Yuefeng Nie

Suppression of superconductivity in FeSe films under tensile strain

We have studied the effect of tensile strain on the superconductivity in FeSe films. 50, 100, and 200 nm FeSe films were grown on MgO, SrTiO3, and LaAlO3 substrates by using a pulsed laser deposition technique. X-ray diffraction analysis showed that the tetragonal phase is dominant in all of our FeSe films. The 50 nm FeSe films on MgO and SrTiO3 are under tensile strain, while the 50 nm FeSe film on LaAlO3 and the other thick FeSe films are unstrained. Superconducting transitions have been observed in unstrained FeSe films with Tonset≈8 K, which is close to the bulk value. However, no sign of superconductivity has been observed in FeSe films under tensile strain down to 5 K. This is evidence to show that tensile strain suppresses superconductivity in FeSe films.

Magnetic phase separation in SrCoOx (2.5 ≤ x ≤ 3)

This work presents a study of the electronic phase separation resulting from oxygen non-stoichiometry in SrCoOx. We report here that for oxygen content 2.88 ≤ x ≤ 3, SrCoOx exhibits a magnetic phase separation while maintaining a single crystallographic phase. Two magnetic components are formed which match those found in SrCoO2.88 and SrCoO3 with TC = 220 K and 280 K, respectively. In addition, a value of TC = 160 K is assigned to the previously identified SrCoO2.75 phase. A magnetic phase diagram with four line phases is proposed for SrCoOx (2.5 ≤ x ≤ 3).

Nanostructured arrays of semiconducting octahedral molecular sieves by pulsed-laser deposition

Cryptomelane-type manganese oxide (OMS-2) has been widely used to explore the semiconducting and catalytic properties of molecular sieves with mixed-valent frameworks. Selective synthesis of patterned thin films of OMS-2 with hierarchical nanostructures and oriented crystals is challenging owing to difficulties in preserving the mixed valence, porosity and crystalline phase. Here, we report that pulsed-laser ablation of OMS-2 in an oxygen-rich medium produces a three-dimensional nanostructured array of parallel and inclined OMS-2 fibres on bare substrates of (001) single-crystal strontium titanate. Both parallel and inclined OMS-2 fibres elongate along the [001](OMS-2) direction. The parallel fibres interact strongly with the substrate and grow epitaxially along <110>(STO) with lattice misfits of less than 4%, whereas the inclined fibres are oriented with (301) parallel to the substrate surface. The spontaneous orientation of the crystalline OMS-2 domains over the STO surface opens up a new avenue in lattice-engineered synthesis of multilayer materials.

Superconductivity Induced in Iron Telluride Films by Low-temperature Oxygen Incorporation

We report superconductivity induced in films of the nonsuperconducting, antiferromagnetic parent material FeTe by low-temperature oxygen incorporation in a reversible manner. X-ray absorption shows that oxygen incorporation changes the nominal Fe valence state from 2+ in the nonsuperconducting state to mainly 3+ in the superconducting state. Thus superconductivity in O-doped FeTe occurs in a quite different charge and strain state than the more common FeTe{sub 1-x}Se{sub x}. This work also suggests a convenient path for conducting doping experiments in situ with many measurement techniques.

Suppression of magnetic phase separation in epitaxial SrCoOX films

Using pulsed laser deposition and a unique fast quenching method, we have prepared SrCoOx epitaxial films on SiTiO3 substrates. As electrochemical oxidation increases the oxygen content from x = 2.75 to 3.0, the films tend to favor the discrete magnetic phases seen in bulk samples for the homologous series SrCoO(3−n/8) (n = 0, 1, 2). Unlike bulk samples, 200 nm thick films remain single phase throughout the oxidation cycle. 300 nm films can show two simultaneous phases during deoxidation. These results are attributed to finite thickness effects and imply the formation of ordered regions larger than approximately 300 nm.

Low temperature crystal structure and large lattice discontinuity at Tc in superconducting FeTeOx films

We report a high resolution x-ray diffraction study of the crystal structure of superconducting FeTeOx films. The crystal symmetry of FeTeOx matches the parent FeTe, unlike most iron-based superconductors. However, at the superconducting transition there is a large change in the c-axis lattice parameter. Such a discontinuity in the thermal expansion is known in normal-to-superconducting phase transitions, but here the effect is far larger than for other iron-based superconductors. Following the typical analysis of such a discontinuity in thermal expansion using the Ehrenfest-relation leads to a prediction of a large enhancement of Tc in strained FeTeOx films.