The Kondo Effect in Mechanically Controllable Break Junctions
We have fabricated C60-based single-molecule devices with the ability to mechanically adjust the spacing between the source and drain electrodes. We use this device geometry to study the Kondo effect, a many-body phenomenon that can arise from the coupling between a localized spin and a sea of conduction electrons. By varying the electrode spacing, we are able to change both the width and height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. We are also able to tune finite-bias Kondo features which appear at the energy of the first C60 intracage vibrational mode.
Joshua Parks, Alexandre Champagne, Geoff Hutchison, Samuel Flores-Torres, Hector Abruña, and Dan Ralph
Our device fabrication begins with a growth of 200 nm SiO2 on 200 µm-thick degenerately doped silicon wafers, which are sufficiently flexible to allow for some mechanical bending. We then strip the oxide within a 75 µm × 75 µm square window and grow a thinner 40 nm thick oxide there. Using a series of photolithography and electron beam lithography steps, we define 32 nm thick Au lines with a 50 nm constriction centered in the window, connected to thicker Au bonding pads lying on the thicker oxide. Using a combination of dry and wet etches, we then strip the 40 nm oxide from under the Au wires to leave Au bridges suspended 40 nm above the silicon substrate.
To incorporate molecules in our devices, we deposit 25 µL of a 100 µM solution of C60 in toluene onto a chip with unbroken wires, wait 1 minute and blow dry. We then cool the chip to 1.6 K. We use electromigration  to create a molecular-scale break in the gold wires before beginning studies as a function of mechanical motion. After electromigration, we find that one or a few molecules can sometimes be found bridging the gap between the electrodes, as inferred from the existence of a Coulomb blockade characteristic in the I-V curve. We choose to study C60 molecules because they are sufficiently durable to survive high temperatures present during electromigration and because previous work on single-molecule C60 devices has observed the Kondo effect [2,3].
Measurements on individual C60 molecules in our device geometry show signatures of the Kondo effect in a quantum dot, namely a zero-bias peak in dI/dV which is suppressed as a function of increasing temperature. By varying the electrode spacing, we are able to tune both the Kondo temperature and the magnitude of the zero-bias conductance signal associated with the Kondo resonance. These changes allow a determination of how the motion modifies the relative coupling of the molecule to the two electrodes. The normalized linear conductance exhibits scaling behavior as a function of temperature, as predicted by theory.
In addition to a zero-bias peak in dI/dV, we have also observed peaks in dI/dV at symmetric values of V near ±33 mV. The energy of 33 meV is known to correspond to the lowest intracage vibrational mode of isolated C60 in which the molecule oscillates between a sphere and a prolate ellipsoid shape . As the electrodes are pulled apart, the positions of the inelastic features increase in |V|, suggesting that the mechanical motion increases the energy of the active vibrational mode.
Figure 1: Scanning electron micrograph of a Au bridge suspended 40 nm above a Si substrate.
Figure 2: Differential conductance traces for a C60 device at various temperatures. Inset: A fit to theory yields a Kondo temperature of 28.2 ± 0.3 K.
Figure 3: Differential conductance traces for the same device at several electrode spacings. Pulling apart the electrodes modifies the height and width of the Kondo resonance. Inset: Normalized conductance data as a function of temperature at several electrode spacings collapse onto a single scaling curve.
Figure 4: d2I/dV2 as a function of bias voltage and electrode spacing for a C60 device exhibiting finite-bias features.
- H. Park et al., Fabrication of metallic electrodes with nanometer separation by electromigration, Appl. Phys. Lett. 75, 301 (1999).
- L. H. Yu and D. Natelson, Kondo physics in C60 Single-Molecule Transistors, Nano Lett. 4, 79-83 (2004).
- A. N. Pasupathy et al., The Kondo effect in the presence of ferromagnetism, Science 306, 86 (2004).
- R. Heid, L. Pintschovius, and J. M. Godard, Eigenvectors of internal vibrations of C60: Theory and experiment, Phys. Rev. B 56, 5925 (1997).
Last updated: 11-July-2007