Bulk Superconductors Research Articles

Study on performance improvement of superconductive-assisted machining (SUAM) with superconducting tape

In this study, we calculated the repulsive force of magnetic levitation for superconductive-assisted machining (SUAM) using the finite element method. Conventionally, SUAM utilizes bulk superconductors; here, we propose a magnetic levitation tool using superconducting tape. To obtain a stronger repulsive force, we fabricated various SUAM models and performed calculations. The results showed that we could obtain a strong repulsive force by arranging six superconducting tape pieces radially at equal intervals from the center of the SUAM tool, shortening the distance between the layers, and making the tape trapezoidal in shape. By increasing the number of layers, we can expect to achieve a greater repulsive force than that obtained with bulk superconductors.

Sharp edged Gd2BaCuO5 particles addition effect on growth and properties of top-seeded melt-grown GdBa2Cu3O7-δ bulk superconductors

We report on top-seeded melt growth of GdBa2Cu3O7-δ (GdBCO) bulks added by sharp edged Gd2BaCuO5 nanoparticles (Gd211U) refined by ultrasonication. GdBCO bulks mixed with 30 mol.% of Gd211U were examined to be grown isothermally at different temperatures in a range of 1015 °C−1035 °C using a top-seeded melt growth technique. Magnetic susceptibility measurements show that all samples become superconducting and especially samples grown at 1020°C and 1025°C have a maximum superconducting transition temperature (T c) of 94.5 K. The critical current density (J c) obtained from magnetic hysteresis loops for magnetic fields parallel to the c-axis is up to 65100 A/cm2 (the self-field at 77 K) for a sample grown at 1025 °C, while J c of a GdBCO bulk with conventional Gd211 particles grown at 1025 °C for comparison is 29000 A/cm2 (the self-field at 77 K).

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering.

In situ MgB2 superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, Jc, which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 105 A/cm2 in the self-field and above 104 A/cm2 at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB2 nanograins in connection to the presence of MgO precipitates and areas rich in boron.

Superconducting gap structure in carrier doped BiCh2-based layered superconductors: A [formula omitted]SR study

The layered bismuth oxy-sulfide materials, which are structurally similar to Fe-pnictides/chalcogenides and cuprates superconductors, have sparked a lot of interest in the physics of reduced-dimensional superconductors. The pairing symmetry of the recently discovered BiCh2-based superconductor, La1−xCexOBiSSe with x = 0.3, was investigated using zero-field and transverse field (TF) muon spin measurements, as well as using magnetization and resistivity measurements. Resistivity and magnetization data confirmed bulk superconductivity below 2.7 K for x = 0.3. The temperature dependence of the magnetic penetration depth has been determined from TF-μSR data, which can be described by an isotropic two-gap s+s wave model compared to a single gap s- or anisotropic s-wave model, which resembles with Fe-pnictides/chalcogenides and MgB2. Furthermore, from the TF-μSR data, we have determined the London’s penetration depth λL(0) = 452(3) nm, superconducting carrier’s density ns = 2.18(1) × 1026 carriers/m3 and effective mass enhancement m∗ = 1.66(1) me, respectively. No signature of spontaneous internal field is found down to 100 mK in ZF-μSR measurement, which suggests that time-reversal symmetry is preserved in this system.

Time-reversal symmetry breaking in the superconducting state of ScS

We have studied the electronic properties of ScS, a transition-metal monochalcogenide with a rocksalt crystal structure, using magnetization, specific heat, transport, and muon spin rotation/relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements. All measurements confirm the bulk superconductivity in ScS with a transition temperature of ${T}_{C}=5.1(5)$ K. Specific heat together with transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements indicate a full gap, while our zero-field $\ensuremath{\mu}\mathrm{SR}$ study reveals the presence of spontaneous static or quasistatic magnetic fields emerging when entering the superconducting state. We discuss various possible microscopic origins of the observed time-reversal symmetry breaking. As none of them can be readily reconciled with a conventional pairing mechanism, this introduces ScS as a candidate material for unconventional superconductivity.

