-
[show abstract]
[hide abstract]
ABSTRACT: The magnetic field response of a mesoscopic superconducting ring with an inside hole positioned off-center is studied by using the multiple-small-tunnel-junction method, by which the strengths of the superconductivity at the narrowest and the widest parts of the ring are detected separately and simultaneously. We observed continuous and reversible transitions between adjacent fluxoid states at temperatures close to the onset of superconductivity. Our findings are in agreement with numerical ones based on the Ginzburg-Landau theory, and show that in such continuous transitions, a different type of vortex nucleates at the narrowest part of the ring in the case of a low flux and at the widest part for a larger flux. The former corresponds to a one-dimensional vortex, which has been predicted theoretically for more than a decade.
Phys. Rev. B. 09/2007; 76(9).
-
[show abstract]
[hide abstract]
ABSTRACT: Experimentally, multivortex states and giant vortex states in mesoscopic superconductors can be distinguished directly by using the multiple-small-tunnel-junctions, and indirectly by studying the temperature dependence of the expulsion fields. These experimental results are compared with the theoretical prediction from the nonlinear Ginzburg- Landau theory.
Journal of Physics Conference Series 07/2006; 43(1):647.
-
[show abstract]
[hide abstract]
ABSTRACT: We study the influence of thermal fluctuations on the magnetic behavior of square mesoscopic superconductors. The strength of thermal fluctuations are parametrized using the Ginzburg number, which is small (Gi≈10−10) in low-Tc superconductors and large in high-Tc superconductors (Gi≈10−4). For low-Tc mesoscopic superconductors we found that the metastable states due to the surface barrier have a large half-life time, which leads to the hysteresis in the magnetization curves as observed experimentally. A very different behavior appears for high-Tc mesoscopic superconductors where thermally activated vortex entrance∕exit through surface barriers is frequent. This leads to a reduction of the magnetization and a noninteger average number of flux quanta penetrating the superconductor. The magnetic field dependence of the probability for the occurrence of the different vortex states and the fluctuations in the number of vortices are studied.
Phys. Rev. B. 06/2005; 71(21).
-
[show abstract]
[hide abstract]
ABSTRACT: We study the influence of thermal fluctuations on the magnetic behavior of square mesoscopic superconductors. The strength of thermal fluctuations are parameterized using the Ginzburg number, which is small ($G_i \approx 10^{-10}$) in low-$T_c$ superconductors and large in high-$T_c$ superconductors ($G_i \approx 10^{-4}$). For low-$T_c$ mesoscopic superconductors we found that the meta-stable states due to the surface barrier have a large half-life time, which leads to the hysteresis in the magnetization curves as observed experimentally. A very different behavior appears for high-$T_c$ mesoscopic superconductors where thermally activated vortex entrance/exit through surface barriers is frequent. This leads to a reduction of the magnetization and a non-integer average number of flux quanta penetrating the superconductor. The magnetic field dependence of the probability for the occurrence of the different vortex states and the fluctuations in the number of vortices are studied. Comment: 9 pages, 6 figures, submitted to Phys. Rev. B
04/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: The response of a mesoscopic superconducting disk to perpendicular magnetic fields is studied by using the multiple-small-tunnel-junction method, in which transport properties of several small tunnel junctions attached to the disk are measured simultaneously. This allows us to make the first experimental distinction between the giant vortex states and multivortex states. Moreover, we experimentally find a magnetic-field induced rearrangement and combination of vortices. The experimental results are well reproduced in numerical results based on the nonlinear Ginzburg-Landau theory.
Physical Review Letters 01/2005; 93(25):257002. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Stable vortex states are studied in large superconducting thin disks (for numerical purposes we considered with radius R = 50 \xi). Configurations containing more than 700 vortices were obtained using two different approaches: the nonlinear Ginzburg-Landau (GL) theory and the London approximation. To obtain better agreement with results from the GL theory we generalized the London theory by including the spatial variation of the order parameter following Clem's ansatz. We find that configurations calculated in the London limit are also stable within the Ginzburg-Landau theory for up to ~ 230 vortices. For large values of the vorticity (typically, L > 100), the vortices are arranged in an Abrikosov lattice in the center of the disk, which is surrounded by at least two circular shells of vortices. A Voronoi construction is used to identify the defects present in the ground state vortex configurations. Such defects cluster near the edge of the disk, but for large L also grain boundaries are found which extend up to the center of the disk. Comment: 15 pages, 10 figures, RevTex4, submitted to Phys. Rev. B
07/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: The distribution of vortices over different vortex shells in mesoscopic superconducting disks with sufficiently large sizes is investigated within the framework of the nonlinear Ginzburg-Landau theory. Keeping the total vorticity fixed, different vortex configurations can be found in which the vortices are distributed differently over the vortex shells. An overview is given of the different possible vortex configurations and the free energies of these states are compared. In general, the difference in the free energy between the possible vortex configurations with the same vorticity is much smaller than the difference in the free energy between vortex states with different vorticity. The transitions between different vortex states with the same and different vorticity are investigated. Contrary to small disks, the change in vorticity can be larger than one with increasing and decreasing magnetic field. Also a combination of a giant vortex state with a multivortex state can nucleate into a stable vortex state. The influence of the sample thickness is briefly studied. Our results are compared with those obtained from the London approximation which clearly shows the limited applicability of the latter.
