Flux Flow Behavior Research Articles

Influences of quenching treatment and cobalt-doping on the thermally activated flux-flow behavior in KCa2(FeCo )4As4F2

Owing to the unique double-FeAs-layered structure between neighboring insulating layers, the 12442-type Fe-based superconductors are expected to host rich vortex phenomena in the mixed state. Here, we report a systematic investigation on the influences of quenching treatment and cobalt-doping on the magnetic vortex dynamics of KCa2(FeCo x )4As4F2 (x = 0, 0.1) single crystals via electrical transport measurements. It is found that the slopes of the upper critical field near Tc , along both directions ( and ), are doubled by the cobalt-doping with x = 0.1. The activation energy is acquired by analyzing the Arrhenius plots, which shows a H −α dependence with a change in the exponent α at around 3 T, indicating the evolution of the dominant pinning mechanisms in different field regions. Importantly, the quenching treatment obviously enhances the magnitude of activation energy and critical current density J c, which can be suppressed by the introduction of cobalt-doping. Possible mechanisms for such evolutions of these important parameters are discussed. Our results emphasize the important role of quenching treatment in enhancing the activation energy and critical current density in 12442 system.

Optimization of the superconducting properties of NbTiN thin films by variation of the N2 partial pressure during sputter deposition

We report the effect of nitrogen partial pressure on the growth of niobium-titanium nitride (NbTiN) thin films through reactive sputtering of the NbTi target in the presence of Ar and N2 atmosphere. The N2 partial pressure was varied from 5.8% to 15.15% with respect to the Ar flow available in the chamber. We measured a critical temperature (T C) as high as 15.77 K for an N2 partial pressure of 6.8% for a 50 nm film deposited on an MgO substrate. The epitaxial growth of the NbTiN films is evident from the observed (200) and (400) XRD peaks with respect to the substrate. The superconducting properties were analyzed with respect to the observed values of T C, ΔT C, temperature dependence of the upper critical field (B C2), coherence length (ξ), diffusion coefficient (D), and the thermally activated flux flow behavior following the Arrhenius equation. The T C variation observed for the entire spectrum of N2 partial pressure variation is ∼11%, with T C getting saturated towards the higher end of the N2 content. While the T C variation with respect to N2 content is non-linear, the variation of B C2(0), diffusion coefficient, and zero temperature coherence length ξ(0) show a linear dependence. While the B C2(0) values show an increasing trend, D and ξ(0) values show a decreasing trend with respect to the increase of N2 partial pressure. It is concluded that the optimum window of N2 partial pressure for the reactive sputtering of NbTiN using a target of Nb0.7Ti0.3 for the present experimental conditions, in terms of epitaxial growth and obtaining the highest T C is within the range of 5.8% to 8.51%.

Numerical investigation of heat transfer and fluid motion in air inflatable textiles: Effect of thickness, surface emissivity and ambient temperature

Air inflatable textile has attracted increasing attention due to the advantage of light, cheap, environmentally friendly and low thermal conductivity. While its thermal insulation is still not yet well understood since the complex interior heat and mass transfer under various thickness and ambient conditions. In this work, a three-dimensional model was developed to investigate the fluid motion and heat transfer in both air inflatable and fiber-filled textiles. The coupled conduction, convection and radiation were modelled by CFD approach. The experimental and numerical results were compared to verify the proposed model. Good agreements were found with the average difference in surface temperature and heat flux less than 2.54% and 9.95%, respectively. Then the effect of various parameters including thickness, surface emissivity and ambient temperature on heat flux, thermal resistance and fluid flow behavior was further analyzed. The simulated interior velocity and temperature distribution suggested that the higher thickness (≥10 mm) was not favorable for better thermal insulation when the air was adopted as thermal insulation layer. Because more available space for air movement would accelerate thermal convection and result in the deteriorated thermal insulation. In addition, inner surface layer with lower emissivity could effectively suppress the thermal radiation in air inflatable textiles and was beneficial to increase the thermal insulation. Besides, the thermal insulation of fiber-filled textile was nearly unaffected when lower ambient temperature was applied, while that of air inflatable textiles significantly decreased due to the aggravated convective heat and flow transfer under large temperature difference.

