Nanoelectronics group of the University of Basel
The nanoelectronics group of the University of Basel does experiments with nanodevices to explore fundamental electrical properties in charge transport through electrically confined geometries.
The devices encompass a size regime from a few 100 of nanometers down to the molecular electronics scale of 1 nanometer. The devices are fabricated, on the one hand, along traditional lines employing state-of-the-art electron-beam lithography and conventional material systems (e.g. semiconductors and metals). On the other hand, alternative approaches, such as the bottom-up assembly, trapping of molecules and clusters in nanojunctions, and the use of nanowires and nanotubes grown by a self-organization process are used as well.
In the field of metallic and semiconducting nanostructures the focus has been on fluctuation phenomena (shot noise) and correlation spectroscopy of quantum-coherent systems. In molecular electronics, key results have been electrical measurements of single molecules trapped in break junctions. There is ample of experience in electrical studies of carbon nanotubes (CNTs), semiconducting nanowires (NWs) and graphene. CNTs and NWs are used as quantum wires and to define quantum dots with major results in the area of spintronics, and the superconducting proximity effect in reduced dimension and in the regime of strong interaction. Finally, we conduct applied research by exploiting CNTs and NWs as new materials for on-chip biochemical sensing.
List of selected publications
Quantum Shot Noise
Carlo Beenakker and C.S., Physics Today, 56, 37 (2003)Physics Today
Copyright (2003) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Electric field control of spin transport
S. Sahoo et al., Nature Physics 1, 99 (2005)Spintronics is an approach to electronics in which the spin of the electrons is exploited to control the electric resistance R of devices. One basic building block is the spin-valve, which is formed if two ferromagnetic electrodes are separated by a thin tunneling barrier. In such devices, R depends on the orientation of the magnetisation of the electrodes. It is usually larger in the antiparallel than in the parallel configuration. The relative difference of R, the so-called magneto-resistance (MR), is then positive. Common devices, such as the giant magneto-resistance sensor used in reading heads of hard disks, are based on this phenomenon. The MR may become anomalous (negative), if the transmission probability of electrons through the device is spin or energy dependent. This offers a route to the realisation of gate-tunable MR devices, because transmission probabilities can readily be tuned in many devices with an electrical gate signal. Such devices have, however, been elusive so far. We report here on a pronounced gate-field controlled MR in devices made from carbon nanotubes with ferromagnetic contacts. Both the amplitude and the sign of the MR are tunable with the gate voltage in a predictable manner. We emphasise that this spin-field effect is not restricted to carbon nanotubes but constitutes a generic effect which can in principle be exploited in all resonant tunneling devices.
Even-Odd Effect in Andreev Transport through a Carbon Nanotube Quantum Dot
A. Eichler et al., Phys. Rev. Lett. 99, 126602(2007)We have measured the current (I)-voltage (V) characteristics of a single-wall carbon nanotube quantum dot coupled to superconducting source and drain contacts in the intermediate coupling regime. Whereas the enhanced differential conductance dI/dV due to the Kondo resonance is observed in the normal state, this feature around zero-bias voltage is absent in the superconducting state. Nonetheless, a pronounced even-odd effect appears at finite bias in the dI=dV subgap structure caused by Andreev reflection. The first-order Andreev peak appearing around V = & Delta; /e is markedly enhanced in gate-voltage regions, in which the charge state of the quantum dot is odd. This enhancement is explained by a "hidden" Kondo resonance, pinned to one contact only. A comparison with a single-impurity Anderson model, which is solved numerically in a slave-boson mean-field approach, yields good agreement with the experiment.
Molecular Junctions based on Aromatic Coupling
Songmei Wu et al., Nature Nanotech. 3, 569 (2008)If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectiﬁcation and switching at the single- molecule level. However, the inﬂuence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligo-phenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic π-π coupling between adjacent molecules is efﬁcient enough to allow for the controlled formation of molecular bridges between nearby electrodes.
Cooper pair splitter realized in a two-quantum-dot Y-junction
Non-locality is a fundamental property of quantum mechanics that manifests itself as correlations between spatially separated parts of a quantum system. A fundamental route for the explora- tion of such phenomena is the generation of Einsteinâ€“Podolskyâ€“ Rosen (EPR) pairs of quantum-entangled objects for the test of so-called Bell inequalities. Whereas such experimental tests of non-locality have been successfully conducted with pairwise entangled photons, it has not yet been possible to realize an elec- tronic analogue of it in the solid state, where spin-1/2 mobile electrons are the natural quantum objects. The difficulty stems from the fact that electrons are immersed in a macroscopic ground stateâ€”the Fermi seaâ€”which prevents the straightforward genera- tion and splitting of entangled pairs of electrons on demand. A superconductor, however, could act as a source of EPR pairs of electrons, because its ground-state is composed of Cooper pairs in a spin-singlet state. These Cooper pairs can be extracted from a superconductor by tunnelling, but, to obtain an efficient EPR source of entangled electrons, the splitting of the Cooper pairs into separate electrons has to be enforced. This can be achieved by having the electrons â€˜repelâ€™ each other by Coulomb inter- action. Controlled Cooper pair splitting can thereby be realized by coupling of the superconductor to two normal metal drain contacts by means of individually tunable quantum dots. Here we demonstrate the first experimental realization of such a tunable Cooper pair splitter, which shows a surprisingly high effi- ciency. Our findings open a route towards a first test of the EPR paradox and Bell inequalities in the solid state.
