Stuart C. Wimbush

Self-assembled, rare earth tantalate pyrochlore nanoparticles for superior flux pinning in YBa2Cu3O7-δ films

Addition of pyrochlore rare earth tantalate phases, RE3TaO7 (RTO, where RE = rare earth, Er, Gd and Yb) to YBa2Cu3O7?? (YBCO) is shown to vastly improve pinning, without being detrimental to the superconducting transition temperature. The closely lattice matched to RTO phase provides a lower interfacial energy with YBCO than BaZrO3 (BZO) and produces very fine (~5?nm) particles with high linearity in their self-assembly along c. Critical current densities of 0.86, 0.38?MA?cm?2 at 1 and 3?T (for fields) parallel to the c axis were recorded at 77?K in 0.5?1.0??m thick films and a transition temperature of 92?K was observed even in the highest level doped sample (8?mol%).

Iron Carbide: An Ancient Advanced Material

Iron carbide ranks amongst the oldest materials known to mankind. As a matter of fact, the combination of iron and carbon was discovered even before the pure metal and what ancient cultures named “iron” was, in reality, an iron/iron carbide composite. The presence of 6.7 wt% C in Fe 3 C in fact changes its properties dramatically: iron carbide is ceramiclike in mechanical behavior and chemically much more inert than pure iron. The so-called “meteorite iron” is rich in Cohenite, cannot be forged, and is apparently “noble” (does not corrode, even on long time scales and in contact with oxygen and water). The presence of Fe 3 C was confi rmed, for example, in ancient Damascene steel, [ 1 ] a 2500-year-old material largely used for swords and daggers due to its very special properties (e.g., superior hardness and lightness), which originates in the modern view from reinforcing the metallic iron with ceramic nanofi llers, more specifi cally iron carbide nanofi bers enwrapped in carbon nanotubes. All of these properties, mechanical and magnetic, as well as the chemical inertness, plus the property that iron is rather sustainable and nontoxic, can open new interest in this material in the form of nanostructures, either as pure iron carbide or in combination with second-phase carbon. Until now, nanosized iron carbide has been mainly observed as a side product in the synthesis of carbon structures, where metallic iron is used as a catalyst, for example, in chemical vapor deposition (CVD) [ 2 ] and pyrolysis processes [ 3 ] during the synthesis of carbon nanotubes. At a time when the literature presents countless procedures for the production of a plethora of nanoparticles and nanostructures (ranging from physical to chemical approaches, in water or solventless, by using a hard template or soft matter), it is surprising that a synthetic pathway to produce basic Fe 3 C nanoparticles in a reproducible, simple, and fast manner is still missing. Iron carbide nanoparticles would indeed be suitable for a variety of applications, from biomedicine (e.g., as a magnetically guided transporter for drugs [ 4 ] or as a contrast agent for magnetic resonance imaging [ 5 ] ) to electronics (e.g.,

Increased Tc in Electrolyte‐Gated Cuprates

Adv. Mater. 2010, 22, 2529–2533 2010 WILEY-VCH Verlag G T IO N We report field-induced doping of the high-Tc superconductor YBa2Cu3O7–x using a polymer-host electrolyte as the gate dielectric in the field-effect transistor configuration. Films of thickness between 10 and 30 nm, grown heteroepitaxially on SrTiO3 substrates, showed an irreversible reduction in superconducting transition temperature, Tc, in the presence of the electrolyte, falling over a period of several hours, but Tc showed a strong increase of above 10K for a negative gate voltage of 3V (corresponding to p-doping of the YBa2Cu3O7–x layer). Thicker films (above 200 nm), thinned by chemical etching using ethylenediaminetetraacetic acid, showed more stable behavior in the presence of electrolyte and showed larger increases in Tc of up to 38K. Starting with the discovery of high-temperature superconductivity in the cuprates, there has been considerable interest in using field-effect transistors (FETs) to induce sufficiently high field-induced carrier density in the conducting channel to control the superconducting transition temperature, Tc. [2,3] Superconductivity in the cuprates typically involves small carrier coherence lengths and carrier densities at least an order of magnitude lower than those found in conventional metals. Consequently, cuprates tend to show larger field-effect in Tc compared to metals such as indium and tin. However, only small changes in the transition temperature are found in cuprate transistors, even using nominally high capacitance gate dielectrics. Devices made using ferroelectric gate dielectrics which can support higher carrier densities in the transistor channel show generally little improvement, with relatively small modulation of the electronic properties, DTc 7K. Use of the field-effect in cuprates has therefore been limited by the capacitance and electric field breakdown strength of the gate dielectric. Electrolytes used as the gate dielectric in inorganic FETs can support field-induced carrier densities that are considerably higher than those achievable with conventional dielectrics. Indeed, the conductivity of lightly doped ZnO films and ‘‘insulating’’ SrTiO3 single crystals has been shown to be enhanced by several orders of magnitude when a polymer electrolyte is used (for sufficiently high gate bias, the SrTiO3 shows an insulator to superconductor transition). More recently, a substantial field-effect has been observed in metallic colossal magnetoresistive manganites, with very high carrier density 10 carriers per cm induced in the transistor channel. One of the problems of working with initially metallic materials is that very thin samples must be used if the bulk conductance of the sample is not to swamp the modulation of the surface layer—a problem recognized as early as 1902. This is obviously even more serious when the bulk of the material is superconducting. We use here principally poly(ethylene oxide)/lithium perchlorate (PEO/LiClO4) polymer electrolyte as gate and gate dielectric to study the resistive superconducting transition in cuprate devices, from 298 to 4.2 K. Figure 1a shows the device structure used including the polymer electrolyte gate dielectric. Epitaxial thin films of the cuprate YBa2Cu3O7–x (YBCO) were selected as the superconducting layer because of their ready availability and nominally high Tc. We measure the channel conductivity in devices using a standard four-probe configuration to avoid contact resistance by placing two long parallel electrodes 0.5mm apart between the source and drain electrodes. The channel length was 2.5mm and the channel width was 2–10mm. In addition, a fifth electrode was present, and this was used as the gate electrode (see Supporting Information). Though YBCO has been reported to show robust double layer capacitance which is unchanged before and after emergence of the superconducting state at low temperature, there is evidence for room temperature electrochemical reduction of high-Tc samples in contact with electrolyte, with consequent decrease in Tc. [12–14]

