Meissner Screening Research Articles

Scanning SQUID microscopy of sparsely twinnedYBa2Cu3O7−δ

Magnetic images of two sparsely twinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals were taken in low applied magnetic fields (${\mathrm{H}}_{\mathrm{a}}$100 mG) with a scanning SQUID microscope. The SQUID sensors have 4 or 8 \ensuremath{\mu}m pickup loops and a magnetic flux resolution of ${10}^{\mathrm{\ensuremath{-}}3}$hc/2e under normal operating conditions. The images showed Meissner screening with a large demagnetization factor and isolated trapped vortices. The trapped vortices carried integral (hc/2e) magnetic flux and were much smaller than the spatial resolution of the sensor. At this level of resolution, there was no evidence for unusual magnetic features, such as fractional vortices, associated with the twin boundaries.

High-Temperature Anyon Superconductivity

The screening of an applied magnetic field in a charged anyon fluid at finite density (μ≠0) and temperature (T≠0) is investigated. Using the semi-infinite sample boundary conditions we find, at densities typical of high-temperature superconducting materials, that the anyon fluid exhibits a superconducting behavior. The total Meissner screening is characterized by two penetration lengths, corresponding to two short-range eigenmodes of propagation within the anyon fluid.

Magneto-optic observation of anomalous Meissner current flow in superconducting thin films with slits

Slits patterned into a YBa2Cu3O7−δ thin film were observed to obstruct Meissner sheet currents leading to an imbalance in the local Meissner screening properties. The new phenomenon was studied with magneto-optic imaging where twin lobes of opposite flux polarity were seen to form near the slits and inside the Meissner region. The lobe closest to the sample edge is always polarized opposite to the applied field. At weak fields, the anomalous flux generation is reversible. At higher fields, but still sufficiently small to keep the vortex penetration front away from the slits, the anomalous current starts nucleating flux lines which become trapped when the field is removed.

Meissner effect in Au induced by the proximity effect in the clean limit

The Meissner effect in Au induced by the proximity effect of Nb has been investigated for Au-clad Nb wire and Au/Nb/Au sandwich over a wide range of Au thicknessdN and the electronic mean free path lN in Au. The Meissner screening length ϱ in Au increases in proportion toT−1 with a decrease in temperature and becomes nearly constant as it approachesdN. Within the temperature range where ϱ is proportional toT−1, ϱ is independent of lN anddN. The typical temperature dependence of ϱ expected in the clean-limit theory has not been observed.

Measurements of Hc1 and Jc in Bi 1.95Sr 1.65La 0.4CuO 6+δ with a micro-Hall probe

Using a Hall-probe technique, the magnetisation of Bi 1.95Sr 1.65La 0.4CuO 6+δ is measured, yielding values of the field of first penetration H p( T) along with the critical current density J c( T) in low fields. Due to reduced surface barriers and a very small critical current density H p is close to H c1. From this, and published values of ξ ab , an upper-bound estimate of the in-plane penetration depth λ ab = 10 000±1000 A ̊ is obtained. Also a reduction of the Meissner screening is observed for 9< T< T c =13 K and is discussed. A quantitative comparison of these results is made with some experiments and recent theories which predict vortex fluctuations in such materials.

Equilibrium magnetic properties and Meissner expulsion of magnetic flux in Bi 2Sr 2CaCu 2O x single crystals

The unusual spatial transformation of the magnetic-flux structure in Bi 2Sr 2CaCu 2O x single crystals during magnetization of zero-field cooled samples and during field cooling (the Meissner flux expulsion) is directly visualized by means of the magneto-optical technique. In both cases similar inhomogeneous flux distributions are observed at high temperatures in a magnetic field applied perpendicularly to the basal plane: The magnetic field in the crystal center is approximately equal to the applied field, but at the crystal perimeter a belt of lower field occurs. A model of such an equilibrium flux structure is developed taking into account the real shape of the crystals. The distribution of the Meissner screening current in this geometry is directly measured for the first time.

Nonlinear electromagnetic rectification of BCS-paired electrons at a superconductor surface.

