Marcelo Goffman

Gigahertz frequency flexible carbon nanotube transistors

We investigate the high frequency performances of flexible field-effect transistors based on carbon nanotubes. A large density of mostly aligned carbon nanotubes deposited on a flexible substrate by dielectrophoresis serves as the channel. The transistors display a constant transconductance up to at least 6GHz and a current gain cutoff frequency (fT) as high as 1GHz at VDS=−700mV. Bending tests show that the devices can withstand a high degree of flexion characterized by a constant transconductance for radius of curvature as small as 3.3mm.

Intrinsic current gain cutoff frequency of 30GHz with carbon nanotube transistors

High frequency capabilities of carbon nanotube field-effect transistors (CNTFETs) are investigated. Structures with a large number of single-walled carbon nanotubes were fabricated using dielecrophoresis to increase the density of nanotubes in the device channel. The authors obtained an intrinsic current gain cutoff frequency of 30GHz establishing state-of-the-art high frequency (hf) potentialities of CNTFETs. The device also showed a maximum stable gain above 10dB at 20GHz. Finally, the parameters of an equivalent circuit model of multitube CNTFET at 20GHz are determined, which open the route to the modeling of nanotubes-based hf electronics.

Coherent manipulation of Andreev states in superconducting atomic contacts

Making and manipulating a weak-link qubit In superconductors, single particles cannot have energies smaller than the superconducting gap. Yet when two superconductors are separated by a thin nonsuperconducting bridge (the “weak link”), quasi-particles can occupy states that are inside the gap, the so-called Andreev bound states (ABSs). Janvier et al. fabricated such a structure out of superconducting aluminum and manipulated the occupation of a pair of ABSs. They observed oscillations in population between two of the energy levels, forming a type of qubit, which they dubbed the Andreev qubit. The results may lead to applications in quantum information processing. Science, this issue p. 1199 States of a superconducting weak link are manipulated in a circuit quantum electrodynamics setup. Coherent control of quantum states has been demonstrated in a variety of superconducting devices. In all of these devices, the variables that are manipulated are collective electromagnetic degrees of freedom: charge, superconducting phase, or flux. Here we demonstrate the coherent manipulation of a quantum system based on Andreev bound states, which are microscopic quasi-particle states inherent to superconducting weak links. Using a circuit quantum electrodynamics setup, we performed single-shot readout of this Andreev qubit. We determined its excited-state lifetime and coherence time to be in the microsecond range. Quantum jumps and parity switchings were observed in continuous measurements. In addition to having possible quantum information applications, such Andreev qubits are a test-bed for the physics of single elementary excitations in superconductors.

Self-assembled molecular monolayers as ultrathin gate dielectric in carbon nanotube transistors

We demonstrate the use of a self-assembled monolayer of octadecanethiol on gold as thin gate dielectric for a single-walled carbon nanotube field-effect transistor. P-type transistors display very steep subthreshold slopes, greatly reduced hysteresis, and band-to-band tunneling. The suppression of the gate efficiency for n-type transistors emphasizes the key role of the electrical dipole of the molecular layer in controlling the switching. Combining the versatility of organic dielectrics with the exceptional electronic and mechanical properties of carbon nanotubes opens interesting ways toward the realization of fully organic nanoscale transistors.

SWNT array resonant gate MOS transistor

We show that thin horizontal arrays of single wall carbon nanotubes (SWNTs) suspended above the channel of silicon MOSFETs can be used as vibrating gate electrodes. This new class of nano-electromechanical system (NEMS) combines the unique mechanical and electronic properties of SWNTs with an integrated silicon-based motion detection. Its electrical response exhibits a clear signature of the mechanical resonance of SWNT arrays (120-150 MHz) showing that these thin horizontal arrays behave as a cohesive, rigid and elastic body membrane with a Youngs modulus in the order of 1-10 GPa and ultra-low mass. The resonant frequency can be tuned by the gate voltage and its dependence is well understood within the continuum mechanics framework.

Synthesis of Palladium Conductive DNA‐based Nanowires

We present here a simple method to metallize DNA by Electroless Plating of palladium, a trusty metal for contacting SWNT devices. Indeed, DNA is a promising scaffolding candidate for molecular electronic bottom‐up self‐assembly approaches of SWNT devices. We report in this work the synthesis and characterization of individual Pd nanowires as thin as 30 nm showing ohmic behavior at room temperature.

Superconducting Atomic Contacts inductively coupled to a microwave resonator

We describe and characterize a microwave setup to probe the Andreev levels of a superconducting atomic contact. The contact is part of a superconducting loop inductively coupled to a superconducting coplanar resonator. By monitoring the resonator reflection coefficient close to its resonance frequency as a function of both flux through the loop and frequency of a second tone we perform spectroscopy of the transition between two Andreev levels of highly transmitting channels of the contact. The results indicate how to perform coherent manipulation of these states.

Superconducting quantum point contacts

Abstract We review our experiments on the electronic transport properties of atomic contacts between metallic electrodes, in particular superconducting ones. Despite ignorance of the exact atomic configuration, these ultimate quantum point contacts can be manipulated and well characterized in-situ. They allow performing fundamental tests of the scattering theory of quantum transport. In particular, we discuss the case of the Josephson effect.

Quantum Noise and Mutiple Andreev Reflections in Superconducting Contacts

The mechanism of multiple Andreev reflections (MAR) leads to a rather complex behavior of the noise spectral density in superconducting quantum point contacts as function of the relevant parameters. In this contribution we analyze recent theoretical and experimental efforts which have permitted to clarify this issue to a great extent. The theoretical description of noise in the coherent MAR regime will be summarized, discussing its main predictions for equilibrium and non-equilibrium current fluctuations. We then analyze noise measurements in well characterized superconducting atomic contacts. These systems allow for a direct test of the theoretical predictions without fitting parameters. In particular, the increase of the effective charge corresponding to the openning of higher order Andreev channels has been verified.