No abstract

Long-range correlations in two-dimensional (2D) systems are significantly altered by disorder potentials. Theory has predicted the existence of disorder-induced phenomena, such as Anderson localization 1 or the emergence of a Bose glass 2 . More recently, it has been shown that when disorder breaks 2D continuous symmetry, long-range correlations can be enhanced 3 . Experimentally, developments in quantum gases have allowed the observation of the e ects of competition between interaction and disorder 4,5 . However, experiments exploring the e ect of symmetry-breaking disorder are lacking. Here, we create a 2D vortex lattice at 0.1 K in a superconducting thin film with a well-defined 1D thickness modulation-the symmetry-breaking disorder-and track the field-induced modification using scanning tunnelling microscopy. We find that the 1D modulation becomes incommensurate with the vortex lattice and drives an order-disorder transition, behaving as a scale-invariant disorder potential. We show that the transition occurs in two steps and is mediated by the proliferation of topological defects. The resulting critical exponents determining the loss of positional and orientational order are far above theoretical expectations for scale-invariant disorder 6-8 and follow instead the critical behaviour describing dislocation unbinding melting 9,10 . Our data show that randomness disorders a 2D crystal, with enhanced long-range correlations due to the presence of a 1D modulation.The competition between order and disorder is a fundamental problem in condensed-matter physics, which directly impacts many different systems, such as crystalline solids 7,11 , electronic or magnetic arrangements 12 , localization in metals and superconductors, or vortex lattices in superconductors and condensates 4,13 . In 2D systems, long wavelength fluctuations induce deviations in the atomic positions from the perfect lattice, with the mean-squared displacement diverging logarithmically at large distances 14 . One major consequence is the so-called MerminWagner-Hohenberg (MWH) theorem 14,15 , which states that no true order exists in 2D systems at any finite temperature. Usually, we can distinguish between static quenched disorder and fluctuations. In the absence of quenched disorder, thermal fluctuations drive the 2D melting transition which is described by Berezinskii-KosterlitzThouless-Halperin-Nelson-Young (BKTHNY) theory through the two-stage proliferation and unbinding of topological defects 9,10,16,17 . Quenched disorder, on the other hand, is expected to suppress long-range correlations more effectively than temperature 18 . It can be classified as pinning with identifiable length scales, such as impurities or defects in 2D crystals, or as scale-invariant (random) disorder, as for example in an amorphous film. Pinning destroys long-range 2D correlations at any strength 19,20 . Scale-invariant disorder produces power-law decaying correlations and a transition to a disordered lattice with exponentially falling correlations above a critical di...

Context. To date, the CoRoT space mission has produced more than 124 471 light curves. Classifying these curves in terms of unambiguous variability behavior is mandatory for obtaining an unbiased statistical view on their controlling root-causes. Aims. The present study provides an overview of semi-sinusoidal light curves observed by the CoRoT exo-field CCDs. Methods. We selected a sample of 4206 light curves presenting well-defined semi-sinusoidal signatures. The variability periods were computed based on Lomb-Scargle periodograms, harmonic fits, and visual inspection. Results. Color-period diagrams for the present sample show the trend of an increase of the variability periods as long as the stars evolve. This evolutionary behavior is also noticed when comparing the period distribution in the Galactic center and anti-center directions. These aspects indicate a compatibility with stellar rotation, although more information is needed to confirm their rootcauses. Considering this possibility, we identified a subset of three Sun-like candidates by their photometric period. Finally, the variability period versus color diagram behavior was found to be highly dependent on the reddening correction.

We present measurements of the superconducting and charge-density-wave (CDW) critical temperatures (T c and T CDW ) as a function of pressure in the transition metal dichalchogenides 2H -TaSe 2 and 2H -TaS 2 . Resistance and susceptibility measurements show that T c increases from temperatures below 1 K up to 8.5 K at 9.5 GPa in 2H -TaS 2 and 8.2 K at 23 GPa in 2H -TaSe 2 . We observe a kink in the pressure dependence of T CDW at about 4 GPa that we attribute to the lock-in transition from incommensurate CDW to commensurate CDW. Above this pressure, the commensurate T CDW slowly decreases, coexisting with superconductivity within our full pressure range.