arXiv subjects

Benjamin J. Topham

Publications and source records attributed to Benjamin J. Topham.

3 recordsLinked to original sources

arXiv2011

Surface dipole of F4TCNQ films: Collective charge transfer and dipole-dipole repulsion in submonolayers

A charge transfer (CT) model is introduced for strong acceptors like A = F4TCNQ that are ionized on surfaces at low coverage θ. Each A forms a CT dimer with a surface state S. Dipole-dipole repulsion grows as θ^(3/2) up to θ = 1 in a full monolayer. Electron transfer ρ(θ) within dimers is found self-consistently and decreases with increasing coverage. The surface dipole and work function shift ΔΦ are proportional to θρ(θ). The CT model has implications for photoemission and accounts for ΔΦ(d) profiles of F4TCNQ films of thickness d on Cu(1,1,1) or on hydrogenated diamond (1,0,0). The CT model also describes ΔΦ(θ) of organic donors on metals and is contrasted to previous treatments of dipoles on a surface.

cond-mat.mtrl-sci↗

arXiv2011

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

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 $V = V_c(U)$ for nearest-neighbor interaction $V$ and on-site repulsion $U$. At high $T$, all three salts have regular stacks of $\rm TCNQ^-$ anion radicals. The $χ(T)$ fits place Na and K in the CDW phase and Rb(II) in the BOW phase with $V \approx V_c$. 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 $\rm TCNQ^-$ 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 $\rm TCNQ^-$ ions. The $χ(T)$ analysis shows that fully relaxed states have reduced model parameters compared to optical or vibration spectra of dimerized or regular $\rm TCNQ^-$ stacks.

cond-mat.mtrl-sci↗

arXiv2010

Ionization potentials of crystalline organic thin films: Position dependence due to molecular shape and charge redistribution

In addition to electronic polarization or charge redistribution, the shape of neutral conjugated molecules yields position-dependent ionization potentials and electron affinities in organic thin films. Self-consistent I(n) and A(n) are computed in each layer n of 10-layer films of prototypical organics on a metal. The depth dependence of I(n) is discussed at surfaces of anthracene, C60 and PTCDA. The shape contribution can be substantial, up to 0.5 eV, and comes primarily from charge-quadrupole interactions.

cond-mat.mtrl-sci↗