Chain reaction explosions in vibrated granular matter
One of the characteristic properties of granular materials--e.g.
sand--is that energy is dissipated at collisions so a continuous
energy input is necessary to keep these materials in motion. In this
paper, we show that granular materials can store energy and release it in
the form of energy bursts--like explosions--to the rest of the system
in a chain reaction manner.
This phenomenon that takes place in vibrated mixtures of heavy and
light grains, confined between two close and parallel plates, has been
observed in experiments and simulations. The analysis shows that the
heavy grains store energy in a rapid synchronized vertical motion that
randomly loses its coherence liberating the energy to the whole
system. In the picture, an energy burst sequence taking place in the
cluster of heavy grains (blue) as seen in the simulations (top) and
experiments (bottom).
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LZ12501
First atom circuit with a tunable weak link
We have created a long-lived (≈ 40 s) persistent current in a toroidal Bose-Einstein condensate held in an all-optical trap. A repulsive optical barrier across one side of the torus creates a tunable weak link in the condensate circuit, which can affect the current around the loop. Super flow stops abruptly at a barrier strength such that the local flow velocity at the barrier exceeds a critical velocity. The measured critical velocity is consistent with dissipation due to the creation of vortex-antivortex pairs. This system is the first realization of an elementary closed-loop atom circuit.
Quantum fluids can exhibit properties such as long range coherence and superfluidity that make them useful for constructing sensors and other devices. For example, superconducting quantum interference devices (SQUIDs)are sensitive magnetic field detectors, and superfluid He circuits have been used to detect rotation. Ultracold atomic-gas analogs of electronic devices and circuits,
or “atomtronics” have been proposed including diodes and transistors. Of particular interest is the realization of an atomic-gas SQUID analog. SQUID circuits
have been realized with either tunnel or weak link junctions. In atomic Bose-Einstein condensates, Josephson junctions have been demonstrated only between
adjacent wells. Here we present the first implementation of a non-trivial, closed-loop atom circuit, and show that it is possible to control the current at the
single-quantum level by changing the strength of a weak link. This is an essential step toward realizing an atomic SQUID analog.
