Superconductor In Magnetic Field Research Articles

Specific heat of high temperature superconductors in high magnetic fields

We review investigations of the mixed-state specific heat (C) of extreme type-II superconductors in magnetic fields B≤16 T, from 1 K to T c. In YBa 2Cu 3O x (Y123) at low temperature T, a scaling law Δ C∞( TB 1 2 )ƒ( T/B 1 2 ) , which is a signature of d-wave symmetry, is observed for the background-free anisotropic component ΔCC(B//c)-C(B//ab). At intermediate T, a C(B)C(0)∞T nBlnB behaviour, which may be explained by the London model, smoothly sets in. Near T c, superconducting fluctuations are investigated using the background-free derivative ∂(C/T)/∂B. 3D-XY scaling properties are verified up to the overdoped regime. Fluctuations obey different laws in Bi 2Sr 2CaCu 2O x (Bi2212), which is strongly affected by its 2D character. Finally, the melting line of the vortex solid, which is now believed to be the only true phase transition above H c1 in the presence of fluctuations, is observed in Y123 either as a 1st- or 2nd-order transition, depending on pinning.

Scaling behavior and 3D-2D dimensional crossover in ceramic Tl 2Ba 2Ca 2Cu 3O 10−δ high- Tc superconductors

The scalng behavior and dimensional crossover in ceramic Tl 2Ba 2Ca 2Cu 3O 10−δ high- T c superconductors in magnetic fields 0 ≤ H ≤ 12000 Oe are determined by means of comprehensive studies of I– V curve characteristics. In the low magnetic field region 0 ≤ H ≤ 2000 Oe, well-defined three-dimensional (3D) vortex-glass phase transition temperatures T g( H) are found. All of the nonlinear I– V curves above and below T g( H) can be collapsed into two universal lines with the dynamic and static critical exponents z = 2.4 ± 0.6, v = 1.0 ± 0.2, clearly showing that the system is in 3D. When H ≈ 2000 Oe and above, the I– V curves on the log-log scale plot begin to be tortuous and this indicates that the system deviates from the 3D behavior. When H ≥ 6000 Oe, all of the I– V curves can be collapsed into one universal line with critical exponents p = 2.3 ± 0.2 and v 2d = 2.0 ± 0.3, which stands for a two dimensional (2D) vortex-glass scaling phenomenon. Our data show that in the quasi-2D region the characteristic temperature T 0( H) continuously decreases as the magnetic field increases. By extensions of 3D vortex-glass phase transition model and quasi-2D scaling process into the intermediate field region (2000 < H < 6000 Oe), we find a correlation between T 0( H) and T g( H). Combining results obtained from the three different regions, a complete phase transition line is determined in the whole field region 0 ≤ H ≤ 12000 Oe.

Magnetic breakdown and quantum interference in the quasi-two-dimensional superconductor in high magnetic fields

Magnetic breakdown phenomena have been investigated in the longitudinal magnetoresistance of the quasi-two-dimensional (Q2D) superconductor in magnetic fields of up to 50 T, well above the characteristic breakdown field. The material is of great interest because its relatively simple Fermi surface, consisting of a closed Q2D pocket and an open Q1D band, is almost identical to the initial hypothetical breakdown network proposed by Pippard. Two frequencies are expected to dominate the magnetoresistance oscillations: the frequency, corresponding to orbits around the closed pocket, and the frequency, corresponding to the simplest classical breakdown orbit. However, a frequency is in fact found to be the dominant high-frequency oscillation in the magnetoresistance. Numerical simulations, employing standard theories for calculating the density of states, indicate that a significant presence of the frequency (forbidden in the standard theories) can result simply from the frequency-mixing effects associated with the pinning of the chemical potential in a quasi-two-dimensional system. While this effect is able to account for the previous experimental observation of frequency oscillations of small amplitude in the magnetization, it cannot explain why such a frequency dominates the high-field magnetotransport spectrum. Instead we have extended the numerical simulations to include a quantum interference model adapted for longitudinal magnetoresistance in a quasi-two-dimensional conductor. The modified simulations are then able to account for most of the features of the experimental magnetoresistance data.

Localization Transitions in Non-Hermitian Quantum Mechanics.

We study the localization transitions which arise in both one and two dimensions when quantum mechanical particles described by a random Schr\"odinger equation are subjected to a constant imaginary vector potential. A path-integral formulation relates the transition to flux lines depinned from columnar defects by a transverse magnetic field in superconductors. The theory predicts that the transverse Meissner effect is accompanied by stretched exponential relaxation of the field into the bulk and a diverging penetration depth at the transition.