A theory describing nonlinear electromagnetic rectification of BCS-paired electrons at a superconductor-vacuum interface by means of a monochromatic, plane electromagnetic wave incident at an oblique angle is presented. On the basis of a recently constructed nonlinear-response tensor, the forced nonlinear dc-current density is analyzed. A fundamental integro-differential equation incorporating the Meissner screening of the nonlinear response is established, and the prevailing dc-current density and the associated magnetostatic field are determined assuming the BCS-paired electrons to be specularly reflected from the surface. A self-consistency requirement for the forced nonlinear current density which has to be obeyed for any specular-reflection model is derived, and polarization selection rules for the incident electromagnetic field are established. For temperatures above the transition temperature, the present theory describes nonlinear electromagnetic rectification in a collisionless unpaired jellium. In the final part of the present work, the possibilities of achieving nonlinear electromagnetic rectification using incident waves of frequencies around the plasma edge, i.e., far above the superconducting gap frequency, are investigated. It is predicted that polariton-plasmon and plasmon-plasmon interactions can give rise to optical rectification.

Meissner screening by induced weak charge in the dense Weinberg-Salam system

The magnetic screening in cold electroweak matter at high density of leptons J L ⩾M W 3 6π 2 is investigated within the approximation known to guarantee the Bose-Einstein condensation of W ±-mesons. It is shown that the photon in this medium acquires a magnetic mass, the electro-magnetic field becoming short-range. It is found that the Meissner damping of a magnetic field is provided in this approximation first of all by an induced polarization charge of weak nature.

Proximity-effect-induced superconductivity at millikelvin temperatures

A systematic study is reported of the proximity effect in superconductivity of thick Cu layers (4≤dn≤105 µm) in contact with Nb or Nb-Ti (46≤Ds≤1041 µm) from measurements of the ac susceptibility and of the thermal conductivity. The induced superconducting properties of Cu are studied as a function of temperature (7 mK≤T≤1 K), field (0.4 mG≤H≤ 10 G), and electronic mean free path (0.2≤l≤14 µm). The Meissner screening length in Cu increases faster than the coherence length with decreasing temperature and eventually saturates, making the Cu fully superconducting. In this case, superconductivity can be destroyed sharply at a breakdown fieldHb; the transition atHb is hysteretic. The data are in agreement with numerical solutions of the de Gennes/Ginzburg-Landau theory, as demonstrated by the dependence of the Meissner screening length and of the breakdown field on temperature, field, and electronic mean free path. The data indicate that observation of a reduction in thermal conductivity requires a substantially larger induced pair potential than is necessary even for total field shielding. The concentration of Cooper pairs in Cu in contact to Nb is larger than in Cu in contact to Nb-Ti, even for Nb-Ti/Cu samples with a larger electron mean free path in the Cu part. No measurable superconducting field screening could be induced in the investigated temperature range in Pd (26≤dn≤72 µm) in contact to Nb/Cu or to Nb (Ds≈5dn). This is explained by the fact that the coherence length is at least an order of magnitude smaller in Pd than in Cu. Evaporating a layer of Fe on Cu in the Nb/Cu samples results in a strong depression of the proximity effect.

Proximity-Effect Induced Superconductivity at Millikelvin Temperatures

We have studied the proximity induced superconductivity of thick Cu layers (up to 105 µm) clad on Nb or Nb-Ti cores by measurements of the ac susceptibility and of the thermal conductivity as a function of temperature (7 mK ≦ T ≦ 1 K), field (0.4 mG ≦ H ≦ loG), and electronic mean free path (0.2 µm ≦ℓ≦14 µm). The dependence of the Meissner screening length and of the hysteretic breakdown field on these parameters agree with numerical solutions of the de Gennes/Ginzburg-Landau theory. Evaporating a layer of Fe on Cu in our Nb/Cu sample results in a strong depression of the proximity effect. No measurable field screening was observed in the investigated temperature range in Pd in contact with Nb/Cu or with Nb. -Experiments to extend these measurements to lower temperatures and thicker Cu layers are in progress.

Critical currents of cross-type superconducting-normal-superconducting junctions in perpendicular magnetic fields.

The maximum supercurrent that can be carried in an applied perpendicular magnetic field by a superconducting--normal-metal--superconducting (SNS) junction consisting of a square normal-metal layer sandwiched between two crossed perpendicular superconducting strips is investigated theoretically and experimentally. For weak applied fields the critical current is suppressed reversibly, as induced Meissner screening currents flow into the SNS sandwich and generate a spatially varying magnetic field largely parallel to the junction, thereby altering the local phase difference. For stronger applied fields the critical current is changed irreversibly as vortices enter and become pinned in the junction. When the pinned vortices in the two superconductors are misaligned, the local magnetic field, which flows mostly parallel to the junction in carrying magnetic flux ${\ensuremath{\varphi}}_{0}$=hc/2e from one vortex to the other, strongly alters the phase difference across the junction near the two vortices. The theory predicts complex patterns of the supercurrent density, which should be directly observable using laser or electron-beam scanning techniques.

Pinning forces in Nb and InPb type II superconductors

Abstract The pinning forces in Nb and Pb-In were determined by mechanically sweeping a magnetic field over it and measuring the resulting force. Three different forces due to the interaction between the magnetic field , the trapped flux lines, the Meissner screening current, and the pinning sites of the specimen were identified. The magnitude of the bulk pinning force was in agreement with that calculated from the magnetization via Irie-Yamafuji model; F=αBr. The temperature dependence of the pinning force was found to be given by Fp(T)=Fp(0) (1−T/Tc) and it is suggested that this can be understood in terms of Anderson's flux creep model.

PROXIMITY EFFECT OF Cu CLAD Nb AND Cu CLAD NbTi WIRES

The Meissner screening length in Cu of Cu clad Nb and Cu clad NbTi wire has been measured over temperature range from Tc down to 25 mK. The temperature dependence of the screening length is expressed by (T+T0)-1/2 from about 1.5 K to the minimum temperature. The length becomes 12 µm for Cu clad Nb and 7 for Cu clad NbTi wire at 25 mK. The measurement has been also done for Cu clad fine multiple NbTi wire. In this case, the screening length shows a drastic increase above 1 K, and the slope becomes remarkably small below 1 K. Some comments for application are also presented.

Meissner Effect in Cu Backed by Superconducting Nb and NbTi Wires

The Meissner screening length in Cu of Cu clad Nb and Cu clad NbTi wire was measured over temperature range from T c down to 25 mK. The temperature dependence of the screening length was found to be expressed by ( T + T 0 ) -1/2 from about 1.5 K to the lowest temperature, rather than by T -1/2 derived by Deutscher and de Gennes for the dirty limit. The length becomes 12 µm for Cu clad Nb and 7 µm for Cu clad NbTi wire. Preliminary measurements of field and frequency dependence are also presented.

Mould for slip-casting ceramic parts: I A Budkin et al, USSR Patent, No 351,704, appl 2 Dec, 1970, publd 10 Oct, 1972

The pinning forces in Nb and Pb-In were determined by mechanically sweeping a magnetic field over it and measuring the resulting force. Three different forces due to the interaction between the magnetic field , the trapped flux lines, the Meissner screening current, and the pinning sites of the specimen were identified. The magnitude of the bulk pinning force was in agreement with that calculated from the magnetization via Irie-Yamafuji model; F=αBr. The temperature dependence of the pinning force was found to be given by Fp(T)=Fp(0) (1−T/Tc) and it is suggested that this can be understood in terms of Anderson's flux creep model.

Change in velocity of high frequency transverse sound at the superconducting transition temperature

The change in velocity of transverse sound at the transition temperature consists of two parts (1) a static part (usually estimated to be 1–10 ppm arising from the difference in the elastic constants between the superconducting and normal states, and (2) a purely dynamical effect due to Meissner Screening of the transverse electromagnetic fields at the transition temperature. It is shown that under suitable conditions, as first suggested by Ozaki and Mikoshiba, the second mechanism can give rise to much larger changes in the velocity of high frequency sound. In fact for Al at a sound wave frequency of 10 6 c s and ql = 10 (here q is the wave number of the sound wave and l is the electron mean free path) the relative change in velocity can be as much as 1 part per thousand. The results of a detailed microscopic calculation of the relative change in velocity (v s−v n) v n are presented for weak coupling type I superconductors.