Anisotropic thermally activated flux-flow behavior in the layered superconductor 2M−WS2

$2M\ensuremath{-}{\mathrm{WS}}_{2}$ is a newly discovered superconductor with a rather high critical transition temperature $({T}_{c}=8.8\phantom{\rule{4pt}{0ex}}\mathrm{K})$ and topological surface states. Recently Majorana bound states were observed in magnetic vortices of this material. Thus uncovering the flux dynamics of the magnetic vortices is essential to further promote the physical understanding and its practical application in topological quantum computation. Here we report an in-depth investigation on this issue by the temperature- and field-dependent electric transport measurements. The magnitudes of activation energy under both field orientations are rather low, revealing a weak pinning strength of the flux in the present system. Moreover, clear anisotropic thermally activated flux-flow behaviors are revealed. Under the in-plane field ($H\ensuremath{\parallel}bc$), the activation energy $U$ shows a $(1\ensuremath{-}T/{T}_{c}){H}^{\ensuremath{-}\ensuremath{\alpha}}$ dependence indicating the three-dimensional feature, while under the out-of-plane field $(H\ensuremath{\perp}bc)$, $U$ is proportional to $(1\ensuremath{-}T/{T}_{c})lnH$, which suggests a two-dimensional liquid state for the vortices.

Thermally Activated Flux Flow Characteristics: MgB2 Tapes

Thermally activated flux flow (TAFF) behavior is studied via magnetoresistance investigations on in situ prepared stainless steel sheathed MgB2 tapes. The thermal activation energy is analyzed using Arrhenius relation and a modified TAFF method. Temperature-dependent prefactor and nonlinear relation of U(H, T) versus T are considered to evaluate apparent activation energy U0. Results from modified TAFF method are found to be in better agreement with experimental data than that from using Conventional Arrhenius method. The results shows that Stainless steel sheathed MgB2 tapes prepared with stoichiometric composition at 800 °C exhibits high U0 of the order of 10 5 K.

Three-Terminal Superconducting Digital Transistor

Discrete array of small Josephson junctions can exhibit flux-flow behavior, which is drastically different from that of long Josephson junctions. In particular, the flux-flow voltage does not monotonically increase with magnetic field; rather, it decreases after a threshold until it reaches zero. Here, using simulation and experimental results, we show that the flux-flow voltage switches abruptly to zero if a magnetic field larger than the threshold is applied to a part of the array. This provides for a digital NOT gate in which the flux-flow voltage corresponds to digital state 1 and zero voltage denotes digital state 0. For a device size of 230 μm, a switching frequency of 15 GHz is evaluated, which can be improved by downsizing the device. Since this device provides gain, its input and output are isolated and has unlimited fan-out, designing and implementing various digital systems is easily accomplished.

Flux-flow instability in 2H-NbSe2 superconducting thin crystals stamped on SiO2/Si substrates

We have investigated the nonlinear flux flow in the mixed state of 2H-NbSe2 superconducting thin crystals stamped on SiO2/Si substrates. The nonlinearity in the flux flow behavior is marked with upward bending and subsequent voltage jump in the current-voltage characteristics. The results were analyzed within the framework of a modified Larkin-Ovchinnikov model, including the estimation of inelastic relaxation time of quasiparticles in the crystal and heat transfer coefficient between the crystal and SiO2/Si substrate. The analysis provides an insight into the quality of acoustic match at the crystal-substrate interface.