Enhanced magnetic properties of YBa2Cu3O7−δ bulk superconductors with tailored structure by a novel seeding assembly

YBa2Cu3O7−δ (YBCO) bulk superconductors are regarded as the most promising alternatives for conventional permanent magnets as they provide high field trapping capacity at low temperatures. However, for further improving their superconducting properties, it remains a significant challenge to enlarge the c growth sector by large-sized seeds or multi-seeds while maintaining the microstructure with a low porosity or clean grain boundaries that enables their performance. Here, we report a novel seeding assembly approach by vertically-connecting strip-seeds for inducing potential [110]-oriented overgrowth. Initially, from naturally right-angled corners, YBCO rapidly crystallized to form kite-liked convex areas, which, together with the original seed, effectively functioned as a large-sized seed to generate a sizable c growth sector. The novel-seeded sample exhibited a promising structure not only free of grain boundaries, but also with a favorable porosity of 20.29%. Such a low value is similar to 19.77% of the small-seeded one and much lower than 34.20% of the large-seeded one. Finally, due to structure tailoring towards an enlarged c growth sector with maintained pore characteristics, high and mono-peak trapped fields of 16 mm diameter YBCO bulks were reliably attained, up to a record value of 0.7025 T. This work offers insights into design principles for boosting magnetic properties of other members in the YBCO family.

Growth and characterization of superconducting Ca1–x Na x Fe2As2 single crystals by NaAs-flux method

High-quality superconducting Ca1−x Na x Fe2As2 single crystals have been successfully grown by the NaAs-flux method, with sodium doping level x = 0.4–0.64. The typical sizes of these crystals are more than 10 mm in ab-plane and ∼ 0.1 mm along c-axis in thickness. X-ray diffraction, resistance and magnetization measurements are carried out to characterize the quality of these crystals. While no signature of magnetic phase transitions is detected in the normal state, bulk superconductivity is found for these samples, with a sharp transition at T c ranging from 19.8 K (x = 0.4) to 34.8 K (x = 0.64). The doping dependences of the c-axis parameter and T c are consistent with previous reports, suggesting a possible connection between the lattice parameters and superconductivity.

Optimizing trapped field in superconductors with perforations

The use of artificial holes can improve the performance of high-temperature bulk superconductors. We report the results of Monte Carlo simulations of the trapped magnetic flux in superconducting samples with different configurations of perforated holes. Vortex trapping and emission at the hole boundary were modeled to account for flux pinning on the holes. It was found that the trapped flux can reach its maximum value, which does not depend on the number of holes. The dependence of the trapped flux on the diameter and number of holes is explored and described by the suggested relations that account for pinning on the holes and the effective hole area. Recommendations are provided to attain the maximum trapped flux for a fixed number or diameter of holes.

Pressure-induced electronic phase transition and superconductivity in spin-ladder compounds BaFe2(S1−x Se x )3

The discovery of high-temperature superconductivity in quasi-one-dimensional spin ladder compound BaFe2S3 has attracted great attention for facilitating study on the interplay among crystalline dimensionality, magnetism, and superconductivity. Here, we report a systematic study on the doping-dependent structure, susceptibility, and high-pressure properties of BaFe2(S1−x Se x )3. The structural analysis manifests a structural transition within the doping level of 0.18 < x< 0.29. Accompanied with this structural transition, the magnetic susceptibility behaviors change substantially, in line with the antiferromagnetic structure change from the stripe-type to the block-type. Furthermore, obvious spin glassy behaviors are observed in the low-temperature region for x⩾ 0.29. High-pressure study has revealed an explicit superconductivity signal in the undoped BaFe2S3 with a pressure of about 12.6 GPa, while no bulk superconductivity is observed for the Se-doped samples. Analysis of the pressure-dependent resistance behavior indicates a pressure-induced electronic phase transition around 18 GPa for all Se-doped samples. Finally, we have discussed the possible origin for the pressure-induced superconductivity in BaFe2(S1−x Se x )3.

