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arXiv:2610.27582

Finite-temperature spectral functions of lattice polarons with process tensors

Qi Gao, Yuan Wan

Published Oct 20, 2026• 0 citations
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Abstract

We develop a process tensor approach for calculating the finite-temperature spectral functions of lattice polarons. We show that the finite-temperature phonon influence functional can be expressed as the elementwise exponential of a low-rank matrix product state, enabling an efficient summation of electron worldlines. An embedding construction embeds worldlines of different propagation times into a single matrix product state. This construction yields the propagator over the full simulated time interval in a single calculation. We benchmark this approach against available finite-temperature density-matrix renormalization group results for the one-dimensional Holstein polaron. We further obtain numerically converged spectra for the two-dimensional Holstein polaron. The spectra reveal a progressive transfer of spectral weight from coherent spectral features into an incoherent background with increasing temperature.

DOI
10.48550/arxiv.2610.27582

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We develop a process tensor approach for calculating the finite-temperature spectral functions of lattice polarons. We show that the finite-temperature phonon influence functional can be expressed as the elementwise exponential of a low-rank matrix product state, enabling an efficient summation of electron worldlines. An embedding construction embeds worldlines of different propagation times into a single matrix product state. This construction yields the propagator over the full simulated time interval in a single calculation. We benchmark this approach against available finite-temperature density-matrix renormalization group results for the one-dimensional Holstein polaron. We further obtain numerically converged spectra for the two-dimensional Holstein polaron. The spectra reveal a progressive transfer of spectral weight from coherent spectral features into an incoherent background with increasing temperature.
This work introduces an efficient numerical tool to calculate thermodynamic and spectral properties for electron-phonon problems at finite temperature in one dimension. Combining a finite-temperature density-matrix renormalization group method with local basis optimization allows one to access system sizes out of reach for other wave-function based approaches. The authors demonstrate the method's capabilities by computing several spectral functions of the Holstein polaron problem. The image shows the electron spectral function for two different system sizes (LLL=21, 101).
We use the density matrix renormalization group (DMRG) to compute the phonon spectral function of a one-dimensional spinless Holstein model doped with a low but finite carrier concentration, x≤0.15x \leq 0.15x≤0.15, as a function of the electron-phonon coupling λλλ. To the best of our knowledge, these are the first such results in this regime, complementing extensive prior work at the single-polaron level (x→0x\to 0x→0). We find that significant phonon spectral weight is transferred both below, all the way down to ω=0ω=0ω=0, and above the bare phonon energy ΩΩΩ, in stark contrast with the Kohn-anomaly phenomenology expected in the Migdal limit, where weight remains centered near ΩΩΩ with a kink at q=2kFq=2k_Fq=2kF​. No signature of this 2kF2k_F2kF​ kink appears in our results. This behavior is captured qualitatively by the Random Phase Approximation (RPA), and semi-quantitatively, at negligible extra computational cost, by a ``dressed RPA'' scheme in which the electron addition propagator is renormalized using the Momentum Average (MA) approximation for the low-density electron-polaron. By contrast, adding the lowest-order vertex correction to this dressed scheme produces unphysical negative spectral weight, signaling that vertex and propagator dressings must be treated consistently once the propagators are dressed nonperturbatively. Our results provide an efficient approximation for the phonon spectral functions of low-density polaron metals, a regime relevant to weakly doped insulators.
We compute the Holstein polaron spectral function on a one-dimensional ring using the finite-temperature TTT Lanczos method. With increasing TTT additional features in the spectral function emerge even at temperatures below the phonon frequency. We observe a substantial spread of the spectral weight towards lower frequencies and the broadening of the quasiparticle (QP) peak. In the weak-coupling regime the QP peak merges with the continuum in the high-TTT limit. In the strong-coupling regime the main features of the low-TTT spectral function remain detectable up to the highest TTT used in our calculations. The effective polaron mass shows a nonmonotonic behavior as a function of TTT at small phonon frequency but increases with TTT at larger frequencies. The self-energy remains kkk independent even at elevated TTT in the frequency range corresponding to the polaron band, while at higher frequencies it develops a distinguishable kkk dependence. Analytical expressions for the first few frequency moments are derived, and they agree well with those extracted from numerical calculations in a wide-TTT regime.
A comprehensive picture of polaron and bipolaron physics is essential to understand the optical absorption spectrum in many materials with electron-phonon interactions. In particular, the finite-temperature properties are of interest since they play an important role in many experiments. Here, we combine the parallel two-site time-dependent variational principle algorithm (p2TDVP) with local basis optimization (LBO) and purification to calculate time-dependent current-current correlation functions. From this information, we extract the optical conductivity for the Holstein polaron and bipolaron with dispersive phonons at finite temperatures. For the polaron in the weak and intermediate electron-phonon coupling regimes, we analyze the influence of phonon dispersion relations on the spectra. For strong electron-phonon coupling, the known result of an asymmetric Gaussian is reproduced for a flat phonon band. For a finite phonon bandwidth, the center of the Gaussian is either shifted to larger or smaller frequencies, depending on the sign of the phonon hopping. We illustrate that this can be well understood by considering the Born-Oppenheimer surfaces. Similar behavior is seen for the bipolaron for strong coupling. For the bipolaron with weak and intermediate coupling strengths and a flat phonon band, we obtain two very different spectra. The latter also has a temperature-dependent resonance at a frequency below the phonon frequency.
We study numerically the real time dynamics of lattice polarons by combining the Feynman path integral and the matrix product state (MPS) approach. By constructing and solving a flow equation, we show that the integrand, viewed as a multivariable function of polaron world line parameters, can be compressed as a low bond dimension MPS, thereby allowing for efficient evaluation of various dynamical observables. We establish the effectiveness of our method by benchmarking the calculated polaron spectral function in one dimension against available results. We further demonstrate its potential by presenting the polaron spectral function in two dimensions and simulating polaron diffusion in both one and two dimensions.
How a polaron loses its quasiparticle coherence with increasing temperature is a long-standing open problem. Holstein addressed it in 1959 only in the extreme antiadiabatic, strong-coupling limit, while more recent numerically exact approaches are largely restricted to one dimension. Here we solve this problem on square and simple cubic lattices across the weak-, intermediate-, and strong-coupling regimes. We show that the polaron effective mass m∗m^*m∗ and inverse lifetime 1/τ1/τ1/τ increase monotonically with temperature until, at T∼0.5 ΩT \sim 0.5\,ΩT∼0.5Ω, the quasiparticle peak dissolves into a broad incoherent thermal continuum. The phonon frequency ΩΩΩ therefore defines a universal coherence scale, independent of dimensionality and coupling strength, validating Holstein's prediction far beyond the regime in which it was derived. These results follow from a finite-temperature generalization of the Momentum Average (MA) approximation, yielding a closed-form, diagrammatically derived self-energy that is asymptotically exact in the strong-coupling limit at all temperatures. Benchmark comparisons demonstrate excellent quantitative agreement of the resulting 1D spectral functions with the numerically exact Variational Exact Diagonalization-Finite-Temperature Lanczos Method (VED-FTLM) and finite-TTT Density Matrix Renormalization Group (DMRG).