Thermally activated transport and vortex-glass phase transition of K0.78Fe1.70Se2 single crystals

The transport property () and flux motion together with the superconductivity of Fe Se2 single crystals are studied. In the transport measurement, contrasting behavior is observed under two different magnetic field orientations, i.e., and , which is ascribed to different vortex configurations. In the flux motion study, the tail of the well fits the description of the vortex-glass (VG) critical behavior, which is confirmed by VG scaling. The strong pinning nature of FeSe2 implies great sustainability under high magnetic fields and the potential for practical application of the system. The relation under Tc follows Arrhenius thermally activated flux flow (TAFF) behavior. The existence of a crossover behavior in a magnetic field is discovered via the magnetic dependence of the effective pinning barrier, which is related to the change in pinning strength of the point defects and the entanglements of the flux lines. Finally, a vortex phase diagram is proposed based on the evolution of upper critical field and other dynamic characteristics.

Thermally activated flux flow in FeSe0.5Te0.5 superconducting single crystal

The current–voltage (J-E) isotherms of single crystal FeSe0.5Te0.5 sample have been measured at several temperatures near the transition temperature (Tc) and under applied magnetic fields (H). A power law (E ∼ Jβ) has been used to fit the data and evaluate the activation energy Uo (T) using β = Uo/kBT. At low current density (J << Jc), the initial behaviour is associated with thermally activated flux Flow (TAFF) while at J >> Jc vortex flux flow (FF) behavior is expected. The effects of applied magnetic field on FF and TAFF also been investigated. We found that Uo(FF) was reduced with by about an order of magnitude in magnetic fields as low as ∼1.5 Tesla-the reduction in Uo(TAFF) is even faster than in Uo(FF)-hence reflecting the low pinning nature (defects, vacancies etc.) of FeSe0.5Te0.5 superconductor.

High temperature and low field regime vortex phase diagram of in-situ prepared stainless steel sheathed MgB2 tapes

A magnetic field-temperature vortex phase diagram has been proposed for MgB2 on the basis of magnetoresistance studies performed on an in-situ prepared stainless steel sheathed MgB2 tapes. Thermally activated flux flow (TAFF) behavior is analyzed using Arrhenius relation. Low dissipative part of magnetoresistance with temperature is well described by vortex glass theory. Above critical region, the resistive behavior is influenced by fluctuation conductivity. In addition, a considerable vortex liquid region below HC2 is also observed but TAFF region is found to be quite narrow.

Numerical modeling of heat transfer and fluid motion in air gap between clothing and human body: Effect of air gap orientation and body movement

Air gap available between the clothing and human body provides significant thermal insulation and hence enhances the protective performance of clothing. A numerical model of fluid motion and heat transfer through the air gap is proposed in this work. The most realistic and detailed model considered in this work shows better accuracy than the previous models and brings several new insights to the combined heat transfer phenomena. Effect of horizontal and vertical air gap orientations on sensor temperature rise, heat flux, thermal protection and fluid flow behavior are analyzed. Results shows that thermal protection with the vertical air gap orientation is significantly better than the protection offered by the horizontal air gap orientation and the difference increases as the air gap width increases. Dynamic mesh technique is used in order to investigate the effect of dynamic air gap which results because of the body movement during heat exposure. Body movement is considered to be such that variation of the air gap with time is sinusoidal. Effect of motion frequency and average air gap width are also studied. Significant difference in convective and total heat flux as well as flow pattern is observed for cases with dynamically varying air gap.

Robust two-dimensional superconductivity and vortex system in Bi2Te3/FeTe heterostructures.

The discovery of two-dimensional superconductivity in Bi2Te3/FeTe heterostructures provides a new platform for the search of Majorana fermions in condensed matter systems. Since Majorana fermions are expected to reside at the core of the vortices, a close examination of the vortex dynamics in superconducting interface is of paramount importance. Here, we report the robustness of the interfacial superconductivity and 2D vortex dynamics in four as-grown and aged Bi2Te3/FeTe heterostructure with different Bi2Te3 epilayer thickness (3, 5, 7, 14 nm). After two years’ air exposure, superconductivity remains robust even when the thickness of Bi2Te3 epilayer is down to 3 nm. Meanwhile, a new feature at ~13 K is induced in the aged samples, and the high field studies reveal its relevance to superconductivity. The resistance of all as-grown and aged heterostructures, just below the superconducting transition temperature follows the Arrhenius relation, indicating the thermally activated flux flow behavior at the interface of Bi2Te3 and FeTe. Moreover, the activation energy exhibits a logarithmic dependence on the magnetic field, providing a compelling evidence for the 2D vortex dynamics in this novel system. The weak disorder associated with aging-induced Te vacancies is possibly responsible for these observed phenomena.

