Tuning superconductivity in FeSe thin films via magnesium doping

In contrast to its bulk crystal, the FeSe thin film or layer exhibits better superconductivity performance, which recently attracted much interest in its fundamental research as well as in potential applications around the world. In the present work, tuning superconductivity in FeSe thin films was achieved by magnesium-doping technique. Tc is significantly enhanced from 10.7 K in pure FeSe films to 13.4 K in optimized Mg-doped ones, which is approximately 1.5 times higher than that of bulk crystals. This is the first time achieving the enhancement of superconducting transition temperature in FeSe thin films with practical thickness (120 nm) via a simple Mg-doping process. Moreover, these Mg-doped FeSe films are quite stable in atmosphere with Hc2 up to 32.7 T and Tc zero up to 12 K, respectively, implying their outstanding potential for practical applications in high magnetic fields. It was found that Mg enters the matrix of FeSe lattice, and does not react with FeSe forming any other secondary phase. Actually, Mg first occupies Fe-vacancies, and then substitutes for some Fe in the FeSe crystal lattices when Fe-vacancies are fully filled. Simultaneously, external Mg-doping introduces sufficient electron doping and induces the variation of electron carrier concentration according to Hall coefficient measurements. This is responsible for the evolution of superconducting performance in FeSe thin films. Our results provide a new strategy to improve the superconductivity of 11 type Fe-based superconductors and will help us to understand the intrinsic mechanism of this unconventional superconducting system

Three-Phase-Based Approach to Develop a River Health Prediction and Early Warning System to Guide River Management

To effectively manage a river system, systematic tracking and diagnosing the change and risks of a river system are essentially required to efficiently conserve or restore its conditions. Hence, this study focuses on how to integrate current status assessment, trend prediction, and cause diagnosis in river health to guide early warning decision-making in river protection and management. This study has presented a three-phase approach by coupling spatial with nonspatial information in a highly systematic and reliable way, and an early warning system has been designed. In phase I, the current health status is assessed and nowcasted by using the order degree of each indicator. In phase II, health predictors, including the single perspective-based health index (HI) (e.g., water quality index (WQI) and index of biotic integrity (IBI)) and multi-perspective-based health index, have been forecasted under normal conditions or emerging conditions using predictive models. In phase III, key causal factors threatening the river health have been identified to enable early notification and to address unexpected events before occurrence. Although different modeling methods can be used in each phase to demonstrate this concept, we tested the model of partial least square regression (PLSR) associated with time series. Additionally, the three-phase approach has been integrated with geographic information system (GIS) and a decision support system (DSS) to develop a river health prediction and early warning system (RHP-EWS), an automatic prediction and decision-making tool. This tool was implemented to deal with the landing of typhoon “Maria” in 2018 into the Shanxi River watershed in China. Because of the timely responses and decisions, the drinking water supply was not influenced. However, the models should be extended to other river systems for testing and improvement at different temporal or spatial scales

In situ hydrostatic pressure induced improvement of critical current density and suppression of magnetic relaxation in Y(Dy-0.5)Ba2Cu3O7-(delta) coated conductors

We report on the effect of in situ hydrostatic pressure on the enhancement of the in-magneticfield critical current density parallel to the crystallographic c-axis and vortex pinning in epitaxial Y(Dy0.5)Ba2Cu3O7−δ coated conductors prepared by metal organic deposition. Our results show that in situ hydrostatic pressure greatly enhances the critical current density at high fields and high temperatures. At 80 K and 5 T we observe a ten-fold increase in the critical current density under the pressure of 1.2 GPa, and the irreversibility line is shifted to higher fields without changing the critical temperature. The normalized magnetic relaxation rate shows that vortex creep rates are strongly suppressed due to applied pressure, and the pinning energy is significantly increased based on the collective creep theory. After releasing the pressure, we recover the original superconducting properties. Therefore, we speculate that the in situ hydrostatic pressure exerted on the coated conductor enhances the pinning of existing extended defects. This is totally different from what has been observed in REBa2Cu3O7−δ melt-textured crystals, where the effect of pressure generates point-like defects

Point defect induced giant enhancement of flux pinning in Co-doped FeSe0.5Te0.5 superconducting single crystals

