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We set up a harmonic lattice model for 2D defect melting which, in contrast to earlier simple-cubic models, lives on a triangular lattice. Integer-valued plastic defect gauge fields allow for the thermal generation of dislocations and disclinations. The model produces universal formulas for the melting temperature expressed in terms of the elastic constants, which are different from those derived for square lattices. They determine a Lindemann-like parameter for two-dimensional melting. In contrast to the square crystal which underwent a first-order melting transition, the triangular model melts in two steps. Our results are applied to the melting of Lennard-Jones and electron lattices.

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We have developed a recently proposed concept of a Josephson traveling-wave parametric amplifier with threewave mixing. The amplifier consists of a microwave transmission line formed by an array of nonhysteretic one-junction SQUIDs. These SQUIDs are flux-biased in a way that the phase drops across the Josephson junctions are equal to 90°. Such a onedimensional metamaterial possesses a large quadratic nonlinearity and zero cubic (Kerr-like) nonlinearity. This property allows phase matching and exponential power gain to take place over a wide frequency range. The proof-of-principle experiment performed at a temperature of T = 4.2 K on Nb trilayer samples has demonstrated that our concept of a practical broadband Josephson parametric amplifier is valid and very promising for achieving quantum-limited performance.