Electron-vibron coupling effects on electron transport via a single-molecule magnet

Abstract

We investigate how the electron-vibron coupling influences electron transport via an anisotropic magnetic molecule, such as a single-molecule magnet (SMM) Fe 4 , by using a model Hamiltonian with parameter values obtained from density-functional theory (DFT). Magnetic anisotropy parameters, vibrational energies, and electron-vibron coupling strengths of the Fe 4 are computed using DFT. A giant spin model is applied to the Fe 4 with only two charge states, specifically a neutral state with the total spin S = 5 and a singly charged state with S = 9/2, which is consistent with our DFT result and experiments on Fe 4 single-molecule transistors. In sequential electron tunneling, we find that the magnetic anisotropy gives rise to new features in conductance peaks arising from vibrational excitations. In particular, the peak height shows a strong, unusual dependence on the direction as well as magnitude of applied B field. The magnetic anisotropy also introduces vibrational satellite peaks whose position and height are modified with the direction and magnitude of applied B field. Furthermore, when multiple vibrational modes with considerable electron-vibron coupling have energies close to one another, a low-bias current is suppressed, independently of gate voltage and applied B field, although that is not the case for a single mode with the similar electron-vibron coupling. In the former case, the conductance peaks reveal a stronger B-field dependence than in the latter case. The new features appear because the magnetic anisotropy barrier is of the same order of magnitude as the energies of vibrational modes with significant electron-vibron coupling. Our findings clearly show the interesting interplay between magnetic anisotropy and electron-vibron coupling in electron transport via the Fe 4 . The similar behavior can be observed in transport via other anisotropic magnetic molecules.

Vibron-assisted spin excitation in a magnetically anisotropic molecule

“…In particular, in magnetic molecules the interplay of vibrational modes, or vibrons, with the spin degrees of freedom is known to impact the spin lifetime [4][5][6]. Since vibrational modes can couple to the electric charge, producing vibron-assisted electron excitations in transport [7][8][9], expectations are that similar effects should be also observed with the electronic spin [10,11].Concerning this last point, experimental work has predominantly focused on a well-known spin-related manybody effect, the Kondo effect. The electron-vibron interaction in Kondo molecules was shown to produce satellite Kondo resonances in the differential conductance spectra at the bias of the vibron's excitation energy [12][13][14].…”

Magnetic field control of the Franck-Condon coupling of few-electron quantum states

“…A large number of extensions to this model has been developed to take into account specific details of transport spectra, see, e.g., Refs. [8,19,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42]; however, for now we focus our analysis on the simplest theoretical case, assuming a single harmonic oscillator mode and fast relaxation into the vibrational ground state. In this case, the current step heights or conductance peak amplitudes follow the Poisson formula [4,10], P n = (e −g g n )/n!, n = 0, 1, 2, .…”

Magnetic and transport properties of Fe4single-molecule magnets: a theoretical insight

“…As the calculations are restricted to molecular systems, they were performed on the isolated Fe 4 complexes using the experimental structures. [46][47][48] For the deposited systems, the molecular structure was taken from the structural optimization with the gold surface, but the zero-field splitting parameters were determined for the molecule alone, without the surface.…”

“…Magnetic anisotropy controlled the electric field as proposed by Zyazin et al 18,33,48 is based on the change of the oxidation state of the ICOCIN Fe 4 complex from the S=5 neutral state to a reduced S=11/2 state with a change in the D parameter from -0.06 to -0.09 meV (-0.48 to -0.27 cm -1 ). We performed DFT calculations using the Gaussian code with the B3LYP functional (see Computational details) to compare the energies of the optimized neutral, reduced and oxidized NIPJEC Fe 4 complex.…”