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The Cooper pairing mechanism that binds single electrons to form pairs in metals allows electrons to circumvent the exclusion principle and condense into a single superconducting or zero-resistance state. We present results from an amorphous bismuth film system patterned with a nanohoneycomb array of holes, which undergoes a thickness-tuned insulator-superconductor transition. The insulating films exhibit activated resistances and magnetoresistance oscillations dictated by the superconducting flux quantum h/2e. This 2e period is direct evidence indicating that Cooper pairing is also responsible for electrically insulating behavior.

Whether a metallic ground state exists in a two-dimensional system beyond Anderson localization remains an unresolved question. We studied how quantum phase coherence evolves across superconductor–metal–insulator transitions through magnetoconductance quantum oscillations in nanopatterned high-temperature superconducting films. We tuned the degree of phase coherence by varying the etching time of our films. Between the superconducting and insulating regimes, we detected a robust intervening anomalous metallic state characterized by saturating resistance and oscillation amplitude at low temperatures. Our measurements suggest that the anomalous metallic state is bosonic and that the saturation of phase coherence plays a prominent role in its formation.

We have investigated the behavior of the superconducting energy gap A in ultrathin films of quench condensed Bi near the superconductor-insulator (SI) transition. From electron tunneling measurements on these films, we conclude that A becomes very small and approaches zero at the SI transition. We studied high-sheet-resistance films with TVo' s as low as 0.19 K. This is a factor of 40 lower than the low-sheet-resistance film Tco of 6.4 K. PACS numbers: 74.65.+n, 74.75.+t, 74.50.+r As the amount of disorder is increased in a metallic system the conduction-electron states begin to localize, and the strength of the effective electron-electron interactions increases. These two effects hinder the formation of the normally superconducting state in a disordered material. The localization of the electrons opposes the formation of a coherent state over distances longer than the localization length. The enhanced repulsive electronelectron interactions will reduce the net attractive interaction that is required for Cooper pairing. In twodimensional systems the effects of increasing disorder on the superconducting state are dramatic and eventually drive a transition from a superconducting to an insulating state [1][2][3][4][5][6].To qualitatively describe this transition it is helpful to consider the superconducting order parameter ^ oc Ao^^e'^ where the amplitude of the order parameter, Ao, is the zero-temperature energy gap and 0 is its phase. If a film is composed of islands that are large enough to have independent superconducting properties then the superconducting state can be weakened by reducing the (Josephson) coupling between the islands. This is accomplished by reducing the tunneling probability between the islands which results in an increase in the normal-state sheet resistance, Ru, of the film. Fluctuations in the phase of the order parameter ensue, which when strong enough destroy the long-range phase coherence across the film [2,6,7]. The situation has been shown to be diflferent for the case where the morphology of the film is more homogeneous, i.e., for disorder on shorter length scales. In systems that are not too close to the superconducting to insulating transition, increases in Ru have been shown to lead to well-defined decreases in the amplitude of the order parameter [7,8] and the mean-field transition temperature, Tco [3][4][5][6][7][8][9][10][11], in direct contrast with the islanded film case. One might expect that very near the superconducting to insulating transition, fluctuations in the amplitude of the order parameter would play a dominant role in driving the transition. Recently, however, it has been argued that even in these systems, phase fluctuations dominate the physics of the superconducting to insulating transition [6,12]. In this paper we present tunneling and transport results on uniformly disordered Bi films that are very close to the superconducting to insulating transition. We find that the energy gap in these films is reduced significantly from its bulk value, showing that the ...

We have levitated, for the first time, living biological specimens, embryos of the frog Xenopus laevis, using a large inhomogeneous magnetic field. The magnetic field/field gradient product required for levitation was 1430 kG2/cm, consistent with the embryo's susceptibility being dominated by the diamagnetism of water and protein. We show that unlike any other earth-based technique, magnetic field gradient levitation of embryos reduces the body forces and gravity-induced stresses on them. We discuss the use of large inhomogeneous magnetic fields as a probe for gravitationally sensitive phenomena in biological specimens.