Bulk superconductivity and non-trivial band topology analysis of Pb2Pd

In this article, we report single crystal growth of superconducting binary compound Pb2Pd. The crystal is well characterized through x-ray diffraction, selected area electron diffraction, transmission electron microscopy, field emission scanning electron microscopy and x-ray photoelectron spectroscopy. The bulk superconducting nature of the synthesized crystal is determined through AC susceptibility and magneto-heat capacity measurements. The specific heat jump at superconducting transition suggests Pb2Pd to be a moderately coupled s-wave superconductor. The topological non-trivial character of Pb2Pd is evidenced through bulk electronic band structure and Z2 invariants, which are calculated under the protocols of density functional theory. Surface states spectrum of Pb2Pd is also studied, which further claims Pb2Pd to have topological non-trivial band structure.

Strong-coupling superconductivity with Tc ∼ 10.8 K induced by P doping in the topological semimetal Mo5Si3

By performing P doping on the Si sites in the topological semimetal Mo5Si3, we discover strong-coupling superconductivity in Mo5Si3−xPx (0.5 ≤ x ≤ 2.0). Mo5Si3 crystallizes in the W5Si3-type structure with space group of I4/mcm (No. 140), and is not a superconductor itself. Upon P doping, the lattice parameter a decreases while c increases monotonously. Bulk superconductivity is revealed in Mo5Si3−xPx (0.5 ≤ x ≤ 2.0) from resistivity, magnetization, and heat capacity measurements. Tc in Mo5Si1.5P1.5 reaches as high as 10.8 K, setting a new record among the W5Si3-type superconductors. The upper and lower critical fields for Mo5Si1.5P1.5 are 14.56 T and 105 mT, respectively. Moreover, Mo5Si1.5P1.5 is found to be a fully gapped superconductor with strong electron-phonon coupling. First-principles calculations suggest that the enhancement of electron-phonon coupling is possibly due to the shift of the Fermi level, which is induced by electron doping. The calculations also reveal the nontrivial band topology in Mo5Si3. The Tc and upper critical field in Mo5Si3−xPx are fairly high among pseudobinary compounds. Both of them are higher than those in NbTi, making future applications promising. Our results suggest that the W5Si3-type compounds are ideal platforms to search for new superconductors. By examinations of their band topologies, more candidates for topological superconductors can be expected in this structural family.

Nanomechanical cat states generated by a dc voltage-driven Cooper pair box qubit

We study a nanoelectromechanical system consisting of a Cooper pair box qubit performing nanomechanical vibrations between two bulk superconductors. We demonstrate that a bias voltage applied to the superconductors may generate states represented by entanglement between qubit states and quantum ’cat states’, i.e. a superposition of the coherent states of the nanomechanical oscillator. We characterize the formation and development of such states in terms of the corresponding Wigner function and entropy of entanglement. Also, we propose an experimentally feasible detection scheme for the effect, in which the average current that attains the specific features created by the entanglement is measured.

Superconductivity in Kagome Metal YRu3Si2 with Strong Electron Correlations

We report the detailed physical properties of YRu3Si2 with the Ru kagome lattice at normal and superconducting states. The results of resistivity and magnetization show that YRu3Si2 is a type-II bulk superconductor with T c ∼ 3.0 K. The specific heat measurement further suggests that this superconductivity could originate from the weak or moderate electron-phonon coupling. On the other hand, both large Kadawaki–Woods ratio and Wilson ratio indicate that there is a strong electron correlation effect in this system, which may have a connection with the featured flat band of kagome lattice.

