F. C. Hsu

Superconductivity in the PbO-type structure α-FeSe

The recent discovery of superconductivity with relatively high transition temperature (Tc) in the layered iron-based quaternary oxypnictides La[O1−xFx] FeAs by Kamihara et al. [Kamihara Y, Watanabe T, Hirano M, Hosono H (2008) Iron-based layered superconductor La[O1-xFx] FeAs (x = 0.05–0.12) with Tc = 26 K. J Am Chem Soc 130:3296–3297.] was a real surprise and has generated tremendous interest. Although superconductivity exists in alloy that contains the element Fe, LaOMPn (with M = Fe, Ni; and Pn = P and As) is the first system where Fe plays the key role to the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to search whether there exists other Fe based planar compounds that exhibit superconductivity. Here, we report the observation of superconductivity with zero-resistance transition temperature at 8 K in the PbO-type α-FeSe compound. A key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency. FeSe, compared with LaOFeAs, is less toxic and much easier to handle. What is truly striking is that this compound has the same, perhaps simpler, planar crystal sublattice as the layered oxypnictides. Therefore, this result provides an opportunity to better understand the underlying mechanism of superconductivity in this class of unconventional superconductors.

Tellurium substitution effect on superconductivity of the α-phase iron selenide

We have carried out a systematic study of the PbO-type compound FeSe1−xTex (x=0–1), where the Te substitution effect on superconductivity is investigated. It is found that the superconducting transition temperature reaches a maximum of Tc=15.2 K at about 50% Te substitution. The pressure-enhanced Tc of FeSe0.5Te0.5 is more than 10 times larger than that of FeSe. Interestingly, FeTe is no longer superconducting. A low-temperature structural distortion changes FeTe from triclinic symmetry to orthorhombic symmetry. We believe that this structural change breaks the magnetic symmetry and suppresses superconductivity in FeTe.

First-order magnetic and structural phase transitions in Fe1+ySexTe1-x

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe1+ySexTe1-x. Fe1.068Te exhibits a first-order phase transition near 67 K with a tetragonal-to-monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of the FeSe1-xTex system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density-functional calculations, where FeSe1-xTex and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density wave AF order.

Crystal orientation and thickness dependence of the superconducting transition temperature of tetragonal FeSe1-x thin films.

Superconductivity was recently found in the tetragonal phase FeSe. A structural transformation from tetragonal to orthorhombic (or monoclinic, depending on point of view) was observed at low temperature, but was not accompanied by a magnetic ordering as commonly occurs in the parent compounds of FeAs-based superconductors. Here, we report the correlation between structural distortion and superconductivity in FeSe(1-x) thin films with different preferred growth orientations. The films with preferred growth along the c axis show a strong thickness dependent suppression of superconductivity and low temperature structural distortion. In contrast, both properties are less affected in the films with (101) preferred orientation. These results suggest that the low temperature structural distortion is closely associated with the superconductivity of this material.

Preparation and Characterization of Thin Film Li4Ti5O12 Electrodes by Magnetron Sputtering

This paper reports that spinel-phase Li 4 Ti 5 O 1 2 thin films were successfully grown by radio frequency (rf) magnetron sputtering on an Au/Ti/SiO 2 /Si substrate. In this process, the buffer layer of gold serves as a template for the texture growth of Li 4 Ti 5 O 1 2 film. The growth temperature affects the microstructure and electrochemical characteristics of the deposited films. In our study, the spinel phase of Li 4 Ti 5 O 1 2 appears at deposition temperatures above 500°C. The redox peaks in the cyclic voltammetry of the Li/Li 4 Ti 5 O 1 2 cell approach the typical value of 1.55 V as raising the deposition temperature. Moreover, the influences of the surface morphology of the film on the capacity were studied. They show that a columnar structure with high porosity was obtained in the film deposited above 650°C. The columnar grains with good crystallinity of the deposited Li 4 Ti 5 O 1 2 enhance the capacity of the electrode. In this work, the capacity of 53 μAh/cm 2 μm can be attained for the film with a thickness of 230 nm deposited at 700°C. This study sheds light on the realization of a solid-state thin film battery and provides a possible solution of electrical power for a mobile integrated circuit chip.