Point defect pinning centers are the key factors responsible for the flux pinning and critical current density in type II superconductors. The introduction of the point defects and increasing their density without any changes to the superconducting transition temperature T c , irreversibility field H irr , and upper critical field H c2, would be ideal to gain insight into the intrinsic point-defect-induced pinning mechanism. In this work, we present our investigations on the critical current density J c , H c2 , H irr , the activation energy U 0 , and the flux pinning mechanism in Fe 1-x Co x Se 0.5 Te 0.5 (x = 0, 0.03 and 0.05) single crystals. Remarkably, we observe that the J c and U 0 are significantly enhanced by up to 12 times and 4 times for the 3at.% Co-doped sample, whereas, there is little change in T c , H irr , and H c2 . Furthermore, charge-carrier mean free path fluctuation, δl pinning, is responsible for the pinning mechanism in Fe 1-x Co x Se 0.5 Te 0.5

Research Progress of Electromagnetic Properties of MgB2 Induced by Carbon-Containing Materials Addition and Process Techniques

© 2020, The Chinese Society for Metals (CSM) and Springer-Verlag GmbH Germany, part of Springer Nature. For the high transition temperature (Tc) and low cost taking both raw materials and fabrication process into account, MgB2 has been a competitive candidate to replace the conventional NiTi superconductor for high-temperature application in fault current limiters, transformers, motors, magnetic resonance imaging, adiabatic demagnetization refrigerators, generators, etc. The carbon-containing materials addition induced high critical current density (Jc) is reviewed based on their influences on the upper critical field (Hc2), flux pinning force, and connectivity. The doping effects were compared in the overview focusing on SiC, organic dopants, and graphene-related dopants. SiC doping is featured for the high-field critical current density, which is caused by the increased Hc2 attributed to the substitution of carbon on boron site and the strong flux pinning force offered by the nanosized secondary phases in the MgB2 matrix. Organic dopants have the advantage over SiC dopant for their relatively homogeneous distribution in the MgB2 matrix based on wet mixing of the organics and the raw boron powders. Low doping level of two-dimensional materials can improve the superconducting properties in all measured fields because of the combined advantages of carbon substitution effect and grain connectivity. MgB2 fabricated with carbon-encapsulated boron also introduces strong flux pinning centers in MgB2, which show weak destruction of the connectivity of the MgB2 grains as reflected by the low-magnetic-supercurrent behavior. High-pressure treatment and diffusion method can fabricate high-density MgB2 superconductors with better connectivity and increase the Jc compared with the in situ and ex situ methods

Enhanced superconductivity induced by several-unit-cells diffusion in an FeTe/FeSe bilayer heterostructure

Unlike monolayer Fe-Chalcogenide (Fe-Ch)/SrTiO3 (STO), which possesses the potential for high-temperature superconductivity (HTS), a regular Fe-Ch thin film grown on a non-STO substrate by the pulsed laser deposition method shows totally different superconducting behavior and a different mechanism. Although regular Fe-Ch thick films grown on CaF2 generally show the highest superconducting transition temperature (Tc) compared with any other substrates, the disappearance of superconductivity always takes place when the thickness of the Fe-Ch film is reduced to a critical value (∼20nm for Fe-Se and ∼30nm for Fe-Se-Te) with the reason still under debate. Here, we report an enhanced Tc≈17.6K in a 7-nm-FeTe/7-nm-FeSe bilayer heterostructure grown on CaF2 substrate. Generally, the Fe-Ch film on CaF2 is supposed to be one order of magnitude greater in thickness to achieve similar performance. Hall measurements manifest the dominant nature of hole-type carriers in the films in this work, which is similar to the case of a pressurized bulk FeSe single crystal, while in sharp contrast to heavily electron-doped HTS Fe-Ch systems. According to the electron energy loss spectroscopy results, we observed direct evidence of nanoscale phase separation in the form of a fluctuation of the Fe-L3/L2 ratio near the FeTe/FeSe interface. In detail, a several-unit-cell-thick Fe(Se,Te) diffusion layer shows a higher Fe-L3/L2 ratio than either an FeTe or an FeSe layer, indicating low Fe 3d electron occupancy, which is, to some extent, consistent with the hole-dominant scenario obtained from the Hall results. It also implies a possible relationship between the state of Fe 3d electron occupancy and the enhanced Tc in this work. Our work clarifies the importance of the FeTe/FeSe interface in reviving the superconductivity in Fe-Ch ultrathin films, contributing to a more unified understanding of unconventional Fe-Ch superconductivity

