Functional methods for quantum thermodynamics
量子热力学的泛函方法
Sibo Wang, Samuel Degen, Haozhao Liang
AI总结 本文通过基准测试泛函重整化群密度泛函理论(FRG-DFT)在单点Bose-Hubbard模型中的精确热力学,验证了从微观哈密顿量构建密度泛函的非微扰方法,并提出了避免虚假自相互作用的两个一般性要求。
Comments 24 pages, 14 figures; comments are welcome
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泛函重整化群提供了一种从微观哈密顿量构建量子多体系统密度泛函的非微扰且可系统改进的途径。在此,我们通过将泛函重整化群密度泛函理论(FRG-DFT)与单点Bose-Hubbard模型的精确热力学进行基准测试,推进了这一程序。该模型是一个理想的测试平台,因为它可解析求解,但在虚时相干态路径积分中仍存在微妙之处,其中朴素的连续处理会产生虚假的自相互作用。我们证明,仔细的Hubbard-Stratonovich推导识别出必须包含在FRG-DFT流中的自相互作用修正项,以恢复精确热力学。然后,我们在密度、温度和相互作用强度的广泛范围内,系统比较了自由能、化学势和连接密度关联函数所得流方程层次结构的几种闭合方式。基准测试表明,自由能相对稳健,而化学势和涨落可观测量提供了层次闭合更敏锐的诊断。最大熵闭合给出了最准确的总体描述,甚至再现了低温下连接双密度关联函数的振荡结构。这些结果确定了量子热力学泛函方法的两个一般性要求:重整化群流方程必须保留等时接触减法以避免虚假自相互作用,并且层次结构的任何闭合都必须保持密度关联函数的统计一致性。这项工作为跨凝聚态物理、超冷原子物理、核物理以及量子化学领域,从第一性原理推导量子多体系统的密度泛函提供了可控基础。
The functional renormalization group provides a nonperturbative and systematically improvable route to constructing density functionals for quantum many-body systems from microscopic Hamiltonians. Here we advance this program by benchmarking functional-renormalization-group density functional theory (FRG-DFT) against the exact thermodynamics of the single-site Bose-Hubbard model. This model provides an ideal testing ground because it is analytically solvable, yet remains subtle in the imaginary-time coherent-state path integral, where a naive continuum treatment generates a spurious self-interaction. We show that a careful Hubbard-Stratonovich derivation identifies the self-interaction correction term that must be included in the FRG-DFT flow to recover the exact thermodynamics. We then systematically compare several closures of the resulting hierarchy of flow equations for the free energy, chemical potential, and connected density correlators over broad ranges of density, temperature, and interaction strength. The benchmark shows that the free energy is comparatively robust, whereas the chemical potential and fluctuation observables provide much sharper diagnostics of the hierarchy closure. A maximum-entropy closure gives the most accurate overall description and reproduces even the low-temperature oscillatory structure of the connected two-density correlator. These results identify two general requirements for functional approaches to quantum thermodynamics: the renormalization group flow equation must retain the equal-time contact subtraction to avoid spurious self-interactions, and any closure of the hierarchy must preserve the statistical consistency of density correlators. This work provides a controlled foundation for deriving ab initio density functionals for quantum many-body systems across condensed-matter, ultracold-atom, and nuclear physics, as well as quantum chemistry.