High-field Superconductors Research Articles

Magnetic flux concentration and magnetic force enhancement using YBCO cylinders

In this paper, we propose new applications of bulk high-temperature superconductors in high magnetic fields. High-temperature superconductors, such as Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+d/ (BSCCO) or YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO), maintain their superconductivity over 30 T at 4.2 K. A field-cooled superconducting hollow cylinder traps the magnetic flux inside it. An increase of the applied magnetic field induces shielding current in the interior of the cylinder to maintain the magnetic flux constant. By suitably arranging the cylinders, we can concentrate the magnetic flux and enhance the gradient of the magnetic field. In order to confirm these concepts, we performed experiments using bulk YBCO hollow cylinders. The cylinders are made of YBa/sub 2/Cu/sub 3/O/sub 7-x/ quasi single-crystals containing Y/sub 2/BaCuO/sub 5/ particles. They have an inner diameter of 5 mm, an outer diameter of 10 mm, and a height of 10 mm. Using three hollow cylinders, a magnetic concentration of 0.49 T was observed in an applied magnetic field of 10.14 T. The energized hollow cylinder with a shielding current modify surrounding magnetic fields and magnetic force fields. A magnetic force field enhanced by using a hollow cylinder was calculated numerically for an applied magnetic field of 10 T and estimated to exceed 6000 T/sup 2//m.

Conduction-cooled Bi-2212-Ag solenoid magnet system with 50-mm RT bore (III): test results

High-field high-temperature superconductor (HTS) conduction-cooled magnet system is considered to be the most promising system for superconducting magnets of the next generation. In order to demonstrate the feasibility, we have been developing a Bi-2212/Ag magnet system that consists of five solenoid coils fabricated with ROtation-Symmetric Arranged Tape-in-tube wire (ROSATwire). ROSATwire shows less field anisotropy on the transport current property than conventional tape conductors; therefore ROSATwire is ideal for solenoid magnets that generate high and precise uniform magnetic field. We succeeded in the fabrication of a 3.5 km long ROSATwire, and the critical current was achieved up to 500 A at 4.2 K, 10 T. Several coils were fabricated. We tested the inner two coils in the conduction cooling system. The maximum current of dc operation test was 120 A (2.17 T). However, the maximum current of ac operation test (repeat high-sweep-rate operation test) was 160 A (2.89 T) with sweep rate of 1 A/s (0.0181 T/s). This property, the maximum current of ac operation being larger than the one of dc operation, is considered to be characteristic of HTS magnets. The operation test results and the thermal stability are also reported.

Introduction to Special Issue of the Proceedings of the 7th Symposium on High Temperature Superconductors in High Frequency Fields

Introduction to Special Issue of the Proceedings of the 7th Symposium on High Temperature Superconductors in High Frequency Fields

Inhomogeneous stripe phase revisited for surface superconductivity.

We consider 2D surface superconductivity in high magnetic fields parallel to the surface. We demonstrate that the spin-orbit interaction at the surface changes the properties of the inhomogeneous superconducting Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state that develops above fields given by the paramagnetic criterion. Strong spin-orbit interaction significantly broadens the range of existence of the LOFF phase, which takes the form of periodic superconducting stripes running along the field direction on the surface, leading to the anisotropy of its properties. Our results provide a tool for studying surface superconductivity as a function of doping.

Liquid-to-liquid phase transition in pancake vortex systems

We study the thermodynamics of a model of pancake vortices in layered superconductors. The model is based on the effective pair potential for the pancake vortices derived from the London approximation of a version of the Lawrence-Doniach model which is valid for extreme type-II superconductors. Using the hypernetted-chain (HNC) approximation, we find that there is a temperature below which multiple solutions to the HNC equations exist. By explicitly evaluating the free energy for each solution we find that the system undergoes a first-order transition between two vortex liquid phases. The low-temperature phase has larger correlations along the field direction than the high-temperature phase. We discuss the possible relation of this phase transition to the liquid-to-liquid phase transition recently observed in Y-Ba-Cu-O superconductors in high magnetic fields in the presence of disorder.

