Mechanism for flux guidance by micrometric antidot arrays in superconducting films

Vestgården, Jørn Inge; Yurchenko, V. V.; Wördenweber, R.; Johansen, T. H.

doi:10.1103/physrevb.85.014516

Cited by 24 publications

(29 citation statements)

References 44 publications

(57 reference statements)

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“…For over sixty years, it has been understood that the ground state vortex structure is a hexagonal lattice 2 , so many methods have been developed to increase the critical current using uniform pinning arrays that incorporate periodicity to match the vortex structure [3][4][5][6][7][8][9][10][11][12][13] . The pinning is enhanced at commensurate fields when the number of vortices equals an integer multiple of the number of pinning sites, but away from these specific matching fields, the enhancement of the critical current is lost 14 .…”

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confidence: 99%

Strongly Enhanced Pinning of Magnetic Vortices in Type-II Superconductors by Conformal Crystal Arrays

Ray¹,

Jankó²,

Reichhardt³

2013

Phys. Rev. Lett.

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Conformal crystals are non-uniform structures created by a conformal transformation of regular two-dimensional lattices. We show that gradient-driven vortices interacting with a conformal pinning array exhibit substantially stronger pinning effects over a much larger range of field than found for random or periodic pinning arrangements. The pinning enhancement is partially due to matching of the critical flux gradient with the pinning gradient, but the preservation of the sixfold ordering in the conformally transformed hexagonal lattice plays a crucial role. Our results can be generalized to a wide class of gradient-driven interacting particle systems such as colloids on optical trap arrays.PACS numbers: 74.25. Wx,74.25.Uv One of the most important problems for applications of type-II superconductors is how to create high critical currents or strong vortex pinning over a wide range of applied magnetic fields 1 . For over sixty years, it has been understood that the ground state vortex structure is a hexagonal lattice 2 , so many methods have been developed to increase the critical current using uniform pinning arrays that incorporate periodicity to match the vortex structure 3-13 . The pinning is enhanced at commensurate fields when the number of vortices equals an integer multiple of the number of pinning sites, but away from these specific matching fields, the enhancement of the critical current is lost 14 . Efforts to enhance the pinning at incommensurate fields have included the use of quasicrystalline substrates 15 or diluted periodic arrays [16][17][18][19][20] , where studies show that new types of non-integer commensurate states can arise in addition to the integer matching configurations. Hyperbolic tessellation arrays were also recently considered 21 .Part of the problem is the fact that under an applied current, the vortex structure does not remain uniform but instead develops a Bean-like flux gradient 22 : the vortex density is highest at the edges of the sample when the magnetic field is increased, and highest in the center of the sample when the magnetic field is removed and only trapped flux remains inside the sample. As a consequence, uniform pinning arrays generally have a portion of the pinning sites that are not fully occupied, suggesting that a more optimal pinning arrangement should include some type of density gradient to match the critical flux gradient. Here we show that a novel type of pinning structure, created using a conformal transformation of a uniform hexagonal lattice, produces a much higher critical current over a much wider range of magnetic fields than any pinning geometry considered up until now. Conformal crystals not only have a density gradient, but also preserve aspects of the hexagonal ordering naturally adopted by the vortex lattice. The pinning enhancement we find is substantial and will be very important for a wide range of superconductor applications and flux control. Our results can also be important for stabilizing novel self-assembled structures created using de...

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confidence: 99%

Strongly Enhanced Pinning of Magnetic Vortices in Type-II Superconductors by Conformal Crystal Arrays

Ray¹,

Jankó²,

Reichhardt³

2013

Phys. Rev. Lett.

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“…At internal boundaries, such as the inner edge of a planar ring, the field can, due to the nonlocal electrodynamics, be in the opposite direction of the applied field [6,7]. Strongly modified behavior is found also in films patterned with regular arrays of small holes (antidots), which tend to guide the flux into the superconductor [8][9][10][11].The response of Ohmic films exposed to varying transverse magnetic fields is also described by nonlocal electrodynamics, but here the material responds linearly. Numerical solutions for strip and disc geometries have shown that the combination of nonlocality and dissipation causes a rapid penetration of a suddenly applied magnetic field [12,13].…”

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confidence: 99%

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confidence: 99%

Nonlocal electrodynamics of normal and superconducting films

Vestgården¹,

Mikheenko²,

Galperin³

et al. 2013

New J. Phys.

