al-doped ZnO and W-doped VO2, which can be optimized over a wide range of optical/electrical properties. This paper will describe the laser deposition of these oxide films and their electrical and optical characterization in the NIR.
It has been known for a long time that many systems, including disordered metals and metallic oxides, undergo a metal/insulator transition (MIT). We have found that the superconducting transition temperature, Tc, of such materials is enhanced in the vicinity of the MIT. We have constructed superconductivity phase diagrams (Tc vs σ, the conductivity) for many materials whose only common feature is their proximity to the MIT and found that they are remarkably similar. These results suggest that there is a common mechanism for the enhancement of superconductivity near the MIT. Following this clue, we developed a simple, heuristic model based on scaling theory near the MIT that accounts for the observed features.
The effect of spin-polarized injection on the superconductivity order parameter is investigated in a device consisting of YBa2Cu3O7-(delta )/Au/Ni0.8Fe0.2 layers. A non-equilibrium theory which qualitatively agrees with the results of measurements made on superconductor/insulator/ferromagnet structures is presented. A quantitative analysis shows that this theory predicts injection currents that are several orders of magnitude too large. Recent results suggest that superconductivity in thin films can be strongly influenced by the injection of a spin- polarized current from a ferromagnetic material. The effect has been found to occur in both low Tc (Sn) and high Tc (YBa2Cu3O7-(delta )) superconductors when either a conventional ferromagnetic metal, permalloy (Ni0.8Fe0.2), or a colossal magnetoresistive material were used as the source of spin polarization. Control experiments showed that unpolarized current from a nonmagnetic metal had comparatively little effect on the same superconductors. A phenomenological model, in which the energy gap of the superconductor is perturbed by the presence of excess spin polarized electrons, has been shown to qualitatively mimic the experimental results. However, an estimate of the current needed to significantly suppress the gap is shown to be several orders of magnitude larger than is observed.
Pulsed laser deposition has been used for the growth of high quality YBa2Cu3O7 and La0.67Sr0.33MnO3 thin films and multilayers for electronic device applications. In particular, YBa2Cu3O7 - (SrTiO3, CeO2) - La0.67Sr0.33MnO3 trilayer devices were fabricated to study the supercurrent suppression by the injection of a spin-polarized quasiparticle current. Our results show that the critical current for a YBa2Cu3O7 - 50 angstroms SrTiO3 - La0.67Sr0.33MnO3 device was found to decrease from 120 mA to 15 mA, for an injection current of 60 mA of spin polarized current yielding a negative current gain of approximately 1.8. The effect of film microstructure on the critical current suppression was investigated. Defects in the SrTiO3 and CeO2 layers were found to control the device properties. Once optimized, spin injection represents a new approach to fabricating superconducting transistors which could impact electronic systems for many important next generation.
Pulsed laser deposition (PLD) has been used to deposit high quality thin films of Ni81Fe19/Au/YBa2Cu3O7- (delta ) onto (100) oriented substrates of MgO and SrTiO3 for the purpose of fabricating a novel high temperature superconducting three terminal device. The ferromagnet-normal metal-superconductor (F-N-S) structure is currently being investigated to determine the effect of the injection of a spin-polarized current on the order parameter of a high temperature superconducting thin film. High quality films with sharp interfaces, free of defects, are required in order to maximize the spin-injection effect. The surface morphology and transport properties of the YBa2Cu3O7-(delta ) films have been investigated using scanning electron microscopy and ac susceptibility measurements, respectively, as a function of increasing laser fluence. Deposition at 2.0 - 2.4 J/cm2, 790 degrees Celsius and 320 m Torr O2 produces films with a sharp superconducting transition and a smooth surface. The growth of Au on YBCO under different PLD conditions has been observed by atomic force microscopy. Surface clustering of Au occurs at elevated temperatures and is attributed to increased surface mobility. The presence or absence of a background gas influences the cluster size. These results are discussed within the framework of the role of excess energy of PLD adatoms with changing laser fluence and background gas.
Structural inhomogeneities in crystals of YBa2Cu3O7-(delta ) are characterized using high resolution x-ray diffraction. Rocking curves and a high resolution specular radial scan show features indicating that the crystals are not truly 'single'. The high resolution radial scan of a crystal that had a superconducting transition width of only one-half Kelvin, as determined by ac susceptibility, and appeared to be free of gross defects, when examined with conventional x-ray diffraction techniques, shows a distribution in the c-axis lattice parameter. These results suggest that crystals with sharp transitions should be carefully examined for the presence of structural anomalies which might influence the superconductive properties.
James Horwitz, Paul Dorsey, N. Koon, M. Rubinstein, J. Byers, D. Gillespie, Michael Osofsky, V. Harris, K. Grabowski, D. Knies, Edward Donovan, Randolph Treece, Douglas Chrisey
The effect of substrate temperature and oxygen deposition pressure on the structure and properties of thin films of LaxCa1-xMnO(delta ) has been investigated. Thin films (approximately 1000 angstroms) of La0.67Ca0.33MnO(delta ) were deposited onto LaAlO3 (100) substrates by pulsed laser deposition at a substrate temperature of 600 and 700 degree(s)C. A series of films were grown on different oxygen pressures, between 15 and 400 mTorr, which systematically changed the oxygen concentrations in the films. As-deposited films exhibited an oriented orthorhombic structure. At low oxygen deposition pressures films were preferentially (202) oriented. At high pressures deposited films had a (040) preferred orientation. A 900 degree(s)C anneal in flowing oxygen of a film deposited at low oxygen pressure resulted in a decrease in the a lattice parameter and a change in the preferred orientation from (202) to (040). Vacuum annealing at 550 degree(s)C resulted in an increase in the a lattice parameter. The resistivity as a function of temperature showed a significant variation as a function of growth conditions. The peak in the resistivity curve (Tm) varied between 73 and 150 K depending upon the growth conditions. The activation energy associated with the semiconducting phase was approximately the same for all films (approximately 100 meV).
Evidence of micron-sized structural inhomogeneities in several high transition temperature (Tc) superconductors is presented. By illuminating samples with high energy, highly collimated x rays produced on a synchrotron wiggler, small changes in the lattice were detected over a spatial scale as small as 10 micrometer. In the YBa2Cu3O7-(delta ) crystals, these changes are interpreted as evidence of variations in the oxygen content and in the Nd2-xCexCuO4-y crystal, as a variation in the cerium content. Each type of inhomogeneity can affect the superconducting properties.
Novel electronic properties are observed in metals in contact with the ferromagnetic insulator europium monoxide (EuO). We have successfully prepared epitaxial bilayers and multilayers involving EuO and the metals V or Ag. With metal layers of 100 angstroms thickness or less, these structures exhibit a slope discontinuity in their resistance versus temperature at 90 K for V/EuO interfaces and at 50 K for Ag/EuO interfaces. For metal layers less than about 40 angstroms, a feature is also apparent in R(T) at 70 K, the Curie temperature of bulk EuO. The slope discontinuities are reminiscent of those observed at the Curie temperature of ferromagnetic metals. Kerr effect measurements confirm that the EuO exhibits ferromagnetic ordering at 70 K. In V/EuO structures, the V still displays superconductivity with a slightly depressed transition temperature. This suggests that such metal/ferromagnetic insulator structures may be used to prepare artificial ferromagnetic superconductors. The resistive transition is incomplete, suggesting small regions of isolated superconductivity within the sample, as has been predicted for certain special cases when a superconductor is in proximity to a ferromagnet.
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