Sharp Superconducting Transitions Research Articles

Structural and vibrational properties of cage-type Sc5Ru6Sn18 superconductor under pressure using synchrotron X-ray diffraction and Raman spectroscopy

We report detailed structural investigations of cage-type quasi-skutterudite compound Sc5Ru6Sn18 using synchrotron single-crystal and powder x-ray diffraction. Single-crystal x-ray diffraction studies at ambient temperature revealed that the Sc5Ru6Sn18 compound has a tetragonal symmetry in the I41/acd space group setting with a = 13.58Å and c = 27.14Å, similar to other R5Rh6Sn18 (R = Y, Sc, Lu) cage-type compounds. The electrical resistivity ρ(T) revealed the presence of a sharp superconducting (SC) transition at Tc = 4.07K. The ρ(T) of Sc5Ru6Sn18 above Tc shows a smooth decrease with decreasing temperature without any evident anomalies. Systematic temperature dependent synchrotron x-ray powder diffraction (XRPD) studies covering 400 to 110K shows that the Sc5Ru6Sn18 system remain in the tetragonal I41/acd structure in this temperature range without any anomalous behaviour in the unit-cell parameters. High-pressure (HP) synchrotron XRPD revealed a smooth reduction in the unit-cell parameters up to ~7GPa which can be well described by a third-order Birch-Murnaghan equation providing the bulk modulus of the system to be ~102.6GPa. This system is found to several Raman modes below 300cm-1. We could follow the pressure evolution of four Raman modes, all of which show a smooth hardening with pressure up to ~7GPa. Our results call for systematic HP studies of the SC properties of the title system.

Pressure Effects on Superconducting Quasiskutterudite Sr3Rh4Sn13 Compared to the Ambient Pressure Properties of Ca3Rh4Sn13 and La3Rh4Sn13 Compounds

Herein, the transport properties of Sr3Rh4Sn13, Ca3Rh4Sn13, and La3Rh4Sn13 quasi‐skutterudites are studied which show sharp superconducting (SC) transition with a transition temperature, Tc, ≈4.7, ≈8, and ≈2 K, respectively. In Sr3Rh4Sn13 system, electrical resistivity data evidence a pronounced anomaly before the SC transition at T* ≈136 K. No such anomalies are observed in the other two systems. Detailed structural investigations using synchrotron single‐crystal and powder diffraction reveal that these systems have cubic structure with Sr3Rh4Sn13 and Ca3Rh4Sn13 being in PmPmn space group setting. The La3Rh4Sn13 shows additional weak reflections which can be described by I4132 space group with almost doubled lattice parameters compared to Sr3Rh4Sn13 system. All three systems are found to show weak Raman modes below 240 cm−1. High‐pressure (HP) resistivity measurements up to 3.2‐GPa on Sr3Rh4Sn13 system reveal that the T* anomaly decreases almost linearly with pressure whereas the superconducting Tc almost linearly increases. HP structural investigations on Sr3Rh4Sn13 compound reveal that the structure remains in the cubic Pmn phase in 0–10 GPa range, following well a second‐order Birch–Murnaghan equation of state. The results provide interesting similarities and differences in the chemical and physical pressure effects on the quasi‐skutterudite X3Rh4Sn13 (x = Sr, Ca, and La) system.

High-performance Fe(Se,Te) films on chemical CeO2-based buffer layers

The fabrication of a Fe-based coated conductor (CC) becomes possible when Fe(Se,Te) is grown as an epitaxial film on a metallic oriented substrate. Thanks to the material’s low structural anisotropy, less strict requirements on the template microstructure allow for the design of a simplified CC architecture with respect to the REBCO multi-layered layout. This design, though, still requires a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template. In this work, Fe(Se,Te) films are grown on chemically-deposited, CeO2-based buffer layers via pulsed laser deposition, and excellent properties are obtained when a Fe(Se,Te) seed layer is used. Among all the employed characterization techniques, transmission electron microscopy proved essential to determine the actual effect of the seed layer on the final film properties. Also, systematic investigation of the full current transport properties J(θ, H, T) is carried out: Fe(Se,Te) samples are obtained with sharp superconducting transitions around 16 K and critical current densities exceeding 1 MA cm−2 at 4.2 K in self-field. The in-field and angular behavior of the sample are in line with data from the literature. These results are the demonstration of the feasibility of a Fe-based CC, with all the relative advantages concerning process simplification and cost reduction.

