Showing papers by "Andre K. Geim published in 2000"
TL;DR: The amount of flux introduced by individual vortices in a superconducting film is measured, finding that the flux always differs substantially from φ0, and ‘negative vortice’ are observed, whose penetration leads to the expulsion of magnetic field.
Abstract: As first pointed out by Bardeen and Ginzburg in the early sixties1,2, the amount of magnetic flux carried by vortices in superconducting materials depends on their distance from the sample edge, and can be smaller than one flux quantum, φ0 = h/2e (where h is Planck's constant and e is the electronic charge). In bulk superconductors, this reduction of flux becomes negligible at sub-micrometre distances from the edge, but in thin films the effect may survive much farther into the material3,4. But the effect has not been observed experimentally, and it is often assumed that magnetic field enters type II superconductors in units of φ0. Here we measure the amount of flux introduced by individual vortices in a superconducting film, finding that the flux always differs substantially from φ0. We have observed vortices that carry as little as 0.001φ0, as well as ‘negative vortices’, whose penetration leads to the expulsion of magnetic field. We distinguish two phenomena responsible for non-quantized flux penetration: the finite-size effect1,2,3,4 and a nonlinear screening of the magnetic field due to the presence of a surface barrier. The latter effect has not been considered previously, but is likely to cause non-quantized penetration in most cases.
142 citations
TL;DR: In this article, the authors report measurements of the amount of flux associated with the entrance of individual vortices in a superconducting film and show that the flux that they bring in, always differs substantially from f0.
Abstract: As first pointed out by Bardeen and Ginzburg in the early sixties, the amount of magnetic flux carried by vortices depends on their distance to the sample edge and can be smaller than one flux quantum, f0 = h/2e. In bulk superconductors, this reduction of flux becomes negligible already at submicron distances from the edge but, in thin films, the effect may survive at much larger distances. In the absence of any experimental observation, such flux reduction is perceived to be an exotic or unimportant effect, and it is often assumed that magnetic field enters type-II superconductors in units of f0. Here we report the measurements of the amount of flux associated with the entrance of individual vortices in a superconducting film and show that the flux that they bring in, always differs substantially from f0. We have observed vortices that carry as little as 0.001 f0 as well as seemingly "negative vortices" whose penetration leads to the expulsion of magnetic field. We distinguish two phenomena responsible for non-quantized flux penetration: the finite-size effect and a non-linear screening of magnetic field in the presence of a surface barrier. The latter effect has previously not been considered but is likely to cause non-quantized penetration in many cases.
134 citations
TL;DR: This work finds clear signatures of first and second order transitions within the states of fluxoid states, which reveal the existence of distinct vortex phases for a fixed number of fluxoids.
Abstract: Each time a vortex enters or exits a small superconductor, a different fluxoid state develops which can be characterized by its vorticity, i.e., the number of fluxoids inside. We have studied magnetization response of such individual states and found clear signatures of first and second order transitions within the states, which reveal the existence of distinct vortex phases for a fixed number of fluxoids. We attribute the transitions to the merger of individual vortices into a single giant vortex and switching between different arrays of vortices.
68 citations
TL;DR: In this paper, the first magnetisation measurements on isolated single crystalline Fe-nanoparticles performed with a ballistic Hall micro-magnetometer are presented, which have a sensitivity of and thus provide the possibility to study the mechanisms of magnetisation reversal in a single nanoparticle.
Abstract: We present here the first magnetisation measurements on isolated single crystalline Fe-nanoparticles performed with a ballistic Hall micro-magnetometer. The measurements have a sensitivity of and thus provide us the possibility to study the mechanisms of magnetisation reversal in a single nanoparticle. The magnetic properties of the nanoparticles are influenced by their crystal structure and shape, and the presence of an oxide surface layer. They exhibit curling of the magnetic moments, but also a novel hysteresis behaviour. The spin configurations found for the system agree well with numerical calculations based on a Heisenberg Hamiltonian including the exchange and dipole interaction and surface anisotropy.
36 citations
TL;DR: In this article, the authors studied the transport of 2D electrons through individual magnetic inhomogeneities of the height up to 1 T and the size down to 100 nm and reported an inversion of the sign of the Hall effect.
Abstract: We have studied transport of 2D electrons through individual magnetic inhomogeneities of the height up to 1 T and the size down to 100 nm. Such magnetic fields were created by placing dysprosium microtablets on top of a near-surface 2D electron gas (2DEG). The cyclotron orbit for such inhomogeneities becomes smaller than their size and incident electrons are strongly deflected. We report an inversion of the sign of the Hall effect: a positively magnetised micromagnet on top of a 2DEG gives rise to a Hall signal which corresponds to a negative field applied to the 2DEG. This dramatic anomaly is attributed to the fact that 2D electrons are not able to reach the central, strongest part of the magnetic field and, therefore, the dominant contribution to the Hall effect comes from a stray field having the opposite sign.
10 citations
TL;DR: In this paper, the authors investigated tunnelling between disordered two-dimensional electron systems in a magnetic field parallel to the current at liquid-helium temperatures, and found that the high magnetic field creates a gap in the tunning density of states that depends linearly on magnetic field.
Abstract: We have investigated tunnelling between disordered two-dimensional electron systems in a magnetic field parallel to the current At liquid-helium temperatures, the high magnetic field creates a gap in the tunnelling density of states that depends linearly on magnetic field The temperature dependence of the magnetic field variation of the equilibrium tunnelling conductance reveals features which could be interpreted as a manifestation of the insulator-quantum Hall-insulator transition
4 citations
TL;DR: In this paper, the authors studied the bistability and discontinuity in the tunnel current of a 12 nm single-barrier GaAs/AlAs p-i-n heterostructure where a system of spatially separated electron and hole (e-h) layers is realized.
Abstract: We studied a novel bistability and discontinuity in the tunnel current of a 12 nm single-barrier GaAs/AlAs p-i-n heterostructure where a system of spatially separated electron and hole (e-h) layers is realized. Both features appear at T < 300 mK and when the in-plane e-e (h-h) distance is comparable to the inter-layer (e-h) onc. Whereas the high-current state (I-ICS) behaves normally and can be identified with the uncoupled e-h gases, the low-current state (LCS) shows some peculiarities: it has low current, a density 10% lower than the HCS at the same bias, a marked phase shift in the current magneto-oscillations (MO), and - contrary to the HCS - does not show any fractional feature. We identify the LCS with a gas of indirect excitons, the binding energy of which we estimate to be similar to 0.5 and similar to 5 meV at B = 0 and 10 T, respectively. We explain the bistability and the discontinuity as a first-order phase transition between a gas of excitons and the uncoupled 2D e-h gases. (C) 2000 Elsevier Science B.V. hll rights reserved.