Superconducting properties of indium-doped topological crystalline insulator SnTe

We report on the superconducting properties of indium-doped SnTe. SnTe has recently been explored as a topological crystalline insulator. Single crystals of Sn0.5In0.5Te have been synthesized by a modified Bridgman method. Resistivity measurement performed in the range 1.6–300 K shows a metallic normal state with onset of superconducting transition at . Bulk superconductivity has also been confirmed by DC magnetization, AC susceptibility and rf penetration depth measurements. Zero-temperature upper critical field, lower critical field, coherence length, and penetration depth are estimated to be 1.6 T, 1 mT, 143.5 Å and 853 nm, respectively. The temperature dependence of the low-temperature penetration depth indicates S-wave fully gapped characteristics with BCS (Bardeen-Cooper-Schrieffer) gap . Hall and Seebeck coefficient measurements confirm the dominance of hole conduction with possible phonon-drag effects around . Resistive transition studied under applied magnetic field shows thermally activated flux flow behaviour.

Flux-flow behavior in high Tc superconductors

We have been exploring flux-flow behavior in high temperature superconductor material by numerically solving Ginzburg-Landau (GL) equation and comparing it to experimental results. Surprisingly, without incorporating phenomenological activation energy and pinning force models and despite the approximate nature of GL equation, the power law IV curves with different exponents are obtained based on the relative magnitudes of normal conductivity σ and κ, the ratio between the London penetration depth (λ) and the coherence length (ξ). Simulations demonstrate that, for typical σ and κ values, vortices of opposite magnetic field polarity are created at two boundaries of the high Tc material and when they reach the center, they pass through and are injected into opposite polarity sides. Injection and what it entails shed light on unexplained high Tc behavior, open ways for practical applications, eliminate the need for phenomenological models, help us to understand and predict high Tc flux-flow properties.

High-Tc Josephson junctions fabricated by focused ion beam direct milling

High-Tc Josephson junctions were successfully fabricated by focused ion beam (FIB) direct milling. The characteristics of the junction barrier were carefully controlled by tuning the thickness of the link region. The optimal remaining thickness for the YBCO is about 70–80 nm even though the YBCO thicknesses are different. The temperature-dependence of the critical current provides good evidence of the superconductor–normal–superconductor weak link of the junctions. Although the flux-flow behavior increasingly dominates at lower temperatures, the voltage–current shows resistively shunted junction-like characteristics at temperatures close to Tc. The Shapiro steps of a single junction irradiated with microwaves were measured, revealing strong Josephson effects in ion-damaged junctions. This method will be of great benefit to nanoSQUIDs for the detection of spin systems.

Properties of FeSe nanobridges prepared by using a focused ion beam

We have fabricated FeSe nanobridges by using a focused ion beam (FIB) etching technique and studied their transport properties. FeSe films were prepatterned into microbridges by using a standard photolithography and argon-ion-milling technique and was subsequently patterned into nanobridges by using a focused Ga+ beam with a beam current of 9 pA to minimize damage to the film. The nominal width and length of the bridges were 300 nm and 100 nm, respectively, and the film thickness was about 1,000 nm. Our film was deposited on an Al2O3(0001) substrate by using a pulsed laser deposition technique and had a (101)-oriented tetragonal phase according to its X-ray diffraction spectrum. We have studied the current-voltage (I–V) characteristics, the temperature-dependent critical current and the normal state resistance. The as-made film exhibited an onset transition temperature (Tc) of 11.0 K and zero resistivity at 6.7 K. Argon-ion milling decreased Tc, Tc,onset = 10.0 K, and Tc0 = 6.0 K, but subsequent FIB nanobridge patterning did not affect Tc. The measured transport properties of the nanobridges showed a flux-flow behavior with thermal fluctuations with negligible pinning effect. The results are compared with those of the MgB2 counterpart with a coherence length of similar size. The details of the fabrication procedure and the measured properties are discussed.

