Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L1: Vortex Ratchet Effect |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Eva Andrei, Rutgers University Room: LACC 152 |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L1.00001: Experimental vortex ratchet in Nb films with magnetic and non-magnetic asymmetric potentials. Invited Speaker: Jose Vicent Electron beam lithography allows growing Nb film on arrays of periodic asymmetric potentials. Injecting an ac current in the sample yields a rectified vortex flow. The applied magnetic field and input current strength tune both the magnitude and polarity of the net vortex flow. We will address several points as the temperature and frequency dependence, magnetic pinning centers (Ni) vs. non-magnetic pinning centers (Cu) and the comparison of the behavior of this vortex ratchet system with different types of ratchet as for instance biological motors. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:42PM |
L1.00002: Vortex Pinning by Symmetric Arrays of Magnetic Nanostructures Invited Speaker: Defects present in a superconducting material can lead to a large variety of static and dynamic vortex phases. In particular, the interaction of a vortex lattice with regular arrays of pinning centers, such as holes or magnetic dots, gives rise to commensurability effects. These commensurability effects can be observed in the magnetoresistance and in critical current dependence with the applied magnetic field. In recent years, experimental results have shown that there is a dependence of the periodic pinning effect on the properties of the vortex lattice and also on the dots characteristics. However, neither the main pinning mechanisms by the magnetic dots nor the dependence on the geometry of the pinning arrays are well understood. To clarify the pinning mechanisms, we studied and compared periodic pinning effects in Nb films with rectangular dot arrays of Ni, Co, Fe and Ni covered with thin Ag layers of varying thicknesses, as well as the pinning effects in a Nb film deposited on a patterned substrate without any magnetic material. We will discuss the differences of pinning phenomena arising from magnetic and structural effects. To clarify the effects of the pinning geometry we studied the vortex-lattice dynamics in Nb films with rectangular arrays of Ni dots. We have performed magnetotransport experiments in which two in-plane orthogonal electrical currents are injected at the same time. This allows selecting the direction and intensity of the resultant driving current on the vortex motion. The background dissipation is angular dependent at low magnetic fields. Increasing the applied magnetic field smears out this angular dependence. The periodic pinning potential locks in the vortex motion along channeling directions. Because of this, the vortex-lattice motion maybe up to 85$^{\circ}$ off the driving force direction. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 4:18PM |
L1.00003: Phase Diagram and Glassy Dynamics of Moving Vortex Lattices Invited Speaker: Much progress was made in understanding the equilibrium properties of vortex systems including the discovery of a topologically ordered Bragg glass phase at low temperatures. By contrast little is known of the dynamics of vortex phases or of their fate once they are driven out of equilibrium and start moving. We describe results of time resolved transport measurements that probe the dynamics of vortex lattices and capture their evolution in response to an applied current pulse. The experiments lead to a dynamic phase diagram consisting of four regions defined by distinctly different response characteristics. In particular it contains a moving Bragg glass state which is the dynamic counterpart of the static Bragg glass. Once the driving force is turned off, the moving Bragg glass relaxes not to the initial stationary state, but to a new state whose properties strongly depend on the relaxation time. This state exhibits simple aging and memory of the direction of the previous moving state indicating that it is a true glass. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:54PM |
L1.00004: Terahertz Generation \& Vortex Motion Control in Superconductors Invited Speaker: A grand challenge is to controllably generate electromagnetic waves in layered superconducting compounds because of its Terahertz frequency range. We propose [1] four experimentally realizable devices for generating continuous and pulsed THz radiation in a controllable frequency range. We also describe [2-4] several novel devices for controlling the motion of vortices in superconductors, including a reversible rectifier made of a magnetic-superconducting hybrid structure [4]. Finally, we summarize a study [5] of the friction force felt by moving vortices. \\ 1) S. Savel'ev, V. Yampol'skii, A. Rakhmanov, F. Nori, Tunable Terahertz radiation from Josephson vortices, preprint \\ 2) S. Savel'ev and F. Nori, Experimentally realizable devices for controlling the motion of magnetic flux quanta, Nature Mat. 1, 179 (2002) \\ 3) S. Savel'ev, F. Marchesoni, F. Nori, Manipulating small particles, PRL 92, 160602 (2004); B. Zhu, F. Marchesoni, F. Nori, Controlling the motion of magnetic flux quanta, PRL 92, 180602 (2004) \\ 4) J.E. Villegas, et al., Reversible Rectifier that Controls the Motion of Magnetic Flux Quanta, Science 302, 1188 (2003) \\ 5) A. Maeda, et al., Nano-scale friction: kinetic friction of magnetic flux quanta and charge density waves, preprint [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:30PM |
L1.00005: Real-time Observation of Vortices in Superconductors by Lorentz Microscopy Invited Speaker: The dynamics of individual quantized vortices in superconducting thin films became observable using coherent Lorentz microscopy with our field-emission transmission electron microscopes (1). The observation principle is based on the Aharonov-Bohm effect (2) Since a phase shift of 2$\pi $ is produced between two electron beams enclosing a magnetic flux of $h$/$e$, a vortex having magnetic flux of $h$/(2$e)$ is a phase object of $\pi $ for an illuminating electron beam, which cannot be observed by in--focus electron microscopy. However, the vortices are observable by holographic interference microscopy (3) and defocused Lorentz microscopy$^{ }$(4). Using Lorenz microscopy, various kinds of vortex motions in superconductors with pinning centers were observed. The vortex motions in niobium thin films were elastic, plastic and even rectified (5) depending on the sample temperature, and also on the distributions and strengths of the pinning centers. In high-$T_{c}$ superconductors, when the sample temperature decreased. The vortex motion changed from hopping to slow migration due to increasing pinning effect of atomic-size defects,which was so strong that even the pinning effect of columnar defects was hidden behind it. (1) A. Tonomura: ?Electron Holography? 2$^{nd}$ Edition, Springer, Heidelberg (1999) (2) M. Peshkin and A. Tonomura : ? The Aharonov-Bohm Effect? Lecture Notes in Physics,\textbf{ 340} (Springer-Verlag, Heidelberg, 1989). (3) J. E. Bonevich \textit{et al}. ? Electron holography observation of vortex lattices in a superconductor? Phys. Rev. Lett. \textbf{70} No. 19 (1993) p. 2952-2955. (4) K. Harada \textit{et al}. ? Real-time observation of vortex lattices in a superconductor by electron microscopy? Nature \textbf{360} ( 5 November 1992) p. 51-53. (5) Y. Togawa \textit{et al. } to be submitted to Phys. Rev. Lett. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700