Spatially resolved electronic structure inside and outside the vortex cores of a high-temperature superconductor.

Puzzling aspects of high-transition-temperature (high-Tc) superconductors include the prevalence of magnetism in the normal state and the persistence of superconductivity in high magnetic fields. Superconductivity and magnetism generally are thought to be incompatible, based on what is known about conventional superconductors. Recent results, however, indicate that antiferromagnetism can appear in the superconducting state of a high-Tc superconductor in the presence of an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines, each of which represents a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. Here we report the results of a high-field nuclear-magnetic-resonance (NMR) imaging experiment in which we spatially resolve the electronic structure of near-optimally doped YBa2Cu3O7-delta inside and outside vortex cores. Outside the cores, we find strong antiferromagnetic fluctuations, whereas inside we detect electronic states that are rather different from those found in conventional superconductors.

Two-step transition of (Nd 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O x superconductors in high fields

Superconducting transitions were measured in field-cooled cooling and field-cooled warming modes in external magnetic fields ranging between 0.1 and 7 T on REBa 2Cu 3O x superconductors (RE: rare earth elements) with fishtail effect. All the samples exhibited a two-step transition when being cooled in high fields. These results show that a two-step transition in magnetization is related to the secondary peak effect.

ＭＨＤ ＥＴＬマークＶ用超電導マグネットの開発 大型高磁界超電導マグネット技術の確立を目指して 第Ｉ部 目標仕様の設定と導体テスト

ＭＨＤ ＥＴＬマークＶ用超電導マグネットの開発 大型高磁界超電導マグネット技術の確立を目指して 第Ｉ部 目標仕様の設定と導体テスト

Magnetic field induced singlet to triplet phase transition in organic superconductors

New experimental results in organic quasi-one-dimensional superconductors suggest the existence of triplet superconductivity in high magnetic fields. When there is competition between singlet and triplet pairing, the superconductors have a singlet phase at low magnetic fields and a triplet phase at high magnetic fields. These two phases are separated by a first order phase transition line where the spin susceptibility changes from smaller values (singlet case) to larger normal state-like values (triplet case) at low temperatures.

Superconducting bulk magnets for magnetic levitation systems

The major applications of high-temperature superconductors have mostly been confined to products in the form of wires and thin films. However, recent developments show that rare-earth REBa 2Cu 3O 7− x and light rare-earth LREBa 2Cu 3O 7− x superconductors prepared by melt processes have a high critical-current density at 77 K and high magnetic fields. These superconductors will promote the application of bulk high-temperature superconductors in high magnetic fields; the superconducting bulk magnet for the Maglev train is one possible application. We investigated the possibility of using bulk magnets in the Maglev system, and examined flux-trapping characteristics of multi-superconducting bulks arranged in array.

Epoxy lead cones for free supported leads

The support of lead wires at the lead exits of epoxy impregnated coils can be an important aspect of the coil design. This is especially true for coils fabricated of the brittle high field superconductors including both the metallic and the oxide superconductors. There is potential damage to a lead wire due to the local Lorentz force and also due to the relative motion of the coil and surrounding support structure, if that relative motion results in mechanical stress on the lead. An extension of the free floating coil concept for epoxy impregnated coils is the free supported lead concept. A free lead is connected to the coil itself and moves with the coil. A free lead is free of rigid connection to the coil supporting structure when that connection can result in stress on the lead. A critical part of the free supported lead is the lead exit from the windings. Support of the lead wire at the lead exit may be accomplished by extending the epoxy-fiber structure of the winding region into a bonded structure that encapsulates and supports the lead exit. The resulting bonded structure tends to be in the shape of a cone. The lead cone concept is introduced through design and processing. The results of a stress analysis are described. Available mechanical test data on model structures are presented.

