The Kondo effect in C60 single-molecule transistors (original) (raw)
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Kondo effects in a C60 single-molecule transistor
physica status solidi (b), 2008
We have used the electromigration technique to fabricate a C60 single-molecule transistor (SMT). We present a full experimental study as a function of temperature, down to 35 mK, and as a function of magnetic field up to 8 T in a SMT with odd number of electrons, where the usual spin-1/2 Kondo effect occurs, with good agreement with theory. In the case of even number of electrons, a low temperature magneto-transport study is provided, which demonstrates a Zeeman splitting of the zero-bias anomaly at energies well below the Kondo scale.
Out-of-Equilibrium Singlet-Triplet Kondo Effect in a Single C60 Quantum Dot
Journal of Low Temperature Physics, 2008
We have used an electromigration technique to fabricate a C60 single-molecule transistor (SMT). Besides describing our electromigration procedure, we focus and present an experimental study of a single molecule quantum dot containing an even number of electrons, revealing, for two different samples, a clear out-of-equilibrium Kondo effect. Low temperature magneto-transport studies are provided, which demonstrates a Zeeman splitting of the finite bias anomaly.
2005
We report Kondo resonances in the conduction of single-molecule transistors based on transition metal coordination complexes. We find Kondo temperatures in excess of 50 K, comparable to those in purely metallic systems. The observed gate dependence of the Kondo temperature is inconsistent with observations in semiconductor quantum dots and a simple single-dot-level model. We discuss possible explanations of this effect, in light of electronic structure calculations. 73.23.Hk,85.65.+h In the Kondo Hamiltonian[1], one of the most wellstudied many-body problems in physics, an unpaired spin localized in a singly occupied electronic level is coupled via tunneling to an electronic bath. On-site Coulomb repulsion forbids real double occupancy of the level, but virtual processes favor antiferromagnetic exchange between the local spin and the electronic bath. As T is reduced below a characteristic Kondo temperature, T K , these exchange processes "screen" the local moment. The Kondo problem has undergone a resurgence, with atomic-scale studies of Kondo physics by scanning tunneling microscopy (STM)[2, 3] and the realization of tunable Kondo systems in semiconductor quantum dots . With the recent development of singlemolecule transistors (SMTs) based on individual small molecules[8], Kondo systems now include organometallic compounds[9, 10] and fullerenes with normal[11] and ferromagnetic[12] leads.
Physical principles of the single-C60transistor effect
Physical Review B
Starting with the physics of tunneling transport through a molecule, we describe the principles underlying electrical amplification effects of a C 60 molecule. We discuss in detail the consequences of intramolecular electronic-level repulsion, an effect induced by compression of the molecule, which leads to an exponential variation of the current for a minute compression of the molecule. This detailed understanding underpins the C 60 amplifier. Using a planar configuration and an independent electromechanical grid, a transistor effect results from this repulsion effect.
Nanomechanical oscillations in a single-C60 transistor
Nature, 2000
Over the last decade, electron transport through quantum dots has attracted considerable attention from the scientific and engineering community. The electronic motion through these structures is strongly modified by single-electron charging and energy level quantization 1,2 . Recently, much effort has been directed toward extending these studies to chemical nanostructures, such as molecules 3-8 , nanocrystals 9-13 , and nanotubes 14-17 . Here we report for the first time the fabrication of single-molecule transistors based on individual C 60 molecules. Transport measurements of single-C 60 transistors provide evidence for coupling between the center-of-mass motion of C 60 and single-electron hopping 18 , a novel conduction mechanism that has not been observed in previous quantum-dot studies. This coupling manifests itself as quantized nano-mechanical oscillations of C 60 against the gold surface. The frequency of this oscillation is determined to be around 1.2 THz, in excellent agreement with a simple theoretical estimate based on van der Waals and electrostatic interactions between C 60 and gold electrodes.
Transport in single-molecule transistors: Kondo physics and negative differential resistance
2004
We report two examples of transport phenomena based on sharp features in the effective density of states of molecular-scale transistors: Kondo physics in C 60based devices, and gate-modulated negative differential resistance (NDR) in "control" devices that we ascribe to adsorbed contamination. We discuss the need for a statistical approach to device characterization, and the criteria that must be satisfied to infer that transport is based on single molecules. We describe apparent Kondo physics in C 60based single-molecule transistors (SMTs), including signatures of molecular vibrations in the Kondo regime. Finally, we report gate-modulated NDR in devices made without intentional molecular components, and discuss possible origins of this property.
A Detailed Experimental and Theoretical Study into the Properties of C 60 Dumbbell Junctions
Small, 2013
full papers electrical coupling and conductance variability, the most commonly explored of which have been monodentate groups. These include thiol, [ 1-5 ] amino [ 4 , 6,7 ] and pyridyl groups, [ 1 , 7-9 ] with nitro [ 10 ] and cyano [ 11 ] groups also scrutinised. However, using the break-junction and related techniques [ 1 , 12,13 ] two main problems exist. First, it remains diffi cult to wire precisely one molecule on-demand. This is in part due to the lack of imaging possibilities and the large number of molecules close to the junction, required to obtain high quality statistics. This means that the most probable conductance value obtained from such measurements may sometimes correspond to more than one wired molecule. [ 4 , 14 ] Equally problematically, it remains diffi cult to control precisely how the anchor group binds to the electrodes and, hence, how the molecule(s) sits in the junction. These two effects lead to signifi cant conductance variation from one junction to the next, and no obvious way exists to differentiate one-molecule from multi-molecule related effects. [ 15-17 ] One solution may lie in the use of molecules with largearea contacts such as fullerenes. In combination with a Scanning Tunneling Microscope (STM), which has the power to image single molecules before they are lifted, [ 18-23 ] such a strategy is highly appealing as larger molecules are easy to image and pinpoint precisely, even under ambient conditions. In this paper, we have examined the wiring of a single C 60 dumbbell molecule, which contains two terminal C 60 groups connected through a fl uorene backbone. The presence of the C 60 groups allows us, through the formation of a highly dilute layer, to target isolated single molecules specifi cally, which are easily determined by their double-lobe appearance. We have previously studied the 'jump into contact' of a different isolated dumbbell molecule between a gold surface and an STM tip. [ 21 ] C 60 , though, remains relatively untested experimentally as an anchoring group, despite the theoretical effort, [ 24,25 ] with only a few examples found in the literature. The fi rst experiments to try and wire a dumbbell molecule were carried out by Martin et al. using a breakjunction setup. [ 26 ] In such break junctions, the presence of C 60 has also been detected by IETS. [ 27 ] Single pristine C 60 molecules sandwiched between metallic contacts have been fairly extensively studied, primarily under UHV-STM conditions, on gold [ 28 ] and copper substrates. [ 29-32 ] Pairs of C 60 molecules have also been pressed together, forming in situ dimers. [ 33,34 ] In the present study, in order to probe the conductance of the contact itself in situ, and also to avoid waiting for the molecule to jump, we have modifi ed our previous "stationary tip" technique so that we now approach one of the C 60 groups with the STM tip fi rst to form a tip-C 60 contact. Once the contact is formed we can withdraw the tip and check to see if a current path exists between the two electrodes via the molecule. We succeeded in lifting a single molecule in this way when one of the C 60 groups became strongly tethered to the tip, which lifted it totally from the surface, leaving it dangling from the tip. With the second C 60 free to bind to the surface, we were then able to form and break the junction many times over a long period. From the analysis of both approaching and retracting traces using 2D histogram representations, two dominant conductance states spaced by almost two orders of magnitude were apparent. We have carried out
Kondo Effect in Single Atom Contacts: The Importance of the Atomic Geometry
Physical Review Letters, 2008
Co single atom junctions on copper surfaces are studied by scanning tunneling microscopy and ab initio calculations. The Kondo temperature of single cobalt atoms on the Cu(111) surface has been measured at various tip-sample distances ranging from tunneling to the point contact regime. The experiments show a constant Kondo temperature for a whole range of tip-substrate distances consistently with the predicted energy position of the spin-polarized d levels of Co. This is in striking difference to experiments on Co=Cuð100Þ junctions, where a substantial increase of the Kondo temperature has been found. Our calculations reveal that the different behavior of the Co adatoms on the two Cu surfaces originates from the interplay between the structural relaxations and the electronic properties in the near-contact regime.