Bakir Babic

Multiple Andreev reflections in a carbon nanotube quantum dot

We report resonant multiple Andreev reflections in a multiwall carbon nanotube quantum dot coupled to superconducting leads. The position and magnitude of the subharmonic gap structure is found to depend strongly on the level positions of the single-electron states which are adjusted with a gate electrode. We discuss a theoretical model of the device and compare the calculated differential conductance with the experimental data.

Intrinsic Thermal Vibrations of Suspended Doubly Clamped Single-Wall Carbon Nanotubes

We report the observation of thermally driven mechanical vibrations of suspended doubly clamped carbon nanotubes, grown by chemical vapor deposition (CVD). Several experimental procedures are used to suspend carbon nanotubes. The vibration is observed as a blurring in images taken with a scanning electron microscope. The measured vibration amplitudes are compared with a model based on linear continuum mechanics.

Observation of Fano resonances in single-wall carbon nanotubes

We have explored the low-temperature linear and nonlinear electrical conductance G of metallic carbon nanotubes (CNTs), which were grown by the chemical-vapor deposition method. The high transparency of the contacts allows to study these two-terminal devices in the high conductance regime. We observe the expected four-fold shell pattern together with Kondo physics at intermediate transparency Gless than or similar to2e(2)/h and a transition to the open regime in which the maximum conductance is doubled and bound by G(max)=4e(2)/h. In the high-G regime, at the transition from a quantum dot to a weak link, the CNT levels are strongly broadened. Nonetheless, sharp resonances appear superimposed on the background which varies slowly with gate voltage. The resonances are identified by their lineshape as Fano resonances. The origin of Fano resonances is discussed along the modeling.

Ambipolar field-effect transistor on as-grown single-wall carbon nanotubes

We use a simultaneous flow of ethylene and hydrogen gases to grow single-wall carbon nanotubes by chemical vapour deposition. Strong coupling to the gate is inferred from transport measurements for both metallic and semiconducting tubes. At low temperatures, our samples act as single-electron transistors where the transport mechanism is mainly governed by Coulomb blockade. The measurements reveal very rich quantized energy level spectra spanning from the valence to the conduction band. The Coulomb diamonds have similar addition energies on both sides of the semiconducting gap. Signatures of the subband population have been observed at intermediate temperature.

Kondo effect in carbon nanotubes at half filling

In a single state of a quantum dot the Kondo effect arises due to the spin-degeneracy, which is present if the dot is occupied with one electron (N=1). The eigenstates of a carbon nanotube quantum dot possess an additional orbital degeneracy leading to a fourfold shell pattern. This additional degeneracy increases the possibility for the Kondo effect to appear. We revisit the Kondo problem in metallic carbon nanotubes by linear and nonlinear transport measurement in this regime, in which the fourfold pattern is present. We have analyzed the ground state of CNTs, which were grown by chemical vapor deposition, at filling N=1, N=2, and N=3. Of particular interest is the half-filled shell, i.e., N=2. In this case, the ground state is either a paired electron state or a state for which the singlet and triplet states are effectively degenerate, allowing in the latter case for the appearance of the Kondo effect. We deduce numbers for the effective missmatch delta of the levels from perfect degeneracy and the exchange energy J. While deltasimilar to0.1-0.2 (in units of level spacing) is in agreement with previous work, the exchange term is found to be surprisingly small: Jless than or similar to0.02. In addition we report on the observation of gaps, which in one case is seen at N=3 and in another is present over an extended sequence of levels.

Magnetophoresis of flexible DNA-based dumbbell structures

Controlled movement and manipulation of magnetic micro- and nanostructures using magnetic forces can give rise to important applications in biomedecine, diagnostics, and immunology. We report controlled magnetophoresis and stretching, in aqueous solution, of a DNA-based dumbbell structure containing magnetic and diamagnetic microspheres. The velocity and stretching of the dumbbell were experimentally measured and correlated with a theoretical model based on the forces acting on individual magnetic beads or the entire dumbbell structures. The results show that precise and predictable manipulation of dumbbell structures is achievable and can potentially be applied to immunomagnetic cell separators.

Induced movement of the magnetic beads and DNA-based Dumbbell in a micro fluidic channel

We have explored controlled movement of magnetic beads and a dumbbell structure composed of DNA, a magnetic and a non-magnetic bead in a micro fluidic channel. Movement of the beads and dumbbells is simulated assuming that a net force is described as a superposition between the magnetic and hydrodynamic drag forces. Trajectories of beads and dumbbells are observed with optical light microscopy. The experimentally measured data show a good agreement with the simulations. This dynamical approach offers the prospect to stretch the DNA within the dumbbell and investigate its conformational changes. Further on, we demonstrate that short sonication can reduce multiple attachments of DNA to the beads.

Suitability of carbon nanotubes grown by chemical vapor deposition for electrical devices

Using carbon nanotubes (CNTs) produced by chemical vapor deposition, we have explored different strategies for the preparation of carbon nanotube devices suited for electrical and mechanical measurements. Though the target device is a single small diameter CNT, there is compelling evidence for bundling, both for CNTs grown over structured slits and on rigid supports. Whereas the bundling is substantial in the former case, individual single‐wall CNTs (SWNTs) can be found in the latter. Our evidence stems from mechanical and electrical measurements on contacted tubes. Furthermore, we report on the fabrication of low‐ohmic contacts to SWNTs. We compare Au, Ti and Pd contacts and find that Pd yields the best results.