Amorphous silicon is a semiconductor with a lower density than the metallic silicon liquid. It is widely believed that the amorphous-liquid transition is a first-order melting transition. In contrast to this, recent computer simulations and the experimental observation of pressure-induced amorphization of nanoporous silicon have revived the idea of an underlying liquid-liquid phase transition implying the existence of a low-density liquid and its glass transition to the amorphous solid. Here we demonstrate that during irradiation with high-energy heavy ions amorphous silicon deforms plastically in the same way as conventional glasses. This behaviour provides experimental evidence for the existence of the low-density liquid. The glass transition temperature for a timescale of 10 picoseconds is estimated to be about 1,000 K. Our results support the idea of liquid polymorphism as a general phenomenon in tetrahedral networks.

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The presence of columnar defects (CDs) in NbSe 3 is shown to induce oscillations in the nonlinear charge density wave (CDW) conductivity as a function of magnetic field, when the field is oriented parallel to the axes of the defects. The period of oscillation corresponds to a change in the magnetic field flux in each CD by an amount f 0 hc͞2e, the superconducting flux quantum. This result is considered as the collective response of the moving CDW to Aharonov-Bohm flux trapped inside the CDs.[S0031-9007 (96)02262-4]