Superconducting Samples Research Articles

The threshold temperature where type-I and type-II interchange in mesoscopic superconductors at the Bogomolnyi limit

In this work we discuss the H−T phase diagram for mesoscopic squared superconducting samples at the Bogomolnyi limit, where the Ginzburg–Landau constant κ=1/2. We calculate Hp(T), the vortex penetration field, and Hu(T) the upper critical field. Through the study of the temperature dependence on the Hp, it is possible to distinguish the region where the magnetic field penetrates into the sample, like a type-I or a type-II superconductor. It permits to determine the threshold temperature T⋆(L,H) where the phase transition from type-I to type-II occurs for some different sizes L of the mesoscopic superconducting samples. The calculation of the upper critical field Hu(T), for these samples, shows that, these two curves, Hp(T) and Hu(T), overlap at the threshold temperature mentioned above. The magnetization of the system was calculated for all sizes studied in this work, and for temperatures above and below T⋆(L,H). This study confirms the existence of the threshold temperature, T⋆(L,H), where type-I and type-II interchange in mesoscopic superconductors at the Bogomolnyi limit.

Anomalous Scaling Relations and Pairing Mechanism of the Fe-based Superconductors

The anomalous scaling behavior of the specific heat jump ΔC vs. , first observed by Bud’ko et al[], has been confirmed with several families of the Fe-based superconducting compounds with various dopings. And more recently, Xing et al[] have reported that the condensation energy (CE) vs. Tc also follows an anomalous power law with about forty different samples of the Fe-based superconductors. These apparently non-BCS power law relations have strongly hinted at a non-BCS pairing mechanism of the Fe-based superconductors. We have studied a minimal two band BCS model to understand these anomalous scaling behaviors and showed that these non-BCS relations of as well as can be well reproduced by the simple two-band BCS model paired by a dominant interband repulsive interaction (Vinter > Vintra > 0). The origin of these anomalous scaling relations are the kinematic constraint of near Tc of the two band BCS model, as realized in the S±-wave pairing state. Our results imply that these seemingly non-BCS-like scaling behaviors, on the contrary to the common expectations, are in fact strong experimental evidences that the pairing mechanism of the Fe-based superconductors is genuinely a BCS mechanism, meaning that the Cooper pairs are formed by the itinerant carriers glued by a pairing interaction.

Peculiarities in the low-temperature specific heat related to nanoscale structural inhomogeneity in fine-crystalline YBa2Cu3O6.93 high-T c superconductors

A comparative study of the low-temperature specific heat for two types of YBa2Cu3O y high-T c superconductor samples is performed within the temperature range of 2−10 K. The samples of the first type are fine-crystalline optimally doped ones with different degrees of nanoscale structural inhomogeneity. The second type includes coarse-crystalline equilibrium samples with different hole doping levels. A similarity in the behavior of different contributions to the specific heat for structurally inhomogeneous and underdoped samples is revealed. The samples of both types exhibit a metal-like contribution linear in temperature to the specific heat ~γT, which is not characteristic of the superconducting phase. It is found that this contribution moderately grows with the decrease in the oxygen content, whereas with the increase in the structural inhomogeneity, such growth of the linear contribution (γT) becomes anomalously large. This leads to the conclusion about the coexistence of metallic and superconducting states in the bulk of the samples under study. Such common feature of electron systems could be related to the formation of the pseudogap regime. It is demonstrated that this regime suppresses just the superconducting states, leaving intact the metallic ones.

SiCサセプターを用いた家庭用電子レンジでのYBa<sub>2</sub>Cu<sub>3</sub>O<sub>y</sub>銅酸化物超伝導体の急速な合成

Nearly single-phase samples of high-Tc superconductors of the Y-123(YBa2Cu3Oy) phase have been prepared using a domestic microwave oven. Since it is significantly important to control temperature of the sample for microwave heating, the samples were placed into thermally insulated box coated with SiC (susceptor). Weighting of Y2O3, BaO, and CuO with a molar ratio were carried out and mixied with agate mortar for 1 hour. Samples (12 mm in diameter) were pressed with mold at 30 MPa and heated by microwave (2.45 GHz, and 700 W) with the susceptor. The temperature in the susceptor was kept at 850 °C for 10 minutes, and the samples of Y-123 were prepared. The produced sample showed superconductive transition temperature (Tc) by 92 K, and had zero resistance by 85 K. These properties for samples prepared by microwave heating were almost consistent with the samples produced by the conventional solid phase process. Properties of Y-123 sintered by various susceptors were mainly investigated. These results showed that microwave sintering was suitable for rapid heat-treatments of a copper oxide superconductor.