Activities in High Field Laboratory for Superconducting Materials at Sendai

Since High Field Laboratory for Superconducting Materials HFLSM was established at Tohoku University in 1981, hybrid magnets such as a 31 T-HM (31.1 T, 32 mm room temperature RT bore) and a 28 T-HM (28.1 T, 52 mm RT bore) have been developed and utilized for shared use in Japan. HFLSM plays a driving role of basic and applied superconducting research using high magnetic fields in Japan. The Tohoku hybrid magnets are being operated so frequently for 18 weeks a year, and have proved powerful in advanced research studies of high-T/sub c/ high field oxide superconductors, conventional high field superconductors and high field superconducting magnet technologies. Magnetism, optical properties and other high field physical properties are also being investigated in fields up to 30 T. Further, HFLSM succeeded in realizing the world's first practical cryogenfree superconducting magnet CSM. Recently, a CSM achieved 15.1 T in a 52 mm RT bore. A CSM is conveniently utilized for research in chemistry, biology and crystal growth in high fields.

Organic Superconductors as Stages for Exploration of Superconductivity in High Magnetic Fields

The low dimensionality of organic superconductors leads to pronounced anisotropy in the upper critical field. In the vicinity of the field direction parallel to the superconducting plane, the critical field shoots out due to the suppression of the orbital pair-breaking effect. Organic superconductors are suitable for the study of the high-field state related to the spin effect under an aligned field, since they are of a high crystalline quality. The reported experimental results covering the behavior at low temperatures are reviewed first, and the breakthrough of the BCS Pauli paramagnetic limit is discussed. The potential of the low-dimensional organic superconductor for study of the effect of electronic spectrum quantization is argued.

Electronic tunneling into the vortex lattice states in superconductors in high magnetic field

Local tunneling current between the normal metal and s-wave and p-wave superconductors in a high quantizing magnetic field is investigated. Differential conductance is calculated as a function of bias voltage at the centre of the vortex, and at various positions of the scanning tunneling microscope tip for a fixed voltage.

High field superconductivity in (sio2)100-xPbx sputtered films

Sputter-quenched SiO2 films containing Pb, which is insoluble to SiO2, have been found to exhibit a remarkably enhanced upper critical field (Hc2), which is about 90 times higher than that of pure Pb The sputtered films consisted of an amorphous SiO2 matrix and fcc Pb particles, dispersed homogeneously in the amorphous matrix. The particle sizes and interparticle distances were about 5 to 350 nm and 5 to 50 nm, respectively The superconducting transition temperature (Tc), upper critical magnetic field (Hc2) at 4.2 K, and residual resistivity (p10) at 10 K were 7.05 K, 4.37 T and 1.4x102 μΩm for (SiO2)29Pb71(vol %), respectively Since thel eff evaluated from linearized Ginzburg-Landau equation was 0.524 nm for x=71%, while theof pure Pb at 4.2 K was valued at 2.3xl05 nm, it can be concluded that thel eff of these SiO2-Pb films was drastically reduced owing to the refinement of Pb particles and internal defects (stress) induced by vapor quenching. Therefore the high field superconductivity can be interpreted mainly due to the significant decrease of coherence length, resulting from the large reduction of the effective mean free path length (l eff) of electrons The dual phase microstructure also had influence on the increase of the fluxoid pinning force and behaved like a type-II superconductor

Specific heat of high temperature superconductors in high fields at Tc: from BCS to the Bose–Einstein condensation

We consider the experimental trends in a database of specific heat measurements near T c in high magnetic fields for type-II superconductors with a large value of κ= λ/ ξ, including mostly high-temperature superconductors. Whereas the BCS limiting case is well established in low- T c superconductors, the exact 3D- XY behavior illustrated by the λ-transition of 4 He applies only in the particular case of optimally doped YBa 2Cu 3O x . Otherwise, transitions are intermediate either between the BCS and the 3D- XY models (e.g., YBa 2Cu 3O 7.00), or between the 3D- XY model and the Bose–Einstein condensation (BEC) (e.g., Bi 2Sr 2CaCu 2O 8). The key parameter in ordering this sequence appears to be the product k F ξ of the Fermi wave number by the coherence length, as evaluated from tunneling spectra in the vortex cores. Such a trend, which is consistent with theoretical descriptions of the strong coupling limit, is visible in the thermodynamics of the phase transition. Implications on the effective mass, the density of pairs just above T c, the pseudo-gap behavior, etc., are discussed.