Chemical CeO2-Based Buffer Layers for Fe(Se,Te) Films

Among other Fe-based superconductors, Fe(Se,Te) is particularly interesting because of the low structural anisotropy, large upper critical fields, low field dependence of the critical current density and low toxicity. It can also be grown as an epitaxial film on a metallic oriented substrate, making the fabrication of a Fe-based coated conductor (CC) possible. Less strict requirements on the template microstructure allow for the design of a simplified design compared to REBCO CCs. This design requires a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template. In this work, CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> based buffer layers are prepared on single crystals via two chemical deposition techniques, metal organic decomposition (MOD) and polymer assisted deposition (PAD). With the design of a suitable thermal treatment, it is possible to obtain oriented buffers with large flat grains and low values of surface roughness. Fe(Se,Te) films are deposited on these templates via laser deposition, and excellent samples are obtained when a Fe(Se,Te) seed layer is used to favour chemical matching with the buffer: sharp superconducting transitions around 16 K and critical current densities exceeding 1 MA cm <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⁻</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K in self-field are observed. These results are the demonstration of the feasibility of a Fe-based CC architecture, with all the relative advantages concerning process simplification and cost reduction.

Realization of epitaxial thin films of the superconductor K-doped BaFe2As2

The iron-based superconductor Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$ thin films by molecular-beam epitaxy with employing a combination of fluoride substrates (CaF$_\text{2}$, SrF$_\text{2}$, and BaF$_\text{2}$) and a low growth temperature (350$-$420$^\circ$C). Our epitaxial thin film grown on CaF$_\text{2}$ showed sharp superconducting transition at an onset critical temperature of 36 K, slightly lower than bulk crystals by ~2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density ($J$$_\text{c}$) determined by the magnetization hysteresis loop is as high as 2.2 MA/cm$^\text{2}$ at 4 K under self-field. In-field $J$$_\text{c}$ characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$.

Type-II superconductivity below 4K in Sn0.4Sb0.6

In this article, we report the occurrence of superconductivity in Sn0.4Sb0.6 single crystal at below 4K. Rietveld refined Powder XRD data confirms the phase purity of as grown crystal, crystallizing in rhombohedral R-3m space group with an elongated (2xc) unit cell in c-direction. Scanning Electron Microscope (SEM) image and EDAX measurement confirm the laminar growth and elemental composition to be near to desired stoichiometry ratio. Raman Spectroscopy data shows the vibrational modes of Sn–Sb and Sb–Sb modes at 110 and 135 cm−1. ZFC (Zero-Field-Cooled) magnetization measurements done at 10Oe showed sharp superconducting transitions at 4K along with a minor step at 3.5K. On the other hand, Paramagnetic Meissner Effect (PME) is observed in FC measurements. Magnetization vs applied field (M − H) plots at 2, 2.2, 2.5, 2.7, 3, 3.2, 3.5, and 3.7K show typical Type-II nature of observed superconductivity with lower and upper critical fields (Hc1 and Hc2) at 69.42Oe and 630Oe respectively at 2K. Type-II superconductivity is also confirmed by calculated Ginzburg-Landau Kappa parameter value of 3.55. Characteristics length viz. coherence length and penetration depth are also calculated. Weak granular coupling is observed from ρ-T plot, in which resistance is not dropping to zero down to 2K.

NbN films on vicinal to the X-cut of LiNbO3 surfaces

We report a study of epitaxial superconductive NbN films on vicinal to the X-cut of single crystal LiNbO3 substrates grown using reactive magnetron sputtering of Nb target in Ar-N2 atmosphere. It is found that the NbN films reveal sharp superconductive transition at TC = 15.2 K and anisotropy of critical current. Critical current measured along the Z direction exceeds JC along the Y direction regardless of the orientation of step edges of the vicinal surfaces. The anisotropy effect is attributed to NbN film uniaxial stress due to the lattice mismatch of NbN (0 0 1) layer along the Z direction of LiNbO3.

Processing and Characterization of Charcoal Added Bulk MgB2 Superconductor

In this study, we focus on methods to further improve the flux pinning and critical current density (Jc) of disk shaped MgB2 bulk superconductors by a comparative study with locally obtained charcoal at optimized processing conditions. The doping of charcoal was varied at 0, 0.2, 0.4 and 0.6 wt%. Bulk MgB2 samples were produced by in-situ solid state reaction in Ar gas and ambient pressures at 775 °C, employing high purity of Mg metal and nano B powder mixed in a fixed ratio of Mg:B = 1:2 and optimal conditions. The phase has been investigated by X-ray diffractometer (XRD). The effects of the dopants on superconducting properties especially the field dependence of Jc have been determined by superconducting quantum interference device (SQUID) and critical temperature (Tc). Measurements of the magnetic susceptibility showed sharp superconducting transitions (ΔTc ∼ 1 K). Jc at self-field and 20 K was determined to be ~280 and 467 kA/cm2 for the 0 and 0.4 wt% charcoal added samples respectively. The structural and superconducting results were correlated with the addition of charcoal in the final product.