The effect of sintering in different atmospheres on superconductivity parameters of YBCO

YBa$_{2}$Cu$_{3}$O$_{7}$ (Y123) is a superconductor material discovered in 1987 and its transition temperature is above the liquid nitrogen temperature. It is known that superconductor samples produced by the solid-state reaction route are very often sintered in air atmosphere. However, in the presented work, Y123 was prepared by the same technique using a sintering ambient composed of a mixture of oxygen and argon gases for the first time. Critical current densities of the samples were calculated using results of magnetization measurements. It was obtained from magnetization analysis that all the samples exhibited superconductor character. It was also observed that the sample sintered in the 50% oxygen and 50% argon atmosphere showed the highest critical current density.

Comparative studies between the influence of single- and multi-walled carbon nanotubes addition on Gd-123 superconducting phase

The effect of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) addition on the phase formation and the superconducting properties of GdBa2Cu3O[Formula: see text] phase has been studied. Therefore, composite superconductor samples of type (CNTs)[Formula: see text] GdBa2Cu3O[Formula: see text], 0.0 [Formula: see text] 0.1 wt.% have been synthesized by a standard solid-state reaction technique. The samples have been characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results of XRD show an enhancement in the phase formation up to 0.06 wt.% and 0.08 wt.% for SWCNTs and MWCNTs, respectively. SEM and TEM reveal that CNTs form an electrical network resulting in well-connected superconducting grains. The electrical properties of the prepared samples have been examined by electric resistivity and I–V measurements, and their results reinforce the XRD, SEM and TEM. Consequently, both [Formula: see text] and [Formula: see text] improve as the addition percentage increases up to 0.06 wt.% and 0.08 wt.% for SWCNTs and MWCNTs, respectively.

Synthesis and Magnetic Properties of Bi1.7Pb0.3Sr2Ca2Cu3O10+δ Added with Nano Y

Bi17Pb0.3Sr2Ca2Cu3O10+δ superconductor samples were synthesized by the conventional solid-state reaction method. To study the effects of the addition of yttrium nanoparticles to the superconducting system, nano Y2O3 was introduced by small weight percentages (0.2, 0.4, 0.6, 0.8, and 1.0 wt%) in the first step of the synthesis process. Phase identification and microstructural characterization of the samples were investigated using X-ray diffraction and scanning electronic microscopy (SEM). Energydispersive X-ray spectroscopy (EDX) analysis was utilized to confirm the presence of the desired elements in the chemical composition of the samples. Moreover, DC electrical resistivity as a function of the temperature, critical current density (J c), AC magnetic susceptibility, and DC magnetization measurements were carried to evaluate the relative performance of samples. XRD analysis showed that both (Bi,Pb)-2223 and Bi-2212 phases coexist in the samples having an orthorhombic crystal structure. Both the onset critical temperatures (T c) (onset) and zero electrical resistivity critical temperatures (T c) (R = 0) of the samples were determined from the DC electrical resistivity measurements. An improvement of the superconducting transition temperature of 3.0 % was obtained with increasing Y2O3 nanoparticles to x = 1.0 wt%, while the critical current density is improved by 200 %. AC magnetic susceptibility measurement showed that the diamagnetic fraction and intergranular coupling of the x = 1.0 wt% sample are greater than those of the others. The variation of magnetization with temperature (M–T curve) of the samples was measured by cooling the sample in zero fields (ZFC) and an applied field of 10 Oe (FC). The results of AC magnetic susceptibility and DC magnetization measurements were in good agreement with DC electrical resistivity measurement.

Strong Electron Correlation behind the Superconductivity in Ce-Free and Ce-Underdoped High-Tc T′-Cuprates

In order to investigate the electronic state of Ce-free and Ce-underdoped high-Tc cuprates with the so-called T' structure, we have performed muon-spin-relaxation (muSR) and specific-heat measurements of Ce-free T'-La_1.8_Eu_0.2_CuO_4+d_ (T'-LECO) polycrystals and Ce-underdoped T'-Pr_1.3-x_La_0.7_Ce_x_CuO_4+d_ (T'-PLCCO) single crystals with x=0.10. The muSR spectra of the reduced superconducting samples of both T'-LECO with Tc=15K and T'-PLCCO with x=0.10 and Tc=27K have revealed that a short-range magnetic order coexists with the superconductivity in the ground state. The formation of a short-range magnetic order due to a tiny amount of excess oxygen in the reduced superconducting samples strongly suggest that the Ce-free and Ce-underdoped T'-cuprates are regarded as strongly correlated electron systems.

