Caviglia et al. [Nature (London) 456, 624 (2008)] have found that the superconducting LaAlO3/SrTiO3 interface can be gate modulated. A central issue is to determine the principal effect of the applied electric field. Using magnetotransport studies of a gated structure, we find that the mobility variation is almost five times as large as the sheet carrier density. Furthermore, superconductivity can be suppressed at both positive and negative gate bias. These results indicate that the relative disorder strength strongly increases across the superconductor-insulator transition.PACS numbers: 74.78.Db, 85.30.Tv The strength of the electric field effect (EFE) in accumulating or depleting carriers in a conducting channel is central not only to many semiconductor devices found ubiquituously in modern electronics, but also in current research into achieving novel physics using tunable materials. Complex oxides are one case where the electronic ground state of the system is highly sensitive to the carrier density [1,2]. Among the commonly studied oxide materials, SrTiO 3 has attracted much attention due to its high electron mobility and electric permittivity at low temperatures, which facilitates large electric field effects [3]. Many recent oxide EFE devices have utilized SrTiO 3 substrates as a crucial component of the experiment: both the metallicity and superconductivity of SrTiO 3 have been modulated [2,4,5,6].Recently Caviglia et al.[2] strikingly demonstrated that the EFE could be used to modulate the superconductivity which appears [7] in the metallic gas formed between the two insulators LaAlO 3 and SrTiO 3 . Since its first discovery [8] the origin and physics of this metallic layer has been intensively investigated. Room temperature scanning electron energy-loss spectroscopy and conducting scanning probe measurements have set an upper limit of ∼7 nm for the gas thickness in annealed or high pressure grown samples [9,10]. However it is still unclear how the gas thickness is correlated to the sheet resistance at low temperatures, which itself can be changed by several orders of magnitude with oxygen pressure during LaAlO 3 growth [8,11], or the EFE [2,5].In this context, Caviglia et al. assumed that the superconductivity suppression by the EFE is due to the reduction of the carrier density of the electron gas. However it was unclear how the mobility of the electron gas changed in these initial EFE experiments, since it was not probed. A direct measurement of the mobility is vital in order to experimentally determine what is the effective tuning parameter of the superconductor-insulator transition. In this Letter, we describe a detailed study of the magnetotransport properties of a LaAlO 3 /SrTiO 3 interface, for which superconductivity can be fully suppressed by both positive and negative gate bias. Normal state magnetotransport measurements were also made above the upper critical field at which superconductivity is destroyed. From these data we find that not only does the carrier density vary with applied ga...
Semiconductor heterostructures provide an ideal platform for studying high-mobility, low-density electrons in reduced dimensions. The realization of superconductivity in heavily doped diamond, silicon, silicon carbide and germanium suggests that Cooper pairs eventually may be directly incorporated in semiconductor heterostructures, but these newly discovered superconductors are currently limited by their extremely large electronic disorder. Similarly, the electron mean free path in low-dimensional superconducting thin films is usually limited by interface scattering, in single-crystal or polycrystalline samples, or atomic-scale disorder, in amorphous materials, confining these examples to the extreme 'dirty limit'. Here we report the fabrication of a high-quality superconducting layer within a thin-film heterostructure based on SrTiO(3) (the first known superconducting semiconductor). By selectively doping a narrow region of SrTiO(3) with the electron-donor niobium, we form a superconductor that is two-dimensional, as probed by the anisotropy of the upper critical magnetic field. Unlike in previous examples, however, the electron mobility is high enough that the normal-state resistance exhibits Shubnikov-de Haas oscillations that scale with the perpendicular field, indicating two-dimensional states. These results suggest that delta-doped SrTiO(3) provides a model system in which to explore the quantum transport and interplay of both superconducting and normal electrons. They also demonstrate that high-quality complex oxide heterostructures can maintain electron coherence on the macroscopic scales probed by transport, as well as on the microscopic scales demonstrated previously.
Carbon nanotubes have long been of interest as additives for increasing the mechanical and electronic properties of polymers, and considerable progress has been made.[1±6] However, melt-phase and solution viscosities ordinarily become too high for conventional processing when the nanotube component exceeds about 10 wt.-%, which limits the nanotube contribution to composite properties. In a groundbreaking development, Vigolo et al. have shown that composite fibers comprising largely nanotubes can be obtained by a process called polyvinyl alcohol (PVA) coagulation spinning. [7±9] In this process, a dilute surfactant-assisted single-walled carbon nanotube (SWNT) dispersion is coagulated into a gel state by spinning it into an aqueous PVA solution; this is followed by conversion into a solid fiber by a slow draw process, during which the water in the gel evaporates. [7±9] We recently reported improvements in this fiber-spinning technique that dramatically increased fiber strength and fiber-spinning rate.[10±12]These improved fibers (comprised of about 60 % SWNTs in a PVA matrix) have a capacity to absorb energy (a specific toughness of about 600 J g ±1 ) that is much higher than any other natural or synthetic organic fiber. Additionally, these SWNT fibers have been successfully utilized in the fabrication of electrochemical devices, such as electromechanical actuators [1,12] and supercapacitors. [10,11] However, unless the polymer is removed by pyrolysis (which degrades the mechanical properties of the fiber), performance of these electrochemical devices is limited by the low electrical conductivity of the nanotube/polymer composite fibers and degradation of mechanical stability when the PVA in these fibers is converted into an ionic conductor. [11] We show here that fibers with useful mechanical properties can be spun if we replace the PVA coagulant with a polyethyleneimine (PEI) coagulant. Although the PEI used is ordinarily a liquid at room temperature, it interacts with the nanotubes to serve as an intertube binding agent. The resulting strain-to-failure and toughness of the PEI-containing fibers are far greater than those of the thermally annealed, binder-free SWNT fibers (which have the advantage of a somewhat higher tensile strength and electrical conductivity) that were spun using the pioneering superacid method developed by the Rice group. [13,14] While the fiber strength and toughness achieved here are far less than those of fibers obtained from the continuous spinning process of Dalton et al. for producing SWNT/PVA composite fibers, [10,11] the prospects of improving the mechanical properties of the SWNT/PEI fibers appear good. Moreover, the electrical conductivity of the SWNT/PEI composite fibers is over a hundred times that of the supertough SWNT/PVA composite fibers. PEI and, in general, amines, are known to effectively interact with carbon nanotubes via physisorption on the nanotubes' sidewalls.[15±21] Thus, a method for separating metallic and semiconducting SWNTs has been developed that uses the hi...
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