Neutron scattering as a probe of unconventional superconductivity in YBa2Cu3O7 (invited) (abstract)

We have carried out a variety of neutron scattering experiments on the high temperature superconductor YBa2Cu3O7 which elucidate the unconventional microscopic properties of the cuprates and illustrate the power and versatility of neutron scattering as a probe of novel materials. Small angle magnetic neutron diffraction is thus far the only technique capable of revealing the structure of the vortex lattice in the cuprate superconductors in magnetic fields of several tesla. We have shown that in YBa2Cu3O7 the vortex lattice in this field range has an oblique structure when the field is applied along the c axis (perpendicular to the CuO2 layers). Recent theoretical work has attributed this unusual structure to the unconventional (d-wave) pairing state realized in the cuprates. We will also discuss the rich behavior of the vortex lattice when the field is tilted with respect to the c axis. We have used inelastic neutron scattering to elucidate the magnetic and lattice vibrational excitations in YBa2Cu3O7. Magnetic and phonon scattering were separated both by detailed calculations of phonon dynamical structure factors and by neutron polarization analysis. A novel resonant magnetic excitation at wave vector q=(π/a,π/a) and energy ℏω=40 meV appears in the superconducting state. The simplest model for this resonance relies on quasiparticle pair creation and implies a sign reversal of the energy gap function on the Fermi surface. Alternative models will also be discussed. We have also observed a strongly wave vector dependent renormalization of the energy and lifetime of a specific phonon in the superconducting state of YBa2Cu3O7. Since the phonon self-energy is sensitive to the anisotropies of the energy gap and Fermi surface via the electron–phonon interaction, our data contain important information about these parameters.

High frequency electromagnetic action on dc magnetic moment of YBaCuO granular superconductor

High frequency (0.3–3GHz) electromagnetic field action on dc magnetic moment of granular YBaCuO superconductor in magnetic field was measured by SQUID magnetometer at temperature 4.2K. It was found that 1) Changing of dc magnetic moment ΔM is proportional to the field amplitudeh e, 2) At low magnetic fieldH dependence ΔM onH is proportionalH 2, 3) Dependence ΔM onH is irreversible and has evolution time about second. This behavior can not be described only by the critical state model.

Resistive noise of YBCO high- Tc granular superconductors in magnetic field

Resistance and resistance-noise experimental results, obtained in the transition region of two YBCO bulk superconductors submitted to a DC magnetic field are reported. The magnetic field was found to influence only the second stage of the transition, enlarging the transition-temperature interval and determining a shift of the noise peak towards lower temperatures. The sample is modelled as a precolating network formed by Josephson junctions and resistance noise is ascribed to the random switching of certain junctions (p-noise) [L.B. Kiss et al. Physica C 207(1993)318]. A junction may be enable to switch either by thermal fluctuations or by random motion of intergranular vortices. The square of the temperature derivative of the resistance was used to control the thermal-fluctuations effect in producing the noise. The magnetic field is believed to influence the resistance and resistance noise via a parameter p (percentage of phase-locked junctions) and p-noise behavior is considered an intrinsic property of the network.

Identification of stacking faults with flux pinning in powder melting processed YBa2Cu3Oy

Highly textured YBCO samples with different densities of stacking faults are prepared by a powder melting process (PMP). Microstructural examination using transmission electron microscopy (TEM) is conducted on the samples. The dc transport critical currents at 77 K in applied magnetic fields up to 22 T and magnetization hysteresis loops at temperatures, between 60 and 90 K in magnetic fields up to 7 T are measured with a SQUID magnetometer. The irreversibility lines are obtained from measurements of magnetoresistance. The results indicate that the stacking faults strongly influence the flux pinning behavior of the melt processed YBCO superconductors in magnetic fields up to 22 T.

Self-consistent microscopic theory of surface superconductivity.

The electronic structure of the superconducting surface sheath in a type-II superconductor in magnetic fields $H_{c2}<H<H_{c3}$ is calculated self-consistently using the Bogoliubov-de Gennes equations. We find that the pair potential $\Delta(x)$ exhibits pronounced Friedel oscillations near the surface, in marked contrast with the results of Ginzburg-Landau theory. The role of magnetic edge states is emphasized. The local density of states near the surface shows a significant depletion near the Fermi energy due to the development of local superconducting order. We suggest that this structure could be unveiled by scanning-tunneling microscopy studies performed near the edge of a superconducting sample.