Influence of Be substitution on the superconducting properties of (Cu0.5Tl0.5)Ba2(Ca2−yBey)(Cu2.5Cd0.5)O10−δ (y = 0, 0.1, 0.2, 0.35, 0.5) samples

Be doped (Cu0.5Tl0.5)Ba2(Ca2−yBey)(Cu2.5Cd0.5)O10−δ (y = 0, 0.1, 0.2, 0.35, 0.5) superconductive samples are synthesized by solid state reaction method at normal pressure. The X-ray diffraction of these samples reveals a suppression in the c-axis length with the enhanced Be contents, reflecting the incorporation of Be at the inter plane Ca sites. The critical temperature Tc (R = 0) and onset temperature of diamagnetism Tc (onset) are found to increase with the increasing Be-contents up to y = 0.2 and decrease beyond this doping level. The room temperature resistivity, ρ(290K), is decreased in y = 0.1 and 0.2 samples, and raised to higher values beyond y = 0.2 in comparison to y = 0 samples. This improvement most likely arises due to the better interplane coupling caused by incorporation of Be at the Ca sites in the unit cell. The magnitude of diamagnetism is suppressed in all the Be-doped samples in comparison to un-doped sample. The FTIR absorption spectra show systematic hardening of the vibrational modes of apical oxygen with the increasing Be concentration. It confirms the incorporation of Be at the interplane Ca sites which results in the enhanced interplane coupling and hence the better superconducting properties in y = 0.1 and 0.2 samples. This hardening of the apical oxygen modes also complements our XRD results. The suppression of superconductivity parameters in samples with higher Be contents, namely y = 0.35 and 0.5, is attributed to the presence of heavier Cd at the Cu planar sites in CuO2/CdO2 planes. Cd atoms suppress the phonon’s population due to anharmonic oscillations it produces. Such anharmonicity is further promoted due to the enhanced interplane coupling caused by Be doping which further suppresses the phonon’s density and hence the superconductivity in the samples with higher Be-contents i.e. y = 0.35 and 0.5. These results stress on the vital role the electron–phonon interactions in the mechanism of superconductivity in cuprate superconductors.

Synchrotron X-ray absorption and 57Fe Mössbauer spectroscopy studies of electronic structure of cobalt-doped SmFeAsO superconductors

We report on the electronic structure of cobalt-doped oxypnictide SmFe1−xCoxAsO superconductor crystals (x=0.1–0.25) by synchrotron X-ray absorption spectroscopy (XAS) and 57Fe Mössbauer spectroscopy. The Mössbauer spectra of the superconductor samples show only paramagnetic singlet patterns indicating the absence of hyperfine magnetic field, and the central shift values, 0.41mm/s referring to α-Fe, reveal coexistence of high- and low-spin Fe2+ in this system. The Fe L2,3-edge and O K-edge XAS spectra for all the samples are found to be similar, though the peaks have different intensity. This indicates that the hybridization of Fe with As becomes weakened as Co content increases from 0.10 to 0.25, leading to decrease of the transition temperature as obtained by the resistance-temperature measurement. The Sm M4,5-edge XAS spectra exhibit two main peaks without any shoulders, but with an abnormal shift as increasing Co concentration, revealing evident influence of the magnetic moment of cobalt on the moment of Sm ions.

Gold Nanoparticles Effect on (Bi,Pb)-2223 Superconducting Thin Films

<p class="1Body">Superconductor samples were prepared by a conventional solid-state reaction method and systematically studied for their superconducting properties. The Au nanoparticles concentration x varied from 0.0 to 1.0 wt% of the sample's total mass. Those samples were used as targets to prepare Au<sub>x</sub>–Bi<sub>1.7</sub>Pb<sub>0.3</sub>Sr<sub>2</sub>Ca<sub>2</sub>Cu<sub>3</sub>O<sub>10+δ</sub> thin films by pulsed laser deposition (PLD), deposited on Si (111) substrates and post-deposition oxygen annealing have been achieved. The effect of Au nanoparticles (20 nm) on the physical properties of superconducting phase was studied. The phase identification/gross structural characteristics of synthesized bulk and thin films samples explored through powder X-ray diffractometer and reveals that all the samples crystallize in orthorhombic structure In addition, phase examination by XRD indicated that Au nanoparticles enhanced the (Bi,Pb)-2223 phase formation. The critical transition temperature (<em>T<sub>c</sub></em>) measured by the standard DC four-probe method and was found to have optimal value at x=1.0 wt%, which had a maximum enhancement in <em>T<sub>c</sub></em> for both bulk and thin films samples. The surface morphology investigated through scanning electron microscope (SEM) and atomic force microscopy (AFM), the granular investigation showed that both number and size of voids decreased while grains size increased as x increases to 1.0 wt%.</p>

