Magnetic susceptibility of alkali-tetracyanoquinodimethane salts and extended Hubbard models with bond order and charge density wave phases

Abstract

The molar spin susceptibilities χ(T ) of Na-TCNQ, K-TCNQ and Rb-TCNQ(II) are fit quantitatively to 450 K in terms of half-filled bands of three one-dimensional Hubbard models with extended interactions using exact results for finite systems. All three models have bond order wave (BOW) and charge density wave (CDW) phases with boundary V = Vc(U ) for nearest-neighbor interaction V and on-site repulsion U . At high T , all three salts have regular stacks of TCNQ − anion radicals. The χ(T ) fits place Na and K in the CDW phase and Rb(II) in the BOW phase with V ≈ Vc. The Na and K salts have dimerized stacks at T < T d while Rb(II) has regular stacks at 100K. The χ(T ) analysis extends to dimerized stacks and to dimerization fluctuations in Rb(II). The three models yield consistent values of U , V and transfer integrals t for closely related TCNQ − stacks. Model parameters based on χ(T ) are smaller than those from optical data that in turn are considerably reduced by electronic polarization from quantum chemical calculation of U , V and t on adjacent TCNQ − ions. The χ(T ) analysis shows that fully relaxed states have reduced model parameters compared to optical or vibration spectra of dimerized or regular TCNQ − stacks.

Direct observations of the photoinduced change in dimerization in K-TCNQ

“…These behaviors imply the occurrence of the generalized Peierls transition and the simultaneous melting of dimerization. The finite value of β in the high-temperature phase suggests that dimerization persists even though x-ray measurement shows no such dimerization [10], which is consistent with previous experimental [19] and theoretical [24] works suggesting "fluctuations" in dimerization.…”

Low temperature transport of a charge transfer complex nanowire grown with an electric field from the vapour phase

“…[8] In such a system the CDW transport occurs due to Peierls transitions in TTF and TCNQ stacks below 50 K. [28] The NWs of TTF:TCNQ in which such a CDW transport was observed were grown from vapor phase between pre-grown contact pads, as has been done here. [30] The transport in the Cu:TCNQ NWs have a linear part G 0 and a bias dependent non-linear part which we write as: Since the conductivity of Cu:TCNQ was not measured below 100 K before, it may be the reason that such a phenomena was not observed or reported.…”

Quantum Phase Diagram of One-Dimensional Spin and Hubbard Models with Transitions to Bond Order Wave Phases

“…We recently proposed that Rb-TCNQ(II), the second polymorph of alkali-tetracyanoquinodimethane salt, is a realization of a BOW phase. 39 The anion radical stacks at 100 and 300 K are equally spaced, 40 not dimerized at low temperature. The magnetic gap is large, E m > 400 K, thereby suppressing excitations 40 below 200 K, and the optical gap is even larger.…”