Ultrathin amorphous Bi films, patterned with a nano-honeycomb array of holes, can exhibit an insulating phase with transport dominated by the incoherent motion of Cooper Pairs (CP) of electrons between localized states. Here we show that the magnetoresistance (MR) of this Cooper Pair Insulator (CPI) phase is positive and grows exponentially with decreasing temperature, T , for T well below the pair formation temperature. It peaks at a field estimated to be sufficient to break the pairs and then decreases monotonically into a regime in which the film resistance assumes the T dependence appropriate for weakly localized single electron transport. We discuss how these results support proposals that the large MR peaks in other unpatterned, ultrathin film systems disclose a CPI phase and provide new insight into the CP localization.Below its transition temperature, T c0 , a conventional superconductor, like Pb, can be driven into its non-superconducting, normal state by applying a magnetic field, H. The temperature, T , dependent sheet resistance, R (T ), of this state joins smoothly to the normal state resistance just above T c0 , R N , and assumes the dependence expected for a simple metal of unpaired electrons [1]. The behavior of this low T normal state changes substantially when these superconductors are made as thin films with R N ∼ R Q = h/4e2 . For example, applying a H to superconducting (SC) films of either Indium oxide For granular Pb films, the formation of a CPI phase has strong intuitive appeal and experimental support. STM experiments show that they consist of islands of grains that can naturally localize CPs [9]. Indeed, tunneling experiments on insulating films confirmed the existence of these localized pairs by showing the energy gap in the density of states that accompanies CP formation [6]. Also, these insulators exhibit giant negative Magneto-Resistance (MR) that can be attributed to the enhancement of inter-island quasi-particle tunneling [10]. By contrast, InO x and TiN films lack any obvious structure that could localize CPs. Moreover, these films exhibit a giant positive MR [3,4,5,11,12], which can peak orders of magnitude above R N at sufficiently low T . The mechanism behind this spectacular giant positive MR [3,4,5,11,12] and whether it is a property of a CPI phase remains unresolved despite significant attention [13,14,15,16].Theories of the positive MR peak presume that CPs spontaneously localize into islands or puddles [13,14,15,16]. On each island, the complex SC order parameter has a well defined amplitude, but electrostatic interactions between islands prevents the development of the long range phase coherence necessary for CP delocalization. A magnetic field induces more phase disorder and localization through the direct coub.

We present in situ scanning tunneling microscopy topographs of Pb films, formed by vapor deposition onto cold ͑T , 20 K͒, inert substrates, near their insulator to metal transition. At the critical mass deposited thickness for conduction, d G Х 5.2 nm, the films consist of approximately two layers of nanoclusters with diameter, 2r ഠ 20 nm and height, 3.5 # h # 5.5 nm. We discuss how the nanocluster size and formation mechanism dictate the need for two layers to form in order for conduction to commence. [S0031-9007 (99)08430-6] PACS numbers: 71.30. + h, 68.55.Jk, 73.61.AtAs the mass per unit area of a film of metal on an insulating substrate increases, it makes a transition from an insulating (dc resistance, R dc `) to a metallic ͑R dc finite͒ phase. The physical factors that control this transition depend strongly on how the film grows and its resulting structure. A simple example is encountered for Au vapor deposited on warm substrates ͑T S $ 700 K͒. Islands nucleate on the substrate, grow laterally, coalesce, and create a conduction path that percolates across the film [1]. By contrast, the processes controlling the conduction onset in films deposited on cold, inert substrates ͑T S ഠ 4 K͒-the so-called granular quench condensed (QC) films-are poorly understood and have been a source of controversy [2][3][4]. The primary obstacle has been the lack of direct information about their mode of growth and corresponding structure. Nevertheless, QC films have been used in numerous studies of superconductivity [2,5], localization [6,7], and quantum phase transitions [3,8]. Figure 1 provides an example of an insulator to metal transition in a QC film system, with Pb films on an oxidized amorphous Ge substrate ͑GeO x ͒ [9]. Like other systems, these films become conducting, hence, metallic, only after a finite mass deposited thickness of material, d G Х 5.2 nm has been quench condensed. After conduction is initiated, the film sheet resistance R S decreases exponentially with additional increments of material. There are two different interpretations of this behavior. The more commonly accepted of the two depicts a film at d G as composed of islands, higher than d G . Interisland electron tunneling controls the transport, and the tunnel gaps decrease linearly with the amount of deposited material [2,3]. The opposing interpretation questions the existence of islands in QC films [4]. Thermally activated mechanisms for island growth freeze out at cryogenic temperatures. Adatoms are believed to stick where they land [4,10-12] and, consequently, QC films are expected to form in a structurally continuous amorphous phase when only a few atomic layers thick. The exciting claim that this phase is microscopically insulating emerges because d G ഠ 20 30 atomic monolayers ¿ few monolayers. At d G , metallic grains grow within the amorphous matrix, and conduction ensues through intergrain tunneling [4].Recent in situ scanning tunneling microscopy (STM) experiments [13] on granular QC Au and Pb films show that their morphology changes dram...

Early cleavages of Xenopus embryos were oriented in strong, static magnetic fields. Third-cleavage planes, normally horizontal, were seen to orient to a vertical plane parallel with a vertical magnetic field. Second cleavages, normally vertical, could also be oriented by applying a horizontal magnetic field. We argue that these changes in cleavage-furrow geometries result from changes in the orientation of the mitotic apparatus. We hypothesize that the magnetic field acts directly on the microtubules of the mitotic apparatus. Considerations of the length of the astral microtubules, their diamagnetic anisotropy, and f lexural rigidity predict the required field strength for an effect that agrees with the data. This observation provides a clear example of a static magnetic-field effect on a fundamental cellular process, cell division.