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arXiv 2401.08031cond-mat.dis-nncond-mat.stat-mechcond-mat.str-elquant-ph
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无序自旋链中的算符增长:多体局域化缺失的迹象

Operator Growth in Disordered Spin Chains: Indications for the Absence of Many-Body Localization

A. Weisse, R. Gerstner, J. Sirker

AI总结:

本文通过算符对易子范数增长分析无序自旋链,证明几乎阶乘式增长与指数局域化不相容,并利用准局域幺正变换和 Schrieffer-Wolff 微扰构造表明相互作用系统中多体局域化可能不存在。

摘要:

我们通过计算 k 重对易子 $[H,[H,[...,[H,A]]]]$ 来考虑哈密顿量为 $H$ 的一维系统中局域算符 $A$ 的传播。我们在自由系统和相互作用系统中,在有、无无序的情况下,推导出该对易子算符范数的界,从而直接将算符增长假设与局域化问题联系起来。我们解析地证明,算符范数的几乎阶乘式增长——正如最近对随机 Ising 模型所证明的那样——与 $A$ 的指数局域化不相容。假设存在一个准局域幺正变换 $U$,它将 $H$ 映射到有效哈密顿量 $\tilde H=UHU^\dagger=\sum_n E_n τ^z_n +\sum_{i,j} J_{ij} τ^z_iτ^z_j+\dots$,我们证明 $\tilde A=UAU^\dagger$ 是一个准局域算符,在多体情形下通常不再保持指数局域化,从而导致几乎阶乘式的范数增长。因此,在具有最大范数增长的多体系统中,幺正变换 $U$ 要么不存在,这类系统始终是遍历的;要么确实存在由 $\tilde H$ 描述的、违反算符增长假设的非遍历奇异相,在这些相中算符会发生传播,意味着输运最终将开始。我们对 Anderson 模型和 Aubry-André 模型进行了数值和符号验证。对于 XXX 情形,符号计算结果与最大范数增长一致。此外,如果幺正变换 $U$ 确实存在,在强无序和低阶对易子情况下预期会出现 $A$ 的指数局域化减弱,但我们没有发现任何迹象。最后,我们尝试通过连续的 Schrieffer-Wolff 变换微扰地构造 $U$。虽然可以直接证明这种构造在 Anderson 情形下收敛,但我们在相互作用情形下没有发现收敛的迹象,这表明 $U$ 不存在,且多体局域化缺失。

英文摘要:

We consider the spreading of a local operator $A$ in one-dimensional systems with Hamiltonian $H$ by calculating the $k$-fold commutator $[H,[H,[...,[H,A]]]]$. We derive bounds for the operator norm of this commutator in free and interacting systems with and without disorder thus directly connecting the operator growth hypothesis with questions of localization. We analytically show that an almost factorial growth of the operator norm - as recently proven for the random Ising model - is inconsistent with an exponential localization of $A$. Assuming that a quasi-local unitary $U$ exists which maps $H$ onto an effective Hamiltonian $\tilde H=UHU^\dagger=\sum_n E_n τ^z_n +\sum_{i,j} J_{ij} τ^z_iτ^z_j+\dots$, we show that $\tilde A=UAU^\dagger$ is a quasi-local operator which in the many-body case does not remain exponentially localized in general leading to an almost factorial norm growth. Therefore the unitary $U$ in many-body systems with maximal norm growth either does not exist and such systems are always ergodic or unusual non-ergodic phases described by $\tilde H$ do exist which violate the operator growth hypothesis and in which operators spread, implying that transport will eventually set in. We analytically and symbolically verify our results for the Anderson and Aubry-André models. For the XXX case, the symbolic calculations are consistent with a maximal norm growth. Furthermore, we find no indication of a weakened exponential localization of $A$, expected for strong disorder and low commutator orders if the unitary $U$ does exist. Finally, we try to perturbatively construct $U$ by consecutive Schrieffer-Wolff transformations. While it is straightforward to show that this construction converges in the Anderson case, we find no indications for a convergence in the interacting case, suggesting that $U$ does not exist and that many-body localization is absent.

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