Critical current enhancement by Lorentz force reduction in superconductor–ferromagnet nanocomposites

Ferromagnetic pinning centres in superconductors form much deeper potential wells than equivalent insulating or metallic non-superconducting inclusions. However, the resultant pinning forces arising from magnetic inclusions are low because the magnetic interaction takes place over the length scale of the magnetic penetration depth which is large in technological superconductors. Nonetheless, we show that a magnetic inclusion can also reduce the Lorentz force on a vortex, yielding a substantially enhanced critical current density for a given pinning force. We calculate this enhancement for a single vortex pinned by a paramagnetic cylinder as well as a vortex lattice interacting with magnetic inclusions, and find that the inclusion of ferromagnetic particles or rods offers a practical means of enhancing the critical currents in oxide high temperature superconductors.

Alginate-mediated routes to the selective synthesis of complex metal oxide nanostructures

The exceptional electronic, magnetic, optical and catalytic properties demonstrated by many ceramic materials when confined to the nanoscale are well established. However, the synthesis of multicomponent metal oxide nanowires and nanoparticles is notoriously problematic due to the difficulty of controlling homogeneity and achieving the correct stoichiometry. In this paper, we demonstrate a selective route to nanowires or nanoparticles of a quaternary metal oxide product using sodium or ammonium alginate respectively. By pre-organizing metal cations within an alginate gel the nucleation and growth of precursor crystalline phases can be constrained to the nanoscale. On further calcination the alginate decomposition products prevent sintering of these precursor nanoparticles prior to conversion to the final product. The cooperative effect of polymer microstructure and decomposition products allows an exceptional level of control over nucleation, growth and transport of the intermediate phases and subsequently on the particle size and morphology of the final product.

Enhanced critical current in YBa2Cu3O7?? thin films through pinning by ferromagnetic YFeO3 nanoparticles

Nanoscale ferromagnetic inclusions of YFeO3 have been incorporated into pulsed laser deposited YBa2Cu3O7 ? ? (YBCO) thin films. The poisoning of the YBCO through the addition of the magnetic material is minor, with 1?mol% doping resulting in an unsuppressed superconducting transition temperature of 90?K. The critical current density of the magnetically doped films is enhanced both in field and at self-field, and values of 3.0?MA?cm ? 2 have been achieved at 77?K, self-field in films 1??m thick, compared to 1.5?MA?cm ? 2 in an undoped film prepared by the same process. Such an enhancement in critical current at such low dopant levels is suggestive of an additional contribution to the flux pinning from the magnetic constituent.

Practical Magnetic Pinning in YBCO

Iron-containing oxide inclusions have been incorporated as nanoscale ferromagnetic precipitate flux pinning centers within pulsed laser deposited YBa2Cu3O7-delta (YBCO) thin films via a simple process utilizing a composite deposition target. The resultant films exhibit a coexistence of superconductivity and ferromagnetism at 77 K, and a consequent significant enhancement in their absolute critical current density, Jc, compared to pure YBCO films prepared under the same conditions. dasiaPoisoningpsila effects due to the incorporation of Fe into the YBCO matrix are avoided at sufficiently low concentrations of inclusions, where the greatest self-field Jc enhancement is seen, but do become increasingly evident as the concentration is increased. The process outlined has practical application in the synthesis of high-Jc YBCO films, with the absolute Jc values obtained in initial samples lying at the upper bound of reported values for samples produced worldwide to date.

Electrical Properties of Pulsed Laser Deposited Y2O3 Gate Oxide on 4H-SiC

Yttrium oxide (Y 2 0 3 ) has been successfully deposited on n-type 4H-SiC substrates using pulsed laser deposition. The effects of postdeposition annealing temperature (400, 500, and 600°C) on the electrical properties of the Y 2 O 3 gate oxide have been studied in comparison with the as-deposited sample. The sample annealed at 600°C possessed the highest dielectric breakdown field of ∼6.5 MV cm -1 at 10- 6 A cm -2 , resulting from the lowest interface trap density and total interface trap density. The Fowler-Nordheim tunneling mechanism has been investigated on all samples and the highest value of barrier height extracted between the semiconductor and oxide conduction band edges was 2.67 eV.

Understanding nanoparticle self-assembly for a strong improvement in functionality in thin film nanocomposites

The striking influence of the growth kinetics and substrate enhanced surface mobility on the control of the self-assembly of rare earth tantalate particles (1.5 mol% of nanoparticles in YBa(2)Cu(3)O(7) thin films) is demonstrated. Strongly enhanced flux pinning, control of the anisotropy property and superior critical current densities were achieved. Owing to the unique ability to probe nanoparticle self-assembly through determination of the nature and extent of the anisotropy of the superconducting properties, this system serves as the perfect model system for understanding how to tune and control functional nanocomposite nanostructures for a wide range of multifunctional applications.

Future Directions for Cuprate Conductors

Superconducting cuprate conductors present several challenges and research opportunities related to the areas of performance and cost. In order to meet these challenges, a combination of radical new approaches is required. This paper proposes several novel ideas and demonstrates early results in some of these areas.