Phys. Rev. B. 02/2004; 69(6).
-
[show abstract]
[hide abstract]
ABSTRACT: On the basis of the time-dependent Ginzburg-Landau equation we studied the dynamics of the superconducting condensate in a wide two-dimensional sample in the presence of a perpendicular magnetic field and applied current. We could identify two critical currents: the current at which the pure superconducting state becomes unstable ($J_{c2}$ \cite{self1}) and the current at which the system transits from the resistive state to the superconducting state ($J_{c1}<J_{c2}$). The current $J_{c2}$ decreases monotonically with external magnetic field, while $J_{c1}$ exhibits a maximum at $H^*$. For sufficient large magnetic fields the hysteresis disappears and $J_{c1}=J_{c2}=J_c$. In this high magnetic field region and for currents close to $J_c$ the voltage appears as a result of the motion of separate vortices. With increasing current the moving vortices form 'channels' with suppressed order parameter along which the vortices can move very fast. This leads to a sharp increase of the voltage. These 'channels' resemble in some respect the phase slip lines which occur at zero magnetic field. Comment: 5 pages, 4 figures, Proceedings of Third European Conference on Vortex Matter in Superconductors
10/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: The nonuniform superconducting state in a ring in which the order parameter vanishing at one point is studied. This state is characterized by a jump of the phase by $\pi$ at the point where the order parameter becomes zero. In uniform rings such a state is a saddle-point state and consequently unstable. However, for non-uniform rings with e.g. variations of geometrical or physical parameters or with attached wires this state can be stabilized and may be realized experimentally. Comment: 6 pages, 7 figures, RevTex 4.0 style
03/2002;
-
[show abstract]
[hide abstract]
ABSTRACT: The distribution of the electric charge density in mesoscopic superconducting disks and cylinders is studied within the phenomenological Ginzburg-Landau approach. We found that even in the Meissner state the mesoscopic sample exhibits a non-uniform charge distribution such that a region near the sample edge becomes negatively charged. When vortices are inside the sample there is a superposition of the negative charge located at the vortex core and this Meissner charge, and, as a result, the charge at the sample edge changes sign as a function of the applied magnetic field.
Czechoslovak Journal of Physics 01/2002; 52(2):303-306. · 0.42 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The nonlinear Ginzburg-Landau equations are solved numerically in order to investigate the vortex structure in thin superconducting disks of arbitrary shape. Depending on the size of the system and the strength of the applied magnetic field giant vortex, multi-vortex and a combination of both of them are found. The saddle points in the energy landscape are identified from which we obtain the energy barriers for flux penetration and expulsion.
01/2002;
-
[show abstract]
[hide abstract]
ABSTRACT: The magnetic coupling between two concentric mesoscopic superconductors with non-zero thickness is studied using the nonlinear Ginzburg-Landau theory. We calculated the free energy, the expelled field, the total field profile, the Cooper-pair density and the current density distribution. By putting a smaller superconducting disk or ring in the center of a larger ring, the properties change drastically. Extra ground state transitions are found, where the total vorticity stays the same, but the vorticity of the inner superconductor changes by one. Due to the magnetic coupling, the current in the external ring exhibits extra jumps at the transition fields where the vorticity of the inner superconductor changes. In this case, for certain temperatures, re-entrant behaviour and switching on and off of the superconducting behavior of the rings are found as function of the magnetic field. A H-T phase diagram is obtained for the situation where the inner ring has a higher critical temperature than the outer ring. An analytic expression for the magnetic coupling is obtained for thin rings and extreme type-II superconductors.
