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A comprehensive picture of polaron and bipolaron physics is essential tounderstand the optical absorption spectrum in many materials withelectron-phonon interactions. In particular, the finite-temperature propertiesare of interest since they play an important role in many experiments. Here, wecombine the parallel two-site time-dependent variational principle algorithm(p2TDVP) with local basis optimization (LBO) and purification to calculatetime-dependent current-current correlation functions. From this information, weextract the optical conductivity for the Holstein polaron and bipolaron withdispersive phonons at finite temperatures. For the polaron in the weak andintermediate electron-phonon coupling regimes, we analyze the influence ofphonon dispersion relations on the spectra. For strong electron-phononcoupling, the known result of an asymmetric Gaussian is reproduced for a flatphonon band. For a finite phonon bandwidth, the center of the Gaussian iseither shifted to larger or smaller frequencies, depending on the sign of thephonon hopping. We illustrate that this can be well understood by consideringthe Born-Oppenheimer surfaces. A similar behavior is seen for the bipolaron forstrong coupling. For the bipolaron with weak and intermediate couplingstrengths and a flat phonon band, we obtain two very different spectra. Thelatter also has a temperature-dependent resonance at a frequency below thephonon frequency.

physical review. b./physical review. b

Finite-temperature optical conductivity with density-matrix  renormalization group methods for the Holstein polaron and bipolaron with  dispersive phonons