Transport mirages in single-molecule devices

Abstract

Molecular systems can exhibit a complex, chemically tailorable inner structure which allows for targeting of specific mechanical, electronic and optical properties. At the single-molecule level, two major complementary ways to explore these properties are molecular quantum-dot structures and scanning probes. This article outlines comprehensive principles of electron-transport spectroscopy relevant to both these approaches and presents a new, high-resolution experiment on a high-spin single-molecule junction exemplifying these principles. Such spectroscopy plays a key role in further advancing our understanding of molecular and atomic systems, in particular the relaxation of their spin. In this joint experimental and theoretical analysis, particular focus is put on the crossover between resonant regime [single-electron tunneling (SET)] and the off-resonant regime [inelastic electron (co)tunneling (IETS)]. We show that the interplay of these two processes leads to unexpected mirages of resonances not captured by either of the two pictures alone. Although this turns out to be important in a large fraction of the possible regimes of level positions and bias voltages, it has been given little attention in molecular transport studies. Combined with nonequilibrium IETSfour-electron pump-probe excitations -these mirages provide crucial information on the relaxation of spin excitations. Our encompassing physical picture is supported by a master-equation approach that goes beyond weak coupling. The present work encourages the development of a broader connection between the fields of molecular quantum-dot and scanning probe spectroscopy.

Five approaches to exact open-system dynamics: Complete positivity, divisibility, and time-dependent observables

“…Notably, this model has been long known to be exactly solvable 44 and features in textbooks on quantum transport and electron transfer 45 . However, we will see that this does not 46,47 imply this problem is fully understood, even though we do not address the role of Coulomb interactions responsible for various manybody effects in transport measurements 43 . These aspects are certainly important but beyond the present scope.…”

“…V. This approach allows for a simultaneous treatment of strong coupling and (the here neglected) strong interaction effects most relevant for molecular quantum dots, see Ref. [43] and the references therein.…”

“…Note that the separation of eigenvectors into subsets k = 1, 4 and k = 2, 3 does not correspond to subsets of Kraus operators in Eq. (43). The operators m1(t) and m TCL 1 (t) are trace-normalized and positive if and only if |g(t)| ≤ 1 and |h(t)| ≤ 1, respectively.…”

“…The function g(t) directly determines the parity evolution (21b), the exponent of the dynamical map (41b) and the Kraus operators (43). By convoluting h(t) with a nonnegative decaying envelope, the oscillations from the correlation function γ(s) are further smoothed out and the sign is as before sgn g(t) = sgn h(t) = sgn ǫ for t ≥ 0.…”

Preface: Special Topic on Frontiers in Molecular Scale Electronics

“…These include aspects of electronic correlations, 6 magnetism, and spin-related effects. [7][8][9] Molecular vibrations, junction mechanics, and associated thermal effects have been analyzed experimentally and theoretically [10][11][12][13][14] and time resolved studies are also under way. [15][16][17] With such a wealth of phenomena, many of which are very specific to molecular junctions, it is not surprising that the field has motivated a significant amount of theoretical a) Electronic addresses: ferdinand.evers@physik.uni-regensburg.de and lv2117@columbia.edu work.…”