Effects of strong magnetic fields on pairing fluctuations in high-temperature superconductors

We present the theory for the effects of superconducting pairing fluctuations on the nuclear spin-lattice relaxation rate, 1/T_1, and the NMR Knight shift for layered superconductors in high magnetic fields. These results can be used to clarify the origin of the pseudogap in high-T_c cuprates, which has been attributed to spin fluctuations as well as pairing fluctuations. We present theoretical results for s-wave and d-wave pairing fluctuations and show that recent experiments in optimally doped YBa_2Cu_3O_{7-d} are described by d-wave pairing fluctuations. In addition, we show that the orthorhombic distortion in YBa_2Cu_3O_{7-d} accounts for an experimentally observed discrepancy between 1/T_1 obtained by NQR and NMR at low field. We propose an NMR experiment to distinguish a fluctuating s-wave order parameter from a fluctuating strongly anisotropic order parameter, which may be applied to the system Nd_{2-x}Ce_xCuO_{4-d} and possibly other layered superconductors.

Development of New High-Field Superconductors.

New (Nb, Ta)3Sn conductors with excellent high-field performance have been fabricated using Ta-Sn powder as a core material. The Ta-Sn powder was easily synthesized by the melt diffusion reaction between Ta and Sn powders. The powder was encased in a Nb or Nb-Ta alloy sheath to form a composite, and then fabricated into wire and tape specimens. The resulting specimens were reacted at 800-925°C in vacuum. After the reaction a thick (Nb, Ta)3Sn layer, several tens micrometers in thickness, is formed between the sheath and the core. No void is formed in the core after the reaction. Ta is incorporated into the (Nb, Ta)3Sn layer from both the Ta-Sn core and the Nb-Ta sheath. The Jc of the (Nb, Ta)3Sn layer reaches 5×104A/cm2 at 23T and 4.2K. The tape specimens show large critical current at 23T due to the large thickness of the (Nb, Ta)3Sn layer.

Revival of superconductivity in high magnetic fields and a possiblep-wave pairing in(TMTSF)2PF6

Revival of superconductivity in high magnetic fields and a possible<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi></mml:math>-wave pairing in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mn /><mml:mo>(</mml:mo><mml:mi mathvariant="normal">TMTSF</mml:mi><mml:mo>)</mml:mo><mml:mn /></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">PF</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Gap Symmetry and a Revival of Superconductivity in High Parallel Magnetic Fields in Q2D and Q1D Organic, High-Tc, and Sr2RuO4 Compounds

From the BCS theory, we derive an equation determining the upper critical field, Hc2(T), parallel to conducting planes of both layered Q2D and Q1D superconductors. It extends the quasiclassical Ginzburg–Landau–Abrikosov–Gor'kov (GLAG) and Lawrence–Doniach descriptions of Hc2(T) to the case of high magnetic fields where the quantum nature of an electron motion along open Fermi surfaces is important. This equation demonstrates two new phenomena: (1) a complete restoration of superconductivity at H ≃ Hc5 > Hc2(0) in the case of p-wave pairing of electrons; (2) a surviving of superconductivity at Hc2(0) < H < H*p in the case of s(d)-wave pairing, where H*p ≠ Hp [Here, Hc2(0) and Hp are the quasiclassical GLAG upper critical field and the paramagnetic Clogston–Chandrasekhar limiting field, correspondingly; Hc5 and H*p are defined in the text]. Our analysis of recent experimental data on (TMTSF)2PF6 by I. J. Lee et al. and M. J. Naughton et al. shows that superconductivity significantly exceeds H*p. This demonstrates a strong suppression of the Pauli pair-breaking effects and may reflect a p-wave pairing of electrons. We propose to measure Hc2(T) in Sr2RuO4 (which is believed to be a p-wave superconductor) to prove a symmetry of a superconducting order parameter.