2026/05/31 by Jun-Cheng Liang, Bo Chen
Physics and Astronomy · #cond-mat.quant-gas #quant-ph
We withdraw this manuscript as the theoretical framework uses a weak-coupling approximation, but the non-thermal fixed point is strongly correlated. Hence, the derived diffusive response, Ward identity, and the predicted $C$ are not justified
arxiv created 2026/08/03 · arxiv updated 2026/08/05
Predicting universal transport coefficients in far-from-equilibrium quantum systems remains a fundamental challenge. A paradigmatic example is the non-thermal fixed point (NTFP) of isolated Bose gases, where coherence spreads as ℓ2(t) = Cℏ t/m with a universal constant C. While the scaling exponent z=2 is well established, the amplitude C has remained elusive because the underlying particle cascade n(k)∼ k-4 leads to a divergent kinetic energy, threatening the very existence of a constant speed limit. Here we resolve this paradox and present the first analytical, parameter-free prediction of a universal amplitude C. A deep interplay between symmetry and dissipation is uncovered. The emergent weak U(1) symmetry at the NTFP enforces a conserved total current, forcing the low-energy phase dynamics to obey a diffusive Langevin equation with noise entering as the divergence of a stochastic current. This structure, combined with dynamical decoherence of high-momentum modes, yields a universal power-law momentum distribution f(v)∼(1+v2)-3 (with v=kℓ) that naturally regularizes the ultraviolet divergence. From this, a parameter-free geometric baseline C=3 is obtained, independent of microscopic details. The experimental value C=3.4(3) [Martirosyan et al., Nature 647, 608 (2025)] is then shown to be quantitatively consistent with universal logarithmic corrections arising from a marginally irrelevant coupling at the fixed point. A new paradigm is thus established for predicting transport coefficients in strongly correlated non-equilibrium systems: symmetry constraints determine the low-energy effective theory, dynamical decoherence provides a natural ultraviolet completion, and scaling analysis delivers testable predictions moving beyond scaling exponents to quantitative amplitude prediction.