Mass Of Bipolaron Research Articles

Periodically driven adiabatic bipolarons

Small lattice bipolarons driven by external harmonic fields are considered in the adiabatic approximation. Resonant excitation of ions modulates the trapping potential and promotes hole transfer between neighboring atomic layers. It leads to a dramatic decrease of the apparent bipolaron mass compared to the undriven case. This effect offers an explanation for dynamic stabilization of superconductivity at high temperatures recently observed in layered cuprates.

Polaron and bipolaron of uniaxially strained one dimensional zigzag ladder

An influence of the uniaxial strains in one dimensional zigzag ladder (1DZL) on the properties of polarons and bipolarons is considered. It is shown that strain changes all the parameters of the system, in particular, spectrum, existing bands and the masses of charge carriers. Numerical results obtained by taking into an account the Poisson effect clearly indicate that the properties of the (bi)polaronic system can be tuned via strain. Mass of bipolaron can be manipulated by the strain too which in turn leads to the way of tuning Bose–Einstein condensation temperature TBEC of bipolarons. It is shown that TBEC of bipolarons in strained 1DZL reasonably correlates with the values of critical temperature of superconductivity of certain perovskites.

Influence of temperature and LO phonon on the effective mass of bipolarons in polar semiconductor quantum dots

The temperature and LO phonon effects of the bipolaron in polar semiconductor quantum dots (QDs) are studied by using the Tokuda modified linear-combination operator method and the Lee-Low-Pines variational method. The expressions for the mean number of LO phonons and the effective mass of the bipolaron are derived. Numerical results show that the mean number of LO phonons of the bipolaron decreases with increasing the temperature and the relative distance r between two electrons, but increases with increasing the electron-phonon coupling strength α. The effective mass of the bipolaron M* increases rapidly with increasing the relative distance r between two electrons when r is smaller, and it reaches a maximum at r ≈ 4.05rp, while after that, M* decreases slowly with increasing r. The effective mass of the bipolaron M* decreases with increasing the temperature. The electron-phonon coupling strength markedly influences the changes of mean number of LO phonons and the effective mass M* with the relative distance r and the temperature parameter γ.

Influence of temperature and LO phonon effects on the effective mass of quasi 0D bipolarons in the strong-coupling limit

In this paper, based on the of Huybrechts strong-coupling polaron model, the Tokuda modified linear-combination operator method, the Lee–Low–Pines variational method and the quantum statistics theory were used to study the temperature dependence of the effective mass of quasi-zero-dimensional strong-coupling bipolarons. The expressions for the effective mass and the mean number of longitudinal optical (LO) phonons of the strong-coupling bipolaron were derived. Numerical results show that the effective mass and the mean number of LO phonons of the bipolaron all decrease with increasing the relative distance between two electrons or the radius of the quantum dot, but they increase with increasing the electron–phonon coupling strength; the effective mass of the bipolaron decreases with the increase of the temperature; the variation of the mean number of LO phonons of the bipolaron with the temperature presents the opposite trend because of the high and low temperature difference.

Gain of the kinetic energy of bipolarons in thet−J-Holstein model based on electron-phonon coupling

With increasing electron-phonon coupling as described within the t-J-Holstein model, bipolaron kinetic energy is lowered in comparison with that of the polaron. This effect is accompanied with "undressing" of bipolaron from lattice degrees of freedom. Consequently, the effective bipolaron mass becomes smaller than the polaron mass. Magnetic as well as lattice degrees of freedom cooperatively contribute to formation of spin-lattice bipolarons.

Superlight small bipolarons

Recent angle-resolved photoemission spectroscopy (ARPES) has identifiedthat a finite-range Fröhlich electron–phonon interaction (EPI) withc-axis polarized optical phonons is important in cuprate superconductors, in agreement withan earlier proposal by Alexandrov and Kornilovitch. The estimated unscreened EPI is sostrong that it could easily transform doped holes into mobile lattice bipolarons innarrow-band Mott insulators such as cuprates. Applying a continuous-time quantumMonte Carlo algorithm (CTQMC), we compute the total energy, effective mass,pair radius, number of phonons and isotope exponent of lattice bipolarons in theregion of parameters where any approximation might fail, taking into account theCoulomb repulsion and the finite-range EPI. The effects of modifying the interactionrange and different lattice geometries are discussed with regards to analyticalstrong-coupling/non-adiabatic results. We demonstrate that bipolarons can besimultaneously small and light, provided suitable conditions on the electron–phononand electron–electron interactions are satisfied. Such light small bipolarons are anecessary precursor to high-temperature Bose–Einstein condensation in solids.The light bipolaron mass is shown to be universal in systems made of triangularplaquettes, due to a novel crab-like motion. Another surprising result is that thetriplet–singlet exchange energy is of the first order in the hopping integral and thattriplet bipolarons are much heavier than singlets in certain lattice structures.Finally, we identify a range of lattices where superlight small bipolarons may beformed, and give estimates for their masses in the anti-adiabatic approximation.