Josephson nanoconstrictions made by AFM plowing of YBa2Cu3O7−x films

Using atomic force microscope (AFM) as a lithography tool, we ploughed (a) constant thickness nanoconstriction (∼492 nm) and (b) variable thickness bridges (∼25 to 50 nm) on YBa2Cu3O7−x thin films. We measured I-V characteristics and observed Shapiro steps, giving positive evidence of Josephson effect in these nanobridges. Further the linear temperature dependence of the critical current (Ic) and the well defined Fraunhofer-type pattern of Ic versus the applied magnetic field, rules out any flux flow behavior and firmly establishes Josephson effect in the constant thickness nanoconstriction type junction. The temperature dependence of Ic is again linear in the variable thickness bridge. However the magnetic field dependence of Ic did not show Fraunhofer-type pattern, possibly due to averaging effects. We believe that the observed low field dependence of Ic in these nanobridges allows one to tune the Josephson energy by small applied fields, making these nanojunctions useful for quantum applications like qubits.

FEM-calculations on the frequency dependence of hysteretic losses in coated conductors

Calculations based on two different finite-element models have been carried out to investigate the flux flow behaviour of High Temperature Superconductors (HTS), in particular of Coated Conductors (CC) based on 123-HTS. The models allow the simulation of the response of the CC to various experimental operating conditions: e.g. a fast ramping of the transport current typically done in measurements of the critical current Ic or sinusoidal changes of an external magnetic field typically used in AC loss measurements. The models also allow calculating the response to arbitrary combinations of current and field changes. The superconductor is modelled by using either a simple power-law E(J) characteristic or one which also accounts for field and temperature dependences. The obtained results go beyond Bean's approximation, which is mostly employed for interpreting such flux penetration effects. One consequence is that hysteretic losses, which in Bean's model are frequency independent, show a dependence on the time scales of current or field changes. The field and frequency ranges where such deviations from Bean's model should be taken into account are discussed.

Simulation of the Effect of Temperature on Flux-Flow Behavior in Stacked Intrinsic Josephson Junctions

We have numerically studied the effect of temperature on the flux-flow behavior in a stack of intrinsic Josephson junctions by using the inductive coupling Josephson junctions model taking into account thermal fluctuations. In the absence of noise, the current-voltage characteristic shows a series of clear Fiske and flux-flow steps depending on an external magnetic field. With increasing temperature the Fiske steps corresponding to the out-of-phase mode remain on the current-voltage characteristic but other steps disappear. This suggests that the out-of-phase cavity mode becomes more stable than other modes due to the fluctuations. Furthermore, it is found that the in-phase flux flow mode is also strongly affected by the thermal noise in contrast to the out-of-phase one and fluctuations disturb the formation of the rectangular lattice of vortices even in the flux-flow mode driven by a strong Lorentz force Our results can qualitatively explain recent experimental observations in intrinsic Josephson junctions in the flux-flow state.

MgB 2 grain boundary nanobridges prepared by focused ion beam

We have fabricated MgB2 grain boundary nanobridges by focused-ion-beam etch and studied their transport properties. Nanobridges with a nominal width and length of 100 nm were patterned across naturally formed single grain boundaries in the microbridges, which were prepatterned by a standard argon ion milling technique. We have studied current-voltage (I-V) characteristics, the temperature-dependent critical current, and the normal-state resistance. The measured properties were interpreted based on a flux flow model. In the I-V curves, a typical resistively shunted-junction characteristic was observed near Tc, however, as temperature decreases, flux-flow behavior became dominant, in accordance with the crossover of the ratio of the bridge length to the coherence length from the single-phased regime to the flux-flow regime. The temperature-dependent critical current was Ic(T)∼(1−T/Tc)1–1.5, similar to that of a superconducting film. The normal-state resistance increased steeply as temperature approaches Tc, in agreement with the flux-flow theory.