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arXiv:2008.03304 · Physical review. B./Physical review. B

Bipolaron liquids at strong Peierls electron-phonon couplings

Alberto Nocera, John Sous, Adrian E. Feiguin, Mona Berciu

Published Nov 17, 2021• 24 citations
arXivHTMLPDF

Abstract

We use the density matrix renormalization group method to study a one-dimensional chain with Peierls electron-phonon coupling, which describes the modulation of the electron hopping by lattice distortions. We demonstrate that this system is stable against phase separation in the dilute density limit. We only find phase separation numerically for large couplings for which the linear approximation for the electron-phonon coupling becomes invalid; this behavior can be stabilized in a narrow sliver of the physical parameter space if the dispersion of the phonons is carefully tuned. These results indicate that in the dilute electron density limit, Peierls bipolaron liquids are generically stable, unlike in other models of electron-phonon coupling. We show that this behavior extends to finite carrier concentrations of up to quarter filling. This stability of low-density, light-mass bipolaron liquids in the Peierls model opens a path to high-${T}_{c}$ superconductivity based on a bipolaronic mechanism, in higher dimensions.

DOI
10.1103/physrevb.104.l201109

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arXiv:2308.13427•Aug 25, 2023•1 citation
We use the density matrix renormalization group method to study a one-dimensional chain with Peierls electron-phonon coupling, which describes the modulation of the electron hopping by lattice distortions. We demonstrate that this system is stable against phase separation in the dilute density limit. We only find phase separation numerically for large couplings for which the linear approximation for the electron-phonon coupling becomes invalid; this behavior can be stabilized in a narrow sliver of the physical parameter space if the dispersion of the phonons is carefully tuned. These results indicate that in the dilute electron density limit, Peierls bipolaron liquids are generically stable, unlike in other models of electron-phonon coupling. We show that this behavior extends to finite carrier concentrations of up to quarter filling. This stability of low-density, light-mass bipolaron liquids in the Peierls model opens a path to high-Tc{T}_{c}Tc​ superconductivity based on a bipolaronic mechanism, in higher dimensions.
We use Density Matrix Renormalization Group to study a one-dimensional chain with Peierls electron-phonon coupling describing the modulation of the electron hopping due to lattice distortion. We demonstrate the appearance of an exotic phase-separated state, which we call Peierls phase separation, in the limit of very dilute electron densities, for sufficiently large couplings and small phonon frequencies. This is unexpected, given that Peierls coupling mediates effective pair-hopping interactions that disfavor phase clustering. The Peierls phase separation consists of a homogenous, dimerized, electron-rich region surrounded by electron-poor regions, which we show to be energetically more favorable than a dilute liquid of bipolarons. This mechanism qualitatively differs from that of typical phase separation in conventional electron-phonon models that describe the modulation of the electron's potential energy due to lattice distortions. Surprisingly, the electron-rich region always stabilizes a dimerized pattern at fractional densities, hinting at a non-perturbative correlation-driven mechanism behind phase separation.
We analyze the transition temperature Tc{T}_{c}Tc​ of bipolaronic superconductivity in a bond Su-Schrieffer-Heeger (bond-SSH) model---also known as a bond Peierls model---where bond phonons modulate the electron hoppings. Using a semiclassical instanton approximation justifiable in the adiabatic limit of slow phonons, we find that the bipolaron mass is only weakly enhanced in contrast to the typically large mass enhancement found in models where the phonons couple to electron density. Strikingly, in the ``atomic'' limit, we find a bond bipolaron can freely slide within a continuously degenerate energy manifold, explaining the origin of the weak mass enhancement in the strong coupling limit. A gas of these bipolarons can undergo a superfluid transition at a critical temperature for which we obtain an upper bound. This bound is exponentially larger than that in the Holstein model. Our study provides an analytical understanding of the mechanism behind the high-Tc{T}_{c}Tc​ bipolaronic superconductivity numerically observed in [Phys. Rev. X 13, 011010 (2023)].
We re-examine the Peierls insulator to Mott insulator transition scenario in the one-dimensional Holstein-Hubbard model where, at half-filling, electron-phonon and electron-electron interactions compete for establishing charge- and spin-density-wave states, respectively. By means of large-scale density-matrix renormalization group calculations we determine the spin, single-particle and two-particle excitation gaps and prove —in the course of a careful finite-size scaling analysis— recent claims for an intervening metallic phase in the weak-coupling regime. We show that for large phonon frequencies the metallic region is even more extended than previously expected, and subdivided into ordinary Luttinger-liquid and bipolaronic-liquid phases.
We review a series of exact, analytical and numerical results obtained on the adiabatic Holstein–Hubbard model, many of which are new and non-trivial. We study next the role of the quantum lattice fluctuations that were initially neglected. The possibility of having high- T c bipolaronic superconductors is analysed on the basis of these results. We suggest that models that involve only electron–phonon coupling are very unlikely to produce bipolaronic superconductivity, which is prevented by the spatial ordering of the bipolarons associated with a lattice instability. This is due to a very large effective mass of the bipolarons that can be related to the large Peierls–Nabarro energy barrier required to move these bipolarons through the lattice in the adiabatic limit. We conjecture that high- T c superconductivity originates specifically from the exceptionally well-balanced competition between electron–phonon coupling and electron–electron repulsion. In the restricted region in which, within the mean field approach, the energy of a bipolaron is close to those of two polarons, a new type of electron pairing occurs by formation of pairs of polarons in the spin singlet state. Such a polaron pair, called a spin resonant bipolaron (SRB), is not the standard bipolaron (both could exist in the adiabatic case). Its Peierls–Nabarro energy barrier can be sharply depressed, almost to zero. As a result of quantum lattice fluctuations, its tunnelling energy is sharply enhanced. Then, superconductivity could persist for a relatively large electron–phonon coupling, with an unusually large critical temperature as a Bose condensate of SRBs before becoming, at larger coupling and in any case, an insulating spin–Peierls polaronic phase.
Standard Peierls theory predicts that the wave-vector of the charge density wave (CDW) induced by the electron-phonon coupling in the ground-state of a one-dimensional conductor is twice the Fermi wave-vector 2k F . At moderately large electron-phonon coupling, beyond the TBA (transition by breaking of analyticity), it is known that this CDW becomes an array of interacting bipolarons. In the simplest models, such as the 1D adiabatic Holstein model, the bipolaron interactions are repulsive and the ground state remains a 2k F CDW of equidistant bipolarons. We show that (apparently) minor changes in the phonon spectrum of the model induce extra elastic forces between the bipolarons, which could break the bipolaronic 2k F CDW ordering. This effect is studied in detail in the modified 1D Holstein model, where the optical phonon has a non-zero dispersion which is chosen positive in order that an extra force that appears between the bipolarons be attractive. The Coulomb forces between the bipolarons are initially neglected. An accurate numerical study modelled on the standard analysis of multiphase points confirms that, in a large part of the phase diagram, the CDW ground state separates in two phases, which correspond to two CDW with different 2k F wave-vectors (which are generally commensurate) and thus different electronic densities. These separations, into phases called either 'parent' or 'non-parent', are studied on the basis of the Farey construction of rational numbers. The long-range Coulomb interaction between the charged bipolarons, forbids any macroscopic phase separation in two phases with different electronic densities. The bipolaron structure then has to be a periodic sequence of alternate domains of the two CDW phases on the microscopic scale. The resulting structure is a new CDW with a modulation wave-vector that is not 2k F . We suggest that the aberrant wave-vector observed in the real CDW system (TaSe 4 ) 2 I could be interpreted along the ideas developed in this paper. Although this compound shares many of the usual properties of quasi-one-dimensional conductors with CDW, the component of the modulation wave-vector 0.085c* in the chain direction is, unusually, quite different from the value 2k F =c* expected from band filling. The CDW in (TaSe 4 ) 2 I could correspond to a nonstandard bipolaron ordering due to a strong attractive elastic interaction between the bipolarons along the chain competing with the long-range Coulomb forces.
Light-mass bipolarons in off-diagonally coupled electron-phonon systems provide a potential route to bipolaronic high-Tc superconductivity. While there has been numerical progress in the physically relevant limit of slow phonons, more insights are needed to fully understand to what extent this mechanism survives at finite densities and in different regimes. We address these questions using advanced tensor-network methods applied to the Su-Schrieffer-Heeger model. Studying both the spectral properties of isolated bipolarons as well as the pairing correlations at small finite densities, we find evidence that the conventional picture of bipolarons as molecular bound states which undergo Bose-Einstein condensation may need to be reconsidered. Instead, our findings suggest correlation-driven formation of a fragmented condensate with spatially separated polaron pairs, stabilized by strong repulsive electron-electron interactions at moderate values of the electron-phonon coupling. These spatially modulated charge clouds exhibit pairing with a typical length-scale set by the Fermi momentum.