Ac Magnetic Susceptibility and EPR Studies of (Co0.5Zn0.5Fe2O4) x /(Cu0.5Tl0.5)-1223 Composites

Cobalt zinc ferrite nanoparticles (Co0.5Zn0. 5Fe2O4)x (∼5–8 nm in size) added to Cu0.5Tl0.5Ba2Ca2Cu3O10−δ superconductor samples, 0.00 ≤x≤ 0.20 wt. %, were prepared using a singlestep solid-state reaction technique. The temperature dependence of the real (χ′) and imaginary (χ″) components of the magnetic susceptibility (ACMS) was measured at various ac magnetic field amplitudes (3–16 Oe). The results were analyzed by using Bean critical state model to estimate the critical current density (Jc) It was found that Jc increases as x increases up to 0.08 and then it decreases with further increase in x. X- and Q-Band (9.3 and 33. GHz) electron paramagnetic resonance (EPR) spectra were measured in the temperature range from 300 to 10 K.The g values of Cu 2+ monomer were found to correspond to that of rhombic site symmetry with gxx = 2.21 ± 0.02, gyy = 2.10 ± 0.02, and gzz = 2.02 ± 0.02.

Superconducting and Mechanical Properties of the Bulk (SnO2) x (Bi1.6Pb0.4)Sr2Ca2Cu3O10−δ Prepared at Different Sintering Times

Solid-state reaction method was used to prepare superconductor samples with nominal composition of (SnO2) x (Bi1.6Pb0.4)Sr2Ca2Cu3O10−δ , (SnO2) x (Bi,Pb)-2223 composite, where 0.0 ≤ x ≤ 0.2 wt%. The prepared samples were characterized by X-ray diffraction (XRD) as well as scanning electron microscopy (SEM). XRD patterns proved that the addition of nano-SnO2 particles to (Bi,Pb)-2223 did not affect the tetragonal structure and the lattice parameters of this phase. Vickers microhardness measurements (H V) for these samples were carried out at room temperature as a function of applied load, heating temperatures, and dwell time. However, to clarify the indentation size effect (ISE) behavior of these composite materials and to estimate the true microhardness values (H 0), different models including Meyer’s law, Hays and Kendall (energy dissipation) model, elastic/plastic deformation model, as well as proportional specimen resistance model were applied. Some important mechanical parameters, such as Young’s modulus (E), yield strength (Y), fracture toughness (K), and brittleness index (B), were estimated from the microhardness curves. It was found that the incorporation of a proper concentration of nano-SnO2 particles has proved their efficiency to enhance the mechanical properties of superconductors. Moreover, it was noted that dwell time and Vickers microhardness were inversely proportional. Indentation creep experiments showed that the operative creep mechanisms in the studied samples were dislocation creep in addition to dislocation interaction.

Excess conductivity analysis for Y3−xNdxBa5−xCaxCu8O18−δ superconducting phase

Superconducting samples of type [Formula: see text] substituted by both [Formula: see text] and [Formula: see text] ions were prepared by a solid-state reaction technique. These substitutions affect the phase formation, lattice parameters, grains formations and superconducting transition temperature [Formula: see text]. The change in these parameters is mainly due to the partial substitution of [Formula: see text] by [Formula: see text] ions rather than the partial substitution of [Formula: see text] by [Formula: see text] ions. The analysis of the electrical resistivity [Formula: see text] versus temperature above [Formula: see text] was carried out using Aslamazov–Larkin (AL) and Lawrence–Doniach (LD) models. Different fluctuation regions starting from high temperature up to [Formula: see text] were estimated, and denoted by short-wave (sw), two-dimensional (2D), three-dimensional (3D) and critical (cr) fluctuations. A distinct crossover temperature [Formula: see text] from 2D to 3D, in the mean field region (MFR), is shifted to lower temperature by increasing substitutions content. The superconducting parameters such as zero temperature coherence length along the [Formula: see text]-axis [Formula: see text], inter-layer coupling strength [Formula: see text], Fermi velocity [Formula: see text] of the carriers and Fermi energy [Formula: see text] were calculated as a function of substitution content.

Dynamics of skyrmions and edge states in the resistive regime of mesoscopic p-wave superconductors

In a mesoscopic sample of a chiral p-wave superconductor, novel states comprising skyrmions and edge states have been stabilized in out-of-plane applied magnetic field. Using the time-dependent Ginzburg–Landau equations we shed light on the dynamic response of such states to an external applied current. Three different regimes are obtained, namely, the superconducting (stationary), resistive (non-stationary) and normal regime, similarly to conventional s-wave superconductors. However, in the resistive regime and depending on the external current, we found that moving skyrmions and the edge state behave distinctly different from the conventional kinematic vortex, thereby providing new fingerprints for identification of p-wave superconductivity.

