High temperature superconducting ball formation in low frequency ac fields

Tao, Ruibao; Xu, Xiaofeng; Amr, E.

doi:10.1103/physrevb.68.144505

Cited by 9 publications

(14 citation statements)

References 15 publications

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“…Jenks et al 7 performed such measurements on an YBCO High-T c superconductor and reported that the penetration depth down to T c was decreasing with temperature as expected from the Thomas-Fermi theory, but once the YBCO was superconductive, the electric field penetration depth increased again with temperature. Tao et al [8][9][10] recently observed the formation of balls when low and High-Tc superconducting particles were exposed to a strong electric field. In their analysis, they had to assume that the electric field penetration depth is at least an order of magnitude higher than the Thomas-Fermi length 9 .…”

Section: Introductionmentioning

confidence: 99%

Electrodynamics in superconductors explained by Proca equations

Tajmar¹

2008

Physics Letters A

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A fully consistent model to study electrodynamics for superconductors in the stationary and non-stationary regime has been developed based on Proca equations and a massive photon. In particular, this approach has been applied to study the electric field penetration depth in superconductors. The model shows a deviation from the charge contribution to an internal electric field compared to previous approaches.

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Section: Introductionmentioning

confidence: 99%

Electrodynamics in superconductors explained by Proca equations

Tajmar¹

2008

Physics Letters A

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“…In fact, some of recent measurements suggests that the electric field penetrates deep into superconductors. 8 Interpretation of these observations is not yet settled, therefore we prefer to assume that the screening in superconductors is similar to the screening in normal metals so that δn is non-zero only on the scale of the Thomas-Fermi screening length. The typical Thomas-Fermi length is less then oneÅngström, while the gap function varies on a scale typical to the BCS kernel ∼ ξ 0 .…”

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confidence: 99%

Ginzburg-Landau theory of superconducting surfaces under the influence of electric fields

et al. 2006

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A boundary condition for the Ginzburg-Landau wave function at surfaces biased by a strong electric field is derived within the de Gennes approach. This condition provides a simple theory of the field effect on the critical temperature of superconducting layers.The critical temperature of a thin superconducting layer is increased or lowered by an electric field applied perpendicular to the layer. [1][2][3][4][5] Similarly to the conductivity of inverse layers in semiconductors, superconductivity of thin metallic layers can thus be controlled by a gate voltage, which makes these structures attractive for applications.In this paper we show that the phase transition in a thin metallic layer is conveniently described by the Ginzburg-Landau (GL) theory, where the electric field E enters the GL boundary condition asBriefly, the logarithmic derivative of the GL function ψ or the gap function ∆ at the surface is a sum of the zerofield part 1/b 0 and the field induced correction E/U s . The zero-field part has been derived by de Gennes 6 from the BCS theory. A typical value b 0 ∼ 1 cm is large on the scale of the GL coherence length, therefore this contribution is usually neglected. This approximation, 1/b 0 ≈ 0, corresponds to the original GL condition ∇ψ = 0.Here we employ the de Gennes approach to derive the field induced correction E/U s . The correction becomes important for the above mentioned experiments, where fields of the order of 10 7 V/cm are realized. Small electric fields appearing e.g. in Josephson junctions do not require such corrections.We start from the conditionderived by de Gennes (Eq. (7-62) in Ref. 6). Here N 0 is the density of states of a bulk material, V is the BCS interaction, and N (x) is the local density of states at position x. The actual gap function ∆(x) has a nontrivial profile close to the surface at x = 0, but it has only slow variation at distances exceeding the BCS coherence length ξ 0 = 0.18hv F /k B T c . For x ∼ ξ 0 it is crudely linear ∆(x) ≈ ∆ 0 (1 + x/b), so that ∆ 0 is not the true surface value but the extrapolation of the gap function towards the surface. In Eq. (2) we have used the GL coherence length at zero temperature ξ(0) = 0.74 ξ 0 for pure metals. In measurements of the field effect on the transition temperature, the zero-field term b 0 is included in the reference zero-bias transition temperature. Accordingly, we can assume a model of the crystal for which 1/b 0 = 0. The simplest model of this kind is a semi-infinite jellium, where for zero field the density of states is steplike, N (x) = N 0 for x > 0 and N (x) = 0 elsewhere. Using that the gap function is restricted to the crystal, ∆(x) = 0 for x < 0, one can check that from (2) follows 1/b 0 = 0. Now we include the electric field. According to the Anderson theorem 7 , the electric field does not change the thermodynamical properties directly but only via the density of states. The change of the density of states is also indirect. The penetrating electric field induces a deviation δn of the electron density. The den...

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“…However, the most important issue is that the water induced ball formation by shaking has nothing to do with the electric field-induced superconducting ball formation. [12][13][14][15][16] When we used liquid nitrogen to work with high temperature superconducting particles, the particles were dried, and the experiments were always conducted in a glove bag with dry nitrogen. No open air was in contact with our particle samples.…”

Section: Water-induced Aggregationmentioning

confidence: 99%

“…No open air was in contact with our particle samples. 12,14 This is the normal procedure, as conducting such experiments in open air would invite moisture and other gases to condense. 5,6 When we worked with Pb, other low temperature superconducting particles, and MgB 2 , the experiments were performed in a helium dewer, which was completely isolated.…”

Section: Water-induced Aggregationmentioning

confidence: 99%

Comment on “Spherical agglomeration of superconducting and normal microparticles with and without applied electric field”

Tao

Amr

2013

Phys. Rev. B

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Ghosh and Hirsch [Phys. Rev. B 86, 054511 (2012)] claimed that many micrometer-size particles in liquid nitrogen, as large as 25 to 32 μm, could be aggregated into balls by shaking. Ghosh and Hirsch performed their experiments with liquid nitrogen in open air; therefore, moisture condensed on the particle surface, leading to ball aggregation by shaking. This phenomenon has nothing to do with the electric field-induced formation of superconducting balls. In addition, their claim that a large electric field still exists in the interior of the capacitor when the electrodes are insulated is flawed. The electric field-induced superconducting ball formation reveals that the area of interaction between electric field and superconductors requires more investigation. However, the phenomenon can be explained within the BCS theory.

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High temperature superconducting ball formation in low frequency ac fields

Cited by 9 publications

References 15 publications

Electrodynamics in superconductors explained by Proca equations

Electrodynamics in superconductors explained by Proca equations

Ginzburg-Landau theory of superconducting surfaces under the influence of electric fields

Comment on “Spherical agglomeration of superconducting and normal microparticles with and without applied electric field”

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