High-pressure synthesis and superconductivity of the novel laves phase BaIr2

Superconductors comprising 5d transition metals of Ir and Pt have been widely explored because they have the potential of unique superconductivity caused by the strong spin-orbit interaction (SOI). We successfully synthesized BaIr2, the last Laves phase remaining unsynthesized in the MgCu2-type AM2 (A = Ca, Sr, Ba; M = Rh, Pd, Ir, Pt). BaIr2 was crystallized at 925 °C under a pressure of 3.3 GPa via a solid-state reaction between Ba and Ir powders; it was found to have the longest a-lattice constant of 8.038 (1) Å among AM2. BaIr2 exhibited bulk superconductivity at a transition temperature (Tc) of ∼2.7 K BaIr2 was found to have a type-II superconductor with an upper critical field of 67.7 kOe, which was above the Pauli paramagnetic limit (∼50 kOe). The electron–phonon coupling constant and normalized specific heat jump were measured to be 0.63 and 1.2, respectively, indicating that BaIr2 is a weak-coupling superconductor. The electronic-structure calculations for BaIr2 revealed that the Ir-5d states are dominant at the Fermi energy (EF) and the density of states at the EF is strongly affected by SOI as in the case of CaIr2 and SrIr2.

Effect of thickness on magnetic properties of single domain GdBCO bulk superconductors

To study the influence of thickness on the magnetic properties of ReBCO (Re = Y, Gd, Sm, Nd, etc.) bulk superconductors, a single domain gadolinium barium copper oxide (GdBCO) bulk superconductor fabricated by the Re + 011 top seeded infiltration growth (Re + 011 TSIG) method was continuously sliced along the bottom to obtain samples of different thickness. The levitation force and attractive force of these samples were tested at 77 K in the zero-field-cooled (ZFC) state. It is found that as the sample thickness decreases, the levitation force decreases gradually whereas the attractive force increases. This is related to the varied ability to resist the penetration of magnetic field occasioned by varying sample thickness, which are deeply revealed by combining with the characteristics of the non-ideal type-II superconductor. Further, the levitation force exhibits a trend of slow initial change followed by rapid change, which may be attributed to the growth of the sample. Measurement of the trapped field shows that a similar distribution of trapped field at the top and bottom surfaces can be achieved by removing some materials from the bottom of the bulk. These results provide a reference for meeting the actual requirements of ReBCO bulks of different thicknesses and greatly contribute to practical designs and applications.

Portable, desktop high-field magnet systems using bulk, single-grain RE–Ba–Cu–O high-temperature superconductors

Bulk high-temperature superconducting materials can trap magnetic fields up to an order of magnitude larger than conventional permanent magnets. Recent advances in pulsed field magnetization (PFM) techniques now provide a fast and cost-effective method to magnetize bulk superconductors to fields of up to 5 T. We have developed a portable, desktop bulk high-temperature superconducting magnet system by combining advanced PFM techniques with state-of-the-art cryocooler technology and single-grain, RE–Ba–Cu–O [(RE)BCO, where RE is a rare-earth element or yttrium] bulk superconducting materials. The base temperature of the system is 41 K and it takes about 1 h for the system to cool down to 50 K from room temperature. A capacitor bank, combined with easily-interchangeable, solenoid- or split-type copper magnetizing coils and an insulated bipolar gate transistor acting as a high-speed switch, allows magnetic pulses to be generated with different pulse profiles. The system is capable of trapping magnetic fields of up to ∼3 T. In this work, we report the results of the magnetization of a range of single-grain Y–Ba–Cu–O, Eu–Ba–Cu–O and Gd–Ba–Cu–O (GdBCO), bulk superconducting discs using this system. A higher trapped field was recorded using a split coil incorporating iron yokes at temperatures of 65 K and above, whereas at lower temperatures, a higher trapped field was obtained using the solenoid coil. The GdBCO sample achieved the highest trapped field for both single-pulse (SP) and two-stage-multi-pulse (TSMP) methods using the solenoid coil. Maximum trapped fields of 2.26 T at 55 K and 2.85 T at 49 K were recorded at the centre of the top surface of the GdBCO sample for the SP and TSMP methods, respectively. The PFM process is substantially an adiabatic process so, therefore, the thermal contact between the sample and sample holder is of critical importance for cooling the bulk sample during application of the pulse. The design of the sample holder can be modified easily to enhance the thermal stability of the sample in order to achieve a higher trapped field.