The origin of the condensation energy scaling of iron-based superconductors

The relation between the condensation energy (CE) and Tc of a phase transition reveals a fundamental nature of the transition. In view of this, the recent experimental observation of the non-BCS scaling relation of the CE vs. with about forty different samples of the Fe-based superconductors (Xing J. et al., Phys. Rev. B, 89 (2014) 140503) was intriguing and strongly hinted at a non-BCS pairing mechanism. In this paper, we have studied the CE and Tc of the multiband BCS model and found that the observed anomalous scaling relation is well reproduced by the two-band BCS model paired by a dominant repulsive interband interaction . Our result implies that this seemingly non-BCS–like scaling behavior of , on the contrary to the common expectations, is in fact a strong experimental evidence that the pairing mechanism of the Fe-based superconductors is genuinely a BCS mechanism, meaning that the Cooper pairs are formed by the itinerant carriers glued by a pairing interaction.

Vortex quasi-crystals in mesoscopic superconducting samples**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274009, 11434011, and 111522436), the National Key Basic Research Program of China (Grant No. 2013CB922000), the Research Funds of Renmin University of China (Grant Nos. 10XNL016 and 16XNLQ03), and the Program of State Key Laboratory of Quantum Optics and Quantum Optics Devices (Grant

There seems to be a one to one correspondence between the phases of atomic and molecular matter (AMOM) and vortex matter (VM) in superfluids and superconductors. Crystals, liquids, and glasses have been experimentally observed in both AMOM and VM. Here, we propose a vortex quasi-crystal state which can be stabilized due to boundary and surface energy effects for samples of special shapes and sizes. For finite sized pentagonal samples, it is proposed that a phase transition between a vortex crystal and a vortex quasi-crystal occurs as a function of magnetic field and temperature as the sample size is reduced.

Role of improving the physical properties of Sm-123 phase by adding nano-magnetic MnFe2O4

Superconducting samples of SmBa2Cu3O7−δ (Sm-123) added with various amounts of nanosized MnFe2O4 addition (0.0−0.20wt%) were investigated. The investigated samples prepared by the solid-state reaction method. The phase formation and microstructure of these samples were examined using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), proton induced X-ray emission (PIXE) and Rutherford backscattering spectrometry (RBS). XRD data indicate that the volume fraction of Sm-123 increases as nanosized MnFe2O4 increases up to 0.02wt%. The elemental distribution and oxygen content were deduced from PIXE and RBS. The oxygen content slightly decreases as MnFe2O4 wt% increases. The superconducting transition temperature (Tc) and critical current density (Jc) of the prepared samples were estimated from electrical resistivity and transport critical current density measurements. It was found that Tc decreases as nanosized MnFe2O4 addition increases, while Jc enhances up to 0.02wt%. Moreover, the temperature dependence of normal state electrical resistivity was studied in view of the pseudogap opening in order to determine the pseudogap temperature T* as a function of nanosized MnFe2O4 addition. T* increases as nanosized MnFe2O4 increased, indicating the enhancement of the pseudogap formation in HTSCs by adding magnetic impurities. The crossover to fluctuation conductivity near the Tc is discussed.

Recovery time of high temperature superconducting tapes exposed in liquid nitrogen

The recovery time is a crucial parameter to high temperature superconducting tapes, especially in power applications. The cooperation between the reclosing device and the superconducting facilities mostly relies on the recovery time of the superconducting tapes. In this paper, a novel method is presented to measure the recovery time of several different superconducting samples. In this method criterion used to judge whether the sample has recovered is the liquid nitrogen temperature, instead of the critical temperature. An interesting phenomenon is observed during the testing of superconducting samples exposed in the liquid nitrogen. Theoretical explanations of this phenomenon are presented from the aspect of heat transfer. Optimization strategy of recovery characteristics based on this phenomenon is also briefly discussed.

Magnetic susceptibility of fine crystalline high-temperature YBa2Cu3O6.93 superconductors. The role of nanoscale structural disordering

Static magnetic susceptibility χ(T) in the normal state (Tc ≤ T ≤ 400 K) and specific heat C(T) near temperature Tc of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa2Cu3Oy superconductor, having y and Tc close to optimal but differing in the degree of nanoscale structural disordering. It is shown that under the influence of structural disordering, there is enhancement of anomalous pseudogap behavior of the studied characteristics and a significant increase in the width of the pseudogap.