Specific heat of Bi 2Sr 2CaCu 2O 8+x in magnetic fields up to 16 Tesla

High resolution specific heat measurements of Bi 2Sr 2CaCu 2O 8+x superconductors in magnetic fields up to 16 Tesla are presented. In a polycrystalline sample the magnetic field leads to a drastic suppression and a pronounced broadening of the anomaly at T c. In contrast, our measurements on a high quality single crystal for fields parallel to the CuO 2-layers do not show any significant suppression of the specific heat at T c. Moreover, the specific heat above T c, which is attributed to superconducting fluctuations, decreases. As a consequence the anomaly at T c appears to become sharper with increasing magnetic field.

Ginzburg-Landau theory of specific heat anomaly of high- Tc superconductors in magnetic field

The change in specific heat by the application of magnetic field ( B a ∼1 T) in the case of the high- T c superconductors is examined phenomenologically by the Ginzburg-Landau theory of anisotropic type-II superconductors. The observed large reduction of specific heat anomaly just below the superconducting transition is explained quantitatively. We have obtained the expression for the change in specific heat in a magnetic field ( ΔC) for any general orientation of B a with respect to the crystallographic c-axis. We also present the result for a polycrystalline sample. ΔC is shown to be linear in ( B a T c ) [ t (1 −t) ] for temperatures slightly below T c, where t= T T c . The slope of this linear behaviour is found to be related to the anisotropy of the effective mass.

Analysis of the specific heat of high-T c superconductors in magnetic fields

We have performed high-resolution specific-heat measurements on an untwinned, high-quality single crystal of LuBa 2Cu 3O 7−x, near the normal-superconducting transition temperature, in applied magnetic fields up to 5 T, along the c-axis. The data has been analyzed in terms of the finite-size scaling approach, Inderhees, et al., (PRL 66, 232, (1991)). We have found that this idea does not adequately describe our results. The problems associated with this model will be detailed. Recently, a model proposed by Tesanovic et al. (PRL 69, 3563 (1992)) has been utilized with some success.

An analogy between superconductors and smectics A

The conformation of a smectic A can be described by a phase function φ( R), the n-th layer corresponding to φ(R) = 2 πn. The role of the phase in smectics A and in superfluids is similar. This analogy leads to the following predictions for a second order smectic A ↔ nematic transition: (1) the transition temperature is lowered if twist, or bend distortions are imposed: these distortions correspond to a magnetic field in superconductors. (2) the Frank coefficients K 2 and K 3 of the nematic phase must show pretransitional anomalies.

A Superconductive Electromagnetic Pump Without Any Mechanical Moving Parts

Mechanical reliability is one of the main obstacles to long-term performance of an artificial heart. To solve this problem a superconductive electromagnetic pump was developed. Two concentric cone-shaped cylinders serve as the pump housings and electrodes. As a current passes through the blood between the inner and outer housings, the blood rotates under the action of a superconductive magnetic field. Therefore, this is a rotary pump without a rotor or any mechanical moving parts. The device was tested in a super-conductive magnetic field with 7 Tesla. By 5 V, and 1 A, a 0.9% saline flow of approximately 1 L/min, and 10 mm H2O was obtained. For further development, a stronger magnetic field with ca. 20 Tesla is desirable. Contrary to traditional thought, blood and saline have almost the same conductibility as any other conductors. For blood damage testing, fresh porcine blood was used. The circulation was maintained by an impeller pump, with the electromagnetic pump serving only as electrodes. Comparing the first and second periods of testing with and without a current of 1 A, the difference in hematologic variations evaluated the blood damage by the electric current passing directly through the blood. Results indicated that the electric current causes no serious blood damage.

Resistance in High-Temperature Superconductors

The effects of magnetic fields on high-temperature superconductors are considered. Lines of flux of magnetic fields in superconductors consist of whirlpools of electric current, or vortices, movement of which can impede the flow of electrons and increase the resistance of superconductors. Vortex movement under various temperature and magnetic field conditions is critical in controlling supercurrent flow. Research on superconductors and the microscopic theory of superconductivity are briefly reviewed. Three magnetic states of superconductors are described and magnetic phase diagrams are presented. Current flow and its interaction with type II superconductors are described. Vortex behavior and the formation of vortex liquid due to vortex lattice melting in high temperature superconductors is discussed. Superconductor microstructure, composition, pinning, and the effects of these factors on vortex solids are considered. Resistance in vortex lattices and vortex glass is discussed. An understanding of these vortex states will assist in the engineering of better superconducting materials. 5 refs.