Point defect induced giant enhancement of flux pinning in Co-doped FeSe0.5Te0.5 superconducting single crystals

Point defect pinning centers are the key factors responsible for the flux pinning and critical current density in type II superconductors. The introduction of the point defects and increasing their density without any changes to the superconducting transition temperature T-c, irreversibility field H-irr, and upper critical field H-c2, would be ideal to gain insight into the intrinsic point-defect-induced pinning mechanism. In this work, we present our investigations on the critical current density J(c), H-c2, H-irr, the activation energy U-0, and the flux pinning mechanism in Fe1-xCoxSe0.5Te0.5 (x = 0, 0.03 and 0.05) single crystals. Remarkably, we observe that the J(c) and U-0 are significantly enhanced by up to 12 times and 4 times for the 3at.% Co-doped sample, whereas, there is little change in T-c, H-irr, and H-c2. Furthermore, charge-carrier mean free path fluctuation, delta l pinning, is responsible for the pinning mechanism in Fe1-xCoxSe0.5Te0.5

In-situ hydrostatic pressure induced significant suppression of magnetic relaxation and enhancement of flux pinning in Fe1−xCoxSe0.5Te0.5 single crystals

We report the first study on the significant effect of in-situ hydrostatic pressure on the magnetic relaxation in Fe1−xCoxSe0.5Te0.5 single crystals. We find that vortex creep rates are significantly suppressed by pressure, and a crossover from elastic to plastic creep is observed. The pressure also induces vortex creep to move from the large bundle to the small bundle region. Our study indicates that in-situ hydrostatic pressure is very effective for not only significantly increasing the pinning energy and the critical current density, but also reducing the size of flux bundles to suppress the decrease in current density from vortex motion

The Interface Structure of FeSe Thin Film on CaF2 Substrate and its Influence on the Superconducting Performance

The investigations into the interfaces in iron selenide (FeSe) thin films on various substrates have manifested the great potential of showing high-temperature-superconductivity in this unique system. In present work, we obtain FeSe thin films with a series of thicknesses on calcium fluoride (CaF2) (100) substrates and glean the detailed information from the FeSe/CaF2 interface by using scanning transmission electron microscopy (STEM). Intriguingly, we have found the universal existence of a calcium selenide (CaSe) interlayer with a thickness of approximate 3 nm between FeSe and CaF2 in all the samples, which is irrelevant to the thickness of FeSe layers. A slight Se deficiency occurs in the FeSe layer due to the formation of CaSe interlayer. This Se deficiency is generally negligible except for the case of the ultrathin FeSe film (8 nm in thickness), in which the stoichiometric deviation from FeSe is big enough to suppress the superconductivity. Meanwhile, in the overly thick FeSe layer (160 nm in thickness), vast precipitates are found and recognized as Fe-rich phases, which brings about degradation in superconductivity. Consequently, the thickness dependence of superconducting transition temperature (Tc) of FeSe thin films is investigated and one of our atmosphere-stable FeSe thin film (127 nm) possesses the highest Tconset/Tczero as 15.1 K/13.4 K on record to date in the class of FeSe thin film with practical thickness. Our results provide a new perspective for exploring the mechanism of superconductivity in FeSe thin film via high-resolution STEM. Moreover, approaches that might improve the quality of FeSe/CaF2 interfaces are also proposed for further enhancing the superconducting performance in this system

Hydrostatic pressure-induced huge enhancement of critical current density and flux pinning in Fe1-xCoxSe0.5Te0.5 single crystals

We performed a systematic study of the hydrostatic pressure (HP) effect on the superconducting transition temperature (T-c), critical current density (J(c)), irreversibility field (H-irr), upper critical field (H-c2), and flux pinning mechanism in un-doped and 3 at.% Co-doped FeSe0.5Te0.5 crystals. We found that T-c is increased from 11.5 to 17 K as HP increases from 0 to 1.2 GPa. Remarkably, the J(c) is significantly enhanced by a factor of 3 to 100 for low and high temperature and field, and the H-irr line is shifted to higher fields by HP up to 1.2 GPa. Based on the collective pinning model, the delta l pinning associated with charge-carrier mean free path fluctuation is responsible for the pinning mechanism of Fe1-xCoxSe0.5Te0.5 samples with or without pressure. A comprehensive vortex phase diagram in the mixed state is constructed and analysed for the 3 at.% Co-doped sample