Abstract
Many-body quantum systems far from equilibrium can exhibit universal scaling dynamics which defy standard classification schemes. Here, we disentangle the dominant excitations in the universal dynamics of highly-occupied $N$-component scalar systems using unequal-time correlators. While previous equal-time studies have conjectured the infrared properties to be universal for all $N$, we clearly identify for the first time two fundamentally different phenomena relevant at different $N$. We find all $N\geq3$ to be indeed dominated by the same Lorentzian ``large-$N$'' peak, whereas $N=1$ is characterized instead by a non-Lorentzian peak with different properties, and for $N=2$ we see a mixture of two contributions.
Highlights
Universality constitutes a powerful tool to understand complex many-body systems
We find all N ≥ 3 to be dominated by the same Lorentzian “large-N” peak, whereas N 1⁄4 1 is characterized instead by a nonLorentzian peak with different properties, and for N 1⁄4 2, we see a mixture of two contributions
We find that the previously believed N-universality breaks up into two clearly distinct universality classes characterized by different phenomena, which are, almost indistinguishable from equal-time correlators and even the dynamical critical exponent z alone
Summary
Universality constitutes a powerful tool to understand complex many-body systems. A remarkable example is equilibrium phase transitions, where theories can be classified into universality classes based on only few system parameters [1]. New farfrom-equilibrium universality classes for isolated quantum systems have been theoretically identified [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24], which have recently started to be probed in cold-atom experiments [25,26,27,28,29] These universality classes can encompass vastly different theories such as gauge and scalar theories [20,30], or relativistic and nonrelativistic theories [12]. These unexpected connections raise the question of what the relevant physics behind the observed universality is The study of these far-from-equilibrium universality classes in isolated systems has so far primarily focused on the properties of equal-time momentum distribution functions, fðt; pÞ. Due to the initial “overoccupation” of mode excitations around Q, the subsequent redistribution dynamics is dominated by transport of particles to lower momenta, and is characterized by universal scaling in the infrared as explained in the
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