Magnetic field dependence of low-temperature specific heat of the spinel oxide superconductor LiTi2O4

Magnetic field dependence of low temperature specific heat of spinel oxide superconductor LiTi_2O_4 has been elaborately investigated. In the normal state, the obtained electronic coefficient of specific heat gamma_n = 19.15 mJ/mol K2, the Debye temperature is 657 K and some other parameters are compared with those reported earlier. The superconducting transition at Tc ~ 11.4 K is very sharp (DeltaTc ~ 0.3 K) and the estimated delataC/gamma_nTc is ~ 1.78. In the superconducting state, the best fit of data leads to the electronic specific heat C_es/Gamma_nTc = 9.87 exp (-1.58 Tc/T) without field and gamma(H) ~ H^0.95 with fields. In addition, Hc2(0) ~ 11.7 T, Hc(0) ~ 0.32 T, xi_GL(0) ~ 5.5 nm, lambda_GL(0) ~ 160 nm, and Hc1(0) ~ 26 mT are estimated from Werthamer-Helfand-Hohenberg (WHH) theory or other relevant relations. All results from the present study indicate that LiTi2O4 can be well described by a typical type-II, BCS-like, moderate coupling, and fully gapped superconductor in the dirty limit. It is further suggested that LiTi2O4 is a moderately electron-electron correlated system.

Texture and Microstructure Development of RF Sputter-Deposited Polycrystalline Lithium Transition Metal Oxide Thin Films

Deposition of polycrystalline lithium transition metal oxide thin films has been performed using radio frequency magnetron sputter deposition on indium tin oxide substrates. Different substrate temperatures, up to 500°C, were applied during deposition. The films deposited at elevated temperature exhibited a strong texture (or preferred orientation). It was found that varying the substrate temperature resulted in changing film texture. As the substrate temperature increased, the film structure changed from noncrystalline to polycrystalline with enhanced texture. The film texture and crystallography were examined using X-ray diffraction and transmission electron microscopy. The cross section and surface morphology were observed using scanning electron microscopy. The compositions of the targets and films were characterized by inductively coupled plasma spectroscopy. The deposition technique was used to prepare textured LiCoO 2 , LiNiO 2 , LiNi 0.8 Co 0.2 O 2 , and LiMn 2 O 4 thin films. The change of texturing can be interpreted by volume strain energy and surface energy minimization. Film texture of lithium transition metal oxide thin films can be controlled by the substrate temperature.

Heterojunction of Fe(Se1−xTex) superconductor on Nb-doped SrTiO3

We report the fabrication of heterojunctions formed by the FeSe0.5Te0.5 (FeSeTe) superconductor and Nb-doped SrTiO3 semiconducting substrate and their properties. At high temperature when FeSeTe is in its normal state, the forward bias I−V curves behave like a metal-semiconductor junction with a low Schottky barrier. Direct tunneling through the thin depletion layer of the junction dominates the reverse bias I−V curves. When FeSeTe film becomes superconducting at low temperature, we observed that the Schottky barrier height of the junction increased but was suppressed by an external magnetic field. This deviation provides an estimate of the superconducting energy gap of the FeSeTe film.

Weak localization in FeSe1-xTex superconducting thin films

We have investigated the magneto-resistivity (MR) of FeSe1 xTex superconducting films on MgO substrate. The MR of pure FeSe and slightly Te-substituted films demonstrates regular Lorentz-type magnetic field dependence, MR B 2 . In highly Te-substituted samples, however, negative MR contribution due to the weak-localization effect gradually dominates at low temperature, which is consistent with the evolution of the temperature dependence of resistivity from a metallic to a weakly semiconductor-like behavior. Furthermore, the negative MR weakens and turns positive as temperature approaches the superconducting transition temperature, which is evidence for the Maki‐Thompson correction in the weak-localization regime. The experimental data can be described very well by the weak-localization theory with the existence of scattering by some magnetic moments. The fitting parameters demonstrate that disorder most likely comes from the excess iron. (Some figures may appear in colour only in the online journal)

The vortex state of FeSe1 − xTex superconducting thin films

We report the vortex dynamics of tellurium substituted FeSe1-xTex superconducting thin films. The electric field versus current density (E-J) curve for films with low Te substitution is still governed by the thermally activated flux flow model at temperatures as low as 0.5T(C)(offset). In contrast, we clearly observed a vortex liquid-glass transition in films with high Te substitution. The E-J curves of these samples fit nicely to the scaling relations based on the 3D vortex glass theory. Our results reveal an enhancement of the vortex pinning as more Te content is introduced, which probably originates from the excess Fe at the interstitial site.