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Critical states in thin planar type-II superconductors in a perpendicular or inclined magnetic field (Review)
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View: Figures


Image of FIG. 1.
FIG. 1.

The functions versus and versus (solid curves), shown schematically in the plane for fixed .45 The points of intersection of the lines correspond to double critical states where and . The directions of the electric field for the critical T and C states are also shown.

Image of FIG. 2.
FIG. 2.

Schematic figure explaining why in an anisotropic superconductor the direction in which a vortex starts to move can differ from the direction of the driving force.53 The ellipse shows the angular dependence of the maximum pinning force . The dashed lines are the projections of the forces acting along on some other directions. All these projections reach the ellipse at angles different from . The thick arrow shows the minimum force of this type, . For the vortex starts to move in the direction .

Image of FIG. 3.
FIG. 3.

Magnetic flux front penetrating into a thin rectangular superconducting plate with increasing magnetic field perpendicular to the plane of the plate.52 The top panel shows the two-dimensional curve which forms the equator of the three-dimensional front , shown in the bottom panel.

Image of FIG. 4.
FIG. 4.

Lines of the current in a thin rectangular superconducting plate with edge ratio 1:4 and isotropic pinning, , for two values of the applied magnetic field (solid lines) and (dashed lines).52 The arrows which are perpendicular to the current lines and whose length is proportional to indicate the direction of the flux lines on the surface of an infinitesimally thin superconductor as well as the flux lines lying at different depth in a plate of small but finite thickness.

Image of FIG. 5.
FIG. 5.

Geometry of a strip and applied magnetic fields (top inset). The flux lines, “stepping” from left to right through the section of a strip (which is regarded as an “infinite” plate) is shown at the times .74 The circles mark the points around which a flux line turns. Here , , which gives and ; is the shift of the flux line over a period of the ac field; is measured from an arbitrary point of the “plate.” The diagram on the right-hand panel shows the profile of the current across the thickness of the strip during the first and second half of the period.

Image of FIG. 6.
FIG. 6.

Shift of a flux line when the magnetic field increases from to .60 The thick solid lines are the projections of the flux line on the plane. As the slope of this line increases, it simultaneously shifts along from to . The arrows with the components and mark the projections of the shifts of the elements of a flux line. These arrows are perpendicular to the local currents (solid arrows) flowing in the plane at angle to the axis.

Image of FIG. 7.
FIG. 7.

Direction of the currents flowing in the plane as a function of .60 Left-hand side: the angle between and the axis. Top: start of relaxation, . Bottom: .

Image of FIG. 8.
FIG. 8.

Orientation of the electric field , constructed on the basis of Eqs. (40)–(43).58 The deviation of the directions of from is greatest with weak currents , and the deviation vanishes when is directed along or . The length of the arrows is proportional to .

Image of FIG. 9.
FIG. 9.

The relaxation of the magnetic moment of rectangular superconducting plates with different edge ratios: .58 Here and . The dashed lines correspond to the transverse and longitudinal shaking of the vortex medium.

Image of FIG. 10.
FIG. 10.

Profiles of the magnetic induction in a plate placed in a field and carrying a transport current for (a), (b), (c).61 Two limiting profiles are shown: (solid lines) and (dashed lines). In the cases (a) and (b) the central part of the profile is “frozen.” In the case (c) the magnetic flux enters the regions 1 and 2 from the left and the region 3 from the right during one-half cycle. In the other half-cycle the flux 1 exits to the left and the fluxes 2 and 3 exit to the right. Thus the flux 2 confined in the parallelogram intersects the plate from left to right.

Image of FIG. 11.
FIG. 11.

Limiting profiles of the magnetic induction in a strip placed in constant and ac fields perpendicular to the strip’s surface and carrying a transport current with ac field amplitudes (a) and (b).61 The solid and dashed lines correspond to and , respectively. In the case (a) the “frozen” flux region has just vanished and the flux lines once again do not intersect the strip, since the area is zero. In the case (b), during each field cycle the magnetic flux confined in the region intersects the strip from left to right. Inset: current profiles , which are identical in the cases (a) and (b).

Image of FIG. 12.
FIG. 12.