Structural and Superconducting Properties of Thermal Treatment-Synthesised Bulk YBa₂Cu₃O7-δ Superconductor: Effect of Addition of SnO₂ Nanoparticles.

YBa2Cu3O7−δ (Y-123) bulk superconductors with the addition of (0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 wt.%) SnO2 nanoparticles were synthesised via a thermal treatment method. The influence of SnO2 addition on the superconducting properties by means of critical temperature, Tc, AC susceptibility, phase formation and microstructures, including its elemental composition analysis, were studied. Sharp superconducting transition, ∆Tc, and diamagnetic transition were obtained for all SnO2-added samples. It was observed that sample x = 0.4 with a Y-123 phase percentage of 95.8% gives the highest Tc, smallest ∆Tc, and the sharpest diamagnetic transition in the normalised susceptibility curves. The microstructure also showed an excess of Sn precipitates on the sample’s surface at x = 0.8 and above. As such, the best superconducting properties were observed at x = 0.4 SnO2 addition inside the Y-123 host sample.

Growth and characterization of HgBa2CaCu2O6+δ and HgBa2Ca2Cu3O8+δ crystals

We report the successful synthesis of single-crystalline cuprate superconductors HgBa$_{2}$CaCu$_{2}$O$_{6+\delta}$ and HgBa$_{2}$Ca$_{2}$Cu$_{3}$O$_{8+\delta}$. These compounds are well-known for their high optimal superconducting critical temperatures of $T_\mathrm{c}$ = 128 K and 134 K at ambient pressure, respectively, and for their challenging synthesis. Using a conventional quartz-tube encapsulation method and a two-layer encapsulation method that utilizes custom-built high-pressure furnaces, we are able to grow single crystals with linear dimensions up to several millimeters parallel to the CuO$_2$ planes. Extended post-growth anneals are shown to lead to sharp superconducting transitions, indicative of high macroscopic homogeneity. X-ray diffraction and polarized Raman spectroscopy are identified as viable methods to resolve the seemingly unavoidable inter-growth of the two compounds. Our work helps to remove obstacles toward the study of these model cuprate systems with experimental probes that require sizable high-quality crystals.

Development of Fe-based superconducting wires for liquid-hydrogen level sensors

We developed liquid-hydrogen (LH2) level sensors with Ba(Fe1-xCox)2As2 superconducting wires (Co-Ba122 wires) as their detection elements. We fabricated Co-Ba122 wires with different Co concentrations x by using the powder-in-tube method. The superconducting transition temperatures of the wires were successfully controlled in the range of 20–25 K by changing x from 0.06 to 0.10. The resistance–temperature curves of the wires exhibited sharp superconducting transitions with widths of 0.5–1.0 K. In addition, we performed an operation test of the Co-Ba122 level sensors with LH2. Close correspondence between the output resistance and the actual LH2 level was observed for a sensor equipped with x = 0.09 wire, demonstrating that this sensor can accurately measure LH2 levels.

Controlled growth of ultrathin Mo2C superconducting crystals on liquid Cu surface

Exhibiting thickness-dependent change in the critical temperature (Tc) for the onset of superconductivity, Mo2C has emerged as an important new member in the family of two-dimensional atomic crystals. Controllable growth in terms of morphology and thickness is necessary to elucidate its intrinsic properties at the 2D limit. Here we demonstrate the chemical vapor deposition of ultrathin Mo2C crystals on liquid Cu surface where the morphology of the crystals can be controlled by tuning the carbon supersaturation. A unique staggered carbon vacancy ordering is discovered in Mo2C crystals having particular geometries. Thickness engineering of the crystal can be achieved by controlling the thickness of the Cu catalyst layer, which affords a facile route to grow ultrathin 2D samples. Ultrathin Mo2C crystals so obtained, have been characterized using aberration corrected scanning transmission electron microscopy annular dark field imaging, where the co-existence of both AA and AB stacking modes is observed. The high crystallinity of the Mo2C crystals synthesized in this work is attested by its characteristic sharp superconducting transition.

