• Matter and Radiation at Extremes
  • Vol. 8, Issue 5, 056601 (2023)
Tobias Dornheim1、2、a), Zhandos A. Moldabekov1、2, Panagiotis Tolias3, Maximilian Böhme1、2、4, and Jan Vorberger2
Author Affiliations
  • 1Center for Advanced Systems Understanding (CASUS), D-02826 Görlitz, Germany
  • 2Helmholtz-Zentrum Dresden-Rossendorf (HZDR), D-01328 Dresden, Germany
  • 3Space and Plasma Physics, Royal Institute of Technology (KTH), Stockholm SE-100 44, Sweden
  • 4Technische Universität Dresden, D-01062 Dresden, Germany
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    DOI: 10.1063/5.0149638 Cite this Article
    Tobias Dornheim, Zhandos A. Moldabekov, Panagiotis Tolias, Maximilian Böhme, Jan Vorberger. Physical insights from imaginary-time density–density correlation functions[J]. Matter and Radiation at Extremes, 2023, 8(5): 056601 Copy Citation Text show less

    Abstract

    An accurate theoretical description of the dynamic properties of correlated quantum many-body systems, such as the dynamic structure factor S(q, ω), is important in many fields. Unfortunately, highly accurate quantum Monte Carlo methods are usually restricted to the imaginary time domain, and the analytic continuation of the imaginary-time density–density correlation function F(q, τ) to real frequencies is a notoriously hard problem. Here, it is argued that often no such analytic continuation is required because by definition, F(q, τ) contains the same physical information as does S(q, ω), only represented unfamiliarly. Specifically, it is shown how one can directly extract key information such as the temperature or quasi-particle excitation energies from the τ domain, which is highly relevant for equation-of-state measurements of matter under extreme conditions [T. Dornheim et al., Nat. Commun. 13, 7911 (2022)]. As a practical example, ab initio path-integral Monte Carlo results for the uniform electron gas (UEG) are considered, and it is shown that even nontrivial processes such as the roton feature of the UEG at low density [T. Dornheim et al., Commun. Phys. 5, 304 (2022)] are manifested straightforwardly in F(q, τ). A comprehensive overview is given of various useful properties of F(q, τ) and how it relates to the usual dynamic structure factor. In fact, working directly in the τ domain is advantageous for many reasons and opens up multiple avenues for future applications.
    S(q,ω)=dteiωtF(q,t),

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    F(q,t)=n̂(q,t)n̂(q,0).

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    F(q,τ)=LS(q,ω)=dωeωτS(q,ω).

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    I(q,ω)=S(q,ω)R(ω).

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    LS(q,ω)=LS(q,ω)R(ω)LR(ω),

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    Ĥ=12l=1Nl2+l<kNϕE(r̂l,r̂k)+N2ξM,

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    Zβ,N,V=1N!N!σSNσSNsgn(σ,σ)×VdRR|eβĤ|π̂σπ̂σR.

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    eβĤ=eϵĤP

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    Zβ,N,V=1N!N!σSNσSNsgn(σ,σ)×VdR0dRP1R0|eϵĤ|R1×R1|RP1|eϵĤ|π̂σπ̂σR0.

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    eϵĤ=eϵK̂eϵV̂+Oϵ2.

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    Z=VdXW(X)=VdXα=0P1WV(Rα)WK(Rα,Rα+1),

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    WV(Rα)=expϵ2klNW(rl,α,rk,α),

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    WK(Rα,Rα+1)=expl=1N(rl,αrl,α+1)22σϵ22πσϵ3N/2.

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    λϵ=λβPandλβ=2πβ.

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    F(q,τ)=n̂(q,0)n̂(q,τ).

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    F(q,τj)=1Zβ,N,V1N!N!σSNσSNsgn(σ,σ)×dR0dRP1R0|n̂(q)eϵĤ|R1R1|eϵĤ|R2Rj|n̂(q)eϵĤ|Rj+1RP1|eϵĤ|π̂σπ̂σR0.

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    S(q,ω)=Imχ(q,ω)πn(1eβω),

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    χ(q,ω)=χ0(q,ω)14πq21G(q,ω)χ0(q,ω),

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    χ(q,0)=n0βdτF(q,τ),

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    S(q,ω)=S(q,ω)eβω,

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    F(q,τ)=0dωS(q,ω)eωτ+eω(βτ)=F(q,βτ).

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    MS(α)=ωαS=dωS(q,ω)ωα.

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    MS(1)=ω1S=q22.

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    dndτnF(q,τ)=(1)ndωωneτωS(q,ω).

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    MS(α)=1ααταF(q,τ)τ=0

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    S(q,ω)=m,lPmnml(q)2δ(ωωlm).

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    F(q,τ)=m,lPmnml(q)2eτωlm=Em<Elnml(q)2Pmeτωlm+Pleτωlm,

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    S(ω)=G(ω;ω0,σ)if ω0,eβωG(ω;ω0,σ)otherwise,

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    G(ω;ω0,σ)=exp(ωω0)22σ22πσ2.

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    F(τ)=A(τ;ω0,σ)+A(βτ;ω0,σ),

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    A(τ;ω0,σ)=eτω0eτ2σ2/21+erfω0σ2τ2σ22.

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    SQP(ω)=δ(ωωQP)+eβωQPδ(ω+ωQP),

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    FQP(τ)=eτωQP+e(βτ)ωQP.

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    MS(1)=χ(q,0)2n.

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    ΔFτ(q)=F(q,0)F(q,τ)F(q,0),

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    Tobias Dornheim, Zhandos A. Moldabekov, Panagiotis Tolias, Maximilian Böhme, Jan Vorberger. Physical insights from imaginary-time density–density correlation functions[J]. Matter and Radiation at Extremes, 2023, 8(5): 056601
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