Small parameter and optical conductivity in the migdal-type description of t-t′-J model.

Sergei I. Mukhin, L.J. de Jongh

Nikolay M. Plakida

Lara Benfatto, J. P. Ćarbotte, F. Marsiglio

We examine the temperature dependence of the optical sum rule in the normal state due to interactions. To be concrete, we adopt a weak-coupling approach that uses an electron-boson exchange model to describe inelastic scattering of the electrons with a boson, in the Migdal approximation. While a number of recent works attribute the temperature dependence in the normal state to that which arises in a Sommerfeld expansion, we show that in a wide parameter regime this contribution can be quite small. Instead, most of the temperature dependence arises from the zeroth-order term in the ``expansion,'' through the temperature dependence of the spectral function and the interaction parameters contained therein. For low boson frequencies, this circumstance causes a linear $T$ dependence in the sum rule. We develop some analytical expressions and understanding of the temperature dependence.

Philip M. Dee, Jennifer Coulter, Kevin G. Kleiner +1 more

Abstract Determining the range of validity of Migdal’s approximation for electron-phonon (e-ph) coupled systems is a long-standing problem. Many attempts to answer this question employ the Holstein Hamiltonian, where the electron density couples linearly to local lattice displacements. When these displacements are large, however, nonlinear corrections to the interaction must also be included, which can significantly alter the physical picture obtained from this model. Using determinant quantum Monte Carlo and the self-consistent Migdal approximation, we compared superconducting and charge-density-wave correlations in the Holstein model with and without second-order nonlinear interactions. We find a disagreement between the two cases, even for relatively small values of the e-ph coupling strength, and, importantly, that this can occur in the same parameter regions where Migdal’s approximation holds. Our results demonstrate that questions regarding the validity of Migdal’s approximation go hand in hand with questions of the validity of a linear e-ph interaction.

Norio Kawakami, Sung-Kil Yang

James S. Walker

The simple, but ad hoc, prescription for bond shifting in the Migdal-Kadanoff approximation is replaced by a systematic calculational procedure. In this method, the free energy is required to be preserved exactly by means of matching series expansions. The resulting renormalization group is characterized by a small parameter, and can be extended by working to higher order. A significant quantitative improvement over the standard Migdal-Kadanoff method is obtained with little increase in calculational effort. This approach is ultimately limited, however, by being restricted to a one-dimensional interaction parameter space.