Holstein and Fröhlich bipolarons

We explore the properties of the bipolaron in a simplified one-dimensional (1D) Fröhlich–Hubbard model in comparison with results in the Holstein–Hubbard model, where the former has a longer range electron–phonon interaction. We solve the model with an exact diagonalization method on an infinite 1D lattice. In the strong coupling regime, the effective mass of the Fröhlich bipolaron is much smaller than the Holstein bipolaron mass. In contrast to the Holstein model where only a singlet bipolaron is bound, in the Fröhlich model, a triplet bipolaron can also form a bound state.

Bipolarons in the extended Holstein Hubbard model

We numerically and analytically calculate the properties of the bipolaron in an extended Hubbard Holstein model, which has a longer range electron-phonon coupling like the Fr\" ohlich model. In the strong coupling regime, the effective mass of the bipolaron in the extended model is much smaller than the Holstein bipolaron mass. In contrast to the Holstein bipolaron, the bipolaron in the extended model has a lower binding energy and remains bound with substantial binding energy even in the large-U limit. In comparison with the Holstein model where only a singlet bipolaron is bound, in the extended Holstein model a triplet bipolaron can also form a bound state. We discuss the possibility of phase separation in the case of finite electron doping.

Small bipolarons in the 2-dimensional Holstein-Hubbard model. II. Quantum bipolarons

We study the effective mass of the bipolarons and essentially the possibility to get both light and strongly bound bipolarons in the Holstein-Hubbard model and some variations in the vicinity of the adiabatic limit. Several approaches to investigate the quantum mobility of polarons and bipolarons are proposed for this model. It is found that the bipolaron mass generally remains very large except in the vicinity of the triple point of the phase diagram, where the bipolarons have several degenerate configurations at the adiabatic limit (single site (S0), two sites (S1) and quadrisinglet (QS)), while the polarons are much lighter. This degeneracy reduces the bipolaron mass significantly. The triple point of the phase diagram is washed out by the lattice quantum fluctuations which thus suppress the light bipolarons. We show that some model variations, for example a phonon dispersion may increase the stability of the (QS) bipolaron against the quantum lattice fluctuations. The triple point of the phase diagram may be stable to quantum lattice fluctuations and a very sharp mass reduction may occur, leading to bipolaron masses of the order of 100 bare electronic mass for realistic parameters. Thus such very light bipolarons could condense as a superconducting state at relatively high temperature when their interactions are not too large, that is, their density is small enough. This effect might be relevant for understanding the origin of the high Tc superconductivity of doped cuprates far enough from half filling.

Mobility of biplasmapolarons and high-T c superconductivity

The mobility of the Frohlich bipolarons is a challenging matter for the bipolaronic theory of high-T c superconductivity. Here we focus our attention on the problem of the effective mass of a bipolaron in a strongly ionic system (like the new H(igh)T cS(uper)C(onductors) are) and in the presence of an electron gas. We show that there is a sizeable renormalization of the bipolaron mass due to the polarization effects of both the optical phonons and plasmons. Moreover, we show that the effective mass decreases increasing the density of carriers, because the screening effects become more important. In all cases, the values of the bipolaron effective mass are not greater than some tens of electronic masses (m e).

Photoinduced absorption and resonant Raman scattering of polythiophene

Resonant Raman scattering (RRS) and photoinduced absorption (PA) measurement in polythiophene are reported. The RRS is analyzed by the amplitude mode model modified to include an additive extrinsic component Δ e. For polythiophene we obtain a ld gap 2 Δ = 2.2 eV and 2 Δ e = 0.22 eV. The dominant photo-carriers generated with an above the gap excitation are charged bipolarons with two correlated electronic PA bands at 0.45 eV and 1.25 eV and several IR active vibrations. The PA spectrum yields for the bipolaron mass M ≅ m e in good agreement with theoretical predictions.