Ultracold atom magnetic field microscopy enables the probing of current flow patterns in planar structures with unprecedented sensitivity. In polycrystalline metal (gold) films we observe longrange correlations forming organized patterns oriented at ±45• relative to the mean current flow, even at room temperature and at length scales orders of magnitude larger than the diffusion length or the grain size. The preference to form patterns at these angles is a direct consequence of universal scattering properties at defects. The observed amplitude of the current direction fluctuations scales inversely to that expected from the relative thickness variations, the grain size and the defect concentration, all determined independently by standard methods. This indicates that ultracold atom magnetometry enables new insight into the interplay between disorder and transport.Thin metal films are the classic environment for studying the effect of geometric constraints [1,2] and crystal defects [3,4] on the transport of electrons. In a perfectly straight long wire that is free from structural defects, a direct current (DC) strictly follows the wire direction and creates a magnetic field in the plane perpendicular to the wire. An obstacle may locally change the direction of the current and consequently locally rotate the magnetic field close to the wire by an angle β in a plane parallel to the plane of the thin film wire.Ultracold atom magnetometry [5,6] on atom chips [7,8,9] allows for the sensitive probing of this angle β (and its spatial variation) with µrad (µm) resolution. Compared to scanning probes having a µm scale spatial resolution and 10 −5 T sensitivity, or superconducting quantum interference devices (SQUIDs) having 10 −13 T sensitivity but a resolution of tens of µm, ultracold atom magnetometry has both high sensitivity (10 −10 T) and high resolution (several µm) [6]. In addition, ultracold atoms enable high resolution over a large length scale (mm) in a single shot. This enables the simultaneous observations of microscopic and macroscopic phenomena, as described in this work.Using cold atoms just above the transition to BoseEinstein Condensation (BEC), we apply ultracold atom magnetometry to study the current deflection in three different precision-fabricated polycrystalline gold wires with a rectangular cross section of 200µm width and different thicknesses and crystalline grain sizes, as summarized in Table I [10]. Choosing the wire length along x, its width along y and thickness along z, Fig. 1 shows the maps of the angular variations β(x, y, z 0 ) = δB x (x, y, z 0 )/B y of the magnetic field created by a current of 180 mA flowing along the wire, measured at z 0 =3.5µm above its center (far from the edges).Even though at ambient temperature scattering by lattice vibrations (phonons) quickly diffuses the electronic motion, long-range correlations (tens of µm) in the current flow patterns can be seen. This is surprising as effects of static defects are usually observed only on a Table I. These fluctuations are...

Magnetic trapping potentials for atoms on atom chips are determined by the current flow in the chip wires. By modifying the shape of the conductor we can realize specialized current flow patterns and therefore micro-design the trapping potentials. We have demonstrated this by nano-machining an atom chip using the focused ion beam technique. We built a trap, a barrier and using a BEC as a probe we showed that by polishing the conductor edge the potential roughness on the selected wire can be reduced. Furthermore we give different other designs and discuss the creation of a 1D magnetic lattice on an atom chip.Comment: 6 pages, 8 figure

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