Vibronic coupling and band gap trends in CuGeO3 nanorods

Abstract

We measured the optical response of CuGeO3 nanorods in order to reveal size effects on the electronic properties. The vibronically-activated d-to-d color band excitations are activated by the 131 and 478 cm −1 phonons, with the relative contribution of the lower frequency O-Cu-O bending mode increasing with decreasing size until it dominates the process. We also uncover trends in the direct band gap, with the charge transfer transfer edge hardening with decreasing size. These findings advance the understanding of size effects in low-dimensional copper oxides.

Vibrational coherent control of localized d–d electronic excitation

“…In order to make the transmissivity map more intelligible and in relation with a dynamical shaping of the three d-d transitions, we have performed a fit of the measured response in the energy-domain and we have repeated this fit for each time delay. In particular, we have firstly fitted the optical absorption of the d-d transitions, which has been measured by O'Neal et al [13], by three Gaussians plus a background in order to obtain a set of initial parameters:…”

“…The onsite optical transition between orbitals of d symmetries should be forbidden in centrosymmetric crystals due to dipole selection rules (∆ = 0), nevertheless, they are visible in absorption spectroscopy even at very low temperatures (< 10K) [12,13]. This can be understood by considering that the optical onsite d-d transitions are accompanied by the creation (and annihilation) of phonons which break the symmetry of the orbital transition thereby removing the optical selection rule [12,13,14]. The involvement of a ground state phonon mode in the optical absorption process is confirmed by the observed strong increase of the oscillator strength with temperature [12].…”

“…The electric field operator can be used to describe an incoherent field as in linear response or the spectral components of a probe pulse in time domain experiments. With this model we can retrieve the total absorption of phonon assisted d-d transitions at equilibrium which depends on the temperature of the system as revealed by equilibrium absorption measurements of the CuGeO 3 as a function of the temperature [12,13] (see SI for the analytical derivation of the temperature dependence eq. 14, [16]).…”

“…Note that this result is consistent with the empirical description given in ref. [13] for the temperature-dependent d-d band absorption measured.…”

Spin–lattice and electron–phonon coupling in 3d/5d hybrid Sr3NiIrO6

“…Here, both spin and parity selection rules are broken due to coupling with a phonon. In this scenario, the temperature dependence of the oscillator strength is modeled as f = f x +f 0 coth(hν/2k B T ), where ν is the frequency of the activating phonon, f 0 is the oscillator strength at base temperature, f x represents oscillator strength from other mixing processes, and h, k B , and T have their usual meanings [45]. This model can be used to determine which phonon activates the electronic transition.…”