aoxing Xi

In situ epitaxial MgB2 thin films for superconducting electronics

The newly discovered 39-K superconductor MgB21 holds great promise for superconducting electronics. Like the conventional superconductor Nb, MgB2 is a phonon-mediated superconductor2, with a relatively long coherence length3. These properties make the prospect of fabricating reproducible uniform Josephson junctions, the fundamental element of superconducting circuits, much more favourable for MgB2 than for high-temperature superconductors. The higher transition temperature and larger energy gap4,5 of MgB2 promise higher operating temperatures and potentially higher speeds than Nb-based integrated circuits. However, success in MgB2 Josephson junctions has been limited because of the lack of an adequate thin-film technology6,7. Because a superconducting integrated circuit uses a multilayer of superconducting, insulating and resistive films, an in situ process in which MgB2 is formed directly on the substrate is desirable. Here we show that this can be achieved by hybrid physical–chemical vapour deposition. The epitaxially grown MgB2 films show a high transition temperature and low resistivity, comparable to the best bulk samples, and their surfaces are smooth. This advance removes a major barrier for superconducting electronics using MgB2.

Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

Thermodynamics of the Mg–B system: Implications for the deposition of MgB2 thin films

We have studied the thermodynamics of the Mg–B system with the calculation of phase diagrams modeling technique using a computerized optimization procedure. Temperature–composition, pressure–composition, and pressure–temperature phase diagrams under different conditions are obtained. The results provide helpful insights into appropriate processing conditions for thin films of the superconducting phase, MgB2, including the identification of the pressure–temperature region for adsorption-controlled growth. Due to the high volatility of Mg, MgB2 is thermodynamically stable only under fairly high-Mg overpressures for likely growth temperatures. This places severe temperature constraints on deposition techniques employing high-vacuum conditions.

Thickness dependence of dielectric loss in SrTiO3 thin films

We have measured the dielectric loss in SrTiO3 thin films grown on SrRuO3 electrode layers with thickness ranging from 25 nm to 2.5 μm. The loss depends strongly on the thickness but differently above and below T≈80 K: as the thickness increases, the loss decreases at high temperatures but becomes higher at low temperatures. Our result suggests that, in the high temperature regime, the interfacial dead layer effect dominates while, in the low temperature regime, the losses related to the structural phase transition and quantum fluctuations are important.

Near single crystal-level dielectric loss and nonlinearity in pulsed laser deposited SrTiO3 thin films

We present low-frequency dielectric loss and nonlinearity measurements in SrTiO3 thin films grown by pulsed laser deposition on SrRuO3 electrode layers. A low loss tangent in the order of 10−4, close to the level found in SrTiO3 single crystals, was observed. Combined with a large tunability, this resulted in a figure of merit for frequency and phase agile materials that can rival that observed in single crystals. The result is potentially significant for tunable microwave device applications, and it points to stress and interface effects as the possible causes for higher dielectric losses in thin films.

Two-band superconductor magnesium diboride

This review focuses on the most important features of the 40 K superconductor MgB2—the weakly interacting multiple bands (the σ and π bands) and the distinct multiple superconducting energy gaps (the σ and π gaps). Even though the pairing mechanism of superconductor MgB2 is the conventional electron–phonon coupling, the prominent influence of the two bands and two gaps on its properties sets it apart from other superconductors. It leads to markedly different behaviors in upper critical field, vortex structure, magnetoresistance and many other superconducting and normal-state properties in MgB2 from single-band superconductors. Further, it gives rise to new physics that does not exist in single-band superconductors, such as the internal Josephson effects between the two order parameters. These unique phenomena depend sensitively on scattering inside and between the two bands, and the intraband and interband scattering can be modified by chemical substitution and irradiation. MgB2 has brought unprecedented attention to two-band superconductivity, which has been found to exist in other old and new superconductors. The legacy of MgB2 will be long lasting because of this, as well as the lessons it teaches in terms of the search for new phonon-mediated higher Tc superconductors.

Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics

The miniaturization and integration of frequency-agile microwave circuits—relevant to electronically tunable filters, antennas, resonators and phase shifters—with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1−xTiO3 have a paraelectric–ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss—Srn+1TinO3n+1 phases—in which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating non-stoichiometry. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics—doping or strain—in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics.

Strain relaxation during in situ growth of SrTiO3 thin films

We report a real-time observation of strain relaxation during in situ growth of SrTiO3 thin films by measuring the in-plane lattice constant at the film surface using reflection high-energy electron diffraction. The initial misfit strain in the SrTiO3 film is tensile on MgO and compressive on LaAlO3 as expected from the lattice mismatches between the film and the substrates. Strain relaxation begins immediately after the deposition starts, but is not complete until the film thickness reaches 500–2500 A depending on the substrate and the deposition temperature. The strain relaxation at the growth temperature influences the film strain at room temperature, which is compressive for both substrates for thin SrTiO3 films.

Properties of MgB2 thin films with carbon doping

We have studied structural and superconducting properties of MgB2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing precursor metalorganic bis(methylcyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the film increases, the resistivity increases, Tc decreases, and the upper critical field increases dramatically as compared to clean films. The self-field Jc in the carbon doped film is lower than that in the clean film, but Jc remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with columnar nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB2 materials for high magnetic-field applications.

In situ Raman scattering studies of the amorphous and crystalline Si nanoparticles

We report on in situ studies of the vibrational properties of Si nanoparticles and ultrathin layers grown by dc magnetron sputtering in ultrahigh vacuum on amorphous MgO and Ag buffer layers. The average thickness of the Si layers ranged from monolayer coverage up to 200 A ˚ . Transmission electron microscopy has been used to determine size and shape of the Si nanoparticles. Changes in the phonon spectra of Si nanoparticles during the crystallization process have been studied by interference enhanced Raman scattering technique. Marked size-dependences in the phonon density of states and the relaxation of the k-vector conservation with decrease in size of the Si nanoparticles have been detected. The transition between crystallineand amorphous-like behavior takes place in the particles with an average number of Si atoms equal toO7 ^ 2U £ 10 2 : q 2000