Magnetic field flux lines near a double ideally screened strip, shown at top-right (, , , the two thick lines in the main part of the figure), in a perpendicular magnetic field .61 The form of the film used in the experiments of Refs. 71 and 72 is shown schematically at the top-left ( and denote the voltage and current contacts).

Image of FIG. 13.
FIG. 13.

Bottom panel: model angular dependence , the relation (51), for the cases of internal pinning (solid line) and pinning by extended defects oriented parallel to the axis (dashed line).113 The top panel shows the corresponding dependences , the relation (52). The field and the current are measured in units of and the angle in degrees. The dependences are shown for the following cases as examples: , and , .

Image of FIG. 14.
FIG. 14.

Some profiles and in a superconducting strip in the field for different dependences , Eq. (52), with the parameters and , and ∞.113 The quantities and are measured in units of . The dashed lines indicate the field and the point at which and . In the limit the field at increases sharply to the value and remains constant for .

Image of FIG. 15.
FIG. 15.

The curves for a superconducting disk, calculated using the relations (27), (64), and (65) for and several sets of the remaining parameters:117 , , (1); , , (2); , , (3); , , (4). The dashed line corresponds to the isotropic case with . Here , and is expressed in units of .

Image of FIG. 16.
FIG. 16.

The distribution of the current in a thin strip with an order-disorder transition, which is described by the model (66).123 The lines and indicate the boundaries of the amorphous and ordered vortex phases, respectively. For there exists a mixture of two vortex phases with . The case shown corresponds to increasing external magnetic field , when .

Image of FIG. 17.
FIG. 17.

Profiles of the magnetic field and current in the strip for , and for the cases of increasing (a) and decreasing (b) .123

Image of FIG. 18.
FIG. 18.

Magnetization loop (solid lines) for a strip in a perpendicular magnetic field (a) and for a plate in a parallel field (b) with .123 Note the different scales on the axis; this is because the total magnetic-flux penetration field for a strip differs strongly from that for a plate. The arrows indicate the direction of change of . The dashed lines show the first and second derivatives of the dimensionless magnetic moment with respect to . Inset: example of the magnetization half-loop for a plate.

Image of FIG. 19.
FIG. 19.

Experimental dependences in a with (○) and without (△) extended defects along the axis.139 The solid line with shows the dependence (75) with , ; the dashed line shows the corresponding angular dependence of the critical current density , the relation (76).140 The dotted line shows in the case of isotropic pinning with . The current density is measured in units of and and in units of . The remaining solid lines give the computed dependences in an inclined magnetic field with and 1 (see Sec. V).

Image of FIG. 20.
FIG. 20.

The curves (dashed lines), relations (76), for , ((a) maximum of for ) and for , ((b) maximum of for ).147 The solid lines show the corresponding with , 0.3, 0.5, 1, 2, and 5. is measured in units of and and in units of .

Image of FIG. 21.
FIG. 21.

The profiles of (a) and the current (b) in a strip, for which is described by the relation (76) with , .147 The fields and are switched on in the order written (third scenario). , 0.6, and 1.2. The magnetic fields and are given in units of . The dotted, dashed, dot-dash, and solid lines show , 0.5, 1, and 2, respectively. For comparison, the solid lines with dots indicate the profiles in the isotropic case . The dotted lines also correspond to the second field-switching scenario.

Image of FIG. 22.
FIG. 22.

The profiles of (a) and the current (b) in a strip with anisotropic pinning, which is described by the relation (76) with , (see Fig. 20).147 Here , 0.6; the dotted, dashed, dot-dash, and solid lines correspond to , 0.24, 0.35, and 0.6, respectively. The magnetic fields and are given in units of . For any the profiles are practically identical to those for , while for they are close to those for . The profiles for are also almost identical to the profiles for isotropic pinning with (solid lines with dots). The profiles and are identical for all three scenarios of switching on .

Image of FIG. 23.
FIG. 23.

The current in a strip with and for different increasing values of .45 The magnetic fields are given in units of .

Image of FIG. 24.
FIG. 24.

The magnetic moment as a function of (top) and as a function of (bottom) for , 0.1, 0.2, 0.5, 1, and 5.45 The dots show the fit by the expression (81). Inset: versus (solid line with circles).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Critical states in thin planar type-II superconductors in a perpendicular or inclined magnetic field (Review)