We compute the universal conductivity of the (2+1)-dimensional XY universality class, which is realized for a superfluid-to-Mott insulator quantum phase transition at constant density. Based on large-scale Monte Carlo simulations of the classical (2+1)-dimensional J-current model and the two-dimensional Bose-Hubbard model, we can precisely determine the conductivity on the quantum critical plateau, σ(∞) = 0.359(4)σQ with σQ the conductivity quantum. The universal conductivity curve is the standard example with the lowest number of components where the bottoms-up AdS/CFT correspondence from string theory can be tested and made to use [R. C. Myers, S. Sachdev, and A. Singh, Phys. Rev. D83, 066017 (2011)]. For the first time, the shape of the σ(iωn) − σ(∞) function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particle-like nature of charge transport. We find that the holographic gauge/gravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.

Textile-based triboelectric nanogenerators (T-TENGs), combining the functions of energy harvesting and self-powered sensing with advantages of breathability and flexibility, have received intensive attention, which is vital to the rapid advancements in smart textiles. However, there exists few reports of T-TENGs applied to fires under the intelligent era of high requirements for devices with versatility and multiscenario practicability. Here, in combination with flame-retardant conductive cotton fabric, polytetrafluoroethylene-coated cotton fabric, and a divider, a low-cost and environmentally friendly flame-retardant textile-based triboelectric nanogenerator (FT-TENG) is developed, which is endowed with excellent fire resistance and outstanding energy harvesting capabilities. The cotton fabrics treated with a layer-by-layer self-assembly method show great self-extinguishing performance. Besides, the maximum peak power density of the FT-TENG can reach 343.19 mW/m2 under the tapping frequency of 3 Hz. Furthermore, the FT-TENG still keeps 49.2% of the initial electrical output even after being burned at 17 different positions; 34.48% of the electrical output is also retained when the FT-TENG is exposed to 220 °C. Moreover, the FT-TENGs are successfully applied as energy harvesters for firefighters and self-powered sensors for forest self-rescue and fire alarm systems. This work may provide a promising potential for multifunctional smart textiles in energy harvesting, self-powered sensing, and life or property security.

We present spectral functions for the magnitude squared of the order parameter in the scaling limit of the two-dimensional superfluid to Mott insulator quantum phase transition at constant density, which has emergent particle-hole symmetry and Lorentz invariance. The universal functions for the superfluid, Mott insulator, and normal liquid phases reveal a low-frequency resonance which is relatively sharp and is followed by a damped oscillation (in the first two phases only) before saturating to the quantum critical plateau. The counterintuitive resonance feature in the insulating and normal phases calls for deeper understanding of collective modes in the strongly coupled (2+1)dimensional relativistic field theory. Our results are derived from analytically continued correlation functions obtained from path-integral Monte Carlo simulations of the Bose-Hubbard model.

No abstract

No abstract