01/2002;
-
[show abstract]
[hide abstract]
ABSTRACT: Using the nonlinear Ginzburg-Landau theory we investigated the dependence of the magnetic coupling between two concentric mesoscopic superconducting rings on their thickness. The size of this magnetic coupling increases with the thickness of the rings. Comment: 3 pages text, 3 ps figures, to be published in Physica C (Proceedings of the 2nd European Conference in School Format "Vortex Matter in Superconductors")
12/2001;
-
[show abstract]
[hide abstract]
ABSTRACT: The influence of the geometry of a thin superconducting sample on the penetration of the magnetic field lines and the arrangement of vortices are investigated theoretically. We compare superconducting disks, squares and triangles with the same surface area having nonzero thickness. The coupled nonlinear Ginzburg-Landau equations are solved self-consistently and the important demagnetization effects are taken into account. We calculate and compare quantities like the free energy, the magnetization, the Cooper-pair density, the magnetic field distribution and the superconducting current density for the three geometries. For given vorticity the vortex lattice is different for the three geometries, i.e. it tries to adapt to the geometry of the sample. This also influences the stability range of the different vortex states. For certain magnetic field ranges we found a coexistence of a giant vortex placed in the center and single vortices toward the corners of the sample. Also the H-T phase diagram is obtained. Comment: 9 pages, 17 figures (submitted to Phys. Rev. B)
06/2001;
-
[show abstract]
[hide abstract]
ABSTRACT: The superconducting state of an infinitely long superconducting cylinder surrounded by a medium which enhances its superconductivity near the boundary is studied within the nonlinear Ginzburg-Landau theory. This enhancement can be due to the proximity of another superconductor or due to surface treatment. Quantities like the free energy, the magnetization and the Cooper-pair density are calculated. Phase diagrams are obtained to investigate how the critical field and the critical temperature depend on this surface enhancement for different values of the Ginzburg-Landau parameter \kappa. Increasing the superconductivity near the surface leads to higher critical fields and critical temperatures. For small cylinder diameters only giant vortex states nucleate, while for larger cylinders multivortices can nucleate. The stability of these multivortex states also depends on the surface enhancement. For type-I superconductors we found the remarkable result that for a range of values of the surface extrapolation length the superconductor can transit from the Meissner state into superconducting states with vorticity L > 1. Such a behaviour is not found for the case of large \kappa, i.e. type-II superconductivity. Comment: submitted to Phys. Rev. B
06/2001;
-
[show abstract]
[hide abstract]
ABSTRACT: The electric charge density in mesoscopic superconductors with circular symmetry, i.e. disks and cylinders, is studied within the phenomenological Ginzburg-Landau approach. We found that even in the Meissner state there is a charge redistribution in the sample which makes the sample edge become negatively charged. In the vortex state there is a competition between this Meissner charge and the vortex charge which may change the polarity of the charge at the sample edge with increasing magnetic field. It is shown analytically that in spite of the charge redistribution the mesoscopic sample as a whole remains electrically neutral. Comment: 8 pages, 7 figures; submitted to Phys. Rev. B
11/2000;
-
[show abstract]
[hide abstract]
ABSTRACT: The transitions between the different vortex states of thin mesoscopic superconducting disks and rings are studied using the non-linear Ginzburg-Landau functional. They are saddle points of the free energy representing the energy barrier which has to be overcome for transition between the different vortex states. In small superconducting disks and rings the saddle point state between two giant vortex states, and in larger systems the saddle point state between a multivortex state and a giant vortex state and between two multivortex states is obtained. The shape and the height of the nucleation barrier is investigated for different disk and ring configurations. Comment: 10 pages, 18 figures
10/2000;
-
[show abstract]
[hide abstract]
ABSTRACT: Phase transitions between different (i.e. giant and multi-vortex) superconducting states and between the superconducting-normal state of mesoscopic disks and rings are studied in the presence of an external magnetic field by solving the two non-linear Ginzburg-Landau equations self-consistently. The flux through a circular disk with a hole in the middle is not quantized. Comment: 8 pages, 10 figures; to appear in Physica C (proceedings of the conference on Vortex matter, Crete (september 1999)
10/1999;
-
[show abstract]
[hide abstract]
ABSTRACT: The superconducting state of a thin superconducting disk with a hole is studied within the non-linear Ginzburg-Landau theory in which the demagnetization effect is accurately taken into account. We find that the flux through the hole is not quantized, the superconducting state is stabilized with increasing size of the hole for fixed radius of the disk, and a transition to a multi-vortex state is found if the disk is sufficiently large. Breaking the circular summetry through a non central location of the hole in the disk enhances the multi-vortex state. Comment: 11 pages, 23 figures (postscript). To appear in Physical Review B, Vol. 61 (2000)
10/1999;
-
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate theoretically how a two-dimensional electron gas can be used to probe local potential profiles using the Hall effect. For small magnetic fields, the Hall resistance is inversely proportional to the average potential profile in the Hall cross and is independent of the shape and the position of this profile in the junction. The bend resistance, on the other hand, is much more sensitive on the exact details of the local potential profile in the cross junction. © 1999 American Institute of Physics.
Applied Physics Letters 04/1999; · 3.84 Impact Factor