Dynamic guidance performance of GdBaCuO and YBaCuO bulk single grain superconductors under a varying external magnetic field

High temperature superconducting (HTS) maglev technology has attracted considerable attention from researchers around the world. It provides a new direction of development for urban rail and high-speed rail transit due to its considerable advantages of frictionless transportation, low noise, and environmental protection. HTS bulk materials are able to provide a significant performance advantage to maglev technology and are a key component to achieving both levitation and vehicle guidance. HTS bulk samples with enhanced flux pinning characteristics can enable improved curve negotiating ability and lateral stability for the maglev vehicle. This paper reports the results of experiments on the dynamic guidance force of single grain Y–Ba–Cu–O (YBCO) and Gd–Ba–Cu–O (GdBCO) bulk HTS single grains in order to explore their potential for enhanced maglev guidance performance. A customized HTS maglev dynamic measurement system (SCML-03) with a rotating, circular permanent magnet guideway (PMG) was employed to simulate the dynamic operation of the HTS vehicle above the PMG at different translational speeds. It was observed from the experimental results that the GdBCO bulk superconductor is more able to resist the attenuation of the dynamic guidance force compared to YBCO under the same operational conditions. In addition, the GdBCO bulk single grain is more able to return to the original equilibrium position following exposure to an external displacement. It is concluded that the GdBCO bulk single grain offers greater potential than YBCO for practical application in HTS maglev given its enhanced guidance performance. The results of these tests provide an important benchmark for the future design of HTS maglev transportation systems.

Effects of Zn doping on superconducting properties of melt textured Y–Ba–Cu–O bulk superconductors

Zn doping which may induce a second peak effect in YBa2Cu3O7−δ (YBCO) has been opted as a regular technique to improve the trapped field and flux pinning stability of YBCO bulk superconductors for practical applications, such as quasi-permanent or maglev magnets. Here, the effects of Zn doping on the flux trapping performance of single domain YBCO bulk superconductors have been systematically investigated. The doping amount x varies from 0 to 1.0 mol%. We analyzed the critical transition temperature (T c), critical current density (J c), trapped field (B t) and the flux relaxation at various temperatures, and measured in addition the levitation force (F L) for comparison. The doping amount of 0.6 mol% has been proven to be the optimal value, which enhances self-field J c by 243.3% at 30 K with an obvious secondary peak effect, resulting in great improvements in trapped field and flux stability. The holistic and local superconducting performances exhibit good agreement, demonstrating Zn-doping an effective method to improve flux pinning properties for practical applications.

Study of a new axial-field superconducting inductor for a synchronous machine

In this work, a novel superconducting (SC) inductor topology for an axial flux synchronous machine is presented and tested. The proposed device combines HTS YBaCuO bulks and coils supplied with DC current to create a variable air gap flux density distribution. In fact, the two SC bulks modulate and redirect the flux lines produced by the coil thanks to their magnetic field shielding property. This results in a periodic space variation of the axial component of the flux density. A 3D electromagnetic modeling based on a finite element solution is developed to demonstrate the relevance of using the magnetic shielding properties of SC bulks. In order to verify the screening properties of the SC bulk, a prototype of the proposed inductor was constructed and tested in the laboratory.

Theory of superconductivity mediated by Rashba coupling in incipient ferroelectrics

Experimental evidence suggests that superconductivity in ${\mathrm{SrTiO}}_{3}$ is mediated by a soft transverse ferroelectric mode which, according to conventional theories, has negligible coupling with electrons. A phenomenological Rashba-type coupling has been proposed on symmetry arguments but a microscopic derivation is lacking. Here, we fill this gap and obtain a linear coupling directly from a minimal microscopic model of the electronic structure. We find that the effective electron-electron pairing interaction has a strong momentum dependence. This yields an unusual situation in which the leading $s$-wave channel is followed by a subleading $p$-wave state which shows a stronger pairing instability than the $d$-wave state. The bare Rashba coupling constant is estimated for the lowest band of doped ${\mathrm{SrTiO}}_{3}$ with the aid of first-principles computations. Extrapolating the estimation to the multiband regime we show that it can explain bulk superconductivity. For densities where the Meissner effect has not been observed an extra softening due to inhomogeneities is needed to explain the zero resistance state.