Pressure-induced phase transition and bi-polaronic sliding in a hole-doped Cu_2O_3 ladder system

TL;DR

This paper investigates how pressure influences polaronic states and phase transitions in a hole-doped Cu_2O_3 ladder system, revealing a transition from single to bi-polaron states and identifying associated vibrational modes.

Contribution

It introduces a model for pressure effects on polaronic states in a Cu_2O_3 ladder, highlighting a phase transition and bi-polaron dynamics not previously characterized.

Findings

Pressure induces a transition from single to bi-polaron states.

Identification of a site- to bond-centered bi-polaron transition.

Detection of a soft mode and charge-sliding mode associated with the transition.

Abstract

We study a hole-doped two-leg ladder system including metal ions, oxygen, and electron-lattice interaction, as a model for Sr_{14-x}Ca_xCu_{24}O_{41-\delta}. Single- and bi-polaronic states at 1/4-hole doping are modeled as functions of pressure by applying an unrestricted Hartree-Fock approximation to a multiband Peierls-Hubbard Hamiltonian. We find evidence for a pressure-induced phase transition between single-polaron and bi-polaron states. The electronic and phononic excitations in those states, including distinctive local lattice vibrational modes, are calculated by means of a direct-space Random Phase approximation. Finally, as a function of pressure, we identify a transition between site- and bond-centered bi-polarons, accompanied by a soft mode and a low-energy charge-sliding mode. We suggest comparisons with available experimented data.