超伝導と高圧力 超電導の新しい応用 微生物の代謝と高磁場

Extensive studies on the effect of a low intensity magnetic field on microbial activity have been reported. However, they are often controversial; the magentic fields are accelerative on the growth, or inhibitory, or noneffective. The inconsistency of the results is mainly because the experimental conditions were improperly controlled and well characterized microbial strains were not used. Following the invention of superconductive materials, high magnetic fields produced by superconductive magnets will be applicable to various microbial reaction control. In this paper, a super high magetic field of 11. 7 tesla (T) was imposed on the auxotrophic bacteria of Escherichia coli and the effect of the magnetic field was investigated under various nutritional and temperature conditions. The 11. 7 T was also imposed on E. coli where the bacteriophage lambda was lysogenic and carries a temperature-sensitive mutation in the phage repressor gene. The transition from lysogenic to lytic process was investigated. A new bioreactor which was operated under 7 T superconductive magnetic field was introduced.

Effect of axial strain on the critical current of Ag-sheathed Bi-based superconductors in magnetic fields up to 25 T

The irreversible strain limit εirrev for the onset of permanent axial strain damage to Ag-sheathed Bi2Sr2Ca1Cu2O8+x and Bi2Sr2Ca2Cu3O10+x superconductors has been measured to be in the range of 0.2%–0.35%. This strain damage onset is about an order of magnitude higher than for bulk sintered Y-, Bi-, or Tl-based superconductors and is approaching practical values for magnet design. The measurements show that the value of εirrev is not dependent on magnetic field, nor does the critical current depend on strain below εirrev at least up to 25 T at 4.2 K. Both of these factors indicate that the observed strain effect in Ag-sheathed Bi-based superconductors is not intrinsic to the superconductor material. Rather, the effect is extrinsic and arises from superconductor fracture. Thus, the damage onset is amenable to further enhancement. Indeed, the data suggest that subdividing the superconductor into fine filaments or adding Ag to the superconductor powder prior to processing significantly enhances the damage threshold εirrev to above 0.6%.

Frustration-induced nonlinear resistivities of high-Tc superconductors in magnetic fields.

We have performed a finite-temperature (T) dynamic simulation to calculate transport properties of a two-dimensional (2D) frustrated Josephson-coupled lattice as a model of high-${\mathit{T}}_{\mathit{c}}$ superconductors in magnetic fields (H/${\mathit{H}}_{\mathit{c}2}$\ensuremath{\ll}1). In the weak-frustration region the I-V characteristics and magnetoresistance obey coexisting power laws with continuously varying exponents, V\ensuremath{\propto}${\mathit{I}}^{\mathrm{\ensuremath{\alpha}}(\mathit{T},}$${\mathit{H}}^{)}$ and V\ensuremath{\propto}${\mathit{H}}^{\mathrm{\ensuremath{\beta}}(\mathit{T},}$${\mathit{I}}^{)}$. These field-dependent nonlinear resistive behaviors are analogous to experimentally observed ones in single crystals of high-${\mathit{T}}_{\mathit{c}}$ oxides, and they are ascribed to the frustration-induced unbinding mechanism of 2D vortex-antivortex pairs. A possible Kosterlitz-Thouless vortex-antivortex unbinding transition in magnetic fields is also addressed based on the temperature dependence of power-law exponents.

Magnetization of anisotropic superconductors in the tilted magnetic field

Abstract The angular and field dependences of reversible magnetization of uniaxial superconductors in magnetic fields tilted to the c-axis are investigated. In contrast to the usual situation with one maximum of M(h) at Hc1, an additional maximum appears in the region of higher field. The peculiarities of the inclined Abrikosov vortex lattice in quasi one-dimensional and quasi two-dimensional superconductors are discussed.

Theoretical study ofμ + site in YBa2Cu3O x site in YBa2Cu3O x

The muon is a useful probe of magnetic fields in superconductors, but knowing the field seen by the muon is often of limited value until we know where the muon is in the crystal lattice. Here we employ two independent theoretical methods to search for candidate muon sites:the potential energy field method, which seeks the minimum of the electrostatic potential of theμ +, and themagnetic dipolar field method, which compares the calculated magnetic field (due to host electronic or nuclear dipolar fields) with the observed local fields at the muon.