Comprehensive scenario for single-crystal growth and doping dependence of resistivity and anisotropic upper critical fields in (Ba1−xKx)Fe2As2(0.22≤x≤1)

Large high-quality single crystals of hole-doped iron-based superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ were grown over a broad composition range $0.22 \leq x \leq 1$ by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and adjusted temperature window of the growth are necessary to obtain single crystals of heavily K doped crystals (0.65$\leq x \leq$ 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the $c$-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity, $\rho(T)$ found doping-independent, constant within error bars, resistivity at room temperature, $\rho(300K)$, in sharp contrast with significant doping dependence in electron and isovalent substituted BaFe$_2$As$_2$ based compositions. The shape of the temperature dependent resistivity, $\rho(T)$, shows systematic doping-evolution, being close to $T^2$ in overdoped and revealing significant contribution of the $T$-linear component at optimum doping. The slope of the upper critical field, $d H_{c2}/dT$, scales linearly with $T_c$ for both $H\parallel c$, $ H_{c2,c}$, and $H \parallel ab$, $H_{c2,ab}$. The anisotropy of the upper critical field, $\gamma \equiv H_{c2,ab} / H_{c2,c}$ determined near zero-field $T_c$ increases from $\sim$2 to 4-5 with increasing K doping level from optimal $x \sim$0.4 to strongly overdoped $x$=1.

Development of Multilayer Readout Wiring TES Calorimeter for Future X-ray Missions

We have fabricated multilayer readout wiring Transition Edge Sensors (TES), which enable us to realize both large effective area and high-energy resolution for future X-ray astrophysical missions, such as DIOS. By sandwiching a SiO2 insulation layer between Al superconducting signal and return lines, self/mutual inductances and self fielding of bias leads are expected to be reduced. We fabricated 4x4 and 20x20 TES array on the multilayer wiring and tested their performance. Under the low temperature condition, several pixels in the TES array showed sharp superconducting transitions at around ~300 mK. We also succeeded in detecting X-ray signals from the 4x4 TES, contrary to the previous results of 20x20 TES. We further investigated the reasons for the differences between the 4x4 TES and the 20x20 TES, and present future plans for improving the multilayer TES array fabrication.

Development of Superconducting Multilayer Wiring for a 400-Pixel TES X-ray Microcalorimeter Array

Aiming at a large array of transition edge sensors for future astrophysical missions such as DIOS, we have been developing a multilayer readout wiring. Superconducting signal and return lines made of Al sandwich a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> insulation layer. With this approach, self and mutual inductances are minimized and self fielding of bias leads can be reduced. We fabricated a 20 × 20 transition-edge sensor (TES) array on the multilayer wiring. A series of TES fabrication processes including absorber deposition and membrane formation were tested. However, resistance measurements at room temperature suggest that process yield is not high (~50 %). At low temperature, sharp superconducting transitions were observed for normal TES pixels, while their residual resistances were high (>;10 mΩ) and the critical currents were low (<;10 μA). Reasons for these phenomena and plans for a future TES array fabrication are discussed.

Splitting of the pairing transition and coherence transition in R1−xCexBa2Cu3O7−δ (R = Y, Er, Ho and Gd)

Systematic conductivity measurements in granular R1−xCexBa2Cu3O7−δ (R = Y, Er, Ho and Gd) superconductors are presented. The focus is the role of the cerium ion on the superconductor–normal transition. The results showed that the resistive transition proceeds in two stages, intragranular and intergranular, as seen by the temperature derivative of the resistivity near TC. Therefore, from the critical exponent analysis, we have also observed two close and sharp genuine superconducting transitions in all samples investigated. Two critical temperatures were identified, but only one is strongly affected by Ce doping. The splitting of the pairing transition was associated with Ce and related to the occurrence of a phase separation. R1−xCexBa2Cu3O7−δ can be regarded as a mixed crystalline solid solution consisting of two crystal unit cells: R-123 (R cell) and Ce-123 (Ce cell), where a two-dimensional (2D) site-percolation structure is built along the ab plane. The ionic size effect of rare earth element R on the superconductor–normal transition has been investigated and it was observed that the temperature splitting Δ varies between approximately 4.2 K for Ho to 12.1 K for Gd.

Superconductivity and magnetic properties of single crystals ofK0.75Fe1.66Se2and Cs0.81Fe1.61Se2

We successfully grew the high-quality single crystals of ${A}_{x}$Fe${}_{2}$Se${}_{2}$ ($A$ $=$ K and Cs) by self-flux method. Sharp superconducting transition was observed for both types of crystals. The crystals show the onset superconducting transition temperatures (${T}_{\mathrm{c}}$) of 31 and 30 K for K and Cs compounds, respectively, with nearly 100$%$ superconductive shielding fraction. The crystals show quite high resistivity in the normal state of more than 160 and 1300 $\mathrm{m}\ensuremath{\Omega}$ cm maximum resistivity for ${\mathrm{K}}_{x}$Fe${}_{2}$Se${}_{2}$ and Cs${}_{x}$Fe${}_{2}$Se${}_{2}$ single crystals, respectively. Much larger upper critical field ${H}_{\mathrm{c}2}$ is inferred from low-temperature isomagnetic-field magnetoresistance in these crystals than in FeSe. The anisotropy ${H}_{\mathrm{c}2}^{\mathit{ab}}$(0)/${H}_{\mathrm{c}2}^{c}$(0) is estimated to be around 3 for both of the two materials. Anisotropic peculiar magnetic behavior in normal state has been found for Cs${}_{x}$Fe${}_{2}$Se${}_{2}$

Development of Multilayer Readout Wiring for Large-Format TES X-Ray Microcalorimeter Arrays

Pursuing the feasibility of a large array of transition-edge sensors for future astrophysical missions, we have undertaken a study of multilayer readout wiring. It is composed of thin superconducting signal and return wires made of Al or Nb that sandwich an insulation SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film. Self and mutual inductances between signal and return wires of pixels, and also self fielding of bias leads can be reduced. Also a necessary spatial resource can be half compared to a normal wiring. We have fabricated four types of 20 × 20 wiring samples whose upper and lower wiring widths were 10 and 15 μm. High process yields of 95 ~ 97% were confirmed for all the four arrays in resistance measurements at room temperature. At low temperature, Al-Al samples showed sharp superconducting transitions, low residual resistances 1 ~ 2 mΩ, and high critical currents of >; 100 μA, which suggests that they can be used as our TES readout wiring. On the other hand, Nb-Al samples showed slow transitions and low critical currents ( <; 10 μA). Energy dispersive X-ray spectroscopy measurements indicated that impurities between Al and Nb films at electrical contacts influence these features.

Nanocrystalline boron‐doped diamond films, a mixture of BCS‐like and non‐BCS‐like superconducting grains

AbstractScanning tunneling topography and spectroscopy are performed below 100 mK on granular nanocrystalline boron‐doped diamond (BDD) films. We found the superconductivity behavior to follow mainly the granular features of the BDD films. The temperature dependence of the local differential conductance spectra shows our nanocrystalline BDD films as made of grains with a supercondutivity either BCS‐like or non‐BCS‐like. Such a distribution is not discernible in transport measurements, which present a sharp macroscopic superconducting transition at a temperature of a few Kelvins. Our local scanning tunneling microscopies also confirm the good coupling between these grains: only a few opaque interfaces are detected. Such a transparency of intergrain interfaces is responsible for a proximity effect in weakly superconductive grains and an inverse proximity effect in strongly superconducting grains.

Flux pinning and superconducting properties of melt-textured NEG-123 superconductor with TiO 2 addition

We studied the effect of TiO 2 doping on flux pinning and superconducting properties of a melt-grown (Nd 0.33Eu 0.33Gd 0.33) Ba 2Cu 3O y + 35 mol% Gd 2BaCuO 5 (70 nm in size) composite (NEG-123) processed in Ar–1% O 2 atmosphere. As indicated by similar, sharp superconducting transitions, the small quantities of TiO 2 used in our experiments did not deteriorate superconducting properties of the NEG material. Transmission electron microscopy (TEM) analysis found 20–50 nm Ti-based particles in the NEG-123 matrix. However, we have not observed the clouds of <10 nm sized particles in the NEG-123 matrix, as in the case of recently reported NEG-123 composites doped by Mo and Nb nanoparticles. Nevertheless, quite a good J c – B performance in the 0.1 mol% Ti-doped sample, namely 550 kA/cm 2 at the self-field and at the secondary peak field (4.5 T) was achieved at 65 K, while 320 kA/cm 2 was obtained at zero-field at 77 K, and 50 kA/cm 2 at 90.2 K. The pinning effectiveness decreased with increasing Ti content above 0.2 mol%. The analysis of the pinning force showed that higher concentration of Ti (>0.2 mol%) increased the amount of normal pins ( δl pinning), indicated by the Fp( h) peak shift from h = 0.42–0.36. The maximum pinning effect in a broad field range could be achieved by optimizing Ti content and adding sub-micron Gd-211 particles.