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Electrically conducting films in a time-varying transverse applied magnetic field are considered. Their behavior is strongly influenced by the self-field of the induced currents, making the electrodynamics nonlocal, and consequently difficult to analyze both numerically and analytically. We present a formalism which allows many phenomena related to superconducting and Ohmic films to be modeled and analyzed. The formalism is based on the Maxwell equations and a material current-voltage characteristics, linear for normal metals and nonlinear for superconductors, plus a careful account of the boundary conditions. For Ohmic films, we consider the response to a delta function sourcefield turned on instantly. As one of few problems in nonlocal electrodynamics, this has an analytical solution, which we obtain in both Fourier and real space. Next, the dynamical behavior of a square superconductor film during ramping up of the field, and subsequently returning to zero, is treated numerically. Then, this remanent state is used as initial condition for triggering thermomagnetic nonlocality implies that induced currents flow in the entire sample [3,4]. Thus, the film behavior is qualitatively different from that of bulks, and magneto-optical imaging (MOI) of thin superconductors has revealed strong piling up of the magnetic field around the sample edges, where values far above H a are reached [5]. At internal boundaries, such as the inner edge of a planar ring, the field can, due to the nonlocal electrodynamics, be in the opposite direction of the applied field [6,7]. Strongly modified behavior is found also in films patterned with regular arrays of small holes (antidots), which tend to guide the flux into the superconductor [8][9][10][11].The response of Ohmic films exposed to varying transverse magnetic fields is also described by nonlocal electrodynamics, but here the material responds linearly. Numerical solutions for strip and disc geometries have shown that the combination of nonlocality and dissipation causes a rapid penetration of a suddenly applied magnetic field [12,13]. Different from superconductors, even regions deep inside an Ohmic film are quickly penetrated by the magnetic field.A phenomenon that involves both the critical-state and Ohmic properties is the occurrence of flux avalanches or flux jumps. These are commonly observed in type-II superconductors at low temperatures, and are caused by a thermomagnetic instability which drives the superconductor from the critical-state to a high resistivity state [14]. The instability is triggered, e.g. by a small temperature fluctuation which reduces the flux pinning locally, and some quantized flux lines, or vortices, will start moving. This creates local heat dissipation and the temperature will increase even further, thus forming a positive feedback loop. The result can be an exponential growth in the temperature and a large-scale runaway of magnetic flux. In superconducting films the thermomagnetic instability is seen by MOI to manifest as abrupt avalanches of mag...

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“…The few existing experimental reports typically involve rather large (R λ) and irregular (uncontrolled shape) indentations. 2,4,19,20 It is then the main objective of this work to present an exhaustive experimental investigation of the flux penetration in rectangular Nb thin films with micron-sized indentations, lithographically defined within a resolution of a few nm. We investigate the effect of shape, size, and periodicity of these artificial cavities through direct visualization of the magnetic flux landscape by magneto-optical imaging.…”

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Magnetic flux penetration in Nb superconducting films with lithographically defined microindentations

et al. 2016

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We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines (d-lines), caused by the abrupt bending of current streamlines around the indentations, depart from the expected parabolic trend close to the defect and depend on the shape and size of the indentation as well as on the temperature. These findings are backed up and compared with theoretical results obtained by numerical simulations and analytical calculations highlighting the key role played by demagnetization effects and the creep exponent n. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border can severely modify the flux front topology and dynamics. Strikingly, in contrast to what has been repeatedly predicted in the literature, we do not observe that indentations act as nucleation spots for flux avalanches, but they instead help to release the flux pressure and avoid thermomagnetic instabilities.

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Mechanism for flux guidance by micrometric antidot arrays in superconducting films

Cited by 24 publications

References 44 publications

Strongly Enhanced Pinning of Magnetic Vortices in Type-II Superconductors by Conformal Crystal Arrays

Strongly Enhanced Pinning of Magnetic Vortices in Type-II Superconductors by Conformal Crystal Arrays

Nonlocal electrodynamics of normal and superconducting films

Magnetic flux penetration in Nb superconducting films with lithographically defined microindentations

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