Ground-State Spin Blockade in a Single-Molecule Junction

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Ground-State Spin Blockade in a Single-Molecule Junction

J. de Bruijckere, P. Gehring, M. Palacios-Corella, M. Clemente-León, E. Coronado, J. Paaske, P. Hedegård, and H. S. J. van der Zant

Phys. Rev. Lett. 122, 197701 – Published 14 May 2019

Abstract

It is known that the quantum mechanical ground state of a nanoscale junction has a significant impact on its electrical transport properties. This becomes particularly important in transistors consisting of a single molecule. Because of strong electron-electron interactions and the possibility of accessing ground states with high spins, these systems are eligible hosts of a current-blockade phenomenon called a ground-state spin blockade. This effect arises from the inability of a charge carrier to account for the spin difference required to enter the junction, as that process would violate the spin selection rules. Here, we present a direct experimental demonstration of a ground-state spin blockade in a high-spin single-molecule transistor. The measured transport characteristics of this device exhibit a complete suppression of resonant transport due to a ground-state spin difference of $3 / 2$ between subsequent charge states. Strikingly, the blockade can be reversibly lifted by driving the system through a magnetic ground-state transition in one charge state, using the tunability offered by both magnetic and electric fields.

Received 4 December 2018
Revised 21 February 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.197701

Physics Subject Headings (PhySH)

Coulomb blockade Exchange interaction Kondo effect Quantum transport Spin blockade

Molecular junctions

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

J. de Bruijckere¹, P. Gehring¹, M. Palacios-Corella², M. Clemente-León², E. Coronado², J. Paaske^3,4, P. Hedegård³, and H. S. J. van der Zant^1,*

¹Kavli Institute of Nanoscience, Delft University of Technology, 2628 CJ Delft, The Netherlands
²Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Catedrático José Beltrán 2, Paterna, 46980, Spain
³Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark
⁴Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark

^*H.S.J.vanderZant@tudelft.nl

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Issue

Vol. 122, Iss. 19 — 17 May 2019

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Images

Figure 1
(a) Sketch of the molecule, gold electrodes, and measurement circuit. The molecule consists of a Mn(III) center (purple) surrounded by six ${MoO}_{6}$ octahedra, which connect on both sides to pyridine-based ligands. Electric current ( $I$ ) through the molecule is recorded as a function of bias voltage ( $V$ ) and gate voltage ( $V_{g}$ ). (b) (Top) $d I / d V$ maps at zero magnetic field (left) and at 8 T (right). At 0 T, no SET lines corresponding to transitions between the ground states of $N$ and $N - 1$ are present, as a result of ground-state spin blockade. At 8 T, these lines do appear and the blockade is lifted. (Bottom) $d I / d V$ traces at zero bias, showing a charge degeneracy peak at 8 T and its complete suppression at 0 T.
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Figure 2
$d I / d V$ maps at 0 T in a wide bias- and gate-voltage range. (a) $d I / d V$ map showing the same features as in Fig. 1, along with a ground-state transition line at $V = - 3 mV$ and additional SET excitation lines with different slopes. The SET lines at negative bias change slope at the ground-state transition. (b) $d I / d V$ map in a larger gate window, where the region marked by dashed brackets is the region shown in (a). The dark parabolic feature indicated by the arrow is the ground state transition line in (a). Three different regions with different ground states are separated by the discontinuity and the parabola, labeled (i), (ii), and (iii). The excitation energy of the COT excitations in (i) are tunable by the gate voltage.
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Figure 3
(a) $d I / d V$ maps of the same region as Fig. 2, at different magnetic fields. The parabolic line separating the two spin ground states in $N - 1$ moves upwards by increasing the magnetic field as the high-spin state becomes energetically more favorable. (b) Magnetic field dependence of the $d I / d V$ spectrum at $V_{g} = - 1.0 V$ . One line originating from the excited multiplet crosses zero bias at $B = 5 T$ and becomes the new ground state above this field. The two ground states are separated by the slanted line indicated by the arrow. (c) Simulated $d I / d V$ spectra for a system of a spin- $1 / 2$ , tunnel coupled to two reservoirs, and exchange coupled to a spin- $3 / 2$ , with a $V$ -dependent exchange coupling.
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Figure 4
Diagram showing the four states of different charge and spin, where the red and green arrows correspond to the ligand spin and the spins in the Mn center, respectively. The observed SET and COT transitions are represented by solid and dashed arrows, respectively. The red cross indicates the blocked SET transition. For charge state $N - 1$ , $S = 1$ (i) and $S = 2$ (ii), and for charge state $N$ , $S = 5 / 2$ (iii) and $S = 3 / 2$ (iv).
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