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Europhysics Letters (EPL)

Light polarons and bipolarons for a highly inhomogeneous electron-boson coupling

Mona Berciu, G. A. Sawatzky

Published Feb 19, 2008• 9 citations
Publisher

Abstract

We derive the exact polaron and bipolaron Green's functions for a model with a highly inhomogeneous electron-boson coupling , where . While the polaron ground-state energy and quasiparticle weight are similar to those of the Holstein polaron with equal effective coupling, the polaron dispersion is very different. Unlike that of a Holstein polaron, which is monotonically increasing with the polaron momentum, the polaron dispersion in this case is folded inside the Brillouin zone. For strong coupling, the polaron and bipolaron effective masses increase linearly with the effective coupling and exhibit no isotope effect, as opposed to an exponential increase and a strong isotope effect, in the Holstein model. As a result, this model exhibits strongly bound yet light bipolarons at strong couplings. Generalizations are discussed.

DOI
10.1209/0295-5075/81/57008

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We derive the exact polaron and bipolaron Green's functions for a model with a highly inhomogeneous electron-boson coupling , where . While the polaron ground-state energy and quasiparticle weight are similar to those of the Holstein polaron with equal effective coupling, the polaron dispersion is very different. Unlike that of a Holstein polaron, which is monotonically increasing with the polaron momentum, the polaron dispersion in this case is folded inside the Brillouin zone. For strong coupling, the polaron and bipolaron effective masses increase linearly with the effective coupling and exhibit no isotope effect, as opposed to an exponential increase and a strong isotope effect, in the Holstein model. As a result, this model exhibits strongly bound yet light bipolarons at strong couplings. Generalizations are discussed.
We have computed static and dynamic properties of an electron coupled to hard-core-boson (HCB) degrees of freedom in one spatial dimension. The polaron, an electron dressed with HCB excitations, remains light even in the strong-coupling limit as its effective mass remains of the order of the free-electron mass. This result is in a sharp contrast to the Holstein model where the electron effective mass increases exponentially with the electron-phonon coupling. HCB degrees of freedom mediate the attractive potential between two electrons that form a bound singlet bipolaron state at any nonzero coupling strength. In the low-frequency regime of the electron spectral function we observe a quasiparticle (QP) band that is separated from the continuum of states only in the central part of the Brillouin zone. The quasiparticle weight approaches zero as the QP band enters the continuum where it obtains a finite lifetime. At finite temperature an electron can annihilate thermally excited HCBs. Such thermally activated processes lead to a buildup of the spectral weight below the QP band. While the investigated model bears a resemblance with the Holstein model, we point out many important differences that originate from the binary HCB excitation spectrum, which in turn mimics spin-12\frac{1}{2}21​ degrees of freedom.
In most cases, as the strength of electron-phonon coupling increases, the effective mass of polarons typically increases. However, in this paper, we uncover a fascinating phenomenon: the presence of light polarons even within the strong coupling regime, where electron-phonon coupling includes both Holstein and bond types (electron-phonon coupling occurring on the hopping bonds). To investigate this, we employ a diagrammatic Monte Carlo method based on the path-integral formulation of the particle sector and the Fock-state representation of the phonon sector. Our study centers on analyzing the impact of bond electron-phonon coupling on the Holstein polaron's essential properties, including its effective mass, ground-state energy, and the average number of phonons. We examine two distinct scenarios: one where the phonon frequency of both Holstein and bond types is the same, and another where the phonon frequency of the Holstein type is twice as large as that of the bond type. In both of these cases, when the Holstein coupling falls within the light mass regime, we observe minimal changes in the effective mass as a function of bond coupling, compared to the case of the bare bond polaron. However, the behavior of the effective mass undergoes a significant change in the heavy mass regime. Most intriguingly, we discover a nonmonotonic dependence of the effective mass on the bond coupling gB{g}_{B}gB​ when the Holstein coupling gH{g}_{H}gH​ falls within the heavy mass regime. This finding holds promise, particularly in the context of bipolarons, where achieving a light effective mass and a large binding energy to get a compact size is crucial.
Bipolaron energies are calculated as a function of wave vector by a variational method of Gurari appropriate for weak or intermediate coupling strengths, for a model with electron-phonon interactions independent of phonon wave vector and a short-ranged Coulomb repulsion. It is assumed that the bare electrons have a constant effective mass. A two-parameter trial function is taken for the relative motion of the two electrons in the bipolaron. The energies of the bipolarons are compared with those of two single polarons as a function of wave vector for various parameter values. Results for the effective masses at the zone center are also obtained. Comparison is made with data of other authors for bipolarons in the Hubbard-Holstein model, which differs mainly from the present model in that it has a tight-binding band structure for the bare electrons.
We investigate the ground-state properties of the bipolaron coupled to quantum dispersive optical phonons in the one-dimensional Holstein-Hubbard model. We concentrate on the interplay between the phonon dispersion and the Coulomb repulsion and their mutual effect on the bipolaron effective mass, the binding energy, and the phase diagram. Most surprisingly, the sign of the curvature of the optical phonon dispersion plays a decisive role in the bipolaron binding energy in the presence of the Coulomb repulsion UUU. In particular, when the sign of the phonon dispersion curvature matches the sign of the electron dispersion curvature, the bipolaron remains bound in the strong-coupling limit even when $U\ensuremath{\rightarrow}\ensuremath{\infty}$ and the binding emanates from the exchange of phonons between two electrons residing on adjacent sites. At moderate electron-phonon coupling a light bipolaron exists up to large values of UUU. Finally, an intuitive explanation of the role of the phonon dispersion on the bipolaron binding energy is derived using the strong-coupling limit where the binding emanates from the exchange of phonons between two electrons residing on adjacent sites which leads to enhanced stability of bipolarons at elevated Coulomb repulsion.
We calculate the spectral properties of the one-dimensional Holstein and breathing polarons using the self-consistent Born approximation. The Holstein model electron-phonon coupling is momentum independent while the breathing coupling increases monotonically with the phonon momentum. We find that for a linear or tight binding electron dispersion: (i) for the same value of the dimensionless coupling the quasiparticle renormalization at small momentum in the breathing polaron is much smaller, (ii) the quasiparticle renormalization at small momentum in the breathing polaron increases with phonon frequency unlike in the Holstein model where it decreases, and (iii) in the Holstein model the quasiparticle dispersion displays a kink and a small gap at an excitation energy equal to the phonon frequency ${\ensuremath{\omega}}_{0}$ while in the breathing model it displays two gaps, one at excitation energy ${\ensuremath{\omega}}_{0}$ and another one at $2{\ensuremath{\omega}}_{0}$. These differences have two reasons: first, the momentum of the relevant scattered phonons increases with increasing polaron momentum and second, the breathing bare coupling is an increasing function of the phonon momentum. These result in an effective electron-phonon coupling for the breathing model which is an increasing function of the total polaron momentum, such that the small momentum polaron is in the weak coupling regime while the large momentum one is in the strong coupling regime. However, the first reason does not hold if the free electron dispersion has low energy states separated by large momentum, as in a higher dimensional system, for example, in this situation the difference between the two models becomes less significant.