E Machler

Doubling the critical temperature of La1.9Sr0.1CuO4 using epitaxial strain

The discovery of high-temperature superconductivity in copper oxides raised the possibility that superconductivity could be achieved at room temperature. But since 1993, when a critical temperature (T c) of 133 K was observed in the HgBa2Ca2Cu3O8+δ (ref. 2), no further progress has been made in raising the critical temperature through material design. It has been shown, however, that the application of hydrostatic pressure can raise T c — up to ∼164 K in the case of HgBa2Ca2Cu3O8+δ (ref. 3). Here we show, by analysing the uniaxial strain and pressure derivatives of T c, that compressive epitaxial strain in thin films of copper oxide superconductors could in principle generate much larger increases in the critical temperature than obtained by comparable hydrostatic pressures. We demonstrate the experimental feasibility of this approach for the compound La1.9Sr0.1CuO4, where we obtain a critical temperature of 49 K in strained single-crystal thin films — roughly double the bulk value of 25 K. Furthermore, the resistive behaviour at low temperatures (but above T c) of the strained samples changes markedly, going from insulating to metallic.

Block‐by‐block deposition: A new growth method for complex oxide thin films

An alternative growth method for high‐Tc oxide thin films employing molecular beam deposition is proposed. Instead of an uncontrolled local nucleation followed by lateral growth and island coalescence, the new method provides substrate coverage by nonreacting constituents before nucleation is initiated, a controlled reaction path, and reduced lateral growth. DyBa2Cu3O6+δ films without precipitates, with a surface roughness of ±1 unit cell and showing finite size oscillations in the x‐ray diffraction spectrum, have been prepared. This method reveals that diffusion dominates the growth process at high substrate temperatures (≂700 °C).

Local determination of the stacking sequence of layered materials

The ability to modify the stacking sequence of ultrathin films offers a unique way to change either the interaction strength or the doping, but demands a careful control of each atomic monolayer. Progress is hampered by the lack of a direct method that allows differentiation on a local scale between the various terminating layers of a crystal. Here, the combination of a vacuum annealing process and friction force microscopy reveals this local distinction on a SrTiO3 surface. Using the friction contrast, we find how the terminating layer of a single crystal profoundly influences the terrace edge structure.

Characterization of a radio frequency plasma source for molecular beam epitaxial growth of high‐Tc superconductor films

The growth of superconducting oxides in vacuum conditions compatible with molecular beam epitaxy (MBE) requires a form of activated oxygen. The activated oxygen species can be either atomic oxygen (O) or ozone (O3). Here we characterize a rf plasma source by measuring the oxidation rate of a silver film on a quartz crystal deposition monitor as a function of the oxygen flow. The initial oxidation rate provides a lower limit for the actual atomic oxygen flux. If the frequency change of the quartz signal is entirely due to the oxidation of silver by atomic oxygen and we assume a detection efficiency of one, then the total cracking efficiency of the source is estimated to be around 30%. The source was used in a MBE machine to deposit thin (200 A) high‐Tc films of DyBa2Cu3O7 on SrTiO3 with a Tonsetc of 88 K and a Tzeroc of 86 K.

Origin of Cu‐rich precipitate formation on superconducting films: A competition between nucleation, oxidation, and growth kinetics

We have unraveled the growth mechanism of Cu‐rich precipitates on the surface of c‐axis‐oriented DyBa2Cu3O7 thin films. Despite the strong oxidation conditions present, the surface of c‐axis DyBa2Cu3O7 acts as an ideal nucleation site for Cu2O crystallites, whose (110) planes are parallel to the substrate surface. Once nucleated, they act as effective sinks for the rest of the deposited copper and form large crystals that revert to monoclinic CuO upon cooling. This result has important implications for all currently used thin film growth processes and indicates that smooth surfaces can only be prepared under stringent composition control.

Electrochemical oxidation of La2CuO4 thin films grown by molecular beam epitaxy

Thin insulating and c-axis oriented films of La2CuO4 are grown using a molecular beam epitaxy technique. Subsequently, these films are oxidized electrochemically using a 1N KOH solution. This approach is used to induce superconductivity, leading to a maximum Tc0 of 31 K,, measured both resistively and inductively. The surface morphology, lattice constants and the resistivity before and after the electrochemical treatment are compared.

Local electrochemical oxidation/reduction: First step towards a new lithography?

Using c‐axis La2CuO4 thin films, we first demonstrate that the electrochemical oxidation mainly occurs along the c axis. Then we induce oxygen‐rich regions electrochemically into an otherwise oxygen‐deficient matrix. On a microscopic scale the extra oxygen introduced creates metallic and/or superconducting regions in the insulating matrix. Contrary to other lithographic techniques in which large amounts of material are either removed or deposited, this technique does not induce significant height differences.

Growth and electrochemical oxidation of MBE-grown c-axis La2CuO4 thin films on different substrates

Abstract The growth and subsequent electrochemical oxygen intercalation of c -axis La 2 CuO 4 + δ thin films on substrates with different lattice mismatch [SrTiO 1 (001) (+3.03%), NdGaO 1 (001) (+1.90%). LaAlO 3 (001) (−0.05%) and SrLaAlO 4 (001) (−0.95%) substrates] are compared. The films grown on the different substrates can all be oxidized electrochemically, but their structural and transport properties differ vastly. The results can roughly be divided into two categories, i.e., those of films grown with a compressive or a tensile lattice mismatch where the former have a larger c -axis latice parameter and better transport properties.

Beam and decomposition properties of copper‐containing metalorganic precursors and their use for CuO thin film preparation in chemical beam epitaxy

The 2,2,6,6‐tetramethyl‐3,5‐heptanedionato and pivaloato precursors of copper have been investigated for their thermodynamic and gas‐phase properties. More specifically, time‐modulated mass spectroscopy has been used to determine the chemical compositions of the effusive beams of metalorganic precursors under various conditions, and to identify the thermal decomposition products resulting from breakdown of the effusive beam on a heated MgO(001) single‐crystal surface. From these results, their relative suitability for use in chemical beam epitaxy is compared. Also, copper oxide films have been grown in a chemical beam epitaxy system and their properties are discussed.

Schiff base precursor compounds for the chemical beam epitaxy of oxide thin films. I. Deposition of CuO on MgO[001] using copper (II) bis(benzoylacetone)‐ethylendiimine

A new precursor compound for the deposition of copper oxide thin films under molecular beam conditions, copper bis(benzoylacetone)‐ethylenediimine, has been characterized by thermal analysis and in situ mass spectrometry. Its stability and decomposition behavior are reported as well as its use for the deposition of epitaxial copper oxide thin films on MgO.