Impedance measurements performed on a weakly frustrated triangular array of Josephson junctions over a wide range of frequencies and at temperatures such that vortex pinning is irrelevant reveal that vortex dynamics in an ideal two-dimensional (2D) superconductor does not obey Drude's classical prediction for a 2D Coulomb gas of free and independent vortex charges. An analysis in terms of a complex vortex dielectric constant implies that the vortex mobility vanishes logarithmically in the limit of small frequencies, thereby pointing to anomalous vortex diffusion.PACS numbers: 74.60. Ge, 74.25.Nf, The concept of vortex is essential to understand the physics of two-dimensional (2D) superfluids [1], In neutral superfluids and, under appropriate conditions, in charged superfluids the interaction between two vortices depends logarithmically on their separation, a feature leading to a natural description of the vortex medium in terms of a 2D Coulomb gas analog [2]. Detailed insight into the physics of 2D superfluids emerging from this picture is provided by studies of their response to a timedependent perturbation. The Andronikashvili torsional oscillator has proven to be quite successful to investigate vortex dynamics near the Kosterlitz-Thouless (KT) transition of liquid-helium films [3]. A corresponding probe for 2D superconductors relies on a two-coil mutual inductance technique [4] which allows us to extract the dynamical properties of the vortices from measurements of the sample's complex sheet impedance [5,6]. However, the experiments performed so far on liquid-helium films could not systematically explore the response as a function of frequency, while the investigations carried out on superconducting films [7] were almost unvariably affected by pinning effects masking the intrinsic 2D Coulomb-gas properties of the vortex medium. In this Letter, we report a study of the complex dielectric constant sico) of a dilute system of vortices created by a small perpendicular magnetic field H in an almost pinning-free triangular array of Josephson junctions. Our data, taken over a wide range of driving angular frequencies co, reveal novel and unexpected aspects of the dynamics of vortex excitations in an ideal, i.e., pinning-free, 2D superconductor.Compared to superconducting films, Josephson junction arrays (JJA) prepared with modern microfabrication techniques provide nearly ideal systems in which vortex pinning due to ever present disorder can be kept at extremely low levels. Moreover, in triangular arrays intrinsic pinning effects resulting from the periodic nature of the system are much weaker than in other lattice structures and become totally irrelevant at temperatures T appreciably lower than the zero-field KT transition temperature TKT [8]. Thus, if T is not too far below TKT and H corresponds to small values (<0.05) of the frustration parameter /, defined as the number of flux quanta per elementary triangular cell, one would expect the vortex medium in a triangular JJA to behave as a 2D Coulomb gas of free (i.e....

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