High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by Pulsed Laser Ablation on different substrates. By varying the film thickness, Tc up to 21K were observed, significantly larger than the bulk value. Structural analyses indicated that the a axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe-(Se,Te) bond length and angle, suggesting the possibility of further enhancement.Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain Ver. 04/12/2009 14:29:00 2 Since the discovery of the iron based superconductors 1 , great efforts have been devoted to the preparation of thin films of the various phases 2 -7 and references therein. Between other reasons, the interest in films deposition is motivated by the strong Tc dependence on external and chemical pressure in iron based pnictides and calcogenides 8 -12 , which has suggested the idea of exploring whether a similar effect can be induced by strain in thin films. Indeed, such expectation turned out to be true: in Ba(Fe1-xCox)2As2 thin films 7 deposited on various substrates, Tc has been observed to increase with the ratio c/a. Also in FeSe0.5Te0.5 thin films an increase of Tc has been obtained by two groups 5,6 in particular a maximum Tc of 17K has been measured. Such increase has been attributed to the observed c axis decrease with respect to the bulk value.Here we present preparation as well as structural, morphological and physical characterization of epitaxial FeSe0.5Te0.5 thin films with different thickness, deposited on different substrates. A maximum Tc =21K was obtained, which is a remarkable 30% increase with respect to the bulk value.The films were grown by Pulsed Laser Ablation Deposition (PLD) in ultra high vacuum system 13 starting from a FeSe0.5Te0.5 (Fe(Se,Te)) target prepared by direct synthesis from high purity materials (Fe 99.9+%, Se 99.9% and Te 99.999%) 5 .With the aim of introducing biaxial strain we deposited films on single crystal substrates with different cell parameters, namely magnesium oxide (MgO a=4.217 Å) , strontium titanate (STO a=3.905 Å ), lantanum aluminate (LAO a=3.789 Å), and yttria stabilized zirconia (ZrO:Y, a=3.637 Å); for all the substrates we used the (001) orientation. The deposition conditions were optimized as reported in a previous paper 5 ; namely, we used a deposition temperature of 550°C at a pressure of 5·10 -Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain Ver. 04/12/2009 14:29:00 3 9 mBar. The laser repetition rate was 3 Hz (248 nm wavelength) and the laser fluency was 2 J/cm 2 (2 mm 2 spot size). The target-substrate distance was maintained at 5 cm.In order to study the residual strain behaviour, films of different thickness from 1.2 nm to 600 nm were deposited; the thickness was calibrated by X-ray reflectometry.XRD analysis allowed to identify the PbO-like tetragon...

We studied the influence of the disorder introduced in polycrystalline MgB 2 samples by neutron irradiation. To circumvent self-shielding effects due to the strong interaction between thermal neutrons and 10 B we employed isotopically enriched 11 B which contains 40 times less 10 B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10 B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1 -2 ϫ 10 18 cm −2 . With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behavior has been carried out to identify what scattering and pinning mechanisms come into play. Finally, the correlation between some characteristic lengths and the transition widths is analyzed.

Advances in growth technology of oxide materials allow single atomic layer control of heterostructures. In particular delta doping, a key materials' engineering tool in today's semiconductor technology, is now also available for oxides. Here we show that a fully electric-field-tunable spin-polarized and superconducting quasi-2D electron system (q2DES) can be artificially created by inserting a few unit cells of delta doping EuTiO3 at the interface between LaAlO3 and SrTiO3 oxides. Spin polarization emerges below the ferromagnetic transition temperature of the EuTiO3 layer (TFM = 6-8 K) and is due to the exchange interaction between the magnetic moments of Eu-4f and of Ti-3d electrons. Moreover, in a large region of the phase diagram, superconductivity sets in from a ferromagnetic normal state. The occurrence of magnetic interactions, superconductivity and spin-orbit coupling in the same q2DES makes the LaAlO3/EuTiO3/SrTiO3 system an intriguing platform for the emergence of novel quantum phases in low-dimensional materials.

Possible ferromagnetism induced in otherwise nonmagnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetism can be stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated with magnetic Ti3+ ions at the interface itself for LaAlO3/SrTiO3 and to Mn3+ ions in the overlayer for BiMnO3/SrTiO3. In LaAlO3/SrTiO3 the induced magnetism is quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in this phenomenon.

We show that the growth of the heterostructure LaGaO 3 / SrTiO 3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO 3 / SrTiO 3 and a superconducting transition is found. The results open the route to widening the field of polar-nonpolar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.The quasi-two-dimensional electron gas ͑q2DEG͒ recently discovered at the LaAlO 3 ͑LAO͒ / SrTiO 3 ͑STO͒ interface 1 is presently envisaged as an ideal system for the realization of nanoscale oxide devices. 2 The electronic reconstruction model attributes the origin of the q2DEG to an electronic relaxation mechanism occurring at the interface between the ͑nominally͒ non-polar ͑001͒ STO substrate and the polar ͑001͒ LAO film. The wide band gap of LAO is considered as crucial in this approach, because it determines the capability of the polar film to transfer charges over the band gap of STO. Ideally, half an electron per areal unit cell ͑Ϸ3.3ϫ 10 14 cm −2 ͒ is expected to be transferred at the TiO 2 -LaO interface, partially filling the 3d Ti levels of the STO conduction band ͑CB͒. Alternatively, a possible active role of oxygen vacancies in STO near the interface was envisaged. 3 Actually, the transport properties of the heterostructure are affected both by oxygen pressure during growth 4,5 and by the application of an oxygen postanneal. 5 Finally, it was argued that a substantial La substitution for Sr during sample growth might drive the insulating surface of STO into a conductor. [6][7][8] Obviously, also LAO poses material issues. 9 In this context, we started the search of novel heterostructures based on a different overlayer. On this basis, we identified as a first test material LaGaO 3 ͑LGO͒, a polar, wide band gap, pseudocubic perovskite.Films of LAO and LGO were deposited on nominally TiO 2 terminated STO substrates, chemically treated in deionized water and buffered-HF. 10,11 The growth was performed by reflection high energy electron diffraction ͑RHEED͒ as-sisted pulsed laser deposition ͑KrF excimer laser, 248 nm͒ with a typical fluence of Ϸ1.5-2.5 J cm −2 at the target, a substrate temperature of 800°C and different oxygen pressures within the 10 −2 -10 −4 mbar range. 12 LAO films presented regular RHEED oscillations typical of layer-by-layer growth and a final pattern reminiscent of a single crystal surface, whereas LGO films showed damped and less regular oscillations, and a streaky 2D pattern at the end of the growth ͑Fig. 1͒.The atomic and electronic structures of LAO/STO and LGO/STO interfaces were investigated by high-resolution scanning transmission electron microscopy ͑STEM͒ and electron energy loss spectroscopy ͑EELS͒ measur...

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Two-terminal multistate memory elements based on VO(2)/TiO(2) thin film microcantilevers are reported. Volatile and non-volatile multiple resistance states are programmed by current pulses at temperatures within the hysteretic region of the metal-insulator transition of VO(2). The memory mechanism is based on current-induced creation of metallic clusters by self-heating of micrometric suspended regions and resistive reading via percolation.

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