Nernst Limit in Dual-Gated Si-Nanowire FET Sensors
O. Knopfmacher et al., Nano Lett. 10, 2268 (2010)Field effect transistors (FETs) are widely used for the label-free detection of analytes in chemical and biological experiments. Here we demonstrate that the apparent sensitivity of a dual-gated silicon nanowire FET to pH can go beyond the Nernst limit of 60 mV/pH at room temperature. This result can be explained by a simple capacitance model including all gates. The consistent and reproducible results build to a great extent on the hysteresis- and leakage-free operation. The dual-gate approach can be used to enhance small signals that are typical for bio- and chemical sensing at the nanoscale.
Ferromagnetic Proximity Effect in a Ferromagnet-Quantum-Dot-Superconductor Device
L. Hofstetter et al., Phys. Rev. Lett. 104, 246804 (2010)The ferromagnetic proximity effect is studied in InAs nanowire based quantum dots strongly coupled to a ferromagnetic (F) and a superconducting (S) lead. The influence of the F lead is detected through the splitting of the spin-1/2 Kondo resonance. We show that the F lead induces a local exchange field on the quantum dot, which has varying amplitude and sign depending on the charge states. The interplay of the F and S correlations generates an exchange field related subgap feature.
Graphene Transistors Are Insensitive to pH Changes in Solution
W. Fu et al., Nano Lett. 11, 3597 (2011)We observe very small gate-voltage shifts in the transfer characteristic of as-prepared graphene field-effect tran- sistors (GFETs) when the pH of the buffer is changed. This observation is in strong contrast to Si-based ion-sensitive FETs. The low gate-shift of a GFET can be further reduced if the graphene surface is covered with a hydrophobic fluorobenzene layer. If a thin Al-oxide layer is applied instead, the opposite happens. This suggests that clean graphene does not sense the chemical potential of protons. A GFET can therefore be used as a reference electrode in an aqueous electrolyte. Our finding sheds light on the large variety of pH-induced gate shifts that have been published for GFETs in the recent literature.
Spontaneously Gapped Ground State in Suspended Bilayer Graphene
F. Freitag et al., Phys. Rev. Lett. 108, 076602 (2012)Bilayer graphene bears an eightfold degeneracy due to spin, valley, and layer symmetry, allowing for a wealth of broken symmetry states induced by magnetic or electric fields, by strain, or even spontaneously by interaction. We study the electrical transport in clean current annealed suspended bilayer graphene. We find two kinds of devices. In bilayers of type B1 the eightfold zero-energy Landau level is partially lifted above a threshold field revealing an insulating ν = 0 quantum-Hall state at the charge neutrality point. In bilayers of type B2 the Landau level lifting is full and a gap appears in the differential conductance even at zero magnetic field, suggesting an insulating spontaneously broken symmetry state. Unlike B1, the minimum conductance in B2 is not exponentially suppressed, but remains finite with a value G ≤ e2/h even in a large magnetic field. We suggest that this phase of B2 is insulating in the bulk and bound by compressible edge states.
Quantum Hall Effect in Graphene with Superconducting Electrodes
P. Rickhaus et al., Nano Letters 12, 1942 (2012)We have realized an integer quantum Hall system with superconducting contacts by connecting graphene to niobium electrodes. Below their upper critical field of 4 T, an integer quantum Hall effect coexists with superconductivity in the leads but with a plateau conductance that is larger than in the normal state. We ascribe this enhanced quantum Hall plateau conductance to Andreev processes at the grapheneâ€“superconductor interface leading to the formation of so-called Andreev edge-states. The enhancement depends strongly on the filling-factor and is less pronounced on the first plateau due to the special nature of the zero energy Landau level in monolayer graphene.
Near-Unity Cooper Pair Splitting Efficiency
J. Schindele et al., Phys. Rev. Lett. 109, 157002 (2012)The two electrons of a Cooper pair in a conventional superconductor form a spin singlet and therefore a maximally entangled state. Recently, it was demonstrated that the two particles can be extracted from the superconductor into two spatially separated contacts via two quantum dots in a process called Cooper pair splitting (CPS). Competing transport processes, however, limit the efficiency of this process. Here we demonstrate efficiencies up to 90%, significantly larger than required to demonstrate interaction-dominated CPS, and on the right order to test Bellâ€™s inequality with electrons. We compare the CPS currents through both quantum dots, for which large apparent discrepancies are possible. The latter we explain intuitively and in a semiclassical master equation model. Large efficiencies are required to detect electron entanglement and for prospective electronics-based quantum information technologies.