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arXiv:cond-mat/0210377 · Journal of Physics B Atomic Molecular and Optical Physics

Anomalous longitudinal magnetic field near the surface of copper conductors

Sebastian Kraft, A G nther, Herwig Ott, D. Wharam, Claus Zimmermann, J Fort gh

Published Oct 28, 2002• 63 citations
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Abstract

We have used ultracold atoms to characterize the magnetic field near the surface of copper conductors at room temperature carrying currents between 0.045 and 2 A. In addition to the usual circular field we find an additional, 1000–10 000 times smaller longitudinal field. The field changes its strength periodically with a period of 200–300 μ m.

DOI
10.1088/0953-4075/35/21/102

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We have used ultracold atoms to characterize the magnetic field near the surface of copper conductors at room temperature carrying currents between 0.045 and 2 A. In addition to the usual circular field we find an additional, 1000–10 000 times smaller longitudinal field. The field changes its strength periodically with a period of 200–300 μ m.
We have investigated Bose-Einstein condensates and ultracold atoms in the vicinity of a surface of a magnetic microtrap. The atoms are prepared along copper conductors at distances to the surface between $300\ensuremath{\mu}\mathrm{m}$ and $20\ensuremath{\mu}\mathrm{m}.$ In this range, the lifetime decreases from 20 s to 0.7 s showing a linear dependence on the distance to the surface. The atoms manifest a weak thermal coupling to the surface, with measured heating rates remaining below 500 nK/s. In addition, we observe a periodic fragmentation of the condensate and thermal clouds when the surface is approached.
Microscopic atom optical devices integrated on atom chips allow to precisely control and manipulate ultra-cold ( T < 1 µK) neutral atoms and Bose-Einstein condensates (BECs) close to surfaces. The relevant energy scale of a BEC is extremely small (down to < 10 −11 eV). Consequently, BECs can be utilized as a sensor for variations of the potential energy of the atoms close to the surface. Here we describe how to use trapped atoms as a measurement device and analyze the performance and flexibility of the field sensor. We demonstrate microscopic magnetic imaging with simultaneous high spatial resolution (3 µm) and high field sensitivity (4 nT). With one dimensional BECs, we probe the magnetic field variations close to the surface at distances down to a few microns. Measurements of the magnetic field of a 100 µm wide current carrying wire imply that the magnetic field variations stem from residual variations of the current flow direction, resulting from local properties of the wire. These disorder potentials found near lithographically fabricated wires are two orders of magnitude smaller than those measured close to electroplated conductors.
Surface based geometries of microfabricated wires or patterned magnetic films can be used to magnetically trap and manipulate ultracold neutral atoms or Bose-Einstein condensates. We investigate the magnetic properties of such atom chips using a scanning magnetoresistive (MR) microscope with high spatial resolution and high field sensitivity. By comparing MR scans of a permanent magnetic atom chip to field profiles obtained using ultracold atoms, we show that MR sensors are ideally suited to observe small variations of the magnetic field caused by imperfections in the wires or magnetic materials which ultimately lead to fragmentation of ultracold atom clouds. Measurements are also provided for the magnetic field produced by a thin current-carrying wire with small geometric modulations along the edge. Comparisons of our measurements with a full numeric calculation of the current flow in the wire and the subsequent magnetic field show excellent agreement. Our results highlight the use of scanning MR microscopy as a convenient and powerful technique for precisely characterizing the magnetic fields produced near the surface of atom chips.
We study a novel millimetre-scale magnetic trap for ultracold atoms, in which the current carrying conductors can be situated outside the vacuum region, a few mm away from the atoms. This design generates a magnetic field gradient in excess of \SI{1000}{G/cm} at a distance of \SI{2}{mm} from the conductors. We perform electromagnetic and thermo-mechanical characterisation using Finite Element Methods (FEM). The predicted behaviour has been verified by electrical and thermal measurements on a prototype, but has not been implemented on an apparatus with cold atoms. Operating this trap at the highest gradient allows for rapid evaporative cooling comparable to that achieved by atom chips.
\nBose-Einstein condensates are trapped in elongated magnetic traps made from\nmicro fabricated current conductors. By bringing the atoms close to the\nsurface of the substrate a periodic anomalous magnetic field is observed. It\nis oriented parallel to the conductor and depends on its current. In a second\nexperiment we study the motion of a condensate in an anharmonic wave guide.\nStrong collective excitations are observed which can be very well described by\ntheory. Simulations predict pronounced chaotic motion within the parameter\nrange of the experiment.\n
A microscopic Ioffe–Pritchard trap is formed using a straight, current-carrying wire, together with suitable auxiliary magnetic fields. By measuring the distribution of cold rubidium atoms held in this trap, we detect a weak magnetic field component Δ B z parallel to the wire. This is produced by the current in the wire and is approximately periodic along the wire with period λ = 230 µm. We have measured this field at distances in the range y = 250–350 µm from the centre of the wire. Over this range we find that the decrease of the field is well described by the Bessel function K 1 (2π y /λ), as one would expect for the far field of a transversely oscillating current within the wire.