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Register a new userEfficient heating of single-molecule junctions for thermoelectric studies at cryogenic temperatures
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Added by
Pascal Gehring P.G.
Publication date
2019
fields
Physics
and research's language is
English
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The energy dependent thermoelectric response of a single molecule contains valuable information about its transmission function and its excited states. However, measuring it requires devices that can efficiently heat up one side of the molecule while being able to tune its electrochemical potential over a wide energy range. Furthermore, to increase junction stability devices need to operate at cryogenic temperatures. In this work we report on a new device architecture to study the thermoelectric properties and the conductance of single molecules simultaneously over a wide energy range. We employ a sample heater in direct contact with the metallic electrodes contacting the single molecule which allows us to apply temperature biases up to $Delta T = 60$K with minimal heating of the molecular junction. This makes these devices compatible with base temperatures $T_mathrm{bath} <2$K and enables studies in the linear ($Delta T ll T_mathrm{molecule}$) and non-linear ($Delta T gg T_mathrm{molecule}$) thermoelectric transport regimes.
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We report results of theoretical studies of thermoelectric efficiency of single-molecule junctions with long molecular linkers. The linker is simulated by a chain of identical sites described using a tight-binding model. It is shown that thermoelectric figure of merit ZT strongly depends on the bridge length, being controlled by the lineshape of electron transmission function within the tunnel energy range corresponding to HOMO/LUMO transport channel. Using the adopted model we demonstrate that ZT may significantly increase as the linker lengthens, and that gateway states on the bridge (if any) may noticeably affect the length-dependent ZT. Temperature dependences of ZT for various bridge lengths are analyzed. It is shown that broad minima emerge in ZT versus temperature curves whose positions are controlled by the bridge lengths.
We study theoretically spin transport through a single-molecule magnet (SMM) in the sequential and cotunneling regimes, where the SMM is weakly coupled to one ferromagnetic and one normalmetallic leads. By a master-equation approach, it is found that the spin polarization injected from the ferromagnetic lead is amplified and highly polarized spin-current can be generated, due to the exchange coupling between the transport electron and the anisotropic spin of the SMM. Moreover, the spin-current polarization can be tuned by the gate or bias voltage, and thus an efficient spin injection device based on the SMM is proposed in molecular spintronics.
In the present work, we theoretically analyze the steady-state thermoelectric transport through a single-molecule junction with a vibrating bridge. Thermally induced charge current in the system is explored using a nonequilibrium Greens functions formalism. We study combined effects of Coulomb interactions between charge carriers on the bridge and electron-phonon interactions on the thermocurrent beyond the linear response regime. It is shown that electron-vibron interactions may significantly affect both magnitude and direction of the thermocurrent, and vibrational signatures may appear.
Comment on the paper: Magnetization Process of Single Molecule Magnets at Low Temperatures of J.F.Fernandez and J.J.Alonso (PRL 91, 047202 (2003)).
Configuration transitions of individual molecules and atoms on surfaces are traditionally described with energy barriers and attempt rates using an Arrhenius law. This approach yields consistent energy barrier values, but also attempt rates orders of magnitude below expected oscillation frequencies of particles in the meta-stable state. Moreover, even for identical systems, the measurements can yield values differing from each other by orders of magnitude. Using low temperature scanning tunnelling microscopy (STM) to measure an individual dibutyl-sulfide molecule (DBS) on Au(111), we show that we can avoid these apparent inconsistencies if we account for the relative position of tip apex and molecule with accuracy of a fraction of the molecule size. Altering the tip position on that scale modifies the transitions barrier and attempt rate in a highly correlated fashion, which on account of the relation between the latter and entropy results in a single-molecular enthalpy-entropy compensation. By appropriately positioning the tip apex the STM tip can be used to select the operating point on the compensation line and modify the transition rates. The results highlight the need to consider entropy in transition rates of a single molecule, even